Drosophila hemocytes are essential for the animal to resist Staphylococcus aureus infections. Phagocytosis is a central component of the hemocyte-mediated immune response. It involves regulated interaction between the phagocytic and the endocytic compartments. Rab GTPases are pivotal for the membrane trafficking and fusion events, and thus are often targets of intracellular pathogens that subvert phagocytosis. An in vivo screen identified Rab2 and Rab14 as candidates for proteins regulating phagosome maturation. Since Rab14 is often targeted by intracellular pathogens, an understanding of its function during phagocytosis and the overall immune response can give insight into the pathogenesis of intracellular microbes. We generated a Drosophila Rab14 mutant and characterized the resulting immune defects in animals and specifically in hemocytes in response to an S. aureus infection. Hemocyte based immunofluorescence studies indicate that Rab14 is recruited to the phagosome and like Rab7, a well-characterized regulator of the phagocytic pathway, is essential for progression of phagosome maturation. Rab14 mutant hemocytes show impaired recruitment of Rab7 and of a lysosomal marker onto S. aureus phagosomes. The defect in phagocytosis is associated with higher bacterial load and increased susceptibility to S. aureus in the animal.
Motility is a fundamental part of cellular life and survival, including for Plasmodium parasites – single‐celled protozoan pathogens responsible for human malaria. The motile life cycle forms achieve motility, called gliding, via the activity of an internal actomyosin motor. Although gliding is based on the well‐studied system of actin and myosin, its core biomechanics are not completely understood. Currently accepted models suggest it results from a specifically organized cellular motor that produces a rearward directional force. When linked to surface‐bound adhesins, this force is passaged to the cell posterior, propelling the parasite forwards. Gliding motility is observed in all three life cycle stages of Plasmodium: sporozoites, merozoites and ookinetes. However, it is only the ookinetes – formed inside the midgut of infected mosquitoes – that display continuous gliding without the necessity of host cell entry. This makes them ideal candidates for invasion‐free biomechanical analysis. Here we apply a plate‐based imaging approach to study ookinete motion in three‐dimensional (3D) space to understand Plasmodium cell motility and how movement facilitates midgut colonization. Using single‐cell tracking and numerical analysis of parasite motion in 3D, our analysis demonstrates that ookinetes move with a conserved left‐handed helical trajectory. Investigation of cell morphology suggests this trajectory may be based on the ookinete subpellicular cytoskeleton, with complementary whole and subcellular electron microscopy showing that, like their motion paths, ookinetes share a conserved left‐handed corkscrew shape and underlying twisted microtubular architecture. Through comparisons of 3D movement between wild‐type ookinetes and a cytoskeleton‐knockout mutant we demonstrate that perturbation of cell shape changes motion from helical to broadly linear. Therefore, while the precise linkages between cellular architecture and actomyosin motor organization remain unknown, our analysis suggests that the molecular basis of cell shape may, in addition to motor force, be a key adaptive strategy for malaria parasite dissemination and, as such, transmission.
Plasmodium spp. and Toxoplasma gondii are important human and veterinary pathogens. These parasites possess an unusual double membrane structure located directly below the plasma membrane named the inner membrane complex (IMC). First identified in early electron micrograph studies, huge advances in genetic manipulation of the Apicomplexa have allowed the visualization of a dynamic, highly structured cellular compartment with important roles in maintaining the structure and motility of these parasites. This review summarizes recent advances in the field and highlights the changes the IMC undergoes during the complex life cycles of the Apicomplexa.
P. aeruginosa infections are commonly associated with cystic fibrosis, pneumonias, neutropenia and burns. The P. aeruginosa quorum sensing molecule N-(3-oxo-dodecanoyl) homoserine lactone (C12) cause multiple deleterious host responses, including repression of NF-κB transcriptional activity and apoptosis. Inhibition of C12-mediated host responses is predicted to reduce P. aeruginosa virulence. We report here a novel, host-targeted approach for potential adjunctive anti-Pseudomonal therapy based on inhibition of C12-mediated host responses. A high-throughput screen was developed to identify C12 inhibitors that restore NF-κB activity in C12-treated, lipopolysaccharide (LPS)-stimulated cells. Triazolo[4,3-a]quinolines with nanomolar potency were identified as C12-inhibitors that restored NF-κB-dependent luciferase expression in LPS- and TNF-stimulated cell lines. In primary macrophages and fibroblasts, triazolo[4,3-a]quinolines inhibited C12 action to restore cytokine secretion in LPS-stimulated cells. Serendipitously, in the absence of an inflammatory stimulus, triazolo[4,3-a]quinolines prevented C12-mediated responses, including cytotoxicity, elevation of cytoplasmic calcium, and p38 MAPK phosphorylation. In vivo efficacy was demonstrated in a murine model of dermal inflammation involving intradermal C12 administration. The discovery of triazolo[4,3-a]quinolines provides a pharmacological tool to investigate C12-mediated host responses, and a potential host-targeted anti-Pseudomonal therapy.
