γ9δ2T cell diversity and the receptor interface with tumor cells

A nonadjuvanted HLA-restricted peptide vaccine induced both T and B cell immunity against SARS-CoV-2 spike protein

During COVID-19 pandemic, cases of postvaccination infections and restored SARS-CoV-2 virus have increased after full vaccination, which might be contributed to by immune surveillance escape or virus rebound. Here, artificial linear 9-mer human leucocyte antigen (HLA)-restricted UC peptides were designed based on the well-conserved S2 region of the SARS-CoV-2 spike protein regardless of rapid mutation and glycosylation hindrance. The UC peptides were characterized for its effect on immune molecules and cells by HLA-tetramer refolding assay for HLA-binding ability, by HLA-tetramer specific T cell assay for engaged cytotoxic T lymphocytes (CTLs) involvement, by HLA-dextramer T cell assay for B cell activation, by intracellular cytokine release assay for polarization of immune response, Th1 or Th2. The specific lysis activity assay of T cells was performed for direct activation of cytotoxic T lymphocytes by UC peptides. Mice were immunized for immunogenicity of UC peptides in vivo and immunized sera was assay for complement cytotoxicity assay. Results appeared that through the engagement of UC peptides and immune molecules, HLA-I and II, that CTLs elicited cytotoxic activity by recognizing SARS-CoV-2 spike-bearing cells and preferably secreting Th1 cytokines. The UC peptides also showed immunogenicity and generated a specific antibody in mice by both intramuscular injection and oral delivery without adjuvant formulation. In conclusion, a T-cell vaccine could provide long-lasting protection against SARS-CoV-2 either during reinfection or during SARS-CoV-2 rebound. Due to its ability to eradicate SARS-CoV-2 virus-infected cells, a COVID-19 T-cell vaccine might provide a solution to lower COVID-19 severity and long COVID-19.

Vγ9Vδ2 T cells recognize butyrophilin 2A1 and 3A1 heteromers

Butyrophilin (BTN) molecules are emerging as key regulators of T cell immunity; however, how they trigger cell-mediated responses is poorly understood. Here, the crystal structure of a gamma-delta T cell antigen receptor (γδTCR) in complex with BTN2A1 revealed that BTN2A1 engages the side of the γδTCR, leaving the apical TCR surface bioavailable. We reveal that a second γδTCR ligand co-engages γδTCR via binding to this accessible apical surface in a BTN3A1-dependent manner. BTN2A1 and BTN3A1 also directly interact with each other in cis, and structural analysis revealed formation of W-shaped heteromeric multimers. This BTN2A1-BTN3A1 interaction involved the same epitopes that BTN2A1 and BTN3A1 each use to mediate the γδTCR interaction; indeed, locking BTN2A1 and BTN3A1 together abrogated their interaction with γδTCR, supporting a model wherein the two γδTCR ligand-binding sites depend on accessibility to cryptic BTN epitopes. Our findings reveal a new paradigm in immune activation, whereby γδTCRs sense dual epitopes on BTN complexes.

Unsynchronized butyrophilin molecules dictate cancer cell evasion of Vγ9Vδ2 T-cell killing

Vγ9Vδ2 T cells are specialized effector cells that have gained prominence as immunotherapy agents due to their ability to target and kill cells with altered pyrophosphate metabolites. In our effort to understand how cancer cells evade the cell-killing activity of Vγ9Vδ2 T cells, we performed a comprehensive genome-scale CRISPR screening of cancer cells. We found that four molecules belonging to the butyrophilin (BTN) family, specifically BTN2A1, BTN3A1, BTN3A2, and BTN3A3, are critically important and play unique, nonoverlapping roles in facilitating the destruction of cancer cells by primary Vγ9Vδ2 T cells. The coordinated function of these BTN molecules was driven by synchronized gene expression, which was regulated by IFN-γ signaling and the RFX complex. Additionally, an enzyme called QPCTL was shown to play a key role in modifying the N-terminal glutamine of these BTN proteins and was found to be a crucial factor in Vγ9Vδ2 T cell killing of cancer cells. Through our research, we offer a detailed overview of the functional genomic mechanisms that underlie how cancer cells escape Vγ9Vδ2 T cells. Moreover, our findings shed light on the importance of the harmonized expression and function of gene family members in modulating T-cell activity.

Engineering a Dual Specificity γδ T-Cell Receptor for Cancer Immunotherapy

γδ T-cells provide immune surveillance against cancer, straddling both innate and adaptive immunity. G115 is a clonal γδ T-cell receptor (TCR) of the Vγ9Vδ2 subtype which can confer responsiveness to phosphoantigens (PAgs) when genetically introduced into conventional αβ T-cells. Cancer immunotherapy using γδ TCR-engineered T-cells is currently under clinical evaluation. In this study, we sought to broaden the cancer specificity of the G115 γδ TCR by insertion of a tumour-binding peptide into the complementarity-determining region (CDR) three regions of the TCR δ2 chain. Peptides were selected from the foot and mouth disease virus A20 peptide which binds with high affinity and selectivity to αvβ6, an epithelial-selective integrin that is expressed by a range of solid tumours. Insertion of an A20-derived 12mer peptide achieved the best results, enabling the resulting G115 + A12 T-cells to kill both PAg and αvβ6-expressing tumour cells. Cytolytic activity of G115 + A12 T-cells against PAg-presenting K562 target cells was enhanced compared to G115 control cells, in keeping with the critical role of CDR3 δ2 length for optimal PAg recognition. Activation was accompanied by interferon (IFN)-γ release in the presence of either target antigen, providing a novel dual-specificity approach for cancer immunotherapy.

Analysis of Butyrophilin-Mediated Activation of γδ T Cells from Human Spleen

There is considerable interest in therapeutically engaging human γδ T cells. However, due to the unique TCRs of human γδ T cells, studies from animal models have provided limited directly applicable insights, and human γδ T cells from key immunological tissues remain poorly characterized. In this study, we investigated γδ T cells from human spleen tissue. Compared to blood, where Vδ2+Vγ9+ T cells are the dominant subset, splenic γδ T cells included a variety of TCR types, with Vδ1+ T cells typically being the most frequent. Intracellular cytokine staining revealed that IFN-γ was produced by a substantial fraction of splenic γδ T cells, IL-17A by a small fraction, and IL-4 was minimal. Primary splenic γδ T cells frequently expressed NKG2D (NK group 2 member D) and CD16, whereas expression of DNAM-1 (DNAX accessory molecule 1), CD28, PD-1, TIGIT, and CD94 varied according to subset, and there was generally little expression of natural cytotoxicity receptors, TIM-3, LAG-3, or killer Ig-like receptors. In vitro expansion was associated with marked changes in expression of these activating and inhibitory receptors. Analysis of functional responses of spleen-derived Vδ2+Vγ9+, Vδ1+Vγ9+, and Vδ1+Vγ9- T cell lines to recombinant butyrophilin BTN2A1 and BTN3A1 demonstrated that both Vδ2+Vγ9+ and Vδ1+Vγ9+ T cells were capable of responding to the extracellular domain of BTN2A1, whereas the addition of BTN3A1 only markedly enhanced the responses of Vδ2+Vγ9+ T cells. Conversely, Vδ1+Vγ9+ T cells appeared more responsive than Vδ2+Vγ9+ T cells to TCR-independent NKG2D stimulation. Thus, despite shared recognition of BTN2A1, differential effects of BTN3A1 and coreceptors may segregate target cell responses of Vδ2+Vγ9+ and Vδ1+Vγ9+ T cells.

