Introduction

In recent years, many studies have shown that cytotoxic γδT cells have a strong killing ability toward autologous, allogeneic or xenogeneic tumor cells, and their concentration is often closely related to the clinical prognosis of patients [1,2,3,4,5]. In contrast to αβT cells, γδT cells can recognize antigens in a way that is not restricted by major histocompatibility complex (MHC) molecules [6], and they mediate the killing of target cells in various ways, participate in immunomodulation, and have a broader tumor cell killing spectrum [7, 8]. As the “bridge” between innate immunity and adaptive immunity as well as the first line of defense against tumors, γδT cells play an important role in the occurrence and development of tumors and can be used to evaluate the prognosis of patients [8,9,10]. Therefore, immunotherapy based on γδT cells has been studied in many kinds of tumors [153]. Therefore, it is very important to further characterize the interaction between γδT cells and BTNs.

BTN3A/CD277 is an indispensable molecule in γδT-cell activation. Administration of a mAbs that activated BTN3A1 can enhance the activation of γδT cells and their cytotoxic effects [63]. By adding anti-CD277 mAbs and knocking down CD277, Harly et al. showed that CD277 plays a unique role in the response of Vγ9Vδ2T cells induced by PAgs, and the mechanism of γδT-cell activation depended on CD277 and was based on TCR signal transduction [154]. Without any other stimulation (such as anti-CD3 mAbs), administration of the soluble mAb 20.1 targeting BTN3A/CD277 indirectly increased the antigen recognition of Vγ9Vδ2T cells and mimicked PAgs to induce complete activation of Vγ9Vδ2T cells, accompanied by activation of Ca2+ signal transduction, upregulation of CD69 expression and increased IFN-γ secretion [154, 155]. The anti-BTN3A mAb 20.1 combined with Vγ9Vδ2T-cell immunotherapy restored the proliferation and cytotoxicity of Vγ9Vδ2T cells in tumors, prevented the exhaustion of adoptively transferred Vγ9Vδ2T cells, improved the survival rate of animals and reduced the tumor burden in blood and bone marrow [152]. Aude De Gassart and coworkers [156] also developed a new humanized anti-BTN3A mAb, ICT01, that can specifically activate Vγ9Vδ2T cells and upregulate the proportion of CD69+ Vγ9Vδ2T cells in a concentration-dependent manner. ICT01 can induce the degranulation of Vγ9Vδ2T cells and promote the production of IFN-γ, TNF-α, IL-8, IL-1β, monocyte chemoattractant protein 1 (MCP-1) and other proinflammatory cytokines. In addition, after the application of ICT01, the tumor growth rate of mice was obviously slowed. Importantly, the mAbs had no obvious effect on normal cells, highlighting the potential clinical value of the anti-BTN3A mAb ICT01.

After blocking BTN proteins, the response of γδT cells to PAgs was blocked, and there was no longer a tumor cell killing effect. Yamashiro et al. found that the expression level of BTN3 was inversely correlated to the activity of lymphocytes, and administration of the mAb 232-5 lead to phosphorylation of the BTN3A3 molecule and transduction of negative signals; these effects caused CD4+ and CD8+ T cells to act like CD4 + CD25+ Tregs, accompanied by a decrease in cell proliferation and cytokine secretion [157]. However, the mAb 20.1 has no significant stimulatory or costimulatory effect on αβT cells [154], suggesting that BTN proteins may jointly inhibit effector T cells through negative signal transmission and that BTN3A3 may be a novel target [158]. In addition, although the mAb 103.2 does not affect the proliferation and activation of CD8+ T cells, it can inhibit the response of γδT cells induced by PAgs and even inhibit the degranulation and cytokine secretion of Vγ9Vδ2T cells, decreasing their antitumor effects. Audrey Benyamine and coworkers also demonstrated that the mAbs 103.2 and 108.5 targeting the BTN3A molecule can completely inhibit the tumor cell lysis mediated by Vγ9Vδ2T cells [65, 152]. In addition to BTN3A, the B30.2 domain of BTN2A is required for the response of γδT cells to PAgs. The expression of the BTN2A1/BTN3A1 complex can trigger the activation of Vγ9Vδ2TCR, and the expression of BTN2A1 in cancer cells is related to the cytotoxicity of Vγ9Vδ2T cells. Anti-BTN2A1 mAbs can significantly inhibit the degranulation of Vγ9Vδ2T cells, and the inhibition of Vγ9Vδ2T-cell cytotoxicity induced by the 7.48 mAb generates an effect similar to the real TME [159].