The facultative intracellular pathogen, Salmonella enterica, triggers its own uptake into nonphagocytic epithelial cells. Invasion is dependent on a Type 3 Secretion System (T3SS), which delivers a cohort of effector proteins across the plasma membrane where they induce dynamic actin-driven ruffling of the membrane and ultimately, internalization of the bacteria into a modified phagosome. In eukaryotic cells, the calcium- and phospholipid-binding protein Annexin A2 (AnxA2) functions as a platform for actin remodeling in the vicinity of dynamic cellular membranes. AnxA2 is mostly found in a stable heterotetramer, with p11, which can interact with other proteins such as the giant phosphoprotein AHNAK. We show here that AnxA2, p11 and AHNAK are required for T3SS-mediated Salmonella invasion of cultured epithelial cells and that the T3SS effector SopB is required for recruitment of AnxA2 and AHNAK to Salmonella invasion sites. Altogether this work shows that, in addition to targeting Rho-family GTPases, Salmonella can intersect the host cell actin pathway via AnxA2.
AHNAK; live-cell imaging; ruffle; type III secretion system
The kinetochore is a multi-protein structure assembled on eukaryotic centromeres mediating chromosome attachment to spindle microtubules. Here we identified the kinetochore proteins Nuf2 and Ndc80 in the apicomplexan parasite Toxoplasma gondii. Localization revealed that kinetochores remain clustered throughout the cell cycle and co-localize with clustered centromeres at the centrocone, a structure containing the spindle pole embedded in the nuclear envelope. Pharmacological disruption of microtubules resulted in partial loss of some kinetochore and centromere clustering, indicating microtubules are necessary but not strictly required for kinetochore clustering. Generation of a TgNuf2 conditional knockdown strain revealed it is essential for chromosome segregation, but dispensable for centromere clustering. The centromeres actually remained associated with the centrocone suggesting microtubule binding is not required for their interaction with the spindle pole. The most striking observation upon TgNuf2 depletion was that the centrosome behaved normally, but that it lost its association with the centrocone. This suggests that microtubules are essential to maintain contact between the centrosome and chromosomes, and this interaction is critical for the partitioning of the nuclei into the two daughter parasites. Finally, genetic complementation experiments with mutated TgNuf2 constructs highlighted an apicomplexan specific motif with a putative role in nuclear localization.
Recent epidemiological studies have revealed a significant association between periodontitis and oral squamous cell carcinoma (OSCC). Furthermore, matrix metalloproteinase 9 (MMP9) is implicated in the invasion and metastasis of tumor cells. We examined the involvement of Porphyromonas gingivalis, a periodontal pathogen, in OSCC invasion through induced expression of proMMP and its activation. proMMP9 was continuously secreted from carcinoma SAS cells, while P. gingivalis infection increased proenzyme expression and subsequently processed it to active MMP9 in culture supernatant, which enhanced cellular invasion. In contrast, Fusobacterium nucleatum, another periodontal organism, failed to demonstrate such activities. The effects of P. gingivalis were observed with highly invasive cells, but not with the low invasive type. P. gingivalis also stimulated proteinase-activated receptor 2 (PAR2) and enhanced proMMP9 expression, which promoted cellular invasion. P. gingivalis mutants deficient in gingipain proteases failed to activate MMP9. Infected SAS cells exhibited activation of ERK1/2, p38, and NF-kB, and their inhibitors diminished both proMMP9-overexpression and cellular invasion. Together, our results show that P. gingivalis activates the ERK1/2-Ets1, p38/HSP27, and PAR2/NFκB pathways to induce proMMP9 expression, after which the proenzyme is activated by gingipains to promote cellular invasion of OSCC cell lines. These findings suggest a novel mechanism of progression and metastasis of OSCC associated with periodontitis.
