Mesenchymal stromal cells immunosuppress osteoarthritis synovial fluid modulated monocytes via IL-6 and CCL2

Mesenchymal stromal cell (MSC) interactions with monocytes/macrophages are central to their therapeutic effects in knee osteoarthritis (KOA); however, mechanisms of these interactions are not fully understood. We hypothesize that MSC soluble factors, particularly interleukin-6 (IL-6) and C-C motif chemokine ligand (CCL2) modulate monocytes in KOA environment. Using healthy donor CD14⁺ monocytes exposed to KOA synovial fluid (SF) in the presence or absence of marrow-derived MSC(M) directly or conditioned medium (CM), we evaluated cell surface markers and signaling via signal transducer and activator of transcription (STAT3), nuclear factor kappa-light-chain-enhancer of activated B (NF-κB) and c-Jun Terminal Kinase (JNK); functional responses were measured by secretion of tumor necrosis factor (TNF) and IL-1, and by phagocytosis of pHrodo Red E-coli. CD14⁺ monocytes demonstrated a mixed phenotype in KOA SF with increased CD163, CD206, and unchanged HLA-DR, CD86 marker expression. This was accompanied by activated STAT3, JNK, and NF-κB signaling. TNF and IL-1 secreted levels were unchanged, but phagocytosis was impaired, indicative of a net dysfunctional repair phenotype and functionality. CD14⁺ monocytes in KOA SF were hyporesponsive to additional lipopolysaccharide re-challenge, based on TNF and IL-1 secretion. Addition of MSC(M) to KOA SF programmed CD14⁺ monocytes resolved the dysfunctional phenotype and functionality, with significant increases in CD163, CD206; significant reductions in HLA-DR and CD86 expression; this was accompanied by significantly increased activated STAT3 and decreased activated JNK and NF-κB. TNF and IL-1 secretion were also significantly reduced, and phagocytic capacity restored. Blocking IL-6, or to a lesser extent, CCL2, partially abrogated MSC(M) soluble factor effects. MSC(M) experienced apoptosis in KOA SF; however, apoptotic bodies did not fully recapitulate MSC(M) soluble factor effects. IL-6, CCL2, other soluble factors and apoptotic bodies from MSC(M) secretome mitigate the dysfunctional effects of KOA SF on CD14⁺ monocytes resulting in immunosuppressed phenotype and functionality.