Mendelian Susceptibility to Mycobacterial Disease (MSMD) is a group of rare inherited immunodeficiencies, typically presenting in infancy or early childhood, caused by mutations in the IL-12/IFN-γ axis that result in selective vulnerability to weakly virulent mycobacteria (such as BCG vaccine strains and nontuberculous mycobacteria) and certain other intracellular organisms.

Mendelian Susceptibility to Mycobacterial Disease (MSMD) — Paediatric Notes

Mendelian susceptibility to mycobacterial disease (MSMD) is a group of rare inborn errors of immunity (IEI) characterised by a selective, inherited vulnerability to poorly virulent mycobacteria — organisms that would not normally cause disease in immunocompetent individuals ^[1]^[2].

Breaking the name down:

Mendelian = inherited in a classical single-gene (monogenic) fashion (autosomal dominant, autosomal recessive, or X-linked)
Susceptibility = predisposition — the immune system cannot contain the organism
Mycobacterial disease = clinical disease caused by mycobacteria, including:
- BCG vaccine strains (Mycobacterium bovis BCG) — the classic paediatric presentation
- Non-tuberculous mycobacteria (NTM) / environmental mycobacteria (e.g., M. avium complex)
- Mycobacterium tuberculosis (some patients also develop severe TB)

MSMD is classified under IUIS Category 6: Defects of Intrinsic and Innate Immunity ^[1]^[2]^[3]. These patients have a fundamentally intact adaptive immune system (normal T-cells, B-cells, immunoglobulins) but a specific defect in the IFN-γ/IL-12 axis — the cytokine circuit that activates macrophages to kill intracellular pathogens.

Core Concept — Why Mycobacteria Specifically?

Mycobacteria are obligate intracellular pathogens that survive inside macrophage phagosomes. Killing them requires macrophage activation via IFN-γ. If IFN-γ signalling is broken at any point — from its induction (IL-12) to its receptor (IFN-γR1/R2) to its downstream signalling (STAT1, IRF8, etc.) — macrophages cannot be "switched on" and mycobacteria proliferate unchecked. This is why the susceptibility is relatively narrow: most other infections are handled by alternative pathways (e.g., type I interferons for viruses, neutrophils for extracellular bacteria).

2. Epidemiology & Risk Factors

Extremely rare — estimated prevalence < 1 per 100,000 worldwide, though likely underdiagnosed, especially in TB-endemic regions where disseminated mycobacterial disease is attributed to "severe TB" rather than host genetics.
Over 20 genes and > 30 genetic aetiologies have been identified to date (IUIS 2024 update) ^[1].
Most cases present in infancy or early childhood (typically after BCG vaccination or environmental mycobacterial exposure).

Universal neonatal BCG vaccination is practised in Hong Kong (given at birth or shortly after). This is critically relevant because:
- BCG dissemination is the classic sentinel event that unmasks MSMD in the paediatric setting ^[2]^[3].
- A neonate with MSMD who receives BCG may develop local BCG-itis, regional lymphadenitis, or disseminated BCG-osis within weeks to months.
Hong Kong also has an intermediate TB incidence (~40–50/100,000), so children with MSMD may also present with severe or disseminated M. tuberculosis disease.
Consanguinity is less common in the Hong Kong Chinese population than in Middle Eastern or South Asian populations, but autosomal recessive forms still occur (de novo mutations, founder effects).

Factor	Explanation
Family history of unexplained death in infancy/childhood	Affected siblings who died of "disseminated TB" or "BCG complications"
Consanguinity	Increases risk of autosomal recessive forms
BCG vaccination	The "challenge" that reveals the defect — not a risk factor per se but the trigger
Environmental mycobacterial exposure	Ubiquitous in water/soil — normal hosts clear them; MSMD patients cannot

3. Anatomy & Function of the IFN-γ/IL-12 Axis

Understanding MSMD requires understanding the IFN-γ/IL-12 positive feedback loop — the central axis of antimycobacterial defence.

Step-by-step:

Macrophage phagocytoses mycobacterium → Pathogen-associated molecular patterns (PAMPs) are recognised by TLRs and intracellular receptors → macrophage produces IL-12 (a heterodimer of IL-12p35 and IL-12p40 subunits).
IL-12 acts on T-cells (CD4+ Th1) and NK cells via the IL-12 receptor (IL-12Rβ1 + IL-12Rβ2) → drives IFN-γ production.
IFN-γ (interferon-gamma, the "type II interferon") feeds back to the macrophage via the IFN-γ receptor (a heterotetramer of IFN-γR1 and IFN-γR2).
IFN-γR engagement → JAK1/JAK2 activation → STAT1 phosphorylation → STAT1 dimerisation (GAF complex) → nuclear translocation → transcription of antimicrobial genes.
Activated macrophage → enhanced phagolysosomal fusion, production of reactive oxygen species (ROS) and reactive nitrogen intermediates (RNI, e.g., NO) → mycobacterial killing.
Granuloma formation — activated macrophages differentiate into epithelioid cells and giant cells, walling off the infection. TNF-α is critical for granuloma integrity (recall: anti-TNF-α biologics reactivate TB for this reason) ^[4].

Component	Gene(s)	Role
IFN-γ Receptor 1	IFNGR1	Ligand-binding chain of the IFN-γ receptor
IFN-γ Receptor 2	IFNGR2	Signal-transducing chain
IL-12 p40 subunit	IL12B	Part of the IL-12 cytokine
IL-12 Receptor β1	IL12RB1	Binds IL-12 (and IL-23, sharing this subunit)
STAT1	STAT1	Transcription factor downstream of IFN-γR
IRF8	IRF8	Interferon regulatory factor 8 — myeloid cell development and IL-12 production
ISG15	ISG15	Interferon-stimulated gene 15 — stabilises IFN-γ signalling
NEMO/IKKγ	IKBKG	NF-κB essential modulator — involved in IL-12 production by macrophages
TYK2	TYK2	Tyrosine kinase downstream of multiple cytokine receptors including IL-12R
CYBB	CYBB	NADPH oxidase — overlap with CGD; some CYBB mutations cause MSMD phenotype
SPPL2a	SPPL2A	Signal peptide peptidase-like 2A — dendritic cell function
RORγ/RORγt	RORC	T-cell differentiation (Th17/IL-17 axis, also IFN-γ)

4. Aetiology & Pathophysiology

4.1 Genetic Aetiologies (Classified by Pathway Disrupted)

MSMD involves mutations in genes of the IFN-γ/IL-12 pathway — STAT1 (AD or AR, loss of function) → IFN-gamma pathway ^[2]^[3].

The genetic defects can be grouped by where in the circuit they act:

Gene	Inheritance	Mechanism
IL12B	AR	No functional IL-12 → T-cells/NK cells receive no signal → reduced IFN-γ production
IL12RB1	AR	IL-12 receptor cannot bind IL-12 → same consequence. Most common genetic cause of MSMD worldwide. Also affects IL-23 signalling → additional susceptibility to Salmonella and chronic mucocutaneous candidiasis (CMC)
IL12RB2	AR	Defective IL-12 signalling (rarer)
TYK2	AR	Downstream kinase of IL-12R; also affects other cytokine receptors (type I IFN, IL-10)
IRF8	AD or AR	Impaired IL-12 production by dendritic cells/macrophages
ISG15	AR	Reduced IFN-γ secretion by NK cells and T-cells
RORC	AR	Impaired Th1/Th17 differentiation → reduced IFN-γ and IL-17

Gene	Inheritance	Mechanism	Clinical Severity
IFNGR1	AR complete	No surface expression → no IFN-γ signalling at all	Most severe — early-onset, often fatal without HSCT
IFNGR1	AR partial	Reduced but not absent surface expression → residual signalling	Moderate — may respond to high-dose IFN-γ
IFNGR1	AD (dominant-negative)	Truncated receptor that reaches the surface but cannot transduce signal and blocks wild-type receptor	Moderate
IFNGR2	AR complete / partial	Same principle as IFNGR1 but rarer	Variable
STAT1	AD LOF (loss of function)	Impaired STAT1-mediated transcription downstream of IFN-γ (also affects IFN-α/β signalling)	Moderate to severe
STAT1	AR complete LOF	No STAT1 function at all → affects IFN-γ AND IFN-α/β → susceptibility to mycobacteria AND viruses	Very severe
JAK1	AR	Defective JAK-STAT signalling	Severe

High Yield — Complete vs Partial Defects

The clinical severity of MSMD correlates with the degree of residual IFN-γ signalling:

Complete deficiency (e.g., AR complete IFNGR1, AR complete STAT1) → no residual signalling → early-onset disseminated BCG-osis or NTM disease, poor granuloma formation, often fatal without haematopoietic stem cell transplant (HSCT).
Partial deficiency (e.g., partial IFNGR1, AD STAT1 LOF, IL12RB1 deficiency) → some residual signalling → later onset, milder disease, may respond to recombinant IFN-γ therapy and anti-mycobacterial drugs.

CYBB (X-linked): Certain CYBB mutations cause an MSMD phenotype without the full CGD picture — macrophages lack the respiratory burst component specifically needed for mycobacterial killing.
NEMO/IKBKG (X-linked): NF-κB pathway defect → impaired IL-12 production + ectodermal dysplasia.
Gain-of-function (GOF) mutations in STAT1: Importantly, GOF STAT1 mutations cause chronic mucocutaneous candidiasis (CMC), NOT MSMD. The LOF mutations cause MSMD. This is a favourite exam trap ^[2]^[3].

Exam Trap — STAT1 LOF vs GOF

STAT1 LOF (loss of function) → MSMD (mycobacteria, Salmonella)
STAT1 GOF (gain of function) → Chronic mucocutaneous candidiasis (CMC) (Candida, fungal infections)

Do NOT confuse these! The direction of the mutation dictates the clinical phenotype.

The unifying pathophysiology is:

Defective macrophage activation → Mycobacteria survive and multiply inside macrophages that cannot mount an adequate microbicidal response.
Defective granuloma formation → In complete defects, granulomas are poorly formed or "lepromatous" type (lots of organisms, few lymphocytes, poorly organised) rather than the well-formed epithelioid granulomas seen in immunocompetent TB.
Dissemination → Without containment, mycobacteria spread via the bloodstream and lymphatics → disseminated disease involving lungs, liver, spleen, bone, lymph nodes, CNS.
Susceptibility to Salmonella → IFN-γ is also critical for macrophage killing of intracellular Salmonella species. About 50% of patients with IL-12Rβ1 deficiency develop invasive salmonellosis ^[1]^[2].
In AR complete STAT1: IFN-α/β signalling is also impaired → additional susceptibility to viral infections (herpes viruses, etc.) ^[1].

Mycobacteria (RECALL CFB TB LECTURE, especially paedi patients with disseminated mycobacterial infection from WEAK mycobacteria such as BCG vaccine and NTM) ^[2]^[3]

5. Classification

MSMD sits within Table 6: Defects of Intrinsic and Innate Immunity of the IUIS classification of Inborn Errors of Immunity ^[1]^[2].

Category	Genetic Examples	Residual IFN-γ Response	Prognosis
Complete deficiency	AR IFNGR1 complete, AR IFNGR2 complete, AR STAT1 complete	None	Poor; HSCT usually required
Partial deficiency	AR IFNGR1 partial, AD IFNGR1 (dominant-negative), AD STAT1 LOF, IL12B, IL12RB1, TYK2	Reduced but present	Better; may respond to antimycobacterials ± recombinant IFN-γ

Pattern	Examples
Autosomal Recessive (AR)	IFNGR1, IFNGR2, IL12B, IL12RB1, IL12RB2, STAT1 (complete), TYK2, ISG15, RORC, SPPL2A
Autosomal Dominant (AD)	IFNGR1 (dominant-negative), STAT1 (partial LOF), IRF8 (AD form)
X-linked	CYBB, NEMO/IKBKG

STAT1 (AD or AR, loss of function) → IFN-gamma pathway ^[2]^[3]

6. Clinical Features

Age of presentation: Typically infancy to early childhood (often within the first 1–2 years of life), coinciding with BCG vaccination and early environmental mycobacterial exposure.
Severity spectrum: From localised BCG-itis (partial defects) to fulminant disseminated BCG-osis (complete defects).
Family history: May have consanguinity, unexplained sibling/childhood relative deaths, known family history of PID ^[5].

