Effects of helicobacter pylori on tumor microenvironment and immunotherapy responses.

Helicobacter pylori is closely associated with gastric cancer. During persistent infection, Helicobacter pylori can form a microenvironment in gastric mucosa which facilitates the survival and colony formation of Helicobacter pylori. Tumor stromal cells are involved in this process, including tumor-associated macrophages, mesenchymal stem cells, cancer-associated fibroblasts, and myeloid-derived suppressor cells, and so on. The immune checkpoints are also regulated by Helicobacter pylori infection. Helicobacter pylori virulence factors can also act as immunogens or adjuvants to elicit or enhance immune responses, indicating their potential applications in vaccine development and tumor immunotherapy. This review highlights the effects of Helicobacter pylori on the immune microenvironment and its potential roles in tumor immunotherapy responses.

Introduction

Helicobacter pylori is a gram-negative, helical, microaerophilic, and flagellated bacteria that colonizes the gastric mucosa in approximately 50% of the world population (1, 2). Helicobacter pylori infection is the main cause of gastric mucosal diseases such as gastric cancer (GC), chronic non-atrophic gastritis, atrophic gastritis, intestinal metaplasia, and dysplasia (3). GC is the fifth most common cancer and the fourth leading cause of cancer-related deaths worldwide (4). H. pylori is classified by the WHO as a class I carcinogen associated with the onset of GC, as chronic H. pylori infection leads to at least 75% of GC cases (5–8). 2% of H. pylori infected patients will develop GC (7).

Tumor growth is supported by oncogene-driven metabolic activities as well as by the microenvironment. Infection with H. pylori promotes gastric tumorigenesis, mainly by influencing the microenvironment (9). Virulence factors such as cytotoxin-associated gene A (CagA), vacuolating cytotoxin A (VacA), urease (Ure), arginase (Arg), lipopolysaccharide (LPS), and neutrophil-activating protein (NAP), enable H. pylori to survive and colonize the gastric mucosa, maintain chronic inflammation, and induce malignant changes within the gastric mucosa (1, 10–12). The immune system plays a pivotal role in eliminating H. pylori infection and controlling inflammation. Throughout a long-term co-existence with human hosts, H. pylori has developed several strategies to maintain a balance between the immune response and immune escape (13, 14). Through regulating tumor stromal cells, immune checkpoints, and other regulatory factors, H. pylori constructs a microenvironment that favors persistent colonization and facilitates tumorigenesis.

However, the influence of H. pylori on responses to immunotherapies and the prognosis of GC remains controversial (15–18). Recent studies have presented that H. pylori infection might affect the curative effect of tumor therapy by the induced immuno-regulation (19, 20). Besides, H. pylori virulence factors such as NAP, VacA, and Ure might elicit or enhance immune responses, which indicates the potential application in vaccine development and tumor immunotherapy (21, 22). These virulence factors are immunodominant antigens of H. pylori and might improve patient prognosis as immunogens or adjuvants in immunotherapy (23). Here, this review describes the mechanisms and effects of H. pylori on the immune microenvironment of GC and tumor immunotherapy responses.

Effects of H. pylori on tumor stromal cells in gastric tumor immune microenvironment

The tumor microenvironment (TME) consists of a continuously evolving complex of tumor cells and stroma. Stroma comprises surrounding non-cancerous fibroblasts, epithelial, immune and blood cells, and extracellular components such as cytokines, growth factors, hormones, and extracellular matrix (ECM) (24, 25). Stroma plays a key role during tumor initiation, progression, and metastasis, meanwhile it significantly influences therapeutic responses and clinical outcomes (26). Helicobacter pylori and its virulence factors can form a microenvironment that facilitates its survival and colony formation by regulating the constituents and functions of the TME. This section summarizes the interactions between H. pylori and tumor stromal cells during GC initiation, progression, and metastasis and describes potential strategies to improve the prognosis (; ).

Figure 1

Effects of H. pylori on tumor stromal cells and tumor-related proteins in gastric tumor immune microenvironment. Arg, arginase; ASK1, apoptosis signal-regulating kinase 1; BM-MSC, Bone marrow-derived mesenchymal stem cells; CAF, cancer-associated fibroblast; Cag A, cytotoxin-associated gene A; CXCL8, chemokine (C-X-C motif) ligand 8; EMT, epithelial-mesenchymal transition; hA-MSC, human adipose-derived mesenchymal stem cells; HH, Hedgehog; HO-1, heme oxygenase-1; H.pylori, Helicobacter pylori; IL-22, Interleukin-22; IRF, interferon regulatory factor; IFN, interferon; KLF4, Krüppel-like factor 4; LPS, lipopolysaccharide; MAPK, mitogen-activated protein kinases; MDSCs, myeloid-derived suppressor cells; MET, mesenchymal-epithelial transition; MHC-II, major histocompatibility complex class II; MMP, matrix metalloproteinase; mTOR, mammalian target of rapamycin; Myh9, myosin heavy chain 9; NF-κB, nuclear factor kappa B; miR, microRNA; MSCs, mesenchymal stem cells; PD-1, programmed death 1; PD-L1, programmed death-ligand 1; PI3K-AKT, phosphatidylinositol 3 kinase-protein kinase B; ROS, reactive oxygen species; SDF, stromal-derived factor; Shh, Sonic hedgehog; SLFN4, Schlafen 4; STAT3, signal transducer and activator of transcription 3; TAMs, tumor-associated macrophages; TGFβ, transforming growth factor β; TLR, Toll-like receptor; Ure, urease; Vac A, vacuolating cytotoxin A.

