Abstract
Canine idiopathic polyarthritis (IPA) is characterized by increased numbers of polymorphonuclear leukocytes (PMNs) in the synovial fluid (SF). In humans, CC chemokine ligand 2 (CCL2) and CXC chemokine ligand 8 (CXCL8) recruit monocytes and neutrophils, respectively, and are involved in various inflammatory disorders. The aim of this study was to assess the roles of these chemokines in driving PMNs infiltration into the joints of dogs with IPA. SF samples were collected from dogs with IPA (n=19) and healthy controls (n=8), and the concentrations of SF CCL2 and CXCL8 were determined by ELISA. Dogs with IPA had significantly higher concentrations of CCL2 (3316±2452pg/ml, mean±SD) and CXCL8 (3668±3879pg/ml) compared with the healthy controls (235±45pg/ml and <15.6pg/ml, respectively). Then, an in vitro chemotaxis assay was performed using a modified Boyden chamber (pore size: 3μm). SF from IPA dogs had a chemoattractant activity for PMNs that purified from the peripheral blood of a healthy dog. We subsequently found that combination treatment with MK-0812 (an antagonist of CCL2 receptor) and repertaxin (an antagonist of CXCL8 receptors) significantly inhibited the migration of PMNs to SF from IPA dogs. Thus, expression of the CCL2 receptor (chemokine (CC motif) receptor 2 (CCR2)) was examined using polymerase chain reaction and immunocytochemistry. Canine peripheral blood PMNs exhibited significantly higher CCR2 mRNA expression levels than those in monocytes. In addition, we observed strong CCR2 expression on PMNs obtained from healthy controls and IPA dogs, although mononuclear cells did not express CCR2. Taken together, the data suggest that CCL2 acts as a canine PMNs chemotactic factor as well as CXCL8 and both CCL2 and CXCL8 facilitate the infiltration of PMNs into the joints of dogs with IPA.