Cell density regulates neutrophil IL-8 synthesis: Role of IL-1 receptor antagonist and soluble TNF receptors

Although cytokine synthesis in polymorphonuclear leukocytes (PMN) was shown to be modulated by soluble mediators, the impact of microenvironmental con ditions has not been elucidated. In this study, we investigated the effect of cell density on cytokine release from human neutrophils. PMN were cultur ed at various cell densities (10 x 10(6) PMN/ml; 60 x 10(6) PMN/ml), and LP S-induced release of cytokines was quantified by ELISA technique. Upon an i ncrease in PLAIN density, secretion of the CXC chemokine IL-8 was progressi vely reduced. This effect was paralleled by a decrease in IL-8 mRNA. In con trast, TNF-alpha and IL-1 beta rose proportionally with increasing cell den sity. The inhibition of IL-8 secretion was reproduced by conditioned media of PMN at high cell density, but was not affected by blocking beta (2) inte grin-dependent adhesion. When analyzing the supernatant of LPS-challenged n eutrophils, large amounts of soluble TNFRs p55 and p75 (sTNFRI, sTNFRII), a nd IL-1R antagonist (IL-IRA), rising constantly with the cell density, were detected. Interestingly, combined blocking of the bioactivities of these m ediators completely restored neutrophil IL-8 secretion at high cell densiti es, with the anti-IL-1RA Ab being the more potent agent. Moreover, combined application of exogenous IL-IRA and sTNFRs to 10 x 10(6) PMN/ml reproduced the suppression of IL-8 generation. We conclude that neutrophil IL-8 synth esis is autoregulated, being suppressed under conditions of high cell densi ty. IL-IRA and sTNFRs, accumulating under these circumstances, seem to be c entrally involved in this regulatory mechanism by interfering with the IL-1 beta- and TNF-alpha -dependent IL-8 generation. This feedback mechanism ma y control further neutrophil recruitment and activation in a neutrophil-ric h environment, thereby preventing tissue destruction.