Danny J. was born a healthy 8 pound baby, the fourth child of Doris and Donald
J. Danny's older brother had died of pneumonia in infancy five years before.
His two older sisters had had only the usual childhood colds and stomach upsets.
Danny's parents brought him to the emergency room when he was about six months
old gasping for breath and with a 104° fever. His lymph nodes were not swollen
and his tonsils were very small. He had previously suffered from only minor respiratory infections. A throat
swab and blood sample were collected for lab analysis. Danny was admitted to
the hospital and treated with broad spectrum antibiotics and oxygen. The throat
culture was positive for Streptococcus pneumoniae, a Gram positive coccus
surrounded by a thick polysaccharide capsule (see micrograph below). A white
blood cell count and total plasma protein levels were slightly below normal.
The infection responded to antibiotics and Danny was able to go home after a
week. Follow-up tests were performed on an outpatient basis.
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Serum immunoelectrophoresis (the separation of serum proteins by charge) revealed the nearly complete absence of gamma globulins (antibodies) from Danny's serum. IgG was present at about 10% of normal levels, and the other isotypes were not detected. Blood and bone marrow were collected for phenotypic analysis, detecting surface proteins on the cells by Flow Cytometry. Less than 0.1% of circulating lymphocytes were IgM+ IgD+, where normally circulating lymphocytes are 5-15% IgM+ IgD+. The rest of the circulating lymphocytes were CD3+ IgM- IgD- (T cells). Granulocytes and monocytes were present in normal numbers.
The bone marrow contained cells which were positive by immunofluorescence for membrane μ, λ5-VpreB, Igα and Igβ (CD 79α and CD 79β), but not membrane or cytoplasmic κ , λ, or δ chain or CD19. Bone marrow was positive by PCR for all structural Ig gene segments, RAG-1, RAG-2, and TdT but was negative for the protein tyrosine kinase btk.
As a progenitor B cell develops in the bone marrow, the enzyme complex of RAG1, RAG2 and TdT join a VH, a DH, and JH segment into the gene for a complete VH. The cell must now express the H chain on its membrane to get a signal to start recombining the gene segments for L chain. H chain cannot be expressed on the membrane without a L chain; the cell uses a surrogate light chain called λ5:VpreB. Igα and Igβ must also be expressed to send the binding signal into the cytoplasm, where it will eventually reach the DNA to change mRNA synthesis. A key enzyme in the signaling cascade is btk (B cell tyrosine kinase), an enzyme that phosphorylates other signaling molecules). Without this signal, pre-B cells do not go on to recombine light chain and become mature B cells; instead, they die. The gene for btk is present on the X chromosome. The genetics of XLA are illustrated in Janeway Fig 11.10 (shown below) and Parham Fig 9.8. Humans normally produce about 30 billion new B cells every day.
The pediatrician explained to Doris and Donald that their son had X-linked agammaglobulinemia (XLA) and would never be able to produce his own antibodies. Danny would be treated with weekly injections of pooled human gamma globulin, which would help him fight infections. Boys with X-linked agammaglobulinemia usually have recurring infections with S. pneumoniae, Streptococcus pyogenes, Staphylococcus aureus, and Haemophilus influenzae, pyogenic (pus-forming) bacteria that often have capsules. Humans normally make opsonizing antibodies to the pyogenic bacteria that promote their phagocytosis by neutrophils. People with XLA do not have problems with most virus infections, although hepatitis A and polio viruses are exceptions.
MIC 419 Case 4 Questions
In class discussion: Friday 2/2-Monday 2/5 Janeway Chapter 7 pp 247-250 and 258-265 or Parham Chapter 4 pp 99-107.
1. Use the case and the reading above to answer the following questions:
2. Where would the physician get the pooled human gamma globulin that would protect Danny from infection? How is it that he can receive antibodies from any other humans but could get a bone marrow transplant from only people with matching HLA (MHC)?
3. Do you think btk is involved in the production of mature T cells? What information in the case is related to this question? How could you test Danny's T cell function? [**Think about this one but don't spend a lot of time on it!]
4. How could Danny have IgG but no other Ig isotypes? [HINT: Think about his age.]
Supplementary Materials: Antibody, Antibody Genes, B Cell Development
Agammaglobulinemia http://www.nlm.nih.gov/medlineplus/ency/article/001307.htm
X-linked agammaglobulinemia http://www.healthatoz.com/healthatoz/Atoz/ency/x-linked_agammaglobulinemia.jsp
