Role of γδT cells in liver diseases and its relationship with intestinal microbiota.

γδT cells are unconventional T lymphocytes that bridge innate and adaptive immunity. Based on the composition of T cell receptor and the cytokines produced, γδT cells can be divided into diverse subsets that may be present at different locations, including the liver, epithelial layer of the gut, the dermis and so on. Many of these cells perform specific functions in liver diseases, such as viral hepatitis, autoimmune liver diseases, non-alcoholic fatty liver disease, liver cirrhosis and liver cancers. In this review, we discuss the distribution, subsets, functions of γδT cells and the relationship between the microbiota and γδT cells in common hepatic diseases. As γδT cells have been used to cure hematological and solid tumors, we are interested in γδT cell-based immunotherapies to treat liver diseases. ©The Author(s) 2020. Published by Baishideng Publishing Group Inc. All rights reserved.

Core tip: γδT cells are unconventional T lymphocytes that bridge innate and adaptive immunity. These cells are enriched in epithelial layers of different tissues and the peripheral blood. They have been used in hematological and solid tumors for immunotherapies. The effects of γδT cells in liver diseases depend on subsets, mechanisms and different stages of diseases. We herein discuss the distribution, subsets, functions of γδT cells and the relationship between the microbiota and γδT cells in common hepatic diseases.

INTRODUCTION

The liver receives approximately 1.5 L of blood from the portal vein and hepatic artery[1]. The liver is the major location of immune responses, metabolic activities and the clearance of toxins[2]. It acts as a sentinel, guarding against pathogen components such as lipopolysaccharide from the gut, and is enriched in innate immune cells, including Kupffer cells, natural killer (NK) cells and natural killer T (NKT) cells and adaptive immune cells, such as T lymphocytes, as well as the cytokines they secrete[3]. In dysregulated hepatic immune conditions, patients die from infections that occur in end-stage liver diseases, such as acute and chronic liver failure (ACLF), liver cirrhosis and liver cancer. Moreover, tumor cells escape immune surveillance, leading to metastasis. γδT cells account for a tiny proportion of immune cells in peripheral blood; however, they are abundant in the human liver. These cells are both protective and pathogenic in different liver diseases[4]. In recent years, γδT cells have attracted increasing attention. In this review, we will explore the properties and roles of γδT cells in the peripheral blood and liver in diverse hepatic diseases (Table 1).

Table 1

The roles of γδT cells in liver diseases

Liver diseases	Mechanisms of γδT cells	Functions
Viral hepatitis	Production of IFN-γ, TNF-α; Expression of CD107a; Downregulation of IL-17, IL-22	Amelioration
	Exhaustion of CD8+T cells; Production of IL-17 and TNF-α	Aggravation
Liver bacterial, fungal and parasitic infections	Production of IL-9, IL-17, TNF-α	Protection
Autoimmune liver diseases	Production of IL-10 and granzymes; Expression of CD39, CD73, NKG2D; Downregulation of NKT cells	Amelioration
	Expression of IL-17; Apoptosis of Tregs	Aggravation
Nonalcoholic fatty liver disease	Expression of IL-17	Aggravation
Liver fibrosis and cirrhosis	Expression of IL-17	Aggravation
	Apoptosis of HSC	Amelioration
Liver tumors	Expression of Ia and LFA-1, CD56 and CD161; Production of IFN-γ, perforin	Anti-tumor

IFN-γ: Interferon-γ; TNF-α: Tumor necrosis factor-α; IL-17: Interleukin-17; NKT: Natural killer T; HSC: Hepatic stellate cells.

γδT CELLS

γδT cells develop before other T cells in all vertebrates. They serve as neonatal protectors when the functions of αβT cells are impaired and act as antigen presenting cells[5]. γδT cells are 15%-25% of the T lymphocyte population and 3%-5% of total lymphocytes in the liver; however, the highest frequency of γδT cells is seen in the gut mucosa[6]. The frequency of γδT cells in the liver outnumbers that in the peripheral blood[1]. γδT cells are unconventional T lymphocytes with unique properties that bridge the innate and adaptive immunity. They express γδT cell receptor (TCR) and do not require antigen presentation with the help of major histocompatibility complex[7]. These cells recognize major histocompatibility complex class I chain-related antigens A and B (MICA and MICB) and nonpeptide metabolites of isoprenoid biosynthesis[7]. γδT cells are also activated by cytokines without TCR stimulation, which allows them to act earlier than αβT cells.

