Chlamydia muridarum evades growth restriction by the IFN-gamma-inducible host resistance factor Irgb10

Small molecule inhibitor of type three secretion suppresses acute and chronic Chlamydia trachomatis infection in a novel urogenital Chlamydia model

Previously, we reported that a compound from a group of thiohydrazides of oxamic acids, CL-55, possessed antichlamydial activity in vitro that was accompanied by a decreased translocation of the type three secretion effector, IncA, into the host cell. In this study, the antichlamydial activity of CL-55 was tested in vivo in DBA/2 mice infected with C. trachomatis serovar D. We found that intravaginal inoculation of DBA/2 mice with the clinically relevant strain, C. trachomatis serovar D, results in a course of infection and pathology similar to that observed in humans. The early stage of infection in this model was characterized by a shedding of Chlamydia in vaginal secretions followed by an ascending infection and inflammation in the upper genital tract. We found that CL-55 possessed antibacterial activity in vivo and was able to control C. trachomatis vaginal shedding, ascending infection, and inflammation in the upper genital organs in DBA/2 mice. Our data provide a proof of concept for the protective effect of the thiadiazinon, CL-55, against chlamydial infection in vivo and support the feasibility of further studies of its potential therapeutic applications.

Identification of the microsporidian Encephalitozoon cuniculi as a new target of the IFNγ-inducible IRG resistance system

The IRG system of IFNγ-inducible GTPases constitutes a powerful resistance mechanism in mice against Toxoplasma gondii and two Chlamydia strains but not against many other bacteria and protozoa. Why only T. gondii and Chlamydia? We hypothesized that unusual features of the entry mechanisms and intracellular replicative niches of these two organisms, neither of which resembles a phagosome, might hint at a common principle. We examined another unicellular parasitic organism of mammals, member of an early-diverging group of Fungi, that bypasses the phagocytic mechanism when it enters the host cell: the microsporidian Encephalitozoon cuniculi. Consistent with the known susceptibility of IFNγ-deficient mice to E. cuniculi infection, we found that IFNγ treatment suppresses meront development and spore formation in mouse fibroblasts in vitro, and that this effect is mediated by IRG proteins. The process resembles that previously described in T. gondii and Chlamydia resistance. Effector (GKS subfamily) IRG proteins accumulate at the parasitophorous vacuole of E. cuniculi and the meronts are eliminated. The suppression of E. cuniculi growth by IFNγ is completely reversed in cells lacking regulatory (GMS subfamily) IRG proteins, cells that effectively lack all IRG function. In addition IFNγ-induced cells infected with E. cuniculi die by necrosis as previously shown for IFNγ-induced cells resisting T. gondii infection. Thus the IRG resistance system provides cell-autonomous immunity to specific parasites from three kingdoms of life: protozoa, bacteria and fungi. The phylogenetic divergence of the three organisms whose vacuoles are now known to be involved in IRG-mediated immunity and the non-phagosomal character of the vacuoles themselves strongly suggests that the IRG system is triggered not by the presence of specific parasite components but rather by absence of specific host components on the vacuolar membrane.

For some time we have studied an intracellular resistance system essential for mice to survive infection with the intracellular protozoan, Toxoplasma gondii, that is based on a family of proteins, immunity-related GTPases or IRG proteins. Immediately after the parasite enters a cell, IRG proteins accumulate on the membrane of the vacuole in which the organism resides. Within a few hours the vacuole membrane breaks down and the parasite dies. A puzzle is why this mechanism works on Toxoplasma, but only on one other organism among the many tested, namely the bacterial species, Chlamydia. What do these widely different parasites have in common that so many other bacteria and protozoa lack? Neither Toxoplasma nor Chlamydia is taken up by conventional phagocytosis. In this paper we suggest that this is an important clue by showing that a microsporidian, Encephalitozoon cuniculi, a highly-divergent fungal parasite, which also invades cells bypassing phagocytosis, is resisted by the IRG system. Therefore, we propose here the “missing self” principle: IRG proteins bind to vacuolar membranes only in the absence of a host derived inhibitor that is present on phagosomal membranes but excluded from the plasma membrane invaginated by IRG target organisms during non-phagosomal entry.

Exploiting ovine immunology to improve the relevance of biomedical models

•

Sheep make a valuable contribution to immunology research.

•

Lessons to be learned from studying infections in the natural host.

•

Factors to consider when selecting biomedical models.

Animal models of human disease are important tools in many areas of biomedicine; for example, in infectious disease research and in the development of novel drugs and medical devices. Most studies involving animals use rodents, in particular congenic mice, due to the availability of a wide number of strains and the ease with which they can be genetically manipulated. The use of mouse models has led to major advances in many fields of research, in particular in immunology but despite these advances, no animal model can exactly reproduce all the features of human disease. It is increasingly becoming recognised that in many circumstances mice do not provide the best model and that alternative species may be more appropriate. Here, we describe the relative merits of sheep as biomedical models for human physiology and disease in comparison to mice, with a particular focus on reproductive and respiratory pathogens.

Influence of the tryptophan-indole-IFNγ axis on human genital Chlamydia trachomatis infection: role of vaginal co-infections

The natural history of genital Chlamydia trachomatis infections can vary widely; infections can spontaneously resolve but can also last from months to years, potentially progressing to cause significant pathology. The host and bacterial factors underlying this wide variation are not completely understood, but emphasize the bacterium's capacity to evade/adapt to the genital immune response, and/or exploit local environmental conditions to survive this immune response. IFNγ is considered to be a primary host protective cytokine against endocervical C.trachomatis infections. IFNγ acts by inducing the host enzyme indoleamine 2,3-dioxgenase, which catabolizes tryptophan, thereby depriving the bacterium of this essential amino acid. In vitro studies have revealed that tryptophan deprivation causes Chlamydia to enter a viable but non-infectious growth pattern that is termed a persistent growth form, characterized by a unique morphology and gene expression pattern. Provision of tryptophan can reactivate the bacterium to the normal developmental cycle. There is a significant difference in the capacity of ocular and genital C. trachomatis serovars to counter tryptophan deprivation. The latter uniquely encode a functional tryptophan synthase to synthesize tryptophan via indole salvage, should indole be available in the infection microenvironment. In vitro studies have confirmed the capacity of indole to mitigate the effects of IFNγ; it has been suggested that a perturbed vaginal microbiome may provide a source of indole in vivo. Consistent with this hypothesis, the microbiome associated with bacterial vaginosis includes species that encode a tryptophanase to produce indole. In this review, we discuss the natural history of genital chlamydial infections, morphological and molecular changes imposed by IFNγ on Chlamydia, and finally, the microenvironmental conditions associated with vaginal co-infections that can ameliorate the effects of IFNγ on C. trachomatis.