Dual targeting of cancer metabolome and stress antigens affects transcriptomic heterogeneity and efficacy of engineered T cells

Few cancers can be targeted efficiently by engineered T cell strategies. Here, we show that γδ T cell antigen receptor (γδ TCR)-mediated cancer metabolome targeting can be combined with targeting of cancer-associated stress antigens (such as NKG2D ligands or CD277) through the addition of chimeric co-receptors. This strategy overcomes suboptimal γ9δ2 TCR engagement of αβ T cells engineered to express a defined γδ TCR (TEGs) and improves serial killing, proliferation and persistence of TEGs. In vivo, the NKG2D-CD28WT chimera enabled control only of liquid tumors, whereas the NKG2D-4-1BBCD28TM chimera prolonged persistence of TEGs and improved control of liquid and solid tumors. The CD277-targeting chimera (103-4-1BB) was the most optimal co-stimulation format, eradicating both liquid and solid tumors. Single-cell transcriptomic analysis revealed that NKG2D-4-1BBCD28TM and 103-4-1BB chimeras reprogram TEGs through NF-κB. Owing to competition with naturally expressed NKG2D in CD8+ TEGs, the NKG2D-4-1BBCD28TM chimera mainly skewed CD4+ TEGs toward adhesion, proliferation, cytotoxicity and less exhausted signatures, whereas the 103-4-1BB chimera additionally shaped the CD8+ subset toward a proliferative state.

γδ-Enriched CAR-T cell therapy for bone metastatic castrate-resistant prostate cancer

Immune checkpoint blockade has been largely unsuccessful for the treatment of bone metastatic castrate-resistant prostate cancer (mCRPC). Here, we report a combinatorial strategy to treat mCRPC using γδ-enriched chimeric antigen receptor (CAR) T cells and zoledronate (ZOL). In a preclinical murine model of bone mCRPC, γδ CAR-T cells targeting prostate stem cell antigen (PSCA) induced a rapid and significant regression of established tumors, combined with increased survival and reduced cancer-associated bone disease. Pretreatment with ZOL, a U.S. Food and Drug Administration-approved bisphosphonate prescribed to mitigate pathological fracture in mCRPC patients, resulted in CAR-independent activation of γδ CAR-T cells, increased cytokine secretion, and enhanced antitumor efficacy. These data show that the activity of the endogenous Vγ9Vδ2 T cell receptor is preserved in CAR-T cells, allowing for dual-receptor recognition of tumor cells. Collectively, our findings support the use of γδ CAR-T cell therapy for mCRPC treatment.

Phosphoantigen sensing combines TCR-dependent recognition of the BTN3A IgV domain and germline interaction with BTN2A1

Vγ9Vδ2 T cells play critical roles in microbial immunity by detecting target cells exposed to pathogen-derived phosphoantigens (P-Ags). Target cell expression of BTN3A1, the "P-Ag sensor," and BTN2A1, a direct ligand for T cell receptor (TCR) Vγ9, is essential for this process; however, the molecular mechanisms involved are unclear. Here, we characterize BTN2A1 interactions with Vγ9Vδ2 TCR and BTN3A1. Nuclear magnetic resonance (NMR), modeling, and mutagenesis establish a BTN2A1-immunoglobulin V (IgV)/BTN3A1-IgV structural model compatible with their cell-surface association in cis. However, TCR and BTN3A1-IgV binding to BTN2A1-IgV is mutually exclusive, owing to binding site proximity and overlap. Moreover, mutagenesis indicates that the BTN2A1-IgV/BTN3A1-IgV interaction is non-essential for recognition but instead identifies a molecular surface on BTN3A1-IgV essential to P-Ag sensing. These results establish a critical role for BTN3A-IgV in P-Ag sensing, in mediating direct or indirect interactions with the γδ-TCR. They support a composite-ligand model whereby intracellular P-Ag detection coordinates weak extracellular germline TCR/BTN2A1 and clonotypically influenced TCR/BTN3A-mediated interactions to initiate Vγ9Vδ2 TCR triggering.

Phenotypic and functional characterization of pharmacologically expanded Vγ9Vδ2 T cells in pigtail macaques

While gaining interest as treatment for cancer and infectious disease, the clinical efficacy of Vγ9Vδ2 T cell-based immunotherapeutics has to date been limited. An improved understanding of γδ T cell heterogeneity across lymphoid and non-lymphoid tissues, before and after pharmacological expansion, is required. Here, we describe the phenotype and tissue distribution of Vγ9Vδ2 T cells at steady state and following in vivo pharmacological expansion in pigtail macaques. Intravenous phosphoantigen administration with subcutaneous rhIL-2 drove robust expansion of Vγ9Vδ2 T cells in blood and pulmonary mucosa, while expansion was confined to the pulmonary mucosa following intratracheal antigen administration. Peripheral blood Vγ9Vδ2 T cell expansion was polyclonal, and associated with a significant loss of CCR6 expression due to IL-2-mediated receptor downregulation. Overall, we show the tissue distribution and phenotype of in vivo pharmacologically expanded Vγ9Vδ2 T cells can be altered based on the antigen administration route, with implications for tissue trafficking and the clinical efficacy of Vγ9Vδ2 T cell immunotherapeutics.

Gamma delta T-cell-based immune checkpoint therapy: attractive candidate for antitumor treatment

As a nontraditional T-cell subgroup, γδT cells have gained popularity in the field of immunotherapy in recent years. They have extraordinary antitumor potential and prospects for clinical application. Immune checkpoint inhibitors (ICIs), which are efficacious in tumor patients, have become pioneer drugs in the field of tumor immunotherapy since they were incorporated into clinical practice. In addition, γδT cells that have infiltrated into tumor tissues are found to be in a state of exhaustion or anergy, and there is upregulation of many immune checkpoints (ICs) on their surface, suggesting that γδT cells have a similar ability to respond to ICIs as traditional effector T cells. Studies have shown that targeting ICs can reverse the dysfunctional state of γδT cells in the tumor microenvironment (TME) and exert antitumor effects by improving γδT-cell proliferation and activation and enhancing cytotoxicity. Clarification of the functional state of γδT cells in the TME and the mechanisms underlying their interaction with ICs will solidify ICIs combined with γδT cells as a good treatment option.

