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-kitLo 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-kitLo 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(-) sIgM-, were also c-kit-. These c-kit- cells, like their c-kitLo counterparts, expressed TdT, proliferated in response to interleukin (IL)-7, and generated sIgM+ 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-kitLo 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.