The macrophage-activating properties of growth hormone

1. We compared the ability of growth hormone (GH) and a wellcharacterized macrophage-activating factor, interferon-γ (IFN-γ) to activate highly purified populations of alveolar macrophages. Both GH and IFN-γ primed macrophages triggered with opsonized zymosan to secrete superoxide anion (O₂^-)in vitro, but IFN-γ was effective at a 40-fold lower concentration. Antibody blocking studies demonstrated that the priming activity of GH was independent of IFN-γ, and the activity of IFN-γ was distinct from that of GH. 2. Both IFN-γ and GH increased the capability of macrophages to kill Pasteurella multocida in vitro. 3. Hypophysectomized rats challenged with Salmonella typhimurium were significantly protected by injections of either GH or recombinant rat IFN-γ in vivo compared to vehicle-treated controls, and the protective effect of GH was increased by incorporation into liposomes. 4. Insulin-like growth factor-I (IGF-I) also primed alveolar macrophages in vitro, which is consistent with the idea that the protective effects of GH in vivo might be mediated by augmenting the synthesis of IGF-I. These data support the concept of reciprocal systems of communication between the neuroendocrine and immune systems.