A hypervariable N-terminal region of Yersinia LcrV determines Toll-like receptor 2-mediated IL-10 induction and mouse virulence

Microenvironmental impact on lung cell homeostasis and immunity during infection

The lung is a vital organ devoted mainly to gas exchange with an external environment that may be contaminated with various life-threatening pathogens and inert particles. Lung immunity must be permanently balanced between costimulatory and coinhibitory signals, thus controlling potential pathogens while avoiding detrimental inflammation. The lung harbors macrophages and dendritic cells (myeloid and plasmacytoid), which orchestrate the primary defense against microbial invaders. During an infection involving host-microbial synapses, microbes either escape by using host cell physiology or are eliminated by a robust immune response. We thus focus on the dynamics of such cellular interactions within the lung and stress the critical role played by airway epithelial cells in modulating immunity.

Current challenges in the development of vaccines for pneumonic plague

Inhalation of Yersinia pestis bacilli causes pneumonic plague, a rapidly progressing and exceptionally virulent disease. Extensively antibiotic-resistant Y. pestis strains exist and we currently lack a safe and effective pneumonic plague vaccine. These facts raise concern that Y. pestis may be exploited as a bioweapon. Here, I review the history and status of plague vaccine research and advocate that pneumonic plague vaccines should strive to prime both humoral and cellular immunity.

Subversion of innate immunity by periodontopathic bacteria via exploitation of complement receptor-3

The capacity of certain pathogens to exploit innate immune receptors enables them to undermine immune clearance and persist in their host, often causing disease. Here we review subversive interactions of Porphyromonas gingivalis, a major periodontal pathogen, with the complement receptor-3 (CR3; CD11b/CD18) in monocytes/macrophages. Through its cell surface fimbriae, P. gingivalis stimulates Toll-like receptor-2 (TLR2) inside-out signaling which induces the high-affinity conformation of CR3. Although this activates CR3-dependent monocyte adhesion and transendothelial migration, P. gingivalis has co-opted this TLR2 proadhesive pathway for CR3 binding and intracellular entry. In CR3-deficient macrophages, the internalization of P. gingivalis is reduced twofold but its ability to survive intracellularly is reduced 1,000-fold, indicating that CR3 is exploited by the pathogen as a relatively safe portal of entry. The interaction of P. gingivalis fimbriae with CR3 additionally inhibits production of bioactive (p70) interleukin-12, which mediates immune clearance. In vivo blockade of CR3 leads to reduced persistence of P. gingivalis in the mouse host and diminished ability to cause periodontal bone loss, the hallmark of periodontal disease. Strikingly, the ability of P. gingivalis to interact with and exploit CR3 depends upon quantitatively minor components (FimCDE) of its fimbrial structure, which predominantly consists of polymerized fimbrillin (FimA). Indeed, isogenic mutants lacking FimCDE but expressing FimA are dramatically less persistent and virulent than the wildtype organism both in vitro and in vivo. This model of immune evasion through CR3 exploitation by P. gingivalis supports the concept that pathogens evolved to manipulate innate immune function for promoting their adaptive fitness.

How the structural gene products of Yersinia pestis relate to virulence

Bubonic plague is the most devastating acute infectious disease known to man. The causative agent, Yersinia pestis, is now more firmly entrenched in sylvatic reservoirs throughout the world than at any time in the past. Consequently, the organism increasingly causes casual human disease and is readily available for use as a bioweapon. Recent attempts to understand the severe nature of plague have focused upon its very recent divergence from Yersinia pseudotuberculosis, an etiological instrument of chronic enteropathogenic infection. This review emphasizes that the invasive nature of plague and its dissemination by fleabite is mediated by plasmids not shared by enteropathogenic yersiniae. The basis for high lethality is considered within the context of chromosomal degeneration causing loss of normal metabolic functions and modification of virulence factors, permitting a terminal anti-inflammatory phase associated with pronounced septicemia.