Table 1

Effects of H. pylori on tumor cells in gastric tumor immune microenvironment.

Tumor cells affected by H. pylori	Roles of H. pylori		Results
TAMs	Simultaneous impairment and induction of M1 macrophage and M2 macrophage differentiation, respectively, or transdifferentiation to M2 macrophages (27)		Promotes tumor progression and invasion by inducing angiogenesis and mediating immunosuppressive signals in solid tumors
	Regulation of specific miRNAs	Downregulates miR-4270 expression (28)	Impairs MHC-II expression and exposure, decreases antigen presentation ability, favors persistent H. pylori infection
		Upregulates let-7i-5p, miR-146b-5p, miR-185-5p, and miR146b expression (29, 30)	Inhibits HLA-II expression, compromises bacterial antigen presentation to Th lymphocytes, impairs immune responses to H. pylori
	Induces production of specific enzymes	Arg2 (31, 32)	Promotes immune escape of H. pylori, mediates macrophage apoptosis,restrains inflammatory responses
		MMP7 (33)	Promotes immune escape of H. pylori
		HO-1 (34)	Reduces M1 population, increases the number of Mregs, promotes immune escape of H. pylori
		MET factor (35)	Elicits uncontrolled activation of macrophages and inflammationinvolved in tumorigenesis and cancer development
	Regulation of some signaling pathway molecules	Upregulation of Jagged 1 expression (36)	Increases secretion of proinflammatory mediators and phagocytosis,decreases bacterial load,confers anti-bacterial activity on macrophages
		Induces SHH release from the stomach (37)	Induces macrophage migration during early H. pylori infection, involved in gastric immune response
MSCs	Upregulates CXCR4 expression and enhances MSCs migration toward SDF-1 (38)		Enhances BM-MSC migration into gastric tissues
	Recruits or induces BM-MSCs and hA-MSCs	Promotes malignant transformation (39–42)	Promotes H. pylori-mediated gastric tumorigenesis and development
		Mediates local and systemic immunosuppression (43, 44)
		Alters THBS expression (45, 46)
CAFs	Induces MSC differentiation into CAFs	Enhances expression of fibroblast markers, CAF activation, and levels of aggression/invasion markers (47, 48)	Promotes survival, proliferation, and migration of GC cell lines, inhibits antitumor functions of T cells in GC TME
	Stimulates BM-MSC differentiation into CAF myofibroblasts	Increases HDGF expression (49)	Enhances tumor cell ability to proliferate, invade, and metastasize (49, 50)
	Induces fibroblast transdifferentiation into myofibroblasts	Upregulates and downregulates HIF-1α and Bax expression, respectively (51)	Promotes gastric tumorigenesis
	Propels EMT via signal pathways and TGF‐β secretion	Induces activation or differentiation of rat gastric fibroblasts by NF-κB and STAT3 signaling (52)	Induces Snail1 expression and propels EMT leading to GC progression
		Secretes TGFβ1 and regulates TGFβR1/R2-dependent signaling in H. pylori-activated gastric fibroblasts (53–55)	Prompts reprogramming normal gastric epithelial cells towards a precancerous phenotype and promotes EMT in normal epithelial cells
MDSCs	Induces differentiation of SLFN4+ MDSCs	HH/Gli1 (56, 57)	Inhibits gastric inflammatory response by H. pylori, suppresses T cell function, immune dysregulation, and tumor progression
		TLR9-MyD88-IRF7- IFN-α pathway (58)
	Interaction between H. pylori and MDSCs is regulated by several factors	MiR130b (59)	Activates SLFN4+ MDSCs and promotes H. pylori-induced metaplasia
		ASK1 (25, 60)	Suppresses inflammation induced by infiltrating immature MDSCs
		IL-22 (61)	Induces expression of proinflammatory proteins, suppresses Th1 cell responses, promotes development of H. pylori-associated gastritis
		PD-L1 (62–64)	Promotes tumor infiltration of MDSCs, mediates resistance to anti-PD-1/PD-L1 therapy
		KLF-4 (65–67)	Promotes recruitment of MDSCs to tumors, creates immunosuppressive microenvironment, promotes tumor growth

Effects of H. pylori on tumor-associated macrophages in gastric tumor immune microenvironment

Changes in immune responses and the immune escape of H. pylori are closely associated with tumor-associated macrophages (TAMs), which are emerging key players in the TME. Macrophages play crucial roles in host defense against bacterial infections and in the regulation of immune responses during H. pylori infection (68). However, macrophages can also induce angiogenesis and suppress the host immune response during cancer development (37, 69). Generally, TAMs comprise M1 and M2 subtypes (27). Proinflammatory activated M1 macrophages promote the type I T helper (Th1) immune response by producing type I proinflammatory cytokines such as IL-1β, IL-1α, and IL-6 to clear pathogens and inhibit tumor progression, while simultaneously suppressing Th2-type responses (27, 70, 71). Activated M2 macrophages contribute to production of ECM and anti-inflammatory effectors such as IL-4 and IL-10 that are involved in the Th2 immune response, promotion of wound healing, and suppression of Th1 responses (72–75). Additionally, a third type called regulatory macrophages (Mregs) secrete abundant IL-10 that limits inflammation but do not secrete ECM (72). Helicobacter pylori and other pathogens might impair M1 macrophage differentiation while inducing M2 macrophage differentiation or M1 transdifferentiation into M2 macrophages, which can promote tumor progression and invasion by inducing angiogenesis and mediating immunosuppressive signals in solid tumors (27).

