Prolactin has been shown to have an immunoregulatory role in the rodent immune response. A prolactinlike molecule has also been found in mouse splenocytes and a human B-lymphoblastoid cell line. We have evaluated whether human peripheral blood mononuclear cells (PBMCs) synthesize and/or secrete prolactin. We used the polymerase chain reaction (PCR) to generate a 276-base-pair prolactin product from human PBMCs, and Southern blot analysis confirmed that it was related to prolactin. Western blotting using a polyclonal antibody to prolactin indicated that cell extracts prepared from human PBMCs contained a high molecular mass (60-kDa) immunoreactive prolactin. To determine whether this PBMC prolactin was being secreted, we developed a highly sensitive and specific hormonal enzyme-linked immunoplaque assay. With this assay, we were able to detect human prolactin secretion from concanavalin A (Con A)-or phytohemagglutinin-stimulated PBMCs but not from unstimulated PBMCs. We next sought to determine whether this secreted prolactin could function as an autocrine growth factor in lymphoproliferation. We observed that anti-human prolactin antiserum significanly inhibited human PBMC proliferation in response to Con A or phytohemagglutinin. We conclude that a prolactin-like molecule is synthesized and secreted by human PBMCs and that it functions in an autocrine manner as a growth factor for lymphoproliferation.Recent studies suggest an immunoregulatory role for prolactin in rodents. In animals, hypophysectomy results in cessation of the growth of the thymus gland (1), decreased antibody titers against sheep red blood cells, and depressed delayed hypersensitivity reaction to chlorodinitrobenzene (2). Bromocryptine-induced hypoprolactinemia in mice injected with Listeria monocytogenes increases mortality that is associated with impaired lymphocyte proliferation and decreased production of macrophage-activating factors by T lymphocytes (3). Further, a prolactin-like molecule is secreted following Con A stimulation of murine lymphocytes (4), and a prolactin-like mRNA as well as a secreted product have been detected in human B-lymphoblastoid cell lines (5, 6). In contrast, Clevenger et al. (7) could not demonstrate prolactin-specific mRNA or prolactin secretion following interleukin 2 stimulation of a mouse T-lymphocyte line.Several investigators (7-9) using rodent lymphoid cell lines or splenocytes have found that a prolactin-like protein is required for lymphocyte mitogenesis. The relevance of these observations to human cellular immunity has not been explored. Here we document that human peripheral blood mononuclear cells (PBMCs) synthesize and secrete a prolactin-like molecule that functions in an autocrine loop as a growth factor for lymphoproliferation.
MATERIALS AND METHODSOligonucleotide Primer Design. The sequences and positions of the two 21-mer primers used for PCR amplification of human prolactin cDNA are shown in Fig. 1 (10). The 5' primer is located within exon 3 of the human prolactin gene and the 3' pri...