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

The mechanisms of mesenchymal stromal cell (MSC) interactions with monocytes/macrophages (M{Phi}s) in knee osteoarthritis (KOA) are not fully understood. We report that synovial fluid (SF) M{Phi}s 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-{kappa}B) and c-Jun Terminal Kinase (JNK) drove CD14+ monocyte activation in late-OA SF. Addition of marrow-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-{kappa}B 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. GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=162 SRC="FIGDIR/small/669875v2_ufig1.gif" ALT="Figure 1"> View larger version (50K): org.highwire.dtl.DTLVardef@474da4org.highwire.dtl.DTLVardef@13a89baorg.highwire.dtl.DTLVardef@282988org.highwire.dtl.DTLVardef@bed003_HPS_FORMAT_FIGEXP M_FIG C_FIG