Effects of 12-o-tetradecanoyl-phorbol-13-acetate (TPA) on the colony growth of human T lymphocytes.

The T colony promoting activity of 12-o-tetradecanoyl-phorbol-13-acetate (TPA) was assessed in a double layer culture assay which is dependent on the simultaneous presence of phytohaemagglutinin (PHA) and a leucocyte rich underlayer. TPA (10(-8) M) incorporated in the overlayer in place of PHA was capable of promoting T cell growth in the form of clusters in all 37 experiments performed and in the form of colonies in more than 50% of the samples tested. However, the T colony promoting activity of TPA alone was markedly less evident and consistent than that of PHA (mean 13 +/- 19.9 s.d. colonies vs 168 +/- 78.6). TPA concentrations of 10(-6) M, 10(-9) M and 10(-10) M were practically ineffective. On the other hand, the number of colonies obtained when both TPA 10(8) M and PHA were incorporated in the overlayer was significantly higher (P less than 0.01) than that observed with PHA alone (mean 250 +/- 108.2 vs 178 +/- 84.5 colonies). When TPA concentrations of 10(-9) M and 10(-10) M were used in addition to PHA, the enhancing effect was less evident, while an inhibition of T colony growth was observed with TPA 10(-6) M + PHA. TPA 10(-8) M was also capable of enhancing T colony growth when incorporated in the leucocyte rich underlayer (222 +/- 98.6 vs 172 +/- 80.9 colonies). In all cultures with TPA the peak of growth was delayed compared with that of control experiments with PHA. These findings demonstrate that TPA, particularly when co-cultured with PHA, is an effective T colony promoting agent. The observation that the number of colonies formed in the presence of TPA plus PHA is higher than the sum of those observed with the two stimulators independently, suggests that their synergistic effect may be mediated via the production of colony stimulating soluble factors.