The role of growth hormone in modulation of the immune response.

Growth hormone was originally discovered because of its ability to promote linear growth in rodents and humans. It is now known that this molecule also augments a number of activities of leukocytes, such as antibody synthesis, cytolytic activity of T lymphocytes, natural killer cell activity, differentiation of neutrophils, production of tumor necrosis factor-alpha (TNF-alpha), and the synthesis of a thymic hormone known as thymulin. We have shown that growth hormone mimics one action of interferon-gamma (IFN-gamma) by augmenting the production of superoxide anion by macrophages and neutrophils. Growth hormone also is synthesized by leukocytes, which creates the possibility that it may act locally as a cytokine in lymphoid tissue. These findings show that a hormone that was originally isolated from the pituitary gland is involved in regulating host defense responses of leukocytes.