Targeting BTN family proteins is beneficial for restoring the antitumor function of γδT cells and promoting their interaction with other immune cells, and they exert their antitumor effects synergistically. It has been reported that melanoma cells can hijack the interaction between pDCs and γδT cells to escape immune control, which is manifested as dysfunction of BTN3A and impaired ability of γδT cells to regulate ICs [160]. Conformational change of BTN3A1 is the key event of PAg perception [161], and BTN2A1 is the key ligand that binds to the Vγ9+ TCR γ chain; it can directly bind to the germline coding region of the Vγ9 chain in Vγ9Vδ2TCR and simultaneously bind to BTN3A1 on the cell surface. The synergistic effect of BTN3A1 and BTN2A1 enhances the recognition of target cells by Vγ9Vδ2T cells and plays an important role in PAg perception [162,163,164,165,166]. BTN3A1 can also recognize the N-mannosylated oligosaccharide in the near-membrane domain of CD45 and anchor the CD45 dimer near TCR, which may physically block the interaction of TCR-peptide-MHC-I complexes (pMHC), thus effectively inhibiting the separation of CD45 molecules from immune synapses and ultimately inhibiting the functions of TCRs and αβT cells [167]. Therefore, CD277-specific antibodies can restore the effector activity of αβT cells and induce BTN3A activity to mediate the synergistic killing of BTN3A1+ tumor cells by αβT cells and γδT cells in a BTN2A1-dependent manner. In summary, BTN family proteins are potential targets for fully exploiting the potential of γδT cells in IC therapy (Fig. 4).

Fig. 4
figure 4

The activation of γδT cells can be modulated by anti-BTN3A antibodies. BTN2A can bind to the γ chain of γδTCR and plays an important role with BTN3A in the activation of γδT cells by phosphoantigens. After the binding of an antagonistic monoclonal antibody to BTN3A, the activation of Vγ9Vδ2T cells will be blocked, and the cytotoxicity of Vγ9Vδ2T cells will decrease or even disappear. In contrast, after the use of an agonist targeting BTN3A, Vγ9Vδ2T cells have increased antigen sensitivity and enhanced ability to kill tumor cells, suggesting that targeting BTN family proteins can significantly regulate the immune response of γδT cells against tumor cells

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Blocking inhibitory IC signaling can reverse the immunosuppressive state of γδT cells

ICIs can restore the proliferation and activation of γδT cells

The proliferation and activation of γδT cells were improved after administration of mAbs against ICs (Fig. 5). ICIs targeting B7-H3 and TIM-3 can improve the proliferation and/or activation of dysfunctional γδT cells. In colorectal cancer, knocking down or blocking B7-H3 can inhibit the apoptosis of Vδ2T cells, promote the proliferation of Vδ2T cells and induce the expression of the activation markers CD25 and CD69 in Vδ2T cells [168]. Caspase-3 participates in the activation of the TIM-3 signaling pathway, so γδT cells with upregulated TIM-3 are more prone to experience early apoptosis. After blocking TIM-3, the proliferation of Vγ9Vδ2T cells, the STAT phosphorylation level and the induction of IL-21 increased significantly [97, 169]. However, the expression of cyclin B1 and cyclin D1, which are related to cell proliferation, was not affected by TIM-3 blockade [170].

