The Yersinia Yops inhibit invasion of Listeria, Shigella and Edwardsiella but not Salmonella into epithelial cells

A TNF-IL-1 circuit controls Yersinia within intestinal pyogranulomas

Tumor necrosis factor (TNF) is a pleiotropic inflammatory cytokine that mediates antimicrobial defense and granuloma formation in response to infection by numerous pathogens. We previously reported that Yersinia pseudotuberculosis colonizes the intestinal mucosa and induces the recruitment of neutrophils and inflammatory monocytes into organized immune structures termed pyogranulomas (PG) that control Yersinia infection. Inflammatory monocytes are essential for the control and clearance of Yersinia within intestinal PG, but how monocytes mediate Yersinia restriction is poorly understood. Here, we demonstrate that TNF signaling in monocytes is required for bacterial containment following enteric Yersinia infection. We further show that monocyte-intrinsic TNFR1 signaling drives the production of monocyte-derived interleukin-1 (IL-1), which signals through IL-1 receptors on non-hematopoietic cells to enable PG-mediated control of intestinal Yersinia infection. Altogether, our work reveals a monocyte-intrinsic TNF-IL-1 collaborative inflammatory circuit that restricts intestinal Yersinia infection.

A TNF-IL-1 circuit controls Yersinia within intestinal granulomas

Tumor necrosis factor (TNF) is a pleiotropic inflammatory cytokine that mediates antimicrobial defense and granuloma formation in response to infection by numerous pathogens. Yersinia pseudotuberculosis colonizes the intestinal mucosa and induces recruitment of neutrophils and inflammatory monocytes into organized immune structures termed pyogranulomas that control the bacterial infection. Inflammatory monocytes are essential for control and clearance of Yersinia within intestinal pyogranulomas, but how monocytes mediate Yersinia restriction is poorly understood. Here, we demonstrate that TNF signaling in monocytes is required for bacterial containment following enteric Yersinia infection. We further show that monocyte-intrinsic TNFR1 signaling drives production of monocyte-derived interleukin-1 (IL-1), which signals through IL-1 receptor on non-hematopoietic cells to enable pyogranuloma-mediated control of Yersinia infection. Altogether, our work reveals a monocyte-intrinsic TNF-IL-1 collaborative circuit as a crucial driver of intestinal granuloma function, and defines the cellular target of TNF signaling that restricts intestinal Yersinia infection.

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 pseudotuberculosis YopE prevents uptake by M cells and instigates M cell extrusion in human ileal enteroid-derived monolayers

Many pathogens use M cells to access the underlying Peyer's patches and spread to systemic sites via the lymph as demonstrated by ligated loop murine intestinal models. However, the study of interactions between M cells and microbial pathogens has stalled due to the lack of cell culture systems. To overcome this obstacle, we use human ileal enteroid-derived monolayers containing five intestinal cell types including M cells to study the interactions between the enteric pathogen, Yersinia pseudotuberculosis (Yptb), and M cells. The Yptb type three secretion system (T3SS) effector Yops inhibit host defenses including phagocytosis and are critical for colonization of the intestine and Peyer's patches. Therefore, it is not understood how Yptb traverses through M cells to breach the epithelium. By growing Yptb under two physiological conditions that mimic the early infectious stage (low T3SS-expression) or host-adapted stage (high T3SS-expression), we found that large numbers of Yptb specifically associated with M cells, recapitulating murine studies. Transcytosis through M cells was significantly higher by Yptb expressing low levels of T3SS, because YopE and YopH prevented Yptb uptake. YopE also caused M cells to extrude from the epithelium without inducing cell-death or disrupting monolayer integrity. Sequential infection with early infectious stage Yptb reduced host-adapted Yptb association with M cells. These data underscore the strength of enteroids as a model by discovering that Yops impede M cell function, indicating that early infectious stage Yptb more effectively penetrates M cells while the host may defend against M cell penetration of host-adapted Yptb.

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.

Role of Host Type IA Phosphoinositide 3-Kinase Pathway Components in Invasin-Mediated Internalization of Yersinia enterocolitica

Many bacterial pathogens subvert mammalian type IA phosphoinositide 3-kinase (PI3K) in order to induce their internalization into host cells. How PI3K promotes internalization is not well understood. Also unclear is whether type IA PI3K affects different pathogens through similar or distinct mechanisms. Here, we performed an RNA interference (RNAi)-based screen to identify components of the type IA PI3K pathway involved in invasin-mediated entry of Yersinia enterocolitica, an enteropathogen that causes enteritis and lymphadenitis. The 69 genes targeted encode known upstream regulators or downstream effectors of PI3K. A similar RNAi screen was previously performed with the food-borne bacterium Listeria monocytogenes The results of the screen with Y. enterocolitica indicate that at least nine members of the PI3K pathway are needed for invasin-mediated entry. Several of these proteins, including centaurin-α1, Dock180, focal adhesion kinase (FAK), Grp1, LL5α, LL5β, and PLD2 (phospholipase D2), were recruited to sites of entry. In addition, centaurin-α1, FAK, PLD2, and mTOR were required for remodeling of the actin cytoskeleton during entry. Six of the human proteins affecting invasin-dependent internalization also promote InlB-mediated entry of L. monocytogenes Our results identify several host proteins that mediate invasin-induced effects on the actin cytoskeleton and indicate that a subset of PI3K pathway components promote internalization of both Y. enterocolitica and L. monocytogenes.

Trypanosoma cruzi Differentiates and Multiplies within Chimeric Parasitophorous Vacuoles in Macrophages Coinfected with Leishmania amazonensis

The trypanosomatids Leishmania amazonensis and Trypanosoma cruzi are excellent models for the study of the cell biology of intracellular protozoan infections. After their uptake by mammalian cells, the parasitic protozoan flagellates L. amazonensis and T. cruzi lodge within acidified parasitophorous vacuoles (PVs). However, whereas L. amazonensis develops in spacious, phagolysosome-like PVs that may enclose numerous parasites, T. cruzi is transiently hosted within smaller vacuoles from which it soon escapes to the host cell cytosol. To investigate if parasite-specific vacuoles are required for the survival and differentiation of T. cruzi, we constructed chimeric vacuoles by infection of L. amazonensis amastigote-infected macrophages with T. cruzi epimastigotes (EPIs) or metacyclic trypomastigotes (MTs). These chimeric vacuoles, easily observed by microscopy, allowed the entry and fate of T. cruzi in L. amazonensis PVs to be dynamically recorded by multidimensional imaging of coinfected cells. We found that although T. cruzi EPIs remained motile and conserved their morphology in chimeric vacuoles, T. cruzi MTs differentiated into amastigote-like forms capable of multiplying. These results demonstrate that the large adaptive vacuoles of L. amazonensis are permissive to T. cruzi survival and differentiation and that noninfective EPIs are spared from destruction within the chimeric PVs. We conclude that T. cruzi differentiation can take place in Leishmania-containing vacuoles, suggesting this occurs prior to their escape into the host cell cytosol.

