The phenomenon of activation-associated growth inhibition of transformed T cells, both in vitro and in vivo, has been studied. In vitro. Activation of transformed T cells results in lymphokine production and inhibition of growth. Growth inhibition, as manifested by a decrease in (3H) thymidine incorporation and a G1/S cell cycle block, is manifested within 1 hr of stimulation. Removal of extracellular Ca2+ with EGTA completely blocks lymphokine secretion but has no effect upon early growth inhibition. However, the cell lysis that usually results 4 to 6 hr after stimulation was blocked by the removal of extracellular Ca2+. Cyclosporine A (CSA), which may exert its effects by antagonizing calcium-dependent intracellular pathways, had effects similar to EGTA. Moreover, in prolonged cultures in CSA (up to 48 hr) it was found that the early G1/S block was reversible. These data indicate that activation-associated growth inhibition is a two step process. The first, a G1/S cell cycle block, does not require extracellular Ca2+, is CSA-resistant, and is reversible. The second, cell lysis, requires extracellular Ca2+ and is CSA- sensitive. In vivo. Treatment of mice bearing antigen-specific T-cell tumors with the appropriate antigen results in tumor elimination. This is due to (1) the direct cytotoxic effect of antigen-stimulation on T-cell growth, and (2) the induction of host immunity. It was found that mice """"""""cured"""""""" of one T-cell hybridoma subsequently rejected the same or related T-cell tumors. Cultured splenocytes from these mice were cytotoxic to the original tumor cell in vitro. Adoptive transfer of splenocytes indicated that Thy-1-positive cells (T cells) were responsible for the immunity. Furthermore, in vivo depletion of either the CD4+ or the CD8+ T- cell subset suggested that the latter, which contain mostly cytotoxic T cells, were critical in this response. These studies increase our understanding of activation-associated growth inhibition, and should enhance our ability to manipulate the phenomenon in vivo.
