Th17 Cells in Helicobacter pylori Infection: a Dichotomy of Help and Harm

Helicobacter pylori is a Gram-negative bacterium that infects the gastric epithelia of its human host. Everyone who is colonized with these pathogenic bacteria can develop gastric inflammation, termed gastritis. Additionally, a small proportion of colonized people develop more adverse outcomes, including gastric ulcer disease, gastric adenocarcinoma, or gastric mucosa-associated lymphoid tissue lymphoma.

ABSTRACT

Helicobacter pylori is a Gram-negative bacterium that infects the gastric epithelia of its human host. Everyone who is colonized with these pathogenic bacteria can develop gastric inflammation, termed gastritis. Additionally, a small proportion of colonized people develop more adverse outcomes, including gastric ulcer disease, gastric adenocarcinoma, or gastric mucosa-associated lymphoid tissue lymphoma. The development of these adverse outcomes is dependent on the establishment of a chronic inflammatory response. The development and control of this chronic inflammatory response are significantly impacted by CD4⁺ T helper cell activity. Noteworthy, T helper 17 (Th17) cells, a proinflammatory subset of CD4⁺ T cells, produce several proinflammatory cytokines that activate innate immune cell antimicrobial activity, drive a pathogenic immune response, regulate B cell responses, and participate in wound healing. Therefore, this review was written to take an intricate look at the involvement of Th17 cells and their affiliated cytokines (interleukin-17A [IL-17A], IL-17F, IL-21, IL-22, and IL-26) in regulating the immune response to H. pylori colonization and carcinogenesis.

INTRODUCTION

Helicobacter pylori is the predominant member of the gastric microbiota in a most of the world’s population (1–3). Depending on the region, an estimated 30 to 80% of the population is colonized with the Gram-negative bacterium. Remarkably, H. pylori colonization can have dichotomous impacts on the host immune response; the impact displayed will depend on the timing of colonization and the environment. H. pylori colonization can lead to protection from some proinflammatory diseases (4–10) or to detrimental outcomes, including gastritis, peptic ulcer disease, and gastric cancer (11, 12). H. pylori-infected individuals are all believed to develop some level of gastritis, but many colonized persons may be asymptomatic or not seek medical care (11).

Detrimental outcomes of H. pylori colonization range from symptomatic gastritis to gastric cancers, including gastric adenocarcinoma and gastric mucosa-associated lymphoid tissue (MALT) lymphoma (11, 12) (Fig. 1). Infection with H. pylori is the single most common risk factor for gastric cancer and, for this reason, was defined by the World Health Organization (WHO) as a class I carcinogen. The Cancer Statistics Center of the American Cancer Society estimated that in 2019 there will have been 27,510 new cases of gastric cancer in the United States with over 11,140 estimated deaths (13). Gastric cancer is the 3rd most common cause of cancer-related deaths in the world, accounting for upwards of 783,000 deaths in 2018, according to the WHO (14).

FIG 1.

Potential significant pathological consequences of H. pylori infection. H. pylori colonization of the gastric mucosa can lead to deleterious consequences, including inflammation of the gastric mucosa (termed gastritis), ulcer disease, or activation of the immunopathological inflammatory cascade, which results in gastric cancer. These detrimental outcomes are influenced by the host’s diet, habits, and genetics and by bacterial strain variation.

There is now evidence that H. pylori colonization protects against pathologies of the esophagus and gastric cardia (8, 15–17), childhood asthma (8, 9, 18), and childhood allergies (19, 20). Moreover, a recent review of the literature and a meta-analysis suggest that there is a protective effect of H. pylori infection on the incidence of inflammatory bowel disease (21, 22). While H. pylori has persistently colonized humans since the origin of the species, studies have found that the outcomes of H. pylori colonization depend on several factors, including, but not limited to, the presence of specific H. pylori virulence factors, diet, and/or host genetics (23–25). Specifically, CD4⁺ T cell responses, including expression of gamma interferon (IFN-γ) and interleukin-17 (IL-17) and regulatory T (Treg) cell development, impact the pathology elicited in response to H. pylori colonization. This review is designed to take an intricate look at the involvement of T helper 17 (Th17) cells and the Th17 cytokines in the immunopathogenesis of H. pylori infection.

INNATE RESPONSE: THE EARLY RESPONSE TO H. PYLORI

The development of an immune response to H. pylori infection has largely been investigated in mouse models of disease. In humans, since early infection is likely asymptomatic or possibly mistaken for a short-lived gastrointestinal infection, knowing when infection occurs in specific individuals is difficult. In some areas where H. pylori colonization is endemic, there is evidence that H. pylori colonization occurs early in childhood (26–30). The mouse model facilitates tractable immunological studies and the use of key technologies to investigate cellular infiltration (and gastritis) in the mouse model.

The course of infection and the development of pathology have been traced using serial evaluations in mice. Flow cytometry was used to characterize the early inflammatory response to H. pylori. In the first 2 to 3 days postinfection, there is an influx of neutrophils and macrophages (31). Then, by 4 days postinfection, the number of macrophages and neutrophils drops significantly. H. pylori induces macrophage apoptosis by the generation of polyamines from ornithine decarboxylase (32, 33), but it is not understood why neutrophil numbers drop so significantly. Subsequently, the H. pylori infection seems to be somewhat quiescent (in terms of gastric immune cell infiltration) for a few more weeks. During this early response, when neutrophil infiltration is reduced, differences in acute inflammation are observed depending on the H. pylori strain’s virulence factor expression. Most notably, the cag type IV secretion system (T4SS) has been shown to modulate the immune response. Strains which harbor a functional cag type IV secretion system activate gastric epithelial cells to produce IL-8 (or IL-8 homologs), a chemokine which recruits neutrophils, and therefore, the cag T4SS can directly induce inflammation through this pathway. Priming of the adaptive response is likely happening during this quiescent stage; the earliest adaptive response is detectable by 2 weeks postinfection, when H. pylori-specific T cells are generated. T cell priming in response to H. pylori infection was demonstrated using paragastric lymph node cells in an antigen-specific enzyme-linked immunosorbent spot assay (31). Unfortunately, the paragastric lymph nodes are difficult to locate in some strains of mice, and while one might expect the cell numbers to expand and lead to the enlargement of local nodes, this does not seem to be the case. Peyer’s patches have become a feasible location to identify H. pylori-specific T cells (34). Identifying sites of T cell priming and activation during H. pylori infection, even in the mouse model, would facilitate more detailed studies about cell differentiation, polarization, migration, and even T cell exhaustion. Nevertheless, it is clear from these studies and others that the gastritis response to H. pylori leads to the activation of dendritic cells (DCs); recruitment of CD4⁺ T cells, including Th1, Th17, and Treg cells; and recruitment and activation of neutrophils and macrophages in the tissue (Fig. 2).

FIG 2.

