Differentiation from thymic B cell progenitors to mature B cells in vitro

The role of the thymic microenvironment in the development of murine thymic B cells has yet to be fully clarified. We therefore investigate the microe nvironment that supports the development of mature thymic B cells (sIg(+)/B 220(+)/CD43(-)B cells) from thymic B cell progenitors with immunophenotypes of sIg(-)/B220(med)/CD43(+) cells. As we have previously reported, thymic B cells generated from these progenitors in the thymus are CD5(+) B cells. We next study the in vitro condition that supports the differentiation of t hymic B cell progenitors. Stromal cells (from the bone marrow or thymus), t hymus-derived cell lines with the character of thymic nurse cells (TNCs) or thymic epithelial cells (TECs), or the bone marrow-derived cell line (MS 5 ) are tested for their ability to support B-lymphopoiesis from thymic B cel l progenitors. Interestingly, thymic stromal cells (but neither stromal cells from the bon e marrow nor stromal cell lines) support the differentiation of thymic B ce ll progenitors into thymic B cells in the presence of IL-7. Cortical epithe lia (but not medullary epithelia, thymic macrophages or dendritic cells) ar e found to contribute to thymic B cell differentiation. Surface phenotype a nd Ig rearrangement analyses reveal that mature B cells generated in this c ondition are primarily CD5(-) B cells, indicating that the thymic microenvi ronment (particularly cortical epithelia) determines the differentiation of thymic B cells.