Cutting edge: urease release by Helicobacter pylori stimulates macrophage inducible nitric oxide synthase

Strategies of Helicobacter pylori in evading host innate and adaptive immunity: insights and prospects for therapeutic targeting

Helicobacter pylori (H. pylori) is the predominant pathogen causing chronic gastric mucosal infections globally. During the period from 2011 to 2022, the global prevalence of H. pylori infection was estimated at 43.1%, while in China, it was slightly higher at approximately 44.2%. Persistent colonization by H. pylori can lead to gastritis, peptic ulcers, and malignancies such as mucosa-associated lymphoid tissue (MALT) lymphomas and gastric adenocarcinomas. Despite eliciting robust immune responses from the host, H. pylori thrives in the gastric mucosa by modulating host immunity, particularly by altering the functions of innate and adaptive immune cells, and dampening inflammatory responses adverse to its survival, posing challenges to clinical management. The interaction between H. pylori and host immune defenses is intricate, involving evasion of host recognition by modifying surface molecules, manipulating macrophage functionality, and modulating T cell responses to evade immune surveillance. This review analyzes the immunopathogenic and immune evasion mechanisms of H. pylori, underscoring the importance of identifying new therapeutic targets and developing effective treatment strategies, and discusses how the development of vaccines against H. pylori offers new hope for eradicating such infections.

Innate immune activation and modulatory factors of Helicobacter pylori towards phagocytic and nonphagocytic cells

Helicobacter pylori is an intriguing obligate host-associated human pathogen with a specific host interaction biology, which has been shaped by thousands of years of host-pathogen coevolution. Molecular mechanisms of interaction of H. pylori with the local immune cells in the human system are less well defined than epithelial cell interactions, although various myeloid cells, including neutrophils and other phagocytes, are locally present or attracted to the sites of infection and interact with H. pylori. We have recently addressed the question of novel bacterial innate immune stimuli, including bacterial cell envelope metabolites, that can activate and modulate cell responses via the H. pylori Cag type IV secretion system. This review article gives an overview of what is currently known about the interaction modes and mechanisms of H. pylori with diverse human cell types, with a focus on bacterial metabolites and cells of the myeloid lineage including phagocytic and antigen-presenting cells.

Immune Biology and Persistence of Helicobacter pylori in Gastric Diseases

Helicobacter pylori is a prevalent pathogen, which affects more than 40% of the global population. It colonizes the human stomach and persists in its host for several decades or even a lifetime, if left untreated. The persistent infection has been linked to various gastric diseases, including gastritis, peptic ulcers, and an increased risk for gastric cancer. H. pylori infection triggers a strong immune response directed against the bacterium associated with the infiltration of innate phagocytotic immune cells and the induction of a Th1/Th17 response. Even though certain immune cells seem to be capable of controlling the infection, the host is unable to eliminate the bacteria as H. pylori has developed remarkable immune evasion strategies. The bacterium avoids its killing through innate recognition mechanisms and manipulates gastric epithelial cells and immune cells to support its persistence. This chapter focuses on the innate and adaptive immune response induced by H. pylori infection, and immune evasion strategies employed by the bacterium to enable persistent infection.

Acute Helicobacter pylori Infection Prevalence Among Renal Failure Patients and Its Potential Roles with Other Chronic Diseases: A Retrospective Cohort Study

Background

Helicobacter pylori (H. pylori) infection is relevant to several chronic human diseases, from digestive diseases to renal, metabolic, and cancer diseases. H. pylori infections and chronic kidney diseases are in increasing, global records; if not well controlled in a specific population, these diseases might lead to more clinical complications.

Methods

In this retrospective study, we investigated the prevalence of acute H. pylori infections among 127 dialysis patients via subjecting their serums to the enzyme-linked immunosorbent assay (ELISA) to detect the human Immunoglobulin M (IgM) against H. pylori infections. Samples were from dialysis patients in a single hemodialysis center in Medina, Saudi Arabia, from January to August 2021.

Results

Our results indicated the significant prevalence of H. pylori acute infections among 33.1% of renal failure patients recruited in this study, chi-squared: 14.559, p-value: 0.0001. In addition, no significant occurrence of acute H. pylori infection among males and females, chi-squared: 1.823, p-value: 0.177. Furthermore, the prevalence of acute H. pylori infection was not significant in different age groups of renal failure patients. Chi-squared: 6.803, p-value: 0.147, despite H. pylori-infected cases predominantly represented in patients above 51 years. Moreover, we noticed that hypertension, followed by diabetes, was the most prevalent underlying medical condition among acute infected H. pylori and renal failure patients.

Conclusion

We documented the significant prevalence of acute H. pylori infection among renal failure patients. We also highlighted and discussed the possible potential roles of H. pylori in renal failure and other chronic diseases. Routine screening and treatment for acute H. pylori infection for chronic kidney diseases, hypertension, and diabetes patients would positively reduce the bacterium’s progressive effects on them. They might even improve the control of these diseases.

A literature review on the potential clinical implications of streptococci in gastric cancer

Streptococcus is widely found in nature and the human body, and most species are not pathogenic. In recent years, studies have found that Streptococcus is associated with gastric cancer. Streptococcus was found to be enriched in the oral cavity, stomach and intestine of gastric cancer patients and found to be increased in gastric cancer tissues, suggesting that Streptococcus may be the pathogenic bacteria underlying gastric cancer. This review discusses the discovery of Streptococcus, the relationship between Streptococcus and gastric cancer, and the possible carcinogenic mechanism of Streptococcus and summarizes the progress of the research on the role of Streptococcus in gastric cancer to provide new ideas for the early detection, diagnosis and treatment of gastric cancer.

The Efficacy of Cancer Immunotherapies Is Compromised by Helicobacter pylori Infection

Helicobacter pylori infects the gastric mucosa of a large number of humans. Although asymptomatic in the vast majority of cases, H pylori infection can lead to the development of peptic ulcers gastric adenocarcinoma and mucosa-associated lymphoid tissue (MALT) lymphoma. Using a variety of mechanisms, H pylori locally suppresses the function of the host immune system to establish chronic infection. Systemic immunomodulation has been observed in both clinical and pre-clinical studies, which have demonstrated that H pylori infection is associated with reduced incidence of inflammatory diseases, such as asthma and Crohn’s disease. The introduction of immunotherapies in the arsenal of anti-cancer drugs has revealed a new facet of H pylori-induced immune suppression. In this review, we will describe the intimate interactions between H pylori and its host, and formulate hypothtyeses describing the detrimental impact of H pylori infection on the efficacy of cancer immunotherapies.

Induction and Regulation of the Innate Immune Response in Helicobacter pylori Infection

Gastric cancer (GC) is the fifth most common cancer and the fourth most common cause of cancer-related death worldwide. The intestinal type of GC progresses from acute to chronic gastritis, multifocal atrophic gastritis, intestinal metaplasia, dysplasia, and carcinoma. Infection of the stomach by Helicobacter pylori, a Gram-negative bacterium that infects approximately 50% of the world's population, is the causal determinant that initiates the gastric inflammation and then disease progression. In this context, the induction of the innate immune response of gastric epithelial cells and myeloid cells by H. pylori effectors plays a critical role in the outcome of the infection. However, only 1% to 3% of infected patients develop gastric adenocarcinoma, emphasizing that other mechanisms regulate the localized non-specific response, including the gastric microbiota and genetic factors. This review summarizes studies describing the factors that induce and regulate the mucosal innate immune response during H. pylori infection.