P. gingivalis; squamous cell carcinoma; periodontitis; MAPK
Intracellular bacterial pathogens often subvert apoptosis signaling to regulate survival of their host cell, allowing propagation of the bacterial population. Coxiella burnetii, the intracellular agent of human Q fever, inhibits host cell apoptosis through several mechanisms, including prevention of mitochondrial cytochrome c release, triggering of an anti-apoptotic transcriptional program, and activation of pro-survival kinases. To control host cell survival, C. burnetii delivers effector proteins to the eukaryotic cytosol using a specialized Dot/Icm type IV secretion system (T4SS). Effectors are predicted to regulate activity of pro-survival host signaling proteins, such as Akt and cAMP-dependent protein kinase (PKA), to control infection. Here, we show that host PKA activity is required for C. burnetii inhibition of macrophage apoptosis. PKA is activated during infection and inhibits activity of the pro-apoptotic protein Bad via phosphorylation. Bad is also phosphorylated at an Akt-specific residue, indicating C. burnetii uses two kinases to fully inactivate Bad. Additionally, Bad and the tethering protein 14-3-3β co-localize at the C. burnetii parasitophorous vacuole (PV) membrane during infection, an event predicted to alter Bad promotion of apoptosis. Inhibiting PKA activity prevents Bad recruitment to the PV, but the protein is retained at the membrane during induction of apoptosis. Finally, PKA regulatory subunit I (RI) traffics to the PV membrane in a T4SS-dependent manner, suggesting a C. burnetii effector(s) regulates PKA-dependent activities. This study is the first to demonstrate subversion of host PKA activity by an intracellular bacterial pathogen to prevent apoptosis and survive within macrophages.
Apicomplexan parasites express various Calcium-Dependent Protein Kinases (CDPKs), and some of them play essential roles in invasion and egress. Five of the six CDPKs conserved in most Apicomplexa have been studied at the molecular and cellular levels in Plasmodium species and/or in Toxoplasma gondii parasites, but the function of CDPK7 was so far uncharacterized. In T. gondii, during intracellular replication, two parasites are formed within a mother cell through a unique process called endodyogeny. Here we demonstrate that the knock-down of CDPK7 protein in T. gondii results in pronounced defects in parasite division and a major growth deficiency, while it is dispensable for motility, egress and microneme exocytosis. In cdpk7-depleted parasites, the overall DNA content was not impaired, but the polarity of daughter cells budding and the fate of several sub-cellular structures or proteins involved in cell division were affected, such as the centrosomes and the kinetochore. Overall, our data suggest that CDPK7 is crucial for proper maintenance of centrosome integrity required for the initiation of endodyogeny. Our findings provide a first insight into the probable role of calcium-dependent signalling in parasite multiplication, in addition to its more widely explored role in invasion and egress.
Apicomplexa; Toxoplasma gondii; Tet-inducible system; Calcium Dependent Protein Kinase; Replication; Calcium; centrosome; kinetochore; apicoplast; Inner Membrane Complex
The presence of bacterial pathogens typically upregulates the host’s production of nitric oxide synthase (NOS) and nitric oxide (NO) as antimicrobial agents. This dramatic response is often mediated by microbe-associated molecular patterns (MAMPs) of the pathogen. In contrast, previous studies of the beneficial Euprymna scolopes/Vibrio fischeri symbiosis demonstrated that symbiont colonization results in an attenuation of host NOS/NO, which occur in high levels in the hatchling light organ. In the present study, we sought to determine whether V. fischeri MAMPs, specifically lipopolysaccharide (LPS) and the peptidoglycan derivative tracheal cytotoxin (TCT), attenuate NOS/NO, and whether this activity mediates the MAMPs-induced light organ morphogenesis. Using confocal microscopy, we visualized and quantified the levels of NOS with immunocytochemistry and NO with a NO-specfic fluorochrome. When added exogenously to seawater containing hatchling animals, V. fischeri LPS and TCT together, but not individually, induced normal NOS/NO attenuation. Further, V. fischeri mutants defective in TCT release did not. Experiments with NOS inhibitors and NO donors provided evidence that NO mediates the apoptosis and morphogenesis associated with symbiont colonization. Attenuation of NOS/NO by LPS and TCT in the squid-vibrio symbiosis provides another example of how the host’s response to MAMPs depends on the context (i.e., beneficial or pathogenic bacteria). These data also provide a mechanism by which symbiont MAMPs regulate host development through NO attenuation.
