Human γδ T cells: From a neglected lymphocyte population to cellular immunotherapy: A personal reflection of 30years of γδ T cell research

Human Vγ9Vδ2 T cell expansion and their cytotoxic responses against cholangiocarcinoma

Human Vγ9Vδ2 T lymphocytes are regarded as promising effector cells for cancer immunotherapy since they have the ability to eliminate several tumor cells through non-peptide antigen recognition. However, the cytotoxic function and the mechanism of Vγ9Vδ2 T cells leading to specific killing of cholangiocarcinoma cells are yet to be confirmed. In this study, we established a protocol for ex vivo expansion of Vγ9Vδ2 T cells from healthy donors' peripheral blood mononuclear cells by culture with zoledronate and addition of IL-2, and IL-15 or IL-18 or neither. Testing the cytotoxic capacity of cultured Vγ9Vδ2 T cells against cholangiocarcinoma cell lines showed higher reactivity than against control cells. Surface expression of CD107 was detected on the Vγ9Vδ2 T cells, suggesting that these cells limit in vitro growth of cholangiocarcinoma cells via degranulation of the perforin and granzyme pathway. Analysis of molecular signaling was used to demonstrate expression of pro- and anti-survival genes and a panel of cytokine genes in Vγ9Vδ2 T cells. We found that in the presence of either IL-15 or IL-18, levels of caspase 3 were significantly reduced. Also, IL-15 and IL-18 stimulated cells contained cytotoxicity against cholangiocarcinoma cells, suggesting that stimulated Vγ9Vδ2 T cells may provide a feasible therapy for cholangiocarcinoma.

Gamma delta T-cell reconstitution after allogeneic HCT: A platform for cell therapy

Allogeneic Hematopoietic stem cell transplantation (allo-HCT) is a curative platform for several hematological diseases. Despite its therapeutic benefits, the profound immunodeficiency associated with the transplant procedure remains a major challenge that renders patients vulnerable to several complications. Today, It is well established that a rapid and efficient immune reconstitution, particularly of the T cell compartment is pivotal to both a short-term and a long-term favorable outcome. T cells expressing a TCR heterodimer comprised of gamma (γ) and delta (δ) chains have received particular attention in allo-HCT setting, as a large body of evidence has indicated that γδ T cells can exert favorable potent anti-tumor effects without inducing severe graft versus host disease (GVHD). However, despite their potential role in allo-HCT, studies investigating their detailed reconstitution in patients after allo-HCT are scarce. In this review we aim to shed lights on the current literature and understanding of γδ T cell reconstitution kinetics as well as the different transplant-related factors that may influence γδ reconstitution in allo-HCT. Furthermore, we will present data from available reports supporting a role of γδ cells and their subsets in patient outcome. Finally, we discuss the current and future strategies to develop γδ cell-based therapies to exploit the full immunotherapeutic potential of γδ cells in HCT setting.

Vγ9Vδ2 T Cells Concurrently Kill Cancer Cells and Cross-Present Tumor Antigens

The human Vγ9Vδ2 T cell is a unique cell type that holds great potential in immunotherapy of cancer. In particular, the therapeutic potential of this cell type in adoptive cell therapy (ACT) has gained interest. In this regard optimization of in vitro expansion methods and functional characterization is desirable. We show that Vγ9Vδ2 T cells, expanded in vitro with zoledronic acid (Zometa or ZOL) and Interleukin-2 (IL-2), are efficient cancer cell killers with a trend towards increased killing efficacy after prolonged expansion time. Thus, Vγ9Vδ2 T cells expanded for 25 days in vitro killed prostate cancer cells more efficiently than Vγ9Vδ2 T cells expanded for 9 days. These data are supported by phenotype characteristics, showing increased expression of CD56 and NKG2D over time, reaching above 90% positive cells after 25 days of expansion. At the early stage of expansion, we demonstrate that Vγ9Vδ2 T cells are capable of cross-presenting tumor antigens. In this regard, our data show that Vγ9Vδ2 T cells can take up tumor-associated antigens (TAA) gp100, MART-1 and MAGE-A3 - either as long peptide or recombinant protein - and then present TAA-derived peptides on the cell surface in the context of HLA class I molecules, demonstrated by their recognition as targets by peptide-specific CD8 T cells. Importantly, we show that cross-presentation is impaired by the proteasome inhibitor lactacystin. In conclusion, our data indicate that Vγ9Vδ2 T cells are broadly tumor-specific killers with the additional ability to cross-present MHC class I-restricted peptides, thereby inducing or supporting tumor-specific αβTCR CD8 T cell responses. The dual functionality is dynamic during in vitro expansion, yet, both functions are of interest to explore in ACT for cancer therapy.