In silico design and in vitro assessment of anti-Helicobacter pylori compounds as potential small-molecule arginase inhibitors

Related to a variety of gastrointestinal disorders ranging from gastric ulcer to gastric adenocarcinoma, the infection caused by the gram-negative bacteria Helicobacter pylori (H. pylori) poses as a great threat to human health; hence, the search for new treatments is a global priority. The H. pylori arginase (HPA) protein has been widely studied as one of the main virulence factors of this bacterium, being involved in the prevention of nitric oxide-mediated bacterial cell death, which is a central component of innate immunity. Given the growing need for the development of new drugs capable of combating the infection by H. pylori, the present work describes the search for new HPA inhibitors, using virtual screening techniques based on molecular docking followed by the evaluation of the proposed modes of interaction at the HPA active site. In vitro studies of minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC), followed by cytotoxicity activity in gastric adenocarcinoma and non-cancer cells, were performed. The results highlighted compounds 6, 11, and 13 as potential inhibitors of HPA; within these compounds, the results indicated 13 presented an improved activity toward H. pylori killing, with MIC and MBC both at 64 µg/mL. Moreover, compound 13 also presented a selectivity index of 8.3, thus being more selective for gastric adenocarcinoma cells compared to the commercial drug cisplatin. Overall, the present work demonstrates the search strategy based on in silico and in vitro techniques is able to support the rational design of new anti-H. pylori drugs.

Gastric Microbiota in a Low-Helicobacter pylori Prevalence General Population and Their Associations With Gastric Lesions

INTRODUCTION:

Non–Helicobacter pylori microbiota might account for some cases with unexplained chronic gastritis that may in a minority eventually progress to gastric cancer through the Correa cascade. We characterized gastric microbiota by describing the normal stomach, compared it with early precancerous lesions and other disease states, and assessed whether H. pylori status affects bacterial diversity.

METHODS:

In a population-based study of those with and without gastrointestinal symptoms, cytology brush samples were collected during endoscopy from 316 individuals. Mucosal status was classified as normal mucosa (171), nonatrophic H. pylori gastritis (33), atrophic gastritis (12), or antral chemical gastritis (61). The 16S rRNA gene sequencing and analysis were performed to characterize the microbiota.

RESULTS:

Microbiota in atrophic gastritis and nonatrophic H. pylori gastritis stomachs were dysbiotic and differed from those in the normal stomach (P = 0.001). The normal stomach had the highest microbial diversity, followed by antral chemical gastritis. The atrophic gastritis and chronic H. pylori gastritis groups had the lowest diversity, a difference that was statistically significant (P = 0.01). Besides H. pylori, non–H. pylori bacteria accounted for group differences. Microbial network analysis showed that the normal group network was most highly connected, whereas the H. pylori gastritis group had the lowest connection. We found an increasing positive co-occurrence of oral bacteria in the stomach because samples deviated from the normal network, some of which were pathogens. The H. pylori–negative group had the highest microbial diversity (Shannon index) compared with the H. pylori–positive group (P = 0.001).

DISCUSSION:

In this low–H. pylori prevalence general population, the gastric mucosal microbiota of the normal stomach differed significantly from those with nonatrophic or atrophic gastritis. There was an increasing abundance of pathogenic bacteria from the normal state to early precancerous states.

Immune response to Helicobacter pylori infection and gastric cancer development

Gastric adenocarcinoma is a global health concern, and Helicobacter pylori (H. pylori) infection is the main risk factor for its occurrence. Of note, the immune response against the pathogen seems to be a determining factor for gastric oncogenesis, and increasing evidence have emphasized several host and bacterium factors that probably influence in this setting. The development of an inflammatory process against H. pylori involves a wide range of mechanisms such as the activation of pattern recognition receptors and intracellular pathways resulting in the production of proinflammatory cytokines by gastric epithelial cells. This process culminates in the establishment of distinct immune response profiles that result from the cytokine-induced differentiation of T naïve cells into specific T helper cells. Cytokines released from each type of T helper cell orchestrate the immune system and interfere in the development of gastric cancer in idiosyncratic ways. Moreover, variants in genes such as single nucleotide polymorphisms have been associated with variable predispositions for the occurrence of gastric malignancy because they influence both the intensity of gene expression and the affinity of the resultant molecule with its receptor. In addition, various repercussions related to some H. pylori virulence factors seem to substantially influence the host immune response against the infection, and many of them have been associated with gastric tumorigenesis.

Orally-administered outer-membrane vesicles from Helicobacter pylori reduce H. pylori infection via Th2-biased immune responses in mice

As the trend of antibiotic resistance has increased, prevention and treatment of Helicobacter pylori infection have been challenged by the fact that no vaccines preventing H. pylori infection are available. Scientists continue to make sustained efforts to find better vaccine formulations and adjuvants to eradicate this chronic infection. In this study, we systemically analyzed the protein composition and potential vaccine function of outer-membrane vesicles (OMVs) derived from gerbil-adapted H. pylori strain 7.13. In total, we identified 169 proteins in H. pylori OMVs and found that outer-membrane, periplasmic and extracellular proteins (48.9% of the total proteins) were enriched. Furthermore, we evaluated the immune protective response of H. pylori OMVs in a C57BL/6 mouse model, and mice were orally immunized with OMVs or the H. pylori whole cell vaccine (WCV) alone, with or without cholera toxin (CT) as an adjuvant. The data demonstrated that oral immunization with OMVs can elicit a strong humoral and significantly higher mucosal immune response than the group immunized with the WCV plus the CT adjuvant. Moreover, our results also confirmed that OMVs predominantly induced T helper 2 (Th2)-biased immune responses that can significantly reduce bacterial loads after challenging with the H. pylori Sydney Strain 1 (SS1). In summary, OMVs as new antigen candidates in vaccine design would be of great value in controlling H. pylori infection.

Study of Antihypertensive Activity of Anvillea radiata in L-Name-Induced Hypertensive Rats and HPLC-ESI-MS Analysis

OBJECTIVE

This study aimed to evaluate the effect of the aqueous extract of Anvillea radiate (A. radiata) aerial parts (AEAR) on arterial blood pressure in normotensive and hypertensive rats.

METHODS

The effect of the acute and sub-chronic administration of AEAR on the following blood pressure parameters: systolic blood pressure (SBP), mean blood pressure (MBP), diastolic blood pressure (DBP), and heart rate (HR) was evaluated in normotensive and L-NAME induced hypertensive rats. In the second experiment, the vasorelaxant effect of AEAR was assessed in isolated aortic rings from rats with functional endothelium pre-contracted with epinephrine (EP) or KCl, and six antagonists/ inhibitors were used to explore the mechanisms of action involved in the vasorelaxant effect. In order to determine the phytochemical contents of Anvillea radiata, HPLC-ESI-MS analysis was conducted.

RESULTS

Daily oral administration of AEAR (100 mg/kg) provoked a significant decrease in SBP, MBP, and DBP without affecting HR in hypertensive rats. In addition, AEAR (0.08-0.64 mg/ml) revealed a vasorelaxant effect in thoracic aortic rings pre-contracted by EP (10 μM) or KCl (80 mM). This effect was reduced in the presence of Nifedipine, L-Name or Methylene blue. The polyphenolic compounds of AEAR were determined.

CONCLUSION

This study revealed that AEAR possesses a potent antihypertensive activity and its vasorelaxant activity seems to be mediated through Ca2+ channels, direct nitric oxide (NO), and NO/cGMP pathways. Chlorogenic acid and caffeic acid identified in A. radiata could be at least partially responsible for the antihypertensive activity of this extract.

