Introduction: Activated platelets exert a proinflammatory action that can be largely ascribed to their ability to interact with monocytes.However, the mechanisms that promote dynamic changes in monocyte subsets in rheumatoid arthritis (RA) have not been clearly identified.The aim of this study was to determine whether platelet activation and the consequent formation of monocyte-platelet aggregates (MPA) might induce a proinflammatory phenotype in circulating monocytes in RA.Methods: The surface phenotype of platelets and the frequencies of monocyte subpopulations in the peripheral blood of RA patients were determined using flow cytometry.Platelets were sorted and co-cultured with monocytes.In addition, monocyte activation was assessed by measuring the nuclear factor kappa B (NF-Κb) pathway.The disease activity was evaluated using the 28-joint disease activity score.Results: Platelet activation, circulating intermediate monocytes (Mon2) and MPA formation were significantly elevated in RA, especially in those with active disease status.Furthermore, Mon2 monocytes showed higher CD147 expression and responded to direct cell contact with activated platelets with higher cytokine production and matrix metallopeptidase 9 (MMP-9)secretion, which increased the expression of CD147.After the addition of specific antibodies for CD 147, those effects were abolished.Furthermore, the NF-Κb-driven inflammatory pathway may be involved in this process.Conclusions: These findings indicate an important role of platelet activation and the consequent formation of MPA in the generation of the proinflammatory cytokine milieu and for the promotion and maintenance of the pathogenically relevant Mon2 monocyte compartment in RA, which is likely to play an important role in the pathogenesis of autoimmunity.