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The effects of monocytes on the transendothelial migration of T lymphocytes

In vivo cell‐mediated immune reactions are characterized by mixtures of monocytes and T cells. The purpose of this study was to investigate the role of monocytes on T‐cell migration and induction of endothelial adhesion molecules. The in vitro model consisted of adding peripheral blood mononuclear cells (PBMC), T cells or mixtures of monocytes and T cells, to endothelial cells on a porous membrane and using flow cytometry to distinguish between the monocyte and lymphocyte components. PBMC and PBMC supernatants were highly potent at upregulating intercellular adhesion molecule‐1 (ICAM‐1) and inducing expression of vascular cell adhesion molecule‐1 (VCAM‐1) and E‐selectin. Induction by supernatants was inhibited by antibodies to tumour necrosis factor‐α (TNF‐α) and interleukin‐1 (IL)‐1β. Using monocyte‐enriched populations, as few as one monocyte to 100 endothelial cells was sufficient to upregulate adhesion molecules. Fixed monocytes also induced adhesion molecules and expressed surface‐bound cytokines. In contrast, highly purified unstimulated T cells were not found to induce adhesion molecules at 4, 6, 24 or 48 hr of coculture. Purified T cells showed low‐level migration through resting (VCAM‐1 negative) endothelium, which was approximately doubled by addition of small numbers of monocytes or TNF‐α. In conclusion, monocytes, via cell surface or released cytokines play an essential role in allowing large‐scale recruitment of T cells to inflammatory sites in vivo.