H. pylori vaccines

Abstract

Helicobacter pylori was appreciated as the major cause of peptic ulcers about 30 y ago and the most significant etiological agent in gastric cancer in the mid-1990s. Since that time, progress in the development of a preventive or therapeutic H. pylori vaccine has been relatively slow. The impediments to rapid advances in the field include a luke-warm enthusiasm among clinicians, research scientists, and public health authorities concerning the need for a vaccine, rudimentary understanding of the correlates of gastric immunity to H. pylori and of gastric mucosal immunology in general, the geographical heterogeneity of the H. pylori genome and insufficient pharmaceutical industry support. Recent enhancements in our understanding of the gastric immune response together with advances in H. pylori genomics now provide the potential to accelerate progress in H. pylori vaccine development. Whether an H. pylori vaccine becomes a reality will likely depend upon our ability to appropriately target the populations at highest risk of the adverse sequelae of infection.

Introduction

The culture of a novel bacterial species from the human stomach in 1982 by Marshall and Warren,¹ and the rapid appreciation that this organism (originally termed Campylobacter pyloridis, now Helicobacter pylori) was responsible for gastritis and peptic ulceration was an iconoclastic discovery in many respects. First, it produced a paradigm shift in the treatment of these common conditions—replacing chronic acid suppression or even gastric surgery with a 1–2 week course of antibiotics.² Second, subsequent epidemiological, clinical and animal studies led to the realization that another consequence of chronic gastric infection by H. pylori was the development of gastric adenocarcinoma and mucosal-associated lymphoid tissue lymphoma, leading to the designation of H. pylori as a definite or class I carcinogen by the World Health Organization in 1994.³ It is now appreciated that about 70% of all gastric cancers (which are the second leading cause of cancer death worldwide) are attributable to infection by this species.⁴ Third, the ability of bacteria to thrive in a niche formerly viewed as inhospitable to microbial colonization has prompted a re-evaluation of the relationship between microbial colonization, symbiosis, infection and inflammation in the stomach and elsewhere. This has helped contribute to a reawakening of interest in investigating the gastrointestinal micro-biome and its link with chronic diseases including cancers, chronic inflammatory states and even diabetes and obesity.⁵

We now recognize that the earliest human beings were colonized with H. pylori, and that this bacterium has co-evolved with and co-migrated with us around the world.⁶ Indeed, other mammalian species have their own Helicobacter-like species, suggesting that gastric Helicobacter infections likely predate the emergence of Homo sapiens. H. pylori is transmitted from person to person in early childhood, probably via the fecal-oral route. Infection is always associated with an intense and persistent inflammatory response in the gastric mucosa, the severity of which is a major determinant of disease predisposition, especially for gastric carcinogenesis.⁷ The prevalence of infection in the developing world remains very high (typically over 70%), whereas it has been declining steadily over the last century in industrialized countries, probably related to improved public health measures, to the point at which the prevalence among children has dropped below 10% in some western populations.⁸ Overall, based upon estimates that about 50% of the global population is infected with H. pylori, and that 10% of H. pylori infected persons will develop peptic ulcers and 1% gastric malignancies, close to a billion people in the world are at risk of premature death and chronic disease from this bacterium.

Because of the high prevalence of H. pylori infection in many resource-poor regions, such as in East Asia, Africa and Central America where gastric cancer and peptic ulcer disease are highly prevalent, mass vaccination against H. pylori may be an attractive and practical strategy to eliminate H. pylori-related disease. Several large pharmaceutical companies, many of which had large vaccine or acid-inhibiting drug portfolios, were early enthusiasts. The second H. pylori genome to be fully sequenced was in a project driven by Astra (before evolving into Astra-Zeneca), a company that already had a massive global market in omeprazole, the first proton pump inhibitor developed.⁹ Despite this early interest, no H. pylori vaccine is on the horizon, or even close to it. What are the hurdles unique to H. pylori that must be overcome to create a marketable H. pylori vaccine?

