NEUROENDOCRINE-IMMUNE INTERACTIONS

The role of the neuroendocrine system in influencing both immune devel opment and function has become an area of active research within many model systems, including the chicken. It is now clear that the neuroen docrine system can exert immediate feedback regulation on the immune s ystem as well as control specific aspects of immune differentiation an d development. The primary lymphoid organs of avian species (i.e., the thymus and the bursa of Fabricius) are also known to function as endo crine organs. These produce hormonal products that influence the devel opment of lymphoid cells and that may feed back on the neuroendocrine system. In conjunction with the endocrine activities of the primary ly mphoid organs, immune and accessory cells are known to produce a varie ty of secreted products or cytokines that have the potential not only for the regulation of immune function but also for mediating neuroendo crine activities. Finally, it has been demonstrated in a variety of sp ecies that leukocytes are capable of producing endocrine mediators pre viously believed to be produced only under the direct control of the h ypothalamic-pituitary axis. Thus, there are numerous possibilities for bidirectional interactions between the immune and neuroendocrine syst ems. This discussion focuses primarily on these interactions with an e mphasis on the means by which the hormonal mediators, growth hormone a nd thyroid hormone, may affect the thymus and the thymic microenvironm ent. The role of the adrenocorticoids and gonadal steroids in regulati ng immune function and their involvement in immune feedback circuits a re also discussed.