Demonstration of C5 cleaving activity in bronchoalveolar fluids and cells: a mechanism of acute and chronic alveolitis.

Recently, we have demonstrated that preformed chemotactic fragments derived from C5 (C5fr), when instilled intratracheally, induce an intense acute alveolitis. An intense intrapulmonary inflammatory reaction, characterized by accumulation of large numbers of neutrophils in the alveolar spaces, also resulted from intratracheal instillation of purified human C5. Since purified C5 does not induce PMN chemotaxis in vitro and presumably native C5 is not phlogistic in vivo, a mechanism for C5 cleavage productive of chemotactic fragments may be operative within the lung. To explore the possibility that pulmonary-mediated fragmentation of C5 occurred, both in vitro and in vivo studies were undertaken. As described in this paper, extensive cleavage of 125I-labeled C5 was demonstrated in the bronchoalveolar lavage fluids (BALF) of hamster lungs previously instilled with 125I-labeled C5. In vitro incubation of 125I-labeled C5 with normal hamster lung lavage components, cells, and lavage fluid also cleaved 125I-labeled C5. Bioassay of such in vitro reactions also revealed potent chemotactic activity for polymorphonuclear leukocytes. A potent C5-cleaving activity was detected in fluid of normal bronchoalveolar lavage (BAL) recovered from normal hamster lungs. This BALF "enzyme activity" selectively cleaved C5 in vitro, producing potent C5-related chemotactic fragmentation products, but it did not fragment C3 nor did it produce C3 chemotactic factors in vitro. Cleavage of C5 by alveolar macrophages that produces chemotactic factors has previously been attributed to an acid protease of lysosomal origin. This "enzyme activity" is apparently distinct from elastase, collagenase, and trypsin since it is not inhibited by alpha 1-antitrypsin. In separate studies, we have recently demonstrated the ability of isolated lung cells to synthesize and secrete C3 and C5 in vitro. Thus, these studies suggest that the lung may intrinsically have the capacity to regulate inflammatory reactions by the localized production of both complement components (C3 and C5) as well as complement-activating systems (e.g., C5-cleaving activity of BALF).