The Challenging Issues in H. pylori Vaccine Development

For simplicity, the hindrances to successful H. pylori vaccine development may be categorized as perceptual, practical and financial.

Perceptual.

The relationship between H. pylori infection and disease causation took many years to establish. While the importance of H. pylori in promoting peptic ulceration was appreciated by the US National Institutes of Health about a decade after the completion of the first pivotal clinical ulcer trials,¹⁰ providing concrete proof that gastric cancer is a preventable infectious disease in humans has been more difficult. This is due to the very long latency between acquisition of infection (in infancy) and the development of cancer some 6–7 decades later. Only a handful of randomized controlled trials of H. pylori eradication and gastric cancer have been performed, none of which was powered sufficiently in terms of patient numbers and length of follow-up. Since the designation of H. pylori as a definite carcinogen,³ it has not been ethically permissible to enter patients into the placebo arm of such studies. Nevertheless, metaanalysis of the completed trials suggests that eradicating H. pylori with antibiotics will reduce the risk of gastric cancer development by about 50%, more so if treatment is given relatively early in the course of infection.¹¹

Gastric cancer was the most common cause of cancer death in the US and many other industrialized countries in the first half of the 20^th century. It has since been declining slowly and steadily in the developed world—a change that predated widespread antibiotic use but paralleled improved sanitation. Now that this tumor has dropped just out of the top 10 causes of cancer deaths in the US (apps.nccd.cdc.gov/uscs/toptencancers.aspx) it is not perceived as a national priority. However, since population growth is most rapid in regions of the world with high H. pylori prevalence,⁴ numbers of cases of gastric cancer worldwide will continue to grow steadily, especially in less industrialized nations. In the developed world, gastric cancer occurs disproportionately among minorities, immigrants and those of low socio-economic status who have relatively limited access to healthcare. Thus while gastric cancer and peptic ulcer disease may not always clamor for our attention, the consequences of untreated H. pylori infection remain a very significant problem on the global scale.

More to the forefront in the Western world, and particularly for obese white males and their gastroenterologists, has been the steady rise in the prevalence of the metaplastic and neoplastic complications of gastro-esophageal reflux disease—namely Barrett's esophagus and esophageal adenocarcinoma. These diseases are inversely associated with H. pylori prevalence, suggesting a “protective” role for H. pylori that led to the recent and unique classification of H. pylori as simultaneously a carcinogen (for the stomach) and an anti-carcinogen (for the lower esophagus).¹² Taking this a step further, it has been proposed that we might even one day want to infect future generations with this bacterium to prevent esophageal diseases as well as other conditions that may result from too little dirt exposure, such as atopic diseases and other consequences of the hygiene hypothesis.¹³ Thus for those in the West who see an epidemic of diseases inversely related to H. pylori and a spontaneous decline in gastric cancer, there may be little enthusiasm and some trepidation at intervention to further decrease the H. pylori burden. This must be contrasted with parts of the world with high H. pylori prevalence where the public health authorities are already developing programs to screen for and eradicate this bacterium.¹⁴

Finally, antibiotic treatments to eradicate H. pylori have been available for many years, and are relatively simple to prescribe. Primary care physicians can now test for H. pylori non-invasively (on blood, stool or breath) and treat with ease.² Post-treatment testing is rarely done, and H. pylori resistance is usually evaluated and appreciated only in research studies. Consequently, most patients and their physicians are unaware of the disturbingly high and rising rates of resistance to antibiotics in recent years (especially to metronidazole and clarithromycin), that can lead to treatment success rates with conventional combination therapies of little more than 50% in practice.¹⁵ More complicated multi-drug combinations are likely to be necessary in the future, especially where achieving cure is critical, such as in patients with complicated peptic ulcer disease or with gastric MALT lymphomas. These latter tumors can be totally cured by H. pylori eradication, with lymphoma remission rates of about 80% at 10-y follow up following successful antibiotic treatment.¹⁶ The development of effective antibiotic therapy in the era of increasingly resistant H. pylori strains will likely remain a significant, though currently under-appreciated, challenge.

