The thymus and the acute phase response

The thymus is a primary lymphoid organ with both endocrine and immune funct ions. There is a large body of evidence indicating the existence of a compl ex neuroendocrine control of the thymus physiology. This is supported by th e historic observation that the thymus becomes involuted during the respons e to stress. The thymus is dramatically affected by the acute phase respons e (APR), a systemic reaction to tissue injury and/or infection accompanied by profound neuroendocrine and metabolic changes. The APR comprises alterat ions in behavior, body temperature, and production and release of cytokines , particularly interleukin (IL)-1, IL-6 and TNF alpha, and glucocorticoids (GCs) and is characterized by suddenly increased production of so-called ac ute phase proteins (APPs). The stimulation of APR activates the hypothalami c-pituitary-adrenal (HPA) axis, resulting in the suppression of specific im munity, which might serve to protect the organism from adverse immune react ions; the immunostimulatory hormones (e.g., PRL, GH, IGF-I) are suppressed, whereas the production of APPs in the liver is stimulated by IL-6, catecho lamines and GCs. The most striking effect of the latter on the immune syste m is the induction of apoptosis in the thymus. In concert with GCs, elevate d levels of catecholamines also selectively suppress immune response mechan isms. APR may be regarded as an emergency response that represents a switch of the host defense from the adaptive immune response which is slow to dev elop and is commanded by the thymus and T-lymphocytes to a less specific, b ut more rapid and intense reaction. Here we discuss the immunoregulatory ch anges during the APR with a special emphasis on the role of thymus in this process.