Nitric oxide; Euprymna scolopes; Vibrio fischeri; Photobacterium; Aliivibrio; symbiosis; apoptosis; lipopolysaccharide; peptidoglycan; tracheal cytotoxin; host-microbe interactions
Cytoadhesion of Plasmodium falciparum infected erythrocytes to host microvasculature is a key virulence determinant. Parasite binding is mediated by a large family of clonally variant adhesion proteins, termed P. falciparum erythrocyte membrane protein 1 (PfEMP1), encoded by var genes and expressed at the infected-erythrocyte surface. Although PfEMP1 proteins have extensively diverged under opposing selection pressure to maintain ligand binding while avoiding antibody-mediated detection, recent work has revealed they can be classified into different groups based on chromosome location and domain composition. This grouping reflects functional specialization of PfEMP1 proteins for different human host and microvascular binding niches and appears to be maintained by gene recombination hierarchies. In one extreme, a specific PfEMP1 variant is associated with placental binding and malaria during pregnancy, while other PfEMP1 subtypes appear to be specialized for infection of malaria naïve hosts. Here, we discuss recent findings on the origins and evolution of the var gene family, the structure-function of PfEMP1 proteins, and a distinct subset of PfEMP1 variants that have been associated with severe childhood malaria.
Shifts in microbial strain structure underlie both emergence of new pathogens and shifts in patterns of infection and disease of known agents. Understanding the selective pressures at a population level as well as the mechanisms at the molecular level represent significant gaps in our knowledge regarding microbial epidemiology. Highly antigenically variant pathogens, which are broadly represented among microbial taxons, are most commonly viewed through the mechanistic lens of how they evade immune clearance within the host. However, equally important are mechanisms that allow pathogens to evade immunity at the population level. The selective pressure of immunity at both the level of the individual host and the population is a driver of diversification within a pathogen strain. Using Anaplasma marginale as a model highly antigenically variable bacterial pathogen, we review how immunity selects for genetic diversification in alleles encoding outer membrane proteins both within and among strains. Importantly, genomic comparisons among strains isolated from diverse epidemiologic settings elucidates the counterbalancing pressures for diversification and conservation, driven by immune escape and transmission fitness, respectively, and how these shape pathogen strain structure.
Legionnaires’ disease is an emerging, severe, pneumonia-like illness caused by the Gram-negative intracellular bacteria Legionella pneumophila, which are able to infect and replicate intracellularly in macrophages. Little is known regarding the mechanisms used by intracellular L. pneumophila for the acquisition of specific nutrients that are essential for bacterial replication. Here, we investigate three L. pneumophila genes with high similarity to the E. coli K+ transporters. These three genes were expressed by L. pneumophila and have been designated kupA, kupB and kupC. Investigation using the L. pneumophila kup mutants revealed that kupA is involved in K+ acquisition during axenic growth. The kupA mutants replicated efficiently in rich axenic media, but poorly in a chemically defined medium. The kupA mutants were defective in the recruitment of polyubiquitinated proteins to the Legionella-containing vacuole that is formed in macrophages and displayed an intracellular multiplication defect during the replication in Acanthamoeba castellanii and in mouse macrophages. We found that bafilomycin treatment of macrophages was able to rescue the growth defects of kupA mutants, but it did not influence the replication of wild-type bacteria. The defects identified in kupA mutants of L. pneumophila were complemented by the expression E. coli trkD/Kup gene in trans, a bona fide K+ transporter encoded by E. coli. Collectively, our data indicate that KupA is a functional K+ transporter expressed by L. pneumophila that facilitates the bacterial replication intracellularly and in nutrient-limited conditions.
