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Divergent antiviral roles of amphibian (Xenopus laevis) macrophages elicited by colony‐stimulating factor‐1 and interleukin‐34

Macrophages are integral to amphibian immunity against RVs, as well as to the infection strategies of these pathogens. Although CSF‐1 was considered to be the principal mediator of macrophage development, the IL‐34 cytokine, which shares no sequence identity with CSF‐1, is now believed to contribute to vertebrate monopoiesis. However, the respective roles of CSF‐1‐ and IL‐34‐derived macrophages are still poorly understood. To delineate the contribution of these macrophage populations to amphibian immunity against the RV FV3, we identified the Xenopus laevis IL‐34 and transcriptionally and functionally compared this cytokine with the previously identified X. laevis CSF‐1. The X. laevis CSF‐1 and IL‐34 displayed strikingly nonoverlapping developmental and tissue‐specific gene‐expression patterns. Furthermore, only CSF‐1 but not IL‐34 was up‐regulated in the kidneys of FV3‐challenged tadpoles. Intriguingly, recombinant forms of these cytokines (rXlCSF‐1, rXlIL‐34) elicited morphologically distinct tadpole macrophages, and whereas rXlCSF‐1 pretreatment decreased the survival of FV3‐infected tadpoles, rXlIL‐34 administration significantly prolonged FV3‐challenged animal survival. Compared with rXlIL‐34‐elicited macrophages, macrophages derived by rXlCSF‐1 were more phagocytic but also significantly more susceptible to in vitro FV3 infections. By contrast, rXlIL‐34‐derived macrophages exhibited significantly greater in vitro antiranaviral activity and displayed substantially more robust gene expression of the NADPH oxidase components (p67phox, gp91phox) and type I IFN. Moreover, FV3‐challenged, rXlIL‐34‐derived macrophages exhibited several orders of magnitude greater up‐regulation of the type I IFN gene expression. This marks the first report of the disparate roles of CSF‐1 and IL‐34 in vertebrate antiviral immunity.