The cellular gastritis response in the stomach during H. pylori infection. H. pylori is detected by the dendritic cells and epithelial cells in the mucosa, and, subsequently, a myriad of immune cells, including proinflammatory macrophages, neutrophils (PMNs), and CD4⁺ helper cells, are recruited to the gastric tissue. Further activation of these recruited cells drives more inflammation of the gastric mucosa. Based on animal model data, when these cells are recruited to the gastric mucosa and the T cells begin producing cytokines to activate macrophages and epithelial cells and to recruit more PMNs, the H. pylori bacterial burden reaches a plateau. Regulatory T cells have been reported in the gastric mucosa and contribute to the control of gastric inflammation in response to H. pylori infection. DC, dendritic cells; PMN, polymorphonuclear neutrophils; Treg, regulatory T cells; Th1, T helper cells making IFN-γ; Th17, T helper 17 cells making IL-17, IL-21, IL-22, and IL-26 (in the human); Mϕ, macrophage.

CYTOKINE DRIVERS OF THE T CELL RESPONSE

From in vitro assays using macrophages, monocytes, and dendritic cells (DCs), studies have indicated that these innate antigen-presenting cells play a significant role in priming and activation of the immune responses. Activation of these cells with live H. pylori or H. pylori lysates results in strong cytokine production, including the production of cytokines involved in the differentiation of Th1, Th17, and Treg cell responses. There is an upregulation of Th1-polarizing cytokines, including IL-12 (35); Treg cell-activating cytokines, including IL-2 and transforming growth factor β (TGF-β); and Th17-polarizing cytokines, including IL-1β, IL-23, and IL-6 (36, 37). For Th17 cells, IL-23 has been shown to be vital for the development of the Th17 response to H. pylori. Specifically, IL-23 plays a key role in the activation of IL-17A and IL-17F expression (38). IL-23 is a heterodimer of IL-23(p19) and IL-12(p40) and signals through the IL-23 receptor (IL-23R)/IL-12Rβ1 (having a common p40 subunit and a common shared receptor subunit with IL-12). IL-23 expression has been reported in the gastric mucosa of uninfected persons, IL-23 expression increases in those infected with H. pylori (39). The role of IL-23 was addressed in a chronic H. pylori infection model. In that study, the level of H. pylori colonization in IL-23(p19)^−/− mice was significantly higher than that in wild-type (WT) mice. Moreover, the expression of IL-17A was reduced in IL-23(p19)^−/− mice, and this reduction correlated with reduced inflammation (36, 40). In relationship to Th1 and Th17 cell development and H. pylori, it has been demonstrated that IL-21, which is required for the expansion and stabilization of Th17 responses, is also important for the maintenance of both Th1 and Th17 responses in the mouse model of H. pylori-induced gastritis (41).

While there is evidence that macrophages and dendritic cells (DCs) respond to H. pylori or H. pylori lysates by the production of proinflammatory cytokines, other studies provide evidence that DCs may play a vital role in Treg cell development. In vivo, H. pylori-DC interactions skew the T cell response toward a Treg cell response rather than a Th17 response (42). In vitro, some bone marrow-derived dendritic cell (BMDC) experiments have demonstrated that H. pylori not only survives within DCs but also alters the DC function by inhibiting antigen presentation, altering cytokine production, and therefore ultimately affecting T cell activation (43). The local cytokine environment may significantly impact the cytokine profile of H. pylori-infected DCs. Moreover, there is a single study which indicates that the phosphorylation of H. pylori’s cytotoxin A (CagA) within DCs can lead to the downregulation of DC cytokine production (44).

HELICOBACTER PYLORI ANTIGENS INVOLVED IN T CELL ACTIVATION AND DIFFERENTIATION

There is evidence in the literature that specific H. pylori proteins, including several virulence factors, influence the T cell response. The influence of these antigens on the T cell response has been investigated from two different perspectives. In some studies, the activation of innate antigen-presenting cells has been measured with different H. pylori proteins, and in other studies, the T cell response to specific antigens has been measured. Only a few studies have simultaneously examined both the activation of antigen-presenting cells and antigen-specific T cell activation. For example, the H. pylori protein urease has been carefully examined in coculture with macrophages. Recombinant urease B (rUreB) induces the expression of IL-6 and IL-23(p19) (which drive Th17 differentiation) in cultured macrophages (45). Moreover, in the same report, the authors provide evidence that when splenic lymphocytes from H. pylori-infected mice are cocultured with rUreB, there was an increase in the number of Th17 cells as well as an increase in the levels of IL-17A. Finally, when using rUreB as an immunization antigen, it elicited increased UreB-specific Th17 cells. Combined, these data indicate that UreB is an important protein which is able to elicit Th17 cell responses both in vivo and in vitro (45). Interestingly, when CD4⁺ T cell epitopes of UreB were combined with CD4⁺ T cell epitopes of H. pylori adhesion A (HpaA) and CagA to create the epitope-based vaccine Epivac, immunizations elicited a Th1 cell-biased immune response which inhibited H. pylori colonization. Unfortunately, this 2012 study did not measure Th17 responses to the vaccination (46).

There is also evidence that CagA influences T cell activation or T cell skewing. Using CagA isogenic mutants, CagA was shown to influence phytohemagglutinin-induced T cell proliferation (47). In human H. pylori-infected patients, the polarization of the T helper cell response was influenced by both the CagA status of the infecting strain and the stage of progression to gastric carcinogenesis. Th1 or Th2 polarization was not skewed in patients infected with CagA-negative H. pylori strains. On the other hand, patients infected with CagA-positive (CagA⁺) strains had Th1-mediated cellular immunity in earlier stages of gastric carcinogenesis, while the advanced stages were dominated by Th2 cell-mediated humoral immunity. Further, these advanced stages and CagA⁺ strains were negatively associated with an abundance of Treg cells (48).

H. pylori affects costimulatory markers associated with T cell responses likely through CagA-dependent mechanisms. For example, H. pylori infection downregulates B7-H2 expression by gastric epithelial cells (GECs) in a CagA-dependent manner (49), and this downregulation is correlated with a decrease in Th17 cell responses in vitro and in a mouse model. B7-H1 is another member of the B7 family of proteins, but unlike B7-H2, which is proinflammatory, B7-H1 (better known as programmed death 1 ligand 1 [PD-L1]) is associated with T cell inhibition on GECs. It has been demonstrated that H. pylori coculture with GECs leads to increased PD-L1 expression; moreover, this induction of PD-L1 influences T cell activation and IL-2 production (50). Using H. pylori WT and isogenic mutant strains, Lina et al. showed that H. pylori requires CagA and peptidoglycan for B7-H1 upregulation in GECs (51). Taken together, these data indicate that a functional cag pathogenicity island (cagPAI) can impact the host response.

Another H. pylori virulence factor which influences T cell responses is H. pylori’s gamma-glutamyl transpeptidase (GGT). GGT converts glutamine into glutamate and ammonia and converts glutathione into glutamate and cysteinylglycine. H. pylori GGT induces immune tolerance through the inhibition of T cell-mediated immunity and dendritic cell differentiation (52). In many of these studies, it is difficult to separate the functions of VacA and GGT, for they seem to have similar effects on dendritic cell activity (toward T cell skewing) and on T cell proliferation.

