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 killPasteurella multocida in vitro. 3. Hypophysectomized rats challenged withSalmonella 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 macrophagesin vitro, which is consistent with the idea that the protective effects of GHin 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.