Human γδT cells can be divided into three groups according to their δ chain expression[7]. Approximately 56.4% of hepatic γδT cells express Vδ2+ chains, whereas approximately 8.9% of hepatic γδT cells express Vδ1+ chains[8]. The Vδ1+ chain usually combines with Vγ2,γ3,γ4,γ5 and γ8 chains[9], and the abovementioned subsets are mainly located in the epithelial layer of the gut, dermis, liver and spleen to maintain the integrity of epithelial tissue[9,10]. The CD1 family members are the ligands for Vδ1 T cells. Moreover, intestinal epithelial Vδ1 recognizes MICA or MICB through the TCR and NKG2D[9]. Vδ1 T cells usually proliferate during intracellular infections, fungal infections, viral infections and celiac disease[1]. Vδ2 T cells exclusively pair with the Vγ9 chain (also termed Vγ9Vδ2 T cells), which are mainly present in the peripheral blood and make up over 90% of peripheral circulating γδT cells[1] and 1%-5% of circulating T cells[7]. Vδ2 T cells usually expand during microbial infections[1]. Vδ2+ T cells can be divided into Vγ9+Vδ2+ and Vγ9-Vδ2+ subsets[11]. The ligands for Vγ9Vδ2 T cells are phosphoantigens on microbes and transformed cells. Intermediate metabolites of microbial isoprenoid biosynthesis, (E)-4-Hydroxy-3-methyl-but-2-enyl pyrophosphate, an isopentenyl pyrophosphate that is produced via the mevalonate pathway by transformed cells, activate these cells in a TCR-dependent manner. F1-ATPase expressed by tumor cells and butyrophilin3A1 are antigen-recognition molecules essential to Vγ9Vδ2 T cells activation[9]. Toll-like receptors (TLRs) and natural killer receptors coordinate with the TCR to stimulate Vγ9Vδ2 T cells. For example, pathogen-associated molecular patterns activate Vγ9Vδ2 T cells through TLRs and induce cytokines and chemokines. Vγ9Vδ2 T cells can also recognize MICA/MICB and UL-16 binding protein through NKG2D. DNAM-1, a kind of natural killer receptor that recognizes nectin-like-5, also participates in Vγ9Vδ2 T cells activation. Toxic shock syndrome toxin and staphylococcal enterotoxins are superantigens that are involved in Vγ9Vδ2 T cell activation[9]. Zoledronate is used to stimulate Vγ9Vδ2 T cells as an immunotherapy against solid tumors and is receiving increasing attention[1]. Activated Vγ9Vδ2 T cells not only play an important role in cytotoxicity and promoting inflammatory processes, but also induce differentiation and maturation of innate immune cells via chemoattractant cytokine ligand 3 (CCL3), CCL4 and chemokine (C-X-C motif) ligand 10 (CXCL10)[1,12]. The third group of γδT cells are Vδ3 T cells, which are approximately 0.2% of circulating T cells. These cells are rich in the liver in healthy individuals and in patients with chronic viral infections, such as cytomegalovirus (CMV) and HIV, and leukemias[9]. Some Vδ3 T cells recognize glycolipids presented by CD1d[1].

Based on the diverse cytokines produced by γδT cells[6], they can be divided into different functional subsets through stimulation. Differentiation requires transcription factors such as T-bet and eomesodermin for interferon-γ (IFN-γ) expression and retinoic acid-related orphan receptor and Runx1 for interleukin-17 (IL-17) expression[13-15]. The IFN-γ-producing subset express the Vδ1 or Vγ9Vδ2 chains[6]. Qureshi et al[16] support the observations that γδT cells and NK cells are the producers of IFN-γ in the early immune response, which is followed by the cellular immune response. γδT cells also act as T regulatory cells (termed γδTreg cells). These cells inhibit peripheral blood mononuclear cell proliferation[9]. Approximately 70%-90% of γδT cells express CD27, and 10%-30% of γδT cells are CD27-[15]. IL-17-secreting γδT cells, also called γδT17 cells, are mainly located in lymphoid organs and peripheral tissues[17,18]. γδT17 cells are CD27- but express C-C motif receptor 6 (CCR6) and CD25[15,19]. γδT17 cells play a pathogenic role in infection and autoimmune diseases. Scavenger receptor SCART2high γδT cells belong to a new subset of activated γδT17 cells[20] and appear under noninflammatory conditions.

