Unraveling the role of IL-36γ in host susceptibility during Toxoplasma gondii infection

The IL-36 cytokines (IL-36α, IL-36β, IL-36γ) are an emerging family of interest that play a multifaceted immune regulatory role in various diseases and infections. Each of the three cytokines bind to the same IL-36R receptor and are therefore thought to have similar biological function. In some contexts, IL-36 plays a proinflammatory role, while in others it promotes an anti-inflammatory effect. Here, we aim to define how IL-36 influences susceptibility and establishment of immunity in the small intestine and mesenteric lymph nodes following oral infection with ME49 Toxoplasma cysts. We first examined expression of these cytokines in intestinal epithelial cells and intestinal epithelial organoids where we found strong upregulation of IL-36γ, but not IL-36α or IL-36β, in response to T. gondii infection. Accordingly, we focused our attention on IL-36γ for subsequent investigation. Survival studies using IL-36γ^+/+ and IL-36γ^-/- mice indicated that the knockout strain displays greater susceptibility, as measured by elevated parasite burden in the small intestine ileum and decreased survival following oral inoculation. As a surrogate indicator of inflammation, we measured lipocalin-2 levels in fecal pellets, ileum contents, and in supernatants of cultured lamina propria cells. We found that IL-36γ^-/- mice have higher levels of lipocalin at steady state than IL-36γ^+/+, and levels were further heightened after cyst inoculation. During acute infection, increased IFN-γ production was found in both the lamina propria and mesenteric lymph nodes of IL-36γ^-/- relative to wild-type mice, with no marked difference in IL-12 production between the two strains. Thus, we are now investigating sources of IFN-γ in both compartments. IL-36γ^-/- mice display no difference in frequency or IFN-γ production by T cells in either compartment. While innate lymphoid cell frequency is increased in the lamina propria of IL-36γ^-/- mice, IFN-γ expression appears to be independent of IL-36γ. Ongoing investigations are assessing other potential IFN-γ sources including natural killer cells and neutrophils. Collectively we report that IL-36γ influences host susceptibility to Toxoplasma infection.