Growth hormone and fish immune system.

This paper reviews the immunomodulatory effects, extra-pituitary expression and paracrine action of growth hormone (GH), and a possible role of GH/insulin-like growth factor-I (IGF-I) axis in the immune system of teleost fish. In some euryhaline fish, the activation of immune functions observed during seawater acclimation appears to be associated with the osmoregulatory action of GH. Administration of GH enhances many aspects of immune functions including non-specific defences; cytotoxic, phagocytic, haemolytic and lysozyme activities. GH also activates immunoglobulin production as a specific defense and increases ceruloplasmin levels as an acute-phase protein. The GH gene is also expressed in many extra-pituitary tissues of fish, especially in lymphoid organs and cells. Several endocrine factors appear to act on immune function through modification of GH secretion from fish leucocytes. Exposure of phagocytic leucocytes of tilapia to IGF-I in vitro stimulated proliferation and superoxide production associated with phagocytosis. Exposure to GH had no significant effect on IGF-I secretion from tilapia leucocytes, despite of the fact that they secreted significant amounts of IGF-I. GH and IGF-I appear to act in a paracrine manner in the regulation of the teleostean immune system. Further studies are necessary to characterize the interactions of GH with other endocrine and paracrine factors.