Identification of mammalian proteins that collaborate with type III secretion system function: involvement of a chemokine receptor in supporting translocon activity

The type III secretion system (T3SS) is a highly conserved protein delivery system found in multiple Gram-negative pathogens, including Yersinia pseudotuberculosis. Most studies of Yersinia species type III intoxication of host cells have focused on the bacterial determinants that promote assembly and function of the secretion system. In this study, we performed a pooled RNA interference (RNAi) screen to identify mammalian host proteins required for the cytotoxic effects associated with the Yersinia translocated substrate YopE, a GTPase-activating protein (GAP) that inactivates the small Rho GTPases. Cell populations were positively selected for short hairpin RNAs (shRNAs) that interfere with YopE activity using a combination of fluorescence resonance energy transfer (FRET) and flow cytometry, and the degree of enrichment was determined by deep sequencing. Analysis of the candidates identified by the enrichment process revealed that many were important for the initial step of Y. pseudotuberculosis T3SS function, YopB/D pore formation. These candidates included shRNA that depleted downstream effectors of RhoA signaling, coated pit formation, and receptors involved in cell signaling, including the chemokine receptor CCR5 (chemokine [C-C motif] receptor 5). Depletion of CCR5 in 293T cells yielded a defect in YopB/D pore formation and effector translocation, while both phenotypes could be complemented by overexpression of CCR5 protein. Yop effector translocation was also decreased in isolated primary phagocytic cells from a Ccr5^−/− knockout mouse. We postulate that CCR5 acts to promote translocation by modulating cytoskeletal activities necessary for proper assembly of the YopB/D translocation pore. Overall, this study presents a new approach to investigating the contribution of the host cell to T3SS in Y. pseudotuberculosis.

Many Gram-negative bacteria require type III secretion systems (T3SS) for host survival, making these highly specialized secretion systems good targets for antimicrobial agents. After the bacterium binds to host cells, T3SS deposit proteins into the cytosol of host cells through a needle-like appendage and a protein translocon channel. Translocation of proteins via this system is highly regulated, and the contribution of the host cell in promoting assembly and insertion of the channel into the plasma membrane, folding of the bacterial proteins, and trafficking of these substrates are all poorly characterized events. In this study, we identified host cell proteins important for activity of YopE, a Yersinia pseudotuberculosis T3SS-delivered protein. The results demonstrate that insertion and assembly of the translocon are complex processes, requiring a variety of membrane trafficking and cytoskeletal processes, as well as a surprising role for cell surface signaling molecules in supporting proper function.

The adhesive protein invasin of Yersinia pseudotuberculosis induces neutrophil extracellular traps via β1 integrins

Yersinia pseudotuberculosis adhesive protein invasin is crucial for the bacteria to cross the intestine epithelium by binding to β1 integrins on M-cells and gaining access to the underlying tissues. After the crossing invasin can bind to β1 integrins on other cell surfaces, however effector proteins delivered by the type III secretion system Y. pseudotuberculosis efficiently inhibit potential immune responses induced by this interaction. Here, we use mutant Y. pseudotuberculosis strains lacking the type III secretion system and additionally invasin-expressing Escherichia coli to analyze neutrophil responses towards invasin. Our data reveals that invasin induces production of reactive oxygen species and release of chromatin into the extracellular milieu, which we confirmed to be neutrophil extracellular traps by immunofluorescence microscopy. This was mediated through β1 integrins and was dependent on both the production of reactive oxygen species and signaling through phosphoinositide 3-kinase. We therefore have gained insight into a potential role of integrins in inflammation and infection clearance that has not previously been described, suggesting that targeting of β1 integrins could be utilized as an adjunctive therapy against yersiniosis.

IscR is essential for yersinia pseudotuberculosis type III secretion and virulence

Type III secretion systems (T3SS) are essential for virulence in dozens of pathogens, but are not required for growth outside the host. Therefore, the T3SS of many bacterial species are under tight regulatory control. To increase our understanding of the molecular mechanisms behind T3SS regulation, we performed a transposon screen to identify genes important for T3SS function in the food-borne pathogen Yersinia pseudotuberculosis. We identified two unique transposon insertions in YPTB2860, a gene that displays 79% identity with the E. coliiron-sulfur cluster regulator, IscR. A Y. pseudotuberculosis iscR in-frame deletion mutant (ΔiscR) was deficient in secretion of Ysc T3SS effector proteins and in targeting macrophages through the T3SS. To determine the mechanism behind IscR control of the Ysc T3SS, we carried out transcriptome and bioinformatic analysis to identify Y. pseudotuberculosis genes regulated by IscR. We discovered a putative IscR binding motif upstream of the Y. pseudotuberculosis yscW-lcrF operon. As LcrF controls transcription of a number of critical T3SS genes in Yersinia, we hypothesized that Yersinia IscR may control the Ysc T3SS through LcrF. Indeed, purified IscR bound to the identified yscW-lcrF promoter motif and mRNA levels of lcrF and 24 other T3SS genes were reduced in Y. pseudotuberculosis in the absence of IscR. Importantly, mice orally infected with the Y. pseudotuberculosis ΔiscR mutant displayed decreased bacterial burden in Peyer's patches, mesenteric lymph nodes, spleens, and livers, indicating an essential role for IscR in Y. pseudotuberculosis virulence. This study presents the first characterization of Yersinia IscR and provides evidence that IscR is critical for virulence and type III secretion through direct regulation of the T3SS master regulator, LcrF.

Bacterial pathogens use regulators that sense environmental cues to enhance their fitness. Here, we identify a transcriptional regulator in the human gut pathogen, Yersinia pseudotuberculosis, which controls a specialized secretion system essential for bacterial growth in mammalian tissues. This regulator was shown in other bacterial species to alter its activity in response to changes in iron concentration and oxidative stress, but has never been studied in Yersinia. Importantly, Y. pseudotuberculosis experiences large changes in iron bioavailability upon transit from the gut to deeper tissues and iron is a critical component in Yersinia virulence, as individuals with iron overload disorders have enhanced susceptibility to systemic Yersinia infections. Our work places this iron-modulated transcriptional regulator within the regulatory network that controls virulence gene expression in Y. pseudotuberculosis, identifying it as a potential new target for antimicrobial agents.

The C-terminal tail of Yersinia pseudotuberculosis YopM is critical for interacting with RSK1 and for virulence

Yersinia spp. undermine the immune responses of infected animals by translocating Yops directly into host cells with a type III secretion system. YopM, a leucine-rich repeat protein, is a critical virulence factor in infection. YopM localizes to both the nucleus and the cytoplasm in cultured cells, interacts with mammalian p90 ribosomal S6 kinase 1 (RSK1), and causes a decrease in NK cell populations in spleens. Little is known about the molecular interaction between YopM and RSK1 and its significance in pathogenesis. We performed a systematic deletion analysis of YopM in Yersinia pseudotuberculosis to determine which regions are required for RSK1 interactions, nuclear localization, virulence, and changes in immune cell populations during infection of mice. Full-length YopM associated with RSK1 in at least two protein complexes in infected cells, and deletion of its C-terminal tail abrogated all RSK1 interactions. The C-terminal tail was required for tissue colonization, as yopM mutants that failed to interact with RSK1 were as defective for tissue colonization as was a DeltayopM mutant; however, nuclear localization of YopM was not dependent on its RSK1 interaction. Mutants expressing YopM proteins which do not interact with RSK1 caused more pathology than did the DeltayopM mutant, suggesting that there are other RSK1-independent functions of YopM. Histopathological and flow cytometric analyses of spleens showed that infection with wild-type Y. pseudotuberculosis caused an influx of neutrophils, while mice infected with yopM mutants had increased numbers of macrophages. Decreases in NK cells after Y. pseudotuberculosis infection did not correlate with YopM expression. In conclusion, the C terminus of YopM is essential for RSK1 interactions and for virulence.

