Phagocytes are attracted to apoptotic cells by “eat me” signals present on the surface or secreted from dying cells. Cullen et al. report that activation of the tumor necrosis factor receptor (TNFR) family member Fas, in addition to inducing apoptosis, leads to production of chemotactic cytokines that help phagocytes find apoptotic cells. As well as triggering apoptosis, activation of Fas with an anti-Fas antibody prompted several types of mammalian primary cells and cell lines to secrete various proinflammatory molecules, including the chemokines interleukin-8 (IL-8) and monocyte chemoattractant protein-1 (MCP-1). Treating HeLa cells with a broad-spectrum caspase inhibitor coadministered with anti-Fas prevented apoptosis but did not prevent the secretion of proinflammatory cytokines. In contrast, small interfering RNA (siRNA)–mediated knockdown of the inhibitor of apoptosis (IAP) proteins cIAP-1 and cIAP-2 increased the proportion of cells that initiated apoptosis in response to low concentrations of anti-Fas but inhibited proinflammatory cytokine production. The kinase RIPK1 and the transcription factor nuclear factor κB (NF-κB), both of which participate in TNFR signaling responses, were also required for maximal Fas-induced cytokine secretion. Coimmunoprecipitation experiments indicated that Fas activation induced the formation of complexes containing RIPK1, caspase-8, and the adaptor protein Fas-associated death domain (FADD) that stimulated NF-κB–mediated cytokine production. When added to the lower well of a standard chemotaxis chamber, conditioned medium from Fas-stimulated HeLa cells induced monocytes in the upper well to migrate toward the filter separating the two wells. Immunodepletion of Fas-stimulated HeLa cell culture supernatant indicated that MCP-1 was primarily responsible for attracting monocytes, with IL-8 playing a minor role, whereas IL-8 was required for the chemotaxis of primary human neutrophils. Treating HeLa cells with an IAP inhibitor or siRNAs targeting RIPK1 in combination with anti-Fas reduced the ability of the conditioned medium to promote monocyte chemotaxis. Thus, proapoptotic signaling through Fas can stimulate cells to release chemokines that attract phagocytes.
S. P. Cullen, C. M. Henry, C. J. Kearney, S. E. Logue, M. Feoktistova, G. A. Tynan, E. C. Lavelle, M. Leverkus, S. J. Martin, Fas/CD95-induced chemokines can serve as “find-me” signals for apoptotic cells. Mol. Cell 49, 1034–1048 (2013). [PubMed]