Furthermore, H. pylori infection might regulate specific microRNAs (miRNAs) to control macrophage function and affect the TME (28, 76). Infection with H. pylori leads to the downregulated expression of miR-4270 by human monocyte-derived macrophages. This favors upregulation of expression of CD300E immune receptors that enhance the proinflammatory potential of macrophages. However, the expression and exposure of major histocompatibility complex class II (MHC-II) molecules on the plasma membrane are simultaneously compromised. Hence, antigen presentation ability is decreased, leading to persistent H. pylori infection (28). The upregulation of let-7i-5p, miR-146b-5p and miR-185-5p, and miR146b expression in macrophages caused by H. pylori infection can similarly decrease HLA-II expression on the plasma membrane, which ultimately compromises bacterial antigen presentation to Th lymphocytes and impairs immune responses against H. pylori (29, 30). Collectively, H. pylori infection mainly downregulates surface recognition factors at the transcriptional level by rendering macrophages fail to degrade the bacteria. Thus, macrophages become a protective niche for H. pylori.

Helicobacter pylori can induce the production of specific enzymes that regulate macrophage function and affect TME. The production of arginase II (Arg2) in macrophages induced by H. pylori infection results in cell apoptosis and restrained proinflammatory cytokine responses, thus promotes H. pylori immune evasion (31, 32). Matrix metalloproteinase 7 (MMP7) plays a pivotal role in H. pylori-mediated immune escape (33). Heme oxygenase-1 (HO-1) expression in macrophages also be induced, resulting in a polarization switch towards a reduction in the M1 population and an increase in the Mreg profile, causing innate and adaptive immune responses failure (34). Transfer exosomes expressing mesenchymal–epithelial transition (MET) factor, a cell-surface receptor tyrosine kinase from H. pylori‐infected GC cells, can elicit uncontrolled macrophage activation and downstream inflammation and might be associated with tumorigenesis and cancer development (35). These findings shed light on how H. pylori influences the gastric microenvironment by inducing the expression of macrophage-associated enzymes in TAMs.

Moreover, H. pylori upregulates the expression of Jagged 1, a ligand of Notch signaling that plays an important role in M1 macrophage activation and bactericidal activity to prevent H. pylori infection. Upregulated Jagged 1 expression induces an increase in the expression of proinflammatory mediators and phagocytosis and a decrease in the bacterial load, which together impart antibacterial activity in macrophages (36). The hedgehog (HH) signaling pathway also plays an important role in the gastric TME. Sonic hedgehog (SHH) induced by H. pylori infection acts as a macrophage chemoattractant, which is a prerequisite in the gastric immune response (37).

In conclusion, H. pylori infection at the early stage can induce the infiltration of polymorphonuclear leukocytes and mononuclear phagocytes in the gastric mucosa as an innate immune response (77). During the advanced stages of GC, H. pylori can escape immune surveillance by impairing the antigen presentation of TAMs or by disrupting the M1/M2 (or Mreg) balance in favor of an M2 (or Mreg) phenotype (34, 72). Immunosuppressive status eventually promotes tumorigenesis and cancer development (78). These mechanisms also provide the potential for investigating novel targeted drugs (79). Specific miRNAs such as let-7i-5p, miR-146b-5p, and miR-185-5p can be targeted to reduce adverse effects on macrophage antigen presentation (29). Targeting specific enzymes including MMP7 and HO-1 or signaling pathways, such as Notch and HH, to regulate the M1/M2 (or Mreg) balance might also warrant investigation (33, 34).

Effects of H. pylori on recruiting and inducing bone marrow-derived mesenchymal stem cells in gastric tumor immune microenvironment

Multipotent mesenchymal stem cells (MSCs) can self-renew and differentiate into various cell types that play key roles in tissue healing, regeneration, and immune regulation (80). Bone marrow-derived mesenchymal stem cells (BM-MSCs) might play important roles in H. pylori-associated gastric tumorigenesis and immunosuppression. Upon sensing signals indicating gastric mucosa damage, BM-MSCs migrate from bone marrow to stomach via the peripheral circulation. BM-MSCs heal damaged mucosa through a paracrine mechanism and directed differentiation (81, 82). H. pylori-induced persistent inflammation is required for BM-MSC migration and tumorigenesis (43, 83). Upregulated C-X-C chemokine receptor type 4 (CXCR4) interacts with its ligand, stromal-derived factor (SDF-1) and then promote BM-MSC migration to the gastric tissues (38).

