A human T-cell antigen with a molecular weight of 16,000 (p16) was expressed predominantly on suppressor T cells, while it had minimal representation on helper T cells. This p16 antigen was expressed on 37 ± 5% of ER+ blood T cells, and was co-expressed on all suppressor T lymphocytes recognized by a monoclonal antibody (OKT8). When ER+ T cells were fractionated on the basis of FcIgG receptor expression, p16 was detected on 62% of ER+FcIgG+ T cells (TG + cells) and 26% of ER+FcIgG- cells. The p16+ and p16- T cells were physicially separated with a fluorescence-activated cell sorter and tested for their functional capability as inducer (helper) cells and suppressor cells in the terminal differentiation of pokeweed mitogen-stimulated B lymphocytes. Almost all of the helper cell activity was in the p16- fraction. Suppressor cell function was analyzed by adding p16+ cells to a "helper cell combination" comprised of ER+ T cells plus B cells. Results in four individuals demonstrated that the p16+ cells could function as suppressor cells since they caused a 60 ± 3% decrease in plasma cell generation. In contrast, adding the p16- cells to the "helper cell combination" further increased the plasma cell response by an additional 10 ± 17%. The helper cell population could be identified in both the FcIgG- and FcIgG+ populations by fractionating these cells with respect to p16 antigen expression.
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