Loss of c-kit accompanies B-lineage commitment and acquisition of CD45R by most murine B-lymphocyte precursors

Using surface markers, we identified two bone marrow (BM) subsets enriched for TdT⁺ cells on the brink of CD45R acquisition. These two populations, Lin^-c-kit(Lo) and Lin^-c-kit^-, consisting of 35.4% and 7.4%, respectively, TdT⁺ cells, generated B-lineage cells in overnight cultures. Approximately half of the c-kit(Lo) B-lineage precursors were bipotential, yielding myeloid and lymphoid progeny, whereas most that were c-kit^- gave rise only to lymphocytes. Analysis of B-lineage progression during a finite culture period showed that the most mature precursors were concentrated in the Lin^-c-kit^- population. Moreover, a majority of the earliest CD45R⁺ pro-B cells in BM, identified as CD45R⁺ CD43⁺ BP-1^- CD25^- natural killer (NK)1.1^- slgM^-, were also c-kit^-. These c-kit^- cells, like their c- kit(Lo) counterparts, expressed TdT, proliferated in response to interleukin (IL)-7, and generated slgM⁺ cells. These data suggest that TdT expression is initiated as c-kit downregulation begins in Lin^- cells, with progressive loss of c-kit during B-lineage differentiation. CD45R expression is initiated during the transition from c-kit(Lo) to c-kit^- with many cells losing c-kit before acquiring CD45R. The ability to isolate highly enriched populations of viable CD45R^- precursors will be instrumental in characterizing the earliest B-lineage cells.