Immunology · Year 2 · from Immunology
Case 2: Hyper-IgM Syndrome (CD40 Ligand Deficiency)
Patient Demographics
- Age: 3 years old
- Sex: Male
- Ethnicity: African American
Chief Complaint
Pneumonia and chronic diarrhea
History of Present Illness
A 3-year-old boy presents with cough, fever, and respiratory distress for 5 days. He has had recurrent respiratory infections since infancy and chronic watery diarrhea for 1 year. His mother notes he has had several episodes of oral sores. BAL from previous pneumonia at age 2 grew Pneumocystis jirovecii.
Past Medical History
- Pneumocystis jirovecii pneumonia at age 2
- Multiple episodes of bacterial sinusitis and otitis media
- Chronic diarrhea (1 year duration)
- Recurrent oral ulcers
- Neutropenia noted intermittently
Family History
- Maternal uncle with recurrent infections, died at age 8
Physical Examination
- General: Thin, ill-appearing child
- Vital Signs: T 39.0C, HR 140, RR 40, SpO2 90% on room air
- Growth: Weight 5th percentile
- HEENT: Oral ulcers, tonsils present but small
- Lymphatics: Palpable cervical lymph nodes
- Lungs: Bilateral crackles, increased work of breathing
- Abdomen: Hepatomegaly, no splenomegaly
Laboratory Workup
| Test | Result | Reference Range |
|---|---|---|
| WBC | 5,500/uL | 6,000-17,500/uL |
| ANC | 800/uL | 1,500-8,500/uL |
| IgG | 45 mg/dL | 345-1236 mg/dL |
| IgA | <5 mg/dL | 14-159 mg/dL |
| IgM | 650 mg/dL | 43-207 mg/dL |
| IgE | <5 IU/mL | <100 IU/mL |
| CD19+ B cells | 15% (normal) | 14-33% |
| CD3+ T cells | Normal | -- |
| CD40 ligand expression | Absent on activated T cells | Present |
| Cryptosporidium stool antigen | Positive | Negative |
Clinical Image
Image showing diffuse bilateral infiltrates characteristic of Pneumocystis pneumonia. Image source: Wikimedia Commons (https://commons.wikimedia.org/wiki/File:Pneumocystis_jirovecii_chest_X-ray.jpg). Licensed under CC BY-SA 4.0.
Diagnosis
X-Linked Hyper-IgM Syndrome (CD40 Ligand Deficiency) with Pneumocystis pneumonia and Cryptosporidium enteritis.
Discussion
Hyper-IgM syndromes are characterized by normal or elevated IgM with markedly decreased IgG, IgA, and IgE due to defective class switch recombination. The most common form is X-linked (CD40L deficiency), which causes combined immunodeficiency:
Why is this a combined immunodeficiency despite being a "B cell" problem?
- CD40L (CD154) on T cells must engage CD40 on B cells for class switch recombination
- CD40L-CD40 interaction is ALSO required for T cell-mediated macrophage activation
- Without this interaction, macrophages cannot effectively kill intracellular pathogens
Clinical features differentiating from pure antibody deficiency:
- Opportunistic infections (PJP, Cryptosporidium, CMV) - suggests T cell/macrophage dysfunction
- Sclerosing cholangitis from Cryptosporidium (major cause of morbidity/mortality)
- Neutropenia (mechanism unclear)
Contrast with Type 2 Hyper-IgM (AID deficiency):
- AID (activation-induced cytidine deaminase) is required for class switch recombination in B cells
- Type 2 affects ONLY B cells, not T cell-macrophage interaction
- Type 2 presents with bacterial infections only, NOT opportunistic infections
Treatment
- Immunoglobulin replacement: IVIG lifelong
- PJP prophylaxis: TMP-SMX (required, unlike pure antibody deficiency)
- Cryptosporidium prevention: Avoid contaminated water sources; no effective treatment
- G-CSF: For symptomatic neutropenia
- Hematopoietic stem cell transplantation: Curative, recommended due to risk of liver disease and malignancy
- Liver monitoring: For sclerosing cholangitis