Targeting of Gr-l+,CCR2+ monocytes in collagen-induced arthritis

Objective. The chemokine receptor CCR2 is highly expressed on monocytes and considered a promising target for treatment of rheumatoid arthritis. However, blockade of CCR2 with a monoclonal antibody (mAb) during progression of collagen -induced arthritis results in a massive aggravation of the disease. In this study we investigated why CCR2 antibodies have proin-flammatory effects, how these effects can be avoided, and whether CCR2+ monocytes are useful targets in the treatment of arthritis. Methods. Arthritis was induced in DBA/1 mice by immunization with type 11 collagen. Mice were treated with mAb against CCR2 (MC-21), IgE, or isotype control antibodies at various time points. Activation of basophils and depletion of monocyte subsets were determined by fluorescence- activated cell sorter analysis and enzyme-linked immunosorbent assay. Results. Crosslinkage of CCR2 activated basophils to release interieukin-6 (IL-6) and IL-4. In vivo, IL-6 release occurred only after exposure to high doses of MC-21, whereas application of low doses of the mAb circumvented the release of IL-6. Regardless of the dose level used, the antibody MC-21 efficiently depleted Gr-1 +,CCR2 + monocytes from the synovial tissue, peripheral blood, and spleen of DBA/1 mice. Activation of basophils with high doses of MC-21 or with antibodies against IgE resulted in a marked aggravation of collagen -induced arthritis and an increased release of IL-6. In contrast, low-dose treatment with MC-21 in this therapeutic setting had no effect on IL-6 and led to marked improvement of arthritis. Conclusion. These results show that depletion of CCR2+ monocytes may prove to be a therapeutic option in inflammatory arthritis, as long as the dose-dependent proinflammatory effects of CCR2 mAb are taken into account.