TNF-α Released by Comigrating Monocytes Promotes Transendothelial Migration of Activated Lymphocytes

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Abstract
We investigated mechanisms that increase motility and transendothelial trafficking of activated lymphocytes. Freshly isolated lymphocytes stimulated with immobilized anti-CD3 for 2 h migrate into polymerized collagen in 1.99 ± 0.25-fold greater numbers and across confluent endothelial monolayers in 4.8 ± 0.5-fold greater numbers compared with leukocytes incubated with nonspecific IgG. Activated lymphocytes form clusters with monocytes, and their increased motility was dependent on the presence of comigrating monocytes. Five lines of evidence support the idea that monocytes modulate lymphocyte motility through the release of TNF-α: 1) flow-cytometric analyses, using highly specific and avid mAbs to probe permeabilized whole blood leukocytes, showed that >80% of circulating monocytes contain intracellular TNF-α, whereas <5% contain IL-1 and none contain IL-6; 2) stimulation with immobilized anti-CD3 that was intended to activate lymphocytes also induced monocytes to release increased quantities of TNF-α; 3) rTNF-α, added in doses of 1 to 20 pg/ml to purified anti-CD3-stimulated lymphocytes, reproduced, in a dose-dependent manner, the motility-enhancing effect of adding monocytes; 4) the transient increase in the expression of TNF R-I on CD3-activated T lymphocytes parallels their transiently increased motility; and 5) addition of anti-TNF-α, anti-TNF R-I, anti-TNF R-II, or soluble TNF R-I decreased the motility of stimulated lymphocytes. These results suggest that T lymphocyte stimulation via the CD3-TCR complex signals nearby monocytes to release TNF-α, which feeds back on the lymphocytes to increase their locomotor activity.

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