Prolactin regulation of lymphocyte proliferation: An early event

Abstract

It has been suggested by immune system studies that pituitary prolactin (PRL) may have an immunomodulatory role and that lymphocytes themselves may produce prolactin-like proteins (Ly-PLP). The studies in this dissertation focused on Ly-PLP production and PRL/Ly-PLP actions in an enriched population of rat splenic T helper (TH) CD4 and cytotoxic CD8 cells. Studies were planned on a dual hypothesis: first, that T-cells produce Ly-PLP which may act in an autocrine manner and second, that Ly-PLP may be necessary for interleukin 2 (IL-2) production. To study the possible mechanisms by which PRL and Ly-PLP regulate lymphocyte proliferation, a PRL-free medium was used and a polyclonal antibody to rat PRL (A-PRL) was used to inactivate the residual PRL and the expressed Ly-PLP in the cell culture system. This antibody inhibited T-cell proliferation in both a time and a concentration-dependent manner. Based on four lines of evidence, the in vitro inhibitory activity of A-PRL on cell proliferation appeared to be modulated in part by inhibition of IL-2 production. First, the A-PRL induced inhibition of T-cell proliferation was overcome by addition of recombinant human IL-2 (rhuIL-2). Second, A-PRL treatment of mitogen stimulated T-cells was shown by RT-PCR analysis to inhibit IL-2 mRNA expression. IL-2 receptor beta (IL-2rB) mRNA, which is constitutively expressed in T-cells, was not inhibited. Third, A-PRL inhibited IL-2 protein production in a concentration dependent manner. Fourth, the A-PRL induced inhibition of IL-2 production was overcome by preincubation of the A-PRL with purified rat pituitary PRL (rPRL). Dot Blot and RT-PCR analysis data in this dissertation were suggestive but were inconclusive in support of the hypothesis that rat lymphocytes and specifically T-cells constitutively express mRNA for PRL and/or Ly-PLP. Western blot studies of T-cell lysates suggested that rat T-cells express a Ly-PLP with the apparent molecular weight 46 kD, as well other variants. In summary the data presented in this dissertation indicate that rat T-cells expressed Ly-PLP and treatment with A-PRL to inactivate Ly-PLP inhibited cell proliferation, expression of IL-2 mRNA, and IL-2 protein expression by CD4 cells. The inhibition of IL-2 production was the likely mechanism by which A-PRL binding of Ly-PLP inhibited mitogen induced T-cell proliferation in rats. The inhibition of IL-2 production suggests that extracellular pituitary PRL or Ly-PLP through a hormone and/or an autocrine mechanism respectively may be necessary but not sufficient for the induction of IL-2 mRNA and IL-2 protein expression in activated rat T-cells.