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) ge nerated from the C-5 component of complement C-5a-type fragments that stimu lated polymorphonuclear leukocyte (PMN) chemotaxis. Absorption of NHP with antiserum against C-5a totally abolished neutrophil chemotactic activity. A sbestos fibers also produced C-5a small peptides in the presence of ethylen e glycol bis(beta-aminoethyl ether) N,N,N',N'-tetraacetic acid (EGTA) but n ot ethylene diamine tetraacetic acid (EDTA). Activation of C-5 was signific antly inhibited when asbestos fibers were pretreated with iron chelators su ch as sodium dithionite (DTN), deferoxamine (DFX), or ascorbate (AA). Conce ntration-related inhibition of C-5 activation was also observed when asbest os fibers were added concurrently to plasma in the presence of DFX, 1,3-dim ethyl-2-thiourea (DMTU), a strong hydroxyl scavenger, or aprotinin (APR), a specific protease inhibitor. Further, chrysotile and crocidolite significa ntly increased plasma kallikrein activity. Data demonstrate that asbestos-i nduced C-5 activation plays a role in inflammatory reactions characteristic of asbestosis through mechanisms involving iron ions, hydroxyl radicals, a nd oxidized C-5-like fragments. The ferrous ions present at the asbestos fi ber surface trigger this activation and catalyze, via Fenton reaction, the production of hydroxyl radicals, which in turn convert native C-5 to an oxi dized 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 functi onal properties as C-5a.