IgG anti-collagen type II (CII) antibodies (Ab) can induce arthritis in healthy mice. Here we have investigated if single monoclonal IgG anti-CII Ab can induce arthritis in CIA-susceptible DBA/1 mice and if there is an IgG subclass dependency. The involvement of Fc receptors for IgG (Fc + R) in anti-CII Ab-mediated arthritis was also investigated by comparing the clinical outcome in DBA/1 mice to those in Fc + R-deficient mice. We demonstrate for the first time that single mAb to naive DBA/1 mice can induce persistent arthritis. Histology of the inflamed joints revealed massive cellular infiltrate and cartilage and bone destruction. All IgG subclasses tested (IgG1, IgG2a and IgG2b) were arthritogenic, with the IgG1 and IgG2b isotypes as the dominating arthritogenic Ab. Pathogenicity was dependent on engagement of activating Fc + R, as FcR + -deficient mice were completely resistant to Ab-mediated arthritis. The arthritis induced with the IgG1 and IgG2b Ab was also inhibited by Fc + RIII disruption, whereas arthritis mediated by the IgG2a Ab was not substantially affected. The arthritic response of the IgG1 and IgG2b isotypes, but not of the IgG2a Ab, was further enhanced in mice lacking the inhibitory Fc + RIIB. These results demonstrate that single IgG anti-CII mAb can induce erosive arthritis and that IgG anti-CII Ab mediate arthritis by engagement of Fc + R.

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The influence of complement receptor 1 and 2 (CR1/2) was investigated on the susceptibility to low-dose collagen-induced arthritis (CIA) in wild-type (WT) and CR1/2-deficient DBA/1 mice. Significantly enhanced CIA was observed in female CR1/2-deficient mice compared with WT female mice, while male mutant and WT mice showed similar arthritis development. The enhanced CIA was accompanied with higher complement levels and a prolonged IgM anti-collagen type II response. When investigating whether estrogen contributed to the different arthritis susceptibility, we found that ovariectomy rendered WT females more sensitive to low-dose CIA and to the same extent as CR1/2-deficient females, while CR1/2-deficient mice were unaffected by ovariectomy. Notably, the ovariectomized WT mice displayed reduced CR1(+) B220(+) B-cell numbers and CR1 expression compared with sham-operated WT mice, suggesting a stimulatory effect of estrogen on CR1. In accordance, a significant correlation was observed between reduced CR1 expression in B cells and increased age in healthy female blood donors but not in male donors. Our findings demonstrate an important role of CR1/2 in suppressing CIA in female mice under low-antigen conditions. The data suggest that estrogen promote CR1 expression in B cells. These findings provide insight to the increased frequency of rheumatoid arthritis in postmenopausal women.