An infant is admitted to the hospital with a history of recurrent and persistent
ID: 217679 • Letter: A
Question
An infant is admitted to the hospital with a history of recurrent and persistent bacterial infections. His physician suspects he has an immunodeficiency disease, and obtains a sample of the patient’s peripheral blood. The white blood cells are analyzed by antibody staining followed by flow cytometry, and the results are shown in figure A on the left. To determine the origin of the peripheral blood cell defect, a bone marrow biopsy is taken from the patient and compared to a healthy control, as shown in figure B on the right.
a). What are populations 1, 2, and 3 in Figure C? Explain your reasoning (just a few words should be sufficient). (1.5 points)
b). In another experiment, the expression of proteins on the surface of cells from a healthy infant are also examined. What is the population of cells that are IgMloIgknegIglneg, as indicated by the arrow in figure D? Explain your reasoning. (0.5 point)
c). Based on these data, where (in terms of the stages of B cell development) does the defect seem to be occurring? Based on the events that occur in the different stages, explain what you think is most likely causing the block between these stages (2 point).
To help further identify the defective or missing protein in the patient’s developing B cells, bone marrow cells are isolated and protein lysates are prepared for Western blots. A series of antibodies are tested and the results are shown in figure E.
Healthy infant Patient Healthy infant Patient Peripheral blood cells Bone marrow cells CD3 CD3 CD19 (B cells) CD19 (B cells) IgM IgMExplanation / Answer
a)
Population 1 are cells that express both low amounts of IgM and Igk antibodies. Both antibodies in those cells have low scattering in the flow cytometry.
Population 2 are cells that express IgM in high amounts and Igk in low amounts. IgM antibodies have higher scattering than Igk in the cell population.
Population 3 are cells that express IgM and Igk in high amounts. Both the antibodies show high scattering in the flow cytometry.
b) In the healthy individual, the cells shown by the arrow are those cells that have low expression of antibodies on the cell surface. This suggests that those are not affected by any antigens.
c) The stage where the defect happens in the pre B cell, where there is a low concentration of Igk and high concentration of IgM. The B cell is retained in the premature stage because the rearrangement of the light chain genes is producing a new IgM with specificity that is self-reactive. Further rearrangements are also being self-reactive leading to the apoptosis of the pre-B cells and causing immunodeficiency.
d) Lack of Igk antibody molecule is responsible for the patient's immunodeficiency disease. This is because all other antibodies molecules expression shown in the Western blots have a similar expression in both the control and patient's bone marrow cells.