Human thymocytes from children less than 6 years of age were tested for their influence on differentiation of normal B cells. The addition of either thymocytes or a culture supernatant from thymocytes to normal peripheral blood lymphocytes (PBL) enhanced pokeweed mitogen-induced B-cell differentiation as tested in a plaque-forming assay for antibody to sheep red blood cells. The thymocytes, however, could not substitute for T lymphocytes in cultures of PBL which had been previously depleted of T lymphocytes. Further, prior treatment of thymocytes with concanavalin A did not result in generation of suppressor cells for either B-cell differentiation or for the responses of PBL to mitogens. Thus, although thymocytes were functionally immature by these assays as compared to mature T lymphocytes they exerted an influence on B-cell differentiation in cultures of normal peripheral blood lymphocytes.
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