102 Enhancement of Macrophage Superoxide

In response to an appropriate stimulus such as phagocytosis of an opsonized particle, macrophages reduce oxygen, first to superoxide anion (O2-), 1 and then to hydrogen peroxide (H202) (1, 2). These metabolites of oxygen are involved in the killing of phagocytosed microorganisms (3). More recently, H202 has also been implicated in macrophage killing of lymphocyte tumor lines (4, 5). Macrophages obtained from the peritoneal cavity of mice infected with Bacille Calmette-Gugrin (BCG) or injected intraperitoneally with bacterial lipopolysaccharide (LPS) display a greatly enhanced ability to generate 02 in response to stimulation by opsonized zymosan or the membrane-reactive agent phorbol myristate acetate (PMA) (6). Such activated macrophages also exhibit enhanced H202 release (2). These observations correlate with studies showing that activated macrophages are better able to kill microorganisms (7, 8) and tumor cells (9, 10). Treatment of macrophages in vitro with LPS has been shown to enhance their ability to kill tumor cells (11-14), and to increase their secretion of collagenase (15) and other hydrolytic enzymes (16). N-Acetylmuramyl-L-alanyl-D-isoglutamine, or muramyl dipeptide (MDP), also enhances macrophage tumor cell killing in vitro (17). This segment of bacterial cell wall peptidoglycan is the minimal fragment that retains adjuvant properties (18). This same material has been shown to cause macrophage migration inhibition when incubated with mononuclear cells (19, 20), to induce granuloma formation in vivo (21), and to confer resistance to death by infection with Klebsiella pneumoniae in mice (22). Macrophages exposed to MDP in vitro produced increased amounts of collagenase, prostaglandin, and a fibroblast proliferation factor (23). We report here that direct contact of cultured peritoneal macrophages with MDP or LPS primes the cells to respond with increased 02 after stimulation with PMA or opsonized zymosan.