1. Introduction
Helicobacter pylori (H. pylori) is one of the most common infections in humans [1, 2, 3]. The microorganism had infected humans for 30,000 years ago and has developed extensive adaptations to survive [2, 4, 5]. Approximately, H. pylori infects the stomach of half of the human population globally [3, 6, 7]. Besides residing in the stomach, abundant H. pylori are also detected in the oral cavity [8]. The colonization is suspected to be started in the childhood period [2, 4, 7] and maybe persisted for decades or life [2, 4]. The presence of spiral microorganisms resembling H. pylori had been identified in the stomachs of the animal during the late nineteenth and early twentieth centuries. Similar spiral bacteria were then isolated in humans, particularly those suffering from peptic ulcer disease or gastric cancer. Previously, the microorganism was named ‘Campylobacter-like organism’, ‘gastric Campylobacter-like organism’, ‘Campylobacter pyloridis’, or ‘Campylobacter pylori’. The fact that this microorganism is different from members of the genus Campylobacter changes the name to H. pylori [9]. The relationship between this microorganism and peptic ulcer disease was established in 1983 [10]. This microorganism causes a wide spectrum of diseases, such as chronic gastritis, peptic ulcer, gastric adenocarcinoma, and mucosa-associated lymphoid tissue lymphoma [6, 7]. In initial reports from all over the world in the first decade after the discovery of H. pylori, approximately 95% of duodenal ulcers and 85% of gastric ulcers occurred in the presence of H. pylori infection [9]. The World Health Organization has classified H. pylori as a class I carcinogen due to its epidemiological link with gastric malignancy [2, 3, 10]. However in some cases, the presence of H. pylori infection is asymptomatic [3, 7, 11]. There is a hypothesis suggesting the role of immune response in the pathogenesis of infection. The immune response toward the infection is ineffective, causing persistent microorganisms and inflammation [6]. In this review, we discuss the host immune response toward H. pylori infection in association with disease chronicity and vaccine development.
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2. Characteristics of H. pylori
H. pylori belongs to the Proteobacteria subdivision, Campylobacterales order, and Helicobacteraceae family. Helicobacter species are divided into two major lineages: the gastric Helicobacter species and the enterohepatic (nongastric) Helicobacter species [9]. H. pylori is a spiral, gram-negative, flagellated, microaerophilic, and facultative acidophilic bacterium [1, 5, 11, 12]. Its envelope consists of an inner (cytoplasmic) membrane, periplasm with peptidoglycan, and an outer membrane that consists of phospholipids and lipopolysaccharide [9]. This microorganism is very sensitive to drying and usual disinfectants [12]. It is transmitted via oral–oral and fecal–oral routes [5, 8, 9, 13]. Contamination of water sources is one major cause of transmission [8, 9, 14]. It is reported that 40% of samples of drinking water in Pakistan are contaminated with H. pylori [8]. Contamination of drinking water is also reported in 20.3% of samples in Peru. Recent transmission hypothesis has suggested that blowflies and houseflies are responsible as they feed with and breed in fecal material [14]. H. pylori extracts nutrients from blood and host cells [5]. The microorganism has extensive genetic diversity resulting in high mutation rates and high recombinant frequency. The virulence factors of H. pylori are also affected by this phenomenon and contribute to immune escape and chronic infection [2, 12]. Several methods of DNA rearrangement along with the introduction and deletion of foreign sequences are responsible for genetic diversity [9].
Some factors contributing to H. pylori infection are younger age, [4] low socioeconomic status, limited living space, sharing of beds, low parental education level, pollution of daily used water, and history of H. pylori infection in family members [4, 8, 13]. Genetic predisposition may play role in the infection of H. pylori. People from African and Pacific Islander ancestries have a higher risk for infection despite adjustment for other risk factors [8]. This is supported by another study which reported a higher prevalence in non-whites compared to non-Hispanic whites in the United States. Higher prevalence was even observed in Alaskan natives [10]. A diet containing less vegetables and fruits along with high consumption of fried food is increasing the risk for infection [14]. Another literature states that age and gender are not related to increased risk of infection [13]. Additionally, the effect of smoking and alcohol is uncertain on the incidence of H. pylori infection [13, 14].
