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Notes: Background Interleukin-4 (IL-4) is an immunoregulatory cytokine which has a wide variety of effects on immune cell function. In addition, recent studies suggest that IL-4 may have effects on other cells including endothelial cells in terms of the regulation of adhesion molecule expression and leucocyte extravasation from the vascular space to sites of tissue inflammation. Consequently, IL-4 may have an important role in the pathogenesis of allergic inflammation and disease.Objective The purpose of this study was to learn more about the potential role of IL-4 in inflammatory disease, specifically in regard to the potential of IL-4 to induce the expression of adhesion molecules on vascular endothelial cells and promote the adherence and transmigration of circulating leucocytes to sites of tissue inflammation.Methods Single subcutaneous injections of human IL-4 were administered to cynomolgus monkeys and tissue biopsy samples were obtained and analysed for adhesion molecule expression on vascular endothehum and inflammatory cell infiltrates. In another series of experiments, multiple subcutaneous injections of human IL-4 were administered (bid on four consecutive days) and the effects on peripheral blood leucocytes and plasma levels of various cytokines and chemokines were examined. Results Intradermal injection of IL-4 induced the expression of vascular cell adhesion molecule-1 (VCAM-l) on cutaneous vascular endothelium that was present at 8hr and persisted out to 24h post injection. The expression of VCAM-1 was associated with an inflammatory cell infiltrate comprised of granulocytes and mononuclear cells. Multiple injections of IL-4 resulted in a dose-related decrease in the relative percentage and total number of circulating lymphocytes and an increase in circulating neutrophils (4.6 ± 1–2.1 ± 0.2 ± 106/mL and 1.7 ± 0.3–7.0 ± 1 ± 106/mL, respectively). Analysis of cell surface markers by flow cytometry revealed a transient decrease in the number of CD4+ T lymphocytes and a sustained decrease in CD16+ cells. In addition. IL-4 administration resulted in a large increase in plasma MCP-1 concentration.Conclusion This is the first study to demonstrate an acute eflect of IL-4 consistent with lymphocyte trafficking out of the vascular space, the induction of VCAM-l expression on vascular endothelium and increases in plasma levels of MCP-1 in vivo. We suggest that IL-4 may be involved in the early recruitment of mononuclear cells to sites of tissue inflammation by the upregulation of VCAM-l expression on vascular endothelium and the generation and release of potent chemoattractants.