Helicobacter pylori lipopolysaccharide hinders polymorphonuclear leucocyte apoptosis

Use of larvae of the wax moth Galleria mellonella as an in vivo model to study the virulence of Helicobacter pylori

Background

Helicobacter pylori is the first bacterium formally recognized as a carcinogen and is one of the most successful human pathogens, as over half of the world’s population is colonized by the bacterium. H. pylori-induced gastroduodenal disease depends on the inflammatory response of the host and on the production of specific bacterial virulence factors. The study of Helicobacter pylori pathogenic action would greatly benefit by easy-to-use models of infection.

Results

In the present study, we examined the effectiveness of the larvae of the wax moth Galleria mellonella as a new model for H. pylori infection. G. mellonella larvae were inoculated with bacterial suspensions or broth culture filtrates from either different wild-type H. pylori strains or their mutants defective in specific virulence determinants, such as VacA, CagA, CagE, the whole pathogenicity island (PAI) cag, urease, and gamma-glutamyl transpeptidase (GGT). We also tested purified VacA cytotoxin. Survival curves were plotted using the Kaplan-Meier method and LD₅₀ lethal doses were calculated. Viable bacteria in the hemocoel were counted at different time points post-infection, while apoptosis in larval hemocytes was evaluated by annexin V staining. We found that wild-type and mutant H. pylori strains were able to survive and replicate in G. mellonella larvae which underwent death rapidly after infection. H. pylori mutant strains defective in either VacA, or CagA, or CagE, or cag PAI, or urease, but not GGT-defective mutants, were less virulent than the respective parental strain. Broth culture filtrates from wild-type strains G27 and 60190 and their mutants replicated the effects observed using their respective bacterial suspension. Also, purified VacA cytotoxin was able to kill the larvae. The killing of larvae always correlated with the induction of apoptosis in hemocytes.

Conclusions

G. mellonella larvae are susceptible to H. pylori infection and may represent an easy to use in vivo model to identify virulence factors and pathogenic mechanisms of H. pylori. The experimental model described can be useful to screen a large number of clinical H. pylori strain and to correlate virulence of H. pylori strains with patients’ disease status.

Neutrophil apoptosis and the resolution of infection

Polymorphonuclear leukocytes (PMNs) are the most abundant white cell in humans and an essential component of the innate immune system. PMNs are typically the first type of leukocyte recruited to sites of infection or areas of inflammation. Ingestion of microorganisms triggers production of reactive oxygen species and fusion of cytoplasmic granules with forming phagosomes, leading to effective killing of ingested microbes. Phagocytosis of bacteria typically accelerates neutrophil apoptosis, which ultimately promotes the resolution of infection. However, some bacterial pathogens alter PMN apoptosis to survive and thereby cause disease. Herein, we review PMN apoptosis and the ability of microorganisms to alter this important process.

Inhibition of apoptosis induced by heat shock preconditioning is associated with decreased phagocytosis in human polymorphonuclear leukocytes through inhibition of Rac and Cdc42

The functionality of polymorphonuclear leukocytes (PMNL) and the exact process of the protective program employed by these cells in response to the heat shock (HS) remain ill-defined and debated. Particularly, the mechanism of phagocytic impairment induced by the HS and the molecular events associated with the delay of apoptosis used by these cells in such condition have given conflictual data. The aim of the present work is to study the consequences of the HS in different pathways involved in human PMNL apoptosis and subsequently in human PMNL phagocytic function. We demonstrated that HS (41 degrees C, 1 h) preconditioning induced inhibition of spontaneous PMNL apoptosis observed at 18 h in control cells incubated at 37 degrees C. This inhibition was characterized by absence of morphological nuclear changes, decrease of DNA fragmentation, low level of annexin V expression and decrease of caspase-3 activity. In parallel, HS increased both Hsp70 and Mcl-1 protein levels in PMNL. Phagocytosis of latex beads by PMNL was inhibited by HS (41 degrees C, 1 h) preconditioning despite an upregulation of CD11b, CD16 and CD47. Moreover, HS induced prolonged F actin depolymerization and inhibited both Rac and Cdc42 activation in PMNL. Finally, our results identify a new function of Mcl-1 in HS protection against apoptosis.

Human polymorphonuclear leukocytes are sensitive in vitro to Helicobacter pylori vaca toxin

BACKGROUND

Interactions between bacterial components and polymorphonuclear leukocytes (PMNL) play a major pathogenic role in Helicobacter pylori-associated diseases. Activation of PMNL can be induced by contact with whole bacteria or by different H. pylori products released in the extracellular space either by active secretion or by bacterial autolysis. Among these products, H. pylori VacA is a secreted toxin inducing vacuolation and apoptosis of epithelial cells.

