Endothelial production of MCP‐1: modulation by heparin and consequences for mononuclear cell activation

Heparin is a polyanionic glycosaminoglycan (GAG) that can bind with high affinity to a range of cytokines including interferon‐γ (IFN‐γ) and members of the chemokine superfamily. This GAG also possesses immunomodulatory activity in vivo and can antagonize the capacity of IFN‐γ to induce class II MHC antigen expression, and to up‐regulate intercellular adhesion molecule‐1, by cultured endothelial cells. Previous studies have shown that binding to cell‐surface heparan sulphate is essential for optimal activity of IFN‐γ and that free heparin competitively inhibits this sequestration process. The present study was performed to increase our understanding of the immunosuppressive activity of heparin by investigation of potential antagonism of the production and function of monocyte chemotactic peptide‐1 (MCP‐1), a chemokine important for mononuclear leucocyte recruitment across vascular endothelium. It was found that mixture of heparin with IFN‐γ inhibited up‐regulation of the signal transducer and activator of transcription protein, STAT‐1 produced normally by treatment of endothelial cells with IFN‐γ. An inhibition of MCP‐1 production was observed that was specifically caused by mixture of IFN‐γ with heparin‐like, and therefore cytokine‐binding, GAGs. It was also shown that mixture of heparin‐like GAGs with MCP‐1 inhibited the rapid tyrosine phosphorylation of phosphatidylinositol 3‐kinase which is normally produced by treatment of mononuclear leucocytes with this chemokine. Blockade of this intracellular signalling event was associated with a reduction in the normal transendothelial migration response towards MCP‐1. Results from this study indicate that soluble, heparin‐like GAGs can block IFN‐γ‐dependent up‐regulation of MCP‐1 production by cultured endothelial cells, and can also antagonize the leucocyte‐activating and migration‐promoting properties of pre‐existing MCP‐1. These activities may contribute to the immunomodulatory properties of heparin.