Monocyte emigration into areas of inflammation is initiated by monocyte adherence to the microvascular endothelium which may be induced by the local production of chemotactic factors at the inflammatory site. However, it is not clear whether such stimuli act on the monocyte and/or the endothelial cell to promote this effect. Accordingly, the effect of the chemotactic peptides C5a des arg and formyl-methionyl-leucyl-phenylalanine (FMLP) on human monocyte adherence to human microvascular endothelial cell monolayers was investigated in vitro. Monocytes (92 to 98% pure) were isolated by discontinuous plasma-Percoll density gradients and cell elutriation, methods designed to minimize monocyte exposure to endotoxin. Mean spontaneous (unstimulated) adherence of 111Indium-tropolonate-radiolabeled monocytes to microvascular endothelial cell monolayers was 19.7% +/- 1.3. Monocyte adherence to microvascular endothelial cell monolayers was stimulated in a dose-response fashion in the presence of C5a des arg or FMLP to a maximum mean adherence of 47.2% +/- 2.9 or 43.8% +/- 2.2, respectively. C5a des arg or FMLP stimulated monocytes to adhere to monolayers of human vascular smooth muscle cells, human dermal fibroblasts, or serum-coated plastic wells in a comparable fashion as to endothelial cells. The simultaneous presence of both chemotactic peptides C5a des arg and FMLP in the assay system stimulated monocyte adherence to the same degree as either stimulus alone. This finding suggested that those monocytes stimulated to adhere by C5a des arg were the same subpopulation responding to FMLP. Spontaneous monocyte adherence (in the absence of chemotactic peptides) to both endothelial cell monolayers and serum-coated plastic wells was reduced in the presence of plasma, but chemotactic peptides induced a significant, albeit reduced, adhesion of monocytes in this circumstance. The pretreatment of monocytes with either C5a des arg or FMLP prior to the adherence assay induced stimulus-specific desensitization of monocyte adherence. Neither a desensitization nor stimulated monocyte adherence occurred when endothelial cell monolayers or serum-coated plastic wells were pretreated with either of the chemotactic peptides. The fixation of endothelial cell monolayers prior to the adherence assay did not alter the degree of spontaneous, C5a des arg-stimulated, or FMLP-stimulated monocyte adherence. These data suggest that the stimulated adhesion of monocytes to endothelial cells by C5a des arg or FMLP represents primarily an effect of these chemotactic peptides on the monocyte.

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