γδT CELLS IN THE IMMUNE SYSTEM

As a kind of unique population, γδT cells act as a bridge between innate and adaptive immunity. Their roles in immune responses depend on many aspects, such as the existing locations, the stimuli used to activate and the period of responses[21]. Their pleiotropy, such as Th1 and Th2 phenotypes, is determined by specific stimuli and cytokines in the microenvironment, and is exhibited at different stages of immune responses[22,23]. Most of the γδT cells are Th1 phenotype. During the early stage of innate immune response, γδT cells sense the stressed epithelial cells or dendritic cells (DCs), then recruit innate cells, including neutrophils and macrophages, by producing IL-17 and CCL2, respectively[24]. During the middle stage of enhanced adaptive response, the interaction between γδT cells and DCs is intensive, leading to the proliferation and polarization of γδT cells and maturation of DCs[24,25]. γδT cells regulate B cells to produce a large number of immunoglobulins in the absence of αβT cells. In addition, human Vγ9Vδ2 T cells act as antigen presenting cells and present antigens to CD4+T cells and CD8+T cells, initiating adaptive responses[22,26]. Whereas, γδT cells play the opposite roles and kill macrophages and αβT cells and promote tissue repair by producing IL-10 during the later stage[21,24].

γδT cells are also involved in antitumor immune responses. The activated cells exert cytotoxic effects by secreting perforin, granzymes, IFN-γ, tumor necrosis factor-α (TNF-α), etc. γδT cells influence DCs and αβT cells which regulate the immune responses in killing tumor cells. The combined regimens consisting of zoledronic acid, IL-2 and Vγ9Vδ2 T cells have shown promising effects in clinical trials[27].

γδT CELLS IN LIVER DISEASES

γδT cells in hepatitis B/C and other hepatic virus infections

Hepatitis B virus (HBV) infection is one of the major liver diseases in Asian people. Approximately 90% of infants with acute infection gradually develop chronic hepatitis B (CHB). Only about 5% of adults develop CHB, and the remaining adults are acutely infected[28]. Among HBV carriers, γδT cells prevented concanavalin A-induced hepatitis in HBV-transgenic mice, indicating that γδT cells can be used to ameliorate liver injury in HBV carriers[29]. Moreover, Jia et al[30] showed that the percentage of γδT cells in peripheral blood was low, whereas the percentage of intrahepatic γδT cells in the inflamed lobular area was high. The proportion of γδT cells in blood was inversely correlated with acute hepatitis B severity. Circulating γδT cells in acute hepatitis B with an activated memory TemRA phenotype exhibited enhanced cytotoxicity against HBV by producing IFN-γ and TNF-α[30]. A study showed that the number of γδT cells, especially Vδ2 T cells, in the liver and peripheral blood of patients with CHB infection decreased during disease progression and during pegylated IFN-α treatment. Vδ2 T cells have increased production of TNF-α and increased expression of CD107a which enhanced the cytotoxicity of these cells in the case of IFN-α therapy[31]. The number of Vδ2 T cells in immune-activated (IA) patients was lower than that in immune-tolerant or healthy subjects and was negatively correlated with disease severity. These cells downregulated IL-17 and IL-22 production in the microenvironment and ameliorated liver injury in IA patients with CHB[32]. Nevertheless, a previous study showed that in an HBV-carrier mouse model, liver γδT cells facilitated HBV-associated tolerance by indirectly inducing antiviral CD8+T cells exhaustion through myeloid-derived suppressor cells infiltration in the liver in an IL-17-dependent manner[33]. Moreover, inhibiting the NKG2A-HLA-E pathway-mediated CD8+T cells response prevented γδT cells from inhibiting HBV replication in CHB patients, while the frequency of γδT cells was negatively correlated with HBeAg seroconversion[32,34]. In addition, Chen et al[35] showed that γδT cells in the blood of HBV-ACLF patients that produced higher amounts of IL-17 and TNF-α were significantly decreased compared to those of patients with CHB and healthy controls. This finding indicated that γδT cells may take part in the pathogenesis of HBV-ACLF due to their inflammatory and cytotoxic properties. Thus, the functions of γδT cells during HBV infection depend on the cell subsets and different stages of disease.