Practical.

Despite considerable effort over the past three decades in both clinical and murine systems, gastric immunology in general and the correlates of immunity to H. pylori in particular remain relatively poorly defined. H. pylori infection stimulates innate and adaptive immune responses in the gastric mucosa.^17,18 The normal gastric inflammatory infiltrate in chronic H. pylori infection comprises neutrophils, T and B lymphocytes, macro-phages, dendritic cells and mast cells.^19,20 H. pylori colonization also induces a strong humoral immune response characterized by an initial rise of IgM, then followed by increased H. pylori-specific Ig A and IgG antibodies in serum.^20,21 However, although the host possesses the necessary armamentarium to eradicate H. pylori, spontaneous eradication is rare and, rather than protect, H. pylori-specific antibodies might even impair host inflammatory responses and facilitate chronic bacterial colonization.²² In the face of this inadequate or ineffective immune response, H. pylori persistence over decades is typical.¹⁸

There is evidence for both humoral and cellular immunity in H. pylori clearance,²⁰ with T cell-mediated cellular immunity the most important element.²³ Gastric CD4⁺ and CD8⁺ T-cell subsets and IFNγ, TNFα, IL-1β, IL-6, IL-8 and IL-17 levels are all increased in the chronically H. pylori-infected gastric mucosa.^20,24 H. pylori induces a mixed T helper 1 (Th1), T helper 17 (Th17) and regulatory T-cell (Treg) response in the gastric mucosa.^25,26 IL-17 is pro-inflammatory while also promoting bacterial growth, as indicated by studies in H. pylori-infected Th17-deficient mice.²⁵ Recent mouse immunization studies^26–28 suggest that Th17 cells are also necessary for vaccine-induced protective immunity and overcoming the immunosuppressive activity of Tregs in H. pylori clearance. Depleting CD25⁺ Tregs reduced H. pylori colonization^26,29 whereas IFNγ-producing Th1 cells and IL-17-producing Th17 cells enhance vaccine-induced H. pylori elimination.^24,28 Taken together, these data suggest that overriding the inhibitory effects of Tregs and stimulating the Th17/Th1 response may be critical for effective vaccine-induced immunity against H. pylori.

A wide diversity of H. pylori vaccine candidates has been explored in animal models. Most investigators have tested them in mice experimentally infected with murine-adapted human H. pylori strains or the related H. felis species.³⁰ A broad range of specific antigen preparations, diverse routes of administration and different adjuvants have been evaluated, using both preventive and therapeutic strategies.^31,32 While many of these studies demonstrate promising results there has been little consensus on defining the correlates of successful immunization. In most cases vaccination stimulates a transient or persistent post-immune gastritis with subsequently reduced bacterial load, but in very few of these studies was complete H. pylori eradication achieved.³⁰ Vaccination trials in several other experimental animal models including rats, gnotobiotic pigs, Mongolian gerbils, cats and non-human primate monkeys have generally been less common and less successful.^30,33–36 For example, in comparison with mice, much lower levels of protective immunity were achieved in endogenously Helicobacter mustelae-infected ferrets.³³

Although humans are the natural hosts of H. pylori, very few clinical vaccination studies have been performed so far. The handful of clinical trials performed have given disappointing results with small numbers of patients studied, diverse vaccine strategies and inconsistent immunization routes testing a variety of antigen and adjuvants.³⁰ While a challenge model may be of great benefit for developing a prophylactic vaccination strategy, infecting humans with a potential gastric carcinogen poses considerable ethical concerns. To circumvent this, Graham et al.³⁷ challenged 20 healthy adult volunteers with a strain of H. pylori lacking the CagA gene, a putative bacterial oncoprotein associated with increased H. pylori virulence. In so doing, they established that a minimum of 10⁵ colony-forming units (CFU) was required to establish infection. Despite possible concerns over carcinogenicity, 34 healthy volunteers were recently experimentally infected with 5 × 10⁶ CFU of a H. pylori Cag A-positive but highly antibiotic-sensitive strain, resulting in the frequent occurrence of moderate dyspeptic symptoms that resolved within 1–2 weeks.³⁸ This model may prove valuable in future clinical H. pylori vaccination trials.

