Interferon-activated neutrophils store a TNF-related apoptosis-inducing ligand (TRAIL/Apo-2 ligand) intracellular pool that is readily mobilizable following exposure to proinflammatory mediators

Neutrophils are versatile cells, which play a role, not only in inflammatory processes but also in immune and antitumoral responses. Recently, we have reported that interferon (IFN)-activated neutro- phils are able to release biologically active tumor necrosis factor (TNF)-related apoptosis-inducing li- gand (TRAIL/APO2 ligand), a molecule exerting se- lective, apoptotic activities toward tumor and virus- infected cells, as well as immunoregulatory functions on activated T lymphocytes. Herein, we show that only a minor fraction of the total TRAIL, newly syn- thesized by IFN-activated neutrophils within 24 h, is released outside, the rest being retained intracellu- larly, mainly in secretory vesicles and light mem- brane fractions. We demonstrate that the intracellu- lar pool of TRAIL present in IFN-pretreated neutro- phils is rapidly mobilizable to the cell surface and can be secreted following exposure to proinflammatory mediators such as TNF-, lipopolysaccharide, formyl- methionyl-leucyl-phenylalanine, CXC chemokine li- gand 8/interleukin-8, insoluble immunocomplexes, and heat shock protein Gp96. These various proin- flammatory agonists functioned as effective secreta- gogue molecules only, in that they failed to augment TRAIL mRNA expression or TRAIL de novo synthe- sis in freshly isolated neutrophils or cultured with or without IFN. In addition, supernatants from IFN- treated neutrophils stimulated with proinflammatory mediators induced the apoptosis of target cells more effectively than supernatants from neutrophils acti- vated with IFNs alone. Collectively, our results un- cover a novel mechanism, whereby the release of soluble TRAIL by neutrophils can be greatly ampli- fied and further reinforce the notion that neutrophils are important cells in tumor surveillance and immunomodulation. J. Leukoc. Biol. 79: 123-132; 2006.