With regard to hepatitis C virus (HCV) infection, approximately 75%-80% of adults who are acutely infected develop chronic hepatitis C infection[36]. γδT cells, especially Vδ1 T cells with an effector phenotype, accumulated in the liver of patients with chronic HCV infection and HIV/HCV-co-infected patients. These Vδ1 T cells that originated from the peripheral blood homed to the HCV-infected liver, showed a Th1-cytokine-secreting pattern and led to liver necroinflammation[10,37,38]. Whereas, Vγ9Vδ2 T cells were decreased in the peripheral blood of these patients[39]. Tseng et al[37] showed that γδT cells from these patients can be stimulated and produced IFN-γ and TNF-α by a cytokine cocktail in vitro and are highly cytotoxic to primary hepatocytes, suggesting a pathogenic role for γδT cells in HCV infection. Moreover, γδT cells isolated from liver tissue with viral infection expanded exclusively in the liver but not in peripheral blood[37]. Therefore, γδT cells display pathogenic function in HCV-infected individuals.

Lu et al[40] demonstrated that liver TCR γδ+ CD4-CD8- (double negative, DN) T cells with an activated phenotype of CD25-CD28-CD69+ were markedly increased in murine hepatitis virus strain 3 infection and were activated to produce TNF-α, IFN-γ, IL-17A and IL-2. These cells were cytotoxic to murine hepatitis virus strain 3-infected hepatocytes via the TNF-α pathway, indicating the critical role of TCR γδ+DN T cells in viral clearance. Ajuebor et al[41] showed that γδT cells accelerated acute liver injury, which was infected with adenovirus expressing the Escherichia coli LacZ gene in a CXCL9-CXCR3-dependent mechanism. The reduced level of IFN-γ and CXCL9 due to the lack of γδT cells and hepatocytes, respectively, may contribute to alleviation of liver injury.

γδT cells in liver bacterial, fungal and parasitic infections

γδT cells were the main producers of IL-17 during Schistosoma japonicum infection in the liver, and they were the first line of defense before T helper cell 17 (Th17) reacted. IL-17 contributed to granulomatous inflammation and fibrosis, which were reduced by an anti-IL-17 monoclonal antibody[42]. Li et al[43] showed that IL-9-producing γδT cells played a part in Schistosoma japonicum infection in C57BL/6 mouse. Mice that were deficient in γδT cells and infected with Listeria monocytogenes developed liver injury due to TNF-α produced by CD8+T cells. This liver pathology was reversed by infusing Vδ4+ γδT cells that secrete IL-10, controlling the proliferation of CD8+ T cells and reducing TNF-α production. Thus, γδT cells protected liver tissue by regulating pathogen-stimulated CD8+ T cells. γδT cells maintained CD8+ T cell homeostasis[44]. A study showed that γδT cells were important early in infections, whereas αβT cells played a part later in infection. IL-17 produced by Vδ4+ γδT cells protected against infection. In parasitic infections, γδT cells were also protective.