METHODS AND RESULTS

We found that non-opsonic human PMNL were sensitive to the vacuolating effect of VacA+ broth culture filtrate (BCF) and of purified VacA toxin. PMNL incubated with VacA+ BCF showed Rab7-positive large intracytoplasmic vacuoles. PMNL preincubation with H. pylori BCF of different phenotypes dramatically potentialized the oxidative burst induced by zymosan, increased phagocytosis of opsonized fluorescent beads, and up-regulated CD11b cell surface expression, but independently of the BCF VacA phenotype. Moreover, by using purified VacA toxin we showed that vacuolation induced in PMNL did not modify the rate of spontaneous PMNL apoptosis measured by caspase 3 activity.

CONCLUSIONS

Taken together, these data showed that human PMNL is a sensitive cell population to H. pylori VacA toxin. However, activation of PMNL (i.e., oxidative burst, phagocytosis, CD11b up-regulation) and PMNL apoptosis are not affected by VacA, raising question about the role of VacA toxin on PMNL in vivo.

Helicobacter pylori vacuolating cytotoxin induces activation of the proapoptotic proteins Bax and Bak, leading to cytochrome c release and cell death, independent of vacuolation

Helicobacter pylori vacuolating cytotoxin, VacA, which causes vacuolation of gastric epithelial cells and other types of cultured cells, is known to stimulate apoptosis via a mitochondria-dependent pathway. In the present study, we examined the mechanisms of VacA-induced mitochondrial damage. Intracellular VacA localization was monitored by immunostaining and confocal microscopy; in AZ-521 cells in which cytochrome c release was stimulated, most of VacA was localized to vacuoles rather than mitochondria. VacA reduced the membrane potential of isolated mitochondria without inducing cytochrome c release, suggesting that it did not act directly to induce cytochrome c release from mitochondria and that in intact cells, VacA-induced cytochrome c release involved apoptosis-related factor(s), such as a proapoptotic Bcl-2 family protein. In agreement, flow cyto-metric analyses using antibodies specific for activated Bax revealed that intracellular Bax was activated by VacA in a concentration- and time-dependent manner. Using active form-specific antibodies, we also observed that the Bcl-2 family protein, Bak, was activated. By confocal microscopy, Bax and Bak were activated in AZ-521 cells in which cyto-chrome c release was induced by VacA. In addition, small interfering RNA-induced silencing of the bax gene resulted in reduction of VacA-stimulated cytochrome c release, consistent with a contribution of VacA-induced Bax activation to cytochrome c release. NH4Cl enhanced both VacA-induced vacuolation and Bax activation, whereas Bax activation was not inhibited by bafilomycin A1, which inhibited vacuolation caused by VacA. These results suggest that VacA acts through different signaling pathways to induce apoptosis via Bax activation, independent of vacuolation.

Increased rate of apoptosis and diminished phagocytic ability of human neutrophils infected with Afa/Dr diffusely adhering Escherichia coli strains

The proinflammatory effect of Afa/Dr diffusely adhering Escherichia coli (Afa/Dr DAEC) strains have been recently demonstrated in vitro by showing that polymorphonuclear leukocyte (PMN) transepithelial migration is induced after bacterial colonization of apical intestinal monolayers. The effect of Afa/Dr DAEC-PMN interaction on PMN behavior has been not investigated. Because of the putative virulence mechanism of PMN apoptosis during infectious diseases and taking into account the high level of expression of the decay-accelerating factor (DAF, or CD55), the receptor of Afa/Dr DAEC on PMNs, we sought to determine whether infection of PMNs by Afa/Dr DAEC strains could promote cell apoptosis. We looked at the behavior of PMNs incubated with Afa/Dr DAEC strains once they had transmigrated across polarized monolayers of intestinal (T84) cells. Infection of PMNs by Afa/Dr DAEC strains induced PMN apoptosis characterized by morphological nuclear changes, DNA fragmentation, caspase activation, and a high level of annexin V expression. However, transmigrated and nontransmigrated PMNs incubated with Afa/Dr DAEC strains showed similar elevated global caspase activities. PMN apoptosis depended on their agglutination, induced by Afa/Dr DAEC, and was still observed after preincubation of PMNs with anti-CD55 and/or anti-CD66 antibodies. Low levels of phagocytosis of Afa/Dr DAEC strains were observed both in nontransmigrated and in transmigrated PMNs compared to that observed with the control E. coli DH5alpha strain. Taken together, these data strongly suggest that interaction of Afa/Dr DAEC with PMNs may increase the bacterial virulence both by inducing PMN apoptosis through an agglutination process and by diminishing their phagocytic capacity.