Salmonella enterica serovar typhimurium invades fibroblasts by multiple routes differing from the entry into epithelial cells

Fibroblasts are ubiquitous cells essential to tissue homeostasis. Despite their nonphagocytic nature, fibroblasts restrain replication of intracellular bacterial pathogens such as Salmonella enterica serovar Typhimurium. The extent to which the entry route of the pathogen determines this intracellular response is unknown. Here, we analyzed S. Typhimurium invasion in fibroblasts obtained from diverse origins, including primary cultures and stable nontransformed cell lines derived from normal tissues. Features distinct to the invasion of epithelial cells were found in all fibroblasts tested. In some fibroblasts, bacteria lacking the type III secretion system encoded in the Salmonella pathogenicity island 1 displayed significant invasion rates and induced the formation of lamellipodia and filopodia at the fibroblast-bacteria contact site. Other bacterial invasion traits observed in fibroblasts were the requirement of phosphatidylinositol 3-kinase, mitogen-activated protein kinase MEK1, and both actin filaments and microtubules. RNA interference studies showed that different Rho family GTPases are targeted by S. Typhimurium to enter into distinct fibroblasts. Rac1 and Cdc42 knockdown affected invasion of normal rat kidney fibroblasts, whereas none of the GTPases tested (Rac1, Cdc42, RhoA, or RhoG) was essential for invasion of immortalized human foreskin fibroblasts. Collectively, these data reveal a marked diversity in the modes used by S. Typhimurium to enter into fibroblasts.

ROS-inhibitory activity of YopE is required for full virulence of Yersinia in mice

YopE, a type III secreted effector of Yersinia, is a GTPase Activating Protein for Rac1 and RhoA whose catalytic activity is critical for virulence. We found that YopE also inhibited reactive oxygen species (ROS) production and inactivated Rac2. How YopE distinguishes among its targets and which specific targets are critical for Yersinia survival in different tissues are unknown. A screen identifying YopE mutants in Yersinia pseudotuberculosis that interact with different Rho GTPases showed that YopE residues at positions 102, 106, 109 and 156 discern among switch I and II regions of Rac1, Rac2 and RhoA. Two mutants, which expressed YopE alleles with different antiphagocytic, ROS-inhibitory and cell-rounding activities, YptbL109A and YptbESptP, were studied in animal infections. Inhibition of both phagocytosis and ROS production were required for splenic colonization, whereas fewer YopE activities were required for Peyer's patch colonization. This study shows that Y. pseudotuberculosis encounters multiple host defences in different tissues and uses distinct YopE activities to disable them.

Innate immune recognition of Yersinia pseudotuberculosis type III secretion

Specialized protein translocation systems are used by many bacterial pathogens to deliver effector proteins into host cells that interfere with normal cellular functions. How the host immune system recognizes and responds to this intrusive event is not understood. To address these questions, we determined the mammalian cellular response to the virulence-associated type III secretion system (T3SS) of the human pathogen Yersinia pseudotuberculosis. We found that macrophages devoid of Toll-like receptor (TLR) signaling regulate expression of 266 genes following recognition of the Y. pseudotuberculosis T3SS. This analysis revealed two temporally distinct responses that could be separated into activation of NFkappaB- and type I IFN-regulated genes. Extracellular bacteria were capable of triggering these signaling events, as inhibition of bacterial uptake had no effect on the ensuing innate immune response. The cytosolic peptidoglycan sensors Nod1 and Nod2 and the inflammasome component caspase-1 were not involved in NFkappaB activation following recognition of the Y. pseudotuberculosis T3SS. However, caspase-1 was required for secretion of the inflammatory cytokine IL-1beta in response to T3SS-positive Y. pseudotuberculosis. In order to characterize the bacterial requirements for induction of this novel TLR-, Nod1/2-, and caspase-1-independent response, we used Y. pseudotuberculosis strains lacking specific components of the T3SS. Formation of a functional T3SS pore was required, as bacteria expressing a secretion needle, but lacking the pore-forming proteins YopB or YopD, did not trigger these signaling events. However, nonspecific membrane disruption could not recapitulate the NFkappaB signaling triggered by Y. pseudotuberculosis expressing a functional T3SS pore. Although host cell recognition of the T3SS did not require known translocated substrates, the ensuing response could be modulated by effectors such as YopJ and YopT, as YopT amplified the response, while YopJ dampened it. Collectively, these data suggest that combined recognition of the T3SS pore and YopBD-mediated delivery of immune activating ligands into the host cytosol informs the host cell of pathogenic challenge. This leads to a unique, multifactorial response distinct from the canonical immune response to a bacterium lacking a T3SS.

Host responses to secreted Shigella virulence factors

Shigella and related enteropathogens deliver effector molecules into the cytoplasm of epithelial cells and macrophages via a type III secretion system. Epithelial cells respond to contact with Shigella by rearranging the cytoskeleton, which leads to uptake of the bacterium. Apart from several cytoskeletal proteins, this process involves the recruitment and activation of kinases, and the small GTPase rho. Macrophages infected with Shigella undergo apoptosis and release mature IL-1beta, a pro-inflammatory cytokine. This apoptotic pathway requires caspase-1 (IL-1beta-converting enzyme). Pro-inflammatory macrophage apoptosis triggers acute shigellosis and might be relevant in other infectious diseases.

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.

High-throughput assays of phagocytosis, phagosome maturation, and bacterial invasion

Ingestion of foreign particles by macrophages and neutrophils and the fate of the vacuole that contains the ingested material are generally monitored by optical microscopy. Invasion of host cells by pathogenic bacteria and their intracellular proliferation are similarly studied by microscopy or by plating assays. These labor-intensive and time-consuming methods limit the number of assays that can be performed. The effort required to test multiple reagents or conditions can be prohibitive. We describe high-throughput assays of phagocytosis and of phagosomal maturation. An automated fluorescence microscope-based platform and associated analysis software were used to study Fcγ receptor-mediated phagocytosis of IgG-opsonized particles by cultured murine macrophages. Phagosomal acidification was measured as an index of maturation. The same platform was similarly used to implement high-throughput assays of invasion of mammalian cells by pathogenic bacteria. The invasion of HeLa cells by Salmonella and the subsequent intracellular proliferation of the bacteria were measured rapidly and reliably in large populations of cells. These high-throughput methods are ideally suited for the efficient screening of chemical libraries to select potential drugs and of small interference RNA libraries to identify essential molecules involved in critical steps of the immune response.