γδT cells in autoimmune liver diseases

Autoimmune liver diseases are chronic liver diseases caused by immune dysfunction. They consist of autoimmune hepatitis (AIH), primary biliary cholangitis, primary sclerosing cholangitis (PSC) and overlap syndrome. AIH is characterized by interface hepatitis, increased transaminase and immunoglobulin G and various au-toantibodies[45]. Primary biliary cholangitis is characterized by destruction of intrahepatic bile ducts and cholestasis. Without proper treatment it will advance to end-stage liver disease[46]. PSC is distinguished by biliary inflammation and fibrosis. The most common symptoms are hepatomegaly and splenomegaly[47]. γδT cells have been shown to play immunoregulatory roles in different studies[48], such as in a mouse model of adriamycin-induced nephropathy[49], and in pulmonary fibrosis[50]. The number of γδT cells was markedly increased in the blood and portal vein and bile duct proliferation areas in patients with PSC and AIH[51]. Peripheral Vδ1 T cells that produce IFN-γ and granzyme B were the main subtype, whereas Vδ2 T cells were low in AIH patients. In a scurfy transfer model, CD62LloCD44hi γδT cells in TCRα-/- mice prevented multi-organ autoimmune diseases before transfer. These cells produced the immunosuppressive cytokine IL-10 and granzymes, and highly expressed CD39 and CD73, which resembled the pattern of ectoenzyme-mediated degradation of ATP to adenosine that was seen in Foxp3+ Tregs cells[52]. γδT cells also exerted suppressive functions via NKG2D, which is expressed by NK cells, NKT cells and γδT cells. γδT17 cells acted as protectors in autoimmune liver diseases. IL-17A produced by Vγ4+ γδT cells prevented concanavalin A-induced fulminant hepatitis by downregulating NKT cells and IFN-γ production[6]. However, γδT17 cells also played an opposite role in the immunopathology of liver diseases[1]. IL-17 produced by these cells, rather than Th17 cells, contributed to hepatic inflammation in a mouse model of biliary atresia (BA) and in BA patients[5]. The pathogenesis of BA is not fully understood; however, experiments show that the autoimmune process may be involved. Neutralization of IL-17 alleviated the severity of liver inflammation in BA and ultimately protects against liver fibrosis[5]. γδT cells also induced Treg cell apoptosis and reversed Treg cell functions to promote effector T cells activities and lead to autoimmune liver diseases[48]. The functions of γδT cells vary from situation to situation. These cells serve as promising immunotherapies in clinical trials for cancer, and whether γδT cells can be used in autoimmune liver disease treatment remains to be elucidated.

γδT cells in non-alcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is one of the metabolic diseases which includes different stages, ranging from hepatic steatosis to steatohepatitis. NAFLD results from multiple factors, and hepatic γδT17 cells that exacerbate steatohepatitis are one of the important mechanisms[53,54]. The γδT cells recruited to the liver were mainly γδT17 cells, which aggravated NAFLD by regulating CD4+T cells[55]. Although He et al[56] only mentioned that the expression of IL-17 by Th17 cells declined, hepatic γδT cells also decreased in lentiviral vectors encoding pre-Mir-26a-treated mice after high fat diet feeding, suggesting an improvement in NAFLD. We hypothesize that IL-17 secreted by γδT cells may also play a part in the pathogenesis of NAFLD. Therefore, γδT17 cells promote NAFLD progression.

γδT cells in liver fibrosis and cirrhosis

Liver cirrhosis is the outcome of many chronic liver diseases, such as HBV infection in Asia and HCV infection and alcohol abuse in developed countries. The process involves necroinflammation, activation and accumulation of hepatic stellate cells (HSC) and fibrosis[57]. γδT cells are a primary cell type found in the portal area of liver cirrhosis patients[58]. These cells produce IL-17, which facilitated fibrosis progression by activating HSC and Kupffer cells[59,60]. In mice infected with Schistosoma japonicum, Zheng et al[61] found that γδT cells recruited neutrophils to the liver and caused liver fibrosis by producing IL-17A. Wang et al[62] also demonstrated that the HMGB1-TLR4-IL23-IL17 axis between macrophages and γδT cells exacerbated liver inflammation. In the presence of IL-23 and IL-1β, the interaction between exosomes and TLR3 in HSCs promoted increased production of IL-17A by γδT cells, which activated HSCs and exacerbated liver fibrosis at an early stage[63]. Ni et al[64] argued that enhanced expression of IL-17A may occur directly from exosomes. Moreover, TLR4 but not TLR3 is expressed in HSCs. A previous study showed that TLR3 is protective against liver fibrosis[65]. However, another study showed that CCR6-expressing γδT17 cells in the injured liver hamper liver inflammation and fibrosis by the induction of HSC apoptosis through Fas/Fas-ligand (FasL) interactions[66]. In addition, γδT cells, especially IFN-γ-producing γδT cells, also exert protective functions against liver fibrosis by killing activated HSCs in a NKp46-, TRAIL- and Fas-ligand-dependent manner directly or indirectly by promoting NK cell-associated cytotoxicity against HSCs[67]. Therefore, the opposite effects of γδT cells on liver fibrosis and cirrhosis are associated with the underlying mechanisms.

