IN VITRO CLEAVAGE BY ASBESTOS FIBERS OF THE FIFTH COMPONENT OF HUMAN COMPLEMENT THROUGH FREE-RADICAL GENERATION AND KALLIKREIN ACTIVATION

Chrysotile and crocidolite fibers incubated in normal human plasma (NHP) generated from the C₅ component of complement C_5a-type fragments that stimulated polymorphonuclear leukocyte (PMN) chemotaxis. Absorption of NHP with antiserum against C_5a totally abolished neutrophil chemotactic activity. Asbestos fibers also produced C_5a small peptides in the presence of ethylene glycol bis(beta-aminoethyl ether) N,N,N',N'-tetraacetic acid (EGTA) but not ethylene diamine tetraacetic acid (EDTA). Activation of C₅ was significantly inhibited when asbestos fibers were pretreated with iron chelators such as sodium dithionite (DTN), deferoxamine (DFX), or ascorbate (AA). Concentration-related inhibition of C₅ activation was also observed when asbestos fibers were added concurrently to plasma in the presence of DFX, 1,3-dimethyl-2-thiourea (DMTU), a strong hydroxyl scavenger, or aprotinin (APR), a specific protease inhibitor. Further, chrysotile and crocidolite significantly increased plasma kallikrein activity. Data demonstrate that asbestos-induced C₅ activation plays a role in inflammatory reactions characteristic of asbestosis through mechanisms involving iron ions, hydroxyl radicals, and oxidized C₅-like fragments. The ferrous ions present at the asbestos fiber surface trigger this activation and catalyze, via Fenton reaction, the production of hydroxyl radicals, which in turn convert native C₅ to an oxidized C₅-like form. This product is then cleaved by kallikrein, activated by the same asbestos fibers, yielding an oxidized C_5a with the same functional properties as C_5a.