C5AR1 and rheumatoid arthritis: Interestingly, our previous studies have shown that C3aR1 or C5aR1 gene targeted mice are resistance to collagen antibody-induced arthritis (54), and there was also a positive correlation between C5aR1 gene expression and clinical disease activity score 28 in synovium from early RA patients (36), suggesting an important role for C5aR1 in inflammation both in mice and humans.