Fig. 5
figure 5

Targeting immune checkpoint molecules can revive dysfunctional γδT cells. Dysfunctional γδT cells express high levels of multiple checkpoint molecules on their surface, a phenotype similar to that of anergic or exhausted T cells. Vδ2T cells can recognize phosphoantigens with the assistance of BTN3A/BTN2A, and some subsets of Vδ1T and Vδ3T cells can recognize lipid antigens presented by CD1d and transmit activation signals through γδTCR. Activated γδT cells express NKG2D and/or other similar costimulatory molecules on their surface. Immunoglobulin-like transcripts (ILTs) or leukocyte immunoglobulin-like receptors (LIRs) belong to the Ig superfamily. ILT2 (LIRB1) binds to HLA-G in addition to recognizing other ligands and can inhibit the immune functions of γδT, NK, and B cells. NKG2A can recognize the nonclassical MHC-I molecule HLA-E and inhibit the stimulatory signal of NKG2D. Inhibitory Siglecs are immune regulatory sialic acid-binding receptors that resemble traditional immune checkpoint molecules with one or more ITIM-like motifs in the intracellular segment. Abnormal signaling of immune checkpoint molecules interferes with the normal function of TCRs, affects the level of intracellular protein phosphorylation through ITIM motifs and SHP-1/2, inhibits the proliferation and activation of γδT cells, and ultimately reduces the cytotoxicity of γδT cells. After blocking the inhibitory signals with monoclonal antibodies targeting immune checkpoint molecules, the ability of γδT cells to kill tumor cells and their interactions with other immune effector cells can be enhanced

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The expression of PD-1 may be one of the reasons for the failure of Vγ9Vδ2T-cell expansion in tumor patients. Blocking the PD-1 signaling pathway can partially restore the damaged proliferation of PD-1+ γδT cells and induce their activation. A study showed that the response of bone marrow-derived Vγ9Vδ2T cells to PAgs stimulation was weakened in MM patients, while the proliferation response of γδT cells was enhanced after zoledronic acid stimulation with anti-PD-1 mAbs in vitro [51]. In addition, researchers have found that the bispecific (PD-L1× CD3) antibody Y111, which can simultaneously recognize PD-L1 and CD3, can effectively connect T cells with tumor cells expressing PD-L1. In the presence of PD-L1+ tumor cells, Y111 can induce the activation of Vγ2Vδ2T cells in a dose-dependent manner [171]. Although some studies have shown that the inhibitory effects of the PD-1 signaling pathway on γδT cells may be reversed by the synergistic effects of TCR and IL-2 signaling, blocking PD-1 signal transduction with anti-PD-L1 antibodies alone does not affect IL-2 production by γδT cells [172, 173], and the inhibitory TME also weakens TCR signal transduction. Blocking the PD-1/PD-L1 signaling pathway, restoring the proliferation and activation of γδT cells and promoting their differentiation into antitumor effector cells in the early stage of γδT cell development are important for effective ICT.

Furthermore, some studies have shown that PD-1 is not the main molecule affecting the proliferation of γδT cells and that the proliferation of γδT cells is strictly regulated by BTLA [174]. BTLA and TCR are clustered at the synapse between Vγ9Vδ2T cells and target cells, and the close localization of BTLA and TCR suggests that BTLA may affect TCR-dependent signal transduction. γδT cells need TCR signals to maintain their stability [175], and it has been suggested that the interaction between BTLA and HVEM inhibits the proliferation of Vγ9VδT cells and their response to lymphoma cells. BTLA on PB γδT cells interacted with HVEM on leukemic cells and caused some cells to stagnate in S phase; however, this did not affect the percentage of G0 cells but did increase the percentage of cells in G2/M phase. After blocking the interaction between BTLA and HVEM with mAbs, PAg/TCR-mediated signal transduction can be enhanced, and the proliferation of γδT cells can be upregulated [51, 98, 176]. However, no synergistic effect was found after the combined blockade of BTLA and PD-1, suggesting that BTLA and PD-1 may have independent effects on the proliferation and cytotoxicity of human PB γδT cells [174]. That is, inhibition of the BTLA signaling pathway can promote the proliferation of γδT cells without affecting their cytotoxicity, the production of IFN-γ or the nontargeted degranulation induced by bromohydrin pyrophosphate (BrHPP) [176].

