Reduced secretion of YopJ by Yersinia limits in vivo cell death but enhances bacterial virulence

Pathogenicity and virulence of Yersinia

The genus Yersinia includes human, animal, insect, and plant pathogens as well as many symbionts and harmless bacteria. Within this genus are Yersinia enterocolitica and the Yersinia pseudotuberculosis complex, with four human pathogenic species that are highly related at the genomic level including the causative agent of plague, Yersinia pestis. Extensive laboratory, field work, and clinical research have been conducted to understand the underlying pathogenesis and zoonotic transmission of these pathogens. There are presently more than 500 whole genome sequences from which an evolutionary footprint can be developed that details shared and unique virulence properties. Whereas the virulence of Y. pestis now seems in apparent homoeostasis within its flea transmission cycle, substantial evolutionary changes that affect transmission and disease severity continue to ndergo apparent selective pressure within the other Yersiniae that cause intestinal diseases. In this review, we will summarize the present understanding of the virulence and pathogenesis of Yersinia, highlighting shared mechanisms of virulence and the differences that determine the infection niche and disease severity.

Translocation of YopJ family effector proteins through the VirB/VirD4 T4SS of Bartonella

The evolutionary conserved YopJ family comprises numerous type-III-secretion system (T3SS) effectors of diverse mammalian and plant pathogens that acetylate host proteins to dampen immune responses. Acetylation is mediated by a central acetyltransferase domain that is flanked by conserved regulatory sequences, while a nonconserved N-terminal extension encodes the T3SS-specific translocation signal. Bartonella spp. are facultative-intracellular pathogens causing intraerythrocytic bacteremia in their mammalian reservoirs and diverse disease manifestations in incidentally infected humans. Bartonellae do not encode a T3SS, but most species possess a type-IV-secretion system (T4SS) to translocate Bartonella effector proteins (Beps) into host cells. Here we report that the YopJ homologs present in Bartonellae species represent genuine T4SS effectors. Like YopJ family T3SS effectors of mammalian pathogens, the "Bartonella YopJ-like effector A" (ByeA) of Bartonella taylorii also targets MAP kinase signaling to dampen proinflammatory responses, however, translocation depends on a functional T4SS. A split NanoLuc luciferase-based translocation assay identified sequences required for T4SS-dependent translocation in conserved regulatory regions at the C-terminus and proximal to the N-terminus of ByeA. The T3SS effectors YopP from Yersinia enterocolitica and AvrA from Salmonella Typhimurium were also translocated via the Bartonella T4SS, while ByeA was not translocated via the Yersinia T3SS. Our data suggest that YopJ family T3SS effectors may have evolved from an ancestral T4SS effector, such as ByeA of Bartonella. In this evolutionary scenario, the signal for T4SS-dependent translocation encoded by N- and C-terminal sequences remained functional in the derived T3SS effectors due to the essential role these sequences coincidentally play in regulating acetyltransferase activity.

Inflammatory monocytes promote granuloma control of Yersinia infection

Granulomas are organized immune cell aggregates formed in response to chronic infection or antigen persistence. The bacterial pathogen Yersinia pseudotuberculosis (Yp) blocks innate inflammatory signalling and immune defence, inducing neutrophil-rich pyogranulomas (PGs) within lymphoid tissues. Here we uncover that Yp also triggers PG formation within the murine intestinal mucosa. Mice lacking circulating monocytes fail to form defined PGs, have defects in neutrophil activation and succumb to Yp infection. Yersinia lacking virulence factors that target actin polymerization to block phagocytosis and reactive oxygen burst do not induce PGs, indicating that intestinal PGs form in response to Yp disruption of cytoskeletal dynamics. Notably, mutation of the virulence factor YopH restores PG formation and control of Yp in mice lacking circulating monocytes, demonstrating that monocytes override YopH-dependent blockade of innate immune defence. This work reveals an unappreciated site of Yersinia intestinal invasion and defines host and pathogen drivers of intestinal granuloma formation.

Yersinia interactions with regulated cell death pathways

Cell death in response to infection is conserved across all kingdoms of life. In metazoans, cell death upon bacterial infection is primarily carried out by the cysteine and aspartate protease and receptor-interacting serine/threonine protein kinase families. The Gram-negative bacterial genus Yersinia includes pathogens that cause disease in humans and other animals ranging from plague to gastrointestinal infections. Pathogenic Yersiniae express a type-III secretion system (T3SS), which translocates effectors that disrupt phagocytosis and innate immune signaling to evade immune defenses and replicate extracellularly in infected tissues. Blockade of innate immune signaling, disruption of the actin cytoskeleton, and the membrane-disrupting activity of the T3SS translocon pore, are all sensed by innate immune cells. Here, we discuss recent advances in understanding the pathways that regulate Yersinia-induced cell death, and how manipulation of these cell death pathways over the course of infection promotes bacterial dissemination or host defense.

Protection Induced by Oral Vaccination with a Recombinant Yersinia pseudotuberculosis Delivering Yersinia pestis LcrV and F1 Antigens in Mice and Rats against Pneumonic Plague

A newly attenuated Yersinia pseudotuberculosis strain (designated Yptb1) with triple mutation Δasd ΔyopK ΔyopJ and chromosomal insertion of the Y. pestis caf1R-caf1M-caf1A-caf1 operon was constructed as a live vaccine platform. Yptb1 tailored with an Asd⁺ plasmid (pYA5199) (designated Yptb1[pYA5199]) simultaneously delivers Y. pestis LcrV and F1. The attenuated Yptb1(pYA5199) localized in the Peyer's patches, lung, spleen, and liver for a few weeks after oral immunization without causing any disease symptoms in immunized rodents. An oral prime-boost Yptb1(pYA5199) immunization stimulated potent antibody responses to LcrV, F1, and Y. pestis whole-cell lysate (YPL) in Swiss Webster mice and Brown Norway rats. The prime-boost Yptb1(pYA5199) immunization induced higher antigen-specific humoral and cellular immune responses in mice than a single immunization did, and it provided complete short-term and long-term protection against a high dose of intranasal Y. pestis challenge in mice. Moreover, the prime-boost immunization afforded substantial protection for Brown Norway rats against an aerosolized Y. pestis challenge. Our study highlights that Yptb1(pYA5199) has high potential as an oral vaccine candidate against pneumonic plague.

The interaction of enteric bacterial effectors with the host engulfment pathway control innate immune responses

Host engulfment protein ELMO1 generates intestinal inflammation following internalization of enteric bacteria. In Shigella, bacterial effector IpgB1 interacts with ELMO1 and promotes bacterial invasion. IpgB1 belongs to the WxxxE effector family, a motif found in several effectors of enteric pathogens. Here, we have studied the role of WxxxE effectors, with emphasis on Salmonella SifA and whether it interacts with ELMO1 to regulate inflammation. In-silico-analysis of WxxxE effectors was performed using BLAST search and Clustal W program. The interaction of ELMO1 with SifA was assessed by GST pulldown assay and co-immunoprecipitation. ELMO1 knockout mice, and ELMO1-depleted murine macrophage J774 cell lines were challenged with WT and SifA mutant Salmonella. Bacterial effectors containing the WxxxE motif were transfected in WT and ELMO1-depleted J774 cells to assess the inflammatory cytokines. ELMO1 generates differential pro-inflammatory cytokines between pathogenic and nonpathogenic bacteria. WxxxE motif is present in pathogens and in the TIR domain of host proteins. The C-terminal part of ELMO1 interacts with SifA where WxxxE motif is important for interaction. ELMO1-SifA interaction affects bacterial colonization, dissemination, and inflammatory cytokines in vivo. Moreover, ELMO1-SifA interaction increases TNF-α and IL-6 production from the macrophage cell line and is associated with enhanced Rac1 activity. ELMO1 also interacts with WxxxE effectors IpgB1, IpgB2, and Map and induces inflammation after challenge with microbes or microbial ligands. ELMO1 generates a differential response through interaction with the WxxxE motif, which is absent in commensals. ELMO1-WxxxE interaction plays a role in bacterial pathogenesis and induction of inflammatory response.