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3. Epidemiology and burden of H. pylori infection
The prevalence of H. pylori infection is low in childhood and has begun to increase to 20% in adults younger than 40 years of age, and to 50% at the age of 60 [9]. In 2015, approximately 4.4 billion individuals were infected with H. pylori [13]. The prevalence rate comprises roughly 4.3% [14]. A systematic review and meta-analysis by Hooi, et al reported that Africa has the highest pooled prevalence of H. pylori infection while Oceania has the lowest rate [10]. The prevalence tends to decrease recently due to improvements in sanitation [9, 10]. Similar result is reported by Sjomina et al. a few years later, showing the minimal change in the epidemiology of H. pylori infection [14]. In Europe, Northern countries report lower prevalence compared to Southern and Eastern countries. The highest prevalence in Europe is reported in Portugal, reaching 84.2%. In the American continent, Mexico holds the highest prevalence (52.2%) similar to Bhutan in the Asia continent (86%). A study from Nigeria reported a very high prevalence of H. pylori infection, which is 93.6% [13]. In the Australia and Oceania region, the highest prevalence is detected on Pacific Island (49%). Minor differences are observed regarding the epidemiology of H. infection in several studies and it is due to the different diagnostic methods utilized from one study to another [8].
H. pylori is associated with 92% of duodenal ulcers and 70% of gastric ulcers. It is also related to 50% of gastric cancer and raises the risk for gastric cancer six times higher compared to those without H. pylori infection [11]. The odd for ulcer disease is even higher, reaching 10 times higher than H. pylori-negative subjects [9]. A study by Plummer et al. reported that 6.2% of estimated 12.7 million new malignancy cases in 2008 are attributed to H. pylori infection [15]. The incidence of gastric cancer is associated with geographical factors, strain diversity, and host immunological responses. The highest incidence of gastric cancer is reported in East Asian countries [16]. In 2017, there were 1.22 million new cases of gastric cancer with 865.000 deaths and 19.1 million disability-adjusted life-years. Not all, but the majority of gastric cancers are related to H. pylori infection therefore the microorganism is responsible for the burden of the disease [17]. Eradication of H. pylori will give a significant impact from an economic perspective [8, 17]. Eradication leads to decreased consultation with medical practitioners and is proven as an effective cost-saving method [8]. Screening and eradication of H. pylori infection in China might prevent one gastric cancer in every four to six cases [10].
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4. Immunology at a glance
There are two major groups of immunity in the human body—innate and adaptive immune responses. Innate immunity comprises immune responses which do not require the previous contact with immune triggers [6, 18]. The response is rapid but not specific and has no memory [18]. Innate immunity acts as the first line of defense against harmful substances. Activation of the innate immune system may eliminate the substance and trigger inflammation by releasing mediators such as cytokines, reactive oxygen species, and nitric oxide [6]. Elimination may also be carried on by cell-dependent mechanisms, such as phagocytosis and cytotoxicity [18]. In the gastrointestinal tract, mucosal defense is classified as an innate immune system that consists of mucosal epithelium, gastric acid, and immune cells (macrophage and dendritic cell) [1, 18]. The innate cellular immune may sense the presence of antigen via pattern recognition receptors (PRR), such as toll-like receptors (TLR) [18].
Adaptive immunity is an immune response toward previously contacted immune triggers. This immune system is specific and has immunologic memory. Activation of adaptive immunity is related to innate immunity. For example, antigen-presenting cells (macrophages and dendritic cells), as a part of the innate immune system, trigger activation and differentiation of T-helper (Th) cells, which marks the initiation of the adaptive immune response [6]. Th cells differentiate into Th1, Th2, Th17, and regulatory T (Treg) cells. Th1 plays role in cell-mediated immunity while Th2 in humoral immunity [4, 5, 6, 7]. The balance between Th1 and Th2 is important in maintaining a normal host’s immune response [6]. Th1 cells secrete tumor necrosis factor (TNF) and interferon (IFN)γ. Th2 cells secrete interleukin (IL)-4, IL-5, and IL-10 which act in suppressing the inflammatory effect of Th1 and in producing antibodies by lymphocyte B cells [3, 4, 5, 7]. Th17 plays role in the immune response toward extracellular bacterial infection by secreting IL-17A, IL-17F, IL21, and IL-22. Treg itself has activity in suppressing effector T cells proliferation and cytokine production, therefore moderating inflammation and preventing autoimmunity. Some cytokines secreted by Th17 are IL-10 and transforming growth factor (TGF)-β [3, 4, 5].