Variability of the protein sequences of lcrV between epidemic and atypical rhamnose-positive strains of Yersinia pestis

Sequencing of lcrV genes and comparison of the deduced amino acid sequences from ten Y. pestis strains belonging mostly to the group of atypical rhamnose-positive isolates (non-pestis subspecies or pestoides group) showed that the LcrV proteins analyzed could be classified into five sequence types. This classification was based on major amino acid polymorphisms among LcrV proteins in the four "hot points" of the protein sequences. Some additional minor polymorphisms were found throughout these sequence types. The "hot points" corresponded to amino acids 18 (Lys --> Asn), 72 (Lys --> Arg), 273 (Cys --> Ser), and 324-326 (Ser-Gly-Lys --> Arg) in the LcrV sequence of the reference Y. pestis strain CO92. One possible explanation for polymorphism in amino acid sequences of LcrV among different strains is that strain-specific variation resulted from adaptation of the plague pathogen to different rodent and lagomorph hosts.

Type III secretion systems and disease

Type III secretion systems (T3SSs) are complex bacterial structures that provide gram-negative pathogens with a unique virulence mechanism enabling them to inject bacterial effector proteins directly into the host cell cytoplasm, bypassing the extracellular milieu. Although the effector proteins vary among different T3SS pathogens, common pathogenic mechanisms emerge, including interference with the host cell cytoskeleton to promote attachment and invasion, interference with cellular trafficking processes, cytotoxicity and barrier dysfunction, and immune system subversion. The activity of the T3SSs correlates closely with infection progression and outcome, both in animal models and in human infection. Therefore, to facilitate patient care and improve outcomes, it is important to understand the T3SS-mediated virulence processes and to target T3SSs in therapeutic and prophylactic development efforts.

Toll-like receptor 2-mediated interleukin-8 expression in gingival epithelial cells by the Tannerella forsythia leucine-rich repeat protein BspA

Tannerella forsythia is a gram-negative anaerobe strongly associated with chronic human periodontitis. This bacterium expresses a cell surface-associated and secreted protein, designated BspA, which has been recognized as an important virulence factor. The BspA protein belongs to the leucine-rich repeat (LRR) and bacterial immunoglobulin-like protein families. BspA is, moreover, a multifunctional protein which interacts with a variety of host cells, including monocytes which appear to respond to BspA through Toll-like receptor (TLR) signaling. Since gingival epithelium forms a barrier against periodontal pathogens, this study was undertaken to determine if gingival epithelial cells respond to BspA challenge and if TLRs play any role in BspA recognition. This study was also directed towards identifying the BspA domains responsible for cellular activation. We provide direct evidence for BspA binding to TLR2 and demonstrate that the release of the chemokine interleukin-8 from human gingival epithelial cells by BspA is TLR2 dependent. Furthermore, the LRR domain of BspA is involved in activation of TLR2, while TLR1 serves as a signaling partner. Thus, our findings suggest that BspA is an important modulator of host innate immune responses through activation of TLR2 in cooperation with TLR1.

Complement receptor 3 blockade promotes IL-12-mediated clearance of Porphyromonas gingivalis and negates its virulence in vivo

The ability of certain pathogens to exploit innate immune function allows them to undermine immune clearance and thereby increase their persistence and capacity to cause disease. Porphyromonas gingivalis is a major pathogen in periodontal disease and is associated with increased risk of systemic conditions. We have previously shown that the fimbriae of P. gingivalis interact with complement receptor 3 (CR3; CD11b/CD18) in monocytes/macrophages, resulting in inhibition of IL-12p70 production in vitro. The in vivo biological implications of this observation were investigated in this study using a CR3 antagonist (XVA143). XVA143 was shown to block CR3 binding of P. gingivalis fimbriae and reverse IL-12p70 inhibition; specifically, CR3 blockade resulted in inhibition of ERK1/2 phosphorylation and up-regulation of IL-12 p35 and p40 mRNA expression. Importantly, mice pretreated with XVA143 elicited higher IL-12p70 and IFN-gamma levels in response to P. gingivalis i.p. infection and displayed enhanced pathogen clearance, compared with similarly infected controls. The notion that CR3 is associated with reduced IL-12p70 induction and impaired P. gingivalis clearance was confirmed using i.p. infected wild-type and CR3-deficient mice. Moreover, XVA143 dramatically attenuated the persistence and virulence of P. gingivalis in experimental mouse periodontitis, as evidenced by reduced induction of periodontal bone loss. Therefore, CR3 blockade may represent a promising immunomodulatory approach for controlling human periodontitis and possibly associated systemic diseases.