Heightened Virulence of Yersinia Is Associated with Decreased Function of the YopJ Protein

Despite the maintenance of YopP/J alleles throughout the human-pathogenic Yersinia lineage, the benefit of YopP/J-induced phagocyte death for Yersinia pathogenesis in animals is not obvious. To determine how the sequence divergence of YopP/J has impacted Yersinia virulence, we examined protein polymorphisms in this type III secreted effector protein across 17 Yersinia species and tested the consequences of polymorphism in a murine model of subacute systemic yersiniosis. Our evolutionary analysis revealed that codon 177 has been subjected to positive selection; the Yersinia enterocolitica residue had been altered from a leucine to a phenylalanine in nearly all Yersinia pseudotuberculosis and Yersinia pestis strains examined. Despite this change being minor, as both leucine and phenylalanine have hydrophobic side chains, reversion of YopJ^F177 to the ancestral YopJ^L177 variant yielded a Y. pseudotuberculosis strain with enhanced cytotoxicity toward macrophages, consistent with previous findings. Surprisingly, expression of YopJ^F177L in the mildly attenuated ksgA^- background rendered the strain completely avirulent in mice. Consistent with this hypothesis that YopJ activity relates indirectly to Yersinia pathogenesis in vivo, ksgA^- strains lacking functional YopJ failed to kill macrophages but actually regained virulence in animals. Also, treatment with the antiapoptosis drug suramin prevented YopJ-mediated macrophage cytotoxicity and enhanced Y. pseudotuberculosis virulence in vivo. Our results demonstrate that Yersinia-induced cell death is detrimental for bacterial pathogenesis in this animal model of illness and indicate that positive selection has driven YopJ/P and Yersinia evolution toward diminished cytotoxicity and increased virulence, respectively.

CpxR regulates the Rcs phosphorelay system in controlling the Ysc-Yop type III secretion system in Yersinia pseudotuberculosis

The CpxRA two-component regulatory system and the Rcs phosphorelay system are both employed by the Enterobacteriaceae family to preserve bacterial envelope integrity and function when growing under stress. Although both systems regulate several overlapping physiological processes, evidence demonstrating a molecular connection between Cpx and Rcs signalling outputs is scarce. Here, we show that CpxR negatively regulates the transcription of the rcsB gene in the Rcs phosphorelay system in Yersinia pseudotuberculosis. Interestingly, transcription of rcsB is under the control of three promoters, which were all repressed by CpxR. Critically, synthetic activation of Cpx signalling through mislocalization of the NlpE lipoprotein to the inner membrane resulted in an active form of CpxR that repressed activity of rcsB promoters. On the other hand, a site-directed mutation of the phosphorylation site at residue 51 in CpxR generated an inactive non-phosphorylated variant that was unable to regulate output from these rcsB promoters. Importantly, CpxR-mediated inhibition of rcsB transcription in turn restricted activation of the Ysc-Yop type III secretion system (T3SS). Moreover, active CpxR blocks zinc-mediated activation of Rcs signalling and the subsequent activation of lcrF transcription. Our results demonstrate a novel regulatory cascade linking CpxR-RcsB-LcrF to control production of the Ysc-Yop T3SS.

Topological analysis of type 3 secretion translocons in native membranes

PFPs (Pore-forming proteins) perforate cellular membranes to create an aqueous pore and allow the passage of ions and polar molecules. The molecular mechanisms for many of these PFPs have been elucidated by combining high resolution structural information of these proteins with biochemical and biophysical approaches. However, some PFPs do not adopt stable conformations and are difficult to study in vitro. An example of these proteins are the bacterial Type 3 Secretion (T3S) translocators. The translocators are secreted by the bacterium and insert into the target cell membrane to form a translocon pore providing a portal for the passage of T3S toxins into eukaryotic cells. Given the important role that the T3S systems play in pathogenesis, methods to study these translocon pores in cellular membranes are needed. Using a combination of protein modifications and methods to selectively permeate and solubilized eukaryotic membranes, we have established an experimental procedure to analyze the topology of the Pseudomonas aeruginosa T3S translocon using P. aeruginosa strain variants and HeLa cell lines.

The Yersinia Type III Secretion System as a Tool for Studying Cytosolic Innate Immune Surveillance

Microbial pathogens have evolved complex mechanisms to interface with host cells in order to evade host defenses and replicate. However, mammalian innate immune receptors detect the presence of molecules unique to the microbial world or sense the activity of virulence factors, activating antimicrobial and inflammatory pathways. We focus on how studies of the major virulence factor of one group of microbial pathogens, the type III secretion system (T3SS) of human pathogenic Yersinia, have shed light on these important innate immune responses. Yersinia are largely extracellular pathogens, yet they insert T3SS cargo into target host cells that modulate the activity of cytosolic innate immune receptors. This review covers both the host pathways that detect the Yersinia T3SS and the effector proteins used by Yersinia to manipulate innate immune signaling.

Gain-of-Function Analysis Reveals Important Virulence Roles for the Yersinia pestis Type III Secretion System Effectors YopJ, YopT, and YpkA

Virulence of Yersinia pestis in mammals requires the type III secretion system, which delivers seven effector proteins into the cytoplasm of host cells to undermine immune responses. All seven of these effectors are conserved across Y. pestis strains, but three, YopJ, YopT, and YpkA, are apparently dispensable for virulence. Some degree of functional redundancy between effector proteins would explain both observations. Here, we use a combinatorial genetic approach to define the minimal subset of effectors required for full virulence in mice following subcutaneous infection. We found that a Y. pestis strain lacking YopJ, YopT, and YpkA is attenuated for virulence in mice and that addition of any one of these effectors to this strain increases lethality significantly. YopJ, YopT, and YpkA likely contribute to virulence via distinct mechanisms. YopJ is uniquely able to cause macrophage cell death in vitro and to suppress accumulation of inflammatory cells to foci of bacterial growth in deep tissue, whereas YopT and YpkA cannot. The synthetic phenotypes that emerge when YopJ, YopT, and YpkA are removed in combination provide evidence that each effector enhances Y. pestis virulence and that YopT and YpkA act through a mechanism distinct from that of YopJ.

The Rickettsial Ankyrin Repeat Protein 2 Is a Type IV Secreted Effector That Associates with the Endoplasmic Reticulum

Strains of Rickettsia rickettsii, the tick-borne agent of Rocky Mountain spotted fever, vary considerably in virulence. Genomic comparisons of R. rickettsii strains have identified a relatively small number of genes divergent in an avirulent strain. Among these is one annotated as Rickettsia ankyrin repeat protein 2 (RARP-2). Homologs of RARP-2 are present in all strains of R. rickettsii, but the protein in the avirulent strain Iowa contains a large internal deletion relative to the virulent Sheila Smith strain. RARP-2 is secreted in a type IV secretion system-dependent manner and exposed to the host cell cytosol. RARP-2 of Sheila Smith colocalizes with multilamellar membranous structures bearing markers of the endoplasmic reticulum (ER), whereas the Iowa protein shows no colocalization with host cell organelles and evidence of proteolytic degradation is detected. Overexpression of Sheila Smith RARP-2 in R. rickettsii Iowa converts this avirulent strain’s typically nonlytic or opaque plaque type to a lytic plaque phenotype similar to that of the virulent Sheila Smith strain. Mutation of a predicted proteolytic active site of Sheila Smith RARP-2 abolished the lytic plaque phenotype but did not eliminate association with host membrane. RARP-2 is thus a type IV secreted effector and released from the rickettsiae into the host cytosol to modulate host processes during infection. Overexpression of Sheila Smith RARP-2 did not, however, restore the virulence of the Iowa strain in a guinea pig model, likely due to the multifactorial nature of rickettsial virulence.

Members of the genus Rickettsia are obligate intracellular bacteria that exhibit a range of virulence from harmless endosymbionts of arthropods to the etiologic agents of severe disease. Despite the growing number of available genomes, little is known regarding virulence determinants of rickettsiae. Here, we have characterized an ankyrin repeat-containing protein, RARP-2, which differs between a highly virulent and an avirulent strain of R. rickettsii, the agent of Rocky Mountain spotted fever. RARP-2 is secreted by a type IV secretion system into the cytosol of the host cell, where it interacts with and manipulates the structure of the endoplasmic reticulum. RARP-2 from the avirulent strain is truncated by the loss of seven of 10 ankyrin repeat units but, although secreted, fails to alter ER structure. Recognition of those rickettsial factors associated with virulence will facilitate understanding of regional and strain-specific variation in severity of disease.

