Activation of Toll-like receptors (TLRs) by pathogens triggers cytokine production and T cell activation, immune defense mechanisms that are linked to immunopathology. Here we show that IFN-γ production by CD4+ TH1 cells during mucosal responses to the protozoan parasite Toxoplasma gondii results in dysbiosis and the elimination of Paneth cells. Paneth cell death led to loss of antimicrobial peptides and occurred in conjunction with uncontrolled expansion of the Enterobacteriaceae family of Gram-negative bacteria. The expanded intestinal bacteria were required for the parasite-induced intestinal pathology. The investigation of cell type-specific factors regulating TH1 polarization during T. gondii infection identified the T cell intrinsic TLR pathway as a major regulator of IFN-γ production in CD4+ T cells responsible for Paneth cell death, dysbiosis and intestinal immunopathology.

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IFN-γ is a major cytokine that is critical for host resistance to a broad range of intracellular pathogens. Production of IFN-γ by natural killer and T cells is initiated by the recognition of pathogens by Toll-like receptors (TLRs). In an experimental model of toxoplasmosis, we have identified the presence of a nonlymphoid source of IFN-γ that was particularly evident in the absence of TLR-mediated recognition of Toxoplasma gondii . Genetically altered mice lacking all lymphoid cells due to deficiencies in Recombination Activating Gene 2 and IL-2Rγ c genes also produced IFN-γ in response to the protozoan parasite. Flow-cytometry and morphological examinations of non-NK/non-T IFN-γ + cells identified neutrophils as the cell type capable of producing IFN-γ. Selective elimination of neutrophils in TLR11 −/− mice infected with the parasite resulted in acute susceptibility similar to that observed in IFN-γ–deficient mice. Similarly, Salmonella typhimurium infection of TLR-deficient mice induces the appearance of IFN-γ + neutrophils. Thus, neutrophils are a crucial source for IFN-γ that is required for TLR-independent host protection against intracellular pathogens.

Toll-like receptors play a central role in the innate recognition of pathogens and the activation of dendritic cells (DCs). Here, we established that, in addition to TLR11, TLR12 recognizes the profilin protein of the protozoan parasite Toxoplasma gondii and regulates IL-12 production by DCs in response to the parasite. Similar to TLR11, TLR12 is an endolysosomal innate immune receptor that colocalizes and interacts with UNC93B1. Biochemical experiments revealed that TLR11 and TLR12 directly bind to T. gondii profilin and are capable of forming a heterodimer complex. We also established that the transcription factor IRF8, not NF-kB, plays a central role in the regulation of the TLR11- and TLR12-dependent IL-12 response of DCs. These results suggest a central role for IRF8-expressing CD8+ DCs in governing the TLR11- and TLR12-mediated host defense against T. gondii.

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BACKGROUND & AIMS Activation of the transcription factor NFκB has been associated with development of inflammatory bowel disease (IBD). COMMD1, a regulator of various transport pathways, has been shown to limit NFκB activation. We investigated the roles of COMMD1 in the pathogenesis of colitis in mice and IBD in humans. METHODS We created mice with specific disruption of Commd1 in myeloid cells (Mye-K/O mice); we analyzed immune cell populations and functions and expression of genes regulated by NFκB. Sepsis was induced in Mye-K/O and wild-type mice by cecal ligation and puncture or intraperitoneal injection of lipopolysaccharide (LPS), colitis was induced by administration of dextran sodium sulfate (DSS), and colitis-associated cancer was induced by administration of DSS and azoxymethane. We measured levels of COMMD1 mRNA in colon biopsies from 29 patients with IBD and 16 patients without (controls), and validated findings in an independent cohort (17 patients with IBD and 22 controls). We searched for polymorphisms in or near COMMD1 that were associated with IBD using data from the International IBD Genetics Consortium and performed quantitative trait locus analysis. RESULTS In comparing gene expression patterns between myeloid cells from Mye-K/O and wild-type mice, we found that COMMD1 represses expression of genes induced by LPS. Mye-K/O mice had more intense inflammatory responses to LPS and developed more severe sepsis and colitis, with greater mortality. More Mye-K/O mice with colitis developed colon dysplasia and tumors than wild-type mice. We observed reduced expression of COMMD1 in colon biopsies and circulating leukocytes from patients with IBD. We associated single nucleotide variants near COMMD1 with reduced expression of the gene and linked them with increased risk for ulcerative colitis. CONCLUSIONS Expression of COMMD1 by myeloid cells has anti-inflammatory effects. Reduced expression or function of COMMD1 could be involved in the pathogenesis of IBD.

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Neutrophils are an emerging cellular source of IFN-γ, a key cytokine that mediates host defense to intracellular pathogens. Production of IFN-γ by neutrophils, in contrast to lymphoid cells, is Toll-like receptor (TLR) and IL-12-independent and the events associated with IFN-γ production by neutrophils are not understood. In this report, we show that mouse neutrophils express IFN-γ during their lineage development in the bone marrow niche at the promyelocyte stage independently of microbes. IFN-γ accumulates in primary neutrophilic granules and is released upon induction of degranulation. The developmental mechanism of IFN-γ production in neutrophils arms the innate immune cells prior to infection and assures the potential for rapid release of IFN-γ upon neutrophil activation, the first step during responses to many microbial infections.