IDENTIFICATION OF THE FUNDAMENTAL FUNCTIONS OF Th17 CELLS

Classically, proinflammatory activated T helper cells were functionally divided into two subsets, Th1 and Th2, based on the cytokines that they produce (reviewed in reference 19), with an understanding that there were also CD4⁺ T regulatory cells playing an immune-modulatory role in inflammation. The cytokines that Th1 and Th2 cells produce, IFN-γ and IL-4, respectively, are necessary for the control of specific types of pathogens based on their lifestyle (40). Th1 responses are key for the control of intracellular pathogens, while Th2 responses are vital for the control of extracellular pathogens, especially helminths (42). Th1 and Th2 responses can also contribute to immune pathogenesis. Th1 cells play an active role under inflammatory conditions, such as inflammatory bowel disease and arthritis, while Th2 cells can contribute to the inflammatory responses in allergy and asthma (43, 44).

Specifically, murine models deficient in IL-12(p35) and IFN-γ demonstrate that cytokine and receptor knockout mice still develop inflammatory conditions, such as experimental autoimmune encephalomyelitis (EAE), a mouse model of multiple sclerosis (53–55). This suggests that another subset of T cells, in addition to Th1 cells, may drive inflammation in this model. Further research demonstrated that IL-23(p19/p40) and another proinflammatory CD4⁺-derived cytokine, IL-17A, are associated with some of the same inflammatory processes as Th1 cell-associated cytokines, including EAE (35, 38, 56). Henceforth, this newly identified lineage of CD4⁺ T cells, involved in immune pathogenesis and in the production of IL-17A, became known as Th17 cells (57). Th17 cells are distinct from Th1 and Th2 cells on the basis of their unique differentiation and transcription factor expression (58). IL-12(p35/p40) and IL-4 have been shown to be the major differentiation factors for Th1 and Th2 cells, respectively. The differentiation of Th1 and Th2 cells is associated with induction of the transcription factors Tbet and GATA-3, respectively, whereas TGF-β, IL-6, and IL-23(p19/p40) are required for the differentiation of Th17 cells through the upregulation of RORγt (18, 59).

Subsequently, it has also been demonstrated that Th17 cells and, specifically, Th17 cytokines are critical for neutrophil recruitment as well as the production of various antimicrobial peptides, matrix metalloproteinases, chemokines, and cytokines, such as IL-6, IL-8, and granulocyte-macrophage colony-stimulating factor (GM-CSF), all of which are essential factors in the control of extracellular pathogens (40, 48, 60–64). Th17 production of IL-17A and IL-17F in murine models plays a similar role in neutrophil and cytokine recruitment. However, while they share a significant degree of sequence homology, studies have suggested that IL-17F may play a more central role in innate responses than IL-17A (48). Th17 also regulates the production of IL-21, which amplifies the Th17 response in an autocrine manner (65), and plays a critical role in the regulation of B cell function (66). A fourth cytokine produced by Th17 cells, IL-22, is important for mucosal immunity against pathogens which infect the intestines, and it enhances the expression of antimicrobial peptides on epithelial cells, especially in skin inflammation and infection (67). Unlike the other cytokines produced by Th17 cells, IL-22 receptor expression is not present on immune cells. Instead, it provides direct communication between the immune system and the tissues, since the receptor is solely present in tissues (reviewed in reference 68). One Th17 cytokine, IL-26, is produced only in humans. This proinflammatory cytokine has been associated with inflammation and may have direct antimicrobial activities. The role(s) of each of these cytokines in the context of controlling H. pylori colonization and immunopathology is discussed below.

ROLES OF Th17 CYTOKINES IN H. PYLORI-DRIVEN IMMUNOPATHOGENESIS

In this section, we summarize the data on the mechanisms by which Th17 cytokines contribute to pathology and the antimicrobial response to H. pylori infection.

IL-17A.

IL-17A has been shown to play a role in inflammation as a result of H. pylori colonization (69). Studies from murine models have demonstrated a significant increase in neutrophil infiltration within the submucosa and lamina propria of the stomach in WT mice infected with H. pylori compared to H. pylori-infected IL-17A^−/− mice (60). The longer that the WT mice are infected with H. pylori, the greater that the level of neutrophil infiltration is. However, the results from H. pylori infection in IL-17A^−/− mice or IL-17RA^−/− mice have shown that at no point during infection is there a significant rise in the level of neutrophils (69, 70). All of these studies suggest that IL-17A is essential for neutrophil infiltration. This is likely through the activation of IL-8 homolog expression in epithelial cells, which subsequently recruits neutrophils. Mizuno et al. investigated IL-17A and IL-8 expression in gastric ulcer and nonulcer patients and found increased levels of IL-17A and IL-8 in the gastric mucosa of H. pylori-infected gastric ulcer and nonulcer patients compared to uninfected nonulcer patients (71). Histologically, they found that both IL-17A and IL-8 were strongly correlated with an increase in neutrophil infiltration in infected patients (71). Prior to that study, Luzza et al. analyzed IL-17A RNA transcripts taken from homogenized gastric mucosa from human subjects complaining of dyspeptic symptoms and found that there was increased IL-17A gene expression in H. pylori-infected patients compared to uninfected patients (72). Luzza et al. also demonstrated that IL-17A specifically induced the production of IL-8 in mucosa by isolating lamina propria mononuclear cells (LMPC) from the gastric mucosa and coculturing them with anti-IL-17A antibodies (Ab). This study found that the levels of IL-8 were downregulated in cultures treated with anti-IL-17A Ab in a dose-dependent manner, with greater concentrations of anti-IL-17A Ab resulting in the release of lower levels of IL-8 (73). Therefore, as IL-17A activity heavily stimulates proinflammatory effects, such as neutrophil infiltration through IL-8 stimulation, interest in targeting IL-17A to reduce inflammation has developed. Since neutrophils are significant antimicrobial effector cells, controlling inflammation through IL-17A in light of its antimicrobial role has proven to be a complex task. In some models, a reduction in IL-17 activity may lift a repression of IFN-γ expression, which can also drive inflammation and lead to the recruitment of innate immune cells. When anti-IL-17 antibody was administered to chronically infected mice (74), there was an increase in inflammation, but the authors reported an increase in IFN-γ, and they hypothesized that this was driven by the Th1 response. Therefore, as IL-17A activity heavily stimulates proinflammatory effects, such as neutrophil infiltration through IL-8 stimulation, this provides a promising therapeutic target to reduce this acute inflammation but may not reduce all chronic inflammation, especially if it is driven by Th1 responses.

Expression of IL-17A and control of H. pylori colonization also appear to be linked. It is difficult to separate the role of IL-17A in recruiting polymorphonuclear leukocytes (PMNs) for their antimicrobial functions from the role of IL-17A in activating antimicrobial product expression by epithelial cells of the gastric mucosa. Studies on murine epithelial cells and human gastric epithelial cell lines suggest that IL-17A only minimally activates antimicrobial responses in these cells but, rather, synergizes with IL-22 to activate the expression of S100A8 and S100A9 (both of which are components of calprotectin [CP], lipocalin [LCN], and some β-defensin proteins [75]). Interestingly, however, IL-17A^−/− mice controlled H. pylori colonization as well as WT mice and arguably did so better than WT mice in the study of Shiomi et al. (69). On the other hand, in IL-17 receptor A knockout (IL-17RA^−/−) mice, the bacterial burden was significantly higher than that in WT mice (70), suggesting that signaling through IL-17RA is important for antimicrobial responses. This is not just evidenced in H. pylori infection models; impaired IL-17 signaling was linked to host susceptibility to a variety of pathogens, including Salmonella enterica, Streptococcus pneumoniae, Listeria monocytogenes, Staphylococcus aureus, Citrobacter rodentium, herpes simplex virus, Trypanosoma cruzi, and Candida albicans (reviewed in reference 76).

