CD147 on platelets stimulates cytokine expression and MMP-9 production of monocytes by NF-κB activation. Isolated human monocytes were pretreated with medium (M), with anti-CD147-blocking antibody (CD147mAb), with IKK inhibitor, with nonblocking control antibody (control mAb) and with lipopolysaccharide (LPS, positive control) for 24 hours. Thereafter, monocytes were washed and contact co-cultured (M + P) or no cellular contact co-cultured (M/P) with freshly isolated platelets from RA patients for an additional 24 hours. Subsequently, intracellular expression of TNF-α (A), IL-6 (B) and IKKβ (C) in monocyte subsets and MMP-9 activity (D) were determined (mean ± standard error of the mean, P >0.05, *P <0.05, **P <0.001 and ***P <0.0001 vs. corresponding controls). IL-6: interleukin 6; LPS: lipopolysaccharide; mAb: monoclonal antibody; MMP: matrix metalloproteinases; MPA: monocyte-platelet aggregates; NF-κB: nuclear factor kappa B; RA: rheumatoid arthritis; TNF-α: tumor necrosis factor alpha.