Gastric epithelial glands become repopulated with BM-MSCs in mice model one year after H. pylori infection (39). After recruitment to stomach, BM-MSCs can become entrapped in a microenvironment containing H. pylori and malignant cells, 25% of which originate from BM-MSCs. Fusion with epithelial cells might render BM-MSCs more susceptible to malignant transformation or lead to the promotion of cancerous processes (40). BM-MSCs gradually acquire a clonal advantage and undergo stepwise transformation to malignant cells (39). During malignant progression, gastric epithelial glandular units undergo monoclonal transformation, resulting in emerging cancer stem cell (CSC) clones and adenocarcinomas (39, 41). Human adipose-derived mesenchymal stem cells (hA-MSCs) also participate in gastric tumorigenesis by increasing tumor cells invasion and metastasis during H. pylori infection (42).

In addition to malignant transformation, MSCs can promote tumorigenesis locally and systemically by compromising cancer immune surveillance or altering tumor stroma. When transplanting BM-MSCs in H. pylori infected mice model, IL-10 and transforming growth factor-β1 (TGF-β1) can be increased, as well as T cells secreting IL-10 and CD4+ CD25+ Foxp3+ regulatory T (Treg) cells in splenic mononuclear cells (43, 44). BM-MSCs can reduce the fraction of T cells that produce IFN-γ, thus inhibiting CD4+ and CD8+ T cell proliferation. Local and systemic immunosuppression mediated by BM-MSCs contributes to GC development induced by H. pylori (43).

MSCs can also promote tumorigenesis by altering tumor stromal components. Thrombospondin (THBS) promotes tumorigenesis through crosstalk with BM-MSCs. Infection with H. pylori significantly upregulates the expression of THBS4 in BM-MSCs. Overexpressed THBS4 then mediates BM-MSC-induced angiogenesis in GC by activating the THBS4/integrin α2/PI3K/AKT pathway (45). Moreover, BM-MSCs can differentiate into pan-cytokeratin-positive (pan-CK+) epithelial cells and alpha-smooth muscle actin (α-SMA+) cancer-associated fibroblasts (CAFs) by secreting THBS2, thus promoting the development of H. pylori-associated GC (46).

BM-MSCs play pivotal roles in H. pylori-associated GC. The immune regulatory functions of MSCs remain obscure. Shedding light on these functions and their mechanisms will provide clues on therapeutic targets for preventing GC development.

Effects of H. pylori on induction of cancer-associated fibroblasts in gastric tumor immune microenvironment

CAFs are activated myofibroblasts that accompany solid tumors and are principal constituents of tumor stroma (84, 85). They play important roles in the TME. They can create a niche for cancer cells and promote cancer progression by stimulating cancer cell proliferation, migration, invasion, and angiogenesis (85–87). Proinflammatory and tumor-associated factors secreted by CAFs might induce persistent inflammation or intervene in tumor immunity, thus mediate tumor immune escape (52, 88). Mainly derived from MSCs, CAFs could induce epithelial-mesenchymal transition (EMT), which enhances the invasive properties of malignant cells (89, 90) that detach from primary tumor site to surrounding tissues (91).

Helicobacter pylori infection can induce MSCs differentiating into CAFs, and upregulate the expression of fibroblast markers, fibroblast activation protein (FAP), CAF activation markers, and aggressive/invasive markers (47). FAP-positive CAFs enhance the survival, proliferation, and migration of GC cell lines and inhibit T cells function (48). H. pylori infection also increases the expression of hepatoma-derived growth factor (HDGF) (49, 50). Exposure to HDGF promotes the recruitment of BM-MSCs, stimulates their differentiation into CAF-myofibroblasts, and enhances tumor cell proliferation, invasiveness, and metastasis (49). Moreover, H. pylori infection can induce fibroblasts transdifferentiating into myofibroblasts, which upregulating the early carcinogenic marker hypoxia-inducible factor 1-alpha (HIF-1α) and downregulating proapoptotic bcl-2-like protein 4 (Bax) expression (51).

CAFs induced by H. pylori propel EMT by nuclear factor kappa B (NF-κB), signal transducer and activator of transcription 3 (STAT3), and TGF-β. Helicobacter pylori might induce the activation or differentiation of rat gastric fibroblasts in vitro, which then activate NF-κB and STAT3 signaling, and upregulate Snail1. This is an EMT-inducing transcription factor (EMT-TF) (52). As a major propeller of EMT in cancer progression and metastasis (53, 54), TGF-β can initiate tumorigenesis by activating EMT-type III initiation in epithelial cell compartments at the early stage of cancer development (55, 92). Gastric fibroblasts activated by H. pylori promote normal gastric epithelial cells to precancerous phenotype, and promote EMT by regulating TGFβ R1/R2-dependent signaling (55). The HH, Wnt, and Notch signaling pathways can interact with TGF-β pathway and induce EMT progression (93–97).

Collectively, persistent H. pylori infection increases the differentiation of CAFs, which propel EMT through NF-κB, STAT3, and TGF-β. As CAFs play key roles in the gastric microenvironment, targeting CAFs might be a potential strategy to improve the prognosis of patients (98, 99).

Effects of H. pylori on myeloid-derived suppressor cells in gastric tumor immune microenvironment

Immature myeloid (progenitor) cells (IMCs) do not mediate immunosuppression in healthy individuals. However, chronic inflammation, infections, and autoimmune diseases impair IMC differentiation and decrease peripheral myeloid cells numbers, resulting in more myelopoiesis (100–103). This eventually results in myeloid-derived suppressor cells (MDSCs) accumulation and immunosuppression (102, 104). MDSCs mediate immune suppression by inducing immunosuppressive cells (105), blocking lymphocyte homing (106), producing reactive oxygen and nitrogen species (107, 108), exhausting critical metabolites for T cell function (109), expressing negative immune checkpoint molecules (110).

