INHIBITION OF HUMAN LYMPHOCYTE STIMULATION BY STEROID-HORMONES - CYTOKINETIC MECHANISMS

Abstract

The steroid hormones estradiol, progesterone, testosterone and cortisol inhibited stimulation of human peripheral blood lymphocytes by phytohemagglutinin (PHA) and Con A [concanavalin A]. This effect upon lymphocyte transformation was assayed by 3 methods: quantitation of [3H]thymidine incorporation into acid precipitable material, microscopic assessment of blastic transformation and determination of the labeling index. Addition of steroid hormones at the initiation of culture resulted in a marked inhibition in all 3 parameters, which was observed with lower concentrations of cortisol than the other hormones. The inhibition was not attributable to cell death and could be partially reversed by removing hormones from the incubation medium after culture for 48-72 h. Late addition of steroid hormones, 52 h after addition of mitogen and 18 h prior to pulse-labeling with [3H]thymidine, also resulted in reduced [3H]thymidine incorporation, accompanied by a nearly 50% reduction in the labeling indices and only a minimal decrease in the per cent transformed cells. Inhibition of lymphocyte stimulation by steroid hormones operates by the following cytokinetic mechanisms: suppressed recruitment of cells from G0 to G1 phase of the cell cycle, as indicated by the diminished per cent blasts; inhibition of progression from G1 phase into S phase, as evidenced by the reduced ratio [labeling index/blasts]; and, in the case of estradiol and progesterone, reduced rate of DNA replication or altered intracellular [3H]thymidine specific activity, as shown by the decreased ([3H]thymidine incorporation/labeling index) ratio. Late addition of steroid hormones to stimulated cultures reduced the per cent of cells in S phase, but did not revert previously transformed cycling lymphocytes to the G0 state.