The making of multivalent gamma delta TCR anti-CD3 bispecific T cell engagers

Introduction

We have recently developed a novel T cell engager concept by utilizing γ9δ2TCR as tumor targeting domain, named gamma delta TCR anti-CD3 bispecific molecule (GAB), targeting the phosphoantigen-dependent orchestration of BTN2A1 and BTN3A1 at the surface of cancer cells. GABs are made by the fusion of the ectodomains of a γδTCR to an anti-CD3 single chain variable fragment (scFv) (γδECTO-αCD3), here we explore alternative designs with the aim to enhance GAB effectivity.

Methods

The first alternative design was made by linking the variable domains of the γ and δ chain to an anti-CD3 scFv (γδVAR-αCD3). The second alternative design was multimerizing γδVAR-αCD3 proteins to increase the tumor binding valency. Both designs were expressed and purified and the potency to target tumor cells by T cells of the alternative designs was compared to γδECTO-αCD3, in T cell activation and cytotoxicity assays.

Results and discussion

The γδVAR-αCD3 proteins were poorly expressed, and while the addition of stabilizing mutations based on finding for αβ single chain formats increased expression, generation of meaningful amounts of γδVAR-αCD3 protein was not possible. As an alternative strategy, we explored the natural properties of the original GAB design (γδECTO-αCD3), and observed the spontaneous formation of γδECTO-αCD3-monomers and -dimers during expression. We successfully enhanced the fraction of γδECTO-αCD3-dimers by shortening the linker length between the heavy and light chain in the anti-CD3 scFv, though this also decreased protein yield by 50%. Finally, we formally demonstrated with purified γδECTO-αCD3-dimers and -monomers, that γδECTO-αCD3-dimers are superior in function when compared to similar concentrations of monomers, and do not induce T cell activation without simultaneous tumor engagement. In conclusion, a γδECTO-αCD3-dimer based GAB design has great potential, though protein production needs to be further optimized before preclinical and clinical testing.

Uncovering the mode of action of engineered T cells in patient cancer organoids

Extending the success of cellular immunotherapies against blood cancers to the realm of solid tumors will require improved in vitro models that reveal therapeutic modes of action at the molecular level. Here we describe a system, called BEHAV3D, developed to study the dynamic interactions of immune cells and patient cancer organoids by means of imaging and transcriptomics. We apply BEHAV3D to live-track >150,000 engineered T cells cultured with patient-derived, solid-tumor organoids, identifying a 'super engager' behavioral cluster comprising T cells with potent serial killing capacity. Among other T cell concepts we also study cancer metabolome-sensing engineered T cells (TEGs) and detect behavior-specific gene signatures that include a group of 27 genes with no previously described T cell function that are expressed by super engager killer TEGs. We further show that type I interferon can prime resistant organoids for TEG-mediated killing. BEHAV3D is a promising tool for the characterization of behavioral-phenotypic heterogeneity of cellular immunotherapies and may support the optimization of personalized solid-tumor-targeting cell therapies.

Cutting Edge: Bispecific γδ T Cell Engager Containing Heterodimeric BTN2A1 and BTN3A1 Promotes Targeted Activation of Vγ9Vδ2+ T Cells in the Presence of Costimulation by CD28 or NKG2D

Vγ9Vδ2⁺ T cell-targeted immunotherapy is of interest to harness its MHC-independent cytotoxic potential against a variety of cancers. Recent studies have identified heterodimeric butyrophilin (BTN) 2A1 and BTN3A1 as the molecular entity providing "signal 1" to the Vγ9Vδ2 TCR, but "signal 2" costimulatory requirements remain unclear. Using a tumor cell-free assay, we demonstrated that a BTN2A1/3A1 heterodimeric fusion protein activated human Vγ9Vδ2⁺ T cells, but only in the presence of costimulatory signal via CD28 or NK group 2 member D. Nonetheless, addition of a bispecific γδ T cell engager BTN2A1/3A1-Fc-CD19scFv alone enhanced granzyme B-mediated killing of human CD19⁺ lymphoma cells when cocultured with Vγ9Vδ2⁺ T cells, suggesting expression of costimulatory ligand(s) on tumor cells is sufficient to satisfy the "signal 2" requirement. These results highlight the parallels of signal 1 and signal 2 requirements in αβ and γδ T cell activation and demonstrate the utility of heterodimeric BTNs to promote targeted activation of γδ T cells.

γδ T Cells in the Tumor Microenvironment-Interactions With Other Immune Cells

A growing number of studies have shown that γδ T cells play a pivotal role in mediating the clearance of tumors and pathogen-infected cells with their potent cytotoxic, cytolytic, and unique immune-modulating functions. Unlike the more abundant αβ T cells, γδ T cells can recognize a broad range of tumors and infected cells without the requirement of antigen presentation via major histocompatibility complex (MHC) molecules. Our group has recently demonstrated parts of the mechanisms of T-cell receptor (TCR)-dependent activation of Vγ9Vδ2+ T cells by tumors following the presentation of phosphoantigens, intermediates of the mevalonate pathway. This process is mediated through the B7 immunoglobulin family-like butyrophilin 2A1 (BTN2A1) and BTN3A1 complexes. Such recognition results in activation, a robust immunosurveillance process, and elicits rapid γδ T-cell immune responses. These include targeted cell killing, and the ability to produce copious quantities of cytokines and chemokines to exert immune-modulating properties and to interact with other immune cells. This immune cell network includes αβ T cells, B cells, dendritic cells, macrophages, monocytes, natural killer cells, and neutrophils, hence heavily influencing the outcome of immune responses. This key role in orchestrating immune cells and their natural tropism for tumor microenvironment makes γδ T cells an attractive target for cancer immunotherapy. Here, we review the current understanding of these important interactions and highlight the implications of the crosstalk between γδ T cells and other immune cells in the context of anti-tumor immunity.

γ9δ2 T-Cell Expansion and Phenotypic Profile Are Reflected in the CDR3δ Repertoire of Healthy Adults

γ9δ2T cells fill a distinct niche in human immunity due to the unique physiology of the phosphoantigen-reactive γ9δ2TCR. Here, we highlight reproducible TCRδ complementarity-determining region 3 (CDR3δ) repertoire patterns associated with γ9δ2T cell proliferation and phenotype, thus providing evidence for the role of the CDR3δ in modulating in vivo T-cell responses. Features that determine γ9δ2TCR binding affinity and reactivity to the phosphoantigen-induced ligand in vitro appear to similarly underpin in vivo clonotypic expansion and differentiation. Likewise, we identify a CDR3δ bias in the γ9δ2T cell natural killer receptor (NKR) landscape. While expression of the inhibitory receptor CD94/NKG2A is skewed toward cells bearing putative high-affinity TCRs, the activating receptor NKG2D is expressed independently of the phosphoantigen-sensing determinants, suggesting a higher net NKR activating signal in T cells with TCRs of low affinity. This study establishes consistent repertoire–phenotype associations and justifies stratification for the T-cell phenotype in future research on γ9δ2TCR repertoire dynamics.

