Question

Case #1

At 3 years old, Daisy Miller was admitted to the Boston Children's Hospital with pneumonia. Her mother had taken her to Dr. James, a pediatrician, because she had a fever and was breathing fast. Her temperature was high, at 40.1°C (104.2°F), her respiratory rate was 40 per minute (normal 20), and her blood oxygen saturation was 88% (normal >98%). Dr. James also noticed that lymph nodes in Daisy's neck and armpits (axillae) were enlarged. A chest X-ray was ordered. It revealed diffuse consolidation (whitened areas of lung due to inflammation, indicating pneumonia) of the lower lobe of her left lung, and she was admitted to the hospital.

Daisy had had pneumonia once before, at 25 months of age, as well as 10 episodes of middle-ear infection (otitis media) that had required antibiotic therapy. Tubes had been placed in her ears to provide adequate drainage and ventilation of the ear infections.

In the hospital a blood sample was taken and was found to contain 13,500 white blood cells/ml, of which 81% were neutrophils and 14% were lymphocytes. A blood culture grew the bacterium Streptococcus pneumoniae.

Because of Daisy's repeated infections, Dr. James consulted an immunologist. She tested Daisy's immunoglobulin levels and found that her serum contained 470 mg/dl of IgM (normal 40-240), undetectable IgA (normal 70-312), and 40 mg/dl of IgG (normal 639-1344). Although Daisy had been vaccinated against tetanus and Haemophilus influenzae, she had no specific IgG antibodies against tetanus toxoid or to the polyribosyl phosphate (PRP) polysaccharide antigen of H. influenzae. Because her blood type was A, she was tested for anti-B antibodies. Her IgM titer of anti-B antibodies was positive at 1:320 (upper limit of normal), whereas her IgG titer was undetectable.

Daisy was started on intravenous antibiotics. She improved rapidly and was sent home on a course of oral antibiotics. Intravenous immunoglobulin (IVIG) therapy was started, which resulted in a marked decrease in the frequency of infections.

Analysis of Daisy's peripheral blood lymphocytes revealed normal expression of CD40L on T cells activated by anti-CD3 antibodies, and normal expression of CD40 on B cells. Nevertheless, her blood cells completely failed to secrete IgG and IgE after stimulation with anti-CD40 antibody (to mimic the effects of engagement of CD40L) and IL-4, a cytokine that also helps to stimulate class switching, although the blood cells proliferated normally in response to these stimuli. cDNAs for CD40 and for the enzyme activation-induced cytidine deaminase (AID) were made and amplified by RT-PCR on mRNA isolated from blood lymphocytes activated by anti-CD40 and IL-4. Sequencing of the cDNAs revealed a point mutation in the AID gene that introduced a stop codon into exon 5, leading to the formation of truncated and defective protein. The CD40 sequence was normal.

Q1: Daisy and other patients with hyper IgM syndrome caused by a mutation in AID do not seem to suffer from opportunistic infections, such as Pneumocystis jirovecii and Cryptosporidium,which are characteristic of CD40L deficiency. Why?

Q2:A three year old girl presents with hyper IgM syndrome, recurrent bacterial infection, and history of Pneumocystis jirovecii pneumonia. Her CD40L and AID genes are normal. What is the potential gene defect in this patient?

Q3: 6 year old girl presents with hyper IgM syndrome, recurrent bacterial infection, and no history of opportunistic infections. Her CD40 and AID genes sequences are normal. What is the potential gene defect in this patient?

Q4: Can you think of a simple test to distinguish hyper IgM syndrome caused by an intrinsic B cell defect from that caused by CD40L deficiency without recourse to DNA sequencing?

Q5: Dr James noted that Daisy had enlarged lymph nodes. Patients with CD40L deficiency do not have enlarged lymph nodes. Can you explain this difference?

Answer 1

1. Patients with hyper IgM syndrome caused by mutation in AID do not suffer from opportunistic infection, however these are characteristic of CD40L deficiency.

In patients with CD40L deficiency, there is an impaired expression of CD40L by activated CD4+ T cells , these affects the monocytes and dendritic cells. There is an impaired secretion of interleukin IL 12 by monocytes, this further results in imapired secretion of interferon IFN gamma by T lymphocytes and defective T cell priming.

These functional abnormalities in the immune system contribute to susceptibility of CD40 L deficient patients to opportunistic infections.

Whereas in case of AID deficiency, the number of circulating B and T cells are normal, the cross linking of CD 40 induces normal proliferation of B cells.

Hence opportunistic infections in AiD deficient patients are not as common.

2. CD 40 deficiency. It was inherited as an autosomal recessive trait and diagnosed on the basis of lack of expression of CD 40 on the surfaces of B lymphocytes and monocytes.

Clinical and immunological findings showed showed recurrent bacterial and opportunistic infections.

• B cells are reduced
• Impaired function of monocyte derived dendritic cells.
• Reduced secretion of IL 12
• Impaired T cell priming
• Diminished release of interferon gamma by T lymphocytes

​​​​​​​​​​3. Intrinsic B cells defects seen in hyper IgM syndrome show the clinical characteristics of AID deficiency, where recurrent bacterial infections are seen with no history of opportunistic infections.

They may be susceptible to autoimmune manifestations and possibly also to tumors.