Role of the 75-kDa TNF receptor in TNF-induced activation of neutrophil respiratory burst

The exclusive role of the 55-kDa TNF receptor (TNF-R55) as the signaling re ceptor in TNF-induced activation of respiratory burst by human polymorphonu clear leukocytes residing on biologic surfaces has been inferred from resul ts obtained with receptor-specific monoclonal and polyclonal Abs. In this w ork, we confirm this assumption by a more direct approach, i.e., by using r eceptor-specific TNF mutants (p55TNF and p75TNF) and, as a novel contributi on, we show that cooperation of the 75-kDa TNF receptor (TNF-R75) is requir ed for a full blown response to the cytokine, This conclusion stems from th ree sets of data: 1) none of the TNF-R55-specific agonists used, i.e., mAbs or p55TNF, induced a respiratory burst comparable with that induced by TNF ; 2) selective down-modulation of TNF-R75 resulted in a diminished response to TNF but not to TNF-R55-specific agonists or to the chemotactic peptide FMLP; and 3) mAbs that either block or stabilize binding of TNF to TNF-R75 inhibited the response to the cytokine, suggesting that cooperation require s not only TNF binding to the receptor but also an appropriate dissociabili ty from it. The inhibitory effect of the Abs increased as the cytokine conc entrations decreased, indicating that cooperation by TNF-R75 becomes more r elevant at low TNF doses. Such a cooperation does not seem to rely on the a ctivation of a TNF-R75-linked signaling pathway independent of TNF-R55, sin ce the response to p55TNF and p75TNF given in combination was not higher th an the response to p55TNF alone. The possible mechanisms of cooperation are discussed.