Generation of effector Vγ9Vδ2 T cells and evaluation of their response to phosphoantigen-loaded cells

Vγ9Vδ2 T cells are non-canonical T cells that use their T cell receptor to detect phosphoantigens bound to the internal domain of the HMBPP receptor (butyrophilin 3/2A1 complex). This protocol describes the expansion and purification of human effector Vγ9Vδ2 T cells from human buffy coat and describes how to assess their activation by antigen-containing target cells. While specifically focused on cytokine production, this protocol can be readily adapted to evaluate other effector functions of activated Vγ9Vδ2 T cells. For complete details on the use and execution of this protocol, please refer to Hsiao et al. (2022) and Hsiao and Wiemer (2018).

Efficiency of bis-amidate phosphonate prodrugs

Bis-amidate derivatives have been viewed as attractive phosphonate prodrug forms because of their straightforward synthesis, lack of phosphorus stereochemistry, plasma stability and nontoxic amino acid metabolites. However, the efficiency of bis-amidate prodrug forms is unclear, as prior studies on this class of prodrugs have not evaluated their activation kinetics. Here, we synthetized a small panel of bis-amidate prodrugs of butyrophilin ligands as potential immunotherapy agents. These compounds were examined relative to other prodrug forms delivering the same payload for their stability in plasma and cell lysate, their ability to stimulate T cell proliferation in human PBMCs, and their activation kinetics in a leukemia co-culture model of T cell cytokine production. The bis-amidate prodrugs demonstrate high plasma stability and improved cellular phosphoantigen activity relative to the free phosphonic acid. However, the efficiency of bis-amidate activation is low relative to other prodrugs that contain at least one ester such as aryl-amidate, aryl-acyloxyalkyl ester, and bis-acyloxyalkyl ester forms. Therefore, bis-amidate prodrugs do not drive rapid cellular payload accumulation and they would be more useful for payloads in which slower, sustained-release kinetics are preferred.

Contemplating Dichotomous Nature of Gamma Delta T Cells for Immunotherapy

γδ T cells are unconventional T cells, distinguished from αβ T cells in a number of functional properties. Being small in number compared to αβ T cells, γδ T cells have surprised us with their pleiotropic roles in various diseases. γδ T cells are ambiguous in nature as they can produce a number of cytokines depending on the (micro) environmental cues and engage different immune response mechanisms, mainly due to their epigenetic plasticity. Depending on the disease condition, γδ T cells contribute to beneficial or detrimental response. In this review, we thus discuss the dichotomous nature of γδ T cells in cancer, neuroimmunology and infectious diseases. We shed light on the importance of equal consideration for systems immunology and personalized approaches, as exemplified by changes in metabolic requirements. While providing the status of immunotherapy, we will assess the metabolic (and other) considerations for better outcome of γδ T cell-based treatments.

Antigen-Specific TCR-T Cells for Acute Myeloid Leukemia: State of the Art and Challenges

The cytogenetic abnormalities and molecular mutations involved in acute myeloid leukemia (AML) lead to unique treatment challenges. Although adoptive T-cell therapies (ACT) such as chimeric antigen receptor (CAR) T-cell therapy have shown promising results in the treatment of leukemias, especially B-cell malignancies, the optimal target surface antigen has yet to be discovered for AML. Alternatively, T-cell receptor (TCR)-redirected T cells can target intracellular antigens presented by HLA molecules, allowing the exploration of a broader territory of new therapeutic targets. Immunotherapy using adoptive transfer of WT1 antigen-specific TCR-T cells, for example, has had positive clinical successes in patients with AML. Nevertheless, AML can escape from immune system elimination by producing immunosuppressive factors or releasing several cytokines. This review presents recent advances of antigen-specific TCR-T cells in treating AML and discusses their challenges and future directions in clinical applications.

The Role of γδ T Cells as a Line of Defense in Viral Infections after Allogeneic Stem Cell Transplantation: Opportunities and Challenges

In the complex interplay between inflammation and graft-versus-host disease (GVHD) after allogeneic stem cell transplantation (allo-HSCT), viral reactivations are often observed and cause substantial morbidity and mortality. As toxicity after allo-HSCT within the context of viral reactivations is mainly driven by αβ T cells, we describe that by delaying αβ T cell reconstitution through defined transplantation techniques, we can harvest the full potential of early reconstituting γδ T cells to control viral reactivations. We summarize evidence of how the γδ T cell repertoire is shaped by CMV and EBV reactivations after allo-HSCT, and their potential role in controlling the most important, but not all, viral reactivations. As most γδ T cells recognize their targets in an MHC-independent manner, γδ T cells not only have the potential to control viral reactivations but also to impact the underlying hematological malignancies. We also highlight the recently re-discovered ability to recognize classical HLA-molecules through a γδ T cell receptor, which also surprisingly do not associate with GVHD. Finally, we discuss the therapeutic potential of γδ T cells and their receptors within and outside the context of allo-HSCT, as well as the opportunities and challenges for developers and for payers.

TGF-β1 potentiates Vγ9Vδ2 T cell adoptive immunotherapy of cancer

Despite its role in cancer surveillance, adoptive immunotherapy using γδ T cells has achieved limited efficacy. To enhance trafficking to bone marrow, circulating Vγ9Vδ2 T cells are expanded in serum-free medium containing TGF-β1 and IL-2 (γδ[T2] cells) or medium containing IL-2 alone (γδ[2] cells, as the control). Unexpectedly, the yield and viability of γδ[T2] cells are also increased by TGF-β1, when compared to γδ[2] controls. γδ[T2] cells are less differentiated and yet display increased cytolytic activity, cytokine release, and antitumor activity in several leukemic and solid tumor models. Efficacy is further enhanced by cancer cell sensitization using aminobisphosphonates or Ara-C. A number of contributory effects of TGF-β are described, including prostaglandin E2 receptor downmodulation, TGF-β insensitivity, and upregulated integrin activity. Biological relevance is supported by the identification of a favorable γδ[T2] signature in acute myeloid leukemia (AML). Given their enhanced therapeutic activity and compatibility with allogeneic use, γδ[T2] cells warrant evaluation in cancer immunotherapy.