Activation and Evasion of Inflammasomes by Yersinia

The innate immune system plays an essential role in initiating the early response against microbial infection, as well as instructing and shaping subsequent responses. Microbial pathogens are enormously diverse in terms of the niches they occupy, their metabolic properties and requirements, and the cellular pathways that they target. Nevertheless, innate sensing of pathogens triggers a relatively stereotyped set of responses that involve transcriptional induction of key inflammatory mediators, as well as post-translational assembly and activation of a multiprotein inflammatory complex termed 'the inflammasome.' Along with classical Pattern Recognition Receptors, the inflammasome activation pathway has emerged as a key regulator of tissue homeostasis and immune defense. Components of the inflammasome generally exist within the cell in a soluble, monomeric state, and oligomerize in response to diverse enzymatic activities associated with infection or cellular stress. Inflammasome assembly triggers activation of the pro-enzyme caspase-1, resulting in the cleavage of caspase-1 targets. The most extensively studied targets are the cytokines of the IL-1 family, but the recent discovery of Gasdermin D as a novel target of caspase-1 and the related inflammatory caspase, caspase-11, has begun to mechanistically define the links between caspase-1 activation and cell death. Cell death is a hallmark of macrophage infection by many pathogens, including the gram-negative bacterial pathogens of the genus Yersinia. Intriguingly, the activities of the Yersinia-secreted effector proteins and the type III secretion system (T3SS) itself have been linked to both inflammasome activation and evasion during infection. The balance between these activating and inhibitory activities shapes the outcome of Yersinia infection. Here, we describe the current state of knowledge on interactions between Yersinia and the inflammasome system, with the goal of integrating these findings within the general framework of inflammasome responses to microbial pathogens.

Immunomodulatory Yersinia outer proteins (Yops)-useful tools for bacteria and humans alike

Human-pathogenic Yersinia produce plasmid-encoded Yersinia outer proteins (Yops), which are necessary to down-regulate anti-bacterial responses that constrict bacterial survival in the host. These Yops are effectively translocated directly from the bacterial into the target cell cytosol by the type III secretion system (T3SS). Cell-penetrating peptides (CPPs) in contrast are characterized by their ability to autonomously cross cell membranes and to transport cargo - independent of additional translocation systems. The recent discovery of bacterial cell-penetrating effector proteins (CPEs) - with the prototype being the T3SS effector protein YopM - established a new class of autonomously translocating immunomodulatory proteins. CPEs represent a vast source of potential self-delivering, anti-inflammatory therapeutics. In this review, we give an update on the characteristic features of the plasmid-encoded Yops and, based on recent findings, propose the further development of these proteins for potential therapeutic applications as natural or artificial cell-penetrating forms of Yops might be of value as bacteria-derived biologics.

Morphogenesis of Experimental Infection Caused by Plasmid Variants of Yersinia pseudotuberculosis

The dynamics of pathomorphological changes in response to infection with plasmid variants of Yersinia pseudotuberculosis was studied in experimental animals. Variability of cell injuries in pseudotuberculosis histopathology depended on the plasmid-associated virulence of the infection agent. Infection with highly virulent two-plasmid strain pYV48:pVM82 MDa and Y. pseudotuberculosis strain with low virulence with the only plasmid pVM82 MDa led to the development of cell destruction (necrosis and apoptosis) in the target organs. Apoptosis predominated in response to infection by plasmid variant pVM82 MDa with low virulence.

Cell-Extrinsic TNF Collaborates with TRIF Signaling To Promote Yersinia-Induced Apoptosis

Innate immune responses that are crucial for control of infection are often targeted by microbial pathogens. Blockade of NF-κB and MAPK signaling by the Yersinia virulence factor YopJ inhibits cytokine production by innate immune cells but also triggers cell death. This cell death requires RIPK1 kinase activity and caspase-8, which are engaged by TLR4 and the adaptor protein TRIF. Nevertheless, TLR4- and TRIF-deficient cells undergo significant apoptosis, implicating TLR4/TRIF-independent pathways in the death of Yersinia-infected cells. In this article, we report a key role for TNF/TNFR1 in Yersinia-induced cell death of murine macrophages, which occurs despite the blockade of NF-κB and MAPK signaling imposed by Yersinia on infected cells. Intriguingly, direct analysis of YopJ injection revealed a heterogeneous population of injection-high and injection-low cells, and demonstrated that TNF expression came from the injection-low population. Moreover, TNF production by this subpopulation was necessary for maximal apoptosis in the population of highly injected cells, and TNFR-deficient mice displayed enhanced susceptibility to Yersinia infection. These data demonstrate an important role for collaboration between TNF and pattern recognition receptor signals in promoting maximal apoptosis during bacterial infection, and demonstrate that heterogeneity in virulence factor injection and cellular responses play an important role in promoting anti-Yersinia immune defense.

A Single Bacterial Immune Evasion Strategy Dismantles Both MyD88 and TRIF Signaling Pathways Downstream of TLR4

During bacterial infections, Toll-like receptor 4 (TLR4) signals through the MyD88- and TRIF-dependent pathways to promote pro-inflammatory and interferon (IFN) responses, respectively. Bacteria can inhibit the MyD88 pathway, but if the TRIF pathway is also targeted is unclear. We demonstrate that, in addition to MyD88, Yersinia pseudotuberculosis inhibits TRIF signaling through the type III secretion system effector YopJ. Suppression of TRIF signaling occurs during dendritic cell (DC) and macrophage infection and prevents expression of type I IFN and pro-inflammatory cytokines. YopJ-mediated inhibition of TRIF prevents DCs from inducing natural killer (NK) cell production of antibacterial IFNγ. During infection of DCs, YopJ potently inhibits MAPK pathways but does not prevent activation of IKK- or TBK1-dependent pathways. This singular YopJ activity efficiently inhibits TLR4 transcription-inducing activities, thus illustrating a simple means by which pathogens impede innate immunity.

The interplay between regulated necrosis and bacterial infection

Necrosis has long been considered as a passive event resulting from a cell extrinsic stimulus, such as pathogen infection. Recent advances have refined this view and it is now well established that necrosis is tightly regulated at the cell level. Regulated necrosis can occur in the context of host-pathogen interactions, and can either participate in the control of infection or favor it. Here, we review the two main pathways implicated so far in bacteria-associated regulated necrosis: caspase 1-dependent pyroptosis and RIPK1/RIPK3-dependent necroptosis. We present how these pathways are modulated in the context of infection by a series of model bacterial pathogens.

Uncovering an Important Role for YopJ in the Inhibition of Caspase-1 in Activated Macrophages and Promoting Yersinia pseudotuberculosis Virulence

Pathogenic Yersinia species utilize a type III secretion system to translocate Yop effectors into infected host cells. Yop effectors inhibit innate immune responses in infected macrophages to promote Yersinia pathogenesis. In turn,Yersinia-infected macrophages respond to translocation of Yops by activating caspase-1, but different mechanisms of caspase-1 activation occur, depending on the bacterial genotype and the state of phagocyte activation. In macrophages activated with lipopolysaccharide (LPS) prior to Yersinia pseudotuberculosis infection, caspase-1 is activated by a rapid inflammasome-dependent mechanism that is inhibited by translocated YopM. The possibility that other effectors cooperate with YopM to inhibit caspase-1 activation in LPS-activated macrophages has not been investigated. Toward this aim, epistasis analysis was carried out in which the phenotype of aY. pseudotuberculosis yopM mutant was compared to that of a yopJ yopM, yopE yopM, yopH yopM, yopT yopM, or ypkA yopM mutant. Activation of caspase-1 was measured by cleavage of the enzyme, release of interleukin-1β (IL-1β), and pyroptosis in LPS-activated macrophages infected with wild-type or mutant Y. pseudotuberculosis strains. Results show enhanced activation of caspase-1 after infection with the yopJ yopM mutant relative to infection by any other single or double mutant. Similar results were obtained with the yopJ, yopM, and yopJ yopM mutants ofY ersinia pestis Following intravenous infection of mice, theY. pseudotuberculosis yopJ mutant was as virulent as the wild type, while the yopJ yopM mutant was significantly more attenuated than the yopM mutant. In summary, through epistasis analysis this work uncovered an important role for YopJ in inhibiting caspase-1 in activated macrophages and in promoting Yersinia virulence.

