Induction of IL-12p40 and type 1 immunity by Toxoplasma gondii in the absence of the TLR-MyD88 signaling cascade.

Lindsay M Snyder,Claire M Doherty,Eric Y Denkers,Heather L Mercer,George S Yap

doi:10.1371/journal.ppat.1009970

Abstract

Toxoplasma gondii is an orally acquired pathogen that induces strong IFN-γ based immunity conferring protection but that can also be the cause of immunopathology. The response in mice is driven in part by well-characterized MyD88-dependent signaling pathways. Here we focus on induction of less well understood immune responses that do not involve this Toll-like receptor (TLR)/IL-1 family receptor adaptor molecule, in particular as they occur in the intestinal mucosa. Using eYFP-IL-12p40 reporter mice on an MyD88-/- background, we identified dendritic cells, macrophages, and neutrophils as cellular sources of MyD88-independent IL-12 after peroral T. gondii infection. Infection-induced IL-12 was lower in the absence of MyD88, but was still clearly above noninfected levels. Overall, this carried through to the IFN-γ response, which while generally decreased was still remarkably robust in the absence of MyD88. In the latter mice, IL-12 was strictly required to induce type I immunity. Type 1 and type 3 innate lymphoid cells (ILC), CD4+ T cells, and CD8+ T cells each contributed to the IFN-γ pool. We report that ILC3 were expanded in infected MyD88-/- mice relative to their MyD88+/+ counterparts, suggesting a compensatory response triggered by loss of MyD88. Furthermore, bacterial flagellin and Toxoplasma specific CD4+ T cell populations in the lamina propria expanded in response to infection in both WT and KO mice. Finally, we show that My88-independent IL-12 and T cell mediated IFN-γ production require the presence of the intestinal microbiota. Our results identify MyD88-independent intestinal immune pathways induced by T. gondii including myeloid cell derived IL-12 production, downstream type I immunity and IFN-γ production by ILC1, ILC3, and T lymphocytes. Collectively, our data reveal an underlying network of immune responses that do not involve signaling through MyD88.

Highlights

Toxoplasma gondii is a widely distributed apicomplexan parasite estimated to be present in 30–50% of the world population, and is a leading cause of foodborne illness-related deaths in the United States [1]
We found that microbiota-specific and T. gondii-specific CD4+ T cells are generated in the lamina propria (LP) regardless of MyD88 expression
T. gondii-induced IL-12 production was similar between mesenteric lymph node (MLN) and LP cells (Fig 2F and S6F Fig); we note that eYFP-IL-12 flow staining, in particular for macrophages, was more robust in the MLN. These results demonstrate that peroral T. gondii infection induces MyD88-independent mucosal immunity and measurable IL-12 production by dendritic cells (DC), neutrophils, and macrophages

Summary

Introduction

Toxoplasma gondii is a widely distributed apicomplexan parasite estimated to be present in 30–50% of the world population, and is a leading cause of foodborne illness-related deaths in the United States [1]. It was long ago established that key to initiating the immune response to T. gondii is early production of IL-12 [8,9]. IL-12 drives induction of NK and TH1 cells that fuel protective immunity through production of IFN-γ [13]. Other cells such as neutrophils and innate lymphoid cells supply IFN-γ during infection [14,15]. While a strong Type 1 cytokine response is required for the host to survive infection, severe immunopathology involving proinflammatory cytokine overproduction, intestinal tissue destruction and bacterial translocation can result in the absence of regulation by cytokines such as IL-10 [18,19,20,21]

Methods

Results

Conclusion