Symptom	Pathophysiological Basis
Prolonged fever (weeks to months, not responding to standard antibiotics)	Ongoing mycobacterial proliferation → persistent macrophage activation → cytokine-driven fever (IL-1, IL-6, TNF-α). The SPUR infection pattern: Serious, Persistent, Unusual, Recurrent ^[5].
Failure to thrive / weight loss	Chronic infection → catabolic state, anorexia, cytokine-mediated wasting (TNF-α = "cachectin"). Failure to thrive is a warning sign of PID ^[5].
Chronic cough	Pulmonary mycobacterial disease → granulomatous inflammation of lung parenchyma. In disseminated disease, miliary pattern throughout both lungs.
Lymph node swelling (especially axillary/cervical on side of BCG injection)	BCG lymphadenitis → failure to contain BCG at the injection site → regional lymph node spread. In disseminated disease, generalised lymphadenopathy.
Skin lesions at BCG site (non-healing ulcer, abscess, sinus formation)	Local BCG-itis: macrophages at the injection site cannot kill BCG → progressive local tissue destruction and caseous necrosis.
Bone/joint pain	Osteomyelitis from haematogenous mycobacterial spread — mycobacteria have tropism for bone (especially vertebral bodies, metaphyses of long bones in children).
Abdominal pain, diarrhoea, hepatosplenomegaly	Disseminated mycobacterial disease involving liver, spleen, mesenteric lymph nodes, and intestinal wall.
Night sweats	Classic TB symptom — cytokine (TNF-α, IL-1)-driven diurnal fever pattern with defervescence during sleep causing drenching sweats.
*Recurrent Salmonella* infections** (enteric fever, bacteraemia)	~50% of patients with IL-12Rβ1 deficiency develop salmonellosis — IFN-γ is essential for macrophage killing of intracellular Salmonella. Loss of IL-12/IL-23 signalling also affects IL-23-dependent gut mucosal immunity.

Sign	Pathophysiological Basis
BCG site abnormalities: non-healing ulcer ( > 12 weeks), cold abscess, fistula, keloid-like scar	Failure of local macrophage killing → progressive local BCG disease
Regional lymphadenopathy (ipsilateral axillary to BCG site, cervical)	Mycobacterial spread to draining lymph nodes; nodes may be matted, fluctuant (caseous necrosis), or with sinus tract formation
Generalised lymphadenopathy	Disseminated mycobacterial disease (haematogenous spread)
Hepatosplenomegaly	Reticuloendothelial system involvement — mycobacteria-laden macrophages infiltrate liver and spleen
Wasting / failure to thrive	Chronic catabolic state; plot on growth chart — weight and height may cross centiles downward
Skin lesions beyond BCG site: papules, nodules, cold abscesses, verrucous lesions	Cutaneous dissemination of mycobacteria
Pallor	Anaemia of chronic disease (hepcidin upregulation → iron sequestration) or bone marrow infiltration
Respiratory signs: tachypnoea, crepitations, bronchial breathing	Pulmonary mycobacterial disease; may present with miliary pattern or lobar consolidation
Bony tenderness / deformity (e.g., gibbus deformity from vertebral involvement)	Mycobacterial osteomyelitis / spondylitis
CNS signs (in disseminated disease): meningism, cranial nerve palsies, altered consciousness	Mycobacterial meningitis from haematogenous spread
Poor or absent granuloma formation on biopsy (especially in complete defects)	Without IFN-γ, macrophages cannot differentiate into epithelioid cells → "lepromatous" or "multibacillary" histology with abundant acid-fast bacilli (AFB) and poorly formed granulomas. This is a pathognomonic clue to MSMD.

High Yield — The 'BCG Red Flag'

BCG → SCID, CGD, Mendelian susceptibility to mycobacterial diseases (MSMD)/STAT1 ^[5]

In Hong Kong, where neonatal BCG is universal, any infant with disseminated or severe local BCG disease should be investigated for MSMD (and SCID/CGD). The differential for BCG complications includes:

MSMD (IFN-γ/IL-12 axis defect)
SCID (severe combined immunodeficiency — T-cell defect → no IFN-γ production)
CGD (chronic granulomatous disease — NADPH oxidase defect → defective mycobacterial killing)
HIV infection (secondary immunodeficiency)

A useful approach: if the child has disseminated BCG AND viral infections → think SCID (T-cell defect). If BCG AND fungal abscesses → think CGD. If BCG/NTM ± Salmonella BUT otherwise well (normal lymphocyte counts, normal immunoglobulins) → think MSMD.

Intracellular pathogens since IFN-gamma (type 2) ^[2]^[3]

Organism	Notes
M. bovis BCG	Most common presenting infection in countries with universal BCG (like HK)
NTM (M. avium, M. abscessus, M. fortuitum, etc.)	Environmental mycobacteria — normally harmless → infection from WEAK mycobacteria ^[2]^[3]
M. tuberculosis	Can present as severe/disseminated TB
Salmonella spp.	Salmonella — especially with IL-12Rβ1 or IL-12B deficiency ^[2]^[3]
Listeria, Nocardia	Other intracellular bacteria occasionally
Histoplasma, Coccidioides (rare)	Dimorphic fungi (intracellular phase)
Viruses (only in AR complete STAT1)	Because STAT1 is also downstream of type I IFN (IFN-α/β) → viral susceptibility

Autoimmunity (rare but described): IFN-γ dysregulation can disrupt immune tolerance.
Lymphoproliferation/granulomatous disease in some cases.

Revisiting the structured PID history taking ^[5]:

History Domain	Specific MSMD-Relevant Points
HPI	Onset after BCG (timing!), site of infection, progression, response to antibiotics
Birth history	When was BCG given? Any neonatal complications?
Immunization history	BCG → MSMD. OPV → not an issue in MSMD (that's SCID/XLA) ^[5]
Developmental history	Failure to thrive?
Family history	Consanguinity, unexplained childhood deaths, known PID
Drug history	Any immunosuppressants? (to exclude secondary immunodeficiency)

Feature	MSMD	SCID	CGD
Lymphocyte count	Normal	Very low (especially T-cells)	Normal
Immunoglobulins	Normal	Low (no T-cell help)	Normal
DHR / NBT test	Normal	Normal	Abnormal (low/absent oxidative burst)
Typical infections	Mycobacteria, Salmonella	All — bacteria, viruses, fungi, Pneumocystis	Catalase-positive bacteria (Staph, Serratia, Aspergillus), mycobacteria
BCG complication	Yes	Yes	Yes
Viral infections	Only if STAT1 AR complete	Yes (severe)	No
Newborn screening	Not on TREC/KREC	Detected by TREC	Not on TREC/KREC

High Yield Summary

MSMD = monogenic defects in the IFN-γ/IL-12 axis → selective vulnerability to weakly virulent mycobacteria (BCG, NTM) and Salmonella.
IUIS Category 6: Defects of Intrinsic and Innate Immunity — adaptive immunity (T-cells, B-cells, Ig) is intact.
IL-12Rβ1 deficiency is the most common genetic cause worldwide.
Complete defects (no residual IFN-γ signalling) → severe, early-onset, poor prognosis, need HSCT. Partial defects → milder, may respond to antimycobacterials ± recombinant IFN-γ.
BCG dissemination in an infant in Hong Kong = think MSMD, SCID, or CGD.
STAT1 LOF → MSMD; STAT1 GOF → chronic mucocutaneous candidiasis (CMC) — do not confuse.
Histology clue: poorly formed granulomas ("lepromatous" pattern) with abundant AFB.
~50% of IL-12Rβ1 deficient patients also get Salmonella infections (IL-12Rβ1 is shared with IL-23R → gut immunity).
Normal CBC, normal immunoglobulins, normal DHR test → advanced functional assays (IFN-γ/IL-12 production, STAT1 phosphorylation, genetic sequencing) required for diagnosis.

Active Recall — MSMD: Definition, Pathophysiology & Clinical Features

Differential Diagnosis of MSMD

The child who prompts consideration of MSMD typically presents with one or more of:

Disseminated or severe local BCG disease after neonatal vaccination
Recurrent or disseminated NTM infection in an otherwise "normal-looking" child
Severe TB disproportionate to exposure or out of keeping with the child's age
Invasive Salmonella disease without an obvious predisposition
SPUR infection pattern: Serious, Persistent, Unusual, Recurrent ^[5]

The differential therefore spans two overlapping questions:

What else causes disseminated mycobacterial disease in a child? (the infection-centred DDx)
What other primary immunodeficiency could produce this picture? (the PID-centred DDx)

We must also exclude secondary (acquired) immunodeficiency and non-immunological explanations before settling on MSMD ^[5].

Systematic Differential Diagnosis

These are the key differentials on an exam stem — a child with BCG dissemination or severe mycobacterial infection and you must pick the most likely PID.

Condition	IUIS Category	Why Mycobacterial Susceptibility?	Key Distinguishing Features
SCID (Severe Combined Immunodeficiency)	Combined T and B cell deficiency	No functional T-cells → no IFN-γ production at all. The most upstream failure in the entire antimycobacterial circuit.	Very low lymphocyte count / absent T-cells on subsets. Presents in first months of life with multiple infection types — bacterial, viral (especially CMV, RSV, adenovirus), fungal (Pneumocystis), AND mycobacterial. BCG → SCID, CGD, MSMD/STAT1 ^[5]. TREC-based newborn screening detects SCID. Failure to thrive is severe and early.
CGD (Chronic Granulomatous Disease)	Phagocyte defect	NADPH oxidase deficiency → no respiratory burst → cannot kill catalase-positive organisms including mycobacteria (which are catalase-positive).	Low dihydrorhodamine reduction (DHR) ^[2]^[3]. Recurrent abscesses (liver, lung, perianal), **Staphylococcus aureus, Serratia, Aspergillus. BCG vaccine will disseminate ^[2]^[3]. Normal lymphocyte counts and immunoglobulins. Granulomas are well-formed (unlike MSMD complete defects). CYBB (X-linked, loss of function)** is the most common genetic cause ^[2]^[3].
IL-12Rβ1 deficiency (technically a form of MSMD but worth knowing as a separate entity)	Intrinsic/innate immunity	Shared subunit for IL-12R AND IL-23R → impaired IFN-γ AND IL-17 production	Most common genetic cause of MSMD worldwide. Also causes Salmonella susceptibility (~50%) and can cause chronic mucocutaneous candidiasis (via IL-23/Th17 loss) ^[2]^[3].
Hyper-IgE Syndrome (HIES / Job Syndrome)	Combined immunodeficiency with syndromic features	Defective Th17 signalling (STAT3 or DOCK8 mutations) → impaired neutrophil chemotaxis and mucosal defence	Very high serum IgE ( > 2000 IU/mL). Classic triad: recurrent staphylococcal skin abscesses ("cold abscesses"), eczema, pneumonia with pneumatoceles. Mycobacterial disease is NOT the dominant feature (more staph/fungal). Coarse facies, retained primary teeth.
DiGeorge Syndrome (22q11.2 deletion)	Combined (thymic defect)	Thymic hypoplasia → variable T-cell deficiency → reduced IFN-γ	Neonatal hypocalcaemia (parathyroid hypoplasia), conotruncal cardiac defects (ToF, truncus arteriosus, IAA type B), facial dysmorphism (low-set ears, short philtrum, micrognathia). Most cases are "partial" DiGeorge with mild T-cell deficiency; severe mycobacterial disease is uncommon unless "complete" DiGeorge ^[5].
STAT1 GOF (Gain-of-Function)	Immune dysregulation	Enhanced STAT1 signalling → paradoxical impairment of Th17 responses → susceptibility to Candida, not mycobacteria	Chronic mucocutaneous candidiasis (CMC) is the hallmark — oral thrush, oesophageal candidiasis, dermatophytosis. Autoimmune manifestations (thyroiditis, cytopenias). NOT mycobacterial disease — this is the exam trap distinguishing it from STAT1 LOF ^[2]^[3].
Leukocyte Adhesion Deficiency (LAD)	Phagocyte defect	Defective neutrophil migration → cannot reach sites of infection	Delayed umbilical cord separation (classically > 30 days), absent pus formation despite severe tissue necrosis, marked leukocytosis (neutrophils cannot leave the bloodstream). Not typically mycobacterial.

AOS Exam Recall — CGD vs MSMD

A 2-year-old boy with recurrent cervical lymphadenitis since 6 months old, BCG scar 2 cm, scars over previous cervical lymphadenitis, palpable axillary lymph node, inflamed gum, perianal sepsis, hepatosplenomegaly. Normal CBC but elevated IgG. Answer: Chronic Granulomatous Disease ^[6].