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5. Immune response toward H. pylori infection
Many diseases, including infection due to H. pylori, involve dysregulation of the immune system. Infection is both active, marked by neutrophilic accumulation, and chronic, marked by lymphocytic deposition [1, 5, 6, 9]. These findings are positive 2 weeks after infection. Anti-H. pylori antibodies are also detected at 4 weeks after initial infection, marked by the high levels of immunoglobulin (Ig) M, IgG, and IgA in gastric mucosa of infected patients [1, 3, 11]. A study in mice demonstrated that transient infiltration of macrophages and neutrophils into the glandular stomach is observed in the first 2 days after infection. By day 10 after infection, the numbers of macrophages and neutrophils are decreased to baseline levels. The adaptive immune response is started to appear in the 3rd week, marked by infiltration of T lymphocytes in paragastric lymph nodes and elevated expression of TNFα and IFNγ [19]. The levels of IgM and IgA anti-H. pylori in biopsy specimens from the gastric antral region of patients infected with H. pylori are 40- to 50-fold higher compared to non-infected subjects [3]. However, the presence of H. pylori in the stomach for a long period of time supports the suspected ineffective immune response [2, 6, 9]. The presence of this microorganism causes a persistent and chronic infection [9]. Chronic infection leads to chronic inflammation, gastritis, peptic ulcer, gastric mucosa-associated lymphoid tissue lymphoma, and ultimately, gastric cancer [1, 6].
H. pylori infection activates innate and adaptive immunity, along with humoral and cellular immunity as the parts of the adaptive immune system. There are cytotoxin-associated gene pathogenicity islands (cag PAI) and vacuolating toxin A (VacA) which act as major virulence factors in H. pylori infection. Cag PAI encodes a type IV bacterial secretion system that injects bacterial products into gastric epithelial cells resulting in inflammation and increased risk of malignancy [1, 6, 7, 9]. Cag PAI is a protein with a molecular mass of 140 kDa. It is highly immunogenic and present in approximately 50–70% of H. pylori strains [9]. VacA, on the other hand, is associated with cellular damage and inflammation [6]. VacA is a protein-sized 95 kDa and secreted from approximately 50% of all H. pylori strains. It damages cells by inducing massive vacuolization. The process ends with apoptosis and immune modulation [9]. H. pylori enters the gastrointestinal tract, penetrates the mucus gastric layer, and interacts with macrophages, dendritic cells, and monocytes [1, 6, 7]. H. pylori adhere to the gastric epithelial cell with the assistance of outer membrane proteins such as BabA, SabA, AlpA, AlpB, and HopZ [1, 5, 20]. After adherence, cag PAI and VacA disrupt gastric epithelial cell polarity, acid secretion (via control of gastrin and H+/K+-ATPase), and induce inflammation [1]. TLR on epithelial cells also recognizes bacterial products, such as flagella and lipopolysaccharide. The interaction elicits inflammation and supports the activation of the adaptive immune response [9]. H. pylori which have been ingested by antigen-presenting cells activate the adaptive immune response [2, 5]. Macrophages and neutrophils may also eliminate H. pylori through nitric oxide (NO)-dependent phagocytosis or reactive oxygen species production [5, 6]. They release cytokines such as IL-12, IL-10, and IL-23 which in turn stimulate naïve Th cells [2, 6]. In the other way, dendritic cells present H. pylori antigen to naïve Th cells. Naïve Th cells then differentiate into Th1 or Th2/Treg [1, 6]. However, Th1 is more prominent compared to Th2/Treg cells. Th1 then produces IFNγ, TNFα, and IL-2 [1, 2, 4, 6]. Elevation of pro-inflammatory cytokines, such as IL-1β, TNFα, IL-8, and IL-6, is observed. The release of cytokines promotes inflammation in the stomach and leads to gastritis [1, 6]. In contrast, the role of lymphocyte B cells in H. pylori infection is indeterminate. Studies reported that antibodies against H. pylori are produced but they might be counterproductive [1, 2, 6]. It is suspected that the immunoglobulins against H. pylori are easily degraded and unstable in structure [1, 6]. Other literature states that the presence of IgA anti-H. pylori gives a protective effect against infection and gastric malignancy [11]. Further investigation is mandatory regarding the role of humoral antibodies in H. pylori infection [6].