The adaptor molecules LAT and SLP-76 are specifically targeted by Yersinia to inhibit T cell activation

T cell responses are critical to the survival of Yersinia-infected animals. Yersinia have the ability to directly suppress T lymphocyte activation through the virulence factor YopH, a tyrosine phosphatase. Using single cell video microscopy and FACS analysis, here we show that even an average of one Yersinia per T cell is sufficient to inhibit or alter T cell responses. This efficient inhibition is traced to specific targeting by YopH of the adaptor proteins, linker for activation of T cells (LAT) and SH2-domain–containing leukocyte protein of 76 kD (SLP-76), which are crucial for T cell antigen receptor (TCR) signaling. A catalytically inactive YopH translocated via the type III secretory pathway from the bacteria into T cells primarily binds to LAT and SLP-76. Furthermore, among the proteins of the TCR signaling pathway, the tyrosine phosphorylation levels of LAT and SLP-76 are the most affected in T cells exposed to low numbers of Yersinia pseudotuberculosis. This is the first example showing that a pathogen targets these adaptor proteins in the TCR signaling pathway, suggesting a novel mechanism by which pathogens may efficiently alter T cell–mediated immune responses.

Repeated cycles of rapid actin assembly and disassembly on epithelial cell phagosomes

We have found that early in infection of the intracellular pathogen Listeria monocytogenes in Madin-Darby canine kidney epithelial cells expressing actin conjugated to green fluorescent protein, F-actin rapidly assembles (approximately 25 s) and disassembles (approximately 30 s) around the bacteria, a phenomenon we call flashing. L. monocytogenes strains unable to perform actin-based motility or unable to escape the phagosome were capable of flashing, suggesting that the actin assembly occurs on the phagosome membrane. Cycles of actin assembly and disassembly could occur repeatedly on the same phagosome. Indirect immunofluorescence showed that most bacteria were fully internalized when flashing occurred, suggesting that actin flashing does not represent phagocytosis. Escherichia coli expressing invA, a gene product from Yersinia pseudotuberculosis that mediates cellular invasion, also induced flashing. Furthermore, polystyrene beads coated with E-cadherin or transferrin also induced flashing after internalization. This suggests that flashing occurs downstream of several distinct molecular entry mechanisms and may be a general consequence of internalization of large objects by epithelial cells.

Analysis of Chlamydia caviae entry sites and involvement of Cdc42 and Rac activity

In epithelial cells, endocytic activity is mostly dedicated to nutrient and macromolecule uptake. To invade these cells, Chlamydiaceae, like other pathogens, have evolved strategies that utilise the existing endocytic machineries and signalling pathways, but little is known about the host cell molecules involved. In this report, we show that within five minutes of infection of HeLa cells by Chlamydia caviae GPIC strain several events take place in the immediate vicinity of invasive bacteria: GM1-containing microdomains cluster, tyrosine-phosphorylated proteins accumulate, and intense actin polymerization occurs. We show that actin polymerization is controlled by the small GTPases Cdc42 and Rac, which become activated upon infection. Expression of dominant negative forms of these GTPases inhibits C. caviae entry and leads to abnormal actin polymerization. In contrast, the small GTPase Rho does not seem essential for bacterial entry. Finally, phosphatidylinositol 3-kinase activity is also required for internalization of C. caviae, probably downstream of the other molecular events reported here. We present the first scheme of the events occurring at the sites of invasion of epithelial cells by a member of the Chlamydiaceae family.

Intracellular signalling and cytoskeletal rearrangement involved in Yersinia pestis plasminogen activator (Pla) mediated HeLa cell invasion

Yersinia pestis, the etiologic agent of plague is a highly invasive organism being able to invade non-phagocytic epithelial cells. Its plasminogen activator (Pla), encoded by the pPCP1 plasmid plays a pivotal role in internalisation of bacteria by HeLa cells. The aim of this study was to analyse the intracellular signalling processes and cytoskeletal rearrangement events associated with invasion. Wortmannin caused a 50% decrease of invasiveness at 50nM concentration pointing to the involvement of phosphatidyl-inosinol-4 kinase (PtINs4). Pre-treatment with staurosporin, a potent inhibitor of protein kinases (PKs) and with genistein, a specific tyrosine kinase inhibitor decreased the number of internalised bacteria about seven-fold and two-fold, respectively, indicating the involvement of PKs including tyrosine kinases in Pla-mediated internalisation. Cytochalasin D, an actin polymerisation inhibitor, C3 exoenzyme of Clostridium botulinum, a specific inhibitor of small GTPase Rho, and NDGA, a 5-lipoxygenase inhibitor also involved in Rho activation strongly reduced the number of internalised bacteria revealing the role of cytoskeletal events in the invasion process. All the tested inhibitors changed the invasion but not the adhesion pattern of the Pla producing recombinant strain. Actin rearrangement could also be visualised also with rhodamin-phalloidin staining.

Differences in host susceptibility to disease progression in the human challenge model of Haemophilus ducreyi infection

With human volunteers inoculated at two sites with Haemophilus ducreyi, outcomes for a subject were not independent. In a reinfection trial, 2 of 11 previous pustule formers and 6 of 10 previous resolvers resolved all sites of infection. There was no correlation between serum bactericidal or phagocytic activity and outcome in the trial. These data indicate that different hosts are differentially susceptible to disease progression versus resolution in the model.

Taking possession: biogenesis of the Salmonella-containing vacuole

The Gram-negative pathogen Salmonella enterica can survive and replicate within a variety of mammalian cells. Regardless of the cell type, internalized bacteria survive and replicate within the Salmonella-containing vacuole, the biogenesis of which is dependent on bacterially encoded virulence factors. In particular, Type III secretion systems translocate bacterial effector proteins into the eukaryotic cell where they can specifically interact with a variety of targets. Salmonella has two distinct Type III secretion systems that are believed to have completely different functions. The SPI2 system is induced intracellularly and is required for intracellular survival in macrophages; it plays no role in invasion but is categorized as being required for Salmonella-containing vacuole biogenesis. In contrast, the SPI1 Type III secretion system is induced extracellularly and is essential for invasion of nonphagocytic cells. Its role in post-invasion processes has not been well studied. Recent studies indicate that Salmonella-containing vacuole biogenesis may be more dependent on SPI1 than previously believed. Other non-SPI2 virulence factors and the host cell itself may play critical roles in determining the intracellular environment of this facultative intracellular pathogen. In this review we discuss the recent advances in determining the mechanisms by which Salmonella regulate Salmonella-containing vacuole biogenesis and the implications of these findings.

Requirement of the Yersinia pseudotuberculosis effectors YopH and YopE in colonization and persistence in intestinal and lymph tissues

The gram-negative enteric pathogen Yersinia pseudotuberculosis employs a type III secretion system and effector Yop proteins that are required for virulence. Mutations in the type III secretion-translocation apparatus have been shown to cause defects in colonization of the murine cecum, suggesting roles for one or more effector Yops in the intestinal tract. To investigate this possibility, isogenic yop mutant strains were tested for their ability to colonize and persist in intestinal and associated lymph tissues of the mouse following orogastric inoculation. In single-strain infections, a yopHEMOJ mutant strain was unable to colonize, replicate, or persist in intestinal and lymph tissues. A yopH mutant strain specifically fails to colonize the mesenteric lymph nodes, but yopE and yopO mutant strains showed only minor defects in persistence in intestinal and lymph tissues. While no single Yop was found to be essential for colonization or persistence in intestinal tissues in single-strain infections, the absence of both YopH and YopE together almost eliminated colonization of all tissues, indicating either that these two Yops have some redundant functions or that Y. pseudotuberculosis employs multiple strategies for colonization. In competition infections with wild-type Y. pseudotuberculosis, the presence of wild-type bacteria severely hindered the ability of the yopH, yopE, and yopO mutants to persist in many tissues, suggesting that the wild-type bacteria either fills colonization niches or elicits host responses that the yop mutants are unable to withstand.

