The role of homotypic interactions in the differentiation of B cell precursors

Numerous studies have demonstrated that B lymphopoiesis is dependent upon a stromal cell microenvironment. Many of the stromal cell-derived factors and cell surface interactions that regulate B cell development have been identified; however, little consideration has been paid to the intimate interactions known to occur among B cell precursors themselves in both the fetal liver and marrow microenvironments. In this study we show that homotypic interactions between B cell precursors play an important role in promoting the development of mature B cells. We used an in vitro assay system to demonstrate that the function of stromal cells can be replaced by culturing B cell precursors in proximity. B cell precursors isolated from bcl-2 transgenic mice were used to rule out the possibility that improved survival, hypothesized to result from culturing precursors in proximity, solely accounted for the observed increase in B cell maturation. The putative maturation signal(s) were shown to be dependent upon direct contact between precursors rather than the release of soluble factors from nearby cells. Upon examination of the potential role of several known cell surface proteins, we found that blocking mu heavy chains with monovalent Fab antibody fragments dramatically inhibited maturation, in a stage-specific manner. Together these results suggest that a major function of stromal cells in vivo may be to act as a docking site to promote critical preB-preB homotypic interactions and ensuing signals. Further, the antibody blocking experiments raise the interesting possibility that interactions between B cell precursors themselves may promote and/or regulate preB cell receptor-driven signals.