Legionella pneumophila; kup genes; macrophage infection; intracellular replication
Microsporidia are obligate intracellular parasites with extremely reduced genomes and a dependence on host-derived ATP. The microsporidium Encephalitozoon cuniculi proliferates within a membranous vacuole and we investigated how the ATP supply is optimized at the vacuole–host interface. Using spatial EM quantification (stereology), we found a single layer of mitochondria coating substantial proportions of the parasitophorous vacuole. Mitochondrial binding occurred preferentially over the vegetative ‘meront’ stages of the parasite, which bulged into the cytoplasm, thereby increasing the membrane surface available for mitochondrial interaction. In a broken cell system mitochondrial binding was maintained and was typified by electron dense structures (< 10 nm long) bridging between outer mitochondrial and vacuole membranes. In broken cells mitochondrial binding was sensitive to a range of protease treatments. The function of directly bound mitochondria, as measured by the membrane potential sensitive dye JC-1, was indistinguishable from other mitochondria in the cell although there was a generalized depression of the membrane potential in infected cells. Finally, quantitative immuno-EM revealed that the ATP-delivering mitochondrial porin, VDAC, was concentrated atthe mitochondria-vacuole interaction site. Thus E. cuniculi appears to maximize ATP supply by direct binding of mitochondria to the parasitophorous vacuole bringing this organelle within 0.020 microns of the growing vegetative form of the parasite. ATP-delivery is further enhanced by clustering of ATP transporting porins in those regions of the outer mitochondrial membrane lying closest to the parasite.
Infection with the malaria parasite, Plasmodium, is associated with a strong inflammatory response and parasite cytoadhesion (sequestration) in several organs. Here, we have carried out a systematic study of sequestration and histopathology during infection of C57Bl/6 mice with Plasmodium chabaudi AS and determined the influence of the immune response. This parasite sequesters predominantly in liver and lung, but not in the brain, kidney or gut. Histopathological changes occur in multiple organs during the acute infection, but are not restricted to the organs where sequestration takes place. Adaptive immunity, and signalling through the IFNγ receptor increased sequestration and histopathology in the liver, but not in the lung, suggesting that there are differences in the adhesion molecules and/or parasite ligands utilized and mechanisms of pathogenesis in these two organs. Exacerbation of pro-inflammatory responses during infection by deletion of the il10 gene resultsin the aggravation of damage to lung and kidney irrespective of the degree of sequestration. The immune response therefore affected both sequestration and histopathology in an organ-specific manner. P. chabaudi AS provides a good model to investigate the influence of the host response on the sequestration and specific organ pathology, which is applicable to human malaria.
The intracellular pathogen Legionella pneumophila is able to strike a balance between the death and survival of the host cell during infection. Despite the presence of high level of active caspase-3, the executioner caspase of apoptotic cell death, infected permissive macrophages are markedly resistant to exogenous apoptotic stimuli. Several bacterial molecules capable of promoting the cell survival pathways have been identified, but proteins involved in the activation of caspase-3 remain unknown. To study the mechanism of L. pneumophila-mediated caspase-3 activation, we tested all known Dot/Icm substrates for their ability to activate caspase-3. Five effectors capable of causing caspase-3 activation upon transient expression were identified. Among these, by using its ability to activate caspase-3 by inducing the release of cytochrome c from the mitochondria, we demonstrated that VipD is a phospholipase A2, which hydrolyzes phosphatidylethanolamine (PE) and phosphocholine (PC) on the mitochondrial membrane in a manner that appears to require host co-factor(s). The lipase activity leads to the production of free fatty acids and 2-lysophospholipids, which destabilize the mitochondrial membrane and may contribute to the release of cytochrome c and the subsequent caspase 3 activation. Furthermore, we found that whereas it is not detectably defectively in caspase 3 activation in permissive cells, a mutant lacking all of these five genes is less potent in inducing apoptosis in dendritic cells. Our results reveal that activation of host cell death pathways by L. pneumophila is a result of the effects of multiple bacterial proteins with diverse biochemical functions.
Type IV secretion; effectors; mitochondrion; cell death; Cytochrome c
Helicobacter pyloriis a bacterial pathogen that colonizes the gastric niche of ~50% of the human population worldwide and is known to cause peptic ulceration and gastric cancer. Pathology of infection strongly depends on a cag pathogenicity island (cagPAI)-encoded type IV secretion system (T4SS). Here, we aimed to identify as yet unknown bacterial factors involved in cagPAI effector function and performed a large-scale screen of an H. pylori transposon mutant library using activation of the pro-inflammatory transcription factor NF-κB in human gastric epithelial cells as a measure of T4SS function. Analysis of ~3000 H. pylori mutants revealed three non-cagPAI genes that affected NF-κB nuclear translocation. Of these, the outer membrane protein HopQ from H. pylori strain P12 was essential for CagA translocation and for CagA-mediated host cell responses such as formation of the hummingbird phenotype and cell scattering. Besides that, deletion of hopQ reduced T4SS-dependent activation of NF-κB, induction of MAPK signalling and secretion of interleukin 8 (IL-8) in the host cells, but did not affect motility or the quantity of bacteria attached to host cells. Hence, we identified HopQ as a non-cagPAI-encoded co-factor of T4SS function.