Unconventional T cells and kidney disease

Unconventional T cells are a diverse and underappreciated group of relatively rare lymphocytes that are distinct from conventional CD4⁺ and CD8⁺ T cells, and that mainly recognize antigens in the absence of classical restriction through the major histocompatibility complex (MHC). These non-MHC-restricted T cells include mucosal-associated invariant T (MAIT) cells, natural killer T (NKT) cells, γδ T cells and other, often poorly defined, subsets. Depending on the physiological context, unconventional T cells may assume either protective or pathogenic roles in a range of inflammatory and autoimmune responses in the kidney. Accordingly, experimental models and clinical studies have revealed that certain unconventional T cells are potential therapeutic targets, as well as prognostic and diagnostic biomarkers. The responsiveness of human Vγ9Vδ2 T cells and MAIT cells to many microbial pathogens, for example, has implications for early diagnosis, risk stratification and targeted treatment of peritoneal dialysis-related peritonitis. The expansion of non-Vγ9Vδ2 γδ T cells during cytomegalovirus infection and their contribution to viral clearance suggest that these cells can be harnessed for immune monitoring and adoptive immunotherapy in kidney transplant recipients. In addition, populations of NKT, MAIT or γδ T cells are involved in the immunopathology of IgA nephropathy and in models of glomerulonephritis, ischaemia-reperfusion injury and kidney transplantation.

Gamma delta TCR anti-CD3 bispecific molecules (GABs) as novel immunotherapeutic compounds

Background

γ9δ2 T cells hold great promise as cancer therapeutics because of their unique capability of reacting to metabolic changes with tumor cells. However, it has proven very difficult to translate this promise into clinical success.

Methods

In order to better utilize the tumor reactivity of γ9δ2T cells and combine this with the great potential of T cell engager molecules, we developed a novel bispecific molecule by linking the extracellular domains of tumor-reactive γ9δ2TCRs to a CD3-binding moiety, creating gamma delta TCR anti-CD3 bispecific molecules (GABs). GABs were tested in vitro and in vivo for ability to redirect T lymphocytes to a variety of tumor cell lines and primary patient material.

Results

GABs utilizing naturally occurring high affinity γ9δ2TCRs efficiently induced αβT cell mediated phosphoantigen-dependent recognition of tumor cells. Reactivity was substantially modulated by variations in the Vδ2 CDR3-region and the BTN2A1-binding HV4-region between CDR2 and CDR3 of the γ-chain was crucial for functionality. GABs redirected αβT cells against a broad range of hematopoietic and solid tumor cell lines and primary acute myeloid leukemia. Furthermore, they enhanced infiltration of immune cells in a 3D bone marrow niche and left healthy tissues intact, while eradicating primary multiple myeloma cells. Lastly, GABs constructed from natural high affinity γ9δ2TCR sequences significantly reduced tumor growth in vivo in a subcutaneous myeloma xenograft model.

Conclusions

We conclude that GABs allow for the introduction of metabolic targeting of cancer cells to the field of T cell engagers.

Adding Help to an HLA-A*24:02 Tumor-Reactive γδTCR Increases Tumor Control

γδT cell receptors (γδTCRs) recognize a broad range of malignantly transformed cells in mainly a major histocompatibility complex (MHC)-independent manner, making them valuable additions to the engineered immune effector cell therapy that currently focuses primarily on αβTCRs and chimeric antigen receptors (CARs). As an exception to the rule, we have previously identified a γδTCR, which exerts antitumor reactivity against HLA-A*24:02-expressing malignant cells, however without the need for defined HLA-restricted peptides, and without exhibiting any sign of off-target toxicity in humanized HLA-A*24:02 transgenic NSG (NSG-A24:02) mouse models. This particular tumor-HLA-A*24:02-specific Vγ5Vδ1TCR required CD8αα co-receptor for its tumor reactive capacity when introduced into αβT cells engineered to express a defined γδTCR (TEG), referred to as TEG011; thus, it was only active in CD8⁺ TEG011. We subsequently explored the concept of additional redirection of CD4⁺ T cells through co-expression of the human CD8α gene into CD4⁺ and CD8⁺ TEG011 cells, later referred as TEG011_CD8α. Adoptive transfer of TEG011_CD8α cells in humanized HLA-A*24:02 transgenic NSG (NSG-A24:02) mice injected with tumor HLA-A*24:02⁺ cells showed superior tumor control in comparison to TEG011, and to mock control groups. The total percentage of mice with persisting TEG011_CD8α cells, as well as the total number of TEG011_CD8α cells per mice, was significantly improved over time, mainly due to a dominance of CD4⁺CD8⁺ double-positive TEG011_CD8α, which resulted in higher total counts of functional T cells in spleen and bone marrow. We observed that tumor clearance in the bone marrow of TEG011_CD8α-treated mice associated with better human T cell infiltration, which was not observed in the TEG011-treated group. Overall, introduction of transgenic human CD8α receptor on TEG011 improves antitumor reactivity against HLA-A*24:02⁺ tumor cells and further enhances in vivo tumor control.

γδ T Cells for Leukemia Immunotherapy: New and Expanding Trends

Recently, many discoveries have elucidated the cellular and molecular diversity in the leukemic microenvironment and improved our knowledge regarding their complex nature. This has allowed the development of new therapeutic strategies against leukemia. Advances in biotechnology and the current understanding of T cell-engineering have led to new approaches in this fight, thus improving cell-mediated immune response against cancer. However, most of the investigations focus only on conventional cytotoxic cells, while ignoring the potential of unconventional T cells that until now have been little studied. γδ T cells are a unique lymphocyte subpopulation that has an extensive repertoire of tumor sensing and may have new immunotherapeutic applications in a wide range of tumors. The ability to respond regardless of human leukocyte antigen (HLA) expression, the secretion of antitumor mediators and high functional plasticity are hallmarks of γδ T cells, and are ones that make them a promising alternative in the field of cell therapy. Despite this situation, in particular cases, the leukemic microenvironment can adopt strategies to circumvent the antitumor response of these lymphocytes, causing their exhaustion or polarization to a tumor-promoting phenotype. Intervening in this crosstalk can improve their capabilities and clinical applications and can make them key components in new therapeutic antileukemic approaches. In this review, we highlight several characteristics of γδ T cells and their interactions in leukemia. Furthermore, we explore strategies for maximizing their antitumor functions, aiming to illustrate the findings destined for a better mobilization of γδ T cells against the tumor. Finally, we outline our perspectives on their therapeutic applicability and indicate outstanding issues for future basic and clinical leukemia research, in the hope of contributing to the advancement of studies on γδ T cells in cancer immunotherapy.

