AbstractProthymosinα(Prota), an immunologically active polypeptide derived initially from rat thymus, and now pig thymus, was tested for its effect on autoantigen-induced human T cell proliferation in vitro. Pig ProTa was found to enhance the autologous mixed lymphocyte response (auto-NLR). Optimum enhancement was achieved at doses which varied among different donors. Treatment of the stimulatory monocytes with ProTa resulted in considerably higher auto-MLR responses as compared to those with non treated monocytes. ProTa was without effect on T lymphocytes. In contrast, T lymphocytes exhibited enhanced proliferative activity when treated with ProTa in the environment of autologous monocytes. Horeover, supernatants from cultures of monocytes incubated with ProTa(ProTa-sup) were also shown to enhance the human auto-NLR either after addition in cultures or after preincubation with responder T lymphocytes. In addition, ProTa-sup did not demonstrate any detectable inter-leukin I (IL 1) or interleukin 2 (1L 2)-like activity Furthermore, ProTa-sup induced an increase in IL 2 production in auto-NLR cultures. The enhancement of T-cell proliferation and IL 2 production by ProTa-sup was maximal when this material was added at the beginning of the auto-MLR, and no effect of ProTa-sup was seen if the latter was added 3 days after initiation of the culture. Finally, Prota-sup was also shown to increase the expression of IL 2 receptors on T lymphocytes activated in the auto-MLR. These studies suggest that ProTa enhances the human auto-HLR through ProTa -sup which is released after interaction of monocytes with ProTa ProTa-sup then increases directly T lymphocyte proliferation by elevating IL 2 production and expression of IL 2 specific receptors on autoactivated T lymphocytes.