Helicobacter pylori induces apoptosis of macrophages in association with alterations in the mitochondrial pathway

Helicobacter pylori is a gastric bacterial pathogen that evades host immune responses in vivo and is associated with the development of gastritis, peptic ulcer disease, and gastric cancers. Induction of macrophage apoptosis is a method employed by multiple pathogens to escape host immune responses. Therefore, we hypothesized that H. pylori induces apoptosis of infected macrophages. RAW 264.7 cells were infected with H. pylori strain 60190, and apoptosis was assessed. Transmission electron microscopy and fluorescence microscopy showed that infected macrophages displayed morphological features characteristic of apoptosis. Quantification by acridine orange-ethidium bromide fluorescent-dye staining showed that apoptosis was dose and time dependent, and apoptosis was further confirmed by increased binding of annexin V-fluorescein isothiocyanate (FITC) to externalized phosphatidylserine of infected but not of control macrophages. Macrophages infected with isogenic mutants of H. pylori strain 60190 deficient in either cagA or vacA induced significantly less apoptosis than the parental strain, as assessed by increased binding of annexin V-FITC. Western blot analysis of whole-cell protein lysates revealed that infection with strain 60190 induced a time-dependent increase in cleavage of procaspase 8 and disappearance of full-length Bid compared with uninfected cells. Furthermore, pharmacological inhibition of caspase 8 caused a decrease in levels of apoptosis. Finally, infection caused a time-dependent increase in mitochondrial-membrane permeability and release of cytochrome c into the cytosol. These results suggest that H. pylori induces apoptosis of macrophages in association with alterations in the mitochondrial pathway. Elimination of this key immunomodulatory cell may represent a mechanism employed by the bacterium to evade host immune responses.

Fascination with bacteria-triggered cell death: the significance of Fas-mediated apoptosis during bacterial infection in vivo

Increasing evidence indicates that bacterial pathogens have developed mechanisms to modulate the apoptotic signaling cascade of host cells and thereby cause disease. The Fas death receptor pathway is one of the most extensively investigated apoptotic signaling pathways. In this review we discuss the role of Fas signaling during the interplay between bacterial pathogens and the host in vivo.

Bartonella quintana lipopolysaccharide effects on leukocytes, CXC chemokines and apoptosis: a study on the human whole blood and a rat model

Bartonella quintana, an emerging gram-negative pathogen, may cause trench fever, endocarditis, cerebral abscess and bacillary angiomatosis usually with the absence of septic shock in humans. B. quintana lipopolysaccharide (LPS), a deep rough endotoxin with strong reactivity in the limulus amebocyte lysate (LAL)-assay, was studied in human whole blood and in a rat model. A significant (P<0.05) increase of interleukin-8 (IL-8) concentration, comparable to the level induced by enterobacterial LPS, was stimulated in the human whole blood by B. quintana LPS. Isolated human neutrophils delayed their apoptotic behavior in the presence of B. quintana LPS. In the rat, B. quintana LPS induced a significant (P<0.001) increase in white blood cell count, both 30 and 60 min after intravenous injection. Such leukocytosis was inhibited by pretreatment with prazosin, an alpha-adrenergic antagonist. B. quintana LPS did not significantly change heart rate (HR), hematocrit (HCT) and platelet count in the above reported in vivo model, and regarding mean blood pressure (MAP) only a very early (5 min after LPS) and mild (yet significant) hypotension was observed. In contrast, a long-lasting decrease of MAP was found in Salmonella minnesota R595 LPS-treated animals. Blood TNFalpha levels did not change significantly from the baseline in rats injected with either saline or with B. quintana LPS, on the contrary S. minnesota R595 LPS-injected animals showed substantial increase of TNFalpha levels up to 2924 pg/ml at 60 min after LPS injection. B. quintana LPS as well as Salmonella LPS-injected rats exhibited an increase of the blood levels of GRO/CINC-1, particularly at 240 min after LPS administration. Apical part of rat gut villi showed several TUNEL-positive cells in tissue sections from B. quintana LPS-treated animals. Taken together, our data demonstrates that B. quintana LPS is able to selectively stimulate some inflammatory mediators. B. quintana LPS-induced leukocytosis appears mediated by an alpha-adrenergic receptor. The delayed apoptotic process of leukocytes and the chemokine increase may explain the apoptotic cells found in the rat gut and the inflammatory reactions in some human Bartonella diseases. This peculiar inflammatory pattern induced by B. quintana LPS, may partially account for the lack of severe septic shock, observed in human B. quintana infections.