Epigenetic modulation of immune synaptic-cytoskeletal networks potentiates γδ T cell-mediated cytotoxicity in lung cancer

γδ T cells are a distinct subgroup of T cells that bridge the innate and adaptive immune system and can attack cancer cells in an MHC-unrestricted manner. Trials of adoptive γδ T cell transfer in solid tumors have had limited success. Here, we show that DNA methyltransferase inhibitors (DNMTis) upregulate surface molecules on cancer cells related to γδ T cell activation using quantitative surface proteomics. DNMTi treatment of human lung cancer potentiates tumor lysis by ex vivo-expanded Vδ1-enriched γδ T cells. Mechanistically, DNMTi enhances immune synapse formation and mediates cytoskeletal reorganization via coordinated alterations of DNA methylation and chromatin accessibility. Genetic depletion of adhesion molecules or pharmacological inhibition of actin polymerization abolishes the potentiating effect of DNMTi. Clinically, the DNMTi-associated cytoskeleton signature stratifies lung cancer patients prognostically. These results support a combinatorial strategy of DNMTis and γδ T cell-based immunotherapy in lung cancer management.

Comparison of a Novel Bisphosphonate Prodrug and Zoledronic Acid in the Induction of Cytotoxicity in Human Vγ2Vδ2 T Cells

Increasing attention has been paid to human γδ T cells expressing Vγ2Vδ2 T cell receptor (also termed Vγ9Vδ2) in the field of cancer immunotherapy. We have previously demonstrated that a novel bisphosphonate prodrug, tetrakis-pivaloyloxymethyl 2-(thiazole-2-ylamino)ethylidene-1,1-bisphosphonate (PTA), efficiently expands peripheral blood Vγ2Vδ2 T cells to purities up to 95–99% in 10–11 days. In the present study, we first examined the effect of PTA on farnesyl diphosphate synthase (FDPS) using liquid chromatography mass spectrometry (LC-MS) to analyze the mechanism underlying the PTA-mediated expansion of Vγ2Vδ2 T cells. We find that the prodrug induced the accumulation of both isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), direct upstream metabolites of FDPS. This indicates that not only IPP but also DMAPP plays an important role in PTA-mediated stimulation of Vγ2Vδ2 T cells. We next analyzed TCR-independent cytotoxicity of Vγ2Vδ2 T cells. When human lung cancer cell lines were challenged by Vγ2Vδ2 T cells, no detectable cytotoxicity was observed in 40 min. The lung cancer cell lines were, however, significantly killed by Vγ2Vδ2 T cells after 4–16 h in an effector-to-target ratio-dependent manner, demonstrating that Vγ2Vδ2 T cell-based cell therapy required a large number of cells and longer time when tumor cells were not sensitized. By contrast, pulsing tumor cell lines with 10–30 nM of PTA induced significant lysis of tumor cells by Vγ2Vδ2 T cells even in 40 min. Similar levels of cytotoxicity were elicited by ZOL at concentrations of 100–300 μM, which were much higher than blood levels of ZOL after infusion (1–2 μM), suggesting that standard 4 mg infusion of ZOL was not enough to sensitize lung cancer cells in clinical settings. In addition, Vγ2Vδ2 T cells secreted interferon-γ (IFN-γ) when challenged by lung cancer cell lines pulsed with PTA in a dose-dependent manner. Taken together, PTA could be utilized for both expansion of Vγ2Vδ2 T cells ex vivo and sensitization of tumor cells in vivo in Vγ2Vδ2 T cell-based cancer immunotherapy. For use in patients, further studies on drug delivery are essential because of the hydrophobic nature of the prodrug.

Electroporation of NKG2D RNA CAR Improves Vγ9Vδ2 T Cell Responses against Human Solid Tumor Xenografts

Vγ9Vδ2 T cell-based anticancer immunotherapy has shown some promise in early-phase clinical trials but there is still large room for improvement. Using the extracellular domain of the human NKG2D, a stimulatory receptor expressed by Vγ9Vδ2 T cells, we constructed NKG2D ligand-specific chimeric antigen receptors (CARs). We adopted a non-viral CAR approach via mRNA electroporation to modify Vγ9Vδ2 T cells and demonstrated that, upon interaction with the NKG2D ligand-positive cancer cells, the CARs substantially enhanced the cytotoxic activity of the modified cells toward multiple cultured solid tumor cell lines, including those resistant to Zometa treatment. Repeated doses of the CAR-expressing cells resulted in tumor regression in mice with established tumors, extending median survival time by up to 132% as compared to the PBS control group. The findings suggest clinical potential for RNA CAR-modified Vγ9Vδ2 T cells to treat a wide variety of NKG2D ligand-expressing cancers.