Role of lipid-mediated signal transduction in bacterial internalization

Receptor-mediated phagocytosis normally represents an important first line of immune defence. Invading microbes are internalized into phagosomes and are typically killed by exposure to a battery of microbicidal agents. To some intracellular pathogens, however, receptor-mediated phagocytosis represents an opportunity to access a protected niche within the host cell. Another type of intracellular pathogen, including Salmonella enterica serovar Typhimurium and Shigella flexneri, invade host cells in a more direct manner. These pathogens deliver effectors into the host cell via a type III secretion apparatus, initiating a ruffling response that leads to their uptake into intracellular vacuoles. Recent studies have demonstrated the importance of lipid signal transduction events in the uptake of pathogenic bacteria by both receptor-mediated phagocytosis and type III secretion-mediated invasion. In this review we highlight some of these discoveries, with a focus on phospholipid-dependent signalling events.

Phosphatidylinositol 3-kinases in carcinoembryonic antigen-related cellular adhesion molecule-mediated internalization of Neisseria gonorrhoeae

Neisseria gonorrhoeae can be internalized by mammalian cells through interactions between bacterial opacity-associated (Opa) adhesins and members of the human carcinoembryonic antigen-related cellular adhesion molecule (CEACAM) family. We examined the role of phosphatidylinositol 3-kinases (PI3Ks) in gonococcal invasion of epithelial cell lines expressing either CEACAM1 or CEACAM3. CEACAM3-mediated internalization, but not that mediated by CEACAM1, was accompanied by localized and transient accumulation of the class I PI3K product phosphatidylinositol 3,4,5-trisphosphate at sites of bacterial engulfment. Inhibition of phosphatidylinositol 3-kinases reduced CEACAM3-mediated uptake but, paradoxically, led to an increase in intracellular survival of bacteria internalized via either CEACAM1 or CEACAM3, suggesting additional roles for PI3K products. Consistent with this finding, the class III PI3K product phosphatidylinositol 3-phosphate accumulated and persisted in the membrane of gonococcal phagosomes after internalization. Inhibition of PI3K blocked phagosomal acquisition of the late endosomal marker lysosome-associated membrane protein 2 and reduced phagosomal acidification. Inhibiting phagosomal acidification with concanamycin A also increased survival of intracellular gonococci. These results suggest two modes of action of phosphatidylinositol 3-kinases during internalization of gonococci: synthesis of phosphatidylinositol 3,4,5-trisphosphate is important for CEACAM3-mediated uptake, while phosphatidylinositol 3-phosphate is needed for phagosomal maturation and acidification, which are required for optimal bacterial killing.

The YadA protein of Yersinia pseudotuberculosis mediates high-efficiency uptake into human cells under environmental conditions in which invasin is repressed

The YadA protein is a major adhesin of Yersinia pseudotuberculosis that promotes tight adhesion to mammalian cells by binding to extracellular matrix proteins. In this study, we first addressed the possibility of competitive interference of YadA and the major invasive factor invasin and found that expression of YadA in the presence of invasin affected neither the export nor the function of invasin in the outer membrane. Furthermore, expression of YadA promoted both bacterial adhesion and high-efficiency invasion entirely independently of invasin. Antibodies against fibronectin and beta(1) integrins blocked invasion, indicating that invasion occurs via extracellular-matrix-dependent bridging between YadA and the host cell beta(1) integrin receptors. Inhibitor studies also demonstrated that tyrosine and Ser/Thr kinases, as well as phosphatidylinositol 3-kinase, are involved in the uptake process. Further expression studies revealed that yadA is regulated in response to several environmental parameters, including temperature, ion and nutrient concentrations, and the bacterial growth phase. In complex medium, YadA production was generally repressed but could be induced by addition of Mg(2+). Maximal expression of yadA was obtained in exponential-phase cells grown in minimal medium at 37 degrees C, conditions under which the invasin gene is repressed. These results suggest that YadA of Y. pseudotuberculosis constitutes another independent high-level uptake pathway that might complement other cell entry mechanisms (e.g., invasin) at certain sites or stages during the infection process.

Disruption of the Salmonella-containing vacuole leads to increased replication of Salmonella enterica serovar typhimurium in the cytosol of epithelial cells

Salmonella enterica serovar Typhimurium is a facultative intracellular pathogen that inhabits a vacuolar compartment, called the Salmonella-containing vacuole (SCV), in infected host cells. Maintenance of the SCV is accomplished by SifA, and mutants of this Salmonella pathogenicity island 2 type III effector replicate more efficiently in epithelial cells. Here we demonstrate that enhanced replication of sifA mutants occurs in the cytosol of these cells. Increased replication of wild-type bacteria was also observed in cells treated with wortmannin or expressing Rab5 Q79L or Rab7 N125I, all of which caused a loss of SCV integrity. Our findings demonstrate the requirement of the host cell endosomal system for maintenance of the SCV and that loss of this compartment allows increased replication of serovar Typhimurium in the cytosol of epithelial cells.

Role of 3-phosphoinositides in the maturation of Salmonella-containing vacuoles within host cells

Salmonella typhimurium invades mammalian cells and replicates within a vacuole that protects it from the host's microbicidal weapons. The Salmonella-containing vacuole (SCV) undergoes a remodelling akin to that of the host cell's endocytic pathway, but SCV progression is arrested prior to fusion with lysosomes. We studied the role of phosphatidylinositol 3-kinase (PI3-K) in SCV maturation within HeLa cells. Phosphatidylinositol 3-phosphate (PI3P), monitored in situ using fluorescent conjugates of FYVE or PX domains, was found to accumulate transiently on the SCV. Wortmannin prevented PI3P accumulation and the recruitment of EEA1 but did not affect the association of Rab5 with the SCV. Importantly, inhibition of PI3-K also impaired fusion of the SCV with vesicles containing LAMP-1. Rab7, which is thought to be required for association of LAMP-1 with the SCV, still associated with SCV in wortmannin-treated cells. We have therefore concluded that a 3-phosphoinositide-dependent step exists following recruitment of Rab7 to the SCV. The data also imply that 3-phosphoinositide-dependent effectors of Rab5 are not an absolute requirement for recruitment of Rab7. Despite failure to acquire LAMP-1, the SCV persists and allows effective replication of Salmonella within wortmannin-treated host cells. These findings imply that PI3-K is involved in the development of the SCV but is not essential for intracellular survival and proliferation of Salmonella.