Characterization and modulation of anti-αβTCR antibodies and their respective binding sites at the βTCR chain to enrich engineered T cells

T cell engineering strategies offer cures to patients and have entered clinical practice with chimeric antibody-based receptors; αβT cell receptor (αβTCR)-based strategies are, however, lagging behind. To allow a more rapid and successful translation to successful concepts also using αβTCRs for engineering, incorporating a method for the purification of genetically modified T cells, as well as engineered T cell deletion after transfer into patients, could be beneficial. This would allow increased efficacy, reduced potential side effects, and improved safety of newly to-be-tested lead structures. By characterizing the antigen-binding interface of a good manufacturing process (GMP)-grade anti-αβTCR antibody, usually used for depletion of αβT cells from stem cell transplantation products, we developed a strategy that allows for the purification of untouched αβTCR-engineered immune cells by changing 2 amino acids only in the TCRβ chain constant domain of introduced TCR chains. Alternatively, we engineered an antibody that targets an extended mutated interface of 9 amino acids in the TCRβ chain constant domain and provides the opportunity to further develop depletion strategies of engineered immune cells.

Targeting butyrophilins for cancer immunotherapy

Vγ9Vδ2⁺ T cells form part of the innate immune repertoire and are activated by phosphorylated antigens produced by many bacteria and tumors. They have long been suggested as promising targets for anti-tumor therapies, but clinical trials so far have not shown major successes. Several recent discoveries could help to overcome these shortfalls, such as those leading to an improved understanding of the role of butyrophilin molecules BTN2A1 and BTN3A1, in Vγ9Vδ2⁺ T cell activation. Moreover, we propose that studies suggesting the presence of live bacteria in a variety of tumors (tumor microbiome), indicate that the latter might be harnessed as a source of high affinity bacterial phosphoantigen to trigger or enhance anti-tumor immune responses.

BTN2A1, an immune checkpoint targeting Vγ9Vδ2 T cell cytotoxicity against malignant cells

The anti-tumor response of Vγ9Vδ2 T cells requires the sensing of accumulated phosphoantigens (pAgs) bound intracellularly to butyrophilin 3A1 (BTN3A1). In this study, we show that butyrophilin 2A1 (BTN2A1) is required for BTN3A-mediated Vγ9Vδ2 T cell cytotoxicity against cancer cells, and that expression of the BTN2A1/BTN3A1 complex is sufficient to trigger Vγ9Vδ2 TCR activation. Also, BTN2A1 interacts with all isoforms of BTN3A (BTN3A1, BTN3A2, BTN3A3), which appears to be a rate-limiting factor to BTN2A1 export to the plasma membrane. BTN2A1/BTN3A1 interaction is enhanced by pAgs and, strikingly, B30.2 domains of both proteins are required for pAg responsiveness. BTN2A1 expression in cancer cells correlates with bisphosphonate-induced Vγ9Vδ2 T cell cytotoxicity. Vγ9Vδ2 T cell killing of cancer cells is modulated by anti-BTN2A1 monoclonal antibodies (mAbs), whose action relies on the inhibition of BTN2A1 binding to the Vγ9Vδ2TCR. This demonstrates the potential of BTN2A1 as a therapeutic target and adds to the emerging butyrophilin-family cooperation pathway in γδ T cell activation.

Allogeneic Stem Cell Transplantation Platforms With Ex Vivo and In Vivo Immune Manipulations: Count and Adjust

Various allogeneic (allo) stem cell transplantation platforms have been developed over the last 2 decades. In this review we focus on the impact of in vivo and ex vivo graft manipulation on immune reconstitution and clinical outcome. Strategies include anti-thymocyte globulin- and post-transplantation cyclophosphamide-based regimens, as well as graft engineering, such as CD34 selection and CD19/αβT cell depletion. Differences in duration of immune suppression, reconstituting immune repertoires, and associated graft-versus-leukemia effects and toxicities mediated through viral reactivations are highlighted. In addition, we discuss the impact of different reconstituting repertoires on donor lymphocyte infusions and post allo pharmacological interventions to enhance tumor control. We advocate for precisely counting all graft ingredients and therapeutic drug monitoring during conditioning in the peripheral blood, and for adjusting dosing accordingly on an individual basis. In addition, we propose novel trial designs to better assess the impact of variations in transplantation platforms in order to better learn from our diversity of "counts" and potential "adjustments." This will, in the future, allow daily clinical practice, strategic choices, and future trial designs to be based on data guided decisions, rather than relying on dogma and habits.

Nuclear Dishevelled targets gene regulatory regions and promotes tumor growth

Dishevelled (DVL) critically regulates Wnt signaling and contributes to a wide spectrum of diseases and is important in normal and pathophysiological settings. However, how it mediates diverse cellular functions remains poorly understood. Recent discoveries have revealed that constitutive Wnt pathway activation contributes to breast cancer malignancy, but the mechanisms by which this occurs are unknown and very few studies have examined the nuclear role of DVL. Here, we have performed DVL3 ChIP-seq analyses and identify novel target genes bound by DVL3. We show that DVL3 depletion alters KMT2D binding to novel targets and changes their epigenetic marks and mRNA levels. We further demonstrate that DVL3 inhibition leads to decreased tumor growth in two different breast cancer models in vivo. Our data uncover new DVL3 functions through its regulation of multiple genes involved in developmental biology, antigen presentation, metabolism, chromatin remodeling, and tumorigenesis. Overall, our study provides unique insight into the function of nuclear DVL, which helps to define its role in mediating aberrant Wnt signaling.

Adoptive γδT-cell transfer alone or combined with chemotherapy for the treatment of advanced esophageal cancer

BACKGROUND AIMS

After therapy with platinum, 5-fluorouracil and taxane, no further recommended therapy is available for recurrent or metastatic esophageal cancer (r/mEC). Here the authors report two phase 1 trials of adoptive γδT-cell therapy, one for treatment-refractory r/mEC (γδT-monotherapy-P1, UMIN000001419) and the other for r/mEC with no prior systemic therapy (DCF-γδT-P1, UMIN000008097).

METHODS

For γδT-monotherapy-P1, patients received four weekly and four biweekly injections of autologous γδT cells. For DCF-γδT-P1, patients received docetaxel, cisplatin and 5-fluorouracil (DCF) chemotherapy consisting of docetaxel (60 mg/m²) and cisplatin (60 mg/m²) on day 1 and continuous injection of 5-fluorouracil (600 mg/m²/day) on days 1-5 of each 28-day cycle; additionally, they received autologous γδT-cell injections on day 15 and day 22 of each cycle.

RESULTS

Twenty-six patients were enrolled for γδT-monotherapy-P1. No severe adverse events were associated with γδT-cell therapy. Median overall survival was 5.7 months (95% confidence interval [CI], 4.3-10.0), and median progression-free survival was 2.4 months (95% CI, 1.7-2.8). Eighteen patients received DCF-γδT-P1. All treatment-related adverse events were associated with DCF chemotherapy, not γδT injection. Median overall survival was 13.4 months (95% CI, 6.7-not reached), and median progression-free survival was 4.0 months (95% CI, 2.5-5.7). The response rate and disease control rate were 39% and 78%, respectively.

