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 C(5) 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 C(5) 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 C(5) 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 C(5) activation plays a role in inflammatory reactions characteristic of asbestosis through mechanisms involving iron ions, hydroxyl radicals, and oxidized C(5)-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 C(5) to an oxidized C(5)-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).