A major issue in H. pylori vaccine design relates to the considerable genetic diversity of H. pylori among different patients and even within the same patient.^6,39 An ideal vaccine may need to comprise multiple highly conserved proteins in order to stimulate a strong but broad-based cellular immune response to achieve lasting H. pylori eradication. To address this, we recently employed an immunoinformatics strategy to develop a multi-epitope H. pylori genome-derived, DNA vaccine. When delivered to mice intra-nasally as a therapeutic vaccine there was a marked reduction in H. pylori colonization and complete H. pylori eradication in 5 of 19 mice.⁴⁰ Further work will explore ex vivo testing of similar multi-epitope vaccines against gastric and peripheral blood lymphocytes extracted from H. pylori-infected patients.

Financial.

Financial challenges in H. pylori vaccine development go hand-in-hand with the perceptual and practical. While there is generally recognized value in vaccine development, there has been little or no urgency to deliver an anti-H. pylori vaccine. H. pylori has not captured the attention that the worldwide community has placed on other infectious diseases, despite its high prevalence. In the developing world in particular, it has not had the devastating impact seen for HIV, TB and malaria and several neglected tropical diseases. Moreover, with the great time-lag between acquisition of infection and manifestation of disease, especially gastric cancer, society does not see H. pylori as a significant health priority and has not demanded that great effort be invested in its prevention. As a result, governments and private foundations have not provided adequate funds for H. pylori vaccine development, and profit-driven companies see little enticement to reformulate their vaccine portfolios, notwithstanding studies that predict vaccine development to be cost-effective.^41,42 Compounding perceptual challenges is the simple fact that conventional strategies have not easily yielded promising vaccine candidates. Rather than grasping at straws, scientists have returned to the bench to learn more the basic biology of H. pylori and its immunopathogenesis and to consider novel approaches to vaccine development. In the coming years, we may see the fruits of these studies translated into a strong vaccine candidate that will renew interest in the field and stimulate new and greater investment.

Conclusions

Efforts to develop a vaccine against H. pylori have stuttered over the almost 30 y since this organism was first recognized as a significant cause of gastric pathology. With improving understanding of the gastric immune response, increasingly sophisticated bioinformatic tools with which to mine the diversity of H. pylori genomes, and the publication of initial H. pylori human challenge models, there is now considerable potential to make substantial progress in the prevention of peptic ulcer disease and gastric cancer through H. pylori eradication. Against this enthusiasm, there remains a fear among some that widespread H. pylori eradication may have adverse effects for conditions that are inversely associated with H. pylori prevalence in western populations.¹³ This may explain some of the lack of enthusiasm among “big pharma” for this field, though Novartis' cag/nap/urease vaccine has at least advanced to some clinical testing.⁴³ It is likely that should a viable H. pylori vaccine emerge in the next few years, it will be targeted initially to the developing world, in regions where gastric cancer remains a scourge and the diseases of affluence many years away.

Funding

US Public Health Service supported This work grant U19AI082642.

Disclosure of Potential Conflicts of Interest

S. Zhang and S. Moss have no competing financial interests. L. Moise is an employee and holds stock options in Epivax Inc., a privately-owned vaccine design company located in Providence, RI. There is therefore a potential conflict of interest related to his relationship with Epivax, but we attest that the views contained in this commentary are free of any bias that might be associated with the commercial goals of the company.