Abstract

Polymorphonuclear leukocytes (PMN) residing on biologic surfaces respond with a vigorous respiratory burst when exposed to tumor necrosis factor alpha (TNF). PMN possess both the p55 and the p75 TNF receptors, but their role in the elicitation of the respiratory burst is not known. We addressed this problem by studying the effect of monoclonal antibodies (MoAbs) directed against the p55 TNF receptor (MoAb H398 and MoAb htr-9) and the p75 TNF receptor (MoAb utr-1) on TNF- induced production of O2- by PMN residing on fibronectin-coated surfaces. Neither the anti-p55 nor the anti-p75 MoAbs affected TNF- induced O2- production despite their known ability to competitively inhibit TNF binding to the corresponding receptor. Experiments with the antibodies alone showed that the anti-p55 MoAbs directly triggered PMN O2- production, whereas no response was elicited by the anti-p75 MoAb. PMN unresponsiveness to the anti-p75 MoAb could not be ascribed to low expression of p75 receptor, because binding of the anti-p75 MoAb utr-1 to PMN was, indeed, even higher than binding of the anti-p55 MoAb htr- 9. The agonistic activity of the anti-p55 MoAbs was comparable with that of TNF and was not or only minimally modified by the simultaneous presence of TNF. Triggering of the respiratory burst by TNF was completely prevented by Fab fragments of the anti-p55 MoAb H398. Moreover, the monovalent Fab fragments, which lacked any stimulatory effect on PMN O2- production, acquired strong agonistic activity on cross-linking with anti Fab antibodies, suggesting that the ability of the anti-p55 antibodies to stimulate PMN O2- production depends on their ability to cross-link the TNF receptors. The agonistic effect of the anti-p55 MoAbs was only observed with cells residing on fibronectin- coated surfaces and not with cells in suspension, and in terms of kinetics, dependence on beta 2 integrin-mediated adherence, microfilament integrity, and sensitivity to elevations of intracellular levels of cAMP, it was virtually indistinguishable from the agonistic effect of TNF. Taken together, these results suggest that the p55 receptor is responsible for TNF-induced triggering of the respiratory burst of PMN residing on biologic surfaces.

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