Associations between the neuroendocrine and immune systems

Organisms respond to infection with complex adaptations involving bidirectional communication between the immune and neuroendocrine systems. The idea of intercellular communication between the neuroendocrine and immune systems via common signal molecules has provided a conceptual framework for such crosstalk. The studies to date show that cells of the immune system contain receptors for neuroendocrine hormones and can also be considered a source of pituitary and hypothalamic peptides. The structure and pattern of synthesis of these peptides by leukocytes appear similar to neuroendocrine hormones, although some differences exist. Once secreted, these peptide hormones may function as endogenous regulators of the immune system as well as conveyors of information from the immune to the neuroendocrine system. The plasma hormone concentrations contributed by lymphocytes usually do not reach the levels required when the pituitary gland is the source, but because immune cells are mobile, they have the potential to locally deposit the hormone at the target site. Likewise, other studies show that cells of the neuroendocrine system contain receptors for cytokines and can also be considered a source of cytokines, particularly interleukin-1 (IL-1) and IL-6. In the pituitary IL-1 beta coexists with thyroid stimulating hormone in a subpopulation of thyrotropes, suggesting it may have a role as a pituitary paracrine factor. The cytokines, including IL-1, IL-2, IL-6, interferon-gamma, and tumor necrosis factor, exert profound effects on hypothalamic pituitary axes. It is our hypothesis that the relay of information to the neuroendocrine system represents a sensory function for the immune system wherein leukocytes recognize stimuli that are not recognizable by the central and peripheral nervous systems (i.e., bacteria, tumors, viruses, and antigens). The recognition of such noncognitive stimuli by immunocytes is then converted into information and a physiological change occurs. Future studies into the physiological role that cytokines and neuroendocrine hormones have in these systems will be of considerable interest for both immunologists and endocrinologists.