CROSS-LINKING CD40 ON HUMAN B-CELL PRECURSORS INHIBITS OR ENHANCES GROWTH DEPENDING ON THE STAGE OF DEVELOPMENT AND THE IL-COSTIMULUS

The function of the cell surface molecule CD40 on B cell precursors (B CP) is not well understood. We now report studies using the L cell/CD4 0 system (anti-CD40 mAb immobilized on CD32(+) mouse L cells) to asses s the potential function of CD40 during human B cell ontogeny. Stimula tion of human B lineage cells with IL-4 in the L cell/CD40 system yiel ded a hierarchy of responsiveness: high density tonsillar B cells > fe tal splenic B cells > fetal bone marrow surface Ig(+) immature B cells > fetal bone marrow surface Ig(-) BCP. Using a microsphere/flow cytom etry growth quantitation assay, we found that substituting IL-3 for IL -4 in the L cell/CD40 system provided a stronger growth stimulus for f etal bone marrow BCP and immature B cells. We also found that FACS-pur ified fetal bone marrow CD1O(+)/CD34(+)/CD40(+)/cytoplasmic mu(-) pro- B cells responded maximally to IL-3 plus IL-7. Surprisingly, anti-CD40 inhibited the pro-B cell response to IL-7. In contrast, FACS-purified fetal bone marrow CD10(+)/CD34(-)/CD40(+)/cytoplasmic mu(+) pre-B cel ls were essentially nonresponsive to IL-3, IL-7, or anti-CD40 alone, b ut were uniquely responsive to IL-3 plus anti-CD40. B-lineage cells de rived after 14 days from IL-7-stimulated pro-B cells were predominantl y CD19(+)/L chain(-), whereas pre-B cells stimulated with IL-3 plus an ti-CD40 were predominantly CD19(+)/L chain(+). The L chain(+) cells fr om pre-B cell cultures were both mu(+)/delta(+) and mu(-)/delta(+). Ou r results demonstrate that the response to CD40 signaling depends upon the BCP developmental stage and the IL costimulus, and indicate that normal human pro-B cells and pre-B cells have different growth factor requirements.