Integrin α4β1 is necessary for CD4+ T cell-mediated protection against genital Chlamydia trachomatis infection

Chlamydia trachomatis infection is the most common sexually transmitted bacterial infection in the United States and a significant health burden worldwide. Protection from Chlamydia infection in the genital mucosa is dependent on IFN-γ derived from CD4(+) Th1 cells. These CD4(+) T cells must home successfully to the genital tract to exert their effector function and decrease C. trachomatis burden. Although adhesion receptors expressed by CD4(+) T cells in the genital tract have been characterized, the integrin receptor required for Chlamydia-specific CD4(+) T cell-mediated protection has not been explored. In this study, we demonstrate that C. trachomatis infection of the upper genital tract results in recruitment of Chlamydia-specific CD4(+) T cells robustly expressing the integrin α4β1. Interfering with α4β1, but not α4β7, function resulted in defective CD4(+) T cell trafficking to the uterus and high bacterial load. We conclude that integrin α4β1 is necessary for CD4(+) T cell-mediated protection against C. trachomatis infection in the genital mucosa. By identifying homing molecules required for successful CD4(+) T cell trafficking to C. trachomatis-infected tissues, we will be better equipped to design vaccines that elicit sterilizing, long-lasting immunity without inducing immune pathologies in the upper genital tract.

The E2-like conjugation enzyme Atg3 promotes binding of IRG and Gbp proteins to Chlamydia- and Toxoplasma-containing vacuoles and host resistance

Cell-autonomous immunity to the bacterial pathogen Chlamydia trachomatis and the protozoan pathogen Toxoplasma gondii is controlled by two families of Interferon (IFN)-inducible GTPases: Immunity Related GTPases (IRGs) and Guanylate binding proteins (Gbps). Members of these two GTPase families associate with pathogen-containing vacuoles (PVs) and solicit antimicrobial resistance pathways specifically to the intracellular site of infection. The proper delivery of IRG and Gbp proteins to PVs requires the autophagy factor Atg5. Atg5 is part of a protein complex that facilitates the transfer of the ubiquitin-like protein Atg8 from the E2-like conjugation enzyme Atg3 to the lipid phosphatidylethanolamine. Here, we show that Atg3 expression, similar to Atg5 expression, is required for IRG and Gbp proteins to dock to PVs. We further demonstrate that expression of a dominant-active, GTP-locked IRG protein variant rescues the PV targeting defect of Atg3- and Atg5-deficient cells, suggesting a possible role for Atg proteins in the activation of IRG proteins. Lastly, we show that IFN-induced cell-autonomous resistance to C. trachomatis infections in mouse cells depends not only on Atg5 and IRG proteins, as previously demonstrated, but also requires the expression of Atg3 and Gbp proteins. These findings provide a foundation for a better understanding of IRG- and Gbp-dependent cell-autonomous resistance and its regulation by Atg proteins.

Irgm1 (LRG-47), a regulator of cell-autonomous immunity, does not localize to mycobacterial or listerial phagosomes in IFN-γ-induced mouse cells

The IFN-inducible protein Irgm1 (LRG-47) belongs to the family of immunity-related GTPases that function in cell-autonomous resistance against intracellular pathogens in mice. Irgm1 deficiency is associated with a severe immunodeficiency syndrome. The protein has been variously interpreted as a direct effector molecule on bacterial phagosomes or on other organelles or as an inducer of autophagy. In this study, we re-examined one of these claims, namely that Irgm1 targets mycobacterial and listerial phagosomes. We found no colocalization of endogenous Irgm1, using two immunofluorescent staining techniques, either in fibroblasts or in macrophages. We demonstrated the predicted existence of two protein isoforms of Irgm1 derived from differential splicing and described immunological reagents for their detection. Both Irgm1 isoforms localize to the Golgi apparatus and weakly to mitochondria; however, only the long Irgm1 isoforms can be detected on endolysosomal membranes. Together with the previous observation that the general immunodeficiency phenotype of Irgm1(-/-) mice is reversed in Irgm1/Irgm3 double-deficient mice, our results argue against a direct effector function of Irgm1 at the bacterial phagosome. We discuss these findings in the context of evidence that Irgm1 functions as a negative regulator of other members of the immunity-related GTPase protein family.

IRG and GBP host resistance factors target aberrant, "non-self" vacuoles characterized by the missing of "self" IRGM proteins

Interferon-inducible GTPases of the Immunity Related GTPase (IRG) and Guanylate Binding Protein (GBP) families provide resistance to intracellular pathogenic microbes. IRGs and GBPs stably associate with pathogen-containing vacuoles (PVs) and elicit immune pathways directed at the targeted vacuoles. Targeting of Interferon-inducible GTPases to PVs requires the formation of higher-order protein oligomers, a process negatively regulated by a subclass of IRG proteins called IRGMs. We found that the paralogous IRGM proteins Irgm1 and Irgm3 fail to robustly associate with "non-self" PVs containing either the bacterial pathogen Chlamydia trachomatis or the protozoan pathogen Toxoplasma gondii. Instead, Irgm1 and Irgm3 reside on "self" organelles including lipid droplets (LDs). Whereas IRGM-positive LDs are guarded against the stable association with other IRGs and GBPs, we demonstrate that IRGM-stripped LDs become high affinity binding substrates for IRG and GBP proteins. These data reveal that intracellular immune recognition of organelle-like structures by IRG and GBP proteins is partly dictated by the missing of "self" IRGM proteins from these structures.

STING-dependent recognition of cyclic di-AMP mediates type I interferon responses during Chlamydia trachomatis infection

STING (stimulator of interferon [IFN] genes) initiates type I IFN responses in mammalian cells through the detection of microbial nucleic acids. The membrane-bound obligate intracellular bacterium Chlamydia trachomatis induces a STING-dependent type I IFN response in infected cells, yet the IFN-inducing ligand remains unknown. In this report, we provide evidence that Chlamydia synthesizes cyclic di-AMP (c-di-AMP), a nucleic acid metabolite not previously identified in Gram-negative bacteria, and that this metabolite is a prominent ligand for STING-mediated activation of IFN responses during infection. We used primary mouse lung fibroblasts and HEK293T cells to compare IFN-β responses to Chlamydia infection, c-di-AMP, and other type I IFN-inducing stimuli. Chlamydia infection and c-di-AMP treatment induced type I IFN responses in cells expressing STING but not in cells expressing STING variants that cannot sense cyclic dinucleotides but still respond to cytoplasmic DNA. The failure to induce a type I IFN response to Chlamydia and c-di-AMP correlated with the inability of STING to relocalize from the endoplasmic reticulum to cytoplasmic punctate signaling complexes required for IFN activation. We conclude that Chlamydia induces STING-mediated IFN responses through the detection of c-di-AMP in the host cell cytosol and propose that c-di-AMP is the ligand predominantly responsible for inducing such a response in Chlamydia-infected cells.

IMPORTANCE

This study shows that the Gram-negative obligate pathogen Chlamydia trachomatis, a major cause of pelvic inflammatory disease and infertility, synthesizes cyclic di-AMP (c-di-AMP), a nucleic acid metabolite that thus far has been described only in Gram-positive bacteria. We further provide evidence that the host cell employs an endoplasmic reticulum (ER)-localized cytoplasmic sensor, STING (stimulator of interferon [IFN] genes), to detect c-di-AMP synthesized by Chlamydia and induce a protective IFN response. This detection occurs even though Chlamydia is confined to a membrane-bound vacuole. This raises the possibility that the ER, an organelle that innervates the entire cytoplasm, is equipped with pattern recognition receptors that can directly survey membrane-bound pathogen-containing vacuoles for leaking microbe-specific metabolites to mount type I IFN responses required to control microbial infections.