Another indication that IL-17A plays a role in the control of bacterial proliferation is through the vaccination studies that have been performed. IL-17 has been identified to be a critical cytokine in vaccine-induced Helicobacter clearance in some studies (77–79), but not in all studies (80, 81). Antibody-mediated depletion of neutrophils abrogated vaccine efficacy, suggesting that PMNs are required for the observed reduced bacterial burden (78). Moreover, treatment of WT immunized mice with anti-IL17A antibodies led to reduced inflammation and increased bacterial colonization after Helicobacter infection compared to the inflammation and colonization seen after treatment of immunized mice with control antibodies. Vaccine-induced reductions in the bacterial load comparable to those in WT mice were observed in IL-12(p35)^−/−, IL-23(p19)^−/−, and IL-17A^−/− mice (80, 82), potentially contradicting the reported requirement for IL-17A activity. This may be explained by the fact that vaccination protocols vary significantly in the mouse models with a variety of antigen types, different adjuvants, and varied routes for immunization. More research is required to interpret correlates of protection. While both Th1 and Th17 responses contribute to the control of extracellular bacterial clearance, only the induced, immunodominant IFN-γ-positive Th1 lymphocytes are required for actual protection against H. pylori (81). An alternative explanation is that Th1 responses may compensate for the absence of a Th17 response if Th1 responses are amplified by administering recombinant cytokines. It has been demonstrated that the use of recombinant IL-12 increased IFN-γ responses to vaccination (83). A review written by D’Elios and Czinn argues that there is “considerable evidence that the key to inducing protection is to pre-empt or override regulatory T-cell activity by promoting Th1 and/or Th17 mediated immunity” (84). This evidence may be worth considering further as vaccines move to clinical trials (ClinicalTrials.gov registration numbers NCT00736476 and NCT02302170).

The role of IL-17A and Th17 cells in activating the inflammatory response and controlling bacterial proliferation may be quite different from the role that IL-17A might play if gastric cancer (GC) develops. The tumor microenvironment (TME) can be immune suppressive, which leads to a bourgeoning interest in exploring whether a proinflammatory environment in the tumor would slow the progression of GC. A recent publication investigating the expression of several cytokines and their relationship with clinicopathological characteristics in GC revealed that IL-17A expression (measured by immunohistochemistry) was associated with decreased survival (85). Further supporting the idea that IL-17A is procarcinogenic, a single study performed on a gastric cancer cell line (AGS cells) found that IL-17A could promote the migration and invasion of GC cells. The data from that study suggest that the upregulated expression and activity of matrix metalloproteinase 2 (MMP-2) and MMP-9 and the downregulated expression of TIMP-1 and TIMP-2 may be the mechanism by which IL-17A promotes invasiveness (86).

On the other hand, the role of IL-17A in different TMEs is not well understood, and there is also some suggestion that increased Th17 responses may lead to better survival. In a herculean effort, Thorsson et al. subtyped more than 10,000 tumors from 33 different cancers by the tumor’s immunological signature (87). The six immune subtypes were type 1 (wound healing), type 2 (IFN-γ), type 3 (inflammatory), type 4 (lymphocyte depleted), type 5 (immunologically quiet), and type 6 (TGF-β dominant) (87). Close to 80% of gastric adenocarcinomas were subtyped into the first two classifications. Type 1 was simply defined as a wound-healing microenvironment, and type 2 was defined as IFN-γ dominant. Both type 1 and type 2 TMEs had high proliferation rates, but type 1 TMEs exhibited elevated angiogenic gene expression and a strong Th2 response, whereas type 2 TMEs not only expressed high levels of IFN-γ but also showed bias toward proinflammatory macrophages and a strong cytotoxic T cell signal. Combining all cancer types, patients with TME defined as immune subtype 3 (elevated Th17 and Th1 signatures with low to moderate proliferation rates) had the best overall survival rates, while patients with the more common subtypes associated with gastric adenocarcinomas (types 1 and 2) had less favorable outcomes (87). It is unclear how commonly immune subtype 3 occurs in gastric cancer and H. pylori infection, indicating a need for a deeper understanding of Th17 cytokines in the development of cancer and in the tumor microenvironment in GC.

IL-17F.

IL-17F is another cytokine that is within the IL-17 family and that is produced by Th17 cells. It has also been implicated in driving inflammation and may contribute to the control of infection. Like IL-17A, the production of IL-17F is regulated by IL-23 (88). IL-17F has been shown to upregulate various proinflammatory cytokines and chemokines, such as IL-2, TGF-β, IL-6, and GM-CSF (89, 90). Research studying the exact role that IL-17F plays in H. pylori infection is minimal in the mouse model. In the gastric mucosa, a compensatory mechanism and/or a redundancy in the immune systems can be found, and these result in similar levels of control of H. pylori when one of these Th17 cytokines is missing.

Expression of IL-17F has been measured in human subjects with H. pylori infection and H. pylori-associated diseases. A study examining the role of IL-17F in functional dyspepsia using human gastric biopsy samples found that an allele of IL-17F, IL-17F 7488T, is positively correlated with the development of functional dyspepsia in H. pylori-infected patients with upper abdominal symptoms who did not show any sign of ulceration of the gastric mucosa. This study also found that the same subjects with the IL-17F 7488T allele had an increase in inflammation in their gastric biopsy samples (91). A meta-analysis of studies focusing on three IL-17 polymorphisms found that these polymorphisms significantly increased the risk of gastric cancer, especially in Chinese participants (92). Another study looked at the expression of IL-17F in H. pylori-infected individuals from Kenya and Germany (93). Kimang’a et al. took stomach biopsy samples from both H. pylori-positive and -negative individuals with dyspeptic symptoms and found that both IL-17A and IL-17F expression was significantly increased in the H. pylori-infected individuals, suggesting a potential link between these two related cytokines (93).

While research on the mechanisms by which IL-17F acts during H. pylori infection has been minimal, some studies have looked at these mechanisms and found that IL-17F is associated with various pathologies in the gastrointestinal tract. Distinct roles for IL-17F and IL-17A in intestinal inflammation have been identified (94). Inhibition of IL-17F, in contrast to inhibition of IL-17A, greatly augmented protection from T cell-induced colitis. This appears to be the result of different regulatory transcription factors and epigenetic modifications between IL-17F and IL-17A; specifically, IL-17F, as opposed to IL-17A, is found to be continuously expressed in naive T cells in colonic cells. Thus, IL-17F can be found in abundance in the intestine (in inflammatory and noninflammatory settings) and has an important function in regulating the microenvironment of the intestinal mucosa. In fact, IL-17F^−/− Rag^−/− mice given IL-17F^−/− T cells showed increased protection against colitis compared with the Rag^−/− mouse recipients (94). These studies also demonstrated that IL-17F plays an important role in activating antimicrobial peptides (AMPs) secreted by Paneth cells. While anti-IL17A treatments showed no benefits in their colitis model, IL-17F, indeed, played a vital role in inflammation of the intestines, specifically, in the colonization of the normal flora and functioning as a target for the treatment of intestinal inflammation (94). Research studying the exact role that IL-17F plays in H. pylori infection is minimal in the mouse model. In the gastric mucosa, there may be some compensatory mechanism and/or redundancy in the immune systems which results in similar levels of control of H. pylori when one of these Th17 cytokines is missing. This is an area ripe for research since the role of IL-17F is undefined and its activities independent of IL-17A in the gastric mucosa are not well understood.