ICIs can enhance the cytotoxicity of exhausted γδT cells

After treatment with ICIs, the cytotoxicity of γδT cells was enhanced (Fig. 5). After blocking PD-1, the antibody-dependent cell-mediated cytotoxicity (ADCC) effects of CD16+ Vγ9T cells on lymphoma cells was improved [177]. However, it has also been reported that in the environment of PD-L1+ tumor cells, blocking or knocking out PD-1 does not significantly increase the cytotoxicity of γδT cells. In contrast, anti-PD-L1 mAbs could enhance the cytotoxicity of γδT cells against some cancer cells, and the expression level of PD-L1 was positively correlated with the cytotoxicity of γδT cells [178].

Increased secretion of antitumor cytokines

After ICs are blocked, γδT cells can produce more inflammatory cytokines, especially IFN-γ and TNF-α. Inhibitory ICs may reduce the production of IFN-γ by inhibiting the key transcription factor Eomes [179]. The PD-1 signaling pathway may be involved in the regulation of IFN-γ production by γδT cells [180]. An in vitro study of γδT cells showed that, similar to that of traditional αβT cells, the cytotoxicity of activated PD1+ Vδ2T cells was inhibited, and the secretion of IFN-γ decreased after PD-L1 binding [172]. Blocking PD-1 with antibodies such as pembrolizumab can promote the secretion and release of IFN-γ and TNF-α by activated γδT cells. After the combined use of anti-LAG-3 and anti-PD-1 antibodies, the secretion of cytokines, especially IFN-γ, in γδT cells increased [92, 171, 177]. However, it should be noted that presensitization of target cells or γδT-cell activation is needed for the production of IFN-γ; that is, the regulatory effects of PD-1 signaling on the proliferation and cytokine secretion of γδT cells depends on costimulatory signals or early activation of γδT cells [92]. The TIM-3 signaling pathway also inhibits the secretion of IFN-γ and TNF-α by γδT cells. In vitro studies have shown that TIM-3+ γδT cells do not produce IFN-γ or TNF-α and have reduced cytotoxicity [96, 181]. In addition, the effects of B7-H3 on the cytokine profile of Vδ2T cells was studied. It was found that B7-H3 inhibited the expression of IFN-γ in Vδ2T cells by inhibiting T-bet [168], suggesting that the ability of γδT cells to produce cytokines would be restored after blocking TIM-3 and B7-H3 with ICIs.

The expression of effector genes in the IFN signaling pathway is negatively correlated with the degree of γδT-cell exhaustion. In exhausted T cells, several inhibitory receptors, including VSIR, KLRG1, LAG3 and TIGIT, as well as the transcription factors NR4A2 and ID2, are significantly upregulated, and IFN response genes, such as IFITM1, STAT1 and IFI6, are also upregulated [182].