Diminished LcrV secretion attenuates Yersinia pseudotuberculosis virulence

Many gram-negative bacterial pathogenicity factors that function beyond the outer membrane are secreted via a contact-dependent type III secretion system. Two types of substrates are predestined for this mode of secretion, namely, antihost effectors that are translocated directly into target cells and the translocators required for targeting of the effectors across the host cell membrane. N-terminal secretion signals are important for recognition of the protein cargo by the type III secretion machinery. Even though such signals are known for several effectors, a consensus signal sequence is not obvious. One of the translocators, LcrV, has been attributed other functions in addition to its role in translocation. These functions include regulation, presumably via interaction with LcrG inside bacteria, and immunomodulation via interaction with Toll-like receptor 2. Here we wanted to address the significance of the specific targeting of LcrV to the exterior for its function in regulation, effector targeting, and virulence. The results, highlighting key N-terminal amino acids important for LcrV secretion, allowed us to dissect the role of LcrV in regulation from that in effector targeting/virulence. While only low levels of exported LcrV were required for in vitro effector translocation, as deduced by a cell infection assay, fully functional export of LcrV was found to be a prerequisite for its role in virulence in the systemic murine infection model.

Prevention and treatment of colitis with Lactococcus lactis secreting the immunomodulatory Yersinia LcrV protein

BACKGROUND & AIMS

The low calcium response V (LcrV) protein synthesized by gram-negative, pathogenic yersiniae participates in bacterial evasion of the host's innate immune response by stimulating synthesis of the anti-inflammatory interleukin (IL)-10 and preventing the activation of proinflammatory cytokines.

METHODS

We genetically engineered the food-grade bacterium Lactococcus lactis to secrete the LcrV protein from the enteropathogenic species Yersinia pseudotuberculosis. The protective and therapeutic potential of orally administered LcrV-secreting L lactis was evaluated in 2 models of acute experimental colitis (induced by trinitrobenzene sulfonic acid [TNBS] and dextran sodium sulfate [DSS], respectively) in wild-type and knockout mice.

RESULTS

Oral administration of LcrV-secreting L lactis led to active delivery of LcrV and induction of IL-10 (via a Toll-like receptor 2-dependent pathway) in the colon and prevented TNBS-induced colitis, in contrast to the L lactis control not producing LcrV. Down-regulation of tissue inflammatory markers correlated well with the reduction in damage to the colonic mucosa. In contrast, TNBS-induced colitis was not prevented in IL-10(-/-) mice pretreated with LcrV-secreting L lactis, thus showing that IL-10 is required for LcrV protection. Administration of LcrV-secreting L lactis also proved to be very effective in preventing and treating acute DSS-induced colitis.

CONCLUSIONS

LcrV-secreting L lactis decreased experimentally induced intestinal inflammation in 2 murine models of colitis. This novel approach highlights the potential of using pathogen-derived immunomodulating molecules in vivo as novel therapeutics for inflammatory bowel diseases.