Graft Engineering and Adoptive Immunotherapy: New Approaches to Promote Immune Tolerance After Hematopoietic Stem Cell Transplantation

Allogeneic hematopoietic stem cell transplantation (HSCT) is a curative therapeutic option for a wide range of immune and hematologic malignant and non-malignant disorders. Once transplanted, allogeneic cells have to support myeloid repopulation and immunological reconstitution, but also need to become tolerant to the host via central or peripheral mechanisms to achieve the desired therapeutic effect. Peripheral tolerance after allogeneic HSCT may be achieved by several mechanisms, though blocking alloreactivity to the host human leukocyte antigens while preserving immune responses to pathogens and tumor antigens remains a challenge. Recently uncovered evidence on the mechanisms of post-HSCT immune reconstitution and tolerance in transplanted patients has allowed for the development of novel cell-based therapeutic approaches. These therapies are aimed at inducing long-term peripheral tolerance and reducing the risk of graft-vs-host disease (GvHD), while sparing the graft-vs-leukemia (GvL) effect. Thus, ensuring effective long term remission in hematologic malignancies. Today, haploidentical stem cell transplants have become a widely used treatment for patients with hematological malignancies. A myriad of ex vivo and in vivo T-cell depletion strategies have been adopted, with the goal of preventing GvHD while preserving GvL in the context of immunogenetic disparity. αβ T-cell/CD19 B-cell depletion techniques, in particular, has gained significant momentum, because of the high rate of leukemia-free survival and the low risk of severe GvHD. Despite progress, better treatments are still needed in a portion of patients to further reduce the incidence of relapse and achieve long-term tolerance. Current post-HSCT cell therapy approaches designed to induce tolerance and minimizing GvHD occurrence include the use of (i) γδ T cells, (ii) regulatory Type 1 T (Tr1) cells, and (iii) engineered FOXP3+ regulatory T cells. Future protocols may include post-HSCT infusion of allogeneic effector or regulatory T cells engineered with a chimeric antigen receptor (CAR). In the present review, we describe the most recent advances in graft engineering and post-HSCT adoptive immunotherapy.

Casting a wider net: Immunosurveillance by nonclassical MHC molecules

Most studies of T lymphocytes focus on recognition of classical major histocompatibility complex (MHC) class I or II molecules presenting oligopeptides, yet there are numerous variations and exceptions of biological significance based on recognition of a wide variety of nonclassical MHC molecules. These include αβ and γδ T cells that recognize different class Ib molecules (CD1, MR-1, HLA-E, G, F, et al.) that are nearly monomorphic within a given species. Collectively, these T cells can be considered “unconventional,” in part because they recognize lipids, metabolites, and modified peptides. Unlike classical MHC-specific cells, unconventional T cells generally exhibit limited T-cell antigen receptor (TCR) repertoires and often produce innate immune cell-like rapid effector responses. Exploiting this system in new generation vaccines for human immunodeficiency virus (HIV), tuberculosis (TB), other infectious agents, and cancer was the focus of a recent workshop, “Immune Surveillance by Non-classical MHC Molecules: Improving Diversity for Antigens,” sponsored by the National Institute of Allergy and Infectious Diseases. Here, we summarize salient points presented regarding the basic immunobiology of unconventional T cells, recent advances in methodologies to measure unconventional T-cell activity in diseases, and approaches to harness their considerable clinical potential.

Decitabine Inhibits Gamma Delta T Cell Cytotoxicity by Promoting KIR2DL2/3 Expression

Gamma delta (γδ) T cells, which possess potent cytotoxicity against a wide range of cancer cells, have become a potential avenue for adoptive immunotherapy. Decitabine (DAC) has been reported to enhance the immunogenicity of tumor cells, thereby reinstating endogenous immune recognition and tumor lysis. However, DAC has also been demonstrated to have direct effects on immune cells. In this study, we report that DAC inhibits γδ T cell proliferation. In addition, DAC increases the number of KIR2DL2/3-positive γδ T cells, which are less cytotoxic than the KIR2DL2/3-negative γδ T cells. We found that DAC upregulated KIR2DL2/3 expression in KIR2DL2/3-negative γδ T cells by inhibiting KIR2DL2/3 promoter methylation, which enhances the binding of KIR2DL2/3 promoter to Sp-1 and activates KIR2DL2/3 gene expression. Our data demonstrated that DAC can inhibit the function of human γδ T cells at both cellular and molecular levels, which confirms and extrapolates the results of previous studies showing that DAC can negatively regulate the function of NK cells and αβ T cells of the immune system.