Positive Association Between Helicobacter pylori Infection and Metabolic Syndrome in a Korean Population: A Multicenter Nationwide Study

BACKGROUND AND AIM

Helicobacter pylori (H. pylori) infection causes extra-gastrointestinal as well as gastric diseases. This analytical cross-sectional study was performed to investigate the association between H. pylori infection and metabolic syndrome in a Korean population.

METHODS

Anthropometric and metabolic data, as well as anti-H. pylori IgG antibodies, were measured in 21,106 subjects who participated in a health checkup between January 2016 and June 2017. The classification of metabolic syndrome followed the revised National Cholesterol Education Program criteria.

RESULTS

After excluding subjects with a history of H. pylori eradication therapy, or gastric symptoms, the seropositivity of H. pylori was 43.2% in 15,195 subjects. H. pylori-positive participants had significantly higher body mass index (BMI), waist circumference, total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), and lower high-density lipoprotein (HDL-C) than did seronegative participants (P < 0.05). After adjusting for confounders, high TC, low HDL-C, and high LDL-C were associated with H. pylori seropositivity. Finally, the prevalence of metabolic syndrome was higher in H. pylori-seropositive subjects than in negative ones (27.2% vs. 21.0%, P < 0.05), and H. pylori seropositivity increased the likelihood of metabolic syndrome (OR 1.19, 95% CI 1.09-1.31, P < 0.001) after adjusting for sex, age, BMI, smoking, residence, household income, and education level. However, the association between H. pylori seropositivity and metabolic syndrome disappeared in those ≥ 65 years old.

CONCLUSIONS

H. pylori infection plays an independent role in the pathogenesis of metabolic syndrome in Koreans under 65 years old.

The Helicobacter pylori Urease Virulence Factor Is Required for the Induction of Hypoxia-Induced Factor-1α in Gastric Cells

Chronic Helicobacter pylori infection increases the risk of gastric cancer and induction of hypoxia-induced factor (HIF), which is frequently associated with the development and progression of several types of cancer. We recently showed that H. pylori activation of the PI3K-AKT-mTOR pathway in gastric cells increased HIF-1α expression. Here, we identified the H. pylori virulence factor responsible for HIF-1α induction. A mutant of the H. pylori 84-183 strain was identified with reduced ability to induce HIF-1α. Coomassie blue staining of extracts from these bacteria separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) revealed poor expression of urease subunits that correlated with reduced urease activity. This finding was confirmed in the 26695 strain, where urease mutants were unable to induce HIF-1α expression. Of note, HIF-1α induction was also observed in the presence of the urease inhibitor acetohydroxamic acid at concentrations (of 20 mM) that abrogated urease activity in bacterial culture supernatants, suggesting that enzymatic activity of the urease is not required for HIF-1α induction. Finally, the pre-incubation of the human gastric adenocarcinoma cell line AGS with blocking antibodies against Toll-like receptor-2 (TLR2), but not TLR4, prevented HIF-1α induction. In summary, these results reveal a hitherto unexpected role for the urease protein in HIF-1α induction via TLR2 activation following H. pylori infection of gastric cells.

Cryptococcus neoformans urease affects the outcome of intracellular pathogenesis by modulating phagolysosomal pH

Cryptococcus neoformans is a facultative intracellular pathogen and its interaction with macrophages is a key event determining the outcome of infection. Urease is a major virulence factor in C. neoformans but its role during macrophage interaction has not been characterized. Consequently, we analyzed the effect of urease on fungal-macrophage interaction using wild-type, urease-deficient and urease-complemented strains of C. neoformans. The frequency of non-lytic exocytosis events was reduced in the absence of urease. Urease-positive C. neoformans manifested reduced and delayed intracellular replication with fewer macrophages displaying phagolysosomal membrane permeabilization. The production of urease was associated with increased phagolysosomal pH, which in turn reduced growth of urease-positive C. neoformans inside macrophages. Interestingly, the ure1 mutant strain grew slower in fungal growth medium which was buffered to neutral pH (pH 7.4). Mice inoculated with macrophages carrying urease-deficient C. neoformans had lower fungal burden in the brain than mice infected with macrophages carrying wild-type strain. In contrast, the absence of urease did not affect survival of yeast when interacting with amoebae. Because of the inability of the urease deletion mutant to grow on urea as a sole nitrogen source, we hypothesize urease plays a nutritional role involved in nitrogen acquisition in the environment. Taken together, our data demonstrate that urease affects fitness within the mammalian phagosome, promoting non-lytic exocytosis while delaying intracellular replication and thus reducing phagolysosomal membrane damage, events that could facilitate cryptococcal dissemination when transported inside macrophages. This system provides an example where an enzyme involved in nutrient acquisition modulates virulence during mammalian infection.

Exhaled nitric oxide as a potential marker for detecting non-ulcer dyspepsia and peptic ulcer disease

Nitric oxide (NO) plays a key role in the development of peptic ulcer disease (PUD). Conversely, the gastric pathogen Helicobacter pylori colonizes the human stomach and contributes to the development of non-ulcer dyspepsia (NUD) and PUD. However, the underlying relation between molecular NO in exhaled breath and H. pylori-associated NUD and PUD remains largely unknown. Here, we found that the excretion kinetics of NO profiles in exhaled breath are altered markedly in H. pylori-infected NUD and PUD subjects. In our observations, PUD led to considerably higher enrichments of NO in exhaled breath compared to NUD, thus revealing a potential link between exhaled NO and ulcer and non-ulcer complications. Our findings therefore suggest that molecular NO in exhaled breath could be used as a potential biomarker for non-invasive diagnosis and selective differentiation of NUD from PUD. Our observations also highlight that alterations of NO in the gastric environment can play an important role in the pathogenesis of peptic ulcers and thus may provide a new strategy for precise evolution of the actual disease state without the need for endoscopic biopsy, even after the eradication of H. pylori infection.

The Impact of Helicobacter pylori Urease upon Platelets and Consequent Contributions to Inflammation

Gastric infection by Helicobacter pylori is considered a risk factor for gastric and duodenal cancer, and extragastric diseases. Previous data have shown that, in a non-enzymatic way, H. pylori urease (HPU) activates neutrophils to produce ROS and also induces platelet aggregation, requiring ADP secretion modulated by the 12-lipoxygenase pathway, a signaling cascade also triggered by the physiological agonist collagen. Here we investigated further the effects on platelets of recombinant versions of the holoenzyme HPU, and of its two subunits (HpUreA and HpUreB). Although HpUreA had no aggregating activity on platelets, it partially inhibited collagen-induced aggregation. HpUreB induced platelet aggregation in the nanomolar range, and also interfered dose-dependently on both collagen- and ADP-induced platelet aggregation. HPU-induced platelet aggregation was inhibited by antibodies against glycoprotein VI (GPVI), the main collagen receptor in platelets. Flow cytometry analysis revealed exposure of P-selectin in HPU-activated platelets. Anti-glycoprotein IIbIIIa (GPIIbIIIa) antibodies increased the binding of FITC-labeled HPU to activated platelets, whereas anti-GPVI did not. Evaluation of post-transcriptional events in HPU-activated platelets revealed modifications in the pre-mRNA processing of pro-inflammatory proteins, with increased levels of mRNAs encoding IL-1β and CD14. We concluded that HPU activates platelets probably through its HpUreB subunit. Activation of platelets by HPU turns these cells into a pro-inflammatory phenotype. Altogether, our data suggest that H. pylori urease, besides allowing bacterial survival within the gastric mucosa, may have an important, and so far overlooked, role in gastric inflammation mediated by urease-activated neutrophils and platelets.