IFN-inducible GTPases in host cell defense

From plants to humans, the ability to control infection at the level of an individual cell-a process termed cell-autonomous immunity-equates firmly with survival of the species. Recent work has begun to unravel this programmed cell-intrinsic response and the central roles played by IFN-inducible GTPases in defending the mammalian cell's interior against a diverse group of invading pathogens. These immune GTPases regulate vesicular traffic and protein complex assembly to stimulate oxidative, autophagic, membranolytic, and inflammasome-related antimicrobial activities within the cytosol, as well as on pathogen-containing vacuoles. Moreover, human genome-wide association studies and disease-related transcriptional profiling have linked mutations in the Immunity-Related GTPase M (IRGM) locus and altered expression of guanylate binding proteins (GBPs) with tuberculosis susceptibility and Crohn's colitis.

CD4+ T cells are necessary and sufficient to confer protection against Chlamydia trachomatis infection in the murine upper genital tract

Chlamydia trachomatis infection is the most common bacterial sexually transmitted disease in the United States. Chlamydia infections that ascend to the upper genital tract can persist, trigger inflammation, and result in serious sequelae such as infertility. However, mouse models in which the vaginal vault is inoculated with C. trachomatis do not recapitulate the course of human disease. These intravaginal infections of the mouse do not ascend efficiently to the upper genital tract, do not cause persistent infection, do not induce significant inflammation, and do not induce significant CD4⁺ T cell infiltration. In this article, we describe a noninvasive transcervical infection model in which we bypass the cervix and directly inoculate C. trachomatis into the uterus. We show that direct C. trachomatis infection of the murine upper genital tract stimulates a robust Chlamydia-specific CD4⁺ T cell response that is both necessary and sufficient to clear infection and provide protection against reinfection.

Host interferon-γ inducible protein contributes to Brucella survival

Brucella spp. are highly adapted intracellular pathogens of mammals that cause chronic infections while surving and replicating in host monocytes and macrophages. Although monocytes are normally susceptible to infection, pretreatment with pro-inflammatory cytokine interferon-γ (IFN-γ) activates cellular defense mechanisms that increase intracellular killing of Brucella and prevents bacterial replication. We examined the contribution of the IFN-γ inducible GTPase, LRG-47, to B. abortus 2308 infection in in vitro and in vivo murine models. Infecting non-activated macrophages from LRG-47(-/-) mice revealed that loss of this host protein negatively effected the intracellular survival and replication of IgG opsonized B. abortus. In contrast, survival and replication of non-opsonized B. abortus was the same in both C57/B6 and LRG-47(-/-) peritoneal macrophages. Following IFN-γ activation of LRG-47(-/-) monocytes, IgG opsonized B. abortus survived better than non-opsonized bacteria. The differential fate of opsonized and non-opsonized B. abortus was only observed in macrophages collected from LRG-47(-/-) mice. Given the specific nature of the relationship between this host protein and the mechanism of Brucella internalization, LRG-47(-/-) mice were infected with B. abortus to assess whether the loss of the lrg47 protein would affect the ability of the bacteria to colonize or persist within the host. B. abortus were able to establish and maintain similar numbers of bacteria in both C57/B6 mice and LRG-47(-/-) through 3 weeks post intraperitoneal infection. By 9 weeks p.i. fewer B. abortus were recovered from LRG-47(-/-) mice than controls, suggesting that the host protein has a positive role in maintaining long term persistence of the bacteria within the host. These observations demonstrating a positive role for a host IFN-γ induced protein defense protein has yet to be reported. These results provide interesting insight into the complex interaction between Brucella and their host.

Interferon-inducible effector mechanisms in cell-autonomous immunity

Interferons (IFNs) induce the expression of hundreds of genes as part of an elaborate antimicrobial programme designed to combat infection in all nucleated cells - a process termed cell-autonomous immunity. As described in this Review, recent genomic and subgenomic analyses have begun to assign functional properties to novel IFN-inducible effector proteins that restrict bacteria, protozoa and viruses in different subcellular compartments and at different stages of the pathogen life cycle. Several newly described host defence factors also participate in canonical oxidative and autophagic pathways by spatially coordinating their activities to enhance microbial killing. Together, these IFN-induced effector networks help to confer vertebrate host resistance to a vast and complex microbial world.

Plac8-dependent and inducible NO synthase-dependent mechanisms clear Chlamydia muridarum infections from the genital tract

Chlamydia trachomatis urogenital serovars replicate predominantly in genital tract epithelium. This tissue tropism poses a unique challenge for host defense and vaccine development. Studies utilizing the Chlamydia muridarum mouse model have shown that CD4 T cells are critical for clearing genital tract infections. In vitro studies have shown that CD4 T cells terminate infection by upregulating epithelial inducible NO synthase (iNOS) transcription and NO production. However, this mechanism is not critical, as iNOS-deficient mice clear infections normally. We recently showed that a subset of Chlamydia-specific CD4 T cell clones could terminate replication in epithelial cells using an iNOS-independent mechanism requiring T cell degranulation. We advance that work using microarrays to compare iNOS-dependent and iNOS-independent CD4 T cell clones. Plac8 was differentially expressed by clones having the iNOS-independent mechanism. Plac8-deficient mice had delayed clearance of infection, and Plac8-deficient mice treated with the iNOS inhibitor N-monomethyl-l-arginine were largely unable to resolve genital tract infections over 8 wk. These results demonstrate that there are two independent and redundant T cell mechanisms for clearing C. muridarum genital tract infections: one dependent on iNOS, and the other dependent on Plac8. Although T cell subsets are routinely defined by cytokine profiles, there may be important subdivisions by effector function, in this case CD4(Plac8).

New challenges for vaccination to prevent chlamydial abortion in sheep

Ovine enzootic abortion (OEA) is caused by the obligate intracellular Gram-negative bacterium Chlamydia abortus. OEA remains a common cause of infectious abortion in many sheep-rearing countries despite the existence of commercially available vaccines that protect against the disease. There are a number of confounding factors that influence the uptake and use of these vaccines, which includes an inability to discriminate between infected and vaccinated animals (DIVA) using conventional serological diagnostic techniques. This suggests that the immunity elicited by current vaccines is similar to that observed in convalescent, immune sheep that have experienced OEA. The existence of these vaccines provides an opportunity to understand how protection against OEA is elicited and also to understand why vaccines can occasionally appear to fail, as has been reported recently for OEA. Interferon-gamma (IFN-γ), the cytokine that classically defines Th1-type adaptive immunity, is a strong correlate of protection against OEA in sheep and has been shown to inhibit the growth of C. abortus in vitro. Humoral immunity to C. abortus is observed in both vaccinated and naturally infected sheep, but antibody responses tend to be used more as diagnostic markers than targets for strategic vaccine design. A future successful DIVA vaccine against OEA should aim to elicit the immunological correlate of protection (IFN-γ) concomitantly with an antibody profile that is distinct from that of the natural infection. Such an approach requires careful selection of protective components of C. abortus combined with an effective delivery system that elicits IFN-γ-producing CD4+ve memory T cells.