Increased secretion of perforin and granzyme

After blocking ICs, the expression of perforin and granzyme is not inhibited, and cytotoxicity is enhanced. In vitro studies showed that after a PD-1 blocking drug was used and ZA was administered as stimulation, the cytotoxicity of Vγ9Vδ2T cells increased nearly 5 times, accompanied by an increase in the expression of the degranulation marker CD107 and an increase in the proportion of CD107+ Vγ9Vδ2T cells [51]. ERK1/2, STAT-3 and Wnt are known to regulate the expression of perforin and granzyme B in various immune cells. Some studies have found that increased expression of members of the TIM-3 pathway can significantly decrease the level of pERK1/2 in Vγ9Vδ2T cells activated by recombinant human (rh) Gal-9 but does not affect the level of pSTAT3 or Wnt [183]. Therefore, blocking TIM-3 can increase the killing effects of Vγ9VδT cells on colon cancer cells by activating the ERK1/2 pathway and upregulating the expression of perforin and granzyme B. B7-H3 also inhibits the cytotoxicity of Vδ2T cells by downregulating the expression of perforin and granzyme B [168], which can be reversed by using B7-H3 blockers. In tumor tissue, compared with PD-1 + LAG-3– cells, PD-1 + LAG-3+ T cells have a weaker ability to produce cytokines and/or undergo degranulation [177]. The cytotoxicity of Vδ2T cells against NSCLC tumor cell lines was enhanced after blocking PD-1 [171, 174], so the degranulation of γδT cells can be improved with the inhibition of the PD-1/PD-L1 or LAG-3 signaling pathway.

KIRs increase the threshold for Vγ9Vδ2T cell antigen-based activation, inhibiting the killing effects of cytotoxic Vγ9Vδ2T cells on MHC-I+ tumor cell lines. Blocking the binding of NKG2A to HLA-E can restore the high responsiveness of NKG2A+ Vδ2T cells. Low expression of NKG2A is usually accompanied by high expression of other ICs, such as PD-1 [141]. NKG2A is often coexpressed with PD-1, CTLA-4, LAG-3 and TIM-3 in CD8+ T cells, but they have different inhibition mechanisms [184]. The safety of humanized anti-NKG2A mAbs has been verified in clinical trials, and studies on HLA-E+ tumor cells have proven that combinations blocking the inhibitory signals of PD-1 and NKG2A have synergistic effects, which are characterized by enhanced ADCC and increased expression of CD107 and degranulation substances [108, 185].

ICIs can promote synergistic antitumor effects of γδT cells and other immune cells

When ICLs on the surface of γδT cells are blocked, the positive interaction between γδT cells and other immune cells becomes more efficient (Fig. 5). Studies have shown that γδT cells are the key source of immunosuppressive ICLs in tumor tissues and may have the ability to regulate αβT cells. Blocking PD-L1 on γδT cells in PDAC can enhance the levels of infiltrating CD4+ and CD8+ T cells and improve immunotherapy efficacy [124]. Data from mouse models also indicate that specific γδT-cell subsets that express PD-L1 can inhibit αβT cell infiltration through PD-1/PD-L1 signaling and promote tumor growth. Vδ2T cells can also activate CTLA-4 and inhibit αβT cells through CD86. In individuals with normal γδT-cell function, the use of PD-L1 or Galectin-9 inhibitors can promote the expansion and activation of CD4+ and CD8+ T cells, but this effect is not observed in the absence of γδT cells [92, 124, 186]. In addition, PD-L1 is a downstream target of HIF1α [187]. Hypoxia and coculture with γδT cells increased the apoptosis rate of CD8+ T cells, suggesting that γδT cells can induce the death of CD8+ T cells, and this effect was significantly changed after blocking PD-1. After blocking BTNL2, the number of cytotoxic CD8+ T cells in the TME increased [126]. Monalizumab can inhibit the newly recognized IC NKG2A [188] and thus activate the antitumor effects of αβT, γδT and NK cells. Therefore, after using ICIs to reverse the immunosuppressive state of γδT cells, αβT cells can better promote tumor cell killing.

Targeting checkpoint molecules on γδT cells with ICIs can also inhibit Tregs. PD-L1 expressed by γδT cells can promote the production of Tregs and enhance the expression of the FOXP3 gene, thus maintaining the expression of TIGIT, which plays an important role in immune regulation; these effects promote Tregs to directly inhibit effector T cells through CTLA-4 and LAG-3 [71, 189]. A high TIGIT/DNAM-1 ratio was detected in Foxp3+ γδT cells of patients with AML and Tregs of patients with melanoma. High expression of TIGIT can promote the stability and inhibitory function of Tregs, which is highly correlated with poor clinical prognosis [190]. Therefore, treatment with anti-TIGIT mAbs can mediate the direct killing of tumor cells in patient tumor samples and preclinical mouse models and can also kill Tregs via Fc receptors (FCRs) [191].

