Endothelial Cells Inhibit Receptor-mediated Superoxide Anion Production by Human Polymorphonuclear Leukocytes via a Soluble Inhibitor

Abstract

Confluent monolayers of bovine pulmonary artery endothelial cells (BPAE) or human umbilical vein endothelial cells (HUVE) inhibited by 80 to 90% the production of O2- by added human neutrophils (PMNs) stimulated by plasma membrane receptor-mediated activators (formylmethionylleucylphenylalanine [fMLP], opsonized zymosan, heat-killed Staphylococci), but not by non-plasma membrane receptor-mediated activators (phorbol myristate acetate and delta-hexachlorocyclohexane). Degranulation induced by fMLP was also inhibited by BPAE. Inhibition was not affected by eicosatetraynoic acid (ETYA) or indomethacin. To assess the role of cell-cell contact, 0.45-microns-pore culture plate inserts were employed to prevent PMN-endothelial cell contact during incubation. A similar amount of inhibition of stimulated PMNs superoxide production was seen as compared to PMN-endothelial incubations where contact occurred. A soluble component released by BPAE monolayers, when added to PMNs, duplicated the inhibition seen by BPAE-PMN co-incubation. Incubation of BPAE with adenosine deaminase did not reduce inhibition of O2- production compared to controls without adenosine deaminase. There was no evidence of endothelial scavenging of O2- generated by hypoxanthine-xanthine oxidase, and inhibition of endothelial superoxide dismutase did not diminish the inhibitory effort. We conclude that cell contact is not required for BPAE inhibition of fMLP-stimulated O2- production by PMN, and that scavenging of superoxide anion is not the mechanism. The inhibitor appears to be a polypeptide with an apparent molecular weight between 1,000 and 10,000 D and does not appear to be adenosine, an arachidonate metabolite, or superoxide dismutase. The mechanism may involve down-regulation of plasma membrane receptor-mediated activation of PMNs.