Yersinia type III effectors perturb host innate immune responses

The innate immune system is the first line of defense against invading pathogens. Innate immune cells recognize molecular patterns from the pathogen and mount a response to resolve the infection. The production of proinflammatory cytokines and reactive oxygen species, phagocytosis, and induced programmed cell death are processes initiated by innate immune cells in order to combat invading pathogens. However, pathogens have evolved various virulence mechanisms to subvert these responses. One strategy utilized by Gram-negative bacterial pathogens is the deployment of a complex machine termed the type III secretion system (T3SS). The T3SS is composed of a syringe-like needle structure and the effector proteins that are injected directly into a target host cell to disrupt a cellular response. The three human pathogenic Yersinia spp. (Y. pestis, Y. enterocolitica, and Y. pseudotuberculosis) are Gram-negative bacteria that share in common a 70 kb virulence plasmid which encodes the T3SS. Translocation of the Yersinia effector proteins (YopE, YopH, YopT, YopM, YpkA/YopO, and YopP/J) into the target host cell results in disruption of the actin cytoskeleton to inhibit phagocytosis, downregulation of proinflammatory cytokine/chemokine production, and induction of cellular apoptosis of the target cell. Over the past 25 years, studies on the Yersinia effector proteins have unveiled tremendous knowledge of how the effectors enhance Yersinia virulence. Recently, the long awaited crystal structure of YpkA has been solved providing further insights into the activation of the YpkA kinase domain. Multisite autophosphorylation by YpkA to activate its kinase domain was also shown and postulated to serve as a mechanism to bypass regulation by host phosphatases. In addition, novel Yersinia effector protein targets, such as caspase-1, and signaling pathways including activation of the inflammasome were identified. In this review, we summarize the recent discoveries made on Yersinia effector proteins and their contribution to Yersinia pathogenesis.

Manipulation of Interleukin-1β and Interleukin-18 Production by Yersinia pestis Effectors YopJ and YopM and Redundant Impact on Virulence

Innate immunity plays a central role in resolving infections by pathogens. Host survival during plague, caused by the Gram-negative bacterium Yersinia pestis, is favored by a robust early innate immune response initiated by IL-1β and IL-18. These cytokines are produced by a two-step mechanism involving NF-κB-mediated pro-cytokine production and inflammasome-driven maturation into bioactive inflammatory mediators. Because of the anti-microbial effects induced by IL-1β/IL-18, it may be desirable for pathogens to manipulate their production. Y. pestis type III secretion system effectors YopJ and YopM can interfere with different parts of this process. Both effectors have been reported to influence inflammasome caspase-1 activity; YopJ promotes caspase-8-dependent cell death and caspase-1 cleavage, whereas YopM inhibits caspase-1 activity via an incompletely understood mechanism. However, neither effector appears essential for full virulence in vivo Here we report that the sum of influences by YopJ and YopM on IL-1β/IL-18 release is suppressive. In the absence of YopM, YopJ minimally affects caspase-1 cleavage but suppresses IL-1β, IL-18, and other cytokines and chemokines. Importantly, we find that Y. pestis containing combined deletions of YopJ and YopM induces elevated levels of IL-1β/IL-18 in vitro and in vivo and is significantly attenuated in a mouse model of bubonic plague. The reduced virulence of the YopJ-YopM mutant is dependent on the presence of IL-1β, IL-18, and caspase-1. Thus, we conclude that Y. pestis YopJ and YopM can both exert a tight control of host IL-1β/IL-18 production to benefit the bacteria, resulting in a redundant impact on virulence.

The inflammasome: Learning from bacterial evasion strategies

The innate immune system plays a critical role in defense against microbial infection and employs germline-encoded pattern recognition receptors to detect broadly conserved microbial structures or activities. Pattern recognition receptors of the nucleotide binding domain/leucine rich repeat (NLR) family respond to particular microbial products or disruption of cellular physiology, and mediate the activation of an arm of the innate immune response termed the inflammasome. Inflammasomes are multiprotein complexes that are inducibly assembled in response to the contamination of the host cell cytosol by microbial products. Individual NLRs sense the presence of their cognate stimuli, and initiate assembly of inflammasomes via the adaptor protein apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) and the effector pro-enzyme caspase-1. Inflammasome activation leads to rapid release of pro-inflammatory mediators of the IL-1 family as well as the release of intracellular alarmins due to a lytic form of programmed cell death termed pyroptosis. Over the past 15 years, a great deal has been learned about the mechanisms that drive inflammasome activation in response to infection by diverse pathogens. However, pathogens have also evolved mechanisms to evade or suppress host defenses, and the mechanisms by which pathogens evade inflammasome activation are not well-understood. Here, we will discuss emerging evidence on how diverse pathogens evade inflammasome activation, and what these studies have revealed about inflammasome biology. Deeper understanding of pathogen evasion of inflammasome activation has the potential to lead to development of novel classes of immunomodulatory factors that could be used in the context of human inflammatory diseases.

Yersinia pseudotuberculosis YopD mutants that genetically separate effector protein translocation from host membrane disruption

The Yersinia type III secretion system (T3SS) translocates Yop effector proteins into host cells to manipulate immune defenses such as phagocytosis and reactive oxygen species (ROS) production. The T3SS translocator proteins YopB and YopD form pores in host membranes, facilitating Yop translocation. While the YopD amino and carboxy termini participate in pore formation, the role of the YopD central region between amino acids 150-227 remains unknown. We assessed the contribution of this region by generating Y. pseudotuberculosis yopD(Δ150-170) and yopD(Δ207-227) mutants and analyzing their T3SS functions. These strains exhibited wild-type levels of Yop secretion in vitro and enabled robust pore formation in macrophages. However, the yopDΔ150-170 and yopD(Δ207-227) mutants were defective in Yop translocation into CHO cells and splenocyte-derived neutrophils and macrophages. These data suggest that YopD-mediated host membrane disruption and effector Yop translocation are genetically separable activities requiring distinct protein domains. Importantly, the yopD(Δ150-170) and yopD(Δ207-227) mutants were defective in Yop-mediated inhibition of macrophage cell death and ROS production in neutrophil-like cells, and were attenuated in disseminated Yersinia infection. Therefore, the ability of the YopD central region to facilitate optimal effector protein delivery into phagocytes, and therefore robust effector Yop function, is important for Yersinia virulence.

Inflammasome activation in response to the Yersinia type III secretion system requires hyperinjection of translocon proteins YopB and YopD

Type III secretion systems (T3SS) translocate effector proteins into target cells in order to disrupt or modulate host cell signaling pathways and establish replicative niches. However, recognition of T3SS activity by cytosolic pattern recognition receptors (PRRs) of the nucleotide-binding domain leucine rich repeat (NLR) family, either through detection of translocated products or membrane disruption, induces assembly of multiprotein complexes known as inflammasomes. Macrophages infected with Yersinia pseudotuberculosis strains lacking all known effectors or lacking the translocation regulator YopK induce rapid activation of both the canonical NLRP3 and noncanonical caspase-11 inflammasomes. While this inflammasome activation requires a functional T3SS, the precise signal that triggers inflammasome activation in response to Yersinia T3SS activity remains unclear. Effectorless strains of Yersinia as well as ΔyopK strains translocate elevated levels of T3SS substrates into infected cells. To dissect the contribution of pore formation and translocation to inflammasome activation, we took advantage of variants of YopD and LcrH that separate these functions of the T3SS. Notably, YopD variants that abrogated translocation but not pore-forming activity failed to induce inflammasome activation. Furthermore, analysis of individual infected cells revealed that inflammasome activation at the single-cell level correlated with translocated levels of YopB and YopD themselves. Intriguingly, LcrH mutants that are fully competent for effector translocation but produce and translocate lower levels of YopB and YopD also fail to trigger inflammasome activation. Our findings therefore suggest that hypertranslocation of YopD and YopB is linked to inflammasome activation in response to the Yersinia T3SS.

The innate immune response is critical to effective clearance of pathogens. Recognition of conserved virulence structures and activities by innate immune receptors such as NLRs constitute one of the first steps in mounting the innate immune response. However, pathogens such as Yersinia actively evade or subvert components of host defense, such as inflammasomes. The T3SS-secreted protein YopK is an essential virulence factor that limits translocation of other Yops, thereby limiting T3SS-induced inflammasome activation. However, what triggers inflammasome activation in cells infected by YopK-deficient Yersinia is not clear. Our findings indicate that hypertranslocation of pore complex proteins promotes inflammasome activation and that YopK prevents inflammasome activation by the T3SS by limiting translocation of YopD and YopB themselves.

Bacterial programming of host responses: coordination between type I interferon and cell death

During mammalian infection, bacteria induce cell death from an extracellular or intracellular niche that can protect or hurt the host. Data is accumulating that associate type I interferon (IFN) signaling activated by intracellular bacteria with programmed death of immune effector cells and enhanced virulence. Multiple pathways leading to IFN-dependent host cell death have been described, and in some cases it is becoming clear how these mechanisms contribute to virulence. Yet common mechanisms of IFN-enhanced bacterial pathogenesis are not obvious and no specific interferon stimulated genes have yet been identified that cause sensitivity to pathogen-induced cell death. In this review, we will summarize some bacterial infections caused by facultative intracellular pathogens and what is known about how type I IFN signaling may promote the replication of extracellular bacteria rather than stimulate protection. Each of these pathogens can survive phagocytosis but their intracellular life cycles are very different, they express distinct virulence factors and trigger different pathways of immune activation and crosstalk. These differences likely lead to widely varying amounts of type I IFN expression and a different inflammatory environment, but these may not be important to the pathologic effects on the host. Instead, each pathogen induces programmed cell death of key immune cells that have been sensitized by the activation of the type I IFN response. We will discuss how IFN-dependent host cell death may increase host susceptibility and try to understand common pathways of pathogenesis that lead to IFN-enhanced bacterial virulence.