Serogroup-related escape of Yersinia enterocolitica YopE from degradation by the ubiquitin-proteasome pathway

Pathogenic Yersinia spp. employ a type III protein secretion system that translocates several Yersinia outer proteins (Yops) into the host cell to modify the host immune response. One strategy of the infected host cell to resist the bacterial attack is degradation and inactivation of injected bacterial virulence proteins through the ubiquitin-proteasome pathway. The cytotoxin YopE is a known target protein of this major proteolytic system in eukaryotic cells. Here, we investigated the sensitivity of YopE belonging to different enteropathogenic Yersinia enterocolitica serogroups to ubiquitination and proteasomal degradation. Analysis of the YopE protein levels in proteasome inhibitor-treated versus untreated cells revealed that YopE from the highly pathogenic Y. enterocolitica serotype O8 was subjected to proteasomal destabilization, whereas the YopE isotypes from serogroups O3 and O9 evaded degradation. Accumulation of YopE from serotypes O3 and O9 was accompanied by an enhanced cytotoxic effect. Using Yersinia strains that specifically produced YopE from either Y. enterocolitica O8 or O9, we found that only the YopE protein from serogroup O8 was modified by polyubiquitination, although both YopE isotypes were highly homologous. We determined two unique N-terminal lysines (K62 and K75) in serogroup O8 YopE, not present in serogroup O9 YopE, that served as polyubiquitin acceptor sites. Insertion of either lysine in serotype O9 YopE enabled its ubiquitination and destabilization. These results define a serotype-dependent difference in the stability and activity of the Yersinia effector protein YopE that could influence Y. enterocolitica pathogenesis.

General and specific host responses to bacterial infection in Peyer's patches: a role for stromelysin-1 (matrix metalloproteinase-3) during Salmonella enterica infection

Salmonella enterica serovar Typhimurium (S. typhimurium) and Yersinia enterocolitica are enteric pathogens capable of colonizing and inducing inflammatory responses in Peyer's patches (PPs) and mesenteric lymph nodes (MLNs). Although the tissue colonization pattern is similar between these two pathogens, their pathogenic lifestyles are quite different. For example, while S. typhimurium is primarily an intracellular pathogen, Y. enterocolitica survives primarily extracellularly. We determined and compared the transcriptional changes occurring in response to S. typhimurium and Y. enterocolitica colonization of PP using Affymetrix GeneChip technology. Both pathogens elicited a general inflammatory response indicated by the upregulation of cytokines and chemokines. However, specific differences were also observed, most notably in the transcriptional regulation of gamma interferon (IFN-gamma) and IFN-gamma-regulated genes in response to S. typhimurium but not Y. enterocolitica. Of particular note, a group of genes encoding matrix metalloproteinases (MMPs) had increased transcript numbers in the PPs following infection with both pathogens. The experiments described here compare oral S. typhimurium or Y. enterocolitica infection in stromelysin-1 (MMP-3)-deficient mice (mmp-3(-/-)) with mice possessing functional MMP-3 (mmp-3(+/+)). There was little difference in the survival of MMP-3-deficient mice infected with Y. enterocolitica when compared with littermate controls. Surprisingly though, mmp-3(-/-) mice were markedly more resistant to S. typhimurium infection than the control mice. S. typhimurium was able to colonize mmp-3(-/-) mice, albeit in a delayed fashion, to equivalent levels as mmp-3(+/+) mice. Nevertheless, significantly lower levels of inflammatory cytokines were detected in tissues and serum in the mmp-3(-/-) mice in comparison with mmp-3(+/+) mice. We hypothesize that MMP-3 is involved in initiating an early and lethal cytokine response to S. typhimurium colonization.