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6. Mechanism of immune evasion of H. pylori
The outer membrane proteins in H. pylori are found to be less immunogenic compared to proteins from other pathogens; therefore, the immune response elicited by the innate immune system is less powerful [7, 9]. It is known that H. pylori’s lipopolysaccharide has a 500- to 1000-fold lower endotoxic activity than lipopolysaccharide from S. typhimurium and E. coli [3]. The presence of arginase enzyme coded by rocF gene in H. pylori may decrease L-arginine, the substrate for NO, level. Decreased NO level will impair phagocytosis by macrophage and prevent H. pylori elimination. Additionally, this will promote the apoptosis of macrophages [1, 2, 3, 6, 7, 20]. H. pylori are also capable of producing urea from L-arginine and further ammonia from urea. The process is mediated by the urease enzyme and α-carbonic anhydrase. Ammonia is known for its ability to neutralize gastric HCl and sustain the survival of H. pylori [1, 5, 6, 20]. Glucosylation of cholesterol also aids H. pylori’s survival. This process protects the microorganism from macrophage phagocytosis [5, 6]. H. pylori may also evade innate immune recognition by avoiding PRR, a subset of pathogen-associated molecular patterns. It modulates its surface molecules including lipopolysaccharide and flagellin to avoid recognition by toll-like receptors on antigen-presenting cells. The molecules are recognized as self molecules and thus do not trigger the immune response [2, 3, 5]. Even after being phagocytized, H. pylori may survive from killing by the aid of cag PAI and VacA. Both delay actin polymerization and phagosome formation inside macrophages [2].
Chronic exposure of dendritic cells to H. pylori decreases the ability of dendritic cells to induce Th1 response and support the persistence of infection [1, 2, 3, 4, 5, 6]. The malfunction of dendritic cells is due to H. pylori-controlled maturation. H. pylori restore transcription factor in dendritic cells and inhibit their maturation [2]. Lewis antigen form H. pylori may also bind dendritic cell-specific ICAM-3-grabbing nonitegrin (DC-SIGN) and blocked Th1 cell recruitment [4]. Examination of patients with chronic H. pylori infection also shows elevation of Treg cells in the gastric tissue compared to healthy subjects [2, 4, 5, 6, 20]. H. pylori are suggested to promote the expansion of the Treg population and their recruitment to the site of infection [4]. As we know that Treg suppresses the activity of memory T cells, it will relieve inflammation and gastritis severity [1, 2, 4, 6] but at the same time hamper the ability of the host to eliminate pathogens, including H. pylori [2, 4, 6, 20]. The condition is hypothesized from the increased level of TGF-β and IL-10 independent of VacA and cag PAI [2, 20]. H. pylori inhibits lymphocyte proliferation via IL-2 inhibitory effect from VacA and induction of cell cycle arrest from VacA-independent produced protein [1, 2, 3, 4, 5, 6]. The process is made possible via an interference signaling pathway at the level of calcium-calmodulin-dependent phosphatase calcineurin [4]. Gamma-glutamyl transpeptidase (GGT) is another low-molecular-weight protein of H. pylori that is capable of inhibiting the proliferation of lymphocytes. The mechanism involves extracellular cleavage of glutathione and the production of reactive oxygen species. The depletion of L-arginine level due to arginase activity of H. pylori is also hampering lymphocyte T cell proliferation [4, 5]. Furthermore, VacA may induce T cell apoptosis by reducing Bcl-2, an anti-apoptotic protein [2, 5].
