Identification of CD19-B220+c-Kit+Flt3/Flk2+ cells as early B lymphoid precursors before pre-B-I cells in juvenile mouse bone marrow

The combined analysis of the expression of receptor tyrosine kinases c-Kit and Flt3/Flk-2 and of the human CD25 gene expressed as a transgene under the regulation of the mouse lambda 5 promoter in the bone marrow of 1-week-old mice allows us to identify three stages of B lymphocyte development before the CD19(+)c-Kit(+) pre-B-I cells. Single-cell PCR analysis of the rearrangement status of the Ig heavy chain alleles allows us to order these early stages of B cell development as follows: (i) B220(+)CD19(-)c-Kit(lo)Flt3/Flk-2(hi)lambda 5(-), (ii) B220(+)CD19(-)c-Kit(lo)Flt3/Flk-2(hi)lambda 5(+) and (iii) B220(+)CD19(+)c-Kit(lo)Flt3/Flk-2(lo)lambda 5(+) before B220(+)CD19(+)c-Kit(lo)Flt3/Flk2-lambda 5(+) pre-B-I cells. All these progenitors are clonable on stromal cells in the presence of IL-7 and can differentiate to CD19(+) c-Kit(-) B-lineage cells. A combination of stem cell factor, Flt3 ligand and IL-7 was also able to support the proliferation and differentiation of the progenitors in a suspension culture. Furthermore, the analyses indicate that the onset of D(H)J(H) rearrangements precedes the expression of the lambda 5 gene. These progenitor populations were characteristic of juvenile mice and could not be detected in the bone marrow of adult mice. Hence the expression pattern, and probably the function, of the receptor tyrosine kinases in early B cell differentiation appears to be different in juvenile and adult mice.