Cross-Kingdom Activation of Vibrio Toxins by ADP-Ribosylation Factor Family GTPases.

Pathogenic Vibrio species use many different approaches to subvert, attack, and undermine the host response. The toxins they produce are often responsible for the devastating effects associated with their diseases. These toxins target a variety of host proteins, which leads to deleterious effects, including dissolution of cell organelle integrity and inhibition of protein secretion. Becoming increasingly prevalent as cofactors for Vibrio toxins are proteins of the small GTPase families. ADP-ribosylation factor small GTPases (ARFs) in particular are emerging as a common host cofactor necessary for full activation of Vibrio toxins. While ARFs are not the direct target of Vibrio cholerae cholera toxin (CT), ARF binding is required for its optimal activity as an ADP-ribosyltransferase. The makes caterpillars floppy (MCF)-like and the domain X (DmX) effectors of the Vibrio vulnificus multifunctional autoprocessing repeats-in-toxin (MARTX) toxin also both require ARFs to initiate autoprocessing and activation as independent effectors. ARFs are ubiquitously expressed in eukaryotes and are key regulators of many cellular processes, and as such they are ideal cofactors for Vibrio pathogens that infect many host species. In this review, we cover in detail the known Vibrio toxins that use ARFs as cross-kingdom activators to both stimulate and optimize their activity. We further discuss how these contrast to toxins and effectors from other bacterial species that coactivate, stimulate, or directly modify host ARFs as their mechanisms of action.

Identification and characterization of Vibrio vulnificus plpA encoding a phospholipase A2 essential for pathogenesis.

The marine bacterium Vibrio vulnificus causes food-borne diseases, which may lead to life-threatening septicemia in some individuals. Therefore, identifying virulence factors in V. vulnificus is of high priority. We performed a transcriptome analysis on V. vulnificus after infection of human intestinal HT29-methotrexate cells and found induction of plpA, encoding a putative phospholipase, VvPlpA. Bioinformatics, biochemical, and genetic analyses demonstrated that VvPlpA is a phospholipase A2 secreted in a type II secretion system-dependent manner. Compared with the wild type, the plpA mutant exhibited reduced mortality, systemic infection, and inflammation in mice as well as low cytotoxicity toward the human epithelial INT-407 cells. Moreover, plpA mutation attenuated the release of actin and cytosolic cyclophilin A from INT-407 cells, indicating that VvPlpA is a virulence factor essential for causing lysis and necrotic death of the epithelial cells. plpA transcription was growth phase-dependent, reaching maximum levels during the early stationary phase. Also, transcription factor HlyU and cAMP receptor protein (CRP) mediate additive activation and host-dependent induction of plpA Molecular biological analyses revealed that plpA expression is controlled via the promoter, P plpA , and that HlyU and CRP directly bind to P plpA upstream sequences. Taken together, this study demonstrated that VvPlpA is a type II secretion system-dependent secretory phospholipase A2 regulated by HlyU and CRP and is essential for the pathogenicity of V. vulnificus.

Cloning and expression analysis of c-type lysozyme gene in golden pompano, Trachinotus ovatus.

It is well known that lysozymes are key proteins to teleosts in the innate immune system and possess high bactericidal properties. In the present study, a c-type lysozyme gene (To-lysC) was cloned from golden pompano, Trachinotus ovatus. The To-lysC cDNA is composed of 743 bp with a 36 bp of 5'-UTR, 432 bp open reading frame (ORF) and 275 bp 3'-UTR, encoding a polypeptide of 144 amino acids (GenBank accession no: KT935522). Phylogenetic analysis revealed that To-lysC showed highest similarity to Perca flavescens lysC. Quantitative real-time PCR (qRT-PCR) analysis showed that To-lysC had relatively high expression level in the head kidney, gill and brain. After Vibrio harveyi infection, transcripts of To-lysC increased and reached its peak at 12 h p.i. These results indicated that To-lysC may play an important role in innate immune response to bacteria.

The mucosal expression signatures of g-type lysozyme in turbot (Scophthalmus maximus) following bacterial challenge.

