Molecular Single-cell Analysis Reveals that CD5-positive Peripheral Blood B Cells in Healthy Humans are Characterized by Rearranged V k 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 1 B cells represent a separate lineage of B lymphocytes that, in contrast to conventional (CD5 2 ) 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 1 B cells in healthy adults did not give rise to a clear picture about the fraction of somatically mutated among all CD5 1 B cells. In this work we used a molecular single-cell analysis to determine reliably the frequency of mutated CD5 1 B cells in healthy humans: single, k light chain–expressing CD5 1 peripheral blood B cells were isolated by flow cytometry, and rearranged V k genes were amplified by PCR. From one donor, CD5 1 CD19 1 B cells were analyzed. Since CD5 1 B cells were found among IgM 1 IgD 1 and IgM 1 IgD 2 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 V k genes revealed that most if not all CD5 1 B cells in healthy humans carry unmutated V region genes. From one of the donors, a novel polymorphic J k 2 gene segment was identified. To explain the discrepancy between the frequent occurrence of disease-associated somatically mutated CD5 1 B cells and the low incidence or absence of somatic mutation in normal CD5 1 B cells, we speculate that CD5 1 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. ( J. Clin. Invest. 1997. 100:1667–1676.)