The rapid immunosuppression in phytohemagglutinin-activated human T cells is inhibited by the proliferative Ca2+ influx induced by progesterone and analogs

Progesterone, an endogenous immunomodulator, suppresses human T-cell activation during pregnancy. A sustained Ca2 + influx is an important signal for T-cell proliferation after crosslinking of T-cell receptor/CD3 complexes by anti-CD3 antibodies or phytohemagglutinin (PHA). Progesterone targets cell membrane sites inducing rapid responses including elevated intracellular free calcium concentration ([Ca2+]i) and suppressed T-cell PHA-activated proliferation. Interestingly, both PHA and progesterone induce [Ca2+]i elevation, but it remains unclear whether the PHA-induced Ca2+ influx is affected by progesterone leading to T-cell immunosuppression. Primary T-cells were isolated from human peripheral blood and the quench effect on intracellular fura-2 fluorescence of Mn2+ was used to explore the responses to Ca2+ influx with cell proliferation being determined by MTT assay. PHA-stimulated Ca2+ influx was dose-dependently suppressed by progesterone and its agonist R5020, which correlated with PHA-activated T-cell proliferation inhibition. A similar dose-dependent suppression effect on cellular Ca2+ influx and proliferation occurred with the TRPC channel inhibitor BTP2 and selective TRPC3 channel inhibitor Pyr3. In addition, two progesterone analogs, Org OD 02-0 and 20α-hydroxyprogesterone (20α-OHP), also produced dose-dependent suppression of Ca2+ influx, but had no effect on proliferation. Finally, inhibition of PHA-activated T-cell proliferation by progesterone is further suppressed by 20α-OHP, but not by Org OD 02-0. Overall, progesterone and R5020 are able to rapidly decrease PHA-stimulated sustained Ca2+ influx, probably via blockade of TRPC3 channels, which suppresses T-cell proliferation. Taken together, the roles of progesterone and its analogs regarding the rapid response Ca2+ influx need to be further explored in relation to cytokine secretion and proliferation in activated T-cells.