B cell BSC

B Cell Interactions BSC

Overview

• Presented by Dr. Towne.

Objectives

• Describe the process of B cell development and maturation.

  • Discuss defects in this process in XLA (X-Linked Agammaglobulinemia).

  • Discuss defects in this process in AID (Activation-Induced Cytidine Deaminase deficiency).

  • Identify genes involved in B cell development and maturation.

• Explore lab values for these deficiencies that might mimic other defects in B-cell development and maturation.

• Examine how IVIG (Intravenous Immunoglobulin) helps these patients.

• Explain why females are typically unaffected in XLA.

• Investigate the types of antibodies affected in AID and their impact on patients.

Case #1: Bill Grignard

• Healthy for the first 4 months of life.

• Following 4 months, developed a series of infections:

  • Pneumonia

  • Otitis media (ear infections)

  • Erysipelas (response to antibiotic treatment).

• Normal growth and development, but recurrent infections at 2 years and 3 months.

  • His mother, a nurse, noted frequent antibiotic use.

• Pediatrician tested serum immunoglobulin levels:

  • Results:

  • Low levels of IgM.

  • Low levels of IgG.

  • No detectable IgA.

Bill’s Immunoglobulin Levels

• Patient: Bill

  • Normal (for a 2-3 y/o):

  • IgM: 10 mg/dL (normal range: 40-160 mg/dL)

  • IgG: 80 mg/dL (normal range: 400-1000 mg/dL)

  • IgA: 0 mg/dL (normal range: 10-120 mg/dL).

• Result: Initiated monthly intramuscular injections of gamma globulin.

Progression of Bill’s Condition

• At 9 years old, presented with:

  • Partially collapsed lung (atelectasis).

  • Chronic cough.

• Examination results:

  • Absence of visible tonsils.

  • Moist crackles (rales) noted at both lung bases.

  • Other health measures appeared normal.

• Family history:

  • 7-year-old brother, John, also had recurrent pneumonia and IgG levels of 150 mg/dL.

  • Mother has two brothers who died of pneumonia at age 2.

  • Two sisters are healthy with no reported defects.

Flow Cytometry Analysis

• Performed to assess B and T cell populations:

  • Markers:

  • CD19 (B cells)

  • CD3 (T cells).

Genetic Findings in Bill

• Genotyping revealed:

  • Diagnosis: XLA (X-Linked Agammaglobulinemia).

  • Genetic defect: Missing BTK (Bruton’s tyrosine kinase).

  • Result: Pre-BCR cannot signal and development is arrested.

Explanation of Normal Lymphocyte Count

• Possible reasons for normal lymphocyte count:

  • a. Neutrophil cell numbers are higher than normal.

  • b. T-cell numbers are higher than normal.

  • c. Neutrophil and T-cell numbers are higher than normal.

  • d. Neutrophil cell numbers are lower than normal.

  • e. T-cell numbers are lower than normal.

  • f. Neutrophil and T-cell numbers are lower than normal.

• Reason for Bill's initial health (first 4 months):

  • a. Passive transfer of immunity through placental IgG.

  • b. Active transfer of immunity through placental IgG.

  • c. Passive transfer of immunity through placental IgA.

  • d. Active transfer of immunity through placental IgA.

Treatment and Outcomes for Bill

• Bill was treated with IVIG:

  • Result: Improvement; rales disappeared.

• Continued health and academic performance:

  • Became a medical student.

  • Occasionally required antibiotics but generally remained healthy.

  • Weekly self-infusion of 10g gamma globulin through a vein in his hand.

Case #2: Daisy Miller

• Admitted to Children’s hospital with pneumonia.

• Enlarged lymph nodes in neck and armpits noted during examination.

• Infection history:

  • Pneumonia at 25 months.

  • 10 episodes of middle ear infections.

• Laboratory findings:

  • IgM: 470 mg/dL (normal: 40-240 mg/dL).

  • IgG: 40 mg/dL (normal: 640-1350 mg/dL).

  • IgA: Undetectable (normal: 70-300 mg/dL).

• High WBC count with abnormal distribution:

  • 81% neutrophils and 14% lymphocytes.

• Consultation with an immunologist was initiated.

Immunological Findings for Daisy

• Vaccination against H. influenzae conducted:

  • No specific antibody detected against the antigens.

• Blood type A:

  • IgM titer of anti-B antibodies positive at 1:320 (upper normal limit).

  • IgG titer of anti-B antibodies undetectable.

• Initiated antibiotics and IVIG therapy.

Daisy's Genetic Findings

• Sequencing outcomes revealed:

  • Point mutation in the AID gene introducing a stop codon.

  • Result: Truncated and defective protein produced.

IgE Production Analysis

• IgE response measured:

  • Results (pg/ml):

  • Normal individual: secretion under various stimulations measured.

  • Comparison of IgE secretion:

    • Normal individual vs. patients with CD40L deficiency and AID deficiency.

  • Key Observations:

  • Stimulation with IL-4 alone versus anti-CD40 + IL-4.

  • Negative control stimulation produced negligible IgE levels.

  • The anti-CD40 and IL-4 combination compensates for CD40L lack but not for AID defects.

Involvement of AID in Immune Processes

• Processes AID is directly involved in (select all that apply):

  • a. Somatic recombination.

  • b. Junctional diversity.

  • c. Isotype switching.

  • d. Somatic hypermutation.

  • e. Affinity maturation.

Clinical Considerations for AID Deficiency

• Answer to why Daisy had enlarged lymph nodes:

  • Enlargement due to stimulation of B-cells leading to germinal center (GC) reaction.

  • Defect in SHM (Somatic Hypermutation) and Isotype switching:

  • B-cells proliferate without effective differentiation.

• Comparison with CD40L deficiency:

  • CD40/CD40L critical for B-cell activation and GC progression; absence leads to no germinal center reactions.

  • Both conditions show Hyper-IgM due to different mechanisms:

  • AID deficiency: avoids mutations but retains IgM function.

  • CD40L deficiency: enhancement through T-independent responses.