No abstract

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.

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.

Toxoplasma gondii is an obligate intracellular parasite of clinical importance, especially in immunocompromised patients. Investigations into the immune response to the parasite found that T cells are the primary effector cells regulating gamma interferon (IFN-␥)-mediated host resistance. However, recent studies have revealed a critical role for the innate immune system in mediating host defense independently of the T cell responses to the parasite. This body of knowledge is put into perspective by the unifying theme that immunity to the protozoan parasite requires a strong IFN-␥ host response. In the following review, we discuss the role of IFN-␥-producing cells and the signals that regulate IFN-␥ production during T. gondii infection.

Toll-like receptor (TLR) activation relies on biochemical recognition of microbial molecules and localization of the TLR within specific cellular compartments. Cell surface TLRs largely recognize bacterial membrane components, and intracellular TLRs are exclusively involved in sensing nucleic acids. Here we show that TLR11, an innate sensor for the Toxoplasma protein profilin, is an intracellular receptor that resides in the endoplasmic reticulum. The 12 membrane-spanning endoplasmic reticulum-resident protein UNC93B1 interacts directly with TLR11 and regulates the activation of dendritic cells in response to Toxoplasma gondii profilin and parasitic infection in vivo. A deficiency in functional UNC93B1 protein abolished TLR11-dependent IL-12 secretion by dendritic cells, attenuated Th1 responses against T. gondii, and dramatically enhanced susceptibility to the parasite. Our results reveal that the association with UNC93B1 and the intracellular localization of TLRs are not unique features of nucleic acid-sensing TLRs but is also essential for TLR11-dependent recognition of T. gondii profilin and for host protection against this parasite. Toll-like receptors (TLRs)2 are a family of type I transmembrane proteins with ectodomains containing leucine-rich repeats that are involved in sensing varied microbial products, including lipids, peptidoglycans, proteins, and nucleic acids (1). TLR activation relies on the ability to sense molecules that are unique to microorganisms (2-4). TLRs involved in sensing bacterial membrane components such as LPS and lipoproteins are expressed and function on the cell surface (5, 6). Host and viral nucleic acids share an additional mechanism for self/nonself discrimination that relies on the localization of nucleic acid-recognizing TLR3, TLR7, and TLR9 within endosomal compartments (7-9). The intracellular localization of these TLRs is important not only for the recognition of viral DNA and RNA but also for the prevention of activation by host-derived nucleic acids (10 -13). Importantly, although the intracellular localization of TLRs can be achieved by distinct targeting sequences, all nucleic acid-recognizing receptors access their ligands in the same intracellular location (14 -16). It has been demonstrated that nucleotide-recognizing TLRs reside in the ER prior to stimulation, and a recently identified protein, UNC93B1, plays a major role in regulating the activation of these TLRs (17, 18). The missense mutation H412R in the UNC93B1 protein completely abolishes the signaling initiated by TLR3, TLR7, and TLR9 (17). Concomitantly, the functions of TLR2, TLR4, and TLR5, which are involved in sensing bacterial membrane and protein components, are not impaired in the absence of functional UNC93B1 protein (17). These data strongly support a surface localization of these "bacterial" innate immune receptors (7). Based on these observations, it has been postulated that nucleic acid-recognizing TLRs are uniquely positioned within intracellular compartments, whereas other TLRs involved in sen...