Bromohydrin pyrophosphate-stimulated Vgamma9delta2 T cells expanded ex vivo from patients with poor-prognosis neuroblastoma lyse autologous primary tumor cells

Advancements in γδT cell engineering: paving the way for enhanced cancer immunotherapy

Comprising only 1-10% of the circulating T cell population, γδT cells play a pivotal role in cancer immunotherapy due to their unique amalgamation of innate and adaptive immune features. These cells can secrete cytokines, including interferon-γ (IFN-γ) and tumor necrosis factor-α (TNF-α), and can directly eliminate tumor cells through mechanisms like Fas/FasL and antibody-dependent cell-mediated cytotoxicity (ADCC). Unlike conventional αβT cells, γδT cells can target a wide variety of cancer cells independently of major histocompatibility complex (MHC) presentation and function as antigen-presenting cells (APCs). Their ability of recognizing antigens in a non-MHC restricted manner makes them an ideal candidate for allogeneic immunotherapy. Additionally, γδT cells exhibit specific tissue tropism, and rapid responsiveness upon reaching cellular targets, indicating a high level of cellular precision and adaptability. Despite these capabilities, the therapeutic potential of γδT cells has been hindered by some limitations, including their restricted abundance, unsatisfactory expansion, limited persistence, and complex biology and plasticity. To address these issues, gene-engineering strategies like the use of chimeric antigen receptor (CAR) T therapy, T cell receptor (TCR) gene transfer, and the combination with γδT cell engagers are being explored. This review will outline the progress in various engineering strategies, discuss their implications and challenges that lie ahead, and the future directions for engineered γδT cells in both monotherapy and combination immunotherapy.

Biological characteristics of γδT cells and application in tumor immunotherapy

Human γδT cells are a special immune cell type which exist in small quantities in the body, do not require processing and presentation for antigen recognition, and have non-major histocompatibility complex (MHC)-restricted immune response. They play an important role in the body's anti-tumor, anti-infection, immune regulation, immune surveillance and maintenance of immune tolerance. This article reviews the generation and development of human γδT cells, genetic characteristics, classification, recognition and role of antigens, and research progress in tumor immunotherapy.

Adoptive Cell Therapy for T-Cell Malignancies

T-cell malignancies are often aggressive and associated with poor prognoses. Adoptive cell therapy has recently shown promise as a new line of therapy for patients with hematological malignancies. However, there are currently challenges in applying adoptive cell therapy to T-cell malignancies. Various approaches have been examined in preclinical and clinical studies to overcome these obstacles. This review aims to provide an overview of the recent progress on adoptive cell therapy for T-cell malignancies. The benefits and drawbacks of different types of adoptive cell therapy are discussed. The potential advantages and current applications of innate immune cell-based adoptive cell therapy for T cell malignancies are emphasized.

Allogeneic Vγ9Vδ2 T-cell immunotherapy exhibits promising clinical safety and prolongs the survival of patients with late-stage lung or liver cancer

Vγ9Vδ2 T cells are promising candidates for cellular tumor immunotherapy. Due to their HLA-independent mode of action, allogeneic Vγ9Vδ2 T cells can be considered for clinical application. To apply allogeneic Vγ9Vδ2 T cells in adoptive immunotherapy, the methodology used to obtain adequate cell numbers with optimal effector function in vitro needs to be optimized, and clinical safety and efficacy also need to be proven. Therefore, we developed a novel formula to improve the expansion of peripheral γδ T cells from healthy donors. Then, we used a humanized mouse model to validate the therapeutic efficacy of expanded γδ T cells in vivo; furthermore, the expanded γδ T cells were adoptively transferred into late-stage liver and lung cancer patients. We found that the expanded cells possessed significantly improved immune effector functions, including proliferation, differentiation, and cancer cell killing, both in vitro and in the humanized mouse model. Furthermore, a phase I clinical trial in 132 late-stage cancer patients with a total of 414 cell infusions unequivocally validated the clinical safety of allogeneic Vγ9Vδ2 T cells. Among these 132 patients, 8 liver cancer patients and 10 lung cancer patients who received ≥5 cell infusions showed greatly prolonged survival, which preliminarily verified the efficacy of allogeneic Vγ9Vδ2 T-cell therapy. Our clinical studies underscore the safety and efficacy of allogeneic Vγ9Vδ2 T-cell immunotherapy, which will inspire further clinical investigations and eventually benefit cancer patients.