Studies from chronic gastritis found that H. pylori may induce autoimmunity which affects gastric parietal cells. Both cellular and humoral antigens damage the cell in patients with gastritis due to H. pylori infection [1, 3, 6, 11]. The origin of autoantibody is suspected from the presence of Lewis x and Lewis y antigens which are similar to the H+/K+-ATPase β subunit of parietal cells. Parietal cell loss occurs via IFNγ-mediated inflammation and Fas-mediated apoptosis or cytotoxicity [2, 3, 6, 7, 9, 11]. The presence of pro-inflammatory cytokines also inhibits acid secretion from parietal cells. IL-1β and TNFα are the most potent inhibitors [6]. The resulting hypochlorhydria situation allows H. pylori to persist and cause prolonged infection [3, 6]. In contrast, those pro-inflammatory cytokines stimulate gastrin secretion by disrupting the negative feedback signal of somatostatin [6].
Coinfection between H. pylori and parasitic helminths will cause an imbalance in Th1 and Th2 responses with predominantly Th2 activity [6, 9, 10]. This situation is clearly observed in the African population and referred to as ‘African enigma’. ‘African enigma’ is marked by low gastric cancer despite a high prevalence of H. pylori in Africa. Lately, it is known that high rate of helminth coinfection is high in the corresponding population [10]. The variation in Lewis antigen also moderates Th1 response and favors Th2 activity [7]. This condition is supported by a study in mice infected with H. pylori showing dysfunctional Th1 response [4]. The imbalance will alleviate inflammation in gastritis but hamper Th1-mediated H. pylori elimination [2, 6, 7].
Host factor also contributes to immune response toward H. pylori infection. Host genetic polymorphisms affecting the IL-1 gene cluster elevate the level of IL-1 and lead to the reduction of gastric acid secretion. Low gastric acid secretion promotes infection and colonization of H. pylori. A similar situation is induced by a polymorphism in the TNF-α gene. In contrast with the IL-10 gene, the polymorphism causes higher expression of IL-10 and favors anti-inflammatory activity [9]. Defects in cytokine coding genes are involved in the persistence of H. pylori infection. Defects in gene coding IL-1 and TNF are associated with decreased cytokines production and increased risk for gastric cancer [7]. Single nucleotide polymorphism in gene coding IL-10 which resulted in increased IL-10 will promote Th2 activity and resulted in prolonged H. pylori infection and an elevated risk for recurrent gastric cancer [16]. The presence of H. pylori in the macrophages alters the expression of miRNA. The alteration in miRNA, particularly miR-4270 causes increased expression of CD300E, a surface protein on macrophages that affects the antigen presentation capacity of macrophages. Increased CD300E expression is negatively correlated with antigen presentation capacity [21]. Shakhatreh, et al conducted a study on the Jordanian population to determine the association between IL-1 gene polymorphism and H. pylori infection. -31T/C polymorphism was found significantly associated with H. pylori infection, particularly the TT genotype [22]. Those statements are reinforced by a meta-analysis by Ma et al. They focused on polymorphism in genes that code IL-1. Increased risk for H. pylori infection is observed in IL-1B-31C/T polymorphism with an odds ratio of 1.134. Furthermore, IL-1B-511C/T and IL-8-251A/T polymorphisms increase the risk for gastric cancer with odds ratios of 1.784 and 1.810, respectively [23]. Zeyaullah et al. also proposed the role of gene polymorphism in gastric cancer. IL-10-592A/C, IL-10-819T/C, and IL-17-197G/A are all found to be related to gastric cancer. Besides polymorphism in cytokine genes, toll-like receptor genes are also involved. TLR4+ 1196C/T polymorphism is one genetic rearrangement that increases the risk of gastric cancer in H. pylori-infected individuals [24].