CONCLUSIONS

The use of γδT-cell immunotherapy with or without chemotherapy was safe and feasible for r/mEC patients. Although the authors failed to demonstrate any clinical benefit of γδT-monotherapy-P1, survival benefits were observed in the DCF-γδT-P1 trial.

Phase II Trial of Maintenance Treatment With IL2 and Zoledronate in Multiple Myeloma After Bone Marrow Transplantation: Biological and Clinical Results

Background

Maintenance treatment after autologous bone marrow transplantation in multiple myeloma improves the outcome of patients. We designed a phase II clinical trial to evaluate the treatment with IL2 and zoledronate after autologous bone marrow transplantation in myeloma patients.

Methods

Patients with a histologically proven diagnosis of multiple myeloma become eligible if achieved a very good partial remission in bone marrow samples after 3 months from autologous bone marrow transplantation. IL2 was administered from day 1 to 7. In the first cycle, the daily dose was 2 × 10⁶ IU, whereas, in subsequent ones the IL2 dose was progressively escalated, with +25% increases at each cycle, until evidence of toxicity or up to 8 × 10⁶ IU. Four mg of zoledronic acid were infused on day 2. Flow cytometry analysis of γδ-lymphocytes was performed at days 1 and 8 of treatment cycles.

Results

Forty-four patients have been enrolled between 2013 and 2016. The median time to progression was 22.5 months (95% CI 9.7–35.2). A complete remission with a negative immunofixation was obtained in 18% of patients and correlated with a significantly longer time to progression (p = 0.015). Treatment was well tolerated without G3 or 4 toxicities. After a week of treatment with IL2 and zoledronate, γδ lymphocytes, Vγ9δ2, CD57+, effector, late effector, and memory γδ increased but in subsequent cycles, there was a progressive reduction of this expansion.

Conclusions

The maintenance treatment with IL2 and Zoledronate has a modest activity in myeloma patients after autologous bone marrow transplantation.

EudraCT Number

2013-001188-22.

Translating Unconventional T Cells and Their Roles in Leukemia Antitumor Immunity

Recently, cell-mediated immune response in malignant neoplasms has become the focus in immunotherapy against cancer. However, in leukemia, most studies on the cytotoxic potential of T cells have concentrated only on T cells that recognize peptide antigens (Ag) presented by polymorphic molecules of the major histocompatibility complex (MHC). This ignores the great potential of unconventional T cell populations, which include gamma-delta T cells (γδ), natural killer T cells (NKT), and mucosal-associated invariant T cells (MAIT). Collectively, these T cell populations can recognize lipid antigens, specially modified peptides and small molecule metabolites, in addition to having several other advantages, which can provide more effective applications in cancer immunotherapy. In recent years, these cell populations have been associated with a repertoire of anti- or protumor responses and play important roles in the dynamics of solid tumors and hematological malignancies, thus, encouraging the development of new investigations in the area. This review focuses on the current knowledge regarding the role of unconventional T cell populations in the antitumor immune response in leukemia and discusses why further studies on the immunotherapeutic potential of these cells are needed.

Innate and adaptive γδ T cells: How, when, and why

γδ T cells comprise the third cell lineage of lymphocytes that use, like αβ T cells and B cells, V(D)J gene rearrangement with the potential to generate a highly diverse T cell receptor (TCR) repertoire. There is no obvious conservation of γδ T cell subsets (based on TCR repertoire and/or function) between mice and human, leading to the notion that human and mouse γδ T cells are highly different. In this review, we focus on human γδ T cells, building on recent studies using high-throughput sequencing to analyze the TCR repertoire in various settings. We make then the comparison with mouse γδ T cell subsets highlighting the similarities and differences and describe the remarkable changes during lifespan of innate and adaptive γδ T cells. Finally, we propose mechanisms contributing to the generation of innate versus adaptive γδ T cells. We conclude that key elements related to the generation of the γδ TCR repertoire and γδ T cell activation/development are conserved between human and mice, highlighting the similarities between these two species.

Diversity in recognition and function of human γδ T cells

As interest increases in harnessing the potential power of tissue-resident cells for human health and disease, γδ T cells have been thrust into the limelight due to their prevalence in peripheral tissues, their sentinel-like phenotypes, and their unique antigen recognition capabilities. This review focuses primarily on human γδ T cells, highlighting their distinctive characteristics including antigen recognition, function, and development, with an emphasis on where they differ from their αβ T cell comparators, as well as from γδ T cell populations in the mouse. We review the antigens that have been identified thus far to regulate members of the human Vδ1 population and discuss what players are involved in transducing phosphoantigen-mediated signals to human Vγ9Vδ2 T cells. We also briefly review distinguishing features of these cells in terms of TCR signaling, use of coreceptor and costimulatory molecules and their development. These cells have great potential to be harnessed in a clinical setting, but caution must be taken to understand their unique capabilities and how they differ from the populations to which they are commonly compared.

The distinct MHC-unrestricted immunobiology of innate-like and adaptive-like human γδ T cell subsets-Nature's CAR-T cells

Distinct innate-like and adaptive-like immunobiological paradigms are emerging for human γδ T cells, supported by a combination of immunophenotypic, T cell receptor (TCR) repertoire, functional, and transcriptomic data. Evidence of the γδ TCR/ligand recognition modalities that respective human subsets utilize is accumulating. Although many questions remain unanswered, one superantigen-like modality features interactions of germline-encoded regions of particular TCR Vγ regions with specific BTN/BTNL family members and apparently aligns with an innate-like biology, albeit with some scope for clonal amplification. A second involves CDR3-mediated γδ TCR interaction with diverse ligands and aligns with an adaptive-like biology. Importantly, these unconventional modalities provide γδ T cells with unique recognition capabilities relative to αβ T cells, B cells, and NK cells, allowing immunosurveillance for signatures of "altered self" on target cells, via a membrane-linked γδ TCR recognizing intact non-MHC proteins on the opposing cell surface. In doing so, they permit cellular responses in diverse situations including where MHC expression is compromised, or where conventional adaptive and/or NK cell-mediated immunity is suppressed. γδ T cells may therefore utilize their TCR like a cell-surface Fab repertoire, somewhat analogous to engineered chimeric antigen receptor T cells, but additionally integrating TCR signaling with parallel signals from other surface immunoreceptors, making them multimolecular sensors of cellular stress.

