CCR2 is a chemokine receptor predominantly expressed on monocytes and is considered proinflammatory in response to inflammation. However, CCR2 deficiency unexpectedly induces severe autoimmune arthritis with accelerated disease onset; and the underlying mechanism is not completely understood. In this study, we show that Th17 cells are selectively expanded prior to disease onset in the draining lymph nodes in collagen-immunized CCR2−/− mice compared to WT controls. Consistently, augmented IL-17A, IL-6, and IL-1β levels are observed in the blood and in the inflamed joints of CCR2−/− mice, which are accompanied by enhanced autoantibody production and neutrophil infiltration. Neutralizing IL-17A has an ameliorating effect on the severe arthritis observed in CCR2−/− mice. Our results suggest a previously unrecognized mechanism that CCR2 may be important in maintaining immunological homeostasis and protecting against collagen-induced arthritis via regulation of Th17 cell responses. Given that monocytes suppress T cell function during autoimmunity, our finding that monocytes are substantially decreased in the spleen but not in the bone marrow and the arthritic joints in CCR2−/− mice may provide a biological link between CCR2 and Th17 cells during the pathogenesis of collagen-induced arthritis.
Accumulated evidence has implicated an important role for Th17 cells in autoimmune disease such as RA 
. Th17 cell differentiation from naïve CD4+ T cells depends on IL-6 and TGF-β production. Given that TGF-β is also required for Treg cell generation and IL-6 can reverse Treg cell-mediated suppression of autoreactive T cells 
, IL-6 is an essential differentiation factor for Th17 polarization. On the other hand, IL-17 activates cells in synovium, such as synovial fibroblasts and monocytes/macrophages, to produce IL-6, IL-1 and TNF-α and thus sustain joint inflammation 
. Consistent with these findings, our data show that early Th17 cell expansion in CCR2−/−
mice is accompanied by elevated IL-6 levels in the blood. Increased levels of IL-17A expression are also observed together with augmented IL-6 and IL-1β abundance in the arthritic joints of CCR2−/−
mice. Additionally, RANKL expression is significantly up-regulated in CCR2-deficient joints, which is consistent with findings that joint IL-17A stimulates osteoclasts to express RANKL that promotes bone erosion 
. Together, our results support the hypothesis that a positive feedback loop between Th17 cells and IL-6 is a key factor involved in tolerance breakdown and tissue injury in autoimmune arthritis. Our data further suggest a potential beneficial role of CCR2 in this process.
Th17 cells promote autoimmune pathology through effects on cellular and humoral immune responses. Indeed, increased Th17 cells are not only accompanied by a substantial expansion of neutrophils in both the spleen and joints but also B cell up-regulation with a consequent elevation of type II collagen-specific antibodies in immunized CCR2−/−
mice. IL-17A is known to stimulate granulopoiesis in vivo 
and stimulates neutrophil-specific chemokines such as MIP-2 and IL-8 
. Blockade of IL-17A in IFN-γ receptor knockout mice with CIA ameliorates joint inflammation and bone erosion by significant reductions of the splenic expansion and joint influx of neutrophils 
, suggesting a critical role of IL-17-stimulated neutrophils in autoimmune pathology. IL-17 receptor is expressed on B cells, and IL-17 has been shown to work together with B cell-activating factor to control the survival and differentiation of B cells into antibody producing cells in autoimmunity 
. In addition, IL-17 is found to drive autoimmune responses by promoting the formation of spontaneous germinal centers 
, and IL-17A-deficient mice have decreased autoantibodies in CIA and EAE 
. These results provide rationales for our data that increased Th17-cell responses contribute to the exacerbated arthritis in CCR2−/−
Recently, Fuji et al. have published a well-designed study showing that ablation of the Ccr2 gene exacerbated polyarthritis in IL-1 receptor antagonist-deficient (Il1rn−/−
) mice 
. Increased neutrophil accumulation and IL-I7 production were observed in the inflamed joints of those double knockout mice. Neutralizing CXCR2, a neutrophil chemokine receptor, reduced arthritis severity in Il1rn−/−
mice. Since our collagen-induced arthritis model is different from the Il1rn-dependent model in which polyarthritis develops spontaneously 
, it is difficult to compare directly which therapeutic method, anti-IL-17A or anti-CXCR2 is more effective. It is possible that the augmented neutrophil infiltration into the joints in CCR2−/−
mice contribute to the aggravated disease process through an additional Th17-independent mechanism. However, since neutrophils are key innate cells against infection, targeting such cells for the treatment of chronic disease like RA needs to be done cautiously.
Monocytes traffic to tissues and differentiate into macrophages or dendritic cells in response to inflammation 
. Recent evidence shows that CCR2 does not mediate monocyte trafficking into the inflamed tissues 
, but is rather involved in the egress of monocytes from the bone marrow 
. Indeed, monocytes are almost absent from the spleen, but are abundant in the bone marrow and inflamed joints in immunized CCR2−/−
mice. Intriguingly, CCR2-expressing monocytes have been recently shown to have an immune regulatory role via suppression of T cell activities under tumor and autoimmune conditions 
. It is conceivable that CCR2 deficiency alters T-cell suppressive monocytes, which results in Th17 cell expansion. CCR2 is also expressed on a subset of Treg cells that inhibit T cell proliferation in vitro 
. However, we find similar numbers of Treg cells between immunized CCR2−/−
and WT mice, suggesting that upregulated Th17 cells in CCR2−/−
mice are Treg-cell-independent. But our data cannot exclude the possibility that a small subset of Treg cells is affected by CCR2 deficiency.
Although CCR2 is a drug target for the treatment of some inflammatory conditions, blocking CCR2 or deleting Ccr2 gene worsens autoimmune arthritis. Our data show that Th17 cell dysregulation is responsible, at least in part, for the exacerbated phenotype in CCR2-deficient mice. CCR2 may have a protective role against autoimmune arthritis. Neutralization of IL-17 is a promising therapeutic approach against autoimmune arthritis possibly by inhibiting the excessive joint recruitment of neutrophils and monocytes as local injection and expression of IL-17 promote migration of both cell types to the joints