The increased IL-17-producing γδT cells promote tumor cell proliferation and migration in neuroblastoma

Neuroblastoma (NB) is the most common solid extracranial malignancy in children with a considerable chance of metastatic progression. Prevalent evidence supports the anti-tumor role of γδT cells and these cells have been testing in clinical trials for constraining tumor growth. A small subpopulation of γδT cells releasing IL-17, however, were demonstrated to exert tumor-promoting effects in many aspects. In this study, we found an augment of IL-17⁺ γδT cells both in in vitro PAM-stimulated γδT-cell expanding culture and circulating γδT cells in NB patients. These patient-origin cells expanded in vitro by PAM in the presence of IL-17 polarizing condition were shown to promote the proliferation and migration of NB cells. Furthermore, an intrinsic preference for IL-17 polarization in NB γδT cells was revealed by mRNA microarray and Western Blot, which pointed to an up-regulated expression of multiple Th17-development related genes in addition to an increased phosphorylation level of STAT3.

Targeting T cell malignancies using CAR-based immunotherapy: challenges and potential solutions

Chimeric antigen receptor (CAR) T cell therapy has been successful in treating B cell malignancies in clinical trials; however, fewer studies have evaluated CAR T cell therapy for the treatment of T cell malignancies. There are many challenges in translating this therapy for T cell disease, including fratricide, T cell aplasia, and product contamination. To the best of our knowledge, no tumor-specific antigen has been identified with universal expression on cancerous T cells, hindering CAR T cell therapy for these malignancies. Numerous approaches have been assessed to address each of these challenges, such as (i) disrupting target antigen expression on CAR-modified T cells, (ii) targeting antigens with limited expression on T cells, and (iii) using third party donor cells that are either non-alloreactive or have been genome edited at the T cell receptor α constant (TRAC) locus. In this review, we discuss CAR approaches that have been explored both in preclinical and clinical studies targeting T cell antigens, as well as examine other potential strategies that can be used to successfully translate this therapy for T cell disease.

Cryoablation and Immunotherapy: An Enthralling Synergy to Confront the Tumors

Treatment of solid tumors by ablation techniques has gained momentum in the recent years due to their technical simplicity and reduced morbidity as juxtaposed to surgery. Cryoablation is one of such techniques, known for its uniqueness to destroy the tumors by freezing to lethal temperatures. Freezing the tumor locally and allowing it to remain in situ unleashes an array of tumor antigens to be exposed to the immune system, paving the way for the generation of anti-tumor immune responses. However, the immune responses triggered in most cases are insufficient to eradicate the tumors with systemic spread. Therefore, combination of cryoablation and immunotherapy is a new treatment strategy currently being evaluated for its efficacy, notably in patients with metastatic disease. This article examines the mechanistic fabric of cryoablation for the generation of an effective immune response against the tumors, and various possibilities of its combination with different immunotherapies that are capable of inducing exceptional therapeutic responses. The combinatorial treatment avenues discussed in this article if explored in sufficient profundity, could reach the pinnacle of future cancer medicine.

The emerging role of γδ T cells in cancer immunotherapy

The recent successes of chimeric antigen receptor T cells in the treatment of hematological malignancies have clearly led to an explosion in the field of adoptive cell therapy for cancer. Current efforts are focused on the translation of this exciting technology to the treatment of solid tumors and the development of allogeneic ‘off-the-shelf’ therapies. γδ T cells are currently gaining considerable attention in this field as their unique biology and established role in cancer immunosurveillance place them in a unique position to potentially overcome these challenges in adoptive cell therapy. Here, we review the relevant aspects of the function of γδ T cells in cancer immunity, and summarize clinical observations and clinical trial results that highlight their emerging role as a platform for the development of safe and effective cancer immunotherapies.

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γδ T cells are a unique subset of T cells combining innate and adaptive features.

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Tissue-resident γδ T cells have important functions in tissue and cancer immunosurveillance.

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γδ T cells are being exploited increasingly for cancer immunotherapy.

CAR-Based Strategies beyond T Lymphocytes: Integrative Opportunities for Cancer Adoptive Immunotherapy

Chimeric antigen receptor (CAR)-engineered T lymphocytes (CAR Ts) produced impressive clinical results against selected hematological malignancies, but the extension of CAR T cell therapy to the challenging field of solid tumors has not, so far, replicated similar clinical outcomes. Many efforts are currently dedicated to improve the efficacy and safety of CAR-based adoptive immunotherapies, including application against solid tumors. A promising approach is CAR engineering of immune effectors different from αβT lymphocytes. Herein we reviewed biological features, therapeutic potential, and safety of alternative effectors to conventional CAR T cells: γδT, natural killer (NK), NKT, or cytokine-induced killer (CIK) cells. The intrinsic CAR-independent antitumor activities, safety profile, and ex vivo expansibility of these alternative immune effectors may favorably contribute to the clinical development of CAR strategies. The proper biological features of innate immune response effectors may represent an added value in tumor settings with heterogeneous CAR target expression, limiting the risk of tumor clonal escape. All these properties bring out CAR engineering of alternative immune effectors as a promising integrative option to be explored in future clinical studies.