The mucosal surfaces constitute the first line of host defense against infection, and also serve as the dynamic interfaces that simultaneously mediate a diverse array of critical physiological processes, while in constantly contact with a wide range of pathogens. The lysozymes are considered as key components for innate immune response to pathogen infection with their strong antibacterial activities. But their activities in mucosal immune responses were always overlooked, especially for g-type lysozymes, whose expression patterns in mucosal tissues following bacterial challenge are still limited. Towards to this end, here, we characterized the g-type lysozymes, Lyg1 and Lyg2 in turbot, and determined their expression patterns in mucosal barriers following different bacterial infection. The phylogenetic analysis revealed the turbot g-type lysozyme genes showed the closest relationship to Cynoglossus semilaevis. The two lysozyme genes showed different expression patterns following challenge. Lyg2 was significantly up-regulated in mucosal tissues following Vibrio anguillarum and Streptococcus iniae challenge, while Lyg1 showed a general trend of down-regulation. The significant mucosal expression signatures of g-type lysozyme genes indicated their key roles to prevent pathogen attachment and entry in the first line of host defense system. Further functional studies should be carried out to better characterize the availability of utilization of g-type lysozyme to increase the disease resistance in the mucosal surfaces and facilitate the disease resistant breeding selection.

Molecular cloning and characterization of lymphocyte cell kinase from humphead snapper (Lutjanus sanguineus).

Lymphocyte cell kinase (LCK) belongs to the Src family of tyrosine kinases, which involves in the proliferation control of lymphocytes. In this study, we cloned the LCK gene of humphead snapper (Lutjanus sanguineus) (designed as LsLCK). Sequence analysis showed that the full-length cDNA of LsLCK was 2279 bp, contained a 1506-bp open reading frame (ORF), encoding a polypeptide of 501 amino acids. The deduced amino acid possessed the typical structural features of known LCK proteins, including four Src homology (SH) domains arranged as the SH1 domain followed by a regulatory C-terminal tail (COOH-domain), SH2 and SH3 adapter domains and SH4 domain which required for membrane attachment and CD4/CD8 binding. Fluorescent quantitative real-time PCR analysis indicated that LsLCK transcripts were expressed mainly in thymus, spleen and head kidney in healthy fish. Moreover, the mRNA expressions in these tissues were significantly up-regulated after challenge with Vibrio harveyi. The results of immunohistochemistry showed that LsLCK protein localized distinctly in cytoplasm of cell in thymus, spleen and head kidney. Taken together, these findings indicated that LsLCK may play an important role in the immune response of humphead snapper against bacterial infection.

Transgenic expression of salmon delta-5 and delta-6 desaturase in zebrafish muscle inhibits the growth of Vibrio alginolyticus and affects fish immunomodulatory activity.

Marine fish are an important nutritional source for highly polyunsaturated fatty acids (PUFAs). PUFA biosynthesis requires the following key enzymes: delta-4 (Δ-4) desaturase, delta-5 (Δ-5) desaturase, delta-6 (Δ-6) desaturase, delta-5 (Δ-5) elongase, and delta-6 (Δ-6) elongase. The effect of overexpressing delta-5 desaturase and/or delta-6 desaturase in zebrafish muscle has not previously been reported. Herein, we investigated the effects of these proteins on antibacterial and immunomodulatory activity in transgenic zebrafish infected with Vibrio alginolyticus. Overexpression of delta-5 and delta-6 desaturase enhanced antibacterial activity at 4 and 12 h after injection of bacteria into muscle, as compared to controls. Furthermore, expression of immune-related genes (IL-1ß, IL-22, and TNF-α) was observed to be altered in transgenic fish after 4 h of bacterial infection, resulting in a significant decrease in the inflammatory response, as compared to control fish. These results demonstrate that muscle-specific expression of transgenic desaturases in zebrafish not only enhance PUFA production, but also enhance antibacterial and anti-inflammatory activity. Overall, these results identify delta-5 and delta-6 desaturase as novel candidate genes for use in aquaculture, to enhance both disease resistance and fish oil production.

Autophagy is induced by the type III secretion system of Vibrio alginolyticus in several mammalian cell lines.

Vibrio alginolyticus is a gram-negative bacterium and has been recognized as an opportunistic pathogen in marine animals as well as humans. Here, we further characterized a cell death mechanism caused by this bacterium in several mammalian cell lines. The T3SS of V. alginolyticus killed HeLa cells by a very similar cell cytolysis mechanism in fish cells, as evidenced by cell rounding and LDH release; however, DNA fragmentation was not observed. Further studies showed that caspase-1 and caspase-3 were not activated during the T3SS-mediated cell death, indicating that the death mechanism is completely independent of pyroptosis and apoptosis in HeLa cells. Conversely, autophagy was detected during the T3SS-mediated cell death by the appearance of MDC-labeled punctate fluorescence and accumulation of autophagic vesicles. Moreover, western blot analysis revealed increase in conversion of LC3-I to LC3-II in infected mammalian cell lines, confirming that autophagy occurs during the process. Together, these data demonstrate that the death process used by V. alginolyticus in mammalian cells is different from that in fish cells, including induction of autophagy, cell rounding and osmotic lysis. This study provides some evidences hinting that differences in death mechanism in responses to V. alginolyticus infection may be attributed to the species of infected cells from which it was derived.