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7. Vaccination and immune response toward H. pylori
The trend of antibiotic resistance in H. pylori is increasing recently [1, 6, 10, 11]. Resistance rates for metronidazole, clarithromycin, and levofloxacin are the highest, surpassing 50%. This condition is worsening in previously treated subjects [25]. Besides, reinfection may occur even after the complete eradication of the previous infection [6]. This condition urges the development of a vaccine against H. pylori [1, 6, 10]. In the past decade, much effort has been devoted to the development of a vaccine as an alternative treatment for H. pylori infection [9]. There are two types of vaccine which are potentially possible: prophylactic vaccine and therapeutic vaccine. A therapeutic vaccine that can both eradicate infection and stimulate long-lasting immunity is the most desired one [10, 11]. Vaccine will significantly cut the economic burden from H. pylori infection even if the vaccine’s efficacy is only 55% [2]. The first report on H. pylori vaccine development was submitted in 2011 by Moss et al., followed by Iankov et al. They conducted trials in mice and showed promising results. Cellular immunity, particularly Th1 response, is able to sterilize the microorganism [20]. The humoral immune response also gains the spotlight for the vaccine platform. The induction of Th2 response is proposed to be the basis of effective vaccination. A trial in mice showed high neutralizing specific salivary IgA and serum IgG after oral immunization. Besides preventing infection, the vaccine was also shown to have therapeutic properties. Gastric inflammation of mice in the trial was alleviated after vaccination [9]. In line with previous literature, Espinosa-Ramos conducted a vaccination trial in mice and observed that plasma secretory IgA and IgG are elevated post-vaccination. The presence of antibodies also protected 100% of mice in the study from virulent H. pylori [26]. The utilization of the vaccine seems promising, but this option still needs further development, especially in humans, considering the immune evasion ability of H. pylori [6, 9]. Public health intervention is still a major concern since preventing is better than treating the infection. Improvement in socioeconomic status together with hygiene and sanitation may decrease the rate of infection as seen in developed countries [9].
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8. Conclusion
H. pylori is the most common infection in humans and has infected humans since 30,000 years ago. The prevalence of H. pylori infection is still high particularly in developing countries. The highest prevalence is reported in Africa, namely Nigeria. H. pylori infection causes a wide spectrum of diseases from chronic gastritis to gastric adenocarcinoma. The disease has high morbidity and mortality rates. The burden from diseases caused by H. pylori is also heavy. The transmission of H. pylori is via oral-oral or fecal-oral routes. Contamination of water sources for drinking is a significant mode of transmission. The transmission is closely related to socioeconomic status, hygiene, and sanitation. H. pylori infection activates both innate and adaptive immunity. In adaptive immunity, Th1 response is dominant compared to Th2. Despite activating the immune system, H. pylori eradication by immune response is ineffective. H. pylori has abilities to escape from the host’s immune system. In the innate immune system, H. pylori can neutralize gastric acid via urease enzyme activity and autoimmune-induced parietal cell loss. H. pylori prevents phagocytosis and promotes apoptosis of macrophages. Its LPS is less immunogenic compared to other gram-negative bacteria. Chronic infection hampers dendritic cell ability and disturbs activation of the adaptive immune response. In the adaptive immune system, H. pylori inhibit lymphocyte proliferation, induces T cell apoptosis, promotes Th2 activity and suppresses Th1 activity via Lewis antigen, and promotes Treg expansion thus dampens inflammation. External factors, such as coinfection with helminths, support the activity of Th2 and hamper H. pylori eradication. Genetic rearrangement is induced by H. pylori or by the host itself. The rearrangement alters immune response and causes ineffective eradication of H. pylori. Multiple antibiotic resistance is observed in H. pylori, particularly against metronidazole, clarithromycin, and levofloxacin. This contributes to persistent H. pylori infection. Vaccination becomes promising alternative management for preventing infection. Additionally, the vaccine may also have a therapeutic effect. However, the development of a vaccine should pay attention to the immune escape mechanism of H. pylori. Public health intervention is still important to holistically manage the infection.
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Conflict of interest
The authors declare no conflict of interest.
Open access peer-reviewed chapter