The clues here: recurrent abscesses (not just mycobacteria), perianal sepsis, inflamed gum (gingivitis/periodontitis is characteristic of CGD), hepatosplenomegaly from granulomatous infiltration, and a large BCG scar (but not disseminated BCG-osis). Elevated IgG reflects chronic antigenic stimulation. Normal DHR would redirect you to MSMD.

Exclude secondary immunodeficiency or non-immunological defects! ^[5]

Condition	Mechanism	How to Differentiate
HIV infection	CD4+ T-cell depletion → impaired IFN-γ production → mycobacterial susceptibility. HIV and TB have a bi-directional relationship ^[4]^[7].	HIV serology / PCR. In neonates: maternal HIV status, PCR at birth/6 weeks/3 months (antibody testing unreliable < 18 months due to maternal IgG).
Malnutrition	Protein-energy malnutrition impairs cell-mediated immunity (thymic atrophy, reduced T-cell function) → "nutritional immunodeficiency"	Anthropometry (weight-for-height < -2 SD), albumin, clinical context. Common in developing countries as a co-factor for severe TB.
Immunosuppressive drugs	Corticosteroids, anti-TNF-α biologics, calcineurin inhibitors, chemotherapy → impaired T-cell / macrophage function	Drug history! Anti-TNF-alpha is the classical example that can increase risk of developing TB ^[4]. Post-transplant patients on tacrolimus/mycophenolate.
Malignancy (especially haematological)	Leukaemia/lymphoma → immune paresis; chemotherapy → neutropenia and lymphopenia	CBC with differential, blood film (blasts), LDH, uric acid
Drug-induced Ig deficiency	Anti-epileptic, immunosuppressant, monoclonal Ab, anti-malarial ^[5]	Drug history and immunoglobulin levels

Condition	How It Mimics	Key Differentiator
Severe TB in immunocompetent child (heavy inoculum, virulent strain)	Disseminated TB can occur in normal infants < 2 years (immature cell-mediated immunity) without a genetic defect	Age: primary TB in infants < 2 years can be severe even without PID. Key: if the child has disease from weakly virulent organisms (BCG, NTM) rather than M. tuberculosis, PID is far more likely. Response to anti-TB treatment is good in immunocompetent children.
Lymphoma / lymphoproliferative disease	Generalised lymphadenopathy, hepatosplenomegaly, fever, weight loss	Blood film, LDH, biopsy showing malignant cells rather than granulomas. No mycobacterial culture positivity.
Sarcoidosis (rare in children)	Non-caseating granulomas, lymphadenopathy, pulmonary infiltrates	Non-caseating granulomas (vs caseating in TB/MSMD), elevated ACE, hypercalcaemia. Mycobacterial cultures negative.
Histiocytic disorders (e.g., Langerhans cell histiocytosis)	Bone lesions, hepatosplenomegaly, skin lesions	Biopsy: CD1a+ / CD207+ histiocytes with Birbeck granules, NOT granulomatous inflammation
Cystic fibrosis	Recurrent pulmonary infections, failure to thrive — but with different organisms (Pseudomonas, Staph) and not mycobacteria predominantly	Sweat chloride test, genetic testing for CFTR mutations. Productive cough and bronchiectasis are prominent.

In Hong Kong, when a neonate/infant presents with adverse BCG reaction, the differential is critically important:

Type of BCG Reaction	Normal or PID?	Explanation
Local BCG ulcer < 2 cm, self-healing	Normal	Expected immune response; heals in 6–12 weeks
Large local ulcer > 2 cm or lasting > 12 weeks	Possible PID (MSMD, CGD) or local inoculation issue	Needs clinical monitoring; consider PID workup if progressive
Ipsilateral regional (axillary) lymphadenitis, non-suppurative	Often normal (occurs in ~1–10% of BCG recipients)	Self-limiting; observe
Suppurative regional lymphadenitis	Warrants PID consideration	May need aspiration; evaluate for MSMD/CGD
Disseminated BCG-osis (multi-organ, distant sites)	Almost always PID — SCID, CGD, or MSMD ^[2]^[3]^[5]	Life-threatening; immediate PID workup essential — lymphocyte subsets, DHR, IFN-γ/IL-12 assays, genetic testing
BCG osteitis / osteomyelitis (bones distant from injection site)	Strongly suggests PID	MSMD, CGD, or SCID. Biopsy + culture + PID workup

High Yield — BCG Complications as PID Sentinel

BCG → SCID, CGD, Mendelian susceptibility to mycobacterial diseases (MSMD)/STAT1 ^[5]

Do not dismiss a severe BCG reaction as "just a vaccine complication." In any infant with disseminated BCG or BCG osteitis, always perform:

Lymphocyte subsets (rule out SCID)
DHR test (rule out CGD)
IFN-γ/IL-12 functional assays (rule out MSMD)
HIV testing (rule out secondary immunodeficiency)

The single most dangerous error is attributing disseminated BCG-osis to "bad luck" and missing the underlying PID.

When you encounter a child with severe/disseminated mycobacterial disease:

Step 1: Exclude secondary immunodeficiency

HIV, drugs, malnutrition, malignancy

Step 2: Basic immune workup

CBC + differential → lymphopenia? neutropenia?
Lymphocyte subsets (CD3, CD4, CD8, CD19, CD16/56) → T-cell deficiency = SCID pathway
Immunoglobulins (IgG, IgA, IgM) → hypogammaglobulinaemia = antibody deficiency pathway

Step 3: Phagocyte function

DHR test → abnormal = CGD ^[2]^[3]

Step 4: IFN-γ/IL-12 axis (if Steps 2–3 normal)

Advanced functional tests ^[2]^[3]:
- Whole-blood IFN-γ production assay (stimulate with BCG/IL-12 and measure IFN-γ)
- IL-12 production assay (stimulate monocytes and measure IL-12p70)
- STAT1 phosphorylation assay
- IFN-γR surface expression (flow cytometry)
Genetic testing (targeted panel or whole-exome sequencing)

Step 5: If IFN-γ/IL-12 axis normal

Consider rarer IEI, structural abnormalities, or immunocompetent child with severe infection

Which pathway is dysregulated in patients with MSMD? → STAT1 → interferon gamma ^[3]

Feature	MSMD	SCID	CGD	HIV	Immunocompetent Child
Age at presentation	Months–years	First weeks–months	Months–years	Variable	Any
Lymphocyte count	Normal	Very low	Normal	Low CD4 (late)	Normal
Immunoglobulins	Normal	Low	Normal (may have ↑IgG)	Variable	Normal
DHR test	Normal	Normal	Abnormal	Normal	Normal
Organisms	Mycobacteria + Salmonella	Everything (bacteria, viruses, fungi, PJP)	Catalase+ bacteria, fungi, mycobacteria	Opportunistic (PJP, crypto, TB, CMV)	M. tuberculosis (virulent strain)
BCG dissemination	Yes	Yes	Yes	Possible	Very rare
Viral infections	Only STAT1 AR complete	Severe	No	Yes	No
Abscess formation	Uncommon	Uncommon	Hallmark	Uncommon	No
Granuloma quality	Poor (complete) / variable	Poor / absent	Excessive (hence the name)	Variable	Well-formed
Treatment	Anti-mycobacterials ± IFN-γ ± HSCT	HSCT (curative)	Septrin + itraconazole + HSCT	ART	Standard anti-TB

High Yield Summary — Differential Diagnosis of MSMD

The three PIDs that present with BCG dissemination in infants: SCID, CGD, MSMD — distinguish by lymphocyte subsets (SCID), DHR (CGD), and IFN-γ/IL-12 assays (MSMD).
Always exclude secondary immunodeficiency first — HIV, drugs (especially anti-TNF-α), malnutrition, malignancy.
MSMD has normal lymphocyte counts, normal immunoglobulins, and a normal DHR test — it is a "functionally specific" defect visible only on advanced IFN-γ/IL-12 assays or genetic testing.
The pathogen pattern is the biggest clue: disease caused by weakly virulent mycobacteria (BCG, NTM) ± Salmonella, without viral or fungal infections → think MSMD.
STAT1 LOF = MSMD (mycobacteria); STAT1 GOF = CMC (Candida) — direction of mutation determines the phenotype.
CGD overlaps significantly (BCG disease, granulomas, hepatosplenomegaly) but features recurrent abscesses with catalase-positive organisms and an abnormal DHR test.
SCID is the most severe PID presenting with BCG disease but has pansusceptibility (all pathogen classes) and lymphopenia / absent T-cells.

Active Recall — Differential Diagnosis of MSMD

References

^[1] Lecture slides: GC 144. A child with recurrent infections Primary immunodeficiencies.pdf (p35 — MSMD section) ^[2] Senior notes: Jerry's immunodeficiencies.pdf (Category 6 — Intrinsic and innate immunity, MSMD; Q13 — STAT1/IFN-gamma) ^[3] Lecture slides: Block C - A child with recurrent infections_ Primary immunodeficiencies.pdf ^[4] Senior notes: Gen Clerk Anaes + Microbiology Summary.pdf (TB in immunocompromised, anti-TNF-alpha and TB) ^[5] Senior notes: Adrian Lui Pediatrics Notes.pdf (p407 — Approach to Immunodeficiency, warning signs of PID, BCG complications) ^[6] AOS material: AOS - Paeds.pdf (Question 9 — CGD case stem) ^[7] Senior notes: Maksim Medicine Notes.pdf (p190 — Tuberculosis section)

Diagnostic Criteria, Algorithm & Investigations for MSMD

Unlike conditions such as CVID (which have formal ESID registry criteria ^[8]), MSMD does not have a single universally codified set of diagnostic criteria with a tick-box checklist. Instead, the diagnosis rests on the convergence of three pillars:

Pillar	Requirement
1. Clinical phenotype	Severe, persistent, or disseminated disease caused by weakly virulent mycobacteria (BCG, NTM) and/or Salmonella, with or without severe M. tuberculosis disease, in the absence of known secondary immunodeficiency
2. Functional immunological defect	Demonstration of a defect in the IFN-γ/IL-12 axis on advanced functional tests ^[1]^[2] — e.g., impaired IFN-γ production, absent IFN-γR surface expression, defective STAT1 phosphorylation
3. Genetic confirmation	Identification of a pathogenic mutation in a known MSMD gene (IFNGR1, IFNGR2, IL12B, IL12RB1, STAT1, IRF8, TYK2, ISG15, RORC, CYBB, NEMO, etc.) via genetic testing ^[8]

Genetic testing has an important role in: Diagnosis (All cases), Family studies, Prognostication, Prenatal diagnosis, Defining new diseases ^[8]

When to Suspect MSMD — Practical Criteria

Suspect MSMD when ALL of the following are present:

A child with disseminated or unusually severe disease from BCG, NTM, or M. tuberculosis, AND/OR recurrent invasive Salmonella — i.e., the SPUR infection pattern: Serious, Persistent, Unusual, Recurrent ^[5]
Normal routine immune workup: normal lymphocyte counts and subsets, normal immunoglobulins, normal DHR test
No secondary immunodeficiency (HIV-negative, no immunosuppressive drugs, no malnutrition, no malignancy)
± Positive family history (consanguinity, sibling death, known PID) ^[5]

If criteria 1–3 are met → proceed to IFN-γ/IL-12 axis functional assays and genetic testing.

Practical "Red Flags" That Should Trigger MSMD Workup

Red Flag	Why It Points to MSMD
Disseminated BCG-osis after neonatal BCG	BCG is a weakly virulent mycobacterium — dissemination implies a specific host defect in mycobacterial defence ^[1]^[2]^[5]
NTM disease in an otherwise well-appearing child	NTM are environmental organisms that rarely cause disease in immunocompetent hosts ^[2]
Severe TB disproportionate to exposure	Suggests host susceptibility factor rather than heavy exposure
*Recurrent non-typhoidal Salmonella* bacteraemia**	Salmonella susceptibility is characteristic, especially in IL-12Rβ1 and IL-12B deficiency ^[1]^[2]
Complications from live vaccine ^[5]	BCG dissemination is the classic live-vaccine complication in MSMD
Poor or absent granuloma formation on biopsy	Indicates defective macrophage activation (IFN-γ-dependent process)

The algorithm follows a stepwise approach — starting from clinical suspicion, through exclusion of secondary causes, basic PID workup, phagocyte assessment, and finally the IFN-γ/IL-12 axis-specific investigations.

3. Investigation Modalities — Detailed Breakdown

The investigations can be organised into five tiers, reflecting the stepwise diagnostic approach above.

These must be done first — you cannot diagnose a primary immunodeficiency without ruling out acquired causes ^[5].