Alveolar macrophages and neutrophils are the primary reservoirs for Legionella pneumophila and mediate cytosolic surveillance of type IV secretion

Legionella pneumophila, an intracellular pathogen responsible for the severe pneumonia Legionnaires' disease, uses its dot/icm-encoded type IV secretion system (T4SS) to translocate effector proteins that promote its survival and replication into the host cell cytosol. However, by introducing bacterial products into the host cytosol, L. pneumophila also activates cytosolic immunosurveillance pathways, thereby triggering robust proinflammatory responses that mediate the control of infection. Thus, the pulmonary cell types that L. pneumophila infects not only may act as an intracellular niche that facilitates its pathogenesis but also may contribute to the immune response against L. pneumophila. The identity of these host cells remains poorly understood. Here, we developed a strain of L. pneumophila producing a fusion protein consisting of β-lactamase fused to the T4SS-translocated effector RalF, which allowed us to track cells injected by the T4SS. Our data reveal that alveolar macrophages and neutrophils both are the primary recipients of T4SS-translocated effectors and harbor viable L. pneumophila during pulmonary infection of mice. Moreover, both alveolar macrophages and neutrophils from infected mice produced tumor necrosis factor and interleukin-1α in response to T4SS-sufficient, but not T4SS-deficient, L. pneumophila. Collectively, our data suggest that alveolar macrophages and neutrophils are both an intracellular reservoir for L. pneumophila and a source of proinflammatory cytokines that contribute to the host immune response against L. pneumophila during pulmonary infection.

Genome-wide mutant fitness profiling identifies nutritional requirements for optimal growth of Yersinia pestis in deep tissue

Rapid growth in deep tissue is essential to the high virulence of Yersinia pestis, causative agent of plague. To better understand the mechanisms underlying this unusual ability, we used transposon mutagenesis and high-throughput sequencing (Tn-seq) to systematically probe the Y. pestis genome for elements contributing to fitness during infection. More than a million independent insertion mutants representing nearly 200,000 unique genotypes were generated in fully virulent Y. pestis. Each mutant in the library was assayed for its ability to proliferate in vitro on rich medium and in mice following intravenous injection. Virtually all genes previously established to contribute to virulence following intravenous infection showed significant fitness defects, with the exception of genes for yersiniabactin biosynthesis, which were masked by strong intercellular complementation effects. We also identified more than 30 genes with roles in nutrient acquisition and metabolism as experiencing strong selection during infection. Many of these genes had not previously been implicated in Y. pestis virulence. We further examined the fitness defects of strains carrying mutations in two such genes—encoding a branched-chain amino acid importer (brnQ) and a glucose importer (ptsG)—both in vivo and in a novel defined synthetic growth medium with nutrient concentrations matching those in serum. Our findings suggest that diverse nutrient limitations in deep tissue play a more important role in controlling bacterial infection than has heretofore been appreciated. Because much is known about Y. pestis pathogenesis, this study also serves as a test case that assesses the ability of Tn-seq to detect virulence genes.

Our understanding of the functions required by bacteria to grow in deep tissues is limited, in part because most growth studies of pathogenic bacteria are conducted on laboratory media that do not reflect conditions prevailing in infected animal tissues. Improving our knowledge of this aspect of bacterial biology is important as a potential pathway to the development of novel therapeutics. Yersinia pestis, the plague bacterium, is highly virulent due to its rapid dissemination and growth in deep tissues, making it a good model for discovering bacterial adaptations that promote rapid growth during infection. Using Tn-seq, a genome-wide fitness profiling technique, we identified several functions required for fitness of Y. pestis in vivo that were not previously known to be important. Most of these functions are needed to acquire or synthesize nutrients. Interference with these critical nutrient acquisition pathways may be an effective strategy for designing novel antibiotics and vaccines.

Caspase-8 mediates caspase-1 processing and innate immune defense in response to bacterial blockade of NF-κB and MAPK signaling

Toll-like receptor signaling and subsequent activation of NF-κB- and MAPK-dependent genes during infection play an important role in antimicrobial host defense. The YopJ protein of pathogenic Yersinia species inhibits NF-κB and MAPK signaling, resulting in blockade of NF-κB-dependent cytokine production and target cell death. Nevertheless, Yersinia infection induces inflammatory responses in vivo. Moreover, increasing the extent of YopJ-dependent cytotoxicity induced by Yersinia pestis and Yersinia pseudotuberculosis paradoxically leads to decreased virulence in vivo, suggesting that cell death promotes anti-Yersinia host defense. However, the specific pathways responsible for YopJ-induced cell death and how this cell death mediates immune defense against Yersinia remain poorly defined. YopJ activity induces processing of multiple caspases, including caspase-1, independently of inflammasome components or the adaptor protein ASC. Unexpectedly, caspase-1 activation in response to the activity of YopJ required caspase-8, receptor-interacting serine/threonine kinase 1 (RIPK1), and Fas-associated death domain (FADD), but not RIPK3. Furthermore, whereas RIPK3 deficiency did not affect YopJ-induced cell death or caspase-1 activation, deficiency of both RIPK3 and caspase-8 or FADD completely abrogated Yersinia-induced cell death and caspase-1 activation. Mice lacking RIPK3 and caspase-8 in their hematopoietic compartment showed extreme susceptibility to Yersinia and were deficient in monocyte and neutrophil-derived production of proinflammatory cytokines. Our data demonstrate for the first time to our knowledge that RIPK1, FADD, and caspase-8 are required for YopJ-induced cell death and caspase-1 activation and suggest that caspase-8-mediated cell death overrides blockade of immune signaling by YopJ to promote anti-Yersinia immune defense.

Caspase-8 and RIP kinases regulate bacteria-induced innate immune responses and cell death

A number of pathogens cause host cell death upon infection, and Yersinia pestis, infamous for its role in large pandemics such as the "Black Death" in medieval Europe, induces considerable cytotoxicity. The rapid killing of macrophages induced by Y. pestis, dependent upon type III secretion system effector Yersinia outer protein J (YopJ), is minimally affected by the absence of caspase-1, caspase-11, Fas ligand, and TNF. Caspase-8 is known to mediate apoptotic death in response to infection with several viruses and to regulate programmed necrosis (necroptosis), but its role in bacterially induced cell death is poorly understood. Here we provide genetic evidence for a receptor-interacting protein (RIP) kinase-caspase-8-dependent macrophage apoptotic death pathway after infection with Y. pestis, influenced by Toll-like receptor 4-TIR-domain-containing adapter-inducing interferon-β (TLR4-TRIF). Interestingly, macrophages lacking either RIP1, or caspase-8 and RIP3, also had reduced infection-induced production of IL-1β, IL-18, TNF, and IL-6; impaired activation of the transcription factor NF-κB; and greatly compromised caspase-1 processing. Cleavage of the proform of caspase-1 is associated with triggering inflammasome activity, which leads to the maturation of IL-1β and IL-18, cytokines important to host responses against Y. pestis and many other infectious agents. Our results identify a RIP1-caspase-8/RIP3-dependent caspase-1 activation pathway after Y. pestis challenge. Mice defective in caspase-8 and RIP3 were also highly susceptible to infection and displayed reduced proinflammatory cytokines and myeloid cell death. We propose that caspase-8 and the RIP kinases are key regulators of macrophage cell death, NF-κB and inflammasome activation, and host resistance after Y. pestis infection.