The IRG protein-based resistance mechanism in mice and its relation to virulence in Toxoplasma gondii

IRG proteins (immunity-related GTPases) provide an early defense mechanism in mice against the protozoal pathogen, Toxoplasma gondii. This is a particularly suitable time to provide a brief review of this host-pathogen interaction because the nature of the IRG resistance system, and to some extent its mode of action, have become known in the past few years. Likewise, forward genetic screens have recently drawn attention to a number of loci contributing to the differential virulence of T. gondii strains in mice. It is now clear that at least some important virulence mechanisms exert their action against components of the IRG resistance system. Thus these two mechanisms form the two poles of a dynamic host-pathogen virulence-resistance relationship with interesting and accessible properties.

Immunity-related GTPase M (IRGM) proteins influence the localization of guanylate-binding protein 2 (GBP2) by modulating macroautophagy

The immunity-related GTPases (IRGs) are a family of proteins induced by interferon-γ that play a crucial role in innate resistance to intracellular pathogens. The M subfamily of IRG proteins (IRGM) plays a profound role in this context, in part because of the ability of its members to regulate the localization and expression of other IRG proteins. We present here evidence that IRGM proteins affect the localization of the guanylate-binding proteins (GBPs), a second family of interferon-induced GTP-binding proteins that also function in innate immunity. Absence of Irgm1 or Irgm3 led to accumulation of Gbp2 in intracellular compartments that were positive for both the macroautophagy (hereafter referred to as autophagy) marker LC3 and the autophagic adapter molecule p62/Sqstm1. Gbp2 was similarly relocalized in cells in which autophagy was impaired because of the absence of Atg5. Both in Atg5- and IRGM-deficient cells, the IRG protein Irga6 relocalized to the same compartments as Gbp2, raising the possibility of a common regulatory mechanism. However, other data indicated that Irga6, but not Gbp2, was ubiquitinated in IRGM-deficient cells. Similarly, coimmunoprecipitation studies indicated that although Irgm3 did interact directly with Irgb6, it did not interact with Gbp2. Collectively, these data suggest that IRGM proteins indirectly modulate the localization of GBPs through a distinct mechanism from that through which they regulate IRG protein localization. Further, these results suggest that a core function of IRGM proteins is to regulate autophagic flux, which influences the localization of GBPs and possibly other factors that instruct cell-autonomous immune resistance.

Chlamydia persistence -- a tool to dissect chlamydia--host interactions

Under stress, chlamydiae can enter a non-infectious but viable state termed persistence. In the absence of a tractable genetic system, persistence induction provides an important experimental tool with which to study these fascinating organisms. This review will discuss examples of: i) persistence studies that have illuminated critical chlamydiae/host interactions; and ii) novel persistence models that will do so in the future.

Compensatory T cell responses in IRG-deficient mice prevent sustained Chlamydia trachomatis infections

The obligate intracellular pathogen Chlamydia trachomatis is the most common cause of bacterial sexually transmitted diseases in the United States. In women C. trachomatis can establish persistent genital infections that lead to pelvic inflammatory disease and sterility. In contrast to natural infections in humans, experimentally induced infections with C. trachomatis in mice are rapidly cleared. The cytokine interferon-γ (IFNγ) plays a critical role in the clearance of C. trachomatis infections in mice. Because IFNγ induces an antimicrobial defense system in mice but not in humans that is composed of a large family of Immunity Related GTPases (IRGs), we questioned whether mice deficient in IRG immunity would develop persistent infections with C. trachomatis as observed in human patients. We found that IRG-deficient Irgm1/m3((-/-)) mice transiently develop high bacterial burden post intrauterine infection, but subsequently clear the infection more efficiently than wildtype mice. We show that the delayed but highly effective clearance of intrauterine C. trachomatis infections in Irgm1/m3((-/-)) mice is dependent on an exacerbated CD4(+) T cell response. These findings indicate that the absence of the predominant murine innate effector mechanism restricting C. trachomatis growth inside epithelial cells results in a compensatory adaptive immune response, which is at least in part driven by CD4(+) T cells and prevents the establishment of a persistent infection in mice.

The IFN-γ-inducible GTPase, Irga6, protects mice against Toxoplasma gondii but not against Plasmodium berghei and some other intracellular pathogens

Clearance of infection with intracellular pathogens in mice involves interferon-regulated GTPases of the IRG protein family. Experiments with mice genetically deficient in members of this family such as Irgm1(LRG-47), Irgm3(IGTP), and Irgd(IRG-47) has revealed a critical role in microbial clearance, especially for Toxoplasma gondii. The in vivo role of another member of this family, Irga6 (IIGP, IIGP1) has been studied in less detail. We investigated the susceptibility of two independently generated mouse strains deficient in Irga6 to in vivo infection with T. gondii, Mycobacterium tuberculosis, Leishmania mexicana, L. major, Listeria monocytogenes, Anaplasma phagocytophilum and Plasmodium berghei. Compared with wild-type mice, mice deficient in Irga6 showed increased susceptibility to oral and intraperitoneal infection with T. gondii but not to infection with the other organisms. Surprisingly, infection of Irga6-deficient mice with the related apicomplexan parasite, P. berghei, did not result in increased replication in the liver stage and no Irga6 (or any other IRG protein) was detected at the parasitophorous vacuole membrane in IFN-γ-induced wild-type cells infected with P. berghei in vitro. Susceptibility to infection with T. gondii was associated with increased mortality and reduced time to death, increased numbers of inflammatory foci in the brains and elevated parasite loads in brains of infected Irga6-deficient mice. In vitro, Irga6-deficient macrophages and fibroblasts stimulated with IFN-γ were defective in controlling parasite replication. Taken together, our results implicate Irga6 in the control of infection with T. gondii and further highlight the importance of the IRG system for resistance to this pathogen.

Diverse requirements for SRC-family tyrosine kinases distinguish chlamydial species

Chlamydiae are well known for their species specificity and tissue tropism, and yet the individual species and strains show remarkable genomic synteny and share an intracellular developmental cycle unique in the microbial world. Only a relatively few chlamydial genes have been linked to specific disease or tissue tropism. Here we show that chlamydial species associated with human infections, Chlamydia trachomatis and C. pneumoniae, exhibit unique requirements for Src-family kinases throughout their developmental cycle. Utilization of Src-family kinases by C. trachomatis includes tyrosine phosphorylation of the secreted effector Tarp during the entry process, a functional role in microtubule-dependent trafficking to the microtubule organizing center, and a requirement for Src-family kinases for successful initiation of development. Nonhuman chlamydial species C. caviae and C. muridarum show none of these requirements and, instead, appear to be growth restricted by the activities of Src-family kinases. Depletion of Src-family kinases triggers a more rapid development of C. caviae with up to an 800% increase in infectious progeny production. Collectively, the results suggest that human chlamydial species have evolved requirements for tyrosine phosphorylation by Src-family kinases that are not seen in other chlamydial species. The requirement for Src-family kinases thus represents a fundamental distinction between chlamydial species that would not be readily apparent in genomic comparisons and may provide insights into chlamydial disease association and species specificity.

