Strikingly, in these mice tumor burden was strongly reduced when compared to wild-type or p40−/−controls, arguing for a pro-tumorigenic role for IL-23, which was ascribed Ferroptosis assay to a reduced

infiltration of cytotoxic CD8+ T cells into the tumor. Given the prominent function of IL-23 during the differentiation of Th17 cells, many researchers focused on the role of Th17 cells in tumor development, but contradictory results have been reported. While several groups attributed increased tumor-killing activity to Th17 cells in both subcutaneous and metastatic mouse melanoma models [103, 104], others have reported the opposite: in a transgenic model of spontaneous intestinal tumorigenesis, the lack of IL-17 abrogated tumor progression [105], and some metastatic melanoma models argue for a pro-tumorigenic function of IL-17 [106], which would fit the data obtained with p19−/−knockouts.

The general consensus seems to argue for tumor-promoting functions of both IL-23 and IL-17, if anything, but further work is needed to clarify their precise roles in anti-tumor immunity. Of note, the presence of GM-CSF has been shown to be beneficial in vaccination approaches during subcutaneous tumor growth [107]. Given that GM-CSF can be expressed check details in an IL-23-dependent fashion by CD4+ T cells, this might be another potential mechanism by which IL-23 can modulate tumor immunosurveillance. Molecular motor The seemingly ubiquitous presence of IL-23 in inflammatory autoimmune disease models and its importance for the associated pathogenesis has significantly elevated the status of this cytokine. IL-23 has undoubtedly risen to prominence because of its unique ability to transform an activated T cell into an encephalitogenic, pro-inflammatory, and potentially self-harming effector cell. Indeed, IL-23 is perhaps the closest immunologists have come to identifying the “”magic bullet”" responsible for autoimmune disorders. This observation has already been translated into a successful clinical application, at least in the treatment of psoriasis. On the other

hand, the initial model of IL-23 only being implicated in the generation of Th17 cells has proven exceedingly (over) simplified. Not only does IL-23 induces a pathogenic T-cell program involving effector cytokines beyond the IL-17 family, but it also acts on additional innate cell types such as γδ T cells and ILCs. Furthermore, the regulation of IL-23 expression itself remains incompletely understood. As the complex network of IL-23-initiated cellular activity becomes more detailed, we will no doubt uncover more features of this cytokine governing the transition from antigen-specificity to auto-aggression. A.L.C. was supported by the EMBO long-term Fellowship ALTF-508–2011, and A.L.C. and F.M. by the Forschungskredit of the University of Zürich. B.B.