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 exported 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 intrathymically, whereas a maximum of similar to 0.8 x 10(4) per day are cells which circulated through the thymus from the periphery. The thymus possesses a significant number of pro-B and pre-B cells that express CD19, VpreB, lambda 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 dendritic 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.