Ablation of the Ccr2 gene exacerbates polyarthritis in interleukin-1 receptor antagonist-deficient mice

The pathogenesis of rheumatoid arthritis (RA) involves cytokines and chemokines. Given the role of intraarticular macrophage infiltration in RA, this study was undertaken to address the pathogenic role of CCR2, a chemokine receptor that is abundantly expressed by macrophages, in Il1rn-deficient mice, a mouse model of RA.

Il1rn-deficient and Il1rn and Ccr2-double-deficient mice were subjected to clinical assessment of arthritis and histologic examination. Bone mineral density was measured with computed tomography. The types of cells infiltrating joints were determined by immunohistochemical analysis and flow cytometric analysis. Osteoclasts in joints were quantified after tartrate-resistant acid phosphatase staining. Cytokine and chemokine levels were measured by enzyme-linked immunosorbent assay and multiplex suspension array assay. The expression patterns of chemokines and osteoclastogenic factors were determined by double-color immunofluorescence analysis. Anti-mouse CXCR2 antibody was injected into Il1rn and Ccr2-double-deficient mice for blocking experiments.

Ablation of the Ccr2 gene actually exacerbated arthritis and intraarticular osteoclastogenesis, while it enhanced intraarticular neutrophil but not macrophage accumulation in Il1rn-deficient mice. Infiltrated neutrophils expressed the osteoclastogenic factors RANKL and ADAM-8, thereby augmenting intraarticular osteoclastogenesis in Il1rn and Ccr2-double-deficient mice. Moreover, the double-deficient mice exhibited enhanced expression of the neutrophilic chemokines keratinocyte chemoattractant and macrophage inflammatory protein 2 (MIP-2), compared with Il1rn-deficient mice. Finally, neutralizing antibodies to CXCR2, the receptor for keratinocyte chemoattractant and MIP-2, dramatically attenuated arthritis in Il1rn and Ccr2-double-deficient mice.

Our findings indicate that CCR2-mediated signals can modulate arthritis in Il1rn-deficient mice by negatively regulating neutrophil infiltration.