Nitric Oxide Synthase Regulates Gut Microbiota Homeostasis by ERK-NF-κB Pathway in Shrimp.

The gut microbiota is a complex group of microorganisms that is not only closely related to intestinal immunity but also affects the whole immune system of the body. Antimicrobial peptides and reactive oxygen species participate in the regulation of gut microbiota homeostasis in invertebrates. However, it is unclear whether nitric oxide, as a key mediator of immunity that plays important roles in antipathogen activity and immune regulation, participates in the regulation of gut microbiota homeostasis. In this study, we identified a nitric oxide synthase responsible for NO production in the shrimp Marsupenaeus japonicus. The expression of Nos and the NO concentration in the gastrointestinal tract were increased significantly in shrimp orally infected with Vibrio anguillarum. After RNA interference of Nos or treatment with an inhibitor of NOS, L-NMMA, NO production decreased and the gut bacterial load increased significantly in shrimp. Treatment with the NO donor, sodium nitroprusside, increased the NO level and reduced the bacterial load significantly in the shrimp gastrointestinal tract. Mechanistically, V. anguillarum infection increased NO level via upregulation of NOS and induced phosphorylation of ERK. The activated ERK phosphorylated the NF-κB-like transcription factor, dorsal, and caused nuclear translocation of dorsal to increase expression of antimicrobial peptides (AMPs) responsible for bacterial clearance. In summary, as a signaling molecule, NOS-produced NO regulates intestinal microbiota homeostasis by promoting AMP expression against infected pathogens via the ERK-dorsal pathway in shrimp.

The type III secretion system of Vibrio alginolyticus induces rapid apoptosis, cell rounding and osmotic lysis of fish cells.

Vibrio alginolyticus is a Gram-negative bacterium and has been recognized as an opportunistic pathogen in humans as well as marine animals. However, the virulence mechanisms for this species of Vibrio have not been elucidated. This study characterized multiple mechanisms that induce cell death in fish cells upon infection with a V. alginolyticus strain, ZJO. The bacterium required its type III secretion system (T3SS) to cause rapid death of infected fish cells. Dying cells exhibited some features of apoptotic cells, such as membrane blebbing, nuclear condensation and DNA fragmentation. Further studies showed that caspase-3 was activated by the T3SS of the ZJO strain, confirming that infection with V. alginolyticus rapidly induces T3SS-dependent apoptosis in fish cells. Infection with the ZJO strain also led to membrane pore formation and release of cellular contents from infected fish cells, as evidenced by lactate dehydrogenase release and the uptake of a membrane-impermeable dye. Importantly, inhibition of apoptosis did not prevent ZJO-infected cells from releasing cellular contents and did not block cell rounding. Taken together, these data demonstrate that infection with V. alginolyticus may promote at least three different T3SS-dependent events, which lead to the death of fish cells. This study provides an important insight into the mechanism used by Vibrio species to cause host-cell death.

A recombinant metalloprotease antigen of Vibrio vulnificus elicits protective antibodies in a murine model.

AIMS: To investigate whether Vibrio vulnificus metalloprotease (VvpE) can induce the production of specific anti-VvpE antibody to confer effective protection against Vibrio vulnificus infection and to evaluate the possibility of VvpE as a potential vaccine candidate against disease caused by V. vulnificus. METHODS AND RESULTS: The gene encoding the 65-kDa VvpE of V. vulnificus was amplified by PCR and cloned into the expression vector pET21(b). The recombinant VvpE of V. vulnificus was expressed in Escherichia coli BL21(DE3). This His(6)-tagged VvpE was purified and injected intramuscularly into mice to evaluate its ability to stimulate immune response. Specific antibody levels were measured by ELISA. The 75% protective efficacy of recombinant VvpE was evaluated by active immunization and intraperitoneal challenge with V. vulnificus in mice. CONCLUSIONS: The recombinant His(6)-tagged VvpE of V. vulnificus is capable of inducing high antibody response in mice to confer effective protection against lethal challenge with V. vulnificus. VvpE might be a potential vaccine candidate to against V. vulnificus infection. SIGNIFICANCE AND IMPACT OF THE STUDY: This study uses His(6)-tagged VvpE to act as vaccine that successfully induces effective and specific anti-VvpE antibody and offers an option for the potential vaccine candidate against V. vulnificus infection.

