What is IL-12 receptor deficiency?

IL-12 receptor beta-1 deficiency is an autosomal recessive immunodeficiency in which T cells and NK cells cannot respond to IL-12 or IL-23, so they produce too little interferon-gamma. With too little interferon-gamma, macrophages are poorly activated to kill the microbes they engulf. It is the most common genetic cause of Mendelian susceptibility to mycobacterial disease (MSMD).

What infections occur in IL-12 receptor beta-1 deficiency?

The typical infections are BCG disease after vaccination and infections with environmental (nontuberculous) mycobacteria, usually beginning in childhood before age 12. About half of patients also have nontyphoidal Salmonella infection (Orphanet puts it at over half). Some patients get Candida infections because the shared receptor chain also carries IL-23 signals needed for Th17 immunity, and tuberculosis has been reported.

How is IL-12 receptor deficiency different from interferon-gamma receptor deficiency?

IL-12 receptor deficiency is a problem of interferon-gamma production, so interferon-gamma output is low and recombinant interferon-gamma therapy (used off-label) helps. Complete interferon-gamma receptor 1 deficiency is a problem of response: plasma interferon-gamma is high but cells cannot respond to it. That form presents in infancy with severe disseminated mycobacterial infection, does not respond to interferon-gamma, and can be cured only by stem cell transplantation.

How is IL-12 receptor deficiency diagnosed?

Routine tests such as the complete blood count, immunoglobulin levels and the DHR neutrophil test are normal, which helps rule out SCID, antibody deficiencies and chronic granulomatous disease. The targeted tests are flow cytometry, which usually shows no IL-12 receptor beta-1 on activated T and NK cells, low interferon-gamma production after whole-blood stimulation with BCG even when IL-12 is added, and genetic testing of IL12RB1.

How is IL-12 receptor deficiency treated?

Treatment combines long-term antimycobacterial therapy with recombinant interferon-gamma, which works because the interferon-gamma receptor is intact; this is an off-label use, because the FDA label covers only chronic granulomatous disease and severe malignant osteopetrosis. Surgery may be needed for localized splenic or mesenteric lesions that drugs penetrate poorly, and BCG vaccination must be avoided. Outcomes vary: many patients do well with treatment, but a 2026 review reported an overall mortality of about 26%.

What is Mendelian susceptibility to mycobacterial disease?

Mendelian susceptibility to mycobacterial disease (MSMD) is a group of rare inborn errors of interferon-gamma immunity that cause disease from weakly virulent mycobacteria, such as BCG vaccine and environmental mycobacteria, in otherwise healthy people with normal routine immune tests. Patients are also prone to salmonellosis, candidiasis and tuberculosis. Almost all known genes converge on the IL-12/IL-23 and interferon-gamma circuit, and IL-12 receptor beta-1 deficiency is the most common cause.

IL-12 Receptor Deficiency USMLE Question: BCG & Salmonella

IL-12 receptor β1 deficiency is the most common genetic cause of Mendelian susceptibility to mycobacterial disease (MSMD): an autosomal recessive defect in which T cells and NK cells cannot respond to IL-12 (or IL-23), so they make too little IFN-γ and macrophages are poorly activated to kill intracellular microbes. The classic stem is a child with BCG disease or disseminated nontuberculous mycobacteria plus nontyphoidal Salmonella, whose blood counts, immunoglobulins and DHR test are normal. Treatment is long-term antimycobacterial therapy plus recombinant IFN-γ, an off-label use that works because the IFN-γ receptor downstream is intact, and BCG vaccine must be avoided.

The Q-bank vignette

A 4-year-old boy has had 3 weeks of fever, night sweats and weight loss. He was born in the Philippines and received BCG vaccine at birth. At 7 months, a draining left axillary lymph node grew Mycobacterium bovis BCG, and at age 2 he was hospitalized for nontyphoidal Salmonella bacteremia. His parents are first cousins; his older sister is healthy. He has hepatosplenomegaly and left axillary lymphadenopathy. His CBC, including neutrophil and lymphocyte counts, is normal; IgG, IgA, IgM and IgE are normal; and dihydrorhodamine (DHR) flow cytometry is normal. A liver biopsy shows granulomas with acid-fast bacilli. When his whole blood is stimulated with BCG, alone or with added IL-12, IFN-γ production is markedly reduced.

Which protein is most likely defective?

A. Adenosine deaminase
B. Bruton tyrosine kinase
C. CD40 ligand
D. Interferon-γ receptor 1 (complete deficiency)
E. Interleukin-12 receptor β1
F. NADPH oxidase
G. STAT3

Answer: E. Interleukin-12 receptor β1.

