Bacterial lipopolysaccharide acts on human endothelial cells to enhance the adherence of peripheral blood monocytes.

The effects of bacterial lipopolysaccharide on the adherence of human peripheral blood monocytes (M0) to endothelial cells (ECs) were investigated in a quantitative adherence assay as an in vitro model of M0-EC interactions. ECs exposed for 2 hours or longer to 0.10 to 10 micrograms/ml lipopolysaccharide demonstrated significant increases in the levels of M0 adherence compared to control cells. By contrast, lipopolysaccharide added directly to adherence assays or preincubated with M0 did not increase M0 adherence to ECs. The effects of interleukin-1 (IL-1) were similar to the effects of lipopolysaccharide in that they acted solely on ECs to enhance M0 adherence and accelerated the rate of EC adherence to M0. Lipopolysaccharide did not increase EC adherence of M0 by causing damage or extracellular release of soluble mediators. The increase in M0-EC adherence by the chemotactic peptide formyl-methionyl-leucylphenylalanine (FMLP) was quite different from that by lipopolysaccharide and IL-1. FMLP rapidly induced M0 to become more adherent to ECs and plastic surfaces but was unable to act on ECs and enhance M0 binding. Thus M0-EC interactions are enhanced by the direct effects of FMLP on M0, whereas the actions of lipopolysaccharide and IL-1 are entirely focused on ECs and can be distinguished by differing characteristics and cellular targets.