Diminished cytolytic activity of γδ T cells with reduced DNAM-1 expression in neuroblastoma patients

Neuroblastoma is one of the children's malignant tumors with poor prognosis, as well as high recurrence and metastasis rates after surgical removal and chemotherapy. γδ T-cell based immunotherapy receives increasing attention thanks to the strong cytolytic activity to tumor cells. Our previous data revealed a significant increase in circulating γδ T-cell frequency in NB patients. In the present study, we found that beside a reduction of IFN-γ in serum of NB patients, DNAM-1 expression decreased in both circulating and PAM-expanded NB γδ T cells. Upon PAM stimulation, NB γδ T cells showed a reduced level of cell proliferation. In addition, the cytolytic activity of NB γδ T cells to NB cell lines was proved to be attenuated in a co-culture system. The fact that DNAM-1 neutralizing antibody abolished the tumor cell killing accentuates the indispensable role of DNAM-1 molecule in γδ T-cell cytolytic function.

Positive & Negative Roles of Innate Effector Cells in Controlling Cancer Progression

Innate immune cells are active at the front line of host defense against pathogens and now appear to play a range of roles under non-infectious conditions as well, most notably in cancer. Establishing the balance of innate immune responses is critical for the “flavor” of these responses and subsequent adaptive immunity and can be either “good or bad” in controlling cancer progression. The importance of innate NK cells in tumor immune responses has already been extensively studied over the last few decades, but more recently several relatively mono- or oligo-clonal [i.e., (semi-) invariant] innate T cell subsets received substantial interest in tumor immunology including invariant natural killer T (iNKT), γδ-T and mucosal associated invariant T (MAIT) cells. These subsets produce high levels of various pro- and/or anti-inflammatory cytokines/chemokines reflecting their capacity to suppress or stimulate immune responses. Survival of patients with cancer has been linked to the frequencies and activation status of NK, iNKT, and γδ-T cells. It has become clear that NK, iNKT, γδ-T as well as MAIT cells all have physiological roles in anti-tumor responses, which emphasize their possible relevance for tumor immunotherapy. A variety of clinical trials has focused on manipulating NK, iNKT, and γδ-T cell functions as a cancer immunotherapeutic approach demonstrating their safety and potential for achieving beneficial therapeutic effects, while the exploration of MAIT cell related therapies is still in its infancy. Current issues limiting the full therapeutic potential of these innate cell subsets appear to be related to defects and suppressive properties of these subsets that, with the right stimulus, might be reversed. In general, how innate lymphocytes are activated appears to control their subsequent abilities and consequent impact on adaptive immunity. Controlling these potent regulators and mediators of the immune system should enable their protective roles to dominate and their deleterious potential (in the specific context of cancer) to be mitigated.

Wnt pathway activator TWS119 enhances the proliferation and cytolytic activity of human γδT cells against colon cancer

γδT cells are a distinct T-cell subset that display unique characteristics regarding T-cell receptor gene usage, tissue tropism and antigen recognition. Adoptive γδT cell transfer therapy has recently been gaining importance as an efficient approach in cancer immunotherapy. However, exploiting γδT cell response for tumour immunotherapy is a challenge due to cell numbers, activities and differentiation states that minimize the clinical therapeutic effects. Previous studies have indicated that the wnt/β-catenin signalling pathway plays a crucial role in the differentiation, survival and enhancement of the immune response of T lymphocytes. In this study, we sought to evaluate whether the activation of the wnt/β-catenin pathway through inhibition of glycogen synthase kinase-3β (GSK-3β) using 4,6-disubstituted pyrrolopyrimidine (TWS119) could be an efficient strategy to improve the proliferation, differentiation and cytolytic activity of γδT cells against colon cancer cells. Remarkably, we found that TWS119 significantly enhanced the proliferation and survival of γδT cells via activation of the mammalian target of rapamycin (mTOR) pathway, upregulation of the expression of the anti-apoptotic protein Bcl-2 and inhibition of cleaved caspase-3 in addition to the Wnt pathway. Our results also showed that enhancement of the cytolytic activity of γδT cells against human colon cancer cells by TWS119 was chiefly associated with upregulation of the expression of perforin and granzyme B in vitro and in vivo. Additionally, TWS119 can induce the expression of CD62L or CCR5 to generate a population of CD62L⁺γδT or CCR5⁺γδT cells in a dose-dependent manner. These findings suggested that TWS119 could be a useful complementary agent for improving γδT cell-based immunotherapy.