CagA; NF-κB; Omp27; adhesin; type IV secretion system
Neisseria gonorrhoeae regulates the expression of epithelial cell genes, activates cytoprotective pathways in the infected cell and protects it from apoptosis. Many of these responses are enhanced by the Type IV pilus (Tfp). We tested the hypothesis that N. gonorrhoeae modulates the innate immune response by inducing expression of ATF3, a transcription factor that negatively regulates the expression of many cytokine genes. We further determined whether Tfp are involved in these events. We found that N. gonorrhoeae induces ATF3 expression in mucosal epithelial cells through activation of mitogen activated protein kinases. Maximal ATF3 expression requires Tfp retraction. Knocking down endogenous levels of ATF3 results in higher levels of IL-6 transcript. Our findings strongly suggest that ATF3 is involved in suppressing cytokine expression during gonococcal infection. We propose a model for the role of ATF3 in the context of N. gonorrhoeae infection.
Enteropathogenic and enterohaemorrhagic Escherichia coli
use a novel infection strategy to colonize the gut epithelium, involving
translocation of their own receptor, Tir, via a type III secretion system and
subsequent formation of attaching and effecting (A/E) lesions. Following
integration into the host cell plasma membrane of cultured cells, and clustering
by the outer membrane adhesin intimin, Tir triggers multiple actin
polymerization pathways involving host and bacterial adaptor proteins that
converge on the host Arp2/3 actin nucleator. Although initially thought to be
involved in A/E lesion formation, recent data have shown that the known
Tir-induced actin polymerization pathways are dispensable for this activity, but
can play other major roles in colonization efficiency, in vivo
fitness and systemic disease. In this review we summarize the roadmap leading
from the discovery of Tir, through the different actin polymerization pathways
it triggers, to our current understanding of their physiological functions.
Nucleo cytoplasmic large DNA viruses (NCLDVs) are a group of double-stranded DNA viruses that replicate their DNA partly or entirely in the cytoplasm in association with viral factories (VFs). They share about 50 genes suggesting that they are derived from a common ancestor. Using transmission electron microscopy (TEM) and electron tomography (ET) we showed that the NCLDV vaccinia virus (VACV) acquires its membrane from open membrane intermediates, derived from the ER. These open membranes contribute to the formation of a single open membrane of the immature virion, shaped into a sphere by the assembly of the viral scaffold protein on its convex side. We now compare VACV with the NCLDV Mimivirus by TEM and ET and show that the latter also acquires its membrane from open membrane intermediates that accumulate at the periphery of the cytoplasmic VF. In analogy to VACV this membrane is shaped by the assembly of a layer on the convex side of its membrane, likely representing the Mimivirus capsid protein. By quantitative ET we show for both viruses that the open membrane intermediates of assembly adopt an ‘open-eight’ conformation with a characteristic diameter of 90 nm for Mimi- and 50 nm for VACV. We discuss these results with respect to the common ancestry of NCLDVs and propose a hypothesis on the possible origin of this unusual membrane biogenesis.
The innate immune system of mammals responds to microbial infection through detection of conserved molecular determinants called “pathogen-associated molecular patterns” (PAMPs). Pathogens use virulence factors to counteract PAMP-directed responses. The innate immune system can in turn recognize signals generated by virulence factors, allowing for a heightened response to dangerous pathogens. Many Gram-negative bacterial pathogens encode type III secretion systems (T3SSs) that translocate effector proteins, subvert PAMP-directed responses and are critical for infection. A plasmid-encoded T3SS in the human-pathogenic Yersinia species translocates seven effectors into infected host cells. Delivery of effectors by the T3SS requires plasma membrane insertion of two translocators, which are thought to form a channel called a translocon. Studies of the Yersinia T3SS have provided key advances in our understanding of how innate immune responses are generated by perturbations in plasma membrane and other signals that result from translocon insertion. Additionally, studies in this system revealed that effectors function to inhibit innate immune responses resulting from insertion of translocons into plasma membrane. Here, we review these advances with the goal of providing insight into how a T3SS can activate and inhibit innate immune responses, allowing a virulent pathogen to bypass host defenses.