Transcriptional responses in spleens from mice exposed to Yersinia pestis CO92

Yersinia pestis is one of the most threatening biological agents due to the associated high mortality and history of plague pandemics. Identifying molecular players in the host response to infection may enable the development of medical countermeasures against Y. pestis. In this study, microarrays were used to identify the host splenic response mechanisms to Y. pestis infection. Groups of Balb/c mice were injected intraperitoneally with 2-257CFU of Y. pestis strain CO92 or vehicle. One group was assessed for mortality rates and another group for transcriptional analysis. The time to death at the 8 and 257CFU challenge doses were 5.0+/-2.3 and 3.8+/-0.4 days, respectively. Gene profiling using Affymetrix Mouse Genome 430 2.0 Arrays revealed no probe sets were significantly altered for all five mice in the low-dose group when compared to the vehicle controls. However, 534 probe sets were significantly altered in the high dose versus vehicle controls; 384 probe sets were down-regulated and 150 probe sets were up-regulated. The predominant biological processes identified were immune function, cytoskeletal, apoptosis, cell cycle, and protein degradation. This study provides new information on the underlying transcriptional mechanisms in mice to Y. pestis infection.

Attachment of LcrV from Yersinia pestis at dual binding sites to human TLR-2 and human IFN-gamma receptor

The virulence antigen (V-antigen, LcrV) of Yersinia pestis, the causative agent of bubonic plague, is an established protective antigen known to regulate, target, and mediate type III translocation of cytotoxic yersiniae outer proteins termed Yops; LcrV also prompts TLR2-dependent upregulation of anti-inflammatory IL-10. In this study, we determined the parameters of specific interaction of LcrV with TLR2 expressed on human transfected HEK293 cells (TLR2+/CD14-), VTEC2.HS cells (TLR2+/CD14-), primary monocytes (TLR2+/CD14+), and THP-1 cells (TLR2+/CD14+). The IRRL314-317 motif of the extracellular domain of human and mouse TLR2 accounted for high-affinity binding of LcrV. The CD14 co-receptor did not influence this interaction. LcrV did not bind to human U937 (TLR2-/CD14-) and alveolar macrophages (TLR2-/CD14+) in the absence of receptor-bound human IFN-gamma or a synthetic C-terminal fragment (hIFN-gamma132-143). The latter, but not mouse IFN-gamma (or synthetic control peptides), shared a GRRA138-141 site necessary for high-affinity specific binding. LcrV of Y. pestis shares the N-terminal LEEL32-35 binding site of Yersinia enterocolitica and also has an exposed internal DEEI203-206 binding site. Comparison of binding constants and consideration of steric restrictions indicate that binding is not cooperative and only the internal site binds LcrV to target cells. Both the LEEL32-35 and DEEI203-206 binding sites are removed by five amino acids from DKN residues associated with biological activity of bound LcrV. LcrV of Y. pestis promoted both TLR2/CD14-dependent and TLR2/CD14-independent amplification of IL-10 and concomitant downregulation of TNF-alpha in human target cells. The ability of LcrV to utilize human IFN-gamma (a major inflammatory effector of innate immunity) to minimize inflammation is insidious and may account in part for the severe symptoms of plague in man.

Evaluation of the role of LcrV-Toll-like receptor 2-mediated immunomodulation in the virulence of Yersinia pestis

Pathogenic members of the Yersinia genus require the translocator protein LcrV for proper function of the type III secretion apparatus, which is crucial for virulence. LcrV has also been reported to play an independent immunosuppressive role via the induction of interleukin-10 (IL-10) through stimulation of Toll-like receptor 2 (TLR2). To investigate the LcrV-TLR2 interaction in vitro, His-tagged recombinant LcrV (rLcrV) from Yersinia pestis was cloned and expressed in Escherichia coli and purified through Ni-nitrilotriacetic acid column chromatography. High concentrations (5 microg/ml) of rLcrV stimulated TLR2 in vitro. Fractionation of rLcrV preparations via gel filtration revealed that only a minor component consisting of high-molecular-weight multimers or aggregates has TLR2 stimulating activity. Dimer and tetramer forms of rLcrV, which constitute the bulk of the material, do not have this activity. To investigate the potential role of LcrV/TLR2 in plague pathogenesis, we infected wild-type and TLR2(-/-) mice with virulent Y. pestis. No discernible difference between the two mouse strains in severity of disease or kinetics of survival after subcutaneous challenge was observed. IL-6, tumor necrosis factor, and IL-10 levels from spleen homogenates; bacterial load; and the extent of inflammation observed in organs from mice infected intravenously were also indistinguishable in both mouse strains. Taken together, our data indicate that the most abundant molecular species of Y. pestis LcrV do not efficiently activate TLR2-signaling and that TLR2-mediated immunomodulation is unlikely to play a significant role in plague.

