Progesterone effects on lymphocytes may be mediated by membrane progesterone receptors.

Luteal cell-induced proliferation of T lymphocytes devoid of the nuclear progesterone receptor (PGR) is inhibited by progesterone. Functional effects of progesterone on bovine lymphocytes and the expression of membrane progesterone receptors (mPRs) alpha (PAQR7), beta (PAQR8), gamma (PAQR5), and progesterone receptor membrane component 1 (PGRMC1) mRNA were analyzed in corpus luteum (CL) and lymphocytes. Progesterone and a cell-impermeable progesterone conjugate caused a dose-dependent decrease in IL2 receptor α-subunit (IL2RA) mRNA and an increase in interleukin 2 (IL2) mRNA concentrations in cultured PBMCs. In luteal tissues, concentrations of PAQR7 and PAQR8 mRNA were lower in CL collected on day 11 compared with day 18, whereas PGRMC1 and PGR mRNA were greater on day 11 than on day 18. The mRNA of all three PAQRs and PGRMC1 were detected in bovine T lymphocytes, but not in B cells/monocytes. Progesterone increased intracellular Ca(++) and reduced the phosphorylation of zeta-chain-associated protein kinase 70 (Zap70). A specific, saturable, and single progesterone binding site with a steroid specificity characteristic of mPRs was demonstrated by saturation and competitive binding assays using T lymphocyte membranes, and PAQR7 receptors were localized on the plasma membranes by immunofluorescence. Thus, progesterone induces specific and rapid functional effects on T lymphocytes in the absence of PGR. The mPRs are potential intermediaries of the cell-surface actions of progesterone because they are expressed in lymphocytes, the actions of progesterone are mimicked by a cell-impermeable form of progesterone, and specific, saturable progesterone binding, which is characteristic of mPRs, is present on lymphocyte membranes.