Interactions between H. pylori and MDSCs are important in gastric immune microenvironment. On one hand, H. pylori can induce the differentiation of myeloid cell differentiation factor Schlafen 4 (SLFN4+) MDSCs (56, 58). This factor marks a subset of MDSCs in the stomach during H. pylori-induced spasmolytic polypeptide-expressing metaplasia (SPEM) (57). During chronic H. pylori infection in mice model, a subset of HH-Gli1-dependent immune cells is recruited to the gastric epithelium, and polarizes into SLFN4+ MDSCs. Overexpression of the SHH ligand in infected WT mice accelerates SLFN4+ MDSCs differentiataion in gastric corpus (57). Furthermore, H. pylori can stimulate plasmacytoid dendritic cells to secrete IFN-α through toll-like receptor 9-myeloid differentiation factor 88-interferon regulatory factor 7 (TLR9-MyD88-IRF7 pathway) (58). Differentiated SLFN4+ MDSCs inhibit gastric inflammatory response induced by H. pylori and suppress T cell function (56–59). Persistent immune dysregulation then favors intestinal metaplasia and neoplastic transformation, which leads to immune disorders and tumor progression.

Several markers, such as MiR130b, apoptosis signal-regulating kinase 1 (ASK1), interleukin 22 (IL-22), programmed death-ligand 1 (PD-L1), and Krüppel-like factor 4 (KLF4) play regulatory roles in the interactions between H. pylori and MDSCs. MiR130b produced by SLFN4+ MDSCs suppress T cells function and promote H. pylori-induced metaplasia (59). ASK1 deficiency promotes a Th1-dependent immune response and recruits immature Gr-1+Cd11b+ MDSCs with H. pylori infection. This could lead to the development of gastric atrophy and metaplasia (25, 60). Moreover, IL-22 secreted by polarized Th22 cells induced by H. pylori can stimulate CXCL2 production from gastric epithelial cells. This causes CXCR2+ MDSCs migration to gastric mucosa, where they produce proinflammatory proteins and suppress Th1 cell responses, contributing to the development of H. pylori-associated gastritis (61). PD-L1 upregulation on the surface of gastric epithelial cells at the early stage of H. pylori infection (62) promotes tumor infiltration of MDSCs (63) and then lead to anti-PD-1/PD-L1 treatment resistance (64). KLF4 is an evolutionarily conserved zinc finger transcription factor and key regulator of diverse cellular processes (111–113). Helicobacter pylori and its virulence factor CagA can influence KLF4 expression. The transduction of CagA or infection with H. pylori downregulates KLF4 expression by inducing CXCL8 expression, and low KLF4 expression further upregulates CXCL8 expression (65). Increased CXCL8 expression promotes MDSCs recruitment to tumors as well as tumor growth, and creates an immunosuppressive microenvironment conducive to resistance against immune response (65–67).

A high abundance of MDSCs in patients correlate with more advanced GC and a poor prognosis (114, 115). MDSCs infiltration induced by H. pylori mediates immunosuppression, immune dysfunction, gastric tumorigenesis, and reduces the effect of chemotherapy and immunotherapy (63). The possibility that combining immunotherapy or chemotherapy with MDSC-targeting therapy might overcome drug resistance and improve prognosis warrants investigation (116–118).

Effects of H. pylori on PD-1/PD-L1 in gastric tumor immune microenvironment

In addition to cells in TME, immune checkpoints are involved in regulating H. pylori-associated TME. ().

Table 2

Effects of H. pylori on tumor-related proteins in gastric tumor immune microenvironment.

Tumor-related proteins affected by H. pylori	Roles of H. pylori	Results
PD-1/PD-L1	Upregulates PD-1/PD-L1 expression (119–121)	Reduces excessive damage induced by H. pylori, reduces T cell-mediated cytotoxicity, promotes GC progression
	Upregulates PD-L1 expression by H. pylori CagA through the SHH pathway (62)	Inhibits T cell proliferation and Treg cell induction from naïve T cells, increases immune escape, promotes GC progression
	Upregulates PD-L1 expression by mTOR-GLI signaling (64)
	Upregulates PD-L1 expression by the p38 MAPK pathway (122, 123)
	Upregulates PD-L1 expression by H. pylori urease subunit through the Myh9/mTORC1 pathway (124)
	Upregulates PD-L1 expression by H. pylori LPS through the NF-κB pathway (125)

The 55 kDa transmembrane protein programmed death 1 (PD-1) is expressed in activated T cells, natural killer (NK) cells, B lymphocytes, macrophages, dendritic cells (DCs), and monocytes. It is abundantly expressed in tumor-specific T cells (126–128). PD-L1 (also known as CD274 or B7-H1) is a 33 kDa type 1 transmembrane glycoprotein that is widely expressed in macrophages, activated T lymphocytes, B cells, DCs, and also expressed in tumor cells (129). Binding of PD-1 and PD-L1 enhances T cell tolerance, inhibits T cell activation and proliferation, increases Th cell transformation to Foxp3+ Treg cell, and prevents T cell cytolysis in tumor cells (130). Thus, interaction between PD-1 and PD-L1 is a double-edged sword. It can inhibit immune responses and promote self-tolerance, while it can also lead to immune escape and tumor progression.

