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Prolactin, Growth Hormone, and Insulin-like Growth Factor-I in the Immune System
Publisher SummaryThis chapter examines whether pituitary hormones prolactin (PRL), growth hormone (GH), and insulin-like growth factors (IGF)-I are indeed full-fledged immunological growth and differentiation factors. The chapter presents ample evidence that the receptors for PRL, GH, and IGF-I are differentially expressed on several leukocyte populations. Moreover, PRL, GH, and IGF-I are produced by minor populations of leukocytes. The role played by these factors in the development and the function of the immune system, ignored until recently, is now the subject of intense investigation. The immune system contains both a PRL- and GH-secreting complex and target cells that express receptors for and respond to PRL and GH. Some effects of GH are mediated through the local production of IGF-I. PRL, GH, and IGF-I are thus paracrine or autocrine immunologic growth and differentiation factors. A weakened version of that hypothesis states that PRL, GH, and IGF-I are immunoregulatory hormones; the immune system is one of their targets but local production is negligible. An extreme view holds that PRL, GH, and IGF-I play little if any role in the development and the function of the human immune system. PRL, GH, and IGF-I hold promise as therapeutic agents in various forms of hemopoietic failure and immune deficiency. Also, dopaminergic agents and somatotropin analogs may be useful in the treatment of autoimmune diseases and the studies of the specific regulation of these hormones in the lymphohemopoietic system can lead to the development of specific clinical tools that interfere with local production of these hormones.