Analysis of Gastric Body Microbiota by Pyrosequencing: Possible Role of Bacteria Other Than Helicobacter pylori in the Gastric Carcinogenesis

Background

Gastric microbiota along with Helicobacter pylori (HP) plays a key role in gastric disease. The aim of our study is to investigate the difference of human gastric microbiota between antrum and body according to disease (control vs. gastric cancer) and HP status.

Methods

Each antrum and body biopsy was collected from 12 subjects at Seoul National University Bundang Hospital. Gastric microbiota was analyzed by bar-coded 454 pyrosequencing of the 16S rRNA gene. Twelve subjects consisted of HP-negative control (n = 2), HP-negative cancer (n = 2), HP-positive control (n = 3), and HP-positive cancer (n = 5). The analysis was focused on non-HP urease-producing bacteria (UB) and non-HP nitrosating or nitroreducing bacteria (NB) between antrum and body.

Results

Gastric body samples showed higher diversity compared to gastric antrum mucosa samples but there was no significant difference. The mean of operational taxonomic units was higher in HP(−) cancer than HP(+) cancer (antrum, 273.5 vs. 228.2, P = 0.439; body, 585.5 vs. 183.2, P = 0.053). The number of non-HP UB and non-HP NB was higher in HP(−) cancer groups than the others. These differences were more pronounced in the body (P = 0.051 and P = 0.081, respectively). Analysis of overlap of non-HP UB and non-HP NB revealed the higher composition of Streptococcus pseudopneumoniae, S. parasanguinis, and S. oralis in HP(−) cancer groups than the others, only in the body (P = 0.030) but not in the antrum (P = 0.123).

Conclusions

Higher diversity and higher composition of S. pseudopneumoniae, S. parasanguinis, and S. oralis in HP(−) cancer group than the other groups in the body suggest that analysis of microbiota from body mucosa could be beneficial to identify a role of non-HP bacteria in the gastric carcinogenesis.

Immune Evasion Strategies and Persistence of Helicobacter pylori

Helicobacter pylori infection is commonly acquired during childhood, can persist lifelong if not treated, and can cause different gastric pathologies, including chronic gastritis, peptic ulcer disease, and eventually gastric cancer. H. pylori has developed a number of strategies in order to cope with the hostile conditions found in the human stomach as well as successful mechanisms to evade the strong innate and adaptive immune responses elicited upon infection. Thus, by manipulating innate immune receptors and related signaling pathways, inducing tolerogenic dendritic cells and inhibiting effector T cell responses, H. pylori ensures low recognition by the host immune system as well as its persistence in the gastric epithelium. Bacterial virulence factors such as cytotoxin-associated gene A, vacuolating cytotoxin A, or gamma-glutamyltranspeptidase have been extensively studied in the context of bacterial immune escape and persistence. Further, the bacterium possesses other factors that contribute to immune evasion. In this chapter, we discuss in detail the main evasion and persistence strategies evolved by the bacterium as well as the specific bacterial virulence factors involved.

Helicobacter pylori Secreted Protein HP1286 Triggers Apoptosis in Macrophages via TNF-Independent and ERK MAPK-Dependent Pathways

Macrophages constitute a powerful line of defense against H. pylori. The final disease outcome is highly dependent on the bacterial ability to modulate the effector functions of activated macrophages. Here, we report that H. pylori secreted protein HP1286 is a novel regulator of macrophage responses. Differential expression and release of HP1286 homologues were observed among H. pylori strains. Recombinant purified HP1286 (rHP1286) had the ability to bind to primary human monocyte-derived macrophages (MDM) and macrophage cell lines. Exposure to rHP1286 induced apoptosis in macrophages in a dose- and time-dependent manner. Although interaction of rHP1286 was observed for several other cell types, such as human monocytes, differentiated neutrophil-like HL60 cells, and the T lymphocyte Jurkat cell line, rHP1286 failed to induce apoptosis under similar conditions, indicating a macrophage-specific effect of the protein. A mutant strain of H. pylori lacking HP1286 protein expression was significantly impaired in its ability to induce apoptosis in macrophages. Significantly higher caspase 3 activity was detected in rHP1286-challenged macrophages. Furthermore, rHP1286-induced macrophages apoptosis was not inhibited in the presence of neutralizing antibodies against TNF. These observations indicate that rHP1286 induced a caspase-dependent and TNF-independent macrophage apoptosis. Pre-treatment of macrophages with U0126, an inhibitor of the ERK MAPK signaling pathway significantly reduced rHP1286-induced apoptosis. Furthermore, nuclear translocation of ERK and phosphorylation of c-Fos was detected in rHP1286-treated macrophages. These results provide functional insight into the potential role of HP1286 during H. pylori infection. Considering the ability of HP1286 to induce macrophage apoptosis, the protein could possibly help in the bacterial escape from the activated macrophages and persistence in the stomach.

Oxidative and nitrosative stress defences of Helicobacter and Campylobacter species that counteract mammalian immunity

Helicobacter and Campylobacter species are Gram-negative microaerophilic host-associated heterotrophic bacteria that invade the digestive tract of humans and animals. Campylobacter jejuni is the major worldwide cause of foodborne gastroenteritis in humans, while Helicobacter pylori is ubiquitous in over half of the world's population causing gastric and duodenal ulcers. The colonisation of the gastrointestinal system by Helicobacter and Campylobacter relies on numerous cellular defences to sense the host environment and respond to adverse conditions, including those imposed by the host immunity. An important antimicrobial tool of the mammalian innate immune system is the generation of harmful oxidative and nitrosative stresses to which pathogens are exposed during phagocytosis. This review summarises the regulators, detoxifying enzymes and subversion mechanisms of Helicobacter and Campylobacter that ultimately promote the successful infection of humans.

Relationship between ureB Sequence Diversity, Urease Activity and Genotypic Variations of Different Helicobacter pylori Strains in Patients with Gastric Disorders

Association of the severity of Helicobacter pylori induced diseases with virulence entity of the colonized strains was proven in some studies. Urease has been demonstrated as a potent virulence factor for H. pylori. The main aim of this study was investigation of the relationships of ureB sequence diversity, urease activity and virulence genotypes of different H. pylori strains with histopathological changes of gastric tissue in infected patients suffering from different gastric disorders. Analysis of the virulence genotypes in the isolated strains indicated significant associations between the presence of severe active gastritis and cagA+ (P = 0.039) or cagA/iceA1 genotypes (P = 0.026), and intestinal metaplasia and vacA m1 (P = 0.008) or vacA s1/m2 (P = 0.001) genotypes. Our results showed a 2.4-fold increased risk of peptic ulcer (95% CI: 0.483-11.93), compared with gastritis, in the infected patients who had dupA positive strains; however this association was not statistically significant. The results of urease activity showed a significant mean difference between the isolated strains from patients with PUD and NUD (P = 0.034). This activity was relatively higher among patients with intestinal metaplasia. Also a significant associa-tion was found between the lack of cagA and increased urease activity among the isolated strains (P = 0.036). While the greatest sequencevariation of ureB was detected in a strain from a patient with intestinal metaplasia, the sole determined amino acid change in UreB sequence (Ala201Thr, 30%), showed no influence on urease activity. In conclusion, the supposed role of H. pylori urease to form peptic ulcer and advancing of intestinal metaplasia was postulated in this study. Higher urease activity in the colonizing H. pylori strains that present specific virulence factors was indicated as a risk factor for promotion of histopathological changes of gastric tissue that advance gastric malignancy.

