Objective. To determine the role of the terminal complement complex (TCC) in the development of experimental antigen-induced arthritis (AIA) and the therapeutic effects of human anti-C5 single-chain Fv (scFv). Methods. Two different anti-C5 scFv, one that inhibits both release of C5a and assembly of the TCC (TS-A 12/22) and another that selectively blocks formation of the TCC (TS-A 8), were injected at the onset of AIA. The effects of these scFv on disease severity were evaluated for up to 21 days and compared with the effects of injection of an unrelated scFv. AIA was also established in C6-deficient and C6-sufficient PVG rats to obtain further information on the role of the TCC in this model. Results. TS-A 12/22 and TS-A 8 proved to be equally effective in reducing joint swelling, cell counts and tumor necrosis factor α levels in synovial lavage fluids, and the degree of histomorphologic changes compared with the effects of the unrelated scFv. TS-A 12/22 and TS-A 8 prevented the deposition of C9 but not that of C3, confirming the ability of the 2 scFv to neutralize C5. Administration of the 2 anti-C5 scFv after AIA onset also reduced disease severity. In C6-deficient rats with AIA, disease activity was reduced markedly compared with that in C6-sufficient rats. Conclusion. These 2 human anti-C5 scFv could represent potential therapeutic reagents to be used in patients with rheumatoid arthritis. In addition, the finding that TS-A 8 was as effective as TS-A 12/22 in reducing disease severity suggests that the TCC is mainly responsible for the joint inflammation and damage observed in AIA.
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