γδT cells in liver tumors

Liver cancers are one of the most common cancers worldwide. Many chronic liver diseases, such as HBV and HCV infection, lead to the pathogenesis of liver cancers[68]. Viey et al[69] showed that γδT cells had the ability to infiltrate tumors. Patients with hepatocellular carcinoma (HCC) had an increased number of CCR2-expressing Vδ1+ γδT cells in the liver[8,70]. γδT cells expressed lymphocyte-activation markers Ia and LFA-1, indicating an activated status in hepatic tumor-bearing subjects[71]. The expression of CD56 and CD161 also increased, revealing cytotoxicity against hepatic tumors[72]. γδT cells lysed hepatoma cells and markedly reduced hepatic tumor cells activity in vitro[73]. Vδ1+ γδT cells produced IFN-γ and exerted cytotoxic effects[74]. CMV-stimulated Vδ1+ γδT cells inhibited primary HT-29 colonic cancer and metastatic foci such as in the liver compared to those of control mice[75,76]. Activated (CD44high) Vγ4+ γδT cells also participated in tumor immune surveillance by secreting increased IFN-γ and perforin in TCRδ-/- mice due to the high level of eomesodermin in Vγ4+ γδT cells, suggesting a protective role in the tumor immune response[77]. Thus, Vγ4+ γδT cells might be a novel therapy in liver diseases[78]. Recently, it was demonstrated that the ratio of peritumoral HSC to γδT cells can be a valuable predictor of the prognosis of HCC after resection and was always positively correlated with tumor progression. In this study, γδT cells inhibited the behavior of progressive HCC[79]. Another study showed that in the HCC group who sequentially used radiofrequency ablation/cellular immunotherapy (CIT), the outcome was efficient and safe compared to that of the group using radiofrequency ablation alone; specifically, NK cells and γδT cells had robust cytotoxicity and may prevent the recurrence of HCC[80]. Qian et al[81] found that the combination of cellular immunotherapy not only favored the progression-free survival of HCV-positive HCC patients but also affected long-lasting viral control. Bispecific antibodies such as MT110 boosted the antitumoral effect of γδT cells. Gustafsson et al[82] reasonably hypothesized that the presentation of γδT cells and TAA to CD8+ cytotoxic T cells may lead to enhanced killing of tumor cells with the help of activated CD4+ helper T cells. Therefore, γδT cells play an antitumor role in liver cancers[6,83].

RELATIONSHIPS BETWEEN LIVER/INTESTINAL γδT CELLS AND INTESTINAL MICROBIOTA

The microbiota plays an important role in maintaining hepatic γδT17 cells homeostasis. The mechanism underlying the abovementioned phenomenon could be attributed to lipid antigens, components of intestinal microbiota, that were presented by hepatocyte CD1d via the portal vein, which activated hepatic γδT cells and produced IL-17A. Activated γδT17 cells have been identified to have pro-inflammation and anti-infection abilities, aggravating liver disease[84,85]. For example, the quantity of microbiota affected γδT17 cells in the liver, thus accelerating the development of NAFLD[2]. In addition, during cholestatic liver diseases the increased intestinal permeability allowed bacterial translocation to the liver, especially Lactobacillus gasseri. Accordingly, hepatic γδT cells responded to the microbial stimulus to secrete IL-17A, exacerbating the cholestatic liver diseases[86]. Hepatic γδT17 cells showed an active and mature status by expressing CD44highCD62L--. A study performed on lung cancer demonstrated that the number of γδT cells in the liver decreased in the absence of commensal microbiota. Li et al[2] demonstrated that Escherichia coli alone could restore hepatic γδT17 cells in a dose-dependent manner. Moreover, supplementation of γδT cells and IL-17A restored immune surveillance in antibiotic-treated mice[87].