Arginase 2 deletion leads to enhanced M1 macrophage activation and upregulated polyamine metabolism in response to Helicobacter pylori infection

We reported that arginase 2 (ARG2) deletion results in increased gastritis and decreased bacterial burden during Helicobacter pylori infection in mice. Our studies implicated a potential role for inducible nitric oxide (NO) synthase (NOS2), as Arg2 (-/-) mice exhibited increased NOS2 levels in gastric macrophages, and NO can kill H. pylori. We now bred Arg2 (-/-) to Nos2 (-/-) mice, and infected them with H. pylori. Compared to wild-type mice, both Arg2 (-/-) and Arg2 (-/-) ;Nos2 (-/-) mice exhibited increased gastritis and decreased colonization, the latter indicating that the effect of ARG2 deletion on bacterial burden was not mediated by NO. While Arg2 (-/-) mice demonstrated enhanced M1 macrophage activation, Nos2 (-/-) and Arg2 (-/-) ;Nos2 (-/-) mice did not demonstrate these changes, but exhibited increased CXCL1 and CXCL2 responses. There was an increased expression of the Th1/Th17 cytokines, interferon gamma and interleukin 17, in gastric tissues and splenic T-cells from Arg2 (-/-), but not Nos2 (-/-) or Arg2 (-/-) ;Nos2 (-/-) mice. Gastric tissues from infected Arg2 (-/-) mice demonstrated increased expression of arginase 1, ornithine decarboxylase, adenosylmethionine decarboxylase 1, spermidine/spermine N (1)-acetyltransferase 1, and spermine oxidase, along with increased spermine levels. These data indicate that ARG2 deletion results in compensatory upregulation of gastric polyamine synthesis and catabolism during H. pylori infection, which may contribute to increased gastric inflammation and associated decreased bacterial load. Overall, the finding of this study is that ARG2 contributes to the immune evasion of H. pylori by restricting M1 macrophage activation and polyamine metabolism.

The Immune Battle against Helicobacter pylori Infection: NO Offense

Helicobacter pylori is a successful pathogen of the human stomach. Despite a vigorous immune response by the gastric mucosa, the bacterium survives in its ecological niche, thus favoring diseases ranging from chronic gastritis to adenocarcinoma. The current literature demonstrates that high-output of nitric oxide (NO) production by the inducible enzyme NO synthase-2 (NOS2) plays major functions in host defense against bacterial infections. However, pathogens have elaborated several strategies to counteract the deleterious effects of NO; this includes inhibition of host NO synthesis and transcriptional regulation in response to reactive nitrogen species, allowing the bacteria to face the nitrosative stress. Moreover, NO is also a critical mediator of inflammation and carcinogenesis. In this context, we review the recent findings on the expression of NOS2 in H. pylori-infected gastric tissues and epithelial cells, the role of NO in H. pylori-related diseases and H. pylori gene expression, and the mechanisms whereby H. pylori regulates NO synthesis by host cells.

Ureases as multifunctional toxic proteins: A review

Ureases are metalloenzymes that hydrolyze urea into ammonia and carbon dioxide. They were the first enzymes to be crystallized and, with them, the notion that enzymes are proteins became accepted. Novel toxic properties of ureases that are independent of their enzyme activity have been discovered in the last three decades. Since our first description of the neurotoxic properties of canatoxin, an isoform of the jack bean urease, which appeared in Toxicon in 1981, about one hundred articles have been published on "new" properties of plant and microbial ureases. Here we review the present knowledge on the non-enzymatic properties of ureases. Plant ureases and microbial ureases are fungitoxic to filamentous fungi and yeasts by a mechanism involving fungal membrane permeabilization. Plant and at least some bacterial ureases have potent insecticidal effects. This entomotoxicity relies partly on an internal peptide released upon proteolysis of ingested urease by insect digestive enzymes. The intact protein and its derived peptide(s) are neurotoxic to insects and affect a number of other physiological functions, such as diuresis, muscle contraction and immunity. In mammal models some ureases are acutely neurotoxic upon injection, at least partially by enzyme-independent effects. For a long time bacterial ureases have been recognized as important virulence factors of diseases by urease-producing microorganisms. Ureases activate exocytosis in different mammalian cells recruiting eicosanoids and Ca(2+)-dependent pathways, even when their ureolytic activity is blocked by an irreversible inhibitor. Ureases are chemotactic factors recognized by neutrophils (and some bacteria), activating them and also platelets into a pro-inflammatory "status". Secretion-induction by ureases may play a role in fungal and bacterial diseases in humans and other animals. The now recognized "moonlighting" properties of these proteins have renewed interest in ureases for their biotechnological potential to improve plant defense against pests and as potential targets to ameliorate diseases due to pathogenic urease-producing microorganisms.

Molecular Mechanisms of Helicobacter pylori Pathogenesis

Helicobacter pylori infects 50% of mankind. The vast majority of H. pylori infection occurs in the developing countries where up to 80% of the middle-aged adults may be infected. Bacterial infection causes an inflammatory response that proceeds through a series of intermediated stages of precancerous lesions (gastritis, atrophy, intestinal metaplasia, and dysplasia). Among infected individuals, approximately 10% develops severe gastric lesions such as peptic ulcer disease, 1-3% progresses to gastric cancer (GC) with a low 5-year survival rate, and 0.1% develops mucosa-associated lymphoid tissue (MALT). GC is one of the most common cancer and the third leading cause of cancer-related deaths worldwide. In this review, we have summarized the most recent papers about molecular mechanisms of H. pylori pathogenesis. The main important steps of H. pylori infection such as adhesion, entry in epithelial gastric cells, activation of intracellular pathways until epigenetic modifications have been described.

Morphological changes in human gastric epithelial cells induced by nuclear targeting of Helicobacter pylori urease subunit A

Nuclear targeting of bacterial proteins and their pathological effects on host cells are an emerging pathogenic mechanism in bacteria. We have previously reported that urease subunit A (UreA) of Helicobacter pylori targets the nuclei of COS-7 cells through nuclear localization signals (NLSs). This study further investigated whether UreA of H. pylori targets the nuclei of gastric epithelial cells and then induces molecular and cellular changes in the host cells. H. pylori 26695 strain produced and secreted outer membrane vesicles (OMVs). UreA was translocated into gastric epithelial AGS cells through outer membrane vesicles (OMVs) and then targeted the nuclei of AGS cells. Nuclear targeting of rUreA did not induce host cell death, but resulted in morphological changes, such as cellular elongation, in AGS cells. In contrast, AGS cells treated with rUreA?NLS proteins did not show this morphological change. Next generation sequencing revealed that nuclear targeting of UreA differentially regulated 102 morphogenesis- related genes, of which 67 and 35 were up-regulated and down-regulated, respectively. Our results suggest that nuclear targeting of H. pylori UreA induces both molecular and cellular changes in gastric epithelial cells.