IMPORTANCE

Chlamydiae are well known for their species specificity and tissue tropism as well as their association with unique diseases. A paradox in the field relates to the remarkable genomic synteny shown among chlamydiae and the very few chlamydial genes linked to specific diseases. We have found that different chlamydial species exhibit unique requirements for Src-family kinases. These differing requirements for Src-family kinases would not be apparent in genomic comparisons and appear to be a previously unrecognized distinction that may provide insights to guide research in chlamydial pathogenesis.

The immunity-related GTPases in mammals: a fast-evolving cell-autonomous resistance system against intracellular pathogens

The immunity-related GTPases (IRGs) belong to the family of large, interferon-inducible GTPases and constitute a cell-autonomous resistance system essential for the control of vacuolar pathogens like Toxoplasma gondii in mice. Recent results demonstrated that numerous IRG members accumulate collaboratively at the parasitophorous vacuole of invading T. gondii leading to the destruction of the vacuole and the parasite and subsequent necrotic host cell death. Complex regulatory interactions between different IRG proteins are necessary for these processes. Disturbance of this finely balanced system, e.g., by single genetic deficiency for the important negative regulator Irgm1 or the autophagic regulator Atg5, leads to spontaneous activation of the effector IRG proteins when induced by IFNγ. This activation has cytotoxic consequences resulting in a severe lymphopenia, macrophage defects, and failure of the adaptive immune system in Irgm1-deficient mice. However, alternative functions in phagosome maturation and induction of autophagy have been proposed for Irgm1. The IRG system has been studied primarily in mice, but IRG genes are present throughout the mammalian lineage. Interestingly, the number, type, and diversity of genes present differ greatly even between closely related species, probably reflecting intimate host-pathogen coevolution driven by an armed race between the IRG resistance proteins and pathogen virulence factors. IRG proteins are targets for polymorphic T. gondii virulence factors, and genetic variation in the IRG system between different mouse strains correlates with resistance and susceptibility to virulent T. gondii strains.

Chlamydiales and the innate immune response: friend or foe?

Pathogenicity of Chlamydia and Chlamydia-related bacteria could be partially mediated by an enhanced activation of the innate immune response. The study of this host pathogen interaction has proved challenging due to the restricted in vitro growth of these strict intracellular bacteria and the lack of genetic tools to manipulate their genomes. Despite these difficulties, the interactions of Chlamydiales with the innate immune cells and their effectors have been studied thoroughly. This review aims to point out the role of pattern recognition receptors and signal molecules (cytokines, reactive oxygen species) of the innate immune response in the pathogenesis of chlamydial infection. Besides inducing clearance of the bacteria, some of these effectors may be used by the Chlamydia to establish chronic infections or to spread. Thus, the induced innate immune response seems to be variable depending on the species and/or the serovar, making the pattern more complex. It remains crucial to determine the common players of the innate immune response in order to help define new treatment strategies and to develop effective vaccines. The excellent growth in phagocytic cells of some Chlamydia-related organisms such as Waddlia chondrophila supports their use as model organisms to study conserved features important for interactions between the innate immunity and Chlamydia.

Phosphorylation of immunity-related GTPases by a Toxoplasma gondii-secreted kinase promotes macrophage survival and virulence

Macrophages are specialized to detect and destroy intracellular microbes and yet a number of pathogens have evolved to exploit this hostile niche. Here we demonstrate that the obligate intracellular parasite Toxoplasma gondii disarms macrophage innate clearance mechanisms by secreting a serine threonine kinase called ROP18, which binds to and phosphorylates immunity-related GTPases (IRGs). Substrate profiling of ROP18 revealed a preference for a conserved motif within switch region I of the GTPase domain, a modification predicted to disrupt IRG function. Consistent with this, expression of ROP18 was both necessary and sufficient to block recruitment of Irgb6, which was in turn required for parasite destruction. ROP18 phosphorylation of IRGs prevented clearance within inflammatory monocytes and IFN-γ-activated macrophages, conferring parasite survival in vivo and promoting virulence. IRGs are implicated in clearance of a variety of intracellular pathogens, suggesting that other virulence factors may similarly thwart this innate cellular defense mechanism.

Chlamydia-specific CD4 T cell clones control Chlamydia muridarum replication in epithelial cells by nitric oxide-dependent and -independent mechanisms

Chlamydia trachomatis serovars D-K are sexually transmitted intracellular bacterial pathogens that replicate in epithelial cells lining the human reproductive tract. It is clear from knockout mice and T cell depletion studies using Chlamydia muridarum that MHC class II and CD4 T cells are critical for clearing bacteria from the murine genital tract. It is not clear how CD4 T cells interact with infected epithelial cells to mediate bacterial clearance in vivo. Previous work using an epithelial tumor cell line showed that a Chlamydia-specific CD4 T cell clone was able to inhibit C. muridarum replication in vitro via induction of epithelial NO production. We have previously shown that Chlamydia-specific CD4 T cell clones can recognize and be activated by infected reproductive tract epithelial cells and block Chlamydia replication in them. We extend those observations by investigating the mechanism used by a panel of CD4 T cell clones to control Chlamydia replication in epithelial cells. We found that Chlamydia-specific CD4 T cell clones were cytolytic, but that cytolysis was not likely critical for controlling C. muridarum replication. For one, CD4 T cell clone-induced epithelial NO production was critical for controlling replication; however, the most potent CD4 T cell clones were dependent on T cell degranulation for replication control with only a minor additional contribution from NO production. We discuss our data as they relate to existing knockout mouse studies addressing mechanisms of T cell-mediated control of Chlamydia replication and their implications for intracellular epithelial pathogens in mouse models.

Towards a Chlamydia trachomatis vaccine: how close are we?

Chlamydia trachomatis is the leading cause of bacterial sexually transmitted infections and preventable blindness worldwide. The incidence of chlamydial sexually transmitted infections has increased rapidly and current antibiotic therapy has failed as an intervention strategy. The most accepted strategy for protection and/or control of chlamydial infections is a vaccine that induces both local neutralizing antibodies to prevent infections by the extracellular elementary bodies and a cell-mediated immune response to target the intracellular infection. This article will discuss the challenges in vaccine design for the prevention of chlamydial urogenital infection and/or disease, including selection of target antigens, discussion of effective delivery systems, immunization routes and adjuvants for induction of protective immunity at the targeted mucosal surface whilst minimizing severe inflammatory disease sequelae.

