Somatolactogens, somatomedins, and immunity.

The neuroendocrine and immune systems participate as active partners in host homeostatic and defense mechanisms. This partnership involves a complex intercommunication system employing an array of shared ligands and receptors. Hormones of the somatolactogen family have marked influences on immune events in vivo, including the maintenance of lymphoid tissue cellularity, the promotion of DNA synthesis in these tissues, and the stimulation of a number of immune effector mechanisms. Both growth hormone and prolactin function to promote erythropoiesis and DNA synthesis in bone marrow precursors. Our results have shown that the somatolactogens and a member of the somatomedin family, IGF-I, are particularly effective in modulating the effector functions in phagocytic cells, including the production of reactive oxygen intermediates and tumor necrosis factor-alpha and the oxygen-dependent killing of bacteria. Evidence indicating a role of IGF-I in modulating immune functions is more recent but nonetheless compelling. Accumulated data suggest that somatolactogenic hormones, as well as one member of the somatomedins, are produced by cells of the immune system and can regulate local immune events. Although the molecular mechanisms by which the somatolactogens and somatomedins exert their effects on immune tissues are only now being explored, the pleiotropic nature of these effects suggests that these hormones participate at endocrine, paracrine, and perhaps autocrine sites of action.