Why it's IL-12 receptor β1: the IL-12/IFN-γ circuit

Macrophages and dendritic cells that meet intracellular microbes secrete IL-12 and IL-23, two cytokines that share the p40 subunit (IL-23 pairs p40 with p19).
IL-12 binds IL-12Rβ1 + IL-12Rβ2 on T cells and NK cells → JAK2/TYK2 → STAT4 → T-bet → Th1 differentiation and IFN-γ secretion.
IFN-γ binds IFN-γR1 + IFN-γR2 on macrophages → JAK1/JAK2 → STAT1 → classically activated (M1) macrophages that kill what they have engulfed.
IFN-γ signaling feeds back to boost the circuit, a positive feedback loop that keeps macrophages switched on.

IL-12Rβ1 is shared: it pairs with IL-12Rβ2 to bind IL-12 and with IL-23R to bind IL-23. Losing it removes both signals, so IFN-γ is low (mycobacteria, Salmonella) and Th17 immunity is weak (some patients also get Candida infections). For the broader T-helper framework, see Th1 vs Th2 immune responses.

Decoding the stem

BCG disease after vaccination: onset is usually in childhood (before age 12), with BCG disease as the usual first infection.
Nontyphoidal Salmonella: affects about half of patients with IL-12Rβ1 deficiency (Orphanet says over half); like mycobacteria, it survives inside macrophages.
Consanguineous parents, healthy sibling: autosomal recessive with incomplete penetrance, so a sibling with the same genotype can stay well.
Normal CBC, immunoglobulins and DHR: MSMD strikes otherwise healthy people with no overt abnormality on routine hematologic and immunologic tests.
Low IFN-γ after BCG stimulation, even with added IL-12: the production arm is broken because T and NK cells cannot respond to IL-12; flow cytometry usually shows no IL-12Rβ1 on activated T and NK cells.

Why every other answer choice is wrong

D. IFN-γ receptor 1, complete deficiency. The closest distractor: same organisms, same circuit, one step later. The defect is in the response, not the production: patients have high plasma IFN-γ, and their cells do not respond to IFN-γ in the lab. Complete autosomal recessive IFN-γR1 deficiency presents in infancy, with severe infections usually before age 3, as disseminated, life-threatening BCG or environmental mycobacterial disease (for example M. fortuitum or M. chelonae). IFN-γ therapy is not effective, and HSCT is the only cure. Low IFN-γ production, even when IL-12 is added, points upstream to IL-12Rβ1.

F. NADPH oxidase (chronic granulomatous disease). Granulomas plus unusual infections make it tempting, and CGD patients can also have severe BCG infection. But CGD infections come from S. aureus, Serratia, Burkholderia cepacia, Nocardia and Aspergillus, and the DHR test is abnormal. A normal DHR excludes it; compare the pattern in the chronic granulomatous disease guide.

A. Adenosine deaminase (SCID). SCID also causes BCG complications, but most infants develop opportunistic infections within the first 3 months of life, and by 6 months most have candidiasis, persistent viral infections, Pneumocystis pneumonia or diarrhea with failure to thrive, with lymphopenia and few or no T cells. A 4-year-old with a normal lymphocyte count does not fit; see the adenosine deaminase deficiency guide.

B. Bruton tyrosine kinase (X-linked agammaglobulinemia). B cells stop maturing, so all immunoglobulins are low and CD19+ B cells are absent. Expect recurrent sinus, lung and skin infections with encapsulated bacteria (S. pneumoniae, H. influenzae) and enteroviral CNS infections, with very small tonsils and no palpable lymph nodes. Normal immunoglobulins and enlarged nodes exclude it.

C. CD40 ligand (X-linked hyper-IgM syndrome). T cells cannot signal B cells to class switch: IgM is normal or high while IgG, IgA and IgE are low. Infants present with Pneumocystis pneumonia, recurrent sinopulmonary infections, Cryptosporidium and neutropenia. Normal IgG, IgA and IgE exclude it.

G. STAT3 (autosomal dominant hyper-IgE, Job syndrome). A Th17 defect: recurrent staphylococcal abscesses of the skin and lungs, pneumatoceles, eczematous dermatitis, retained primary teeth, coarse facies and IgE above 2,000 IU/mL. Normal IgE and no staphylococcal abscesses exclude it; the full picture is in the Job syndrome flowchart.

