Mechanism of action of a suppressor-activating factor (SAF) produced by a human T cell line

We previously described a potent suppressor-activating factor (SAF) produced constitutively by a 6-thioguanine-resistant mutant of the human T cell line CEM. In the present study, we investigated the mechanism of action of SAF. After a brief (4- to 18-hr) exposure to SAF at 37 degrees C, T lymphocytes (either unseparated, or purified OKT4+ and OKT8+ subpopulations), but not B lymphocytes, suppressed allogeneic and syngeneic T cells in co-culture experiments, apparently via the release of a suppressor activity. The total T cell-released suppressor activity (TRSA) accumulated after 3 days culture post-treatment was about 100- to 500-fold higher than the original suppressor activity (SAF) added to trigger the release. Arresting protein or DNA synthesis, or even killing the cells did not affect the release of TRSA by T lymphocytes, but lowering the incubation temperature to 4 degrees C reduced it drastically. Pre-treatment of T lymphocytes with the metabolic inhibitor, sodium azide, or the adenylate cyclase stimulator, prostaglandin E2, or the addition of exogenous dibutyryl cAMP, all suppressed the release of TRSA. The presence of monoclonal antibody OKT3, but not OKT4 or OKT8, enhanced the release of TRSA. The presence of OKT11 blocked the release of SAF. The functional characteristics of TRSA appeared to be identical to those of SAF. However, unlike SAF, interaction of T lymphocytes with TRSA triggered only marginal enhancement of suppressor activity. In addition, the kinetics of the suppression mediated by SAF showed a much larger increment as a function of time than that mediated by TRSA. Taken together, the data suggest that SAF might represent an activated form of SAF, and that the continuous activation of SAF by lymphocytes in culture may account for its high potency in suppressing T cell proliferation in vitro.