Yersinia effectors target mammalian signalling pathways

Animals have an immune system to fight off challenges from both viruses and bacteria. The first line of defence is innate immunity, which is composed of cells that engulf pathogens as well as cells that release potent signalling molecules to activate an inflammatory response and the adaptive immune system. Pathogenic bacteria have evolved a set of weapons, or effectors, to ensure survival in the host. Yersinia spp. use a type III secretion system to translocate these effector proteins, called Yops, into the host. This report outlines how Yops thwart the signalling machinery of the host immune system.

The invasion-associated type III secretion system of Salmonella enterica serovar Typhimurium is necessary for intracellular proliferation and vacuole biogenesis in epithelial cells

Type III secretion systems (TTSS) are used by Gram-negative pathogens to translocate proteins into eukaryotic host cells. Salmonella enterica serovar Typhimurium (S. Typhimurium) has two of these specialized systems, which are encoded on separate Salmonella pathogenicity islands (SPI-1 and SPI-2) and translocate unique sets of effectors. The specific roles of these systems in Salmonella pathogenesis remain undefined, although SPI-1 is required for bacterial invasion of epithelial cells and SPI-2 for survival/replication in phagocytic cells. However, because SPI-1 TTSS mutants are invasion-incompetent, the role of this TTSS in post-invasion processes has not been investigated. In this study, we have used two distinct methods to internalize a non-invasive SPI-1 TTSS mutant (invA) into cultured epithelial cells: (i) co-internalization with wild-type S. Typhimurium (SPI-1-dependent) and (ii) complementation with the Yersinia pseudotuberculosis invasin (inv) gene (SPI-1-independent). In both cases, internalized invA mutants were unable to replicate intracellularly, indicating that SPI-1 effectors are essential for this process and cannot be complemented by wild-type bacteria in the same cell. Analysis of the biogenesis of SCVs showed that vacuoles containing mutant bacteria displayed abnormal maturation that was dependent on the mechanism of entry. Manipulation of Salmonella-containing vacuole (SCV) biogenesis by pharmacologically perturbing membrane trafficking in the host cell increased intracellular replication of wild-type but not mutant S. Typhimurium This demonstrates a previously unknown role for SPI-1 in vacuole biogenesis and intracellular survival in non-phagocytic cells.

Membrane dynamics in phagocytosis

Engulfment of particles by phagocytes involves remodeling of the plasma membrane. We review recent work that suggests that focal exocytosis of endomembranes plays an important role in pseudopod extension during phagocytosis.

Inhibition of attaching and effacing lesion formation following enteropathogenic Escherichia coli and Shiga toxin-producing E. coli infection

Enteropathogenic Escherichia coli (EPEC) and Shiga toxin-producing E. coli (STEC) induce cytoskeletal changes in infected epithelial cells. To further characterize host cytosolic responses to infection, a series of specific cell-signaling inhibitors were employed. Initial bacterial adhesion to HEp-2 epithelial cells was not reduced, whereas alpha-actinin accumulation in infected cells was blocked by a phosphoinositide-specific phospholipase C inhibitor (ET-18-OCH3), phosphoinositide 3-kinase inhibitors (wortmannin and LY294002), and a 5-lipoxygenase inhibitor, nordihydroguaretic acid. A cyclooxygenase-2 inhibitor (NS-398), however, did not block alpha-actinin reorganization in response to EPEC and STEC infections. Understanding signal transduction responses to enteric pathogens could provide the basis for the development of novel therapeutic strategies.

Efficient uptake of Yersinia pseudotuberculosis via integrin receptors involves a Rac1-Arp 2/3 pathway that bypasses N-WASP function

Efficient uptake of Yersinia pseudotuberculosis into cultured mammalian cells is the result of high-affinity binding of invasin to beta1 chain integrins. We demonstrate here that uptake requires Rac1 and Arp 2/3 function. Bacterial uptake was stimulated by GTPgammaS, but was inhibited in mammalian cells transfected with the interfering Rac1-N17 derivative. Rac1 was found to be activated in response to integrin engagement by invasin, whereas Rac1 and Arp 2/3 were found to be intensely localized around phagosomes bearing bacteria, indicating a specific role for Rac1 signalling from the nascent phagosome to downstream effectors. To determine whether the Arp 2/3 complex was a component of this proposed pathway, cells overproducing various derivatives of Scar1/WAVE1, an Arp 2/3-binding protein, were analysed. Sequestration of Arp 2/3 away from the phagocytic cup as a result of Scar1/WAVE1 overproduction dramatically inhibited uptake. To determine whether signalling from Rac1 to Arp 2/3 occurred via N-WASP, uptake was analysed in a cell line lacking expression of WASP and N-WASP. Uptake was unaffected by the absence of these proteins, indicating that beta1 integrin signalling from Rac1 to Arp 2/3 can occur in the absence of N-WASP function.

A bacterial type III secretion system inhibits actin polymerization to prevent pore formation in host cell membranes

The bacterial pathogen Yersinia pseudotuberculosis uses type III secretion machinery to translocate Yop effector proteins through host cell plasma membranes. A current model suggests that a type III translocation channel is inserted into the plasma membrane, and if Yops are not present to fill the channel, the channel will form a pore. We examined the possibility that Yops act within the host cell to prevent pore formation. Yop- mutants of Y.pseudotuberculosis were assayed for pore-forming activity in HeLa cells. A YopE- mutant exhibited high levels of pore-forming activity. The GTPase-downregulating function of YopE was required to prevent pore formation. YopE+ bacteria had increased pore-forming activity when HeLa cells expressed activated Rho GTPases. Pore formation by YopE- bacteria required actin polymerization. F-actin was concentrated at sites of contact between HeLa cells and YopE- bacteria. The data suggest that localized actin polymerization, triggered by the type III machinery, results in pore formation in cells infected with YopE- bacteria. Thus, translocated YopE inhibits actin polymerization to prevent membane damage to cells infected with wild-type bacteria.

Modulation of Rho GTPases and the actin cytoskeleton by Yersinia outer proteins (Yops)

Pathogenic species of the genus Yersinia employ a type III secretion apparatus to inject up to six effector proteins (Yersinia outer proteins; Yops) into host cells. Thereby yersiniae disarm the immune cell system of the host to proliferate extracellularly. At least four of the Yop effectors (YopE, YpkA/YopO, YopT and YopH) are involved in the rearrangement of the actin cytoskeleton: YopE, YopT and YpkA/YopO modulate the activity of actin-regulating Rho GTP-binding proteins, whereas YopH dephosphorylates phospho-tyrosine residues in focal adhesion proteins. In this review we will focus on recent evidence implicating Rho GTPases and the actin cytoskeleton as major targets of Yersinia Yops.

