REGULATION OF NEUTROPHIL APOPTOSIS BY TUMOR-NECROSIS-FACTOR-ALPHA - REQUIREMENT FOR TNFR55 AND TNFR75 FOR INDUCTION OF APOPTOSIS IN-VITRO

Granulocyte apoptosis is an important mechanism underlying the removal of redundant neutrophils from an inflammatory focus. The ability of m any proinflammatory agents to impede this event suggests that such age nts act not only in a priming or secretagogue capacity but also increa se neutrophil longevity by delaying apoptosis, We have examined whethe r this hypothesis holds true for all neutrophil priming agents, in par ticular tumor necrosis factor-alpha (TNF-alpha), which has been variab ly reported to either induce, delay, or have no effect on neutrophil a poptosis. After 20 hours coincubation TNF-alpha inhibited neutrophil a poptosis; however, more detailed analysis demonstrated its ability to promote apoptosis in a subpopulation of cells at earlier (2 to 8 hours ) times. Formyl-Met-Leu-Phe, platelet-activating factor, inositol hexa kisphosphate, lipopolysaccharide, leukotriene B-4, and granulocyte-mac rophage colony-stimulating factor all inhibited apoptosis at 6 and 20 hours, The early proapoptotic effect of TNF-alpha was concentration-de pendent (EC50 2.8 ng/ mt), abolished by TNF-alpha neutralizing antibod y, and was not associated with any change in cell viability or recover y. Of relevance to the inflamed site, the ability of TNF-alpha to acce lerate apoptosis was lost if neutrophils were primed with 1 mu mol/L P AF or aged for 6 hours before TNF-alpha addition. The TNFR55-selective TNF-alpha mutants (E146K, R32W-S86T) induced neutrophil apoptosis but with a potency 14-fold lower than wild-type TNF-alpha, Although the T NFR75-selective mutant (D143F) did not induce apoptosis, blocking anti bodies to both receptor subtypes abolished TNF-alpha-stimulated apopto sis. Hence, TNF-alpha has the unique ability to induce apoptosis in hu man neutrophils via a mechanism where TNFR75 facilitates the dominant TNFR55 death effect, This may be an important mechanism controlling ne utrophil iongevity and clearance in vivo, (C) 1997 by The American Soc iety of Hematology.