Molecular single-cell analysis reveals that CD5-positive peripheral blood B cells in healthy humans are characterized by rearranged Vkappa genes lacking somatic mutation.
B cells expressing the CD5 cell surface antigen are involved in certain B cell malignancies and autoimmune diseases. From studies in the mouse, it emerged that CD5+ B cells represent a separate lineage of B lymphocytes that, in contrast to conventional (CD5-) B cells, are not driven into T cell-dependent immune responses in which rearranged variable (V) region genes are diversified by somatic hypermutation. Against this background it came as a surprise that human disease-involved CD5-positive autoreactive B cells as well as B cell chronic lymphocytic leukemias can harbor somatically mutated V region genes. Recent V gene analyses on CD5+ B cells in healthy adults did not give rise to a clear picture about the fraction of somatically mutated among all CD5+ B cells. In this work we used a molecular single-cell analysis to determine reliably the frequency of mutated CD5+ B cells in healthy humans: single, kappa light chain-expressing CD5+ peripheral blood B cells were isolated by flow cytometry, and rearranged Vkappa genes were amplified by PCR. From one donor, CD5+CD19+ B cells were analyzed. Since CD5+ B cells were found among IgM+IgD+ and IgM+IgD- cells (but almost not among class-switched cells) from two other donors, individual cells corresponding to these IgM-expressing subsets were investigated separately. The sequence analysis of rearranged Vkappa genes revealed that most if not all CD5+ B cells in healthy humans carry unmutated V region genes. From one of the donors, a novel polymorphic Jkappa2 gene segment was identified. To explain the discrepancy between the frequent occurrence of disease-associated somatically mutated CD5+ B cells and the low incidence or absence of somatic mutation in normal CD5+ B cells, we speculate that CD5+ B cells usually do not participate in germinal center reactions, but if they occasionally do so, they may be at an increased risk to become involved in autoimmune diseases or B cell malignancies.
Fischer
M
M
Institute for Genetics, University of Cologne, 50931 Cologne, Germany.
Klein
U
U
Küppers
R
R
D000328:Adult; D018941:Antigens, CD19; D016175:B-Lymphocyte Subsets / Q000166:cytology / Q000276:immunology*; D018956:CD5 Antigens; D005434:Flow Cytometry; D015328:Gene Rearrangement, B-Lymphocyte, Light Chain; D006801:Humans; D007072:Immunoglobulin D / Q000235:genetics; D007075:Immunoglobulin M / Q000235:genetics; D007135:Immunoglobulin Variable Region / Q000235:genetics*; D015451:Leukemia, Lymphocytic, Chronic, B-Cell / Q000209:etiology; D015463:Leukemia, Prolymphocytic / Q000209:etiology; D008297:Male; D009154:Mutation; D016133:Polymerase Chain Reaction; D017422:Sequence Analysis, DNA
D018941:Antigens, CD19; D018956:CD5 Antigens; D007072:Immunoglobulin D; D007075:Immunoglobulin M; D007135:Immunoglobulin Variable Region