Association of Helicobacter pylori and iNOS production by macrophages and lymphocytes in the gastric mucosa in chronic gastritis

Helicobacter pylori is one of the most common causes of chronic gastritis. With the development of the disease cellular inflammatory infiltrates composed of lymphocytes, plasma cells, and macrophages are formed in epithelium and lamina propria of the stomach. These cells are capable of secreting a number of active substances, including inducible nitric oxide synthase (iNOS). We examined the relationship between H. pylori and secretion of iNOS by cells of inflammatory infiltrates in chronic gastritis by light microscopy and immunohistochemistry. The data obtained indicate that stimulation of H. pylori immune system cells of the host organism during development of chronic gastritis causes increase in number of macrophages and lymphocytes in the inflammatory infiltrate of the gastric mucosa. This is accompanied with increased expression of inducible NO-synthase with excess free radicals in the tissues, which leads to secondary alterations and exacerbates the inflammation with impaired regeneration processes.

Heme oxygenase-1 dysregulates macrophage polarization and the immune response to Helicobacter pylori

Helicobacter pylori incites a futile inflammatory response, which is the key feature of its immunopathogenesis. This leads to the ability of this bacterial pathogen to survive in the stomach and cause peptic ulcers and gastric cancer. Myeloid cells recruited to the gastric mucosa during H. pylori infection have been directly implicated in the modulation of host defense against the bacterium and gastric inflammation. Heme oxygenase-1 (HO-1) is an inducible enzyme that exhibits anti-inflammatory functions. Our aim was to analyze the induction and role of HO-1 in macrophages during H. pylori infection. We now show that phosphorylation of the H. pylori virulence factor cytotoxin-associated gene A (CagA) in macrophages results in expression of hmox-1, the gene encoding HO-1, through p38/NF (erythroid-derived 2)-like 2 signaling. Blocking phagocytosis prevented CagA phosphorylation and HO-1 induction. The expression of HO-1 was also increased in gastric mononuclear cells of human patients and macrophages of mice infected with cagA(+) H. pylori strains. Genetic ablation of hmox-1 in H. pylori-infected mice increased histologic gastritis, which was associated with enhanced M1/Th1/Th17 responses, decreased regulatory macrophage (Mreg) response, and reduced H. pylori colonization. Gastric macrophages of H. pylori-infected mice and macrophages infected in vitro with this bacterium showed an M1/Mreg mixed polarization type; deletion of hmox-1 or inhibition of HO-1 in macrophages caused an increased M1 and a decrease of Mreg phenotype. These data highlight a mechanism by which H. pylori impairs the immune response and favors its own survival via activation of macrophage HO-1.

At the Bench: Helicobacter pylori, dysregulated host responses, DNA damage, and gastric cancer

Helicobacter pylori infection is the strongest known risk factor for the development of gastric cancer. Given that ∼50% of the global population is infected with this pathogen, there is great impetus to elucidate underlying causes that mediate progression from infection to cancer. Recent evidence suggests that H. pylori-induced chronic inflammation and oxidative stress create an environment conducive to DNA damage and tissue injury. DNA damage leads to genetic instability and eventually, neoplastic transformation. Pathogen-encoded virulence factors induce a robust but futile immune response and alter host pathways that lower the threshold for carcinogenesis, including DNA damage repair, polyamine synthesis and catabolism, antioxidant responses, and cytokine production. Collectively, such dysregulation creates a protumorigenic microenvironment within the stomach. This review seeks to address each of these aspects of H. pylori infection and to call attention to areas of particular interest within this field of research. This review also seeks to prioritize areas of translational research related to H. pylori-induced gastric cancer based on insights garnered from basic research in this field. See related review by Dalal and Moss, At the Bedside: H. pylori, dysregulated host responses, DNA damage, and gastric cancer.

Spermine oxidase is a regulator of macrophage host response to Helicobacter pylori: enhancement of antimicrobial nitric oxide generation by depletion of spermine

The gastric pathogen Helicobacter pylori causes peptic ulcer disease and gastric cancer. We have reported that in H. pylori-activated macrophages, nitric oxide (NO) derived from inducible NO synthase (iNOS) can kill the bacterium, iNOS protein expression is dependent on uptake of its substrate L-arginine (L-Arg), the polyamine spermine can inhibit iNOS translation by inhibiting L-Arg uptake, and inhibition of polyamine synthesis enhances NO-mediated bacterial killing. Because spermine oxidase (SMO), which back-converts spermine to spermidine, is induced in macrophages by H. pylori, we determined its role in iNOS-dependent host defense. SMO shRNA knockdown in RAW 264.7 murine macrophages resulted in a marked decrease in H. pylori-stimulated iNOS protein, but not mRNA expression, and a 90% reduction in NO levels; NO production was also inhibited in primary murine peritoneal macrophages with SMO knockdown. There was an increase in spermine levels after H. pylori stimulation that rapidly decreased, while SMO knockdown caused a greater increase in spermine that was sustained. With SMO knockdown, L-Arg uptake and killing of H. pylori by macrophages was prevented. The overexpression of SMO by transfection of an expression plasmid prevented the H. pylori-stimulated increase in spermine levels, and led to increased L-Arg uptake, iNOS protein expression and NO production, and H. pylori killing. In two human monocytic cell lines, U937 and THP-1, overexpression of SMO caused a significant enhancement of NO production with H. pylori stimulation. By depleting spermine, SMO can abrogate the inhibitory effect of polyamines on innate immune responses to H. pylori by enhancing antimicrobial NO production.

Role of childhood infection in the sequelae of H. pylori disease

The persistence of Helicobacter pylori infection plays a fundamental role in the development of H. pylori-associated complications. Since the majority of infected persons acquire the bacteria during early childhood, an examination of the immunobiology of H. pylori infection in children compared with that of adults may help identify host factors that contribute to persistent infection. Therefore, we begin our review of the role of persistence in H. pylori disease with an assessment of the clinical features of H. pylori infection in children. We next review the bacterial factors that promote colonization and evasion of host defense mechanisms. We then focus our attention on the early host immunological factors that promote persistence of the infection and its complications in humans and mouse models. We also highlight topics in which further research is needed. An examination of how immunological factors cause divergent manifestations of H. pylori infection in children compared with adults may provide new insight for therapeutic modification or prevention of persistent H. pylori infection and its complications.

Chronic inflammation and oxidative stress: the smoking gun for Helicobacter pylori-induced gastric cancer?

Helicobacter pylori is the leading risk factor associated with gastric carcinogenesis. H. pylori leads to chronic inflammation because of the failure of the host to eradicate the infection. Chronic inflammation leads to oxidative stress, deriving from immune cells and from within gastric epithelial cells. This is a main contributor to DNA damage, apoptosis and neoplastic transformation. Both pathogen and host factors directly contribute to oxidative stress, including H. pylori virulence factors, and pathways involving DNA damage and repair, polyamine synthesis and metabolism, and oxidative stress response. Our laboratory has recently uncovered a mechanism by which polyamine oxidation by spermine oxidase causes H 2O 2 release, DNA damage and apoptosis. Our studies indicate novel targets for therapeutic intervention and risk assessment in H. pylori-induced gastric cancer. More studies addressing the many potential contributors to oxidative stress, chronic inflammation, and gastric carcinogenesis are essential for development of therapeutics and identification of gastric cancer biomarkers.