A glance over the fence: Using phylogeny and species comparison for a better understanding of antigen recognition by human γδ T-cells

Both, jawless and jawed vertebrates possess three lymphocyte lineages defined by highly diverse antigen receptors: Two T-cell- and one B-cell-like lineage. In both phylogenetic groups, the theoretically possible number of individual antigen receptor specificities can even outnumber that of lymphocytes of a whole organism. Despite fundamental differences in structure and genetics of these antigen receptors, convergent evolution led to functional similarities between the lineages. Jawed vertebrates possess αβ and γδ T-cells defined by eponymous αβ and γδ T-cell antigen receptors (TCRs). "Conventional" αβ T-cells recognize complexes of Major Histocompatibility Complex (MHC) class I and II molecules and peptides. Non-conventional T-cells, which can be αβ or γδ T-cells, recognize a large variety of ligands and differ strongly in phenotype and function between species and within an organism. This review describes similarities and differences of non-conventional T-cells of various species and discusses ligands and functions of their TCRs. A special focus is laid on Vγ9Vδ2 T-cells whose TCRs act as sensors for phosphorylated isoprenoid metabolites, so-called phosphoantigens (PAg), associated with microbial infections or altered host metabolism in cancer or after drug treatment. We discuss the role of butyrophilin (BTN)3A and BTN2A1 in PAg-sensing and how species comparison can help in a better understanding of this human Vγ9Vδ2 T-cell subset.

Adoptive transfer of zoledronate-expanded autologous Vγ9Vδ2 T-cells in patients with treatment-refractory non-small-cell lung cancer: a multicenter, open-label, single-arm, phase 2 study

BACKGROUND

Not all non-small cell lung cancer (NSCLC) patients possess drug-targetable driver mutations, and response rates to immune checkpoint blockade therapies also remain unsatisfactory. Therefore, more effective treatments are still needed. Here, we report the results of a phase 2 clinical trial of adoptive cell therapy using zoledronate-expanded autologous Vγ9Vδ2 T-cells for treatment-refractory NSCLC.

METHODS

NSCLC patients who had undergone at least two regimens of standard chemotherapy for unresectable disease or had had at least one treatment including chemotherapy or radiation for recurrent disease after surgery were enrolled in this open-label, single-arm, multicenter, phase 2 study. After preliminary testing of Vγ9Vδ2 T-cell proliferation, autologous peripheral blood mononuclear cells were cultured with zoledronate and IL-2 to expand the Vγ9Vδ2 T-cells. Cultured cells (>1×10⁹) were intravenously administered every 2 weeks for six injections. The primary endpoint of this study was progression-free survival (PFS), and secondary endpoints included overall survival (OS), best objective response rate (ORR), disease control rate (DCR), safety and immunomonitoring. Clinical efficacy was defined as median PFS significantly >4 months.

RESULTS

Twenty-five patients (20 adenocarcinoma, 4 squamous cell carcinoma and 1 large cell carcinoma) were enrolled. Autologous Vγ9Vδ2 T-cell therapy was administered to all 25 patients, of which 16 completed the foreseen course of 6 injections of cultured cells. Median PFS was 95.0 days (95% CI 73.0 to 132.0 days); median OS was 418.0 days (179.0-479.0 days), and best overall responses were 1 partial response, 16 stable disease (SD) and 8 progressive disease. ORR and DCR were 4.0% (0.1%-20.4%) and 68.0% (46.5%-85.1%), respectively. Severe adverse events developed in nine patients, mostly associated with disease progression. In one patient, pneumonitis and inflammatory responses resulted from Vγ9Vδ2 T-cell infusions, together with the disappearance of a massive tumor.

CONCLUSIONS

Although autologous Vγ9Vδ2 T-cell therapy was well tolerated and may have an acceptable DCR, this trial did not meet its primary efficacy endpoint.

TRIAL REGISTRATION NUMBER

UMIN000006128.

Cancer immunotherapy with γδ T cells: many paths ahead of us

γδ T cells play uniquely important roles in stress surveillance and immunity for infections and carcinogenesis. Human γδ T cells recognize and kill transformed cells independently of human leukocyte antigen (HLA) restriction, which is an essential feature of conventional αβ T cells. Vγ9Vδ2 γδ T cells, which prevail in the peripheral blood of healthy adults, are activated by microbial or endogenous tumor-derived pyrophosphates by a mechanism dependent on butyrophilin molecules. γδ T cells expressing other T cell receptor variable genes, notably Vδ1, are more abundant in mucosal tissue. In addition to the T cell receptor, γδ T cells usually express activating natural killer (NK) receptors, such as NKp30, NKp44, or NKG2D which binds to stress-inducible surface molecules that are absent on healthy cells but are frequently expressed on malignant cells. Therefore, γδ T cells are endowed with at least two independent recognition systems to sense tumor cells and to initiate anticancer effector mechanisms, including cytokine production and cytotoxicity. In view of their HLA-independent potent antitumor activity, there has been increasing interest in translating the unique potential of γδ T cells into innovative cellular cancer immunotherapies. Here, we discuss recent developments to enhance the efficacy of γδ T cell-based immunotherapy. This includes strategies for in vivo activation and tumor-targeting of γδ T cells, the optimization of in vitro expansion protocols, and the development of gene-modified γδ T cells. It is equally important to consider potential synergisms with other therapeutic strategies, notably checkpoint inhibitors, chemotherapy, or the (local) activation of innate immunity.

Mistletoe-Extract Drugs Stimulate Anti-Cancer Vγ9Vδ2 T Cells

Human phosphoantigen-reactive Vγ9Vδ2 T cells possess several characteristics, including MHC-independent recognition of tumor cells and potent killing potential, that make them attractive candidates for cancer immunotherapeutic approaches. Injectable preparations from the hemi-parasite plant Viscum album L. (European mistletoe) are commonly prescribed as complementary cancer therapy in European countries such as Germany, but their mechanism of action remains poorly understood. Here, we investigated in-depth the in vitro response of human T cells towards mistletoe-extract drugs by analyzing their functional and T-cell-receptor (TCR) response using flow cytometry and high-throughput sequencing respectively. Non-fermented mistletoe-extract drugs (AbnobaViscum), but not their fermented counterparts (Iscador), induced specific expansion of Vγ9Vδ2 T cells among T cells. Furthermore, AbnobaViscum rapidly induced the release of cytotoxic granules and the production of the cytokines IFNγ and TNFα in Vγ9Vδ2 T cells. This stimulation of anti-cancer Vγ9Vδ2 T cells was mediated by the butyrophilin BTN3A, did not depend on the accumulation of endogenous phosphoantigens and involved the same Vγ9Vδ2 TCR repertoire as those of phosphoantigen-reactive Vγ9Vδ2 T cells. These insights highlight Vγ9Vδ2 T cells as a potential target for mistletoe-extract drugs and their role in cancer patients receiving these herbal drugs needs to be investigated.