Helicobacter pylori infection could upregulate PD-1/PD-L1 expression in gastric ulcers and GC patients (119), which might be related with poor prognosis (131, 132). Chronic H. pylori infection could cause excessive damage to gastric mucosa. Upregulated PD-1/PD-L1 is launched to avoid such damage, meanwhile this also reduces T cell-mediated cytotoxicity and promotes GC progression (119–121). SHH pathway is involved in PD-L1 upregulating (62). As an HH transcriptional effector, zinc finger protein GL1, mediates mammalian target of rapamycin (mTOR)-induced PD-L1 expression in GC organoids (64). Kinds of H. pylori virulence factors are reported in this process. H. pylori T4SS components activate p38 MAPK pathway and upregulate PD-L1 expression, thus inhibiting T cell proliferation and inducing Treg differentiation from naïve T cells, which lead to immune escape (122, 123). Helicobacter pylori urease B subunit mediates PD-L1 upregulation via myosin heavy chain 9 (Myh9) or mTORC1 signaling in bone marrow-derived macrophages (BMDMs) and, and regulates CD8+ T cells infiltration and activation (124). Helicobacter pylori LPS induces PD-L1 expression via NF‐κB pathway in GC cells and eventually promotes GC progression (125).

Overall, PD-1/PD-L1 play vital roles in H. pylori-infected GC, which presents an opportunity and challenge for treatment. However, numerous unknown mechanisms of PD-1/PD-L1 expression might be the basis for overcoming drug resistance and developing novel immunotherapies (133). The mechanisms and functions of PD1/PD-L1 with H. pylori infection requires further investigation (132, 134–136).

Immunotherapy stimulates the immune system against neoplasms and harnesses the specificity of innate immune to fight cancer, particularly by activating T-cell mediated immunity (137, 138). With the wide application of immune therapy, the immune checkpoint inhibitors (ICIs) targeting immune checkpoint molecules such as PD-1 and CTLA-4, and other immune therapies such as cancer vaccine, the immune cells input, antigen vaccine, oncolytic viruses, and recombinant cytokines, have been receiving worldwide attention and have made a certain progress (139–147). However, as lack of optimal criteria selecting suitable patients until now, the objective response rate of immunotherapy remains low (148, 149). Hence, factors that influence the effectiveness of tumor immunotherapy need to be identified. In this section, we focused on the effects and potential applications of H. pylori infection on tumor immunotherapies (; ).

Figure 2

Effects and applications of H. pylori and its factors in tumor immunotherapies. Bab A, blood-group antigen-binding adhesin gene A; Cag A, cytotoxin-associated gene A; Chi-rNap, rNAP coated chitosan nanoparticles; DCs, dendritic cells; DLBCL, diffuse large B-cell lymphoma; HP-NAP, H. pylori neutrophil-activating protein; MDSCs, myeloid-derived suppressor cells; MV-NAP-uPAR, recombinant measles virus-NAP-urokinase-type plasminogen activator receptor; NSCLC, non-small cell lung cancer; OVs, oncolytic viruses; PD-L1, programmed death-ligand 1; rHP-NAP, recombinant H. pylori neutrophil-activating protein; rMBP-NAP, recombinant HP-NAP with the maltose-binding protein of Escherichia coli; Th cells, T helper cells; TIL-T cells, tumor-infiltrating T lymphocytes; TME, tumor microenvironment; Vac A, vacuolating cytotoxin A; VV-GD2m-NAP, vaccinia virus - neuroblastoma-associated antigen disialoganglioside mimotope.

Table 3

Effects of H. pylori on tumor immunotherapy responses.

Cancer targeted by immunotherapy affected by H. pylori	Roles of H. pylori		Effects and applications
Gastric cancer	Induces PD-L1 expression and MDSC infiltration (62–64, 150)		Mediates immune escape by cancer cells, causing resistance to immunotherapy
	Enhances tumor immunity by virulence factors	CagA, VacA and BabA	Increases levels of CagA, VacA, and BabA autoantibodies, enhances antigen processing and presentation and T-cell activation and proliferation, and improves host immune status (151)
			DNA vaccine from CagA, VacA and BabA induces a shift from Th1 to Th2 response and activates CD3+ T cells to inhibit GC xenograft growth in vivo (152)
		HP-NAP	HP-NAP promotes maturation of DCs and stimulates neutrophils and monocytes to enhance antigen-specific T cell responses (153)
			Oral NapA vaccination promotes Th17 and Th1 polarization, exerts anti-H. pylori and antitumor effects, enhances immune responses (154)
Non-small cell lung carcinoma	Decreases immune responses, inhibits antitumoral CD8+ T cell responses (19)		Partially blocks the activity of ICIs and vaccine-based cancer immunotherapy
DLBCL	Causes increased numbers of tumor-infiltrating T lymphocytes and persistent activation of autoimmune Th cells (155)		Results in a benign tumor immune microenvironment
Mouse subcutaneous hepatoma and sarcoma	rMBP-NAP promotes Th1 differentiation and increases the number of CD4+ IFN-γ-secreting cells (156)		rMBP-NAP has antitumor potential
Lung cancer	rMBP-NAP increases the number of IFN−γ-secreting cells and CTL activity of PBMCs (157)
Mouse metastatic lung cancer	rMBP-NAP restricts tumor progression by triggering antitumor immunity (158)
Mouse breastand bladder cancers	HP-NAP enhances immune response and inhibits tumor growth (137, 159)		HP-NAP has antitumor potential
Melanoma	rHP-NAP promotes the maturation of dendritic cells in dendritic cell-based vaccines (160)		rHP-NAP has potential as an adjuvant
Mouse neuroendocrine tumor	HP-NAP improves median survival (161)		HP-NAP is a powerful source of immune-stimulatory agonists that can boost OV immunogenicity and enhance ICI effects (162, 163)
Mouse subcutaneous neuroblastoma	HP-NAP enhances antitumor efficacy of oncolytic vaccinia virus (164, 165)
Glioblastoma	MVs-NAP-uPAR improves tumor immunotherapy efficacy (163)

