Although mast cells (MCs) often are abundant in the synovial tissues of patients with
rheumatoid arthritis, the contribution of MCs to joint
inflammation and cartilage loss remains poorly understood. MC-restricted
tryptase/
heparin complexes have proinflammatory activity, and significant amounts of human
tryptase beta (hTryptase-beta) are present in
rheumatoid arthritis synovial fluid. Mouse MC protease-6 (mMCP-6) is the ortholog of hTryptase-beta, and this
serine protease is abundant in the synovium of arthritic mice. We now report that C57BL/6 (B6) mice lacking their
tryptase/
heparin complexes have attenuated arthritic responses, with mMCP-6 as the dominant
tryptase responsible for augmenting neutrophil infiltration in the K/BxN mouse serum-transfer
arthritis model. While
inflammation in this
experimental arthritis model was not dependent on
protease-activated receptor-2, it was dependent on the
chemokine receptor CXCR2. In support of the latter data, exposure of synovial fibroblasts to hTryptase-
beta/heparin or mMCP-6/
heparin complexes resulted in expression of the neutrophil
chemotactic factors CXCL1/KC, CXCL5/LIX, and CXCL8/IL-8. Our proteomics, histochemistry, and immunohistochemistry data also revealed substantial loss of cartilage-derived
aggrecan proteoglycans in the arthritic joints of wild-type B6 mice but not mMCP-6-null B6 mice. These observations demonstrate the functional contribution of MC-restricted
tryptase/
heparin complexes in the K/BxN mouse
arthritis model and connect our mouse findings with
rheumatoid arthritis pathophysiology.