Mesenchymal Stromal Cells Immunosuppress Osteoarthritis Synovial Fluid Tolerized Monocytes via IL-6

The mechanisms of mesenchymal stromal cell (MSC) interactions with monocytes/macrophages (MΦs) in knee osteoarthritis (KOA) are not fully understood. We report that synovial fluid (SF) MΦs are tolerized. Healthy and OA patient donor-derived peripheral CD14 ⁺ monocytes exposed to late-stage OA SF are semi-tolerized with stable phenotype, including CD86 expression with additional lipopolysaccharide (LPS) re-challenge; there were slight differences in healthy vs. OA CD14 ⁺ monocyte tolerization profiles, indicative of systemic differences. Notably, both healthy and OA CD14 ⁺ monocytes demonstrated non-significant or significant increases respectively in tumor necrosis factor (TNF), suggestive of an activated profile. Late-OA SF is a complex mixture of multiple factors resulting in a mixed immunosuppressed, activated, tolerized profile of CD14 ⁺ peripheral monocytes. We explored the in-depth roles of interleukin (IL)-6 and toll-like receptor (TLR)4 signaling in late-OA SF using healthy CD14 ⁺ monocytes. Classical IL-6 signaling immunosuppressed CD14 ⁺ monocytes via signal transducer and activator of transcription (STAT3) and suppressor of cytokine signaling (SOCS3), while TLR4 via nuclear factor kappa-light-chain-enhancer of activated B (NF-κB) and c-Jun Terminal Kinase (JNK) drove CD14 ⁺ monocyte activation in late-OA SF. Addition of m arrow-derived MSC( M ) soluble factors enhanced immunosuppression through IL-6 signaling and nullified late-OA SF activation effects by decreasing p-JNK and reducing TLR-mediated NF-κB and TNF. MSC(M) immunosuppressed CD14 ⁺ monocytes in late-OA SF were also functionally rescued with improved phagocytosis. We thus demonstrated that soluble factors from MSC(M) mitigate the activating effects of late-OA SF through IL-6 dependent and independent pathways resulting in CD14 ⁺ monocytes with increased immunosuppressive phenotypes and function.