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Rationale: Monocytes play critical roles in the pathogenesis of arthritis by contributing to the inflammatory response and bone erosion. Among genes involved in regulating monocyte functions, miR-146a negatively regulates the inflammatory response and osteoclast differentiation of monocytes. It is also the only miRNA reported to differentially regulate the cytokine response of the two classical Ly6C high and non-classical Ly6C low monocyte subsets upon bacterial challenge. Although miR-146a is overexpressed in many tissues of arthritic patients, its specific role in monocyte subsets under arthritic conditions remains to be explored. Methods: We analyzed the monocyte subsets during collagen-induced arthritis (CIA) development by flow cytometry. We quantified the expression of miR-146a in classical and non-classical monocytes sorted from healthy and CIA mice, as well as patients with rheumatoid arthritis (RA). We monitored arthritis features in miR-146a -/- mice and assessed in vivo the therapeutic potential of miR-146a mimics delivery to Ly6C high monocytes. We performed transcriptomic and pathway enrichment analyses on both monocyte subsets sorted from wild type and miR-146a -/- mice. Results: We showed that the expression of miR-146a is reduced in the Ly6C high subset of CIA mice and in the analogous monocyte subset (CD14 + CD16 - ) in humans with RA as compared with healthy controls. The ablation of miR-146a in mice worsened arthritis severity, increased osteoclast differentiation in vitro and bone erosion in vivo . In vivo delivery of miR-146a to Ly6C high monocytes, and not to Ly6C low monocytes, rescues bone erosion in miR-146a -/- arthritic mice and reduces osteoclast differentiation and pathogenic bone erosion in CIA joints of miR-146a +/+ mice, with no effect on inflammation. Silencing of the non-canonical NF-κB family member RelB in miR-146a -/- Ly6C high monocytes uncovers a role for miR-146a as a key regulator of the differentiation of Ly6C high , and not Ly6C low , monocytes into osteoclasts under arthritic conditions. Conclusion: Our results show that classical monocytes play a critical role in arthritis bone erosion. They demonstrate the theranostics potential of manipulating miR-146a expression in Ly6C high monocytes to prevent joint destruction while sparing inflammation in arthritis., Competing Interests: Competing Interests: The authors have declared that no competing interest exists.

The article presents an abstract of a research paper that characterize monocytes subsets and monocyte-derived cells during the development of autoimmune arthritis in mice, submitted at the 5th European Workshop on Immune-Mediated Inflammatory Diseases in Sitges-Barcelona, Spain from December 1-3, 2010.