In conclusion, after blocking ICR-ICL signaling with ICIs, γδT cells can gradually recover their functions and produce synergistic antitumor immune effects.

Relationship between ICI resistance and γδT cells

The rate of response to ICIs is related to many factors [192, 193], and ICT can be a good option for cancer patients who are likely to have an active immune response. However, tumor cells may evolve other mechanisms of resistance after a period of treatment [78, 194,195,196,197,198]. The mechanisms of drug resistance may not be failure of targeted drugs but rather compensatory changes in γδT cells leading to acquired resistance to drugs [72, 199]. For example, in EGFR- and KRAS-mutant mice and human lung cancer specimens, therapeutic PD-1-blocking antibodies still bind to T cells as the disease progresses; this finding suggests that mAbs still play a role when drug resistance occurs, so attention should be given to pathways other than PD-1/PD-L1 in adaptive drug resistance [200].

During the course of treatment with mAbs, compensatory and alternative suppressor receptors on γδT cells are upregulated, indicating that γδT cells may also be involved in the development of ICI tolerance in cancer patients. Some data show that anti-PD-1 inhibition significantly increases the frequency of TIM-3+ Vδ2T cells, indicating compensatory upregulation of TIM-3; additionally, combined inhibition of TIM-3 and PD-1 can significantly increase the production of TNF-α and IFN-γ [222]. For example, the upregulation of ICs does not mean the absolute exhaustion of γδT cells [141, 143]. As such, it remains to be determined how the specific epigenetic state of γδT cells changes as dysfunction develops. Furthermore, when chromatin remodeling occurs, is it reversible? How do γδT cells exhibit opposite effects under the influence of different TMEs, and can these effects be exploited? Are the changes in the expression patterns of γδT-cell ICs similar to those of αβT cells? If future research can identify a method for selective targeting of certain subsets, γδT cells will be able to play a more efficient antitumor role.

Tumor models used for the study of γδT cells urgently need to be improved for several reasons. First, the groups of γδT cells are different between humans and mice [37]. Therefore, the subgroup results obtained in mice cannot be perfectly applied to humans. Second, BTNs similar to those in humans have not been found in mice. Therefore, rodent models cannot completely represent the actual environment in humans [223]. Furthermore, the TME is different in different tumor models and is also different from that in humans.

The therapeutic prospects of targeting checkpoints on the surface of γδT cells are broad. However, resistance to ICT based on γδT can occur, which necessitates researches on the mechanism underlying resistance to γδT-cell therapy and multitarget combination therapy. Furthermore, the role of γδT cells in irAEs needs to be further clarified, which will pave the way for the inclusion of γδT cells in the arsenal of immunotherapy options.

First, MHC molecule independence and glycolipid antigen recognition activity are great advantages for γδT cells. In ICI-resistant tumor cells, a similarity between cells with ICI resistance and a hypermetabolic phenotype was observed. This phenotype included synergistic upregulation of glycolysis and mitochondrial oxidative phosphorylation, which assists tumor cells in maintaining vigorous growth under hypoxia and resisting ICT [195, 224]. The associations between checkpoint molecules related to the internal metabolism of tumor cells and ICs is also an interesting topic [225].

Second, ICs can be either costimulatory or coinhibitory molecules. Agents targeting inhibitory molecules combined with αβT cell agonists have been shown to have synergistic antitumor effects [226,227,228,229,230]. As dysfunctional γδT cells can also express multiple ICs, it may be possible to restore the antitumor potential of γδT cells. In addition, given that the mechanism of action of CD39/CD73 is relatively independent of traditional ICs and that CD39/CD73 and its related ADO pathway have been proven to be related to the survival and prognosis of patients [231], the rationality of ICI combined with other targets, such as agents targeting CD39/CD73, deserves further discussion.