Low-dose bortezomib increases the expression of NKG2D and DNAM-1 ligands and enhances induced NK and γδ T cell-mediated lysis in multiple myeloma

Multiple myeloma (MM) is an incurable hematological malignancy, although bortezomib has markedly improved its outcomes. Growing clinical evidence indicates that enhancing induced natural killer (NK) or γδ T cells for infusion is useful in the treatment of MM. However, whether combination treatment with bortezomib and induced NK and γδ T cells further improves outcomes in MM, and how the treatments should be combined, remain unclear. Herein, we found that low-dose bortezomib did not suppress the viability of induced NK and γδ T cells, but did induce MM cell apoptosis. Importantly, low-dose bortezomib increased the expression of NKG2D and DNAM-1 ligands on MM cells, which sensitized the multiple myeloma cells to lysis by induced NK and γδ T cells. Our results suggested that combination treatment with low-dose bortezomib and induced NK or γδ T cells had a synergistic cytotoxic effect on MM cells. This study provided a proof of principle for the design of future trials and investigation of this combination therapeutic strategy for MM treatment.

Large-scale expansion of Vγ9Vδ2 T cells with engineered K562 feeder cells in G-Rex vessels and their use as chimeric antigen receptor-modified effector cells

Vγ9Vδ2 T cells are a minor subset of lymphocytes in the peripheral blood that has been extensively investigated for their tolerability, safety and anticancer efficacy. A hindrance to the broad application of these cells for adoptive cellular immunotherapy has been attaining clinically appropriate numbers of Vγ9Vδ2 T cells. Furthermore, Vγ9Vδ2 T cells exist at low frequencies among cancer patients. We, therefore, sought to conceive an economical method that allows for a quick and robust large-scale expansion of Vγ9Vδ2 T cells. A two-step protocol was developed, in which peripheral blood mononuclear cells (PBMCs) from healthy donors or cancer patients were activated with Zometa and interleukin (IL)-2, followed by co-culturing with gamma-irradiated, CD64-, CD86- and CD137L-expressing K562 artificial antigen-presenting cells (aAPCs) in the presence of the anti-CD3 antibody OKT3. We optimized the co-culture ratio of K562 aAPCs to immune cells, and migrated this method to a G-Rex cell growth platform to derive clinically relevant cell numbers in a Good Manufacturing Practice (GMP)-compliant manner. We further include a depletion step to selectively remove αβ T lymphocytes. The method exhibited high expansion folds and a specific enrichment of Vγ9Vδ2 T cells. Expanded Vγ9Vδ2 T cells displayed an effector memory phenotype with a concomitant down-regulated expression of inhibitory immune checkpoint receptors. Finally, we ascertained the cytotoxic activity of these expanded cells by using nonmodified and chimeric antigen receptor (CAR)-engrafted Vγ9Vδ2 T cells against a panel of solid tumor cells. Overall, we report an efficient approach to generate highly functional Vγ9Vδ2 T cells in massive numbers suitable for clinical application in an allogeneic setting.

Vγ9Vδ2 T cell activation by strongly agonistic nucleotidic phosphoantigens

Human Vγ9Vδ2 T cells can sense through their TCR tumor cells producing the weak endogenous phosphorylated antigen isopentenyl pyrophosphate (IPP), or bacterially infected cells producing the strong agonist hydroxyl dimethylallyl pyrophosphate (HDMAPP). The recognition of the phosphoantigen is dependent on its binding to the intracellular B30.2 domain of butyrophilin BTN3A1. Most studies have focused on pyrophosphate phosphoantigens. As triphosphate nucleotide derivatives are naturally co-produced with IPP and HDMAPP, we analyzed their specific properties using synthetic nucleotides derived from HDMAPP. The adenylated, thymidylated and uridylated triphosphate derivatives were found to activate directly Vγ9Vδ2 cell lines as efficiently as HDMAPP in the absence of accessory cells. These antigens were inherently resistant to terminal phosphatases, but apyrase, when added during a direct stimulation of Vγ9Vδ2 cells, abrogated their stimulating activity, indicating that their activity required transformation into strong pyrophosphate agonists by a nucleotide pyrophosphatase activity which is present in serum. Tumor cells can be sensitized with nucleotide phosphoantigens in the presence of apyrase to become stimulatory, showing that this can occur before their hydrolysis into pyrophosphates. Whereas tumors sensitized with HDMAPP rapidly lost their stimulatory activity, sensitization with nucleotide derivatives, in particular with the thymidine derivative, induced long-lasting stimulating ability. Using isothermal titration calorimetry, binding of some nucleotide derivatives to BTN3A1 intracellular domain was found to occur with an affinity similar to that of IPP, but much lower than that of HDMAPP. Thus, nucleotide phosphoantigens are precursors of pyrophosphate antigens which can deliver strong agonists intracellularly resulting in prolonged and strengthened activity.