Evidence for regulatory roles of IL-17 signaling.

The Th17 cytokines IL-17A, IL-17F, and heterodimer IL-17A/F signal through a shared receptor. These three cytokines, IL-17A, IL-17F, and IL-17A/F, utilize the IL-17R, which is a multimeric receptor with an unknown stoichiometry. While IL-17RA is expressed constitutively on most cells, the signaling through IL-17 has been best characterized in nonhematopoietic cells, including stromal cells, fibroblasts, and epithelial cells. In the mouse model, IL-17RA^−/− mice do not respond to H. pylori infection like the single-cytokine-deficient (IL-17A^−/−) mice (69, 70, 77). As expected, PMN recruitment to the gastric mucosa was reduced in IL-17RA^−/− mice, and the mice did not control H. pylori levels as well as WT mice. Loss of IL-17 receptor signaling led to increased chronic inflammation in the gastric mucosa concomitant with the increased infiltration of CD4⁺ T cells and a very significant B cell infiltration into the tissue. The B lymphocytes organized into lymphoid follicles with germinal centers. These data indicate that IL-17 signaling, while having proinflammatory activity to drive PMN infiltration and antimicrobial production, may exhibit some regulatory activity. This suggests that some ligation of the receptor by either IL-17A or IL-17F may provide a negative feedback loop within T lymphocytes to inhibit Th17 cytokine production. While there is evidence in the literature for this negative feedback loop, the mechanism by which it functions has not been elucidated. Garg and Gaffen have identified at least two pathways which restrict IL-17 signaling in fibroblasts (95). The investigators demonstrated that the ubiquitin-editing enzyme A20 inhibits IL-17 signaling by deubiquitinating tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6), thereby limiting NF-κB and mitogen-activated protein kinase activation (95). Since IL-17 upregulates A20 expression, this demonstrates that this pathway of regulation is an established negative feedback loop that restricts IL-17-driven inflammation (96). Moreover, they also reported that the A20-binding inhibitor of NF-κB activation 1 (ABIN-1) negatively regulates the IL-17-induced production of Il6, Lcn2, and a few other NF-κB-dependent genes in fibroblasts and stromal cells (97). We speculate that similar signaling pathways may regulate the downstream activation of T cell cytokine production.

Interestingly, in an Aspergillus airway inflammation model, the IL-17F/IL-17RC axis is functional under conditions of an IL-17RA deficiency model (98). Aspergillus-infected IL-17RA^−/− mice had an elevated inflammatory response after infection. However, in this model, in mice lacking IL-17RA, the interaction of IL-17F with IL-17RC on epithelial cells in the upper airways contributed to inflammatory allergy, despite providing protection against local colonizers. It was reported that IL-17F is produced by the lung epithelium, which would be an unusual source of this Th17 cytokine. It has yet to be determined if IL-17F could function in this way in the absence of IL-17RA during H. pylori infection.

These data raise the question: is IL-17 signaling protective for people infected with H. pylori? Despite its reputation for driving inflammation, in the context of H. pylori infection, IL-17 expression may help regulate antimicrobial functions and chronic inflammation. Overexpression or uncontrolled IL-17A expression may contribute to cancer, but there is conflicting evidence as to whether expression of IL-17A or IL-17RA is associated with a better prognosis (87, 99).

IL-17RA can also complex with the IL-17RB subunit. When IL-17RA and IL-17RB complex, they form the receptor for IL-17E (also known as IL-25). The IL-25 signaling pathway has been reported to play a role in Th2 cell-driven inflammation. Furthermore, IL-17B shares the receptor IL-17RB with IL-17E, and the function of IL-17B has not been fully determined. In mouse models of H. pylori infection, the role of IL-17RB has been investigated in both the early and the late stages. During acute infection, H. pylori infection can lead to the decreased expression of IL-17RB on gastric epithelial cells in a cag-dependent fashion. This decreased expression impaired CD11b⁺ CD11c⁻ myeloid cell accumulation in the gastric mucosa and disrupted the early activation of antimicrobial RegIIIα (100). In a separate study, at the chronic stage of H. pylori infection, IL-17RB^−/− mice controlled H. pylori colonization as well as WT mice did, but IL-17RB^−/− mice did not develop H. pylori-specific antibodies (101). These data suggest that IL-17E/IL-25 is required for the development of the Th2 cell response to support B cell activation and antibody production.

IL-21.

IL-21 is another cytokine produced by Th17 cells. It is also produced by T follicular helper (Tfh) cells and may be produced in smaller quantities from NK T cells or Th1 cells. IL-21 deficiency in the mouse model has very profound effects on H. pylori-induced gastritis compared to those in WT mice (41). This is likely due to its pleotropic activities in various tissues (Fig. 3 and 4). At 3 months postinfection, IL-21^−/− mice do not control H. pylori colonization as well as WT mice, and the bacteria localize more frequently in the gastric glands. IL-21^−/− mice exhibit less inflammation than H. pylori-infected WT mice (41). The data also suggest that IL-21 is a key cytokine for maintenance of both the Th1 and Th17 responses during H. pylori infection in the gastric mucosa (Fig. 3). To further investigate T helper cell responses and to delineate the molecular mechanisms behind IL-21-driven signaling during chronic H. pylori infection, computational modeling and mechanistic experimental studies were used (see the data in the supplemental material associated with the published paper for more details [41]). Starting with a published model of CD4⁺ T cell differentiation (102), the computational model was calibrated and IL-21-specific pathways were investigated in situ. The CD4⁺ T cell model predicted that proinflammatory markers were positively correlated with IL-21, whereas regulatory T cell responses were negatively correlated. The computational model’s predictions were validated using the IL-21^−/− animal model. CD4⁺ T cells from H. pylori-infected IL-21^−/− mice had lower phosphoactivation of STAT1 and STAT3 than CD4⁺ T cells from H. pylori-infected WT mice. The STAT1 and STAT3 pathways are important for Th1 and Th17 responses, respectively (41). Also, H. pylori infection in IL-21^−/− mice led to lower levels of expression of tbx21 and rorc, which encode Tbet and RORγt, respectively, than infection in WT mice (41). Moreover, as predicted, CD4⁺ T cells from H. pylori-infected IL-21^−/− mice expressed higher levels of IL-10 than CD4⁺ T cells from H. pylori-infected WT mice. These data demonstrate not only that IL-21 plays a critical role in the development or maintenance of Th1 and Th17 responses to H. pylori but also that IL-21 may affect Treg cell responses.

FIG 3.

