ACQUISITION OF CD40 EXPRESSION DURING MURINE B-CELL DIFFERENTIATION

Expression of CD40 on mouse cells was investigated comparing the bindi ng to cells of a monoclonal antibody against CD40, to that of a solubl e fusion protein consisting of the extracellular domains of the mouse CD40-ligand. The analysis of a series of established cell lines failed to demonstrate expression of CD40 on pro- or pre-B cells, and indicat ed that CT40 expression was restricted to cells that had undergone pro ductive heavy- and light-chain gene rearrangements, and expressed surf ace Ig. In cells from normal mice, CD40 first becomes detectable, alth ough at low levels, on a subset of small pre-B-II cells in bone marrow , the levels of CD40 expression increasing thereafter during B-cell ma turation. Thus, immature B cells (IgM(+) IgD(lo) B220(lo)) express int ermediate levels of CD40, and mature B cells (IgM(+) IgD(hi) B220(hi)) express high levels of CD40. Anatomical location also seems to correl ate with the levels of CD40 expression, as B cells expressing the high est levels of CD40 were found in lymph nodes and Peyer's patches.