Adult BM generates CD5+ B1 cells containing abundant N‐region additions

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Abstract
The self‐renewing capacity of B1 cells infers homeostatic regulation; however, previous work suggests the low level of N‐region addition characterizing B1 cells early in life increases with age, which implies that the B1‐cell population is not a closed system. To explore this, we evaluated N‐region addition in CD5+ B1 cells generated from adult BM. Adult BM cells were marked with GFP introduced by mouse stem cell virus transduction, and were then adoptively transferred into lethally irradiated recipients. Within 2–3 months, we found GFP‐marked CD5+ B cells in the peritoneal cavities of recipients, which we demonstrate here meet a variety of criteria for B1‐cell traits including Mac‐1 surface expression; annexin, elfin, and Pax‐5 gene expression; mitogenic responsiveness to phorbol ester; and spontaneous immunoglobulin secretion. Notably, we found by single‐cell PCR that this population of BM‐derived CD5+ B1 cells expressed immunoglobulin with abundant N‐region addition (and little VH11/VH12 skewing), unlike CD5+ B1 cells obtained from unmanipulated animals but reminiscent of B2 cells. Further, we confirmed that native CD5+ B1 cells from older mice contain more N‐region additions than native CD5+ B1 cells from younger mice. These results suggest that adult BM progenitors contribute to the peritoneal CD5+ B1‐cell pool over time.