In vivo administration of recombinant growth hormone or gamma interferon activates macrophages: Enhanced resistance to experimental Salmonella typhimurium infection is correlated with generation of reactive oxygen intermediates

Purified and recombinant forms of growth hormone (GH) as well as of recombinant rat gamma interferon (IFN-γ) enhance the survival of rats deprived of endogenous pituitary GH secretion by hypophysectomy (HX rats) and infected with virulent Salmonella typhimurium. Macrophages obtained from rats with intact pituitaries (pituitary-intact rats) or HX rats that were treated in vivo with either GH or the closely related hormone prolactin released elevated (P < 0.05) levels of superoxide anion (O₂^-) after in vitro opsonized-zymosan stimulation compared with those from placebo-treated animals. These levels of O₂^- release were similar in magnitude to those of macrophages from rats treated in vivo with IFN-γ. In time course in vivo macrophage activation studies, both IFN-γ and GH significantly increased O₂^- secretion within 24 h, with maximal secretion occurring at day 3. Macrophages obtained from pituitary-intact and HX rats injected in vivo with GH also released elevated (P < 0.05) levels of hydrogen peroxide (H₂O₂) and displayed enhanced (P < 0.01) phagocytic activity toward opsonized Listeria monocytogenes in vitro. The mechanism of action of GH in vivo is likely to be a direct one because resident peritoneal macrophages from rats could be primed in vitro for enhanced secretion of O₂^- following triggering of these cells with opsonized zymosan. These data show that in vivo administration of two closely related pituitary hormones, GH and prolactin, can effectively prime macrophages, which is consistent with the hypothesis that GH mediates resistance to S. typhimurium by a direct stimulatory action on macrophages.