Topological journey of parasite-derived antigens for presentation by MHC class I molecules

Within cells of their host, many bacteria and parasites inhabit specialized compartments, such as a modified phagosome for Mycobacterium tuberculosis or a parasitophorous vacuole for Toxoplasma gondii. These locations could exclude microbial material from entry into the MHC class I surveillance pathway. Remarkably, however, under these circumstances, cells can still signal the presence of invading pathogens to circulating CD8(+) T cells, which typically play a key role in protection against such intracellular organisms. Here, we review MHC I presentation pathways in various contexts, ranging from model antigens in non-infectious settings to pathogen-infected cells. We suggest that presentation of intracellular pathogens can be described as not just one, but several distinct pathways; perhaps because diverse pathogens have evolved different strategies to interact with host cells.

Immunological paradigms and the pathogenesis of ovine chlamydial abortion

Successful mammalian pregnancy involves complex immunological interactions between the mother and foetus that are not yet fully understood. A number of immunological paradigms have been established to explain the failure of the maternal immune system to reject the semi-allogeneic foetus, mainly based on studies in mice and humans. However, as placental structure, gestation periods and number of concepti per pregnancy can vary greatly between mammals, it is not always clear how applicable these immunological paradigms are to reproduction in other species. Here, we discuss the predictions of three important immunological paradigms in relation to the pathogenesis of ovine enzootic abortion (OEA), a common cause of infectious abortion in sheep and other ruminants. OEA is caused by the intracellular Gram-negative bacterium Chlamydophila abortus that exhibits a tropism for placental trophoblast. The paradigms of particular relevance to the pathogenesis of OEA are as follows: (i) intracellular bacterial infections are controlled by T(H)1-type CD4(+ve) T cells; (ii) indoleamine 2,3-dioxygenase is expressed in the placenta to prevent immunological rejection of the semi-allogeneic foetus; and (iii) pregnancy is a maternal T(H)2-type phenomenon. We discuss the relevance and validity of these paradigms for chlamydial abortion and reproductive immunology in sheep.

Increased inflammation and impaired resistance to Chlamydophila pneumoniae infection in Dusp1(-/-) mice: critical role of IL-6

The MAPK phosphatase DUSP1 is an essential negative regulator of TLR-triggered innate immune activation. Here, we have investigated the impact of DUSP1 on inflammatory and antimicrobial host responses to the intracellular pathogen Chlamydophila pneumoniae. Following nasal infection, DUSP1-deficient mice mounted an enhanced pulmonary cytokine (IL-1beta, IL-6) and chemokine response (CCL3, CCL4, CXCL1, CXCL2), leading to increased leukocyte infiltration. Of interest, the increased inflammatory response, in the absence of DUSP1, was associated with higher bacterial numbers in the lungs, although the expression of IFN-gamma and critical antichlamydial effector molecules, such as iNOS, was intact. Blockade of IL-6 trans-signaling by injection of a soluble gp130-Fc fusion protein corrected the overshooting chemokine production as well as the increased chlamydial load in Dusp1(-/-) mice. Furthermore, IL-6 enhanced the replication of C. pneumoniae in embryonic fibroblasts in vitro. These data show that DUSP1 is required to achieve a balanced response to chlamydial infection and identify IL-6 as critical for amplifying inflammation and benefiting chlamydial growth through direct effects on infected cells.

cPLA2 regulates the expression of type I interferons and intracellular immunity to Chlamydia trachomatis

Infection with the obligate bacterial intracellular pathogen Chlamydia trachomatis leads to the sustained activation of the small GTPase RAS and many of its downstream signaling components. In particular, the mitogen-activated protein kinase ERK and the calcium-dependent phospholipase cPLA(2) are activated and are important for the onset of inflammatory responses. In this study we tested if activation of ERK and cPLA(2) occurred as a result of RAS signaling during infection and determined the relative contribution of these signaling components to chlamydial replication and survival. We provide genetic and pharmacological evidence that during infection RAS, ERK, and, to a lesser extent, cPLA(2) activation are uncoupled, suggesting that Chlamydia activates individual components of this signaling pathway in a non-canonical manner. In human cell lines, inhibition of ERK or cPLA(2) signaling did not adversely impact C. trachomatis replication. In contrast, in murine cells, inhibition of ERK and cPLA(2) played a significant protective role against C. trachomatis. We determined that cPLA(2)-deficient murine cells are permissive for C. trachomatis replication because of their impaired expression of beta interferon and the induction of immunity-related GTPases (IRG) important for the containment of intracellular pathogens. Furthermore, the MAPK p38 was primarily responsible for cPLA(2) activation in Chlamydia-infected cells and IRG expression. Overall, these findings define a previously unrecognized role for cPLA(2) in the induction of cell autonomous cellular immunity to Chlamydia and highlight the many non-canonical signaling pathways engaged during infection.

Cross-presentation of peptides from intracellular pathogens by MHC class I molecules

Many prokaryotic and eukaryotic parasites multiply in specialized subcellular niches in the host cell. The invading microbes hijack key cellular functions to establish the intracellular niches but, unlike viruses, do not need the protein synthesis machinery of host cells to replicate. Circulating CD8+ T cells provide protective immunity by recognizing pathogen-derived peptide major histocompatibility complex class I molecules (pMHC I) expressed by infected cells. Here, we review studies on the complex and varied pathways that produce the appropriate pMHC I as ligands for the CD8+ T cells. We also discuss possible explanations for the curious observations that CD8+ T cells are specific for fewer pMHC I ligands in parasite infections compared to the diversity of pMHC I ligands in viral infections.

Toxoplasma gondii and the Immunity-Related GTPase (IRG) resistance system in mice: a review

The Immunity Related GTPases (IRG proteins) constitute a large family of interferon-inducible proteins that mediate early resistance to Toxoplasma gondii infection in mice. At least six members of this family are required for resistance of mice to virulent T. gondii strains. Recent results have shown that the complexity of the resistance arises from complex regulatory interactions between different family members. The mode of action against T. gondii depends on the ability of IRG proteins to accumulate on the parasitophorous vacuole of invading tachyzoites and to induce local damage to the vacuole resulting in disruption of the vacuolar membrane. Virulent strains of T. gondii overcome the IRG resistance system, probably by interfering with the loading of IRG proteins onto the parasitophorous vacuole membrane. It may be assumed that T. gondii strains highly virulent for mice will be disadvantaged in the wild due to the rapid extinction of the infected host, while it is self-evident that susceptibility to virulent strains is disadvantageous to the mouse host. We consider the possibility that this double disadvantage is compensated in wild populations by segregating alleles with different resistance and susceptibility properties in the IRG system.

Coordinated loading of IRG resistance GTPases on to the Toxoplasma gondii parasitophorous vacuole

The immunity-related GTPases (IRGs) constitute an interferon-induced intracellular resistance mechanism in mice against Toxoplasma gondii. IRG proteins accumulate on the parasitophorous vacuole membrane (PVM), leading to its disruption and to death of the parasite. How IRGs target the PVM is unknown. We show that accumulation of IRGs on the PVM begins minutes after parasite invasion and increases for about 1 h. Targeting occurs independently of several signalling pathways and the microtubule network, suggesting that IRG transport is diffusion-driven. The intensity of IRG accumulation on the PVM, however, is reduced in absence of the autophagy regulator, Atg5. In wild-type cells IRG proteins accumulate cooperatively on PVMs in a definite order reflecting a temporal hierarchy, with Irgb6 and Irgb10 apparently acting as pioneers. Loading of IRG proteins onto the vacuoles of virulent Toxoplasma strains is attenuated and the two pioneer IRGs are the most affected. The polymorphic rhoptry kinases, ROP16, ROP18 and the catalytically inactive proteins, ROP5A–D, are not individually responsible for this effect. Thus IRG proteins protect mice against avirulent strains of Toxoplasma but fail against virulent strains. The complex cooperative behaviour of IRG proteins in resisting Toxoplasma may hint at undiscovered complexity also in virulence mechanisms.

