Secondary necrotic neutrophils release interleukin-16C and macrophage migration inhibitory factor from stores in the cytosol

• Published in: Cell death discovery 2015-11, Vol.1 (1), p.15056-15056, Article 15056
• Main Authors: Roth, S, Agthe, M, Eickhoff, S, Möller, S, Karsten, C M, Borregaard, N, Solbach, W, Laskay, T
• Format: Article
• Language: English
• Subjects: 631/250/127; 631/250/1932; Apoptosis; Biochemistry; Biomedical and Life Sciences; Cell Biology; Cell Cycle Analysis; Life Sciences; Stem Cells
• ISSN: 2058-7716; 2058-7716
• DOI: 10.1038/cddiscovery.2015.56

Neutrophils harbor a number of preformed effector proteins that allow for immediate antimicrobial functions without the need for time-consuming de novo synthesis. Evidence indicates that neutrophils also contain preformed cytokines, including interleukin (IL)-1ra, CXCL8 and CXCL2. In the search for additional preformed cytokines, a cytokine array analysis identified IL-16 and macrophage migration inhibitory factor (MIF) as preformed cytokines in lysates from human primary neutrophils. Both IL-16 and MIF are unconventional cytokines because they lack a signal sequence. Using confocal immunofluorescence microscopy as well as western blot analysis of subcellular fractions, IL-16 and MIF were found to be stored in the cytosol rather than in the granules of human neutrophils, which implies an unconventional secretion mechanism for both cytokines. IL-16 is synthesized and stored as a precursor (pre-IL-16). We present evidence that the processing of pre-IL-16 to the biologically active IL-16C is mediated by caspase-3 and occurs during both spontaneous and UV-induced apoptosis of human neutrophils. Although IL-16 processing occurs during apoptosis, IL-16C and MIF release was observed only during secondary necrosis of neutrophils. Screening a panel of microbial substances and proinflammatory cytokines did not identify a stimulus that induced the release of IL-16C and MIF independent of secondary necrosis. The data presented here suggest that IL-16 and MIF are neutrophil-derived inflammatory mediators released under conditions of insufficient clearance of apoptotic neutrophils, as typically occurs at sites of infection and autoimmunity.