Loss of c-kit Accompanies B-Lineage Commitment and Acquisition of CD45R by Most Murine B-Lymphocyte Precursors

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Abstract
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-kitLoB-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-kitLocounterparts, 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.