Genome-wide analysis of small RNAs expressed by Yersinia pestis identifies a regulator of the Yop-Ysc type III secretion system

Small noncoding RNA (sRNA) molecules are integral components of the regulatory machinery for many bacterial species and are known to posttranscriptionally regulate metabolic and stress-response pathways, quorum sensing, virulence factors, and more. The Yop-Ysc type III secretion system (T3SS) is a critical virulence component for the pathogenic Yersinia species, and the regulation of this system is tightly controlled at each step from transcription to translocation of effectors into host cells. The contribution of sRNAs to the regulation of the T3SS in Yersinia has been largely unstudied, however. Previously, our lab identified a role for the sRNA chaperone protein Hfq in the regulation of components of the T3SS in the gastrointestinal pathogen Yersinia pseudotuberculosis. Here we present data demonstrating a similar requirement for Hfq in the closely related species Yersinia pestis. Through deep sequencing analysis of the Y. pestis sRNA-ome, we found 63 previously unidentified putative sRNAs in this species. We identified a Yersinia-specific sRNA, Ysr141, carried by the T3SS plasmid pCD1 that is required for the production of multiple T3SS proteins. In addition, we show that Ysr141 targets an untranslated region upstream of yopJ to posttranscriptionally activate the synthesis of the YopJ protein. Furthermore, Ysr141 may be an unstable and/or processed sRNA, which could contribute to its function in the regulation of the T3SS. The discovery of an sRNA that influences the synthesis of the T3SS adds an additional layer of regulation to this tightly controlled virulence determinant of Y. pestis.

Allelic variation in Salmonella: an underappreciated driver of adaptation and virulence

Salmonella enterica causes substantial morbidity and mortality in humans and animals. Infection and intestinal colonization by S. enterica require virulence factors that mediate bacterial binding and invasion of enterocytes and innate immune cells. Some S. enterica colonization factors and their alleles are host restricted, suggesting a potential role in regulation of host specificity. Recent data also suggest that colonization factors promote horizontal gene transfer of antimicrobial resistance genes by increasing the local density of Salmonella in colonized intestines. Although a profusion of genes are involved in Salmonella pathogenesis, the relative importance of their allelic variation has only been studied intensely in the type 1 fimbrial adhesin FimH. Although other Salmonella virulence factors demonstrate allelic variation, their association with specific metadata (e.g., host species, disease or carrier state, time and geographic place of isolation, antibiotic resistance profile, etc.) remains to be interrogated. To date, genome-wide association studies (GWAS) in bacteriology have been limited by the paucity of relevant metadata. In addition, due to the many variables amid metadata categories, a very large number of strains must be assessed to attain statistically significant results. However, targeted approaches in which genes of interest (e.g., virulence factors) are specifically sequenced alleviates the time-consuming and costly statistical GWAS analysis and increases statistical power, as larger numbers of strains can be screened for non-synonymous single nucleotide polymorphisms (SNPs) that are associated with available metadata. Congruence of specific allelic variants with specific metadata from strains that have a relevant clinical and epidemiological history will help to prioritize functional wet-lab and animal studies aimed at determining cause-effect relationships. Such an approach should be applicable to other pathogens that are being collected in well-curated repositories.

YopP-expressing variant of Y. pestis activates a potent innate immune response affording cross-protection against yersiniosis and tularemia [corrected]

Plague, initiated by Yersinia pestis infection, is a rapidly progressing disease with a high mortality rate if not quickly treated. The existence of antibiotic-resistant Y. pestis strains emphasizes the need for the development of novel countermeasures against plague. We previously reported the generation of a recombinant Y. pestis strain (Kim53ΔJ+P) that over-expresses Y. enterocolitica YopP. When this strain was administered subcutaneously to mice, it elicited a fast and effective protective immune response in models of bubonic, pneumonic and septicemic plague. In the present study, we further characterized the immune response induced by the Kim53ΔJ+P recombinant strain. Using a panel of mouse strains defective in specific immune functions, we observed the induction of a prompt protective innate immune response that was interferon-γ dependent. Moreover, inoculation of mice with Y. pestis Kim53ΔJ+P elicited a rapid protective response against secondary infection by other bacterial pathogens, including the enteropathogen Y. enterocolitica and the respiratory pathogen Francisella tularensis. Thus, the development of new therapies to enhance the innate immune response may provide an initial critical delay in disease progression following the exposure to highly virulent bacterial pathogens, extending the time window for successful treatment.

Subversion of inflammasome activation and pyroptosis by pathogenic bacteria

Activation of the inflammasome occurs in response to a notably high number of pathogenic microbes and is a broad innate immune response that effectively contributes to restriction of pathogen replication and generation of adaptive immunity. Activation of these platforms leads to caspase-1- and/or caspase-11-dependent secretion of proteins, including cytokines, and induction of a specific form of cell death called pyroptosis, which directly or indirectly contribute for restriction of pathogen replication. Not surprisingly, bona fide intracellular pathogens developed strategies for manipulation of cell death to guarantee intracellular replication. In this sense, the remarkable advances in the knowledge of the inflammasome field have been accompanied by several reports characterizing the inhibition of this platform by several pathogenic bacteria. Herein, we review some processes used by pathogenic bacteria, including Yersinia spp., Pseudomonas aeruginosa, Vibrio parahaemolyticus, Chlamydia trachomatis, Francisella tularensis, Shigella flexneri, Legionella pneumophila, and Coxiella burnetii to evade the activation of the inflammasome and the induction of pyroptosis.

c-KIT signaling is targeted by pathogenic Yersinia to suppress the host immune response

Background

The pathogenic Yersinia species exhibit a primarily extracellular lifestyle through manipulation of host signaling pathways that regulate pro-inflammatory gene expression and cytokine release. To identify host genes that are targeted by Yersinia during the infection process, we performed an RNA interference (RNAi) screen based on recovery of host NF-κB-mediated gene activation in response to TNF-α stimulation upon Y. enterocolitica infection.

Results

We screened shRNAs against 782 genes in the human kinome and 26 heat shock genes, and identified 19 genes that exhibited ≥40% relative increase in NF-κB reporter gene activity. The identified genes function in multiple cellular processes including MAP and ERK signaling pathways, ion channel activity, and regulation of cell growth. Pre-treatment with small molecule inhibitors specific for the screen hits c-KIT and CKII recovered NF-κB gene activation and/or pro-inflammatory TNF-α cytokine release in multiple cell types, in response to either Y. enterocolitica or Y. pestis infection.

Conclusions

We demonstrate that pathogenic Yersinia exploits c-KIT signaling in a T3SS-dependent manner to downregulate expression of transcription factors EGR1 and RelA/p65, and pro-inflammatory cytokines. This study is the first major functional genomics RNAi screen to elucidate virulence mechanisms of a pathogen that is primarily dependent on extracellular-directed immunomodulation of host signaling pathways for suppression of host immunity.

Modulation of host immune defenses by Aeromonas and Yersinia species: convergence on toxins secreted by various secretion systems

Like other pathogenic bacteria, Yersinia and Aeromonas species have been continuously co-evolving with their respective hosts. Although the former is a bonafide human pathogen, the latter has gained notararity as an emerging disease-causing agent. In response to immune cell challenges, bacterial pathogens have developed diverse mechanism(s) enabling their survival, and, at times, dominance over various host immune defense systems. The bacterial type three secretion system (T3SS) is evolutionarily derived from flagellar subunits and serves as a vehicle by which microbes can directly inject/translocate anti-host factors/effector proteins into targeted host immune cells. A large number of Gram-negative bacterial pathogens possess a T3SS empowering them to disrupt host cell signaling, actin cytoskeleton re-arrangements, and even to induce host-cell apoptotic and pyroptotic pathways. All pathogenic yersiniae and most Aeromonas species possess a T3SS, but they also possess T2- and T6-secreted toxins/effector proteins. This review will focus on the mechanisms by which the T3SS effectors Yersinia outer membrane protein J (YopJ) and an Aeromonas hydrophila AexU protein, isolated from the diarrheal isolate SSU, mollify host immune system defenses. Additionally, the mechanisms that are associated with host cell apoptosis/pyroptosis by Aeromonas T2SS secreted Act, a cytotoxic enterotoxin, and Hemolysin co-regulated protein (Hcp), an A. hydrophila T6SS effector, will also be discussed.

Early apoptosis of macrophages modulated by injection of Yersinia pestis YopK promotes progression of primary pneumonic plague

Yersinia pestis causes pneumonic plague, a disease characterized by inflammation, necrosis and rapid bacterial growth which together cause acute lung congestion and lethality. The bacterial type III secretion system (T3SS) injects 7 effector proteins into host cells and their combined activities are necessary to establish infection. Y. pestis infection of the lungs proceeds as a biphasic inflammatory response believed to be regulated through the control of apoptosis and pyroptosis by a single, well-conserved T3SS effector protein YopJ. Recently, YopJ-mediated pyroptosis, which proceeds via the NLRP3-inflammasome, was shown to be regulated by a second T3SS effector protein YopK in the related strain Y. pseudotuberculosis. In this work, we show that for Y. pestis, YopK appears to regulate YopJ-mediated apoptosis, rather than pyroptosis, of macrophages. Inhibition of caspase-8 blocked YopK-dependent apoptosis, suggesting the involvement of the extrinsic pathway, and appeared cell-type specific. However, in contrast to yopJ, deletion of yopK caused a large decrease in virulence in a mouse pneumonic plague model. YopK-dependent modulation of macrophage apoptosis was observed at 6 and 24 hours post-infection (HPI). When YopK was absent, decreased populations of macrophages and dendritic cells were seen in the lungs at 24 HPI and correlated with resolution rather than progression of inflammation. Together the data suggest that Y. pestis YopK may coordinate the inflammatory response during pneumonic plague through the regulation of apoptosis of immune cells.