Investigation	What You're Looking For	Key Findings / Interpretation
HIV serology (children > 18 months) / HIV PCR (infants < 18 months)	HIV infection causing CD4 depletion	Positive → secondary immunodeficiency; treat with ART. In neonates, maternal IgG crosses placenta → serology unreliable < 18 months; use HIV DNA/RNA PCR instead.
Drug history	Immunosuppressive medications	Anti-TNF-alpha ^[4], corticosteroids, calcineurin inhibitors, chemotherapy, anti-epileptics, immunosuppressants, monoclonal antibodies, anti-malarials → drug-induced Ig deficiency ^[5]
Nutritional assessment	Protein-energy malnutrition	Weight-for-height z-score < -2 SD, low albumin, clinical signs of kwashiorkor/marasmus
Blood film	Leukaemia / malignancy	Blasts, abnormal cells → refer haematology

Phenotypic investigations: IgG, IgA, IgM levels; Vaccine responses; T, B, NK cell enumerations; Lymphocyte subsets ^[8]

Investigation	What It Tests	Expected Result in MSMD	Abnormal Result → Suggests
CBC with differential white cell count	Total lymphocyte count, neutrophil count, platelet count	Normal in MSMD	Lymphopenia → SCID; Neutropenia → congenital neutropenia; Pancytopenia → bone marrow infiltration/failure
Peripheral blood film	Morphology of cells	Normal	Blasts → leukaemia; Howell-Jolly bodies → asplenia
Lymphocyte subsets by flow cytometry (CD3+ T-cells, CD4+, CD8+, CD19+ B-cells, CD16+/56+ NK cells)	Enumerate the major lymphocyte populations	All normal in MSMD — T-cells, B-cells, and NK cells are present in normal numbers and proportions	Very low / absent CD3+ T-cells → SCID; Low CD19+ → XLA/agammaglobulinaemia; Low NK → NK-cell deficiency
Serum immunoglobulins (IgG, IgA, IgM ± IgE) — must interpret against age-appropriate reference ranges (paediatric Ig levels change significantly with age)	Humoral immunity	Normal in MSMD	Low IgG + IgA ± IgM → CVID, XLA, Hyper-IgM; Very high IgE → Hyper-IgE syndrome or atopy
Vaccine antibody responses (e.g., tetanus, diphtheria, Haemophilus, pneumococcal polysaccharide)	Functional antibody production	Normal in MSMD (B-cells and T-cell help are intact)	Poor vaccine responses → antibody deficiency (CVID, specific antibody deficiency)
TREC / KREC (if newborn screening available)	T-cell receptor excision circles / Kappa-deleting recombination excision circles — markers of recent thymic output and B-cell development	Normal in MSMD	Low/absent TREC → SCID; Low KREC → XLA/agammaglobulinaemia ^[2]

Key Point — MSMD Has a Normal Basic Workup

This is what makes MSMD tricky to diagnose: the entire routine PID workup is normal. CBC is normal, lymphocyte subsets are normal, immunoglobulins are normal, vaccine responses are normal, and the DHR test is normal. The defect is functionally specific to the IFN-γ/IL-12 axis and only becomes apparent on advanced functional tests ^[1]^[2]. This is why clinical suspicion based on the pathogen pattern (weakly virulent mycobacteria, Salmonella) is so critical.

Investigation	What It Tests	Expected Result in MSMD	Abnormal Result → Suggests
DHR (Dihydrorhodamine) test — the gold standard for CGD screening	Measures oxidative burst in neutrophils/monocytes. DHR 123 is a non-fluorescent dye → converted to fluorescent rhodamine 123 by reactive oxygen species (ROS) → detected by flow cytometry	Normal (normal fluorescence shift after PMA stimulation)	Low / absent fluorescence → CGD ^[2]^[3]. CGD has defective NADPH oxidase → cannot produce ROS → DHR stays non-fluorescent.
NBT (Nitroblue Tetrazolium) test — older test, largely replaced by DHR	NBT is a yellow dye → reduced to blue-purple formazan by ROS in neutrophils → read visually under microscope	Normal in MSMD	Negative (no colour change) → CGD

Why test DHR? Because CGD and MSMD can look clinically similar — both cause BCG dissemination and mycobacterial disease. The DHR test rapidly and reliably separates them. Think of it as the "CGD gatekeeper" — if normal, move on to IFN-γ/IL-12 axis testing ^[2]^[3].

This is the diagnostic core for MSMD. These are the advanced functional tests referred to in both the GC lecture and senior notes ^[1]^[2]^[8].

Investigation	Principle	What It Detects	Key Findings in MSMD
Whole-blood IFN-γ production assay	Stimulate whole blood with BCG + IL-12 (or PHA as control) → measure IFN-γ in supernatant by ELISA	Defects in the IL-12 → IFN-γ arm of the pathway	Reduced/absent IFN-γ production after BCG+IL-12 stimulation → suggests defect in IL-12 signalling or IFN-γ production. If IFN-γ is low with BCG alone but corrects with exogenous IL-12 → IL-12 production defect. If IFN-γ remains low even with IL-12 → IL-12R or downstream defect.
IL-12 (p70) production assay	Stimulate monocytes/whole blood with BCG or LPS + IFN-γ → measure IL-12p70 in supernatant	Defects in IL-12 production (macrophage/DC side)	Reduced IL-12p70 → IL12B deficiency, IRF8 deficiency, NEMO deficiency. If IL-12 production is normal but IFN-γ is still low → the defect is on the T-cell/NK side (receptor or signalling).
IFN-γR1/R2 surface expression by flow cytometry	Stain cells with anti-IFN-γR1 or anti-IFN-γR2 monoclonal antibodies → quantify surface expression	Complete IFN-γR deficiency (no surface expression) vs partial deficiency (reduced) vs dominant-negative (normal/increased surface expression but non-functional)	Absent → AR complete IFNGR1/R2 deficiency (most severe form). Normal/increased → consider AD IFNGR1 (dominant-negative form — the truncated receptor reaches the cell surface and is detectable but is non-functional and blocks wild-type signalling).
STAT1 phosphorylation assay by flow cytometry	Stimulate cells with IFN-γ (or IFN-α) → measure phospho-STAT1 (pSTAT1) by intracellular staining	Defects in STAT1 signalling downstream of IFN-γR	Absent/reduced pSTAT1 after IFN-γ stimulation → STAT1 LOF. If pSTAT1 is also absent after IFN-α → AR complete STAT1 deficiency (affects both type I and type II IFN signalling → mycobacterial + viral susceptibility). If only IFN-γ-induced pSTAT1 is reduced → AD partial STAT1 LOF.
IFN-γ receptor binding / signalling assay	Radiolabelled or fluorescent IFN-γ binding to cells + downstream readouts	Functional integrity of the entire IFN-γ receptor complex	Abnormal in IFN-γR or STAT1 defects

Interpretation framework for the functional assays:

High Yield — The Logic of Functional Assays

The beauty of these assays is that they walk you through the IFN-γ/IL-12 circuit step by step:

Can the system make IFN-γ? (stimulate with BCG → measure IFN-γ)
Is the problem IL-12 production or IL-12 response? (add exogenous IL-12 → does IFN-γ normalise?)
Is the IFN-γ receptor present? (flow cytometry for surface expression)
Does the receptor signal properly? (pSTAT1 assay)

Each "no" answer at each step localises the defect to a specific part of the pathway, guiding genetic testing.

Genetic testing has an important role in: Diagnosis (All cases), Family studies, Prognostication, Prenatal diagnosis, Defining new diseases ^[8]

Modality	Details	Advantages	Limitations
Targeted gene panel (MSMD-specific)	Sequences all known MSMD genes (IFNGR1, IFNGR2, IL12B, IL12RB1, IL12RB2, STAT1, IRF8, ISG15, TYK2, RORC, CYBB, IKBKG, SPPL2A, JAK1) in one run	Fast, cost-effective, high coverage of known genes	Misses novel genes not on the panel
Whole-exome sequencing (WES)	Sequences all protein-coding regions (~1.5% of genome)	Detects mutations in known AND novel genes; increasingly used first-line in PID centres	Misses intronic/regulatory variants; variant interpretation can be challenging
Whole-genome sequencing (WGS)	Sequences entire genome	Most comprehensive — detects structural variants, deep intronic mutations	Expensive; enormous data analysis burden; many variants of uncertain significance
Sanger sequencing (of individual gene)	Traditional sequencing of a specific gene	Gold standard for confirming a variant found on WES/WGS; useful for family segregation studies	Only practical when the candidate gene is already known

Interpretation of genetic results:

Pathogenic / Likely pathogenic variant in a known MSMD gene + consistent clinical and functional phenotype → Diagnosis confirmed
Variant of uncertain significance (VUS) → Requires functional validation (does the variant impair IFN-γ/IL-12 signalling?) and family segregation analysis
No variant found → Consider deep intronic variants, structural variants, novel genes, or alternative diagnosis

Family studies and genetic counselling (Paediatric-specific considerations):

Autosomal recessive forms: both parents are carriers (25% recurrence risk per pregnancy); crucial for prenatal diagnosis in subsequent pregnancies
Autosomal dominant forms: may be de novo or inherited; 50% risk to offspring
X-linked (CYBB, NEMO): carrier mother; 50% risk to sons
Cascade genetic screening of siblings — especially important if a younger sibling is about to receive BCG ^[5]^[8]

Clinical Pearl — Hold BCG if Sibling Has MSMD

If a child is diagnosed with MSMD, all younger siblings should NOT receive BCG vaccination until they have been tested and cleared of the same genetic defect. This is a life-saving intervention in Hong Kong where BCG is given at birth. Genetic testing or at minimum functional assays should be performed urgently in the neonatal period.

While the above tiers diagnose the immunodeficiency, you simultaneously need to diagnose and characterise the infection itself.

Investigation	Specimen	Purpose	Key Points
AFB smear (Ziehl-Neelsen stain)	Tissue, pus, sputum, gastric aspirate, blood, CSF, bone marrow	Rapid detection of acid-fast bacilli	Staining with Ziehl-Neelsen to confirm Mycobacterium genus ^[4]. May be positive in disseminated disease (high organism burden).
Mycobacterial culture (solid: Lowenstein-Jensen / liquid: BACTEC MGIT)	Same specimens	Gold standard for definitive species identification and drug susceptibility testing	Obtain body specimen then send for smear and culture → either solid or liquid ^[4]. Slow: solid media 4–8 weeks; liquid media 1–3 weeks. Essential because NTM and BCG require different treatment than M. tuberculosis.
NAAT / PCR (e.g., GeneXpert MTB/RIF, line probe assays)	Same specimens	Rapid species identification and resistance detection	Rapid diagnosis requiring 1–2 days only; differentiate M. tuberculosis from NTM with species identification possible; detection of genes encoding drug resistance ^[9]. Does NOT replace culture ^[9].
MALDI-TOF MS	Culture isolate	Rapid species-level identification from positive culture	MALDI-TOF MS, molecular techniques for species identification ^[4]. Particularly useful for NTM speciation.
Histopathology (biopsy of affected tissue: lymph node, liver, bone, skin)	Tissue biopsy	Granuloma assessment + AFB detection	Caseating granulomatous inflammation ^[9]. In MSMD (especially complete defects): poorly formed or "lepromatous" granulomas with abundant AFB — this histological pattern is a pathognomonic clue to MSMD vs immunocompetent TB.
Early morning gastric aspirate	Gastric aspirate (for young children who cannot produce sputum)	Specimen for AFB smear/culture/PCR	Early morning gastric aspirate (for children or uncooperative patients) ^[7]. Children swallow sputum overnight → aspirate stomach contents in early morning → send for AFB testing.
Blood culture for mycobacteria	Blood (in mycobacteria-specific bottles)	Detect mycobacteraemia in disseminated disease	Positive mycobacteraemia is unusual in immunocompetent children but common in MSMD with disseminated disease.
TST / IGRA	Skin / Blood	Evidence of immune response to mycobacteria	TST: intradermal injection of purified protein derivative → Mantoux test, delayed type hypersensitivity ^[4]^[7]. IGRA: assess release of interferon gamma in response to MTB-specific antigens ^[4]. Caveat in MSMD: these tests depend on intact IFN-γ production — affected by underlying immune status / immunocompromised ^[4]. In complete MSMD, TST may be falsely negative (no IFN-γ → no Type IV hypersensitivity → no induration) and IGRA may also be falsely negative (no IFN-γ production).

