GL7 defines the cycling stage of pre-B cells in murine bone marrow

We have identified a novel subset of early B lineage cells in the mouse bone marrow (BM) by GL7 expression on cell surface. GL7+B220low BM cells have a large cell size and are CD43–to low, CD95–, Sca-1–, I-Alow, IgM– and IgD–, suggesting that they are large pre-B cells. These BM cells express λ5 and VpreB but notterminal deoxytransferase (TdT) and Bcl-2, and approximately 50 % of them are in cell cycle. This fraction was not detected in BM cells of Rag-1-deficient and Scid mice, supporting that GL7+B220low BM cells belong to fraction C′ and D according to Hardy's criteria or to an early large pre-B-II fraction according to Melchers-Rolink's criteria. Furthermore, GL7+B220low BM cells can differentiate into IgM+ immature B cells in co-culture with stromal cells. These results suggest that B lymphocytes pass through the GL7+ pre-B cell stage during differentiation in the BM. Thus, GL7 is the critical marker to define the proliferation stage of large pre-B cells.