γδT cells make up 10%-30% of CD3+ T cells in the intestine of healthy subjects[11]. γδT cells are important in maintaining homeostasis of the intestinal barrier in order to kill pathogens and prevent bacteria from translocating to the liver. The commensal microbiota residing on the epithelial surface of the gastrointestinal tract modulated intestinal mucosal γδT cells[88]. These microbiota induced-γδT cells exert their roles in many pathological processes in the initial stage or in the later stage[85]. For example, IL-17A produced by γδT cells affected intestinal immunity. In addition, some studies suggested that IL-17A was protective and maintained barrier functions by regulating the tight junction protein occludin in a DSS-induced colitis model[89]. A recent study suggested that Lactobacillus breves DM9218 directly stimulated γδT17 cells by expressing TLR2 in the colon, leading to beneficial effects on colitis[88]. Another study also suggested that the bacterial consortium markedly stimulated the proliferation of γδT17 cells in the colonic lamina propria. Specific beneficial microorganisms, such as Bifidobacterium and Bacillus spp, promoted TLR2 expression on γδT cells, which led to enhancement of barrier functions[90]. In vitro, only the Bacillus strains but not Bifidobacterium promoted TLR2 and IL-17 expression. However, bacteria constituting the families Prevotellaceae, Rhodospirillaceae, and Flavobacteriaceae were inversely related to γδT17 cells in the intestine. Bacteria in the family Bifidobacteriaceae were positively correlated with γδT17 cells[90]. The communication between intestinal epithelial cells (IEC) and γδ intraepithelial lymphocytes (IEL) relied on microbiota and served to maintain homeostasis of intestinal immunity. γδIEL depended on IL-15 produced by IEC which were stimulated by microbiota to sustain their presence and functions. In turn, γδIEL promoted IEC functions, such as maintenance of the epithelial barrier and lysis of invasive pathogens. Moreover, microbial localization impacted the biological behaviors of γδIEL, for instance, enhanced cytotoxicity against deleterious bacteria[85]. Microbiota was also involved in the relationship between γδT cells and cutaneous carcinogenesis. Microbial infection resulted in Vδ2-γδT cells residing in tissue epithelial layers. The Vδ2-γδT cells proliferated and secreted IFN-γ upon encountering antigens, leading to cancer cell death. On the other hand, the anti-tumor subset could be transformed into the pro-tumor subset with the help of IL-23. Due to a larger intercellular space and subsequent bacterial translocation, γδT17 cells expanded, causing tumorigenesis. IL-17 also inhibited effector T cells through myeloid-derived suppressor cells indirectly[85].

CONCLUSION

In brief, the effects of γδT cells in liver disease depend on subsets, mechanisms and different stages of diseases. γδT cells not only show cytotoxicity against viral hepatitis but also exacerbate CHB and HBV-ACLF. These cells also play an antitumor role in liver cancers. This minority of cells is mainly used to treat hematological and solid tumors through the production of IFN-γ, TNF, cytotoxic granules, as well as the functions of γδT17 cells. γδT cells that function as carriers for chimeric antigen receptors are being explored due to their reduced risk of side effects. T cells that are engineered with defined γδTCRs are outstanding in cancer treatment. The functions of γδT cells have also been important in various virus infections, especially CMV. We should emphasize their beneficial roles and engineer this type of immune cell in adoptive immunotherapies, such as targeting specific receptors and expanding cytotoxic anticancer immune cells, and then apply these immune treatments not only for liver diseases but also other systemic diseases.

ACKNOWLEDGEMENTS

The authors are grateful to the Department of Infectious Diseases, State Key Laboratory for Diagnosis and Treatment of Infectious Diseases, National Clinical Research Center for Infectious Diseases, Collaborative Innovation Center for Diagnosis and Treatment of Infectious Diseases, the First Affiliated Hospital, College of Medicine, Zhejiang University for their contribution to this manuscript.