D-dopachrome tautomerase from Japanese sea bass ( Lateolabrax japonicus) is a chemokine-like cytokine and functional homolog of macrophage migration inhibitory factor.

D-dopachrome tautomerase (DDT), a member of the macrophage migration inhibitory factor (MIF) protein superfamily, is a newly described cytokine with chemokine-like characteristics. However, research on fish DDT remains limited. In this study, we identified a DDT homolog (LjDDT) from the Japanese sea bass, Lateolabrax japonicus. Sequence analysis showed that LjDDT had typical sequence features of known DDT and MIF homologs and was most closely related to DDT of rock bream ( Oplegnathus fasciatus). LjDDT transcripts were detected in all tested tissues of healthy Japanese sea bass, with the highest expression found in the liver. Upon infection with Vibrio harveyi, LjDDT transcripts were significantly down-regulated in the three tested tissues, including the liver, spleen, and head kidney. Recombinant LjDDT (rLjDDT) and the corresponding antibody (anti-rLjDDT) were subsequently prepared. The administration of 100 µg/g anti-rLjDDT had a statistically significant protective effect on the survival of V. harveyi-infected fish. Moreover, rLjDDT was able to induce the migration of monocytes/macrophages (MO/MФ) and lymphocytes both in vitro and in vivo, but without significant influence on the migration of neutrophils. rLjDDT exhibited chemotactic activity for lipopolysaccharide (LPS) -stimulated M1-type MO/ MΦ in vitro, but not for cAMP-stimulated M2-type MO/MΦ. Furthermore, the knockdown of LjCD74, but not LjCXCR4, significantly down-regulated the rLjDDT-enhanced migration of MO/MΦ and relieved the rLjMIF-inhibited migration of MO/MΦ. These results indicate that LjCD74 may be the major chemotactic receptor of LjDDT and LjMIF in Japanese sea bass MO/MΦ. Combined rLjDDT+ rLjMIF treatment had no significant effect on the migration of MsiRNA, LjCD74si-, or LjCXCR4sitreated MO/MΦ compared to the control group, suggesting that the roles of LjDDT and LjMIF may be antagonistic. In conclusion, our study demonstrates for the first time that DDT may play a role in the immune responses of fish against bacterial infection through chemotactic recruitment of MO/MΦ via mediation of CD74 as an antagonist of MIF.

Two prophenoloxidases are important for the survival of Vibrio harveyi challenged shrimp Penaeus monodon.

Phenoloxidase (PO) plays an important role in arthropod melanization. Previously, a prophenoloxidase (PmproPO1) gene was cloned and characterized from the hemocytes of the black tiger shrimp, Penaeus monodon. In the present study, we report a novel proPO gene (PmproPO2) belonging to the proPO family identified from the P. monodon EST database (http://pmonodon.biotec.or.th). The full-length sequence of PmproPO2 consists of 2513bp encoding a predicted 689 amino acid residues with a calculated molecular mass and pI of 79.21kDa and 6.69, respectively. It is predicted to possess all the expected features of proPO members, including two putative tyrosinase copper-binding motifs with six histidine residues and a thiol ester-like motif, sharing 67% amino acid sequence identity with PmproPO1. Tissue distribution analyses revealed that the two proPO genes are primarily expressed in the hemocyte. Gene silencing of either PmproPO1 or PmproPO2 or both by RNA interference (RNAi) resulted in a significant decrease in the respective endogenous proPO mRNA level in hemocytes and a reduction of total PO enzyme activity by 75, 73 and 88%, respectively. Experimental infection of P. monodon with the pathogenic bacterium, Vibrio harveyi, revealed that PmproPO silenced shrimps were more susceptible to bacterial infection than the control GFP injected shrimps, and suggesting that the two proPOs are important components in the shrimp immune defense.

Vibrio vulnificus-induced cell death of human mononuclear cells requires ROS-dependent activation of p38 and ERK 1/2 MAPKs.

