The signals that regulate the fate of circulating monocytes remain unknown. In the present study, we demonstrate that triggering of the NOD2 receptor by muramyl dipeptide (MDP) converts inflammatory Ly6C monocytes into patrolling Ly6C monocytes. Administration of MDP to Nr4a1 mice, which lack Ly6C monocytes, or to Ly6C-depleted mice led to the emergence of blood-patrolling monocytes with a profile similar to that of Ly6C monocytes, including high expression of CX3CR1 and LFA1. Using intravital microscopy in animal models of inflammatory diseases, we also found that converted Ly6C monocytes patrol the endothelium of blood vessels and that their presence contributes to a reduction in the inflammatory response following MDP injection. Our results demonstrate that NOD2 contributes to the regulation of blood monocytes and suggest that it could be therapeutically targeted to treat inflammatory diseases.

Monocytes are central to the physiopathology of arthritis, but their roles in progression and resolution of the disease remain to be clarified. Using NR4A1 mice, which lack patrolling lymphocyte antigen 6C (Ly6C ) monocytes, we found that inflammatory Ly6C monocytes contribute to rapid development of arthritis in a serum transfer-induced arthritis (STIA) model. Our experiments suggest that patrolling monocytes do not promote the initiation and progression of arthritis in mice, as severity of symptoms was amplified in NR4A1 mice. Moreover, we show that treatment of arthritic wild type (WT) mice with cytosporone B (Csn-B), a NR4A1-specific agonist, significantly reduces severity of disease. Effects of Csn-B were absent in monocyte-depleted mice treated with clodronate until Ly6C monocytes were restored. Adoptive transfer of Ly6C monocytes in arthritic NR4A1 mice treated with Csn-B reduces joint inflammation, supporting the regulatory role of Ly6C subset on disease development. Our results also reveal that administration of Csn-B to arthritic mice enhances levels of circulating CD4 CD25 FoxP3 Treg cells, a process requiring the presence of Ly6C monocytes. Together, these data indicate that Ly6C monocytes are involved in the initiation and progression of arthritis and Ly6C monocytes contribute to reduce joint inflammation through the mobilization of Treg cells.