B lymphopoiesis in the thymus

The thymus has been regarded as the major site of T cell, differentiation. We find that in addition to alpha beta and ya T cells, a significant number (similar to 3 x 10(4) per day) of B220(+)IgM(+) mature B cells are exporte d from the thymus of C57BL/6 mice. Of these emigrating B cells, we estimate that at least similar to 2 x 10(4) per day are cells which developed intra thymically, whereas a maximum of similar to 0.8 x 10(4) per day are cells w hich circulated through the thymus from the periphery. The thymus possesses a significant number of pro-B and pre-B cells that express CD19, VpreB, la mbda 5, and pax-5, These B cell progenitors were found in the thymic cortex , whereas increasingly mature B cells were found in the corticomedullar and medullary regions. Other lymphoid cells, including NK cells and lymphoid d endritic cells, are not exported from the thymus at detectable levels, Thus , the thymus contributes to the formation of peripheral pools of B cells as well as of alpha beta and gamma delta T cells.