IL-21 plays a significant role the development of gastritis. IL-21 is required for the presence of activated Th1 and Th17 cells in the gastric mucosa. Th1 and Th17 cells are activated by IL-21 to increase the production of proinflammatory cytokines, including IFN-γ, IL-17, and IL-22. IFN-γ then activates macrophages, while IL-17A and IL-22 activate epithelial cells, release antimicrobial peptides, and produce the neutrophil-recruiting chemokine IL-8. In addition, IL-21 stimulates B cells to isotype switch and produce antibody.

FIG 4.

IL-21’s pleotropic effects can be seen in the lymph node. IL-21’s release from T follicular helper (Tfh) cells stimulates the differentiation of T cell subsets and potentially downregulates the DC response through reduction of proinflammatory cytokine production. IL-21 can reduce the proinflammatory cytokine production of DCs (IL-12, IL-1β, IL-6, and IL-23) activated by H. pylori (101). Green lines and arrows represent the stimulatory activity of IL-21, while red lines with blunted ends represent the inhibitory activity of IL-21. These pleotropic roles of IL-21 further demonstrate why it has such an important influence on the immune response to H. pylori gastritis.

The expression of IL-21 has been associated with H. pylori infection in studies of infected humans (103, 104). An expression analysis of Th17-associated markers during H. pylori infection and gastric cancer demonstrated a strong positive correlation of RORγt and IL-17A with IL-21 in both H. pylori-infected tissues and cancer tissues (105). These data suggest that IL-21 may be a key T cell-derived cytokine promoting a more pathogenic T cell response by driving strong Th1 and Th17 responses and associated pathologies and potentially inhibit Treg cell responses. The concentration of IL-21 in peripheral blood, in fact, is higher in gastric cancer patients than in healthy controls and is associated with the development, metastasis, and maturation of gastric cancer in later stages of disease (106).

IL-21 does signal B lymphocytes to proliferate and switch isotypes; therefore, IL-21 plays a critical role in activation of antigen-specific antibody responses. H. pylori infection of WT mice does induce H. pylori-specific antibody responses, but in H. pylori-infected IL-21^−/− mice, H. pylori-specific antibodies were not detected in the serum (41). This could be due to the direct effects of IL-21 on the B cell response or could possibly be due to IL-21 promoting the differentiation of Tfh cells, which then subsequently support B cell activity (107, 108). Although the data are correlative, chronically infected IL-17RA^−/− mice, which express very high levels of IL-21, also have significantly higher levels of H. pylori-specific antibodies in their serum than WT mice (70), suggesting that IL-21 may drive the antibody response in IL-17RA^−/− mice. These H. pylori-infected IL-17RA^−/− mice also develop very significant B cell infiltration and lymphoid follicles with germinal centers in their gastric tissue, a pathological outcome which is rare in WT C57BL/6 mice. The significance of high or low H. pylori-specific antibodies is not well understood. Using B cell-deficient mice, it was demonstrated that H. pylori-specific antibodies inhibit the development of gastritis, may enable bacterial colonization, and actually lead to increased resistance against H. pylori infection (109). As such, the data demonstrating that higher levels of H. pylori-specific antibodies in IL-17RA^−/− mice correlate with increased inflammation do not contradict the finding that B cell-deficient mice which lack antigen-specific responses had less inflammation. Rather, the data suggest that the correlation between antibodies and gastritis may support the hypothesis that high H. pylori-specific antibody responses may be a marker of increased inflammation, and there may be a way to use this correlative biomarker to predict when a patient may experience a more detrimental outcome of H. pylori infection. In fact, although potentially looking at it from a different perspective, Epplein et al. have found that seropositivity to a number of specific H. pylori antigens was associated with an increase in the odds of gastric cancer in several prospective cohorts (110–112). The relationship between the inflammatory response, IL-21 expression, IL-21 polymorphisms, and antibody levels in these cohorts has not been investigated.

IL-21 is expressed in lymph nodes or in Peyer’s patches (107, 108) and functions to maintain Tfh cells and B cell function (Fig. 4). Interestingly, while IL-21^−/− mice infected with H. pylori had reduced Ifng and reduced Il17 expression in their gastric mucosa compared to WT mice (41), the T cells in the Peyer’s patches and mesenteric lymph nodes of the H. pylori-infected IL-21^−/− mice produced significantly more IL-17A (113). Moreover, there was an increase in Th17-directing cytokines, including IL-1β and IL-6, in the IL-21^−/− mice than in the WT mice. In vitro assays performed on dendritic cells demonstrated that recombinant IL-21 significantly reduces the H. pylori-induced expression of many proinflammatory cytokines, including IL-1β, IL-6, IL-12, and IL-23, but not IL-10 (113). These data suggest that IL-21 has an immune-modulatory role in the context of dendritic cell responses.

The proinflammatory cytokine IL-21, being pleiotropic, may also signal through gastric epithelial cells. There is only one group thus far which has addressed the ability of IL-21 to activate gastric epithelial cells. Caruso et al. have reported that IL-21 upregulates expression of matrix metalloprotease 1 (MMP-1) and MMP-3 on the AGS cell line (104). In other chronic inflammation settings, such as rheumatoid arthritis, IL-21 induced the migration and invasion of fibroblast-like synoviocytes (114), as well as induced MMP-1 and MMP-3 production in these cells (115). Additional studies utilizing other epithelial cells (i.e., intestinal epithelial cells) have shown that IL-21 can activate the expression of CCL20 (macrophage inflammatory protein 3a), a T cell-recruiting chemokine (116). While not proven in gastric cancer, some cancer cell lines are more invasive and migratory when MMP expression is up and when the cells are treated with IL-21, including some breast cancer cell lines (117). IL-21 binding to its receptor activates the JAK/STAT pathway; in fact, in gastric cancer patients, increased IL-21 receptor expression is correlated with survival and the recurrence of disease. A decrease in IL-21R expression is associated with the reduced growth/spread of gastric cancer, while an increase in IL-21R expression inactivates antitumor medications, such as oxymatrine (OMT), which work through the inactivation of JAK/STAT signaling (118). It has been suggested that since the development of gastric cancer relies upon inflammation, IL-21 could increase the risk of gastric cancer, but more studies of this key cytokine are needed (119). Despite the concern that increased IL-21 signaling may activate inflammation and drive carcinogenesis, there is interest in boosting IL-21 signaling during immune therapies because IL-21 also augments CD8⁺ T cell functions and could improve antitumor cytotoxic T cell responses. Moreover, one study designed to address how IL-21 directly impacts gastric cancer cells used IL-21 alone and in combination with 5-fluorouracil on gastric cancer cell line SGC-7901 (120). The data from that study demonstrated that IL-21 inhibited the proliferation and growth of the cell line (120).

IL-22.

Interleukin-22, a member of the IL-10 cytokine family (121), was first described to be a Th1 CD4⁺ T cell-derived cytokine. Later studies attributed IL-22’s expression to predominantly that of the Th17 cell subset; and then Th22 cells, innate lymphoid cells (ILCs), and γδ cells were added to the list of IL-22 producers (122–124). All of these cell types produce IL-22 primarily in response to IL-23. In 2015, Zhuang et al. suggested that an H. pylori infection could elicit a Th22 cell response and IL-22 expression (113). While they also attributed the IL-22 expression to IL-23, they did not rule out Th17 cells as the source of IL-22 in that instance.