Exam Pearl — TST/IGRA in MSMD

TST and IGRA both depend on intact IFN-γ production by T-cells. In MSMD:

Complete IFN-γ pathway defects → TST and IGRA may be falsely negative because the T-cells cannot produce IFN-γ in response to mycobacterial antigens.
Partial defects → may produce a weak positive.

A negative TST/IGRA in a child with disseminated mycobacterial disease does NOT exclude TB infection — it may actually be a clue to the underlying immunodeficiency.

Investigation	Purpose	Relevance to MSMD
CXR	Assess pulmonary involvement	Miliary pattern, hilar lymphadenopathy, consolidation, cavitation. In BCG-osis: bilateral infiltrates.
CT chest/abdomen	Detailed assessment of disseminated disease	Hepatosplenic microabscesses, lymphadenopathy (mediastinal, mesenteric), bone lesions
MRI spine/brain	If CNS involvement suspected	TB meningitis (basal enhancement, hydrocephalus), spondylodiscitis
Bone scan / PET-CT	Map extent of disseminated disease	Osteomyelitis foci, occult disease sites
Liver function tests, renal function tests	Baseline organ function + pre-treatment baseline	Pre-Tx baseline: LRFT ^[7]. Anti-TB drugs are hepatotoxic (isoniazid, pyrazinamide, rifampicin) and nephrotoxic (aminoglycosides).
CBC + inflammatory markers (CRP, ESR)	Monitor infection severity and treatment response	Elevated CRP/ESR in active infection; anaemia of chronic disease

Step	Investigation	Looking For	Result in MSMD
1	HIV PCR, drug Hx, nutritional assessment, blood film	Secondary immunodeficiency	Negative / normal
2	CBC, lymphocyte subsets, Ig levels, vaccine responses	SCID, XLA, CVID, Hyper-IgM	All normal
3	DHR test	CGD	Normal
4	IFN-γ production assay, IL-12 assay, IFN-γR expression, pSTAT1	MSMD — IFN-γ/IL-12 axis defect	Abnormal — the diagnostic step
5	Genetic testing (panel / WES / WGS)	Specific gene mutation	Pathogenic variant identified
Parallel	AFB smear, culture, PCR, histopathology	Identify and characterise the mycobacterial infection	Positive for mycobacteria (BCG, NTM, or MTB)

High Yield Summary — Diagnosis of MSMD

No formal tick-box diagnostic criteria exist for MSMD — diagnosis requires the triad of: (a) clinical phenotype (disseminated weakly virulent mycobacteria ± Salmonella), (b) functional IFN-γ/IL-12 axis defect, (c) genetic confirmation.
The routine PID workup is entirely normal — CBC, lymphocyte subsets, immunoglobulins, DHR test are all normal. Only advanced functional tests reveal the defect ^[1]^[2].
The stepwise approach: exclude secondary causes → basic PID workup → DHR (rule out CGD) → IFN-γ/IL-12 axis assays → genetic testing.
TST and IGRA may be falsely negative in MSMD because they depend on IFN-γ production by T-cells.
Histopathology clue: poorly formed granulomas with abundant AFB (especially in complete defects) — distinct from the well-formed caseating granulomas of immunocompetent TB.
Genetic testing is essential for all cases — for diagnosis, family counselling, prognostication, and prenatal diagnosis ^[8].
Hold BCG for unscreened siblings of an MSMD patient until genetic status is clarified.

Active Recall — Diagnosis & Investigations of MSMD

References

^[1] Lecture slides: GC 144. A child with recurrent infections Primary immunodeficiencies.pdf (p35 — MSMD section) ^[2] Senior notes: Jerry's immunodeficiencies.pdf (Category 6 — Intrinsic and innate immunity, MSMD; advanced functional tests) ^[3] Lecture slides: Block C - A child with recurrent infections_ Primary immunodeficiencies.pdf ^[4] Senior notes: Gen Clerk Anaes + Microbiology Summary.pdf (Diagnosis of mycobacterial infection, TST, IGRA, ZN stain, culture) ^[5] Senior notes: Adrian Lui Pediatrics Notes.pdf (p407 — Approach to Immunodeficiency, warning signs of PID, BCG complications) ^[7] Senior notes: Maksim Medicine Notes.pdf (p190 — TB investigations, pre-treatment baseline, gastric aspirate) ^[8] Lecture slides: Investigations of Imm Disorders 2025.pdf (Approach to diagnosis of PID, genetic testing roles, ESID criteria) ^[9] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p167 — NAAT/PCR, histopathology of TB)

Management of MSMD

Management of MSMD requires two parallel streams running simultaneously:

Treat the active infection — anti-mycobacterial therapy (and anti-Salmonella therapy if applicable)
Address the underlying immunodeficiency — ranging from supportive measures and adjunctive immunotherapy (recombinant IFN-γ) through to definitive cure with haematopoietic stem cell transplant (HSCT) in severe cases

The severity of the genetic defect dictates the balance between these streams:

Complete defects (e.g., AR complete IFNGR1) → antimicrobials alone will not suffice; HSCT is usually required for long-term survival
Partial defects (e.g., IL12RB1, AD STAT1 LOF, partial IFNGR1) → antimicrobials ± recombinant IFN-γ may control disease; HSCT reserved for refractory cases

Treatment: antimycobacterials ^[1]^[2]

Stream 1: Treatment of Active Mycobacterial Infection

The approach differs fundamentally from treating TB in an immunocompetent child, because:

The host cannot mount an adequate IFN-γ response to assist drug-mediated killing → drugs must do more of the work
Treatment duration is much longer — often 12–24 months or longer, sometimes lifelong in complete defects without HSCT
Drug susceptibility must be determined because the organism may be BCG (intrinsically resistant to pyrazinamide), NTM (often multidrug-resistant), or MTB
Combination therapy is essential to prevent resistance emergence, just as in standard TB treatment

Organism	Recommended Regimen	Duration	Key Notes
M. bovis BCG (post-vaccination BCG-osis)	Isoniazid (H) + Rifampicin (R) + Ethambutol (E) ± Fluoroquinolone (moxifloxacin/levofloxacin)	Minimum 12 months beyond clinical/microbiological resolution; often 18–24 months total	BCG is intrinsically resistant to pyrazinamide (Z) — do NOT include Z in BCG treatment. This is a key difference from standard MTB regimens. Ethambutol replaces pyrazinamide.
M. tuberculosis	HRZE × 2 months + HR × 4–10 months ^[7] (modified duration depending on disease extent and immune status)	Minimum 9–12 months in MSMD (longer than standard 6 months for immunocompetent children)	DOTS (directly observed treatment, short-course): HRZE × 2 months + HR × 4 months is the standard for immunocompetent patients ^[7]. In MSMD, extend continuation phase due to impaired host immunity. Extrapulmonary TB: longer duration — e.g., TBM 12 months ^[7].
NTM (M. avium complex, M. abscessus, etc.)	Species-dependent — guided by susceptibility testing. Typically macrolide (azithromycin/clarithromycin) + ethambutol + rifampicin for MAC; macrolide + amikacin + cefoxitin/imipenem for M. abscessus	At least 12 months beyond culture conversion	NTM are notoriously difficult to treat ^[10]. M. abscessus is particularly challenging — check for inducible macrolide resistance. In MSMD, NTM often requires even more prolonged therapy than in non-immunodeficient NTM pulmonary disease.

All doses must be weight-based with paediatric formulations where available.

Drug	Paediatric Dose	Key Toxicity	Monitoring
Isoniazid (H)	10–15 mg/kg/day (max 300 mg/day)	Hepatotoxicity, peripheral neuropathy (pyridoxine deficiency)	LFTs monthly for first 3 months; always co-prescribe pyridoxine (vitamin B6) 1–2 mg/kg/day to prevent neuropathy
Rifampicin (R)	10–20 mg/kg/day (max 600 mg/day)	Hepatotoxicity, orange discolouration of body fluids, drug interactions (potent CYP3A4 inducer)	LFTs; warn family about orange urine/tears (not harmful). Rifampicin interacts with many drugs — check all co-medications.
Ethambutol (E)	15–25 mg/kg/day	Optic neuritis (dose-related, reversible if caught early) — "ethambutol → eye"	Refer Eye for baseline visual acuity ^[7] before starting. Monthly visual acuity and colour vision in cooperative children. Challenging in very young children who cannot report visual changes.
Pyrazinamide (Z)	30–40 mg/kg/day	Hepatotoxicity, hyperuricaemia	LFTs, urate. NOT used for BCG (intrinsic resistance).
Fluoroquinolones (moxifloxacin, levofloxacin)	Moxifloxacin: 7.5–10 mg/kg/day; Levofloxacin: 10–20 mg/kg/day	Tendinopathy (rare in children), QTc prolongation, cartilage toxicity (theoretical concern in growing children — but increasingly used in paediatric TB)	ECG if using moxifloxacin; clinical monitoring for joint/tendon symptoms
Amikacin (for severe/disseminated disease or NTM)	15–30 mg/kg/day IV	Nephrotoxicity, ototoxicity	Renal function, therapeutic drug monitoring (trough and peak levels), audiometry
Macrolides (azithromycin, clarithromycin — for NTM)	Azithromycin: 10 mg/kg/day; Clarithromycin: 15 mg/kg/day divided BD	GI upset, hepatotoxicity, QTc prolongation (clarithromycin)	LFTs, ECG if combining with other QTc-prolonging drugs

Pre-Tx baseline: LRFT, urate, HBsAg ± anti-HIV, refer Eye for baseline visual acuity ^[7]

Paediatric Prescribing Pearl — BCG ≠ Standard TB Regimen

The most common prescribing error in BCG-osis is adding pyrazinamide to the regimen. M. bovis BCG is intrinsically resistant to pyrazinamide because it lacks the pyrazinamidase enzyme (encoded by the pncA gene, which is mutated in BCG strains). Using pyrazinamide adds hepatotoxicity without anti-mycobacterial benefit. The correct regimen is HRE (± fluoroquinolone), NOT HRZE.

Patients with MSMD (especially IL-12Rβ1 and IL-12B deficiency) are susceptible to Salmonella ^[1]^[2]
Empiric therapy: IV ceftriaxone (50–80 mg/kg/day, max 4 g/day) or IV ciprofloxacin (10–15 mg/kg/day) — guided by local resistance patterns
Duration: typically 2–4 weeks for bacteraemia; longer for deep-seated infections (osteomyelitis, meningitis)
Secondary prophylaxis: co-trimoxazole or ciprofloxacin may be considered for recurrent salmonellosis

Stream 2: Addressing the Underlying Immunodeficiency

Aspect	Details
What it is	Recombinant human IFN-γ — provides exogenous IFN-γ to partially compensate for the endogenous deficiency
Mechanism	Binds to IFN-γR on macrophages → activates JAK-STAT1 signalling → enhances macrophage microbicidal function (phagolysosomal fusion, ROS/RNI production)
Indication	Partial MSMD defects where the IFN-γ receptor and downstream signalling retain some function (e.g., IL-12B, IL-12RB1, AD STAT1 LOF, partial IFNGR1). Also used as bridge therapy while awaiting HSCT in complete defects.
Dose	50 μg/m² BSA, subcutaneous injection, three times per week
Efficacy	Reduces frequency and severity of mycobacterial infections in partial defects. Variable response — monitor clinically and microbiologically.
Contraindication / Limitation	Complete IFNGR1 or IFNGR2 deficiency — there is NO functional receptor for exogenous IFN-γ to act on, so it is ineffective. This is the single most important contraindication. Also: AR complete STAT1 (no downstream signalling even if receptor is present).
Side effects	Fever, chills, myalgia, fatigue, injection-site reactions (usually mild and self-limiting). Dose-related — may need dose reduction.

Key Concept — Why IFN-γ Works in Partial But Not Complete Defects

Think of it this way:

Partial defect = the "lock" (receptor/signalling) is partially functional → giving extra "key" (exogenous IFN-γ) helps because some of it gets through
Complete defect = the "lock" is entirely absent or non-functional → no matter how many "keys" you provide, they have nothing to bind → IFN-γ therapy is futile

This is why genetic characterisation (complete vs partial) directly determines the treatment approach.