Listeria pathogenesis and molecular virulence determinants

The gram-positive bacterium Listeria monocytogenes is the causative agent of listeriosis, a highly fatal opportunistic foodborne infection. Pregnant women, neonates, the elderly, and debilitated or immunocompromised patients in general are predominantly affected, although the disease can also develop in normal individuals. Clinical manifestations of invasive listeriosis are usually severe and include abortion, sepsis, and meningoencephalitis. Listeriosis can also manifest as a febrile gastroenteritis syndrome. In addition to humans, L. monocytogenes affects many vertebrate species, including birds. Listeria ivanovii, a second pathogenic species of the genus, is specific for ruminants. Our current view of the pathophysiology of listeriosis derives largely from studies with the mouse infection model. Pathogenic listeriae enter the host primarily through the intestine. The liver is thought to be their first target organ after intestinal translocation. In the liver, listeriae actively multiply until the infection is controlled by a cell-mediated immune response. This initial, subclinical step of listeriosis is thought to be common due to the frequent presence of pathogenic L. monocytogenes in food. In normal individuals, the continual exposure to listerial antigens probably contributes to the maintenance of anti-Listeria memory T cells. However, in debilitated and immunocompromised patients, the unrestricted proliferation of listeriae in the liver may result in prolonged low-level bacteremia, leading to invasion of the preferred secondary target organs (the brain and the gravid uterus) and to overt clinical disease. L. monocytogenes and L. ivanovii are facultative intracellular parasites able to survive in macrophages and to invade a variety of normally nonphagocytic cells, such as epithelial cells, hepatocytes, and endothelial cells. In all these cell types, pathogenic listeriae go through an intracellular life cycle involving early escape from the phagocytic vacuole, rapid intracytoplasmic multiplication, bacterially induced actin-based motility, and direct spread to neighboring cells, in which they reinitiate the cycle. In this way, listeriae disseminate in host tissues sheltered from the humoral arm of the immune system. Over the last 15 years, a number of virulence factors involved in key steps of this intracellular life cycle have been identified. This review describes in detail the molecular determinants of Listeria virulence and their mechanism of action and summarizes the current knowledge on the pathophysiology of listeriosis and the cell biology and host cell responses to Listeria infection. This article provides an updated perspective of the development of our understanding of Listeria pathogenesis from the first molecular genetic analyses of virulence mechanisms reported in 1985 until the start of the genomic era of Listeria research.

Identification of attenuated Yersinia pseudotuberculosis strains and characterization of an orogastric infection in BALB/c mice on day 5 postinfection by signature-tagged mutagenesis

Yersinia pseudotuberculosis localizes to the distal ileum, cecum, and proximal colon of the gastrointestinal tract after oral infection. Using signature-tagged mutagenesis, we isolated 13 Y. pseudotuberculosis mutants that failed to survive in the cecum of mice after orogastric inoculation. Twelve of these mutants were also attenuated for replication in the spleen after intraperitoneal infection, whereas one strain, mutated the gene encoding invasin, replicated as well as wild-type bacteria in the spleen. Several mutations were in operons encoding components of the type III secretion system, including components involved in translocating Yop proteins into host cells. This indicates that one or more Yops may be necessary for survival in the gastrointestinal tract. Three mutants were defective in O-antigen biosynthesis; these mutants were also unable to invade epithelial cells as efficiently as wild-type Y. pseudotuberculosis. Several other mutations were in genes that had not previously been associated with growth in a host, including cls, ksgA, and sufl. In addition, using Y. pseudotuberculosis strains marked with signature tags, we counted the number of different bacterial clones that were present in the cecum, mesenteric lymph nodes, and spleen 5 days postinfection. We find barriers in the host animal that limit the number of bacteria that succeed in reaching and/or replicating in the mesenteric lymph nodes and spleen after breaching the gut mucosa.

YopE of Yersinia, a GAP for Rho GTPases, selectively modulates Rac-dependent actin structures in endothelial cells

Yersinia spp. inject effector proteins (Yersinia outer proteins, Yops) into target cells via a type III secretion apparatus. The effector YopE was recently shown to possess GAP activity towards the Rho GTPases RhoA, Rac and CDC42 in vitro. To investigate the intracellular, 'in vivo' targets of YopE we generated a Yersinia enterocolitica strain [WA(pYLCR+E)] that injects 'life-like' amounts of YopE as only effector. Primary human umbilical vein endothelial cells (HUVEC) were infected with WA(pYLCR+E) and were then stimulated with: (i) bradykinin to induce actin microspikes followed by ruffles as an assay for CDC42 activity followed by CDC42 stimulated Rac activity; (ii) sphingosine-1-phosphate to form ruffles by direct Rac activation; or (iii) thrombin to generate actin stress fibres through Rho activation. In WA(pYLCR+E)-infected HUVEC microspike formation stimulated with bradykinin remained intact but the subsequent development of ruffles was abolished. Furthermore, ruffle formation after stimulation with sphingosine-1-phosphate or thrombin induced production of stress fibres was unaltered in the infected cells. These data suggest that YopE is able to inhibit Rac- but not Rho- or CDC42-regulated actin structures and, more specifically, that YopE is capable of blocking CDC42Hs dependent Rac activation but not direct Rac activation in HUVEC. This provides evidence for a considerable specificity of YopE towards selective Rac-mediated signalling pathways in primary target cells of Yersinia.

Enteropathogenic Escherichia coli mediates antiphagocytosis through the inhibition of PI 3-kinase-dependent pathways

The extracellular pathogen enteropathogenic Escherichia coli (EPEC) uses a type III secretion system to inhibit its uptake by macrophages. We show that EPEC antiphagocytosis is independent of the translocated intimin receptor Tir and occurs by preventing F-actin polymerization required for bacterial uptake. EPEC-macrophage contact triggered activation of phosphatidylinositol (PI) 3-kinase, which was subsequently inhibited in a type III secretion-dependent manner. Inhibition of PI 3-kinase significantly reduced uptake of a secretion-deficient mutant, without affecting antiphagocytosis by the wild type, suggesting that EPEC blocks a PI 3-kinase-dependent phagocytic pathway. EPEC specifically inhibited Fc gamma receptor- but not CR3-receptor mediated phagocytosis of opsonized zymosan. We showed that EPEC inhibits PI 3-kinase activity rather than its recruitment to the site of bacterial contact. Phagocytosis of a secretion mutant correlated with the association of PI 3-kinase with tyrosine-phosphorylated proteins, which wild-type EPEC prevented. These results show that EPEC blocks its uptake by inhibiting a PI 3-kinase-mediated pathway, and translocates effectors other than Tir to interfere with actin-driven host cell processes. This constitutes a novel mechanism of phagocytosis avoidance by an extracellular pathogen.

Requirement for N-ethylmaleimide-sensitive factor activity at different stages of bacterial invasion and phagocytosis

Bacterial invasion, like the process of phagocytosis, involves extensive and localized protrusion of the host cell plasma membrane. To examine the molecular mechanisms of the membrane remodeling that accompanies bacterial invasion, soluble NSF attachment protein receptor (SNARE)-mediated membrane traffic was studied in cultured cells during infection by Salmonella typhimurium. A green fluorescent protein-tagged chimera of VAMP3, a SNARE characteristic of recycling endosomes, was found to accumulate at sites of Salmonella invasion. To analyze the possible role of SNARE-mediated membrane traffic in bacterial infection, invasion was measured in cells expressing a dominant-negative form of N-ethylmaleimide-sensitive factor (NSF), an essential regulator of membrane fusion. Inhibition of NSF activity did not affect cellular invasion by S. typhimurium nor the associated membrane remodeling. By contrast, Fcgamma receptor-mediated phagocytosis was greatly reduced in the presence of the mutant NSF. Most important, dominant-negative NSF significantly impaired the fusion of Salmonella-containing vacuoles with endomembranes. These observations indicate that the membrane protrusions elicited by Salmonella invasion, unlike those involved in phagocytosis, occur via an NSF-independent mechanism, whereas maturation of Salmonella-containing vacuoles is NSF-dependent.