IL-22 acts primarily on nonhematopoietic cells. It acts through a heterodimeric receptor of IL-22R1 and IL-10R2, which is present primarily on nonhematopoietic cells of the skin, respiratory system, liver, and gastrointestinal tract (126–130). At mucosal surfaces, its functions include antimicrobial defense, wound healing, and protection against tissue damage (127, 131–133). Specifically, when IL-22 was overexpressed using adenovirus, IL-22 induced a systematic acute-phase response, activating fibrinogen and increasing platelet counts and PMN responses in the blood (134). In the skin lesions of psoriasis patients, higher levels of IL-22 were associated with increased expression of some antimicrobials, namely, S100 proteins (A7, A8, and A9) and increased expression of matrix metalloprotease 1 (135). IL-22 acts independently and synergistically with a number of cytokines, including TNF, IL-1β, and IL-17A (136, 137). It is its synergistic activity with IL-17A that has been a major focus of research, which indicates that, together, these cytokines activate antimicrobial responses in the mucosa, including lipocalin-2, S100A8, S100A9 (and calprotectin A8/A9), and serum amyloid A-3 (SAA3) (134–138).

In H. pylori-infected humans and H. pylori-infected mice, IL-22 expression correlates with increased gastritis (75, 125). Many proinflammatory cytokines are activated on the same time course as IL-22, and several even activate STAT3; therefore, identifying an independent role for IL-22 that is not compensated for has been difficult. The role of IL-22 in the activation of antimicrobial responses was examined in models of H. pylori infection using human gastric epithelial cell lines and the mouse model of H. pylori infection (75). Recapitulating what had been observed in several epithelial cell lines, when both IL-17A and IL-22 were cultured with human cell lines, primary mouse gastric epithelial cells (GEC), or gastroids, and antimicrobials, including components of calprotectin (CP), lipocalin (LCN), and some β-defensins, were induced (75). In vivo IL-22 deficiency alone did not render mice more susceptible to H. pylori infection or significantly change the pathological consequences of the infection (75). These data conflict with data that were published by Zhuang et al., where they attributed IL-22 expression to Th22 cells rather than Th17 cells (125). These experimental design differences are worth noting. WT BALB/c mice and IL-22^−/− mice in the BALB/c background were used for the studies of Zhuang et al. (125), whereas in the study of Dixon et al. (75), C57BL/6 mice and IL-22-/- in the C57BL/6 background were used. This is relevant because C57BL/6 and BALB/c mice respond differently to infection (139). Therefore, mice on the C57BL/6 background may have produced higher levels of Th1 cytokines, such as IFN-γ and IL-21, and compensated for the IL-22 deficiency in the experiments performed by Dixon et al. (75). Another difference between the experimental designs was the choice of H. pylori strains. While both studies used cagPAI-positive strains (strain NCTC 11637 [125] versus strain PMSS1 [75]), there may be undefined differences between these strains that could activate epithelial cells directly and induce inflammation differently. A key signaling pathway that may need to be activated to compensate for IL-22 deficiency is the phosphorylation of STAT3. Since several proinflammatory cytokines can activate STAT3 (140), expression of these cytokines may be enough to compensate for IL-22 deficiency. Also, IL-22 or IL-17A may enhance cytokine receptor expression for other proinflammatory cytokines, therefore enhancing epithelial cell sensitivity. Thus, in vivo IL-22 deficiency may be compensated for by several other activation signals that appear to converge on similar signaling components.

IL-21 is not the only Th17 cytokine to impact matrix metalloprotease expression. IL-22 impacts matrix metalloprotease 10 expression and, therefore, impacts gastritis in humans and mouse models. MMP-10 expression was found to be positively correlated with IL-22 expression in the gastric mucosa of H. pylori-infected patients. Moreover, in vitro H. pylori and IL-22 synergistically induce MMP-10 expression in a cagA-dependent pathway (141), suggesting that IL-22 impacts angiogenesis and/or tissue repair.

In a study out of Taiwan, polymorphisms in IL22 were found to be significantly associated with gastric MALT lymphoma (142). Interestingly, H. pylori-infected patients with higher IL-22 expression were more likely to respond to therapy. Consistent with data that IL-22 activates antimicrobial protein expression (75), that study demonstrated that IL-22 expression increased the expression of RegIIIα and LCN2. On the other hand, several recent studies suggest that IL-22 can promote carcinogenesis. IL-22 increased gastric cancer cell invasion through activation of the STAT3/Erk pathway (143) and increased the metastatic activity of human gastric cancer cells through increased AKT activation and MMP-9 production (138). Interestingly, in the tumor microenvironment, IL-22 production may be not only from T lymphocytes but also from cancer-associated fibroblasts (143).

IL-26.

The human IL-26 gene, IL26, maps to chromosome region 12q15 between two other cytokine genes, IFNG and IL22 (144). IL26 is often coexpressed with IL22 at high levels by human Th17 cells (145, 146). High IL-26 expression is strongly associated with inflammatory activity in the synovia of individuals with rheumatoid arthritis, in psoriatic skin lesions, and in the colonic lesions of individuals with inflammatory bowel disease (147–150). A risk locus containing IL26 and single-nucleotide polymorphisms within the IL26 gene region have been associated with several of these inflammatory diseases (151, 152). The IL-26 gene is conserved in most vertebrate species but, curiously, is absent in mice. At the time of this review, our literature review was unable to find any studies addressing whether H. pylori infection enhances the expression of IL-26 in humans. A recent article published results which supported the hypothesis that IL-26 promotes gastric cancer. Immunohistochemical staining of human gastric tissues revealed increased levels of IL-26 and activation of STAT3 (153). Moreover, ex vivo analysis of tumor-infiltrating lymphocytes (TILs) indicated that IL-26 is expressed by both Th17 cells and NKT cells in the tumor. Furthermore, that study demonstrated that IL-26 promotes the proliferation and survival of human gastric cancer cells (MKN45 and SGC-7901 cells) by regulating the balance of STAT1 and STAT3 activation.

IL-26 has also received some attention as an antimicrobial protein. The cytokine has cationic amphipathic properties, and at concentrations greater than 2 mM, IL-26 can kill extracellular bacteria (including Pseudomonas aeruginosa, Escherichia coli, S. aureus, and Klebsiella pneumoniae) (148) via membrane pore formation. Moreover, recent reports suggest that IL-26 may act in a manner similar to that of LL-37 and inhibit S. aureus growth and biofilm formation (154). There is no evidence to date that IL-26 can inhibit H. pylori growth.

PLASTICITY OF T CELL RESPONSES

Defining T helper subsets is helpful for the field to describe cell types, the cytokines that they produce, and their downstream responses, but in reality, there is plenty of evidence to suggest that T helper cells have the ability to switch the cytokine production patterns under some circumstances (reviewed in reference 155). Moreover, the activation of T cell subsets in vivo likely leads to significant heterogeneity within a population of cells. The heterogeneity may be a result of cells being at different points along the pathway to terminal differentiation (if terminal differentiation even exists). Fate- mapping experiments in mouse models have been used to address plasticity, but few fate-mapping experiments have been performed with H. pylori. In the Helicobacter hepaticus infection model, it was found to be the loss of IL-17A expression—in IFN-γ-producing ex-Th17 cells—which plays a pathogenic role in the disease (156). It is common in intestinal inflammation models that IFN-γ and IL-17 dual-producing Tbet-positive RORγt-positive T cells are associated with more severe disease. Adoptive transfer models utilizing both single and double knockout T cells have demonstrated that both IFN-γ and IL-17 contribute to H. pylori gastritis (157).