The weak interaction of LcrV and TLR2 does not contribute to the virulence of Yersinia pestis

Yersinia pestis and the enteropathogenic Yersinia pseudotuberculosis and Yersinia enterocolitica share the virulence-antigen LcrV. Previously, using reverse genetics we have proven that LcrV contributes to the virulence of Y. enterocolitica serotype O:8 by inducing IL-10 via Toll-like receptor 2 (TLR2). However, both the ability of Y. pestis LcrV to activate TLR2 and a possible role of TLR2-dependent IL-10 induction by LcrV in Y. pestis are not yet known. To eliminate interference from additional protein sequences, we produced LcrVs without affinity tags from Y. pestis and from Y. enterocolitica O:8 (LcrVO:8). LcrVO:8 was much more potent in TLR2-activity than Y. pestis LcrV. To analyse the role of TLR2 in plague, we infected both wild-type and TLR2-/- mice subcutaneously with Y. pestis GB. While TLR2-/- mice exhibited lower blood levels of IL-10 (day 2 post-infection) and of the pro-inflammatory cytokines TNF-alpha, IFN-gamma and MCP-1 (day 4) than wild-type mice, there was no significant difference in survival. The low TLR2-activity of Y. pestis LcrV and associated cytokine expression might explain why - in contrast to Y. enterocolitica O:8 infection - TLR2-deficient mice are not more resistant than wild-type mice in a bubonic plague model.

Growth of Yersinia pseudotuberculosis in mice occurs independently of Toll-like receptor 2 expression and induction of interleukin-10

Pathogenic Yersinia translocates effector proteins into target cells via a type III secretion system (TTSS), modulating the host immune response. A component of the TTSS translocon, LcrV, has been implicated in preventing inflammation through Toll-like receptor 2 (TLR2) by inducing expression of the anti-inflammatory cytokine interleukin-10 (IL-10). TLR2(-/-) mice were reported to be less susceptible to the enteropathogen Yersinia enterocolitica. To determine whether TLR2 also plays a role in recognition of the enteropathogen Yersinia pseudotuberculosis and whether this results in an immune response that is detrimental to the host, we evaluated the macrophage cytokine response to live Y. pseudotuberculosis and analyzed the susceptibility of TLR2(-/-) mice to enteropathogenic Yersinia. We find that Yersinia induction of macrophage IL-10 occurs independently of TLR2 and LcrV and is blocked by the TTSS. In particular, the TTSS effector protein YopJ, which inhibits production of the inflammatory cytokine tumor necrosis factor alpha (TNF-alpha), also inhibits IL-10 expression. Consistent with these results, IL-10 is undetectable in Y. pseudotuberculosis-infected mouse tissues until advanced stages of infection. In addition, we find that TLR2(-/-) mice (derived independently from those used in previous studies) do not display altered susceptibility to enteropathogenic Yersinia compared to wild-type mice. Tissue levels of IL-10, as well as the inflammatory cytokines TNF-alpha, IL-6, and gamma interferon and the chemokine macrophage chemotactic protein 1, are similar in TLR2(+/+) and TLR2(-/-) mice during enteropathogenic Yersinia infection. Therefore, the absence of TLR2 alone does not affect the cytokine response of macrophages to, or the in vivo growth and survival of, enteropathogenic Yersinia.

