UNC93B1 Is Essential for TLR11 Activation and IL-12-dependent Host Resistance to Toxoplasma gondii

Abstract

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...

TLR-independent neutrophil-derived IFN-γ is important for host resistance to intracellular pathogens

“…IL-12-independent functions of MyD88 are also supported by the inability of recombinant IL-12 to rescue T. gondii-infected MyD88 −/− mice (16). Although this treatment protects DC-specific MyD88-deficient mice as well as 3d mice with impaired functions in all endosomal TLRs (20), exogenous IL-12 fails to protect complete MyD88-deficient mice from T. gondii (16,21). In this study, we established that IL-1R-mediated MyD88 activation cooperates with TNF to regulate IL-12-independent IFN-γ.…”

“…IFN-γ production from NK and T cells is regulated by TLR11, UNC93B1, and TLR12-dependent induction of interleukin-12 (IL-12) (15,16,21,23). The appearance of IFN-γ + neutrophils in the absence of TLR11 or UNC93B1 raised the question of whether IL-12 plays a role in triggering a neutrophilderived IFN-γ response.…”

“…Nevertheless, depletion of NK cells in T. gondii-infected TLR11 −/− mice did not affect the survival of these mice, strongly suggesting that NK cells play little or no significant role in TLR11-independent IFN-γ-dependent host resistance to T. gondii. Furthermore, TLR11, as well as TLR12 inactivation, or combined deficiencies in TLR11, TLR12, TLR7, and TLR9 signaling pathways caused by mutation in UNC93B1, abolished IL-12 production in vivo and in vitro, but had a minor effect on the levels of IFN-γ observed in T. gondiiinfected mice (18,21). This unexpected disconnect between TLRmediated pathogen recognition and IFN-γ-dependent host resistance is not unique to T. gondii; it has been observed with other intracellular pathogens and is frequently interpreted as a functional redundancy among TLRs in the activation of NK or T cells.…”

Avirulent strains of Toxoplasma gondii infect macrophages by active invasion from the phagosome

“…The persistence of infection requires not only the acute induction of immune mechanisms that control parasite hyperproliferation but also efficient dissemination of parasites into deep tissues, where they establish a chronic niche. A concept that is now gaining acceptance is that induction of proinflammatory cytokines essential for in vivo control of acute Toxoplasma infection involves intracellular signaling from endosomal and cytoplasmic pattern recognition receptors (PRRs) (22)(23)(24)(25)(26). Interestingly, the inflammatory cytokines IL-12 and IL-1β are both induced differentially by avirulent, but not virulent, Toxoplasma strains (9,27,28), presumably through intracellular activation of TLRs and other PRRs.…”

Toxoplasmosis and Public Health Genomics

“…TLR is linked to infertility [33], pregnancy disorders, and placental dysfunction [34]. In toxoplasmosis TLR2, TLR4, TLR9, and TLR11 have been found important for recognition of ligands expressed by T. gondii [1,[35][36][37]. Andrade et al [38] have demonstrated TLR7 and TLR9 sense T. gondii-associated nucleic acids (DNA or RNA) while TLR11 and TLR12 sense Toxoplasma profiling.…”