Aspect	Details
Rationale	Replace the patient's defective haematopoietic and immune system with a donor system that has intact IFN-γ/IL-12 signalling. The transplanted macrophages, T-cells, and NK cells will have functional IFN-γR/STAT1 → can kill mycobacteria.
Indications	(1) Complete defects (AR complete IFNGR1, AR complete IFNGR2, AR complete STAT1) — HSCT is the only curative option because IFN-γ therapy is ineffective. (2) Partial defects with refractory disease despite optimal antimicrobials + IFN-γ. (3) Recurrent life-threatening infections despite all medical therapy.
Donor source	HLA-matched sibling donor (MSD) is ideal. If unavailable: matched unrelated donor (MUD), haploidentical donor (with post-transplant cyclophosphamide), or cord blood.
Conditioning	Myeloablative conditioning (e.g., busulfan + cyclophosphamide ± fludarabine) — needed to fully replace the defective immune compartment. Reduced-intensity conditioning (RIC) has been used but has higher rates of mixed chimerism and potential graft failure in MSMD.
Timing	Ideally after active infection is controlled (or at least stabilised) with antimicrobials. Transplanting during fulminant disseminated infection carries high mortality. Bridge with anti-mycobacterials ± IFN-γ while awaiting donor/preparation.
Outcomes	Variable; overall survival 50–75% in reported series. Better outcomes with: (a) MSD, (b) pre-transplant infection control, (c) myeloablative conditioning achieving full donor chimerism. Worse outcomes with: ongoing active disseminated BCG-osis at time of transplant, mismatched/haploidentical donors.
Complications	Standard HSCT complications: graft-versus-host disease (GVHD), graft failure/rejection, infections during neutropenic phase, veno-occlusive disease, organ toxicity from conditioning. Immune reconstitution inflammatory syndrome (IRIS) — paradoxical worsening of mycobacterial inflammation as the new donor immune system "wakes up" and mounts a vigorous IFN-γ response against residual mycobacteria → can cause flare of lymphadenitis, hepatitis, fevers. Managed with corticosteroids as needed.
Post-HSCT monitoring	Chimerism studies (donor vs recipient cell proportion), immune reconstitution (lymphocyte subsets, Ig recovery), mycobacterial cultures, LFTs, growth and development.

IRIS After HSCT — The Paradox of Immune Reconstitution

After HSCT, the donor immune cells begin producing IFN-γ for the first time in this child's body. The macrophages now become activated and "discover" all the residual mycobacteria that have been sitting dormant or slowly proliferating → a massive inflammatory response ensues. This is analogous to the paradoxical reaction seen when anti-TNF-α is stopped in a patient with TB ^[4] — the return of immune function causes a temporary clinical worsening before improvement. Anticipate IRIS and manage with steroids if severe.

Aspect	Details
Concept	Correct the specific genetic defect in the patient's own haematopoietic stem cells using viral vectors (lentiviral) carrying the functional gene → autologous transplant of corrected cells
Current status (2025–2026)	Preclinical and early clinical trials for some MSMD genes (particularly IFNGR1). Not yet standard of care.
Advantages	No need for an HLA-matched donor; avoids GVHD (since cells are autologous); avoids the immunological complications of allogeneic HSCT
Challenges	Risk of insertional mutagenesis, variable transgene expression, long-term durability of correction unknown, limited availability

Stream 3: Supportive and Preventive Measures

These measures are just as important as the antimicrobials and are critical to family-centred paediatric care.

Some PID cannot handle live vaccines: BCG → SCID, CGD, Mendelian susceptibility to mycobacterial diseases (MSMD)/STAT1 ^[5]

Vaccine	Risk in MSMD	Action
BCG	Disseminated BCG-osis — the very trigger that unmasked the disease	Absolutely contraindicated. If already given and tolerated (in partial defects), no further action for this vaccine specifically, but monitor for late complications.
OPV (oral polio vaccine)	Not specifically contraindicated in MSMD (OPV risk is for SCID/XLA), but inactivated IPV is preferred for any PID	Use IPV instead
Rotavirus vaccine	Live vaccine — theoretical risk in severe T-cell defects but MSMD has normal T-cells; still avoid in complete defects out of caution	Case-by-case; generally avoid in complete defects
MMR, varicella, live influenza	MSMD patients have intact T-cell and B-cell function → these vaccines are generally safe in partial defects	Can be given in partial defects with intact T-cell immunity. Avoid in complete STAT1 AR (because type I IFN signalling also impaired → viral susceptibility).

Killed/inactivated vaccines (e.g., DTaP, IPV, Hib, PCV, hepatitis B, influenza inactivated) → safe and recommended — these children benefit from all routine immunisations that do not contain live organisms.

Agent	Indication	Rationale
Azithromycin (5 mg/kg 3×/week or daily)	Prophylaxis against NTM (especially MAC) in patients with recurrent NTM disease	Macrolides have activity against MAC and serve as suppressive therapy
Isoniazid (10 mg/kg/day) + pyridoxine	TB chemoprophylaxis if significant MTB exposure	Prevents MTB reactivation/progression; however, in MSMD, the host cannot independently control even latent TB → lower threshold for prophylaxis
Co-trimoxazole (trimethoprim 5 mg/kg/day)	Prophylaxis against Salmonella in patients with recurrent salmonellosis	Co-trimoxazole also provides broad-spectrum prophylaxis against some bacterial and protozoal infections

Chronic infection causes catabolic wasting → ensure adequate caloric intake, protein, and micronutrients
Plot growth (weight, height/length, head circumference for age) at every clinical encounter — failure to thrive is both a warning sign of PID ^[5] and a marker of treatment response
Dietician referral for children with significant weight loss or faltering growth
Supplemental feeds (oral calorie-dense formulas or NG feeds) if oral intake insufficient

Action	Details
Genetic counselling	Explain inheritance pattern (AR, AD, or X-linked), recurrence risk, carrier testing for parents, implications for future pregnancies
Cascade genetic screening of siblings	Especially critical for younger siblings who have not yet received BCG — hold BCG until genetic status confirmed ^[5]
Prenatal / preimplantation genetic diagnosis	Available for families with known mutations planning future pregnancies
Psychosocial support	Chronic disease in a child → significant family stress. Link with social worker, patient support groups, clinical psychologist as needed. Address school attendance, developmental needs.
Communication with child	Age-appropriate explanation of their condition. Adolescents should be involved in discussions about treatment, including HSCT risks/benefits (assent).

Precaution	Rationale
Boil or filter water (especially if MAC endemia)	NTM (including MAC) are ubiquitous in water supplies → reduce environmental mycobacterial exposure
Avoid soil/garden contact without gloves	Soil is a reservoir for NTM
Avoid unpasteurised dairy	M. bovis (non-BCG) can be present in unpasteurised milk
Good hand hygiene, food hygiene	Reduce Salmonella exposure (especially important if IL-12Rβ1 deficiency)
Household TB screening	If any household contacts have active TB → treat promptly to reduce child's exposure

	Partial Defect	Complete Defect
Examples	IL12RB1, IL12B, AD STAT1 LOF, partial IFNGR1, TYK2	AR complete IFNGR1, AR complete IFNGR2, AR complete STAT1
Anti-mycobacterial Rx	Yes — prolonged (12–24+ months)	Yes — as bridge to HSCT
Recombinant IFN-γ	Yes — mainstay adjunctive therapy (receptor is functional)	Ineffective (no functional receptor) — may still try empirically if receptor status uncertain
HSCT	Reserved for refractory disease	First-line definitive therapy — plan early
Antimicrobial prophylaxis	Yes — macrolide ± co-trimoxazole ± isoniazid	Yes — while awaiting HSCT
Live vaccines	Avoid BCG; others case-by-case	Avoid all live vaccines
Prognosis	Generally good with antimicrobials ± IFN-γ; most survive long-term	Poor without HSCT; 50–75% survival with HSCT

High Yield Summary — Management of MSMD

Treatment: antimycobacterials ^[1]^[2] — prolonged multi-drug regimens tailored to organism (BCG uses HRE without pyrazinamide; MTB uses standard regimen with extended duration; NTM uses macrolide-based regimens guided by susceptibility).
Recombinant IFN-γ is the key adjunctive immunotherapy for partial defects — it is ineffective in complete receptor defects (no receptor to bind).
HSCT is the only curative option for complete defects — timing is critical (control infection first if possible). IRIS is an expected complication post-HSCT.
Avoid BCG in diagnosed patients and unscreened siblings — BCG → SCID, CGD, MSMD/STAT1 ^[5].
Pre-treatment baseline: LFTs, RFTs, urate, HBsAg, baseline visual acuity (for ethambutol) ^[7].
Antimicrobial prophylaxis (macrolide for NTM, co-trimoxazole for Salmonella, isoniazid for TB exposure) is part of long-term management.
Family-centred care: genetic counselling, cascade sibling screening, prenatal diagnosis for future pregnancies, psychosocial support.
Gene therapy is emerging but not yet standard of care (as of 2025–2026).

Active Recall — Management of MSMD

References

^[1] Lecture slides: GC 144. A child with recurrent infections Primary immunodeficiencies.pdf (p35 — MSMD: treatment = antimycobacterials) ^[2] Senior notes: Jerry's immunodeficiencies.pdf (Category 6 — MSMD treatment: anti-mycobacterial drugs; HSCT) ^[3] Lecture slides: Block C - A child with recurrent infections_ Primary immunodeficiencies.pdf (p33 — MSMD reference) ^[4] Senior notes: Gen Clerk Anaes + Microbiology Summary.pdf (Anti-TNF-alpha and TB paradoxical reaction; NTM management) ^[5] Senior notes: Adrian Lui Pediatrics Notes.pdf (p407 — BCG complications in PID, live vaccine avoidance, warning signs) ^[7] Senior notes: Maksim Medicine Notes.pdf (p190 — Anti-TB regimens, DOTS, pre-treatment baseline, extrapulmonary TB duration) ^[10] Senior notes: Ryan Ho Respiratory.pdf (p94 — NTM management, M. abscessus regimen, macrolide resistance)

Complications of MSMD

Complications in MSMD arise from three sources, and it is helpful to think of them systematically:

The disease itself — consequences of unchecked mycobacterial (and Salmonella) infection in a child whose IFN-γ/IL-12 axis cannot contain intracellular pathogens
The treatment — adverse effects of prolonged anti-mycobacterial drugs, recombinant IFN-γ, and HSCT
The underlying immunodeficiency — long-term sequelae of living with a defective innate immune pathway

A. Complications of the Mycobacterial Infection

These complications mirror those of disseminated TB and NTM disease but are more frequent and more severe in MSMD because the host cannot form effective granulomas to contain the organism.

Complication	Pathophysiology	Clinical Features
Disseminated BCG-osis	After neonatal BCG vaccination, the live attenuated M. bovis BCG cannot be contained at the injection site → haematogenous spread to virtually every organ. This is the most feared complication and often the presenting event that unmasks MSMD.	Multi-organ involvement: miliary lung disease, hepatosplenomegaly, generalised lymphadenopathy, bone marrow infiltration (pancytopenia), cutaneous dissemination. BCG vaccine will disseminate in CGD and MSMD ^[2]^[5]. Can be rapidly fatal in complete defects.
Disseminated NTM disease	Environmental mycobacteria (MAC, M. abscessus, etc.) enter via respiratory or GI tract → cannot be cleared → haematogenous spread	Similar to BCG-osis. Often insidious onset. May coexist with BCG-osis if BCG was given.
Miliary TB	M. tuberculosis disseminates haematogenously → seeding of all organs with tiny (1–2 mm) granulomas ("millet seed" appearance)	Diffuse fine micronodular shadows ~1–2 mm in diameter on CXR ^[7]. Fever, wasting, hepatosplenomegaly. May present with pancytopenia from bone marrow infiltration.

