The complement system, especially the alternative pathway (AP), plays essential roles in the induction of injury in collagen antibody-induced arthritis (CAIA) in mice. The goal of the current study was to directly compare the roles of receptors for C3a and C5a, as well as the membrane attack complex (MAC), as effector mechanisms in the pathogenesis of CAIA. Clinical disease activity (CDA) in C3aR−/−, C5aR−/−, and C6 deficient (C6-def) mice was decreased by 52%, 94%, and 65%, respectively, as compared with WT mice. Decreases in histopathologic injury as well as in IgG and C3 deposition paralleled the CDA. A decrease in the percentage of synovial neutrophils was observed in C3aR−/−, C5aR−/−, and C6-def mice, and a decrease in macrophages was observed in C3aR−/− and C5aR−/−, but not in C6-def, mice. Synovial mRNA obtained by laser capture microdissection exhibited a decrease in TNF-α in C5aR−/− mice and in IL-1β in both C5aR−/− and C6-def mice, while C3aR−/− mice demonstrated no change in either cytokine. Our findings show that absent C3aR-, C5aR- or MAC-initiated effector mechanisms each decreases susceptibility to CAIA, with clinical effects most pronounced in C5aR deficient mice. Although the absence of C3aR, C5aR, or C6 led to differential deficiencies in effector mechanisms, decreased proximal joint IgG and C3 deposition was common to all three genotypes in comparison to WT mice. These data suggest the existence of positive feedback amplification pathways downstream of all three effectors that promote additional IgG deposition and C3 activation in the joint.

Mannose-binding lectin-associated serine proteases-1/3 (MASP-1/3) are essential in activating the alternative pathway (AP) of complement through cleaving pro-factor D (pro-Df) into mature Df. MASP are believed to require binding to mannose binding lectins (MBL) or ficolins (FCN) to carry out their biological activities. Murine sera have been reported to contain MBL-A, MBL-C, and FCN-A, but not FCN-B that exists endogenously in monocytes and is thought not to bind MASP-1. We examined some possible mechanisms whereby MASP-1/3 might activate the AP. Collagen antibody-induced arthritis, a murine model of inflammatory arthritis dependent on the AP, was unchanged in mice lacking MBL-A, MBL-C, and FCN-A (MBL−/−/FCN A−/− mice) in comparison to wild-type mice. The in vitro induction of the AP by adherent mAb to collagen II was intact using sera from MBL−/−/FCN A−/− mice. Furthermore, sera from MBL−/−/FCN A−/− mice lacked pro-Df and possessed only mature Df. Gel filtration of sera from MBL−/−/FCN A−/− mice showed the presence of MASP-1 protein in fractions containing proteins smaller than the migration of MBL-A and MBL-C in sera from C4−/− mice, suggesting possible binding of MASP-1 to an unknown protein. Lastly, we show that FCN-B was present in the sera of MBL−/−/FCN A−/−mice and that it was bound to MASP-1. We conclude that MASP-1 does not require binding to MBL-A, MBL-C, or FCN-A to activate the AP. MASP-1 may cleave pro-Df into mature Df through binding to FCN-B or to an unknown protein, or may function as an unbound soluble protein.

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