TOLERANCE THROUGH Treg CELL RESPONSES

While the focus of this review is on the dichotomous roles of Th17 cells in H. pylori infection and inflammation, the impact of Th17 responses on H. pylori cannot be fully considered without some discussion of the other heterogeneous CD4⁺ T cell populations, including the T regulatory (Treg) cell response. Entire reviews have certainly been dedicated to the potential roles of Treg cells in reducing inflammation, potentially facilitating the persistence of H. pylori and potentially inhibiting anticancer immune responses (158–160). Studies have suggested that it is not the level of the Th17 or Treg cell response in patients which may dictate their prognosis but, rather, the imbalance of Th17 cells and Treg cells which drives chronic inflammation. Treg cells are induced during H. pylori infection. In fact, a few groups have demonstrated that DCs not only activate cytokines which can drive Th1 or Th17 responses but also produce IL-10 and IL-18, which can induce Foxp3 expression and Treg differentiation (161, 162). To characterize whether Treg cells may prevent pathological outcomes, experiments were performed on athymic C57BL/6 nu/nu mice. The C57BL/6 nu/nu mice were reconstituted with either all lymph node cells or lymph node cells depleted of CD25⁺ T cells. These reconstituted mice were compared to mice that were not reconstituted at all (163). The result of this adoptive transfer experiment demonstrates that in the absence of Treg cells (cells depleted of CD25⁺ cells), H. pylori colonization is reduced, but at the cost of inducing an earlier, increased severity of gastritis. Moreover, using a different model, depletion of CD25⁺ Treg cells also led to a reduced H. pylori bacterial burden, which may be due to the enhanced peripheral H. pylori-specific Th17 cells observed in those mice (161). The literature suggests that this balance of Th17 and Treg cells may change as people age. In several studies focusing on H. pylori-infected children, there was evidence that Treg cell responses positively correlate with bacterial density, while Th17 responses negatively correlate with bacterial density (164). In a study which compared adults and children, Serrano et al. reported that children had more IL-10⁺ cells than adults, and, accordingly, it was reported that they had reduced gastric Th17 responses, which correlated with reduced gastritis (165).

Interestingly, Treg cells may be more plastic than other CD4⁺ T cell subsets and under some conditions have been shown to become proinflammatory. In vitro, when culturing Treg cells with IL-6, especially in the absence of TGF-β, they upregulate RORγt and produce IL-17 (166). In addition, Foxp3⁺ IL-17⁺ T cells have been identified in vivo (167).

CONCLUSIONS

These studies led us to recognize that the field needs a clearer understanding of the dynamic interactions of T cells with the gastric mucosa. For example, Th17 responses (especially IL-22) are important for maintaining the mucosal barrier and repairing the intestinal mucosa; how the Th17 cell-produced cytokines contribute to repair of the gastric mucosa during H. pylori-induced ulcer disease or injury has not been investigated. Moreover, the roles of IL-17F and IL-26 are virtually unexplored and may be underappreciated in this complex microenvironment. There is a lack of understanding of how common dual cytokine (IFN-γ and IL-17)-producing T cells, which are pathogenic in some inflammatory diseases, are activated in response to H. pylori infection, and whether there is a differential role for these dual cytokine-producing cells from that of Th1 and Th17 cells has not been studied in the context of infection. Further, since the IL-17 signaling pathway is being targeted by immunotherapeutics, a better comprehension of the immune-modulatory mechanisms regulating IL-17RA signaling is required. In psoriasis patients, there has been great success with reducing disease pathology through inhibition of the IL-17 pathway, but in Crohn’s disease patients, the data suggest that anti-IL-17 therapies worsen the disease. With up to 50% of the world’s population being colonized by H. pylori, we need a better understanding of how these therapies would impact the outcomes of H. pylori colonization both in the short term (gastritis) and in the long term (carcinogenesis). Finally, an area ripe for research investigation is the impact of Th17 cells in the gastric tumor microenvironment and how the role for Th17 and IL-17 might shift as carcinogenesis develops. Th17 cells may contribute significantly to many of these processes, as they are central to inducing inflammation, activating antimicrobial responses, and impacting cell infiltration in tumor environments.

Biographies

Beverly R. E. A. Dixon began her journey in biological research at Fisk University, where her academic interest shifted from Computer Information Systems and International Management to Biology. Upon completing her bachelor’s degree at Fisk, she enrolled in the Masters of Agricultural Science program at Tennessee State University. There, she focused on the gut microbiota in poultry, developing a deep appreciation for the astounding capabilities of microbes―especially those that provide significant benefits to the host. This interest in microbe-host interaction led Dixon to the Algood laboratory at Vanderbilt University Medical Center where she serves as a Sr. Research Scientist. Currently, she champions the goals of the Algood laboratory in seeking to answer questions that arise from the interaction between Helicobacter pylori and its human host.

Rafat Hossain graduated from Vanderbilt University in 2017, where he majored in psychology on a pre-medicine track. His interest in medicine, especially the aspects of immunity and microbiology, led him back to Vanderbilt in the summer of 2018, to the Algood laboratory in the Department of Medicine, Division of Infectious Diseases. During this research internship, he worked alongside Dr. Algood and other research assistants exploring the body's immune reaction to inflammation. Rafat is currently a 3rd year medical student at LMU-DeBusk College of Osteopathic Medicine, where he is starting rotations and exploring the various medical fields.

Rachna V. Patel is currently an Assistant Professor of Hospitalist Medicine at Temple University Hospital in Philadelphia, PA. She completed medical school and earned her M.D. at Vanderbilt University Medical School. While at Vanderbilt, she worked under Dr. Holly Algood and helped investigate the role of IL-17 cytokines in the stimulation of antimicrobial responses during H. pylori infection in a mouse model. She completed her residency in internal medicine at Hahnemann University Hospital/Drexel University School of Medicine and then went on to do a sleep medicine fellowship at Thomas Jefferson University Hospital. She will be starting a pulmonary critical care fellowship at Cooper University Hospital.

Holly M. Scott Algood is currently an Associate Professor of Medicine, and Pathology, Microbiology and Immunology at Vanderbilt University Medical Center. She holds a dual appointment with the Department of Veteran’s Affairs Hospital. Dr. Algood established her long-standing interest in microbe-host interactions as an undergraduate student at Mount Union College in Alliance, Ohio. It was there that she did her first experiments on the pathogenic yeast Cryptococcus neoformans. Holly completed her Ph.D. at the University of Pittsburgh in the laboratory of JoAnne Flynn working on animal models of Mycobacterium tuberculosis and the role of TNF on granuloma formation. This training piqued her curiosity and she specifically started investigating how chronic infections challenge the immune system to balance its need for immunity with its need to limit inflammation. Dr. Algood has been working on Helicobacter pylori and mucosal inflammation models since her post-doctoral fellowship began in 2004.