In this work, we studied the mechanism whereby bacteria manipulate innate immune responses by controlling host cell death. Yersinia pestis, the causative agent of plague, requires effector Yops of the Type III Secretion System (T3SS) to evade the innate immune system during infection. We show that Yersinia induces apoptosis of macrophages through two distinct mechanisms, each through the activity of the well-characterized T3SS effector YopJ, yet regulated in an opposing manner through the activity of a second effector protein YopK. In a murine pneumonic plague model, we found evidence that YopK regulates apoptosis of macrophages during the early stage of infection, leading to uncontrolled inflammation and disease. In contrast, the absence of YopK-regulated apoptosis allowed recruitment of lymphocytes and CCR2⁺ immune cells which led to bacterial clearance and resolution of inflammation. Together the data suggest that Yersinia YopK modulates apoptosis of immune cells to control the inflammatory response during plague.

Cell death programs in Yersinia immunity and pathogenesis

Cell death plays a central role in host-pathogen interactions, as it can eliminate the pathogen's replicative niche and provide pro-inflammatory signals necessary for an effective immune response; conversely, cell death can allow pathogens to eliminate immune cells and evade anti-microbial effector mechanisms. In response to developmental signals or cell-intrinsic stresses, the executioner caspases-3 and -7 mediate apoptotic cell death, which is generally viewed as immunologically silent or immunosuppressive. A proinflammatory form of cell death that requires caspase-1, termed pyroptosis, is activated in response to microbial products within the host cytosol or disruption of cellular membranes by microbial pathogens. Infection by the bacterial pathogen Yersinia has features of both apoptosis and pyroptosis. Cell death and caspase-1 processing in Yersinia-infected cells occur in response to inhibition of NF-κB and MAPK signaling by the Yersinia virulence factor YopJ. However, the molecular basis of YopJ-induced cell death, and the role of different death pathways in anti-Yersinia immune responses remain enigmatic. Here, we discuss the role that cell death may play in inducing specific pro-inflammatory signals that shape innate and adaptive immune responses against Yersinia infection.

The role of innate immunity in the host defense against intestinal bacterial pathogens

Eradication of infectious disease is our global health challenge. After encountering intestinal infection with a bacterial pathogen, the host defense program is initiated by local antigen-presenting cells (APCs) that eliminate invading pathogens by phagocytosis and establish localized inflammation by secreting cytokines and chemokines. These pathogen-experienced APCs migrate to the mesenteric lymph nodes, where host immune responses are precisely orchestrated. Initiation and regulation of this defense program appear to be largely dependent on innate immunity which is antigen non-specific and provides a rapid defense against broader targets. On the other hand, many bacterial enteropathogens have evoked abilities to modify the host defense program to their advantage. Therefore, better understanding of the host-pathogen interactions is essential to establish effective eradication strategies for enteric infectious diseases. In this review, we will discuss the current understanding of innate immune regulation of the host defense mechanisms against intestinal infection by bacterial pathogens.

Yersinia pseudotuberculosis effector YopJ subverts the Nod2/RICK/TAK1 pathway and activates caspase-1 to induce intestinal barrier dysfunction

Yersinia pseudotuberculosis is an enteropathogenic bacteria that disrupts the intestinal barrier and invades its host through gut-associated lymphoid tissue and Peyer's patches (PP). We show that the Y. pseudotuberculosis effector YopJ induces intestinal barrier dysfunction by subverting signaling of the innate immune receptor Nod2, a phenotype that can be reversed by pretreating with the Nod2 ligand muramyl-dipeptide. YopJ, but not the catalytically inactive mutant YopJ(C172A), acetylates critical sites in the activation loops of the RICK and TAK1 kinases, which are central mediators of Nod2 signaling, and decreases the affinity of Nod2 for RICK. Concomitantly, Nod2 interacts with and activates caspase-1, resulting in increased levels of IL-1β. Finally, IL-1β within PP plays an essential role in inducing intestinal barrier dysfunction. Thus, YopJ alters intestinal permeability and promotes the dissemination of Yersinia as well as commensal bacteria by exploiting the mucosal inflammatory response.

YopJ-induced caspase-1 activation in Yersinia-infected macrophages: independent of apoptosis, linked to necrosis, dispensable for innate host defense

Yersinia outer protein J (YopJ) is a type III secretion system (T3SS) effector of pathogenic Yersinia (Yersinia pestis, Yersinia enterocolitica and Yersinia pseudotuberculosis) that is secreted into host cells. YopJ inhibits survival response pathways in macrophages, causing cell death. Allelic variation of YopJ is responsible for differential cytotoxicity in Yersinia strains. YopJ isoforms in Y. enterocolitica O:8 (YopP) and Y. pestis KIM (YopJ^KIM) strains have high cytotoxic activity. In addition, YopJ^KIM-induced macrophage death is associated with caspase-1 activation and interleukin-1β (IL-1β secretion. Here, the mechanism of YopJ^KIM-induced cell death, caspase-1 activation, and IL-1β secretion in primary murine macrophages was examined. Caspase-3/7 activity was low and the caspase-3 substrate poly (ADP-ribose) polymerase (PARP) was not cleaved in Y. pestis KIM5-infected macrophages. In addition, cytotoxicity and IL-1β secretion were not reduced in the presence of a caspase-8 inhibitor, or in B-cell lymphoma 2 (Bcl-2)-associated X protein (Bax)/Bcl-2 homologous antagonist/killer (Bak) knockout macrophages, showing that YopJ^KIM-mediated cell death and caspase-1 activation occur independent of mitochondrial-directed apoptosis. KIM5-infected macrophages released high mobility group protein B1 (HMGB1), a marker of necrosis, and microscopic analysis revealed that necrotic cells contained active caspase-1, indicating that caspase-1 activation is associated with necrosis. Inhibitor studies showed that receptor interacting protein 1 (RIP1) kinase and reactive oxygen species (ROS) were not required for cytotoxicity or IL-β release in KIM5-infected macrophages. IL-1β secretion was reduced in the presence of cathepsin B inhibitors, suggesting that activation of caspase-1 requires cathepsin B activity. Ectopically-expressed YopP caused higher cytotoxicity and secretion of IL-1β in Y. pseudotuberculosis-infected macrophages than YopJ^KIM. Wild-type and congenic caspase 1 knockout C57BL/6 mice were equally susceptible to lethal infection with Y. pseudotuberculosis ectopically expressing YopP. These data suggest that YopJ-induced caspase-1 activation in Yersinia-infected macrophages is a downstream consequence of necrotic cell death and is dispensable for innate host resistance to a strain with enhanced cytotoxicity.

Host innate recognition of an intestinal bacterial pathogen induces TRIF-dependent protective immunity

TRIF signaling triggers the amplification of macrophage bactericidal activity sufficient to eliminate invading intestinal pathogens through the sequential induction of IFN-β and IFN-γ from macrophages and NK cells, respectively.

Toll-like receptor 4 (TLR4), which signals through the adapter molecules myeloid differentiation factor 88 (MyD88) and toll/interleukin 1 receptor domain-containing adapter inducing IFN-β (TRIF), is required for protection against Gram-negative bacteria. TRIF is known to be important in TLR3-mediated antiviral signaling, but the role of TRIF signaling against Gram-negative enteropathogens is currently unknown. We show that TRIF signaling is indispensable for establishing innate protective immunity against Gram-negative Yersinia enterocolitica. Infection of wild-type mice rapidly induced both IFN-β and IFN-γ in the mesenteric lymph nodes. In contrast, TRIF-deficient mice were defective in these IFN responses and showed impaired phagocytosis in regional macrophages, resulting in greater bacterial dissemination and mortality. TRIF signaling may be universally important for protection against Gram-negative pathogens, as TRIF-deficient macrophages were also impaired in killing both Salmonella and Escherichia coli in vitro. The mechanism of TRIF-mediated protective immunity appears to be orchestrated by macrophage-induced IFN-β and NK cell production of IFN-γ. Sequential induction of IFN-β and IFN-γ leads to amplification of macrophage bactericidal activity sufficient to eliminate the invading pathogens at the intestinal interface. Our results demonstrate a previously unknown role of TRIF in host resistance to Gram-negative enteropathogens, which may lead to effective strategies for combating enteric infections.