Localisation and mislocalisation of the interferon-inducible immunity-related GTPase, Irgm1 (LRG-47) in mouse cells

Irgm1 (LRG-47) is an interferon-inducible Golgi membrane associated GTPase of the mouse whose disruption causes susceptibility to many different intracellular pathogens. Irgm1 has been variously interpreted as a regulator of homologous effector GTPases of the IRG family, a regulator of phagosome maturation and as an initiator of autophagy in interferon-induced cells. We find that endogenous Irgm1 localises to late endosomal and lysosomal compartments in addition to the Golgi membranes. The targeting motif known to be required for Golgi localisation is surprisingly also required for endolysosomal localisation. However, unlike Golgi localisation, localisation to the endolysosomal system also requires the functional integrity of the nucleotide binding site, and thus probably reflects transient activation. Golgi localisation is lost when Irgm1 is tagged at either N- or C-termini with EGFP, while localisation to the endolysosomal system is relatively favoured. N-terminally tagged Irgm1 localises predominantly to early endosomes, while C-terminally tagged Irgm1 localises to late endosomes and lysosomes. Both these anomalous distributions are reversed by inactivation of the nucleotide binding site, and the tagged proteins both revert to Golgi membrane localisation. Irgm1 is the first IRG protein to be found associated with the endolysosomal membrane system in addition to either Golgi (Irgm1 and Irgm2) or ER (Irgm3) membranes, and we interpret the result to be in favour of a regulatory function of IRGM proteins at cellular membrane systems. In future analyses it should be borne in mind that tagging of Irgm1 leads to loss of Golgi localisation and enhanced localisation on endolysosomal membranes, probably as a result of constitutive activation.

Brucella abortus induces Irgm3 and Irga6 expression via type-I IFN by a MyD88-dependent pathway, without the requirement of TLR2, TLR4, TLR5 and TLR9

The innate immune system senses bacterial pathogens by pattern recognition receptors, such as the well-characterised Toll-like Receptors (TLR). The activation of TLR signalling cascades depends on several adaptor proteins, among which MyD88 plays a key role in triggering innate immune responses. Here, we show in murine macrophages that Brucella abortus triggers expression of the interferon-inducible resistance proteins (IRGs, p47 GTPases) via type-I IFN secretion at late time points, when Brucella has reached its replication niche. This induction requires the adaptor molecule MyD88 but does not involve the TLRs normally implicated in sensing Gram-negative bacteria, namely TLR2, TLR4, TLR5 and TLR9. Brucella mutants lacking the functional VirB type-IV secretion system were not capable of inducing Irgm3 and Irga6 expression, suggesting that the type-IV secretion system is part of the triggering of the activation process. Our data suggest that Brucella is recognized intracellularly by an unknown receptor, different from the conventional ones used for Gram-negative sensing, but one that nevertheless signals through MyD88.

Chlamydia muridarum-specific CD4 T-cell clones recognize infected reproductive tract epithelial cells in an interferon-dependent fashion

During natural infections Chlamydia trachomatis urogenital serovars replicate predominantly in the epithelial cells lining the reproductive tract. This tissue tropism poses a unique challenge to host cellar immunity and future vaccine development. In the experimental mouse model, CD4 T cells are necessary and sufficient to clear Chlamydia muridarum genital tract infections. This implies that resolution of genital tract infection depends on CD4 T-cell interactions with infected epithelial cells. However, no laboratory has shown that Chlamydia-specific CD4 T cells can recognize Chlamydia antigens presented by major histocompatibility complex class II (MHC-I) molecules on epithelial cells. In this report we show that MHC-II-restricted Chlamydia-specific CD4 T-cell clones recognize infected upper reproductive tract epithelial cells as early as 12 h postinfection. The timing of recognition and degree of T-cell activation are dependent on the interferon (IFN) milieu. Beta IFN (IFN-beta) and IFN-gamma have different effects on T-cell activation, with IFN-beta blunting IFN-gamma-induced upregulation of epithelial cell surface MHC-II and T-cell activation. Individual CD4 T-cell clones differed in their degrees of dependence on IFN-gamma-regulated MHC-II for controlling Chlamydia replication in epithelial cells in vitro. We discuss our data as they relate to published studies with IFN knockout mice, proposing a straightforward interpretation of the existing literature based on CD4 T-cell interactions with the infected reproductive tract epithelium.

Human guanylate binding proteins potentiate the anti-chlamydia effects of interferon-gamma

Chlamydiae are obligate intracellular pathogens that are sensitive to pro-inflammatory cytokine interferon-gamma. IFN-gamma-inducible murine p47 GTPases have been demonstrated to function in resistance to chlamydia infection in vivo and in vitro. Because the human genome does not encode IFN-gamma-inducible homologues of these proteins, the significance of the p47 GTPase findings to chlamydia pathogenesis in humans is unclear. Here we report a pair of IFN-gamma-inducible proteins, the human guanylate binding proteins (hGBPs) 1 and 2 that potentiate the anti-chlamydial properties of IFN-gamma. hGBP1 and 2 localize to the inclusion membrane, and their anti-chlamydial functions required the GTPase domain. Alone, hGBP1 or 2 have mild, but statistically significant and reproducible negative effects on the growth of Chlamydia trachomatis, whilst having potent anti-chlamydial activity in conjunction with treatment with a sub-inhibitory concentration of IFN-gamma. Thus, hGBPs appear to potentiate the anti-chlamydial effects of IFN-gamma. Indeed, depletion of hGBP1 and 2 in cells treated with IFN-gamma led to an increase in inclusion size, indicative of better growth. Interestingly, chlamydia species/strains harboring the full-length version of the putative cytotoxin gene, which has been suggested to confer resistance to IFN-gamma was not affected by hGBP overexpression. These findings identify the guanylate binding proteins as potentiators of IFN-gamma inhibition of C. trachomatis growth, and may be the targets of the chlamydial cytotoxin.

