Complement component C5 is required for release of alveolar macrophage-derived neutrophil chemotactic activity

The influx of neutrophils into the alveolar structures can be induced by stimulation of the resident lung phagocyte, the alveolar macrophage, to release a potent neutrophil chemoattractant(s). We hypothesized that the fifth component of complement (C5) on the cell surface may be required for activation of the alveolar macrophage to release neutrophil chemotactic activity. C5 was identified on guinea pig alveolar macrophages by epifluorescent microscopy, flow cytometry, and enzyme-linked immunoabsorbent assay of eluted macrophages. When cultured for 4 h with stimuli that induce the release of chemotactic activity or for 24 h without added stimuli, purified Fab' fragments of a goat anti-C5 antibody significantly inhibited the ability of macrophages to release chemotactic activity as determined by a blindwell chamber method (p less than 0.001, all comparisons). This inhibition of chemotactic activity was not detected when anti-C5 antibody was added after the culture period. In contrast, anti-C3 antibody had no inhibitory effect at 4 h or at 24 h (p greater than 0.2, all comparisons). Partial characterization of released chemotactic activity revealed it was of low molecular weight, partially lipid soluble, and not inhibited by C5a chemotactic factor inactivator. These studies suggest that C5 may have a regulatory role in the release of chemotactic activity by alveolar macrophages.