Vibrio vulnificus is a Gram-negative bacterium that multiplies rapidly in host tissue and causes extensive tissue damage. Human peripheral blood mononuclear cells (PBMC) were shown to be readily killed by exposure to live V. vulnificus. V. vulnificus induced production of intracellular reactive oxygen species (ROS) and nitric oxide (NO) in PBMC. Pretreatment of PBMC with diphenyleneiodonium chloride (DPI) abolished ROS generation upon exposure to V. vulnificus and decreased the bacterial ability to cause cell death. In contrast, pretreatment of these cells with inhibitors of inducible nitric oxide synthase (iNOS) blocked V. vunificus-induced NO production, but did not significantly alter cell death by V. vulnificus. V. vulnificus also triggered phosphorylation of mitogen-activated protein kinases (MAPKs), including p38 and ERK1/2 in PBMC. Inactivation of these MAPKs by selective inhibitors caused a reduction both in ROS generation and cell death induced by V. vulnificus. It was further shown that an inhibitor of ROS generation (DPI) blocked V. vulnificus-induced phosphorylation of p38 and ERK1/2 MAPK. This study demonstrates that V. vulnificus induces death of PBMC via ROS-dependent activation of p38 MAPK and ERK1/2 MAPK.

Caspase -1, -3, -8 and antioxidant enzyme genes are key molecular effectors following Vibrio parahaemolyticus and Aeromonas veronii infection in fish leukocytes.

Caspases are a family of proteases involved in many important biological processes including apoptosis and inflammation. In order to get insights into the caspase gene family and antioxidant enzymes in Totoaba macdonaldi during bacterial infection, an in vitro assay was performed involving three different types of caspases (Casp-1, Casp-3 and Casp-8) and antioxidant enzymes (catalase, gluthathione peroxidase 1 and 4) after Vibrio parahaemolyticus and Aeromonas veronii infection, using head-kidney and spleen leukocytes from the teleost fish totoaba at 12 and 24 h post-exposure. Characterization of caspases by bioinformatics analyses showed that TmCas-1, TmCas-3 and TmCas-8 shared overall sequence identities of 82-61%, 85-97% and 77-63%, respectively, with other teleost fish. Caspase-1, -3 and -8 proteins revealed a conserved penta-peptide sequence at the catalytic site and three amino acid residues involved in the catalysis (H, G and C), as well as two conserved domains. The expression levels of the three caspases were detected in a wide range of fish tissues; however, they varied among tissues and caspases, which were highly up-regulated in immune organs, such as head-kidney, liver and/or spleen. The pathogen-induced gene expression pattern revealed two interesting facts; first, that the expression of all the caspase genes and antioxidant enzyme genes evaluated in this study were strongly induced following V. parahaemolyticus infection; second, these up-regulations reached a maximum level at 24 h post-infection in head-kidney whereas in spleen leukocytes, it was observed at 6-h post-infection. In conclusion, based on these observations, the acute toxic effects of V. parahaemolyticus are associated to cell death and release of free radicals. This information provides a better understanding of the effects and nature of early immune response against common bacterial infections in totoaba leukocytes.

Phospholipase A as a potent virulence factor of Vibrio vulnificus.

Vibrio vulnificus infection has attracted special interest because of its high mortality rate. However, the identification of its major pathogenic determinant still remains obscure. In this study, a cytolysin-negative mutant strain of V. vulnificus CVD707 was used to determine the role of phospholipase A (PLA) in the pathogenesis of this bacterial infection. The mutant strain caused the lysis of erythrocytes in vitro and elevated plasma hemoglobin during the infection in mice. Both the hemolytic and PLA activities were dependent on calcium. Inhibition of hemolysis by PLA inhibitors including tetracyclin and the PLA substrate phosphatidylcholine also supports the possibility of membranous PLA as a major hemolytic factor in the cytolysin-deficient mutant. To identify the role of PLA in the pathogenesis of V. vulnificus infection, the effects of tetracycline on bacteria-induced macrophage cytotoxicity and lethality were compared with those of penicillin, an antibiotic with no inhibitory effect on PLA. Both the macrophage cytotoxicity and the lethality of V. vulnificus CVD707 to mice were significantly attenuated by tetracycline, but not by penicillin. However, bacterial counts in culture medium and mouse blood revealed that penicillin was more effective than tetracycline in killing bacteria under our experimental conditions. These results indicate that PLA activity is important in V. vulnificus-induced cytotoxicity and lethality, suggesting a crucial role for PLA in the pathogenesis of V. vulnificus infection.

Vibrio vulnificus infection and metalloprotease.

