[Immunological properties of prolactin]

A profound bi-directional interaction exists between the hormone prolactin and the immune system. Even the name "hormone" seems to be inadequate, since prolactin is clearly a growth factor and in fact it functions as an immune co-mitogenic cytokine using autocrine, paracrine and obviously endocrine mechanisms. Prolactin (PRL) stimulates lymphocyte proliferation in response to antigen and mitogens. In addition, prolactin is locally secreted by immune cells, and the pituitary production of prolactin is partially under the control of pro-inflammatory cytokines. These reciprocal influences imply the presence of specific receptors for prolactin in many immune cells, such as lymphocytes and other accessory cells. The PRL-binding to its receptor stimulates the synthesis and secretion of lymphocyte cytokines. In addition, it is a growth factor essential for at least one lymphoid cell line. The PRL-corresponding mRNA has been demonstrated in the cytoplasm of mitogen-stimulated lymphocytes, and the secretion of PRL has been well documented in lymphoid cells. Moreover, PRL acts on NK cells to induce their differentiation to prolactin-activated killer cells (PAK cells) in a dose-dependent way (activation at physiological concentrations, and cytotoxicity inhibition at tenfold higher concentrations). PRL also shows a well known capacity to induce IFN-gamma and IL-2 synthesis, suggesting their participation in the genesis of Th1-responses. These PRL immunological properties strongly support PRL as a cytokine. PRL involvement in both the normal immune response and in many pathological conditions raises important considerations regarding potential diagnostic and therapeutic applications.