Regulation of the expression of ICAM-1 on human monocytes and monocytic tumor cell lines.

In this study we investigated the mechanisms regulating expression of ICAM-1 that plays an important role for Ag presentation, on peripheral blood monocytes, and three myelomonocytic cell lines representing different stages of monocyte maturation. ICAM-1 expression on freshly isolated monocytes was low in all donors tested. After 16- to 20-h incubation, about a 20-fold increase was observed, whereas ICAM-1 expression on lymphocytes did not change. This marked increase in Ag expression was specific for ICAM-1, because expression of other monocyte Ag remained unchanged (alpha-chain of lymphocyte function-associated Ag-1) or decreased (ICAM-2, alpha-chains of CR3 and CR4, their common beta-chain and Fc gamma RI) during overnight incubation; only HLA-DR showed a slight increase. Enhanced expression of ICAM-1 was not caused by adherence; it was also not due to trace amounts of IFN-gamma possibly present under our culture conditions. Endotoxin contamination of the medium was excluded as a cause for the enhanced expression, and neither LPS nor its antagonist polymyxin B were able to alter ICAM-1 expression. Whole blood culture almost completely prevented up-regulation of ICAM-1 expression, thus apparently mimicking the in vivo situation. Culture of monocytes together with other PBMC had no significant influence on monocyte ICAM-1 levels. From 12 different cytokines tested for their ability to modulate "spontaneous" up-regulation of ICAM-1 in culture, only IFN-gamma was found to increase expression about twofold. This effect was also observed in whole blood culture. All other cytokines had no significant effect. In contrast, ICAM-1 expression on two of the three cell lines (KG1 and HL60) was inducible by TNF-alpha to a much higher degree than by IFN-gamma, whereas the results with U937 were comparable with those obtained for normal monocytes. The protein- and RNA-synthesis inhibitors cycloheximide and actinomycin D prevented expression of ICAM-1 in a dose-dependent fashion. In Northern blot experiments no difference in the amounts of mRNA was observed between freshly isolated and cultured monocytes, indicating that "spontaneous" induction of ICAM-1 is regulated at a posttranscriptional level. In contrast, stimulation with IFN-gamma caused a significant increase of detectable RNA comparable to that observed for Ag expression. These results disclose a very special regulation of ICAM-1 expression on monocytes which could be consistent with the view that ICAM-1 is actively down-regulated in vivo until the monocyte leaves the circulation.