Rho GTPase activity modulates Pseudomonas aeruginosa internalization by epithelial cells

The Gram-negative pathogen Pseudomonas aeruginosa invades epithelial cells in vivo and in vitro. We have examined the pathway(s) by which epithelial cells internalize P. aeruginosa strain PA103 using Madin-Darby canine kidney (MDCK) cells. We have recently demonstrated that P. aeruginosa internalization occurs by an actin-dependent Toxin B-inhibited pathway which becomes downregulated as epithelial cells become polarized, suggesting that one or more of the Rho family GTPases is involved in bacterial internalization. Here, we demonstrate that activation of the Rho family GTPases by cytotoxic necrotizing factor 1 (CNF-1) stimulates P. aeruginosa internalization. Examination of the roles of the individual Rho family GTPases in internalization shows that expression of a constitutively active allele of RhoA (RhoAV14), but not of constitutively active Rac1 (Rac1V12) or Cdc42 (Cdc42V12), is sufficient to increase uptake of PA103pscJ. This relative increase persists when bacterial infection is established at the basolateral surface of polarized cells, suggesting that the effect of RhoAV14 is not simply due to its known ability to disrupt tight junction integrity in polarized cells. RhoAV14-mediated stimulation of bacterial uptake is actin dependent as it is abrogated by exposure to latrunculin A. We also find that endogenous Rho GTP levels in epithelial cells are increased by infection with an internalized strain of P. aeruginosa; conversely, a poorly internalized isogenic strain expressing the bacterial anti-internalization protein ExoT causes decreased Rho GTP levels. Experimental inhibition of Rho, either by expressing dominant negative RhoAN19 or by inhibiting native Rho using a membrane permeable fusion construct of a Rho-specific inhibitor, C3 ADP-ribosyltransferase, does not inhibit PA103pscJ internalization in MDCK or HeLa cells. Models consistent with these data are presented.

Subversion of integrins by enteropathogenic Yersinia

Enteropathogenic Yersinia are gram-negative bacterial species that translocate from the lumen of the intestine and are able to grow within deep tissue sites. During the earliest stages of disease, the organism is able to bind integrin receptors that are presented on the apical surface of M cells in the intestine, which allows its internalization and subsequent translocation into regional lymph nodes. The primary integrin substrate is the outer-membrane protein invasin, which binds with extraordinarily high affinity to at least five different integrins that have the (beta)(1) chain. Bacterial uptake into host cells is modulated by the affinity of receptor-substrate interaction, receptor concentration and the ability of the substrate to aggregate target receptors.

The RhoGAP activity of the Yersinia pseudotuberculosis cytotoxin YopE is required for antiphagocytic function and virulence

A variety of pathogenic bacteria use type III secretion pathways to translocate virulence proteins into host eukaryotic cells. YopE is an important virulence factor that is translocated into mammalian cells via a plasmid-encoded type III system in Yersinia spp. YopE action in mammalian cells promotes the disruption of actin filaments, cell rounding and blockage of phagocytosis. It was reported recently that two proteins with sequence similarity to YopE, SptP of Salmonella typhimurium and ExoS of Pseudomonas aeruginosa, function as GTPase-activating proteins (GAPs) for Rho GTPases. YopE contains an 'arginine finger' motif that is present in SptP, ExoS and other Rho GAPs and is essential for catalysis by this class of proteins. We show here that a GST-YopE fusion protein stimulated in vitro GTP hydrolysis by the Rho family members Cdc42, RhoA and Rac1, but not by Ras. Conversion of the essential arginine in the arginine finger motif to alanine (R144A) eliminated the in vitro GAP activity of GST-YopE. Infection assays carried out with a Yersinia pseudotuberculosis strain producing YopER144A demonstrated that GAP function was essential for the disruption of actin filaments, cell rounding and inhibition of phagocytosis by YopE in HeLa cells. Furthermore, the GAP function of YopE was important for Y. pseudotuberculosis pathogenesis in a mouse infection assay. Transfection of HeLa cells with a vector that produces a constitutively active form of RhoA (RhoA-V14) prevented the disruption of actin filaments and cell rounding by YopE. Production of an activated form of Rac1 (Rac1-V12), but not RhoA-V14, in HeLa cells interfered with YopE antiphagocytic activity. These results demonstrate that YopE functions as a RhoGAP to downregulate multiple Rho GTPases, leading to the disruption of actin filaments and inhibition of bacterial uptake into host cells.

Type 1 pilus-mediated bacterial invasion of bladder epithelial cells

Most strains of uropathogenic Escherichia coli (UPEC) encode filamentous adhesive organelles called type 1 pili. We have determined that the type 1 pilus adhesin, FimH, mediates not only bacterial adherence, but also invasion of human bladder epithelial cells. In contrast, adherence mediated by another pilus adhesin, PapG, did not initiate bacterial internalization. FimH-mediated invasion required localized host actin reorganization, phosphoinositide 3-kinase (PI 3-kinase) activation and host protein tyrosine phosphorylation, but not activation of Src-family tyrosine kinases. Phosphorylation of focal adhesin kinase (FAK) at Tyr397 and the formation of complexes between FAK and PI 3-kinase and between alpha-actinin and vinculin were found to correlate with type 1 pilus-mediated bacterial invasion. Inhibitors that prevented bacterial invasion also blocked the formation of these complexes. Our results demonstrate that UPEC strains are not strictly extracellular pathogens and that the type 1 pilus adhesin FimH can directly trigger host cell signaling cascades that lead to bacterial internalization.

GAP activity of the Yersinia YopE cytotoxin specifically targets the Rho pathway: a mechanism for disruption of actin microfilament structure

The YopE cytotoxin of Yersinia pseudotuberculosis is an essential virulence determinant that is injected into the eukaryotic target cell via a plasmid-encoded type III secretion system. Injection of YopE into eukaryotic cells induces depolymerization of actin stress fibres. Here, we show that YopE exhibits a GTPase-activating protein (GAP) activity and that the presence of YopE stimulates downregulation of Rho, Rac and Cdc42 activity. YopE has an arginine finger motif showing homology with those found in other GAP proteins. Exchange of arginine 144 with alanine, located in this arginine finger motif, results in an inactive form of YopE that can no longer stimulate GTP hydrolysis by the GTPase. Furthermore, a yopE(R144A) mutant is unable to induce cytotoxicity on cultured HeLa cells in contrast to the corresponding wild-type strain. Expression of wild-type YopE in cells of Saccharomyces cerevisiae inhibits growth, while in contrast, expression of the inactive form of YopE, YopE(R144A), does not affect the yeast cells. Co-expression of proteins belonging to the Rho1 pathway of yeast, Rho1, Rom2p, Bck1 and Ste20, suppressed the growth phenotype of YopE in yeast cells. These results provide evidence that YopE exhibits a GAP activity to inactivate RhoGTPases, leading to depolymerization of the actin stress fibres in eukaryotic cells and growth inhibition in yeast.