In addition to considering the combination of multitarget therapy with ICT, whether combination immunotherapy based on γδT cells can have a synergistic effect is also worth exploring. Vγ9Vδ2T cells can release a large number of cytokines and chemokines to induce the activation of other bystander immune cells after being activated by amino bisphosphonic acids such as ZA. With the help of MCP-2, γδT cells can activate the effector function of granulocytes [232]. Some studies have shown that the maturation of immature DCs may mainly result from a bystander process because immature DCs rely on the activation of Vγ9Vδ2T cells by TCR signal transduction and are able to promote Vγ9Vδ2T cells to secrete cytokines required for the maturation process [233]. Bystander activation of T cells is a type of antigen-independent activation that can occur through different receptors and costimulatory signals and has been observed in cancers [234]. Microorganism-related pattern recognition receptors, such as Toll-like receptors (TLRs), and cytokines such as IL-12, IL-15, and IL-18 are associated with activating bystander T-cell responses [235]. Thus, both the innate immune properties of γδT cells and their excellent cytokine and chemokine secretion abilities may aid in the recruitment of other immune cells [236] or interactions with other bystander cells in the TME and may be involved in bystander activation. Bystander T cells rarely show an exhausted status and have excellent innate killing ability [237]; therefore, reversing the immunosuppressive state of γδT cells may allow γδT cells to promote ICI responsiveness by activating bystander cells. ICT based on γδT cells combined with strategies that target bystander T cells (such as intratumoral injection of viral peptides) may have additional beneficial effects on antitumor immunity, but this hypothesis needs to be further experimentally verified and mechanistically explored.

In clinical applications, adoptive cell infusion based on γδT cells combined with ICIs or CAR-γδT cells may be promising future research directions. The feasibility and antitumor ability of CAR-γδT cells have been preliminarily proven [238, 239], and it has been shown that CAR-γδT cells can successfully migrate into the TME and cross-present tumor-associated antigens, providing more stable and durable antitumor immunity than conventional CAR-T cells in the solid tumor environment [240,241,242]. Compared with CAR-αβT cells, CAR-γδT cells have no alloreactivity and lower incidences of off-target toxicities and cytokine release syndrome [243, 244]. Arming CAR-T cells with bacterial-derived virulence factors with strong immunomodulatory properties can mediate bystander immunity through epitope spreading and can expand the therapeutic spectrum [245]. Antigen recognition by γδT cells is closely related to microbial metabolites, indicating that CAR-γδT cells may benefit from microbial engineering. CAR-γδT cells have just taken the first step toward clinical application [246, 247], and more clinical trials are required to determine the potential of CAR-γδT cells. In clinical trials, excellent antitumor activity and good patient tolerance were demonstrated for adoptive γδT-cell-based therapies after γδT cells were expanded in the presence of appropriate doses of ZA or BrHPP or infused with IL-2/IL-21 [248]. One study showed that Vδ2T cells were able to maintain a low PD-1 expression state in vivo after administration of immunotherapy [249]. However, not all patients respond to adoptive Vγ9Vδ2T-cell therapies. Considering that γδT cells present in the suppressive TME can display exhaustion or anergy, combination with ICIs might improve the cytotoxicity of γδT-cell immunotherapy and lead to better antitumor effects [250].

First describing the interaction between the TME and γδT cells, this paper has summarized the potential of γδT cells to participate in ICT by describing the functional regulation of ICs on γδT cells. Agents targeting ICs can significantly regulate the proliferation, activation and cytotoxicity of γδT cells, which provides a strategy to reverse the immunosuppressive state of γδT cells and supports the use of ICT based on γδT cells in clinical applications in the future.