Protease-activated receptor (PAR) 1 and PAR2 have been implicated in RA, however their exact role is unclear. Here, we detailed the mechanistic impact of these receptors on the onset and development of inflammatory arthritis in murine CIA and antigen-induced arthritis (AIA) models.
CIA or AIA was induced in PAR1 or PAR2 gene knockout (KO) and matched wild type mice. The onset and development of arthritis was monitored clinically and histologically. Immune cells, cytokines and MMPs were detected by ELISA, zymography, flow cytometry, western blot or immunohistochemistry.
In CIA, PAR1KO and PAR2KO exacerbated arthritis, in opposition to their effects in AIA. These deficient mice had high plasma levels of IL-17, IFN-γ, TGF-β1 and MMP-13, and lower levels of TNF-α; T cells and B cells were higher in both KO spleen and thymus, and myeloid-derived suppressor cells were lower only in PAR1KO spleen, when compared with wild type cells. Th1, Th2 and Th17 cells were lower in PAR1KO spleens cells, whereas Th1 and Th2 cells were lower and Th17 cells higher in both KO thymus cells, when compared with wild type cells. PAR1KO synovial fibroblasts proliferated faster and produced the most abundant MMP-9 amongst three type cells in the control, lipopolysaccharides or TNF stimulated conditions.
This is the first study demonstrated that deficiency of PAR1 or PAR2 aggravates inflammatory arthritis in CIA. Furthermore, the protective functions of PAR1 and PAR2 in CIA likely occur via differing mechanisms involving immune cell differentiation and cytokines/MMPs.

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