Differential roles of ASK1 and TAK1 in Helicobacter pylori-induced cellular responses

The mitogen-activated protein kinase (MAPK) signaling pathway regulates various cellular functions, including those induced by Helicobacter pylori. TAK1 is an upstream MAPK kinase kinase (MAP3K) required for H. pylori-induced MAPK and NF-κB activation, but it remains unclear whether other MAP3Ks are involved in H. pylori-induced cellular responses. In this study, we focused on the MAP3K ASK1, which plays a critical role in gastric tumorigenesis. In gastric epithelial cells, H. pylori activates ASK1 in a reactive oxygen species (ROS)- and cag pathogenicity island-dependent manner, and ASK1 regulates sustained JNK activation and apoptosis induced by H. pylori. In contrast, TAK1 regulates H. pylori-mediated early JNK activation and cytokine production. We also found reciprocal regulation between ASK1 and TAK1 in H. pylori-related responses, whereby inhibition of TAK1 or downstream p38 MAPK activates ASK1 through ROS production, and ASK1 suppresses TAK1 and downstream NF-κB activation. We identified ROS/ASK1/JNK as a new signaling pathway induced by H. pylori, which regulates apoptotic cell death. The balance of ASK1-induced apoptosis and TAK1-induced antiapoptotic or inflammatory responses may determine the fate of epithelial cells infected with H. pylori and thus be involved in the pathogenesis of gastritis and gastric cancer.

Overexpression of Helicobacter pylori VacA N-terminal fragment induces proinflammatory cytokine expression and apoptosis in human monocytic cell line through activation of NF-κB

Vacuolating cytotoxin (VacA) is an important virulence factor in the pathogenesis of Helicobacter pylori-related diseases. The aim of this study was to investigate the function of the amino-terminal 476 residue fragment (p52) of VacA and the possible molecular mechanisms responsible for its induction of proinflammatory cytokines secretion and apoptosis. Human acute monocytic leukemia cell line THP-1 was used as an in vitro model to study proinflammatory cytokines secretion and apoptosis induced by transfection of a recombinant plasmid encoding the amino-terminal 476 residue fragment (p52) of VacA. The results showed that VacA p52 overexpression induced the production of tumor necrosis factor alpha (TNF-α), interleukin-1 beta (IL-1β), nitric oxide, and reactive oxygen species in THP-1 cells in a time-dependent manner. VacA p52 overexpression also promoted THP-1 cells apoptosis. In addition, VacA p52 triggered the activation of nuclear factor kappa B (NF-κB), indicating a possible mechanism for its induction of proinflammatory cytokines secretion and cell apoptosis. Our study demonstrated that the induction of cytokines secretion and apoptosis by VacA p52 in THP-1 cells could be mediated through activation of nuclear factor kappa B.

Lower expression of inducible nitric oxide synthase and higher expression of arginase in rat alveolar macrophages are linked to their susceptibility to Toxoplasma gondii infection

Rats are naturally resistant to Toxoplasma gondii infection, particularly the RH strain, while mice are not. Previous studies have demonstrated that inducible nitric oxide synthase (iNOS) and arginase-1 of rodent peritoneal macrophages are linked to the mechanism of resistance. As an increasing number of studies on human and animal infections are showing that pulmonary toxoplasmosis is one of the most severe clinical signs from T. gondii infection, we are interested to know whether T. gondii infection in alveolar macrophages of rats is also linked to the levels of iNOS and arginase-1 activity. Our results demonstrate that T. gondii could grow and proliferate in rat alveolar macrophages, both in vitro and in vivo, at levels higher than resistant rat peritoneal macrophages and at comparable levels to sensitive mouse peritoneal macrophages. Lower activity and expression levels of iNOS and higher activity and expression levels of arginase-1 in rat alveolar macrophages were found to be linked to the susceptibility of T. gondii infection in these cells. These novel findings could aid a better understanding of the pathogenesis of clinical pulmonary toxoplasmosis in humans and domestic animals.

Immunohistochemical detection of Helicobacter pylori without association of TLR5 expression in oral squamous cell carcinoma

BACKGROUND

Approximately 15% of human deaths from cancer are associated with chronic viral or bacterial infections. Helicobacter pylori (HP), a flagellated, Gram-negative, spiral, microaerophilic bacteria is considered to be the most common chronic bacterial infection in humans. Toll-like receptor 5 (TLR5) is involved in recognition of bacterial flagella and is thought to promote tumour growth through inflammation-dependent mechanisms in epithelial cells.

METHODS

Expression of HP and TLR5 was analysed in OSCC specimen (n = 191) by immunohistochemistry. TLR5 expression specificity was conducted by Western blotting in cancer cell lines (BICR3, BICR56). TLR5-stained sections were scanned and digitally analysed using ImageJ and the immunomembrane plug-in. HP expression and TLR5 expression were associated with clinicopathological characteristics and impact on survival.

RESULTS

Helicobacter pylori detection was significantly associated with recurrence of the tumour, whereas TLR5 expression was not. Multivariate analysis demonstrated HP expression as an independent prognostic factor (P = 0.0260). TLR5 specificity was confirmed by Western blot analysis.

CONCLUSIONS

For the first time, this study provides evidence that immunohistochemically detected HP expression in OSCC is associated with reduced disease-free survival in a large patient cohort. Although TLR5 was not associated with any clinicopathological characteristics or impact on survival, investigation of the TLR family seems to be reasonable due to the possible existence of other pathogenic bacterial or viral compounds in oral cavity cancer.

Microbial biofilms and gastrointestinal diseases

The majority of bacteria live not planktonically, but as residents of sessile biofilm communities. Such populations have been defined as 'matrix-enclosed microbial accretions, which adhere to both biological and nonbiological surfaces'. Bacterial formation of biofilm is implicated in many chronic disease states. Growth in this mode promotes survival by increasing community recalcitrance to clearance by host immune effectors and therapeutic antimicrobials. The human gastrointestinal (GI) tract encompasses a plethora of nutritional and physicochemical environments, many of which are ideal for biofilm formation and survival. However, little is known of the nature, function, and clinical relevance of these communities. This review summarizes current knowledge of the composition and association with health and disease of biofilm communities in the GI tract.

Differences in iNOS and arginase expression and activity in the macrophages of rats are responsible for the resistance against T. gondii infection

Toxoplasma gondii infects humans and warm blooded animals causing devastating disease worldwide. It has long been a mystery as to why the peritoneal macrophages of rats are naturally resistant to T. gondii infection while those of mice are not. Here, we report that high expression levels and activity of inducible nitric oxide synthase (iNOS) and low levels of arginase-1 (Arg 1) activity in the peritoneal macrophages of rats are responsible for their resistance against T. gondii infection, due to high nitric oxide and low polyamines within these cells. The opposite situation was observed in the peritoneal macrophages of mice. This discovery of the opposing functions of iNOS and Arg 1 in rodent peritoneal macrophages may lead to a better understanding of the resistance mechanisms of mammals, particularly humans and livestock, against T. gondii and other intracellular pathogens.

Ceramide and Toll-like receptor 4 are mobilized into membrane rafts in response to Helicobacter pylori infection in gastric epithelial cells

Helicobacter pylori infection is thought to be involved in the development of several gastric diseases. Two H. pylori virulence factors (vacuolating cytotoxin A and cytotoxin-associated gene A) reportedly interact with lipid rafts in gastric epithelial cells. The role of Toll-like receptor (TLR)-mediated signaling in response to H. pylori infection has been investigated extensively in host cells. However, the receptor molecules in lipid rafts that are involved in H. pylori-induced innate sensing have not been well characterized. This study investigated whether lipid rafts play a role in H. pylori-induced ceramide secretion and TLR4 expression and thereby contribute to inflammation in gastric epithelial cells. We observed that both TLR4 and MD-2 mRNA and protein levels were significantly higher in H. pylori-infected AGS cells than in mock-infected cells. Moreover, significantly more TLR4 protein was detected in detergent-resistant membranes extracted from H. pylori-infected AGS cells than in those extracted from mock-infected cells. However, this effect was attenuated by the treatment of cells with cholesterol-usurping agents, suggesting that H. pylori-induced TLR4 signaling is dependent on cholesterol-rich microdomains. Similarly, the level of cellular ceramide was elevated and ceramide was translocated into lipid rafts after H. pylori infection, leading to interleukin-8 (IL-8) production. Using the sphingomyelinase inhibitor imipramine, we observed that H. pylori-induced TLR4 expression was ceramide dependent. These results indicate the mobilization of ceramide and TLR4 into lipid rafts by H. pylori infection in response to inflammation in gastric epithelial cells.

