MULTIPLE RECEPTORS ON HUMAN MONOCYTES ARE INVOLVED IN ADHESION TO CULTURED HUMAN ENDOTHELIAL-CELLS

Monocytes exhibit significant basal (unstimulated) adherence to human umbilical vein endothelium (HUVE), which is only partially inhibited by an anti-CD18 monoclonal antibody (mAb) (60.3). We examined factors modulating the residual, CD18-independent monocyte binding to HUVE by pretreating monocytes with mAb 60.3 to eliminate CD18-dependent binding. Basal adherence was reduced from 32% ± 2% to 14% ± 2% with mAb 60.3 (means ± SE of eight experiments; P < 0.01). mAb 60.3-treated monocytes were incubated with tumor necrosis factor-γ (TNF-α), interleukin-1 (IL-1), lipopolysaccharide (LPS), N-formylmethionyl-leucyl-phenylalanine (FMLP), or phorbol myristate acetate (PMA). Only PMA affected CD18-independent binding. Pretreatment with PMA alone reduced adherence to 21% ± 2% (mean ± SE of eight experiments; P < 0.01). In conjunction with mAb 60.3, PMA virtually eliminated monocyte adherence to HUVE (7% ± 1%, mean ± SE of eight experiments; P < 0.01). We also examined CD18-independent monocyte binding to endothelial-leukocyte adhesion molecules (E-LAMs) induced by pretreatment of HUVE with LPS. Monoclonal antibody 60.3-treated monocytes increased adherence from 14% ± 2% with unstimulated HUVE to 37% ± 2% with LPS-stimulated HUVE (mean ± SE of four experiments; P < 0.01). Monocytes pretreated with both mAb 60.3 and PMA increased adherence from 5% ± 1% with the unstimulated HUVE to 18% ± 1% with the LPS-stimulated HUVE (mean ± SE of four experiments; P < 0.01). This result implies the presence of a CD18-independent and PMA-insensitive receptor on human monocytes for an E-LAM induced by LPS. In summary, we have identified two CD18-independent mechanisms of monocyte adherence to HUVE; a PMA-sensitive mechanism mediating basal adherence and a PMA-insensitive mechanism involved in binding to E-LAMs.