Organ System	Complication	Pathophysiology	Key Features
Pulmonary	Bronchiectasis	Chronic mycobacterial pneumonia → parenchymal destruction and bronchial wall damage → irreversible dilatation of airways. Progressive destruction and fibrosis of lung parenchyma leads to localised bronchial dilation ^[9].	Chronic productive cough, recurrent secondary bacterial infections, haemoptysis. CXR may show bronchiectasis ^[2]. Once established, bronchiectasis is irreversible and requires long-term airway clearance (physiotherapy, mucolytics).
Pulmonary	Pulmonary fibrosis / restrictive lung disease	End-stage granulomatous inflammation → fibrotic remodelling of lung parenchyma	Progressive dyspnoea, reduced FVC on spirometry (in cooperative older children), cor pulmonale in late stages.
Pulmonary	Pneumothorax	Rupture of peripheral cavity or subpleural caseous focus with liquefaction into pleural space ^[9]	Acute chest pain, dyspnoea, absent breath sounds. May require chest tube drainage.
Skeletal	Mycobacterial osteomyelitis / spondylodiscitis	Haematogenous seeding of mycobacteria to bone (especially vertebral bodies in children; also metaphyses of long bones). Why bone? Mycobacteria have tropism for areas of high blood flow and high oxygen tension — the vertebral body's rich vascular supply makes it vulnerable.	Bone pain, tenderness, swelling. Vertebral involvement → gibbus deformity (angular kyphosis from anterior vertebral body collapse), spinal cord compression → paraplegia. Pott's disease (tuberculous spondylitis).
CNS	Tuberculous meningitis (TBM)	Mycobacteria seed the meninges via haematogenous spread → basal meningitis (predilection for basal cisterns due to slow CSF flow and high protein concentration)	Triphasic illness: prodrome → meningitic → paralytic ^[11]. Basal meningeal adhesions → cranial nerve palsies (CN III, IV, VI, VIII), hydrocephalus (obstruction of CSF flow at basal cisterns). Endarteritis obliterans → cerebral infarction ^[11]. Mortality and morbidity extremely high.
CNS	Tuberculoma / cerebral abscess	Space-occupying granulomatous lesion in brain parenchyma	Seizures, focal neurological deficits, raised intracranial pressure (headache, vomiting, papilloedema).
Hepatic	Granulomatous hepatitis	Mycobacteria-laden macrophages infiltrate the liver → granuloma formation (or in complete defects: poorly formed granulomas with abundant AFB) → hepatic inflammation	Hepatomegaly, elevated transaminases and ALP, jaundice (if severe). May contribute to failure to thrive through impaired synthetic/metabolic function.
Splenic	Massive splenomegaly / splenic abscess	Reticuloendothelial involvement → mycobacterial proliferation within the spleen	Splenomegaly (palpable below costal margin), hypersplenism (cytopenias from splenic sequestration), risk of splenic rupture (rare).
Haematological	Bone marrow infiltration → pancytopenia	Disseminated mycobacteria replace normal marrow elements	Anaemia (pallor, fatigue), neutropenia (secondary bacterial infections), thrombocytopenia (bleeding tendency). Must distinguish from treatment-related cytopenias.
Lymphatic	Suppurative / caseous lymphadenitis	Regional lymph nodes overwhelmed by mycobacteria → caseous necrosis, liquefaction → abscess formation, sinus tracts	Fluctuant lymph nodes, discharging sinuses, chronic wound healing problems. May require surgical drainage.
Adrenal	Adrenal insufficiency	Bilateral adrenal gland destruction by mycobacteria (TB adrenalitis) — the adrenal glands are a classic site of TB haematogenous seeding	Addisonian crisis (hypotension, hypoglycaemia, hyperkalaemia, hyponatraemia), chronic fatigue, skin hyperpigmentation. May be insidious — check cortisol and ACTH in any child with MSMD and unexplained hypotension/electrolyte disturbance.

Complication	Pathophysiology	Key Features
Salmonella bacteraemia / sepsis	Defective IFN-γ → macrophages cannot kill intracellular Salmonella → bacteraemia → sepsis cascade	Fever, rigors, shock. Salmonella susceptibility especially in IL-12Rβ1 and IL-12B deficiency ^[1]^[2].
Salmonella osteomyelitis	Haematogenous seeding from bacteraemia (classic association in sickle cell disease, but also in MSMD due to impaired intracellular killing)	Bone pain, swelling, fever. Diaphyseal involvement.
Salmonella meningitis	CNS seeding from bacteraemia	Meningism, altered consciousness, seizures. High mortality in neonates and young infants.
Recurrent / relapsing salmonellosis	Cannot eradicate intracellular organisms even after appropriate antibiotics → relapses once antibiotics stopped	Requires prolonged treatment courses and possibly lifelong prophylaxis.

Drug	Complication	Pathophysiology	Prevention / Monitoring
Isoniazid	Hepatotoxicity	Metabolised by N-acetyltransferase → toxic metabolites (acetylhydrazine) damage hepatocytes. Risk increased in slow acetylators and with concurrent rifampicin.	Monthly LFTs for first 3 months; hold if ALT > 3× ULN with symptoms or > 5× ULN asymptomatic.
Isoniazid	Peripheral neuropathy	Isoniazid depletes pyridoxine (vitamin B6) → impaired nerve function. "Isoniazid steals B6."	Always co-prescribe pyridoxine (1–2 mg/kg/day) ^[7].
Rifampicin	Hepatotoxicity	Induces CYP enzymes → alters drug metabolism; direct hepatotoxicity through cholestatic mechanism	LFTs; also a potent CYP3A4 inducer → drug interactions (reduces levels of anticonvulsants, oral contraceptives, azole antifungals, calcineurin inhibitors — critical post-HSCT).
Ethambutol	Optic neuritis	Disrupts mitochondrial function in retinal ganglion cells → axonal degeneration of the optic nerve. Dose-related and usually reversible if detected early.	Refer Eye for baseline visual acuity ^[7]. Monthly visual acuity testing in cooperative children. Challenging in infants/toddlers — rely on clinical observation (reduced visual tracking, nystagmus).
Pyrazinamide	Hepatotoxicity, hyperuricaemia	Hepatotoxic metabolites; inhibits renal urate excretion. Not used for BCG (intrinsic resistance).	LFTs, urate.
Aminoglycosides (amikacin)	Nephrotoxicity, ototoxicity	Accumulates in proximal tubular cells and inner ear hair cells → irreversible damage	Therapeutic drug monitoring (trough levels), renal function, audiometry (especially critical in developing children — hearing loss affects language development).
Fluoroquinolones	Tendinopathy, QTc prolongation	Inhibition of collagen synthesis in tendons; block hERG potassium channels	ECG monitoring, clinical assessment of joint/tendon symptoms. Theoretical cartilage toxicity in growing children — increasingly accepted in paediatric practice for severe infections but used cautiously.

Paediatric-Specific Drug Monitoring

In MSMD, children are on anti-mycobacterial drugs for much longer than typical TB treatment (often 12–24+ months). This prolonged exposure means cumulative drug toxicity is a major concern:

Ethambutol optic neuritis may be insidious in young children who cannot verbalise visual changes → objective visual testing (visual evoked potentials) may be needed.
Aminoglycoside ototoxicity in a pre-verbal infant/toddler can go undetected for months → regular brainstem auditory evoked response (BAER) testing.
Hepatotoxicity from multiple hepatotoxic drugs (H + R + Z) → monthly LFTs; have a low threshold to modify the regimen.

Complication	Pathophysiology	Clinical Features & Management
Graft-versus-host disease (GVHD)	Donor T-cells recognise host tissues as foreign → immune attack on skin, gut, liver	Acute GVHD: skin rash, diarrhoea, jaundice, typically within first 100 days. Chronic GVHD: sclerodermatous skin changes, sicca syndrome, bronchiolitis obliterans. Managed with immunosuppression (steroids, calcineurin inhibitors).
Graft failure / rejection	Host residual immune system rejects donor graft, or donor stem cells fail to engraft	Persistent pancytopenia, failure of immune reconstitution. May require second transplant or donor lymphocyte infusion. More common with reduced-intensity conditioning and mismatched donors.
Immune reconstitution inflammatory syndrome (IRIS)	Donor immune cells produce IFN-γ → macrophage activation against residual mycobacteria → paradoxical inflammatory flare	Fever, lymphadenitis exacerbation, hepatitis, new or worsening pulmonary infiltrates — despite falling mycobacterial burden. Expected complication. Managed with corticosteroids while continuing antimycobacterials.
Infections during neutropenic phase	Post-conditioning aplasia → no neutrophils → susceptibility to bacterial and fungal infections	Standard transplant infection prophylaxis (antifungals, aciclovir, co-trimoxazole for PJP). Managed in protective isolation with HEPA-filtered rooms.
Veno-occlusive disease (VOD) / Sinusoidal obstruction syndrome (SOS)	Conditioning chemotherapy damages hepatic sinusoidal endothelium → microthrombosis → hepatic congestion	Hepatomegaly, jaundice, weight gain (fluid retention), ascites. Managed with defibrotide.
Long-term effects of conditioning	Alkylating agents (busulfan, cyclophosphamide) → gonadotoxicity, secondary malignancy risk, growth impairment	Endocrine follow-up (puberty, thyroid, growth hormone axis), fertility counselling in older children/adolescents, long-term cancer surveillance.

Side Effect	Mechanism	Management
Fever, chills, myalgia	IFN-γ activates macrophages and induces pyrogenic cytokines (IL-1, TNF-α) — a pharmacological version of the physiological febrile response	Usually mild, dose-related. Pre-medicate with paracetamol. Dose reduction if severe.
Injection-site reactions	Local inflammation at SC injection site	Rotate injection sites. Usually self-limiting.
Reversible neutropenia / thrombocytopenia	IFN-γ can suppress myelopoiesis at high doses	Monitor CBC; dose adjustment if significant cytopenias.

Sequela	Pathophysiology	Impact on the Child
Failure to thrive	Chronic infection → catabolic state (TNF-α/IL-6-driven wasting), anorexia, malabsorption from GI involvement	Failure to thrive is a warning sign of PID ^[5]. Persistent growth faltering even after infection control may indicate ongoing subclinical disease or end-organ damage. Requires dietitian input and regular growth plotting.
Chronic lung disease / bronchiectasis	Repeated or prolonged pulmonary mycobacterial infection → structural lung damage	Persistent cough, recurrent secondary bacterial infections, reduced exercise tolerance. CXR may show bronchiectasis ^[2]. Managed with chest physiotherapy, airway clearance techniques, prompt antibiotic treatment of bacterial superinfection.
Neurodevelopmental sequelae	TBM or cerebral tuberculoma → brain injury. Ototoxic drugs (aminoglycosides) → hearing loss → language delay.	Developmental assessment at regular intervals. Speech and language therapy if hearing loss. Educational support (special education needs). Early intervention is key in paediatrics.
Orthopaedic deformity	Mycobacterial osteomyelitis / Pott's disease → vertebral collapse → gibbus deformity, limb deformity	May require orthopaedic intervention (bracing, surgical correction). Spinal cord compression → paraplegia is the most devastating complication.
Adrenal insufficiency	Bilateral adrenal destruction → permanent cortisol deficiency	Lifelong steroid replacement (hydrocortisone). Stress-dose steroids during illness/surgery. MedicAlert bracelet.
Psychosocial impact	Chronic illness in childhood → school absence, social isolation, anxiety, depression; family stress (financial burden of prolonged treatment, impact on siblings)	Clinical psychologist involvement. Social worker for financial/practical support. Peer support groups. Normalise school attendance as much as possible. Adolescent transition planning.
Risk of secondary/recurrent infections	The underlying genetic defect persists (unless cured by HSCT/gene therapy) → lifelong susceptibility to mycobacteria and Salmonella	Lifelong antimicrobial prophylaxis in patients who do not undergo HSCT. Vigilance for new infections with each environmental mycobacterial exposure. Adherence counselling.

IRIS deserves special emphasis because it occurs at two critical junctures in MSMD management:

Trigger	Mechanism	Timing	Management
Post-HSCT	Donor-derived immune cells begin producing IFN-γ → activated macrophages mount vigorous inflammatory response against residual mycobacteria in tissues → tissue-damaging inflammation	Weeks to months after engraftment, coinciding with immune reconstitution	Corticosteroids (prednisolone 1–2 mg/kg/day, taper over weeks); continue antimycobacterials; supportive care
After starting recombinant IFN-γ	Exogenous IFN-γ partially restores macrophage function → inflammatory response to organisms that were previously "invisible" to the immune system	Days to weeks after starting IFN-γ therapy	As above; may need temporary IFN-γ dose reduction

Why does IRIS happen? Think of it from first principles: in MSMD, the child's macrophages have been "blind" to mycobacteria (cannot be activated without IFN-γ). Mycobacteria have been multiplying quietly with minimal inflammation. When IFN-γ is suddenly restored (either from transplanted donor cells or from a syringe), the macrophages suddenly "see" all the mycobacteria that have accumulated → a massive, sometimes destructive, inflammatory response ensues. It is the immunological equivalent of turning on the lights in a room full of intruders — the alarm goes off all at once.