IL-10 deficiency promotes increased Borrelia burgdorferi clearance predominantly through enhanced innate immune responses

Borrelia burgdorferi is capable of persistently infecting a variety of hosts despite eliciting potent innate and adaptive immune responses. Preliminary studies indicated that IL-10-deficient (IL-10(-/-)) mice exhibit up to 10-fold greater clearance of B. burgdorferi from target tissues compared with wild-type mice, establishing IL-10 as the only cytokine currently known to have such a significant effect on spirochetal clearance. To further delineate these IL-10-mediated immune effects, kinetic studies indicated that spirochete dissemination to target tissues is similar in both wild-type and IL-10(-/-) mouse strains, and that enhanced clearance of B. burgdorferi in IL-10(-/-) mice is correlated with increased B. burgdorferi-specific Ab as early as 2 wk postinfection. Immunoblot analysis indicated that Abs produced by infected IL-10(-/-) and wild-type mice recognize similar ranges of spirochetal Ags. Immune sera from IL-10(-/-) and wild-type mice also exhibited similar bactericidal activity in vitro, and passive transfer of these immune sera into B. burgdorferi-infected SCID mice caused similar reductions of bacterial numbers in target tissues. Infectious dose studies indicated that 8-fold more B. burgdorferi were needed to efficiently infect naive IL-10(-/-) mice, suggesting these animals possess higher innate barriers to infection. Moreover, macrophages derived from IL-10(-/-) mice exhibit enhanced proinflammatory responses to B. burgdorferi stimulation compared with wild-type controls, and these responses are not significantly affected by the presence of immune serum. These findings confirm that B. burgdorferi clearance by innate immune responses is more efficient in the absence of IL-10, and these activities are not directly related to increased levels of B. burgdorferi-specific Ab.

Immunogenicity and protective immunity against bubonic plague and pneumonic plague by immunization of mice with the recombinant V10 antigen, a variant of LcrV

In contrast to Yersinia pestis LcrV, the recombinant V10 (rV10) variant (lacking residues 271 to 300) does not suppress the release of proinflammatory cytokines by immune cells. Immunization with rV10 generates robust antibody responses that protect mice against bubonic plague and pneumonic plague, suggesting that rV10 may serve as an improved plague vaccine.

Type III secretion gets an LcrV tip

Type III secretion is used by many Gram-negative pathogenic bacteria to inject effector proteins into eukaryotic host cells. Effector delivery requires a secretion apparatus, called an injectisome or needle complex, and the assembly of a translocation pore in a target-cell membrane. Recent work provides evidence that enlightens the view of how pore assembly might occur and of how the injectisome and the pore might be linked.

Yersinia's stratagem: targeting innate and adaptive immune defense

In contrast to Salmonella and Shigella, enteropathogenic Yersinia species are extracellular multiplying Gram-negative bacteria. This life style requires a sophisticated anti-host strategy, which is implemented by the Yersinia virulence plasmid. This plasmid encodes the type 3 secretion system (injectisome), at least six microinjected anti-host effector proteins, a trimeric coiled coil outer membrane protein (Yersinia adhesin) with cell adhesin and protective functions against complement and defensins, and the released V antigen, which has Toll-like receptor 2 agonist activity.

Anti-Immunology: Evasion of the Host Immune System by Bacterial and Viral Pathogens

Multicellular organisms possess very sophisticated defense mechanisms that are designed to effectively counter the continual microbial insult of the environment within the vertebrate host. However, successful microbial pathogens have in turn evolved complex and efficient methods to overcome innate and adaptive immune mechanisms, which can result in disease or chronic infections. Although the various virulence strategies used by viral and bacterial pathogens are numerous, there are several general mechanisms that are used to subvert and exploit immune systems that are shared between these diverse microbial pathogens. The success of each pathogen is directly dependant on its ability to mount an effective anti-immune response within the infected host, which can ultimately result in acute disease, chronic infection, or pathogen clearance. In this review, we highlight and compare some of the many molecular mechanisms that bacterial and viral pathogens use to evade host immune defenses.