Mesenchymal stem cells (MSC) have been suggested to have protective effects through modulation of immune responses. We examined the hypothesis that MSC can modify the ability of endothelial cells (EC) to recruit neutrophils and lymphocytes. Using flow-based assays, leukocytes were perfused over HUVEC that had been cultured with MSC, either on opposite sides of porous filers or in direct contact for 24 h, and then stimulated with tumour necrosis factor-α (TNF-α) for 4 h or TNF-α and interferon-γ for 24 h. Cytokine-stimulated EC supported neutrophil and lymphocyte recruitment, and a combination of rolling, firm adhesion and transmigration was observed. EC cultured with MSC supported significantly lower total adhesion of both types of leukocytes, and the proportion of adherent leukocytes that migrated was reduced compared to EC monocultures. The effects were dependent on the number of MSC added, but were evident for ratios of MSC to EC <<0.1. Interestingly, antibody neutralisation of interleukin-6 (IL-6) or transforming growth factor (TGF)-β during co-culture largely abolished the inhibitory effects of MSC on recruitment of both types of leukocyte. These antibodies did not modify recruitment to cytokine-treated EC cultured alone. We conclude that MSC can adhere to and integrate with EC monolayers, and then inhibit the inflammatory recruitment of leukocytes. The inhibitory effects may be due to linked production and effects of IL-6 and TGF-ï□¢ in the co-culture milieu.