High Yield Summary — Complications of MSMD

Disease-related: Disseminated BCG-osis/NTM disease is the sentinel complication. Organ-specific sequelae include bronchiectasis, TBM with hydrocephalus and CN palsies, mycobacterial osteomyelitis (Pott's disease), granulomatous hepatitis, adrenal insufficiency, and bone marrow infiltration causing pancytopenia.
Infection-related: Recurrent invasive salmonellosis (bacteraemia, osteomyelitis, meningitis) occurs in ~50% of IL-12Rβ1-deficient patients.
Treatment-related: Prolonged anti-mycobacterial therapy → hepatotoxicity (H, R, Z), optic neuritis (E), ototoxicity (amikacin), neuropathy (H). HSCT → GVHD, graft failure, VOD, IRIS, conditioning-related gonadotoxicity.
IRIS is a characteristic complication occurring after HSCT or initiation of IFN-γ therapy — paradoxical inflammatory worsening as immune function is restored. Managed with corticosteroids.
Long-term sequelae: Chronic lung disease, neurodevelopmental delay, orthopaedic deformity, adrenal insufficiency, psychosocial burden, and lifelong infection susceptibility unless cured by HSCT.
Failure to thrive is both a warning sign and an ongoing complication — monitor growth at every visit ^[5].

Active Recall — Complications of MSMD

References

^[1] Lecture slides: GC 144. A child with recurrent infections Primary immunodeficiencies.pdf (p35 — MSMD: pathogens mycobacteria and Salmonella; treatment antimycobacterials) ^[2] Senior notes: Jerry's immunodeficiencies.pdf (Category 6 — MSMD; BCG dissemination; Salmonella; CXR bronchiectasis) ^[3] Lecture slides: Block C - A child with recurrent infections_ Primary immunodeficiencies.pdf (p33 — MSMD reference) ^[5] Senior notes: Adrian Lui Pediatrics Notes.pdf (p407 — Warning signs of PID including failure to thrive; complications from live vaccine; BCG → MSMD) ^[7] Senior notes: Maksim Medicine Notes.pdf (p190 — TB complications; pre-treatment baseline; ethambutol visual acuity) ^[9] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p177 — TB complications: pneumothorax, bronchiectasis, atelectasis) ^[11] Senior notes: Ryan Ho Neurology.pdf (p144 — TBM: triphasic illness, basal meningeal adhesions, CN palsies, endarteritis obliterans, hydrocephalus)

High Yield Summary

MSMD = monogenic defects in the IFN-γ/IL-12 axis → selective vulnerability to weakly virulent mycobacteria (BCG, NTM) and Salmonella.
IUIS Category 6: Defects of Intrinsic and Innate Immunity — adaptive immunity (T-cells, B-cells, Ig) is intact.
IL-12Rβ1 deficiency is the most common genetic cause worldwide.
Complete defects (no residual IFN-γ signalling) → severe, early-onset, poor prognosis, need HSCT. Partial defects → milder, may respond to antimycobacterials ± recombinant IFN-γ.
BCG dissemination in an infant in Hong Kong = think MSMD, SCID, or CGD.
STAT1 LOF → MSMD; STAT1 GOF → chronic mucocutaneous candidiasis (CMC) — do not confuse.
Histology clue: poorly formed granulomas ("lepromatous" pattern) with abundant AFB.
~50% of IL-12Rβ1 deficient patients also get Salmonella infections (IL-12Rβ1 is shared with IL-23R → gut immunity).
Normal CBC, normal immunoglobulins, normal DHR test → advanced functional assays (IFN-γ/IL-12 production, STAT1 phosphorylation, genetic sequencing) required for diagnosis.

High Yield Summary — Differential Diagnosis of MSMD

The three PIDs that present with BCG dissemination in infants: SCID, CGD, MSMD — distinguish by lymphocyte subsets (SCID), DHR (CGD), and IFN-γ/IL-12 assays (MSMD).
Always exclude secondary immunodeficiency first — HIV, drugs (especially anti-TNF-α), malnutrition, malignancy.
MSMD has normal lymphocyte counts, normal immunoglobulins, and a normal DHR test — it is a "functionally specific" defect visible only on advanced IFN-γ/IL-12 assays or genetic testing.
The pathogen pattern is the biggest clue: disease caused by weakly virulent mycobacteria (BCG, NTM) ± Salmonella, without viral or fungal infections → think MSMD.
STAT1 LOF = MSMD (mycobacteria); STAT1 GOF = CMC (Candida) — direction of mutation determines the phenotype.
CGD overlaps significantly (BCG disease, granulomas, hepatosplenomegaly) but features recurrent abscesses with catalase-positive organisms and an abnormal DHR test.
SCID is the most severe PID presenting with BCG disease but has pansusceptibility (all pathogen classes) and lymphopenia / absent T-cells.

High Yield Summary — Diagnosis of MSMD

No formal tick-box diagnostic criteria exist for MSMD — diagnosis requires the triad of: (a) clinical phenotype (disseminated weakly virulent mycobacteria ± Salmonella), (b) functional IFN-γ/IL-12 axis defect, (c) genetic confirmation.
The routine PID workup is entirely normal — CBC, lymphocyte subsets, immunoglobulins, DHR test are all normal. Only advanced functional tests reveal the defect ^[1]^[2].
The stepwise approach: exclude secondary causes → basic PID workup → DHR (rule out CGD) → IFN-γ/IL-12 axis assays → genetic testing.
TST and IGRA may be falsely negative in MSMD because they depend on IFN-γ production by T-cells.
Histopathology clue: poorly formed granulomas with abundant AFB (especially in complete defects) — distinct from the well-formed caseating granulomas of immunocompetent TB.
Genetic testing is essential for all cases — for diagnosis, family counselling, prognostication, and prenatal diagnosis ^[8].
Hold BCG for unscreened siblings of an MSMD patient until genetic status is clarified.

High Yield Summary — Management of MSMD

Treatment: antimycobacterials ^[1]^[2] — prolonged multi-drug regimens tailored to organism (BCG uses HRE without pyrazinamide; MTB uses standard regimen with extended duration; NTM uses macrolide-based regimens guided by susceptibility).
Recombinant IFN-γ is the key adjunctive immunotherapy for partial defects — it is ineffective in complete receptor defects (no receptor to bind).
HSCT is the only curative option for complete defects — timing is critical (control infection first if possible). IRIS is an expected complication post-HSCT.
Avoid BCG in diagnosed patients and unscreened siblings — BCG → SCID, CGD, MSMD/STAT1 ^[5].
Pre-treatment baseline: LFTs, RFTs, urate, HBsAg, baseline visual acuity (for ethambutol) ^[7].
Antimicrobial prophylaxis (macrolide for NTM, co-trimoxazole for Salmonella, isoniazid for TB exposure) is part of long-term management.
Family-centred care: genetic counselling, cascade sibling screening, prenatal diagnosis for future pregnancies, psychosocial support.
Gene therapy is emerging but not yet standard of care (as of 2025–2026).

High Yield Summary — Complications of MSMD

Disease-related: Disseminated BCG-osis/NTM disease is the sentinel complication. Organ-specific sequelae include bronchiectasis, TBM with hydrocephalus and CN palsies, mycobacterial osteomyelitis (Pott's disease), granulomatous hepatitis, adrenal insufficiency, and bone marrow infiltration causing pancytopenia.
Infection-related: Recurrent invasive salmonellosis (bacteraemia, osteomyelitis, meningitis) occurs in ~50% of IL-12Rβ1-deficient patients.
Treatment-related: Prolonged anti-mycobacterial therapy → hepatotoxicity (H, R, Z), optic neuritis (E), ototoxicity (amikacin), neuropathy (H). HSCT → GVHD, graft failure, VOD, IRIS, conditioning-related gonadotoxicity.
IRIS is a characteristic complication occurring after HSCT or initiation of IFN-γ therapy — paradoxical inflammatory worsening as immune function is restored. Managed with corticosteroids.
Long-term sequelae: Chronic lung disease, neurodevelopmental delay, orthopaedic deformity, adrenal insufficiency, psychosocial burden, and lifelong infection susceptibility unless cured by HSCT.
Failure to thrive is both a warning sign and an ongoing complication — monitor growth at every visit ^[5].

Mendelian Susceptibility To Mycobacterial Disease

Mendelian Susceptibility to Mycobacterial Disease (MSMD) — Paediatric Notes

2. Epidemiology & Risk Factors

3. Anatomy & Function of the IFN-γ/IL-12 Axis

4. Aetiology & Pathophysiology

4.1 Genetic Aetiologies (Classified by Pathway Disrupted)

5. Classification

6. Clinical Features

Active Recall — MSMD: Definition, Pathophysiology & Clinical Features

Differential Diagnosis of MSMD

Systematic Differential Diagnosis

Active Recall — Differential Diagnosis of MSMD

References

Diagnostic Criteria, Algorithm & Investigations for MSMD

Practical "Red Flags" That Should Trigger MSMD Workup

3. Investigation Modalities — Detailed Breakdown

Active Recall — Diagnosis & Investigations of MSMD

References

Management of MSMD

Stream 1: Treatment of Active Mycobacterial Infection

Stream 2: Addressing the Underlying Immunodeficiency

Stream 3: Supportive and Preventive Measures

Active Recall — Management of MSMD

References

Complications of MSMD

A. Complications of the Mycobacterial Infection

Active Recall — Complications of MSMD

References

On this page

Mendelian Susceptibility To Mycobacterial Disease

1. Definition

2. Epidemiology & Risk Factors

2.1 Incidence and Prevalence

2.2 Hong Kong Context

2.3 Risk Factors

3. Anatomy & Function of the IFN-γ/IL-12 Axis

3.1 The Normal Pathway (From First Principles)

3.2 Key Players (Each Can Be Defective in MSMD)

4. Aetiology & Pathophysiology

4.1 Genetic Aetiologies (Classified by Pathway Disrupted)

A. Defects in IFN-γ Production (Upstream)

B. Defects in IFN-γ Response (Downstream)

C. Overlap Phenotypes

4.2 Pathophysiology — Why These Children Get Sick

5. Classification

5.1 IUIS Classification (2024)

5.2 Functional Classification (By Residual IFN-γ Signalling)

5.3 By Inheritance Pattern

6. Clinical Features

6.1 General Principles

6.2 Symptoms (With Pathophysiological Basis)

6.3 Signs (With Pathophysiological Basis)

6.4 Infection Pattern

6.5 Non-Infectious Features

6.6 Key History Points for MSMD (Paediatric Ward Round)

7. Key Distinguishing Features from Other PIDs

Active Recall — MSMD: Definition, Pathophysiology & Clinical Features

References

Framing the Clinical Scenario

Differential Diagnosis Algorithm

Systematic Differential Diagnosis

A. Other Primary Immunodeficiencies That Present with Mycobacterial Disease

B. Secondary (Acquired) Immunodeficiency — Must Exclude First

C. Non-Immunological Conditions Mimicking MSMD Presentation

D. Differential for the Specific "BCG Complication" Presentation

Approach to Differentiating — The "Stepwise Sieve"

Summary Table: Key Differentials at a Glance

Active Recall — Differential Diagnosis of MSMD

1. Diagnostic Criteria

2. Diagnostic Algorithm

3. Investigation Modalities — Detailed Breakdown

Tier 1: Excluding Secondary Immunodeficiency

Tier 2: Basic PID Workup (First-Line Immune Assessment)

Tier 3: Phagocyte Function Assessment

Tier 4: IFN-γ/IL-12 Axis — Advanced Functional Tests

Tier 5: Genetic Confirmation

Supporting Investigations: Confirming the Mycobacterial Infection

Additional Supporting Investigations

4. Summary — The Diagnostic Pathway in One Table

Active Recall — Diagnosis & Investigations of MSMD

Overarching Principles

Management Algorithm

Stream 1: Treatment of Active Mycobacterial Infection

1A. General Principles of Anti-Mycobacterial Therapy in MSMD

1B. Regimen Selection by Organism

1C. Paediatric Drug Dosing

1D. Treatment of Invasive Salmonellosis

Stream 2: Addressing the Underlying Immunodeficiency

2A. Recombinant IFN-γ (Interferon-Gamma-1b / Actimmune®)

2B. Haematopoietic Stem Cell Transplant (HSCT) — Definitive Cure

2C. Gene Therapy — Emerging Approach

Stream 3: Supportive and Preventive Measures

3A. Avoidance of Live Vaccines

3B. Antimicrobial Prophylaxis

3C. Nutritional Support and Growth Monitoring

3D. Family-Centred Care and Genetic Counselling

3E. Infection Prevention — Environmental Precautions

Summary Table — Management by Defect Severity

Active Recall — Management of MSMD

Framework for Understanding Complications

A. Complications of the Mycobacterial Infection

A1. Disseminated Mycobacterial Disease

A2. Organ-Specific Complications

A3. Invasive Salmonellosis Complications

B. Treatment-Related Complications

B1. Anti-Mycobacterial Drug Toxicity

B2. HSCT-Related Complications

B3. Recombinant IFN-γ Side Effects

C. Long-Term Sequelae of the Immunodeficiency