An increasingly large body of evidence implicates PRL as an im-munoregulatory molecule. While most of the data relate to PRL levels and immunocompetence in vivo, we have shown that PRL is mitogenic for splenocytes from ovariectomized rats and rats in certain other hormonal states. This finding suggests that these lymphocytes express PRL receptors. Here, we wished to determine whether all or only a subset of splenocytes were PRL receptor positive. By using polyclonal as well as monoclonal antibodies to PRL receptor, we determined that as many as 20% of the primary splenocytes expressed PRL receptors. In a culture of Nb2 cells, a PRL receptor-positive lymphoid line, as many as 70% were PRL receptor positive. Dual labeling for lymphoid-specific antigen surface markers and PRL receptor indicated that about one third of the PRL receptor-positive splenocytes were x-light chainpositive B-cells, while the others stained with antibodies to T-cell markers, CD4 or CD8. These data confirm that lymphocytes express PRL receptors and show for the first time that PRL receptor-positive lymphocytes are a heterogenous subset of total primary splenocytes. These cells may be the target for PRL-mediated immunoregulation.
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