Vibrio vulnificus is ubiquitous in aquatic environments; however, it occasionally causes serious and often fatal infections in humans. These include invasive septicemia contracted through consumption of raw seafood, as well as wound infections acquired through contact with brackish or marine waters. In most cases of septicemia, the patients have underlying disease(s), such as liver dysfunction or alcoholic cirrhosis, and the secondary skin lesions including cellulitis, edema and hemorrhagic bulla appear on the limbs. Although V. Vul produces various virulent factors including polysaccharide capsule, type IV pili, hemolysin and proteolytic enzymes, the 45-kDa metalloprotease may be a causative factor of the skin lesions, because the purified protease enhances vascular permeability through generation of chemical mediators and also induces serious hemorrhagic damage through digestion of the vascular basement membrane. As well as other bacteria, V. Vul can regulate the protease production through the quorum-sensing system depending on bacterial cell density. However, this system operates efficiently at 25 degrees C, but not at 37 degrees C. Therefore, V. vulnificus may produce sufficient amounts of the protease only in the interstitial tissue of the limbs, in which temperature is lower than the internal temperature of the human body.

Effects of Pyrogallol on Growth and Cytotoxicity of Wild-Type and katG Mutant Strains of Vibrio vulnificus.

Vibrio vulnificus is a causative agent of fatal septicemia and necrotic wound infection and the pathogen infection became an important public health problem in many counties. Vibrio vulnificus causes RtxA1 toxin-induced acute cell death. We tried to identify natural products that inhibit the acute cytotoxicity of V. vulnificus using a lactate hydrogenase assay. A polyphenol pyrogallol protected HeLa cells from V. vulnificus-induced cytotoxicity. Pyrogallol also decreased the growth of V. vulnificus; this inhibitory effect was more significant during log phase than stationary phase. To further elucidate the inhibitory mechanism, pyrogallol-induced toxicity was compared between a V. vulnificus catalase-peroxidase mutant (katG-) and the isogenic wild-type MO6-24/O strains. No growth was observed for the katG- mutant in the presence of pyrogallol (50 µg/mL) even after 24 h, whereas the wild-type strain demonstrated growth recovery following a prolonged lag phase. Pyrogallol-mediated growth inhibition of the katG- mutant strain was partially rescued by exogenous catalase treatment. These results indicate that the mechanism by which pyrogallol inhibits the growth and cytotoxicity of V. vulnificus likely involves polyphenol-induced prooxidant damage. Taken together, these results suggest that pyrogallol has potential for development as a new paradigm drug to treat infectious diseases.

Cystine aminopeptidase activity (oxytocinase) in pregnant guinea pigs: normal and infected with Campylobacter fetus.

The cystine aminopeptidase (CAP) activity was determined in plasma from guinea pigs at various stages of gestation and in pregnant guinea pigs infected with Campylobacter fetus subspecies intestinalis. The CAP activity decreased with length of gestation, which is in contrast to the changes in activity in persons. Infected guinea pigs near the time of abortion can be distinguished from normal guinea pigs on the basis of plasma CAP activity, if the plasma samples are taken 1 to 2 days before abortion. Among pregnant guinea pigs at the same stage of gestation, CAP activity was lower for infected than for normal pregnant animals. This test might be used to indicate probable disruption of placentation and the onset of abortion in the guinea pig.

Cell death mediated by Vibrio parahaemolyticus type III secretion system 1 is dependent on ERK1/2 MAPK, but independent of caspases.

Vibrio parahaemolyticus, which causes gastroenteritis, wound infection, and septicemia, has two sets of type III secretion systems (TTSS), TTSS1 and TTSS2. A TTSS1- deficient vcrD1 mutant of V. parahaemolyticus showed an attenuated cytotoxicity against HEp-2 cells, and a significant reduction in mouse lethality, which were both restored by complementation with the intact vcrD1 gene. V. parahaemolyticus also triggered phosphorylation of mitogenactivated protein kinases (MAPKs) including p38 and ERK1/2 in HEp-2 cells. The ability to activate p38 and ERK1/2 was significantly affected in a TTSS1-deficient vcrD1 mutant. Experiments using MAPK inhibitors showed that p38 and ERK1/2 MAPKs are involved in V. parahaemolyticus-induced death of HEp-2 cells. In addition, caspase-3 and caspase-9 were processed into active forms in V. parahaemolyticus-exposed HEp-2 cells, but activation of caspases was not essential for V. parahaemolyticusinduced death of HEp-2 cells, as shown by both annexin V staining and lactate dehydrogenase release assays. We conclude that secreted protein(s) of TTSS1 play an important role in activation of p38 and ERK1/2 in HEp-2 cells that eventually leads to cell death via a caspaseindependent mechanism.