IL-12 receptor vs IFN-γ receptor deficiency

Feature	IL-12Rβ1 deficiency	Complete IFN-γR1 deficiency	AD partial IFN-γR1 deficiency
Inheritance	Autosomal recessive	Autosomal recessive	Autosomal dominant
Broken step	IFN-γ production: T and NK cells ignore IL-12 and IL-23	IFN-γ response: no functional receptor	Reduced response
Onset	Childhood, usually before age 12	Infancy; severe infections usually before age 3	Around adolescence (about 13 years)
Typical infections	BCG and environmental mycobacteria; nontyphoidal Salmonella in about half; some Candida or tuberculosis	Disseminated, life-threatening BCG or environmental mycobacteria	BCG or M. avium, often osteomyelitis; salmonellosis in about 5%
IFN-γ	Low production after BCG stimulation	High in plasma; cells unresponsive	Residual response
Recombinant IFN-γ (off-label)	Helps, with antimycobacterial drugs	Not effective	Used if needed
Outlook	Variable; many do well with treatment, but overall mortality is about 26%	Poor; HSCT is the only cure	Usually controlled with long-term antimicrobials

One-line rule: low IFN-γ = production problem (IL-12, IL-12 receptor); high IFN-γ = response problem (IFN-γ receptor).

Workup and treatment

Routine screen: CBC, immunoglobulins and DHR are normal in MSMD, which is exactly what rules out SCID, antibody defects and CGD.
Targeted tests: IL-12Rβ1 expression on activated T and NK cells by flow cytometry, IFN-γ production after whole-blood stimulation with BCG and with BCG plus IL-12, plasma IFN-γ (high in complete IFN-γR1 deficiency) and gene testing (IL12RB1).
Treatment: long-term antimycobacterial therapy combined with recombinant IFN-γ; surgery for localized splenic or mesenteric lesions that drugs penetrate poorly. IFN-γ is used off-label here: its FDA-approved indications are CGD and severe malignant osteopetrosis.
Prevention: avoid BCG vaccination.
Prognosis: variable. Orphanet calls it usually good with proper treatment, with most patients reaching adulthood, but a 2026 review reports an overall mortality of about 26%, so HSCT is occasionally considered.

Exam traps

BCG disease is not specific. SCID and CGD also cause BCG complications; the lymphocyte count and the DHR are the tie-breakers.
A single neck node in a toddler is usually not MSMD. Chronic submandibular or cervical lymphadenitis at ages 1 to 5 is commonly due to MAC or M. scrofulaceum. Dissemination, recurrence, BCG disease or Salmonella is what raises the question.
The adult look-alike is advanced HIV: disseminated MAC is a late complication, and MAC prophylaxis is indicated when the CD4 count is below 50/µL in patients who are not receiving antiretroviral therapy, remain viremic on it, or cannot be fully suppressed (it is not recommended for patients who start ART right away).
Candida does not argue against IL-12Rβ1 deficiency. The shared IL-23 receptor chain means Th17 immunity is impaired too.
BCG in the stem usually means birth abroad. BCG is not generally used in the United States but is given to infants in countries where TB is common.

Follow-up question

IL-12Rβ1 deficiency is confirmed. In addition to antimycobacterial drugs, which therapy is most appropriate?

A. Intravenous immune globulin
B. Recombinant interferon-γ
C. TMP-SMX plus itraconazole prophylaxis
D. Repeat BCG vaccination
E. G-CSF

Answer: B. Recombinant IFN-γ supplies the signal that his T and NK cells cannot make, and his macrophages still have working IFN-γ receptors. This is an off-label use; the FDA-approved indications are CGD and severe malignant osteopetrosis. Immune globulin replaces antibodies in XLA or hyper-IgM, and his antibody levels are normal. TMP-SMX plus itraconazole is CGD prophylaxis. BCG is a live vaccine that caused his first infection and must be avoided. G-CSF treats severe neutropenia (as in X-linked hyper-IgM), and his neutrophil count is normal.

Key takeaways

IL-12 (macrophages, dendritic cells) → IL-12Rβ1/β2 on T and NK cells → IFN-γ → IFN-γR on macrophages → intracellular killing.
IL-12Rβ1 deficiency is autosomal recessive and the most common genetic cause of MSMD: BCG and environmental mycobacteria, nontyphoidal Salmonella in about half, sometimes Candida.
Routine immune tests are normal; diagnose with IL-12Rβ1 flow cytometry, IFN-γ production assays and gene testing.
Production vs response: IL-12Rβ1 deficiency has low IFN-γ and responds to (off-label) IFN-γ therapy; complete IFN-γR1 deficiency has high IFN-γ, does not respond, and needs HSCT.
Tie-breakers: normal DHR (not CGD), normal lymphocytes (not SCID), normal immunoglobulins (not XLA or hyper-IgM), normal IgE (not Job syndrome).
Never give BCG to a patient with an IL-12/IFN-γ axis defect.