Effects and applications of H. pylori and its factors on GC immunotherapy

The 5-year survival rate of advanced GC patients is <30%. Although platinum-fluoropyrimidine combination chemotherapy is the standard first-line treatment for advanced GC, its low complete response rate and severe adverse reactions have limited its application (63, 166). Novel effective therapies are urgently required. For example, PD-1 inhibitor pembrolizumab received accelerated approval from the US Food and Drug Administration (FDA) in 2017 to treat recurrent advanced or metastatic gastric or gastroesophageal junction adenocarcinomas expressing PD-L1 (63, 167–169).

Helicobacter pylori is a class I carcinogen associated with GC (170–172). The overall survival of GC diagnosis is reported to be higher for patients with H. pylori infection (17). Helicobacter pylori infection induces PD-L1 expression and MDSC infiltration that mediate immune escape. HH signaling activated by H. pylori infection induces PD-L1 expression and tumor cell proliferation in GC, resulting in cancer cell resistance to immunotherapy (150). In addition, Helicobacter pylori and its virulence factors can act as antigens or adjuvants to enhance tumor immunity.

Helicobacter pylori virulence factors, such as CagA, VacA, blood-group antigen-binding adhesin gene (BabA), and H. pylori neutrophil-activating protein (HP-NAP), can act as antigens or adjuvants to enhance tumor immunity. The stimulation of autoantibodies during antigen processing and presentation and subsequent T-cell activation and proliferation improves the host immune status, which can kill cancer cells and even suppress metastasis (151). Moreover, H. pylori DNA vaccines encoding fragments of CagA, VacA, and BabA can induce Th1 shift to Th2 response in immunized BALB/c mice, which mimics the immune status of GC patients with chronic H. pylori infection. Stimulated CD3+ T cells inhibit the proliferation of human GC cells in vitro, and the adoptive infusion of CD3+ T cells inhibits the growth of GC xenografts in vivo (152).

HP-NAP is a major virulence factor in H. pylori infection and colony formation, and it can also act as a protective factor (173, 174). As a Toll-like receptor-2 (TLR2) agonist, HP-NAP can bind to TLR2 of neutrophils (161, 175). Furthermore, HP-NAP promotes the maturation of DCs with Th1 polarization and improves migration of mature DCs. Stimulating neutrophils and monocytes by HP-NAP induces IL-12 and IL-23 expression, thus shifting antigen-specific T cell responses from the Th2 to the Th1 phenotype which characterized by abundant IFN-γ and TNF-α expression (153). Vaccination with HP-NAP A subunit (NapA) promotes Th17 and Th1 polarization. Such vaccines have potential effects as an anti-H. pylori oral vaccine candidate and a mucosal immunomodulatory agent, which could be used in antitumor strategies (154).

Effects and applications of H. pylori and its factors in other tumor immunotherapies

In addition to GC, the influence of H. pylori on other tumor immunotherapies is also paid much attention recently. Helicobacter pylori infection might disrupt the immune system and exert detrimental effects on the outcomes of cancer immunotherapies (19).

Helicobacter pylori seropositivity could reduce anti-PD-1 immunotherapy effect in non-small cell lung cancer (NSCLC) patients. Helicobacter pylori infection partially blocks the activities of ICIs and vaccine-based cancer immunotherapies. Helicobacter pylori suppresses the innate and adaptive immune responses of infected hosts and inhibits antitumor CD8+ T cell responses by altering the cross-presentation activity of DCs (19). In contrast, a significantly high proportion of tumor-infiltrating T lymphocytes in H. pylori-positive de novo diffuse large B-cell lymphoma (DLBCL) patients preliminarily indicates a benign TME. Inflammation induced by H. pylori confers persistent activation of autoimmune Th cells, which would explain the benign TME (155). More researches are necessary to elucidate how H. pylori infection status influences the effects of tumor immunotherapies.