A protective epitope in type III effector YopE is a major CD8 T cell antigen during primary infection with Yersinia pseudotuberculosis

Virulence in human-pathogenic Yersinia species is associated with a plasmid-encoded type III secretion system that translocates a set of Yop effector proteins into host cells. One effector, YopE, functions as a Rho GTPase-activating protein (GAP). In addition to acting as a virulence factor, YopE can function as a protective antigen. C57BL/6 mice infected with attenuated Yersinia pestis generate a dominant H2-Kb-restricted CD8 T cell response to an epitope in the N-terminal domain of YopE (YopE69-77), and intranasal vaccination with the YopE69-77 peptide and the mucosal adjuvant cholera toxin (CT) elicits CD8 T cells that are protective against lethal pulmonary challenge with Y. pestis. Because YopE69-77 is conserved in many Yersinia strains, we sought to determine if YopE is a protective antigen for Yersinia pseudotuberculosis and if primary infection with this enteric pathogen elicits a CD8 T cell response to this epitope. Intranasal immunization with the YopE69-77 peptide and CT elicited a CD8 T cell response that was protective against lethal intragastric Y. pseudotuberculosis challenge. The YopE69-77 epitope was a major antigen (∼30% of splenic CD8 T cells were specific for this peptide at the peak of the response) during primary infection with Y. pseudotuberculosis, as shown by flow cytometry tetramer staining. Results of infections with Y. pseudotuberculosis expressing catalytically inactive YopE demonstrated that GAP activity is dispensable for a CD8 T cell response to YopE69-77. Determining the features of YopE that are important for this response will lead to a better understanding of how protective CD8 T cell immunity is generated against Yersinia and other pathogens with type III secretion systems.

A Yersinia effector with enhanced inhibitory activity on the NF-κB pathway activates the NLRP3/ASC/caspase-1 inflammasome in macrophages

A type III secretion system (T3SS) in pathogenic Yersinia species functions to translocate Yop effectors, which modulate cytokine production and regulate cell death in macrophages. Distinct pathways of T3SS-dependent cell death and caspase-1 activation occur in Yersinia-infected macrophages. One pathway of cell death and caspase-1 activation in macrophages requires the effector YopJ. YopJ is an acetyltransferase that inactivates MAPK kinases and IKKβ to cause TLR4-dependent apoptosis in naïve macrophages. A YopJ isoform in Y. pestis KIM (YopJ^KIM) has two amino acid substitutions, F177L and K206E, not present in YopJ proteins of Y. pseudotuberculosis and Y. pestis CO92. As compared to other YopJ isoforms, YopJ^KIM causes increased apoptosis, caspase-1 activation, and secretion of IL-1β in Yersinia-infected macrophages. The molecular basis for increased apoptosis and activation of caspase-1 by YopJ^KIM in Yersinia-infected macrophages was studied. Site directed mutagenesis showed that the F177L and K206E substitutions in YopJ^KIM were important for enhanced apoptosis, caspase-1 activation, and IL-1β secretion. As compared to YopJ^CO92, YopJ^KIM displayed an enhanced capacity to inhibit phosphorylation of IκB-α in macrophages and to bind IKKβ in vitro. YopJ^KIM also showed a moderately increased ability to inhibit phosphorylation of MAPKs. Increased caspase-1 cleavage and IL-1β secretion occurred in IKKβ-deficient macrophages infected with Y. pestis expressing YopJ^CO92, confirming that the NF-κB pathway can negatively regulate inflammasome activation. K⁺ efflux, NLRP3 and ASC were important for secretion of IL-1β in response to Y. pestis KIM infection as shown using macrophages lacking inflammasome components or by the addition of exogenous KCl. These data show that caspase-1 is activated in naïve macrophages in response to infection with a pathogen that inhibits IKKβ and MAPK kinases and induces TLR4-dependent apoptosis. This pro-inflammatory form of apoptosis may represent an early innate immune response to highly virulent pathogens such as Y. pestis KIM that have evolved an enhanced ability to inhibit host signaling pathways.

Pathogenic bacteria in the genus Yersinia use multiple virulence determinants to counteract innate immunity and facilitate infection. A type III system in Yersinia translocates an effector called YopJ that elicits cell death in macrophages. YopJ inhibits the production of survival factors in naïve macrophages, causing them to die by apoptosis, which is generally considered to be immunologically silent. However, recent studies show that caspase-1, a key regulator of pro-inflammatory responses, is activated in Yersinia-infected macrophages undergoing apoptosis. How caspase-1 is activated during YopJ-induced macrophage apoptosis is not known. We have identified a distinct isoform of YopJ in Y. pestis (YopJ^KIM) that induces high levels of apoptosis and caspase-1 activation in infected macrophages. In this study, the molecular basis for the increased activity of YopJ^KIM was studied with the goal of better understanding the underlying mechanism of caspase-1 activation. The data show that YopJ^KIM has two amino acid changes that give it an enhanced ability to inhibit survival signals in macrophages. The increased apoptosis may cause membrane permeability, resulting in efflux of ions and activation of caspase-1. Therefore, apoptosis of naïve macrophages inflicted by highly virulent pathogens may not be immunologically silent.

Destabilization of YopE by the ubiquitin-proteasome pathway fine-tunes Yop delivery into host cells and facilitates systemic spread of Yersinia enterocolitica in host lymphoid tissue

Pathogenic Yersinia species inject a panel of Yop virulence proteins by type III protein secretion into host cells to modulate cellular defense responses. This enables the survival and dissemination of the bacteria in the host lymphoid tissue. We have previously shown that YopE of the Y. enterocolitica serogroup O8 is degraded in the host cell through the ubiquitin-proteasome pathway. YopE normally manipulates rearrangements of the actin cytoskeleton and triggers phagocytosis resistance. To shed light into the physiological role of YopE inactivation, we mutagenized the lysine polyubiquitin acceptor sites of YopE in the Y. enterocolitica serogroup O8 virulence plasmid. The resulting mutant strain escaped polyubiquitination and degradation of YopE and displayed increased intracellular YopE levels, which was accompanied by a pronounced cytotoxic effect on infected cells. Despite its intensified activity on cultured cells, the Yersinia mutant with stabilized YopE showed reduced dissemination into liver and spleen following enteral infection of mice. Furthermore, the accumulation of degradation-resistant YopE was accompanied by the diminished delivery of YopP and YopH into cultured, Yersinia-infected cells. A role of YopE in the regulation of Yop translocation has already been described. Our results imply that the inactivation of YopE by the proteasome could be a tool to ensure intermediate intracellular YopE levels, which may effectuate optimized Yop injection into host cells. In this regard, Y. enterocolitica O8 appears to exploit the host ubiquitin proteasome system to destabilize YopE and to fine-tune the activities of the Yop virulence arsenal on the infected host organism.

Mathematical relationship between cytokine concentrations and pathogen levels during infection

The relationship between concentrations of cytokines and microbial pathogen levels during infection is not clear. In a sub-lethal murine infection model using Gram-negative bacterial pathogen Yersinia pseudotuberculosis, the serum concentrations (C) of pro-inflammatory cytokines tumor necrosis factor α (TNFα), interferon γ (IFNγ), interleukine-1β (IL-1β) and interleukine-18 (IL-18) formed a mathematical relationship with the splenic pathogen levels (P) as measured by colony forming unit. Naming parameters "m" and "k" for magnitude and kinetics, respectively, the relationship is depicted as C=mP(k). When reanalyzing the TNFα and IFNγ concentrations and the bacterial levels that were determined by other groups during infection with another strain of Y. pseudotuberculosis or with Yersinia pestis, this relationship was maintained. Interestingly, the changes in the values of "m" and "k" were consistent with the progress of the host immune response during infection; while deviation from this relationship was observed in individuals that seemed to be unable to control infection. Furthermore, in a murine model of ricin intoxication the local concentrations of the cytokine monocyte chemotactic protein 1 (MCP-1) and the concentrations of injected castor bean toxin ricin also conform to this relationship. C=mP(k) could be a general relationship in host cytokine response to pathogens or pathogen-associated molecular patterns. If confirmed, this type of analysis will be very useful in identifying the steps in a host immune response with which a pathogen interferes. It will also help to determine the specific functions of a host factor in the immune response.