Cationic amino acid transporter 2 enhances innate immunity during Helicobacter pylori infection

Once acquired, Helicobacter pylori infection is lifelong due to an inadequate innate and adaptive immune response. Our previous studies indicate that interactions among the various pathways of arginine metabolism in the host are critical determinants of outcomes following infection. Cationic amino acid transporter 2 (CAT2) is essential for transport of L-arginine (L-Arg) into monocytic immune cells during H. pylori infection. Once within the cell, this amino acid is utilized by opposing pathways that lead to elaboration of either bactericidal nitric oxide (NO) produced from inducible NO synthase (iNOS), or hydrogen peroxide, which causes macrophage apoptosis, via arginase and the polyamine pathway. Because of its central role in controlling L-Arg availability in macrophages, we investigated the importance of CAT2 in vivo during H. pylori infection. CAT2(-/-) mice infected for 4 months exhibited decreased gastritis and increased levels of colonization compared to wild type mice. We observed suppression of gastric macrophage levels, macrophage expression of iNOS, dendritic cell activation, and expression of granulocyte-colony stimulating factor in CAT2(-/-) mice suggesting that CAT2 is involved in enhancing the innate immune response. In addition, cytokine expression in CAT2(-/-) mice was altered from an antimicrobial Th1 response to a Th2 response, indicating that the transporter has downstream effects on adaptive immunity as well. These findings demonstrate that CAT2 is an important regulator of the immune response during H. pylori infection.

Difluoromethylornithine is a novel inhibitor of Helicobacter pylori growth, CagA translocation, and interleukin-8 induction

Helicobacter pylori infects half the world's population, and carriage is lifelong without antibiotic therapy. Current regimens prescribed to prevent infection-associated diseases such as gastroduodenal ulcers and gastric cancer can be thwarted by antibiotic resistance. We reported that administration of 1% d,l-α-difluoromethylornithine (DFMO) to mice infected with H. pylori reduces gastritis and colonization, which we attributed to enhanced host immune response due to inhibition of macrophage ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis. Although no ODC has been identified in any H. pylori genome, we sought to determine if DFMO has direct effects on the bacterium. We found that DFMO significantly reduced the growth rate of H. pylori in a polyamine-independent manner. Two other Gram-negative pathogens possessing ODC, Escherichia coli and Citrobacter rodentium, were resistant to the DFMO effect. The effect of DFMO on H. pylori required continuous exposure to the drug and was reversible when removed, with recovery of growth rate in vitro and the ability to colonize mice. H. pylori exposed to DFMO were significantly shorter in length than those untreated and they contained greater internal levels of ATP, suggesting severe effects on bacterial metabolism. DFMO inhibited expression of the H. pylori virulence factor cytotoxin associated gene A, and its translocation and phosphorylation in gastric epithelial cells, which was associated with a reduction in interleukin-8 expression. These findings suggest that DFMO has effects on H. pylori that may contribute to its effectiveness in reducing gastritis and colonization and may be a useful addition to anti-H. pylori therapies.

Methods to evaluate alterations in polyamine metabolism caused by Helicobacter pylori infection

Helicobacter pylori is a Gram-negative bacteria that infects the human stomach of half of the world's -population. Colonization is followed by infiltration of the gastric mucosa by lymphocytes and myeloid cells. These cells are activated by various bacterial factors, causing them to produce immune/inflammatory mediators, including reactive nitrogen species and polyamines that contribute to cellular damage and the pathogenesis of H. pylori-associated gastric cancer. In vitro experiments have revealed that H. pylori induces macrophage polyamine production by upregulation of the arginase 2/ornithine decarboxylase (ODC) metabolic pathway and enhances hydrogen peroxide synthesis through the activity of spermidine oxidase (SMO). In this chapter, we present a survey of the methods used to analyze the induction and the role of the enzymes related to polyamine metabolism, i.e., arginase, ODC, and SMO in H. pylori-infected macrophages.

Helicobacter pylori and gastric cancer: factors that modulate disease risk

Helicobacter pylori is a gastric pathogen that colonizes approximately 50% of the world's population. Infection with H. pylori causes chronic inflammation and significantly increases the risk of developing duodenal and gastric ulcer disease and gastric cancer. Infection with H. pylori is the strongest known risk factor for gastric cancer, which is the second leading cause of cancer-related deaths worldwide. Once H. pylori colonizes the gastric environment, it persists for the lifetime of the host, suggesting that the host immune response is ineffective in clearing this bacterium. In this review, we discuss the host immune response and examine other host factors that increase the pathogenic potential of this bacterium, including host polymorphisms, alterations to the apical-junctional complex, and the effects of environmental factors. In addition to host effects and responses, H. pylori strains are genetically diverse. We discuss the main virulence determinants in H. pylori strains and the correlation between these and the diverse clinical outcomes following H. pylori infection. Since H. pylori inhibits the gastric epithelium of half of the world, it is crucial that we continue to gain understanding of host and microbial factors that increase the risk of developing more severe clinical outcomes.

Polyamines Impair Immunity to Helicobacter pylori by Inhibiting L-Arginine Uptake Required for Nitric Oxide Production

BACKGROUND & AIMS

Helicobacter pylori-induced immune responses fail to eradicate the bacterium. Nitric oxide (NO) can kill H pylori. However, translation of inducible NO synthase (iNOS) and NO generation by H pylori-stimulated macrophages is inhibited by the polyamine spermine derived from ornithine decarboxylase (ODC), and is dependent on availability of the iNOS substrate L-arginine (L-Arg). We determined if spermine inhibits iNOS-mediated immunity by reducing L-Arg uptake into macrophages.

METHODS

Levels of the inducible cationic amino acid transporter (CAT)2, ODC, and iNOS were measured in macrophages and H pylori gastritis tissues. L-Arg uptake, iNOS expression, and NO levels were assessed in cells with small interfering RNA knockdown of CAT2 or ODC, and in gastric macrophages. The ODC inhibitor, α-difluoromethylornithine, was administered to H pylori-infected mice for 4 months after inoculation.

RESULTS

H pylori induced CAT2 and uptake of L-Arg in RAW 264.7 or primary macrophages. Addition of spermine or knockdown of CAT2 inhibited L-Arg uptake, NO production, and iNOS protein levels, whereas knockdown of ODC had the opposite effect. CAT2 and ODC were increased in mouse and human H pylori gastritis tissues and localized to macrophages. Gastric macrophages from H pylori-infected mice showed increased ODC expression, and attenuated iNOS and NO levels upon ex vivo H pylori stimulation versus cells from uninfected mice. α-Difluoromethylornithine treatment of infected mice restored L-Arg uptake, iNOS protein expression, and NO production in gastric macrophages, and significantly reduced both H pylori colonization levels and gastritis severity.

CONCLUSIONS

Up-regulation of ODC in gastric macrophages impairs host defense against H pylori by suppressing iNOS-derived NO production.