The immunomodulatory activity and potential applications of NAP in tumor immunotherapy have been investigated. Recombinant HP-NAP with the maltose-binding protein of Escherichia coli (rMBP-NAP) can mediate T helper lymphocytes differentiation into the Th1 phenotype and significantly increase the number of CD4+ IFN-γ-secreting T cells. This induces antitumor effects through a TLR-2-dependent mechanism in subcutaneous hepatoma and sarcoma mice model (156). rMBP-NAP can significantly increase peripheral blood mononuclear cells (PBMCs) that secrete IFN-γ, and prominently increases the cytotoxic activity of PBMCs derived from lung cancer patients (157). Treatment with rMBP-NAP restricts the progression of metastatic lung cancer in mice model by triggering antitumor immunity (158). A therapeutic nanocomplex of HP-NAP altered the production rate of cytokines and increase tumoricidal activities of the immune system, leading to decreased breast tumor growth in mice (137). Local administration of HP-NAP inhibits tumor growth by triggering tumor cell necrosis in bladder cancer mice model (159). Recombinant HP-NAP has potential effects as an adjuvant in DC-based vaccines for treating melanoma (160).

Because of its ideal immunogenicity, NAP has recently been applied as an immune adjuvant to enhance the antitumor immune response. When combined with oncolytic viruses (OVs), HP-NAP can activate the immune response. The intratumoral administration of adenovirus armed with secretory HP-NAP can improve the median survival rate of nude mice xenografted with neuroendocrine tumors (161). A recombinant vaccinia virus (VV) neuroblastoma-associated antigen disialoganglioside mimotope (GD2m)-NAP significantly improved therapeutic efficacy. Helicobacter pylori-NAP might help to overcome virus-mediated suppressive immune responses, resulting in improved anti-GD2 antibody production and a better therapeutic outcome (164, 165). Moreover, recombinant measles virus (MV)-NAP-urokinase-type plasminogen activator receptor (uPAR) can improve immunotherapeutic effects on glioblastoma with a better tumor prognosis and increased susceptibility to CD8+ T cell-mediated lysis. Overall, HP-NAP represents a potential immunostimulatory agonists which can boost the immunogenicity of OVs and enhance ICIs effects (162, 163).

In conclusion, H. pylori and its virulence factors could be closely related with personalized treatment strategies during tumor immunotherapies. The mechanisms of H. pylori infection in tumor immunotherapies requires further elucidation, and the translation of research findings to clinical applications should be accelerated.

Summary

This review summarized current knowledge of the effects of H. pylori on the immune microenvironment of GC and tumor immunotherapy responses. Helicobacter pylori elicits powerful immune responses during surviving and colonizing gastric mucosa. Helicobacter pylori has also developed several strategies to evade recognition and disrupt immune function. The constituents and functions of stroma are regulated by H. pylori and its virulence factors to facilitate its survival and colony. Persistent H. pylori infection can induce immune evasion and tumorigenesis.

The stroma provides TME for tumor initiation and development after H. pylori persistent infection. Immunotherapy targeting tumor-associated immune cells is more mature and improved, particularly immunotherapy targeting T cells, such as ICIs. PD-1 inhibitor pembrolizumab has received approval from the US FDA in 2017 to treat recurrent advanced or metastatic gastric or gastroesophageal junction adenocarcinomas (167). While some clinical trials targeting non-immune cells in TME such as CAFs, MSCs, have failed to show promising efficacy in cancer patients (176–178). The main reason might be a lack of deep understanding of the fundamental mechanisms of stromal cells and elements as well as a lack of reliable biomarkers to guide stroma-targeted therapies (176). Of course, because of the important roles of regulating the immune response in TME, targeting TAMs is getting more and more attraction. For example, targeting colony-stimulating factor 1 receptor (CSF1R) signaling and the CCL2-CCR2 axis are developing drugs (179, 180). And there are some developing drugs to reprogram TAMs from a pro-tumor phenotype to an anti-tumor phenotype and interrupt the bad cycle between TAMs and tumor cells (176, 177), such as agonistic anti-CD40 antibodies (181), PI3Kγ inhibitors (182). These ongoing researches show good prospects in immunotherapy. Based on these, it seems that immunotherapy intervening tumor-associated immune cells may be more appropriate currently. However, we should also pay attention to the study of non-immune cells in TME. Further research on these cells may provide clues for developing new therapies in the future.

H. pylori infection might affect the tumor immunotherapy. Although H. pylori infection has been reported as a protective factor in GC immunotherapy while in NSCLC as a negative factor, the mechanisms and effect of H. pylori on GC immunotherapy still remains unclear (19, 183). Helicobacter pylori virulence factors can act as immunogens or adjuvants to elicit or enhance immune responses. Some H. pylori virulence factors such as HP-NAP, have been applied as adjuvants or combined with drugs in pan-tumor treatment to improve immunotherapeutic efficiency. The effects of H. pylori in TME should be further explored, and clinical applications should be performed to select the proper features of population for better immunotherapy benefits.

Author contributions

RD and HZ searched the literature and wrote the manuscript. HC and ML re-checked the literature. YS and SD designed this study and revised the manuscript. All authors contributed to the article and approved the submitted version.

Funding

This study was funded by the National Natural Science Foundation of China (Grant No. 81700496 and 81870386), Peking University Medicine Fund of Fostering Young Scholars’ Scientific and Technological Innovation (BMU2021PY002), and Key laboratory for Helicobacter pylori infection and upper gastrointestinal diseases, Beijing Key Laboratory (No.BZ0371).

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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