A case for the involvement of PRL in the regulation of the immune system is strong. However, no mechanism by which PRL exerts this regulation has yet been identified. We studied the in vitro effects of PRL on splenocytes from ovariec- tomized (OVX) rats and discovered that PRL induced the for-mation of interleukin-2 (IL-2) cell surface receptors. However, PRL did not induce IL-2 secretion. This response, which was dependent on the concentration of PRL, also depended upon the estrogen status of the splenocyte donor; thus, splenocytes from OVX rats or rats in diestrus responded to PRL, whereas those from estrogen-treated OVX rats or rats in estrus did not. We propose that in vivo exposure of PRL, under certain physiological conditions, may prime a pool of splenocytes to express IL-2 cell surface receptors, allowing these cells to be responsive to variations in local concentrations of IL-2.
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