T cell responses in the absence of IFN-gamma exacerbate uterine infection with Chlamydia trachomatis

Infection with the obligate intracellular bacterium Chlamydia trachomatis is controlled primarily by IFN-gamma and Th1 immunity. In this study, we used cells from a Chlamydia-specific CD4(+) TCR-transgenic mouse to assess the role of IFN-gamma in development of Th1 immunity. We show that secretion of host IFN-gamma or the ability of host cells to respond to secreted IFN-gamma is not required to initiate a Th1 immune response. Additionally, we found that Ag-specific CD4(+) cells that were preskewed toward Th1 confer protection, whereas cells preskewed toward Th2 cause a previously unreported exacerbation of disease leading to higher bacterial load. Chlamydia-specific Th1 cells transferred into an IFN-gamma(-/-) recipient mouse demonstrate protective effects, but the same cells exacerbate bacterial burden when transferred into IFN-gammaR(-/-) mice. Thus, we demonstrate that the secretion of IFN-gamma is necessary for protection against C. trachomatis and that in the absence of host cell IFN-gammaR expression, both Th1 and Th2 cells lead to increased burden of C. trachomatis.

Virulent Toxoplasma gondii evade immunity-related GTPase-mediated parasite vacuole disruption within primed macrophages

Cytokine-activated macrophages restrain the replication of intracellular parasites and disrupt the integrity of vacuolar pathogens. In this study, we show that inducible nitric oxide synthase and the immunity-related GTPase (IRG) family member Irgm3, respectively, are required for the ability of in vivo primed macrophages to restrain the growth of Toxoplasma gondii and to destroy the parasite's intracellular niche. Remarkably, virulent Type I strains of T. gondii evade IRG-dependent vacuolar disruption, while remaining susceptible to iNOS-dependent restriction. The ability of virulent T. gondii to escape killing by macrophages is controlled at the level of the individual vacuole and is associated with differential permissiveness for association of the IRG proteins Irga6 (IIGP1) and Irgb6 (TGTP) to the vacuolar membrane. Surprisingly, expression of the Type I ROP-18 virulence determinant in an avirulent strain did not confer the evasive phenotype. These results pinpoint evasion of vacuolar disruption by IRG proteins as a new determinant of pathogen virulence.

IFN-gamma-inducible Irga6 mediates host resistance against Chlamydia trachomatis via autophagy

Chlamydial infection of the host cell induces Gamma interferon (IFNgamma), a central immunoprotector for humans and mice. The primary defense against Chlamydia infection in the mouse involves the IFNgamma-inducible family of IRG proteins; however, the precise mechanisms mediating the pathogen's elimination are unknown. In this study, we identify Irga6 as an important resistance factor against C. trachomatis, but not C. muridarum, infection in IFNgamma-stimulated mouse embryonic fibroblasts (MEFs). We show that Irga6, Irgd, Irgm2 and Irgm3 accumulate at bacterial inclusions in MEFs upon stimulation with IFNgamma, whereas Irgb6 colocalized in the presence or absence of the cytokine. This accumulation triggers a rerouting of bacterial inclusions to autophagosomes that subsequently fuse to lysosomes for elimination. Autophagy-deficient Atg5-/- MEFs and lysosomal acidification impaired cells surrender to infection. Irgm2, Irgm3 and Irgd still localize to inclusions in IFNgamma-induced Atg5-/- cells, but Irga6 localization is disrupted indicating its pivotal role in pathogen resistance. Irga6-deficient (Irga6-/-) MEFs, in which chlamydial growth is enhanced, do not respond to IFNgamma even though Irgb6, Irgd, Irgm2 and Irgm3 still localize to inclusions. Taken together, we identify Irga6 as a necessary factor in conferring host resistance by remodelling a classically nonfusogenic intracellular pathogen to stimulate fusion with autophagosomes, thereby rerouting the intruder to the lysosomal compartment for destruction.

Disruption of the Toxoplasma gondii parasitophorous vacuole by IFNgamma-inducible immunity-related GTPases (IRG proteins) triggers necrotic cell death

Toxoplasma gondii is a natural intracellular protozoal pathogen of mice and other small mammals. After infection, the parasite replicates freely in many cell types (tachyzoite stage) before undergoing a phase transition and encysting in brain and muscle (bradyzoite stage). In the mouse, early immune resistance to the tachyzoite stage is mediated by the family of interferon-inducible immunity-related GTPases (IRG proteins), but little is known of the nature of this resistance. We reported earlier that IRG proteins accumulate on intracellular vacuoles containing the pathogen, and that the vacuolar membrane subsequently ruptures. In this report, live-cell imaging microscopy has been used to follow this process and its consequences in real time. We show that the rupture of the vacuole is inevitably followed by death of the intracellular parasite, shown by its permeability to cytosolic protein markers. Death of the parasite is followed by the death of the infected cell. The death of the cell has features of pyronecrosis, including membrane permeabilisation and release of the inflammatory protein, HMGB1, but caspase-1 cleavage is not detected. This sequence of events occurs on a large scale only following infection of IFNgamma-induced cells with an avirulent strain of T. gondii, and is reduced by expression of a dominant negative mutant IRG protein. Cells infected by virulent strains rarely undergo necrosis. We did not find autophagy to play any role in the key steps leading to the death of the parasite. We conclude that IRG proteins resist infection by avirulent T. gondii by a novel mechanism involving disruption of the vacuolar membrane, which in turn ultimately leads to the necrotic death of the infected cell.

Autophagy and pattern recognition receptors in innate immunity

Autophagy is a physiologically and immunologically controlled intracellular homeostatic pathway that sequesters and degrades cytoplasmic targets including macromolecular aggregates, cellular organelles such as mitochondria, and whole microbes or their products. Recent advances show that autophagy plays a role in innate immunity in several ways: (i) direct elimination of intracellular microbes by digestion in autolysosomes, (ii) delivery of cytosolic microbial products to pattern recognition receptors (PRRs) in a process referred to as topological inversion, and (iii) as an anti-microbial effector of Toll-like receptors and other PRR signaling. Autophagy eliminates pathogens in vitro and in vivo but, when aberrant due to mutations, contributes to human inflammatory disorders such as Crohn's disease. In this review, we examine these relationships and propose that autophagy is one of the most ancient innate immune defenses that has possibly evolved at the time of alpha-protobacteria-pre-eukaryote relationships, leading up to modern eukaryotic cell-mitochondrial symbiosis, and that during the metazoan evolution, additional layers of immunological regulation have been superimposed and integrated with this primordial innate immunity mechanism.

Inactive and active states of the interferon-inducible resistance GTPase, Irga6, in vivo

Irga6, a myristoylated, interferon-inducible member of the immunity-related GTPase family, contributes to disease resistance against Toxoplasma gondii in mice. Accumulation of Irga6 on the T. gondii parasitophorous vacuole membrane is associated with vesiculation and ultimately disruption of the vacuolar membrane in a process that requires an intact GTP-binding domain. The role of the GTP-binding domain of Irga6 in pathogen resistance is, however, unclear. We provide evidence that Irga6 in interferon-induced, uninfected cells is predominantly in a GDP-bound state that is maintained by other interferon-induced proteins. However, Irga6 that accumulates on the parasitophorous vacuole membrane after Toxoplasma infection is in the GTP-bound form. We demonstrate that a monoclonal antibody, 10D7, specifically detects GTP-bound Irga6, and we show that the formation of the 10D7 epitope follows from a GTP-dependent conformational transition of the N terminus of Irga6, anticipating an important role of the myristoyl group on Irga6 function in vivo.