Insights into Local Tumor Microenvironment Immune Factors Associated with Regression of Cutaneous Melanoma Metastases by Mycobacterium bovis Bacille Calmette-Guérin

Mycobacterium bovis bacille Calmette–Guérin (BCG) is listed as an intralesional (IL) therapeutic option for inoperable stage III in-transit melanoma in the National Comprehensive Cancer Network Guidelines. Although the mechanism is unknown, others have reported up to 50% regression of injected lesions, and 17% regression of uninjected lesions in immunocompetent patients after direct injection of BCG into metastatic melanoma lesions in the skin. BCG and other mycobacteria express ligands capable of stimulating the γ9δ2 T cells. Therefore, we hypothesized that γ9δ2 T cells play a role in promoting BCG-mediated antitumor immunity in patients treated with IL-BCG for in-transit cutaneous melanoma metastases. Indeed, we found γ9δ2 T cell infiltration in melanoma skin lesions during the course of IL-BCG treatment. Gene expression analysis revealed that BCG injection elicits the expression of a vast array of chemokines in tumor lesions, including strong expression of CXCL9, 10, and 11, a set of chemokines that attract T cells expressing the CXCR3 chemokine receptor. In corroboration with our hypothesis, approximately 85% of γδ T cells express high levels of CXCR3 on their surface. Importantly, the injected tumor lesions also express genes whose protein products are the antigenic ligands for γδ T cells (BTN3A1 and MICB), and the cytokines that are the typical products of activated γδ T cells. Interestingly, we also found that γδ T cells infiltrate the regressed lesions that did not receive BCG injections. Our study suggests that γ9δ2 T cells may contribute to melanoma regression induced by IL-BCG treatment.

In vivo expansion and activation of γδ T cells as immunotherapy for refractory neuroblastoma: A phase 1 study

INTRODUCTION

CD3+ γδ+ T cells comprise 2% to 5% of circulating T cells with Vγ9Vδ2+ cells the dominant circulating subtype. Vγ9Vδ2+ cells recognize non-peptide phosphoantigens and stress-associated NKG2D ligands expressed on malignant cells. Strategies that incorporate the tumoricidal properties of γδ T cells represent a promising immunotherapeutic strategy for treatment of solid malignancies including neuroblastoma (NB). In this prospective, non-randomized Phase I trial, we assessed whether circulating Vγ9Vδ2+ cells could be safely expanded using intravenous ZOL (Zoledronate [Zometa]) and subcutaneous Interleukin-2 (IL-2) in patients with refractory NB.

METHODS

Patients 2 to 21 years of age with refractory neuroblastoma with no known curative therapeutic options received ZOL on day 1, and IL-2 on days 1 to 5 and 15 to 19 of each 28-day cycle (n = 4). Lymphocyte immunophenotyping was assessed weekly. Immunophenotyping studies from the treatment group were compared with healthy pediatric controls (n = 16; range, 5y-15y) and of untreated NB disease controls (n = 9; range, 4m-18y).

RESULTS

Treatment was well tolerated with no unexpected grade 3 and 4 toxicities. Lymphocyte subset counts did not differ significantly between volunteers and disease controls with the exception of γδ+ T cell counts that were significantly higher in healthy volunteers (212 + 93 vs. 89 + 42, P = 0.05). Study patients showed increases in circulating γδ+ T cell count (3-10 fold) after the first week, increasing into the range seen in healthy volunteers (125 + 37, P = 0.1940). Interestingly, all ZOL + IL-2 treated patients showed significant increases in CD3+CD4+CD27CD127 T cells that rose weekly in 2 patients throughout the 4 weeks of observation (maximum 41% and 24% of total CD3+CD4+ T cells, respectively).

CONCLUSIONS

In summary, combined ZOL and IL-2 is well tolerated and restored γδ+ T cell counts to the normal range with a moderate expansion of Natural Killer cells. Progressive increases in circulating CD4+ T cells with a regulatory phenotype cells may offset beneficial effects of this therapy.

Harnessing the power of Vδ2 cells in cancer immunotherapy

γδ T cells are a subset of T lymphocytes that have been implicated in immunosurveillance against infections and tumours. In the peripheral blood of humans the γδ T cell pool is made up predominantly of Vδ2 cells, which can detect both foreign and self-metabolites of the isoprenoid biosynthesis pathway. This unique axis of antigen recognition enables Vδ2 cells to respond to a range of pathogenic infections as well as perturbations in endogenous isoprenoid biosynthesis that can occur during cell stress and malignant transformation. There has been growing interest in Vδ2 cells as a potential avenue for cancer immunotherapy, and a number of strategies have been utilized in an attempt to boost the anti-tumour response of Vδ2 cells in patients. In this review we discuss critically the evidence that Vδ2 cells contribute to the cytotoxic response against tumours and evaluate current immunotherapeutic approaches that target these cells in cancer patients, with specific focus on their shortcomings and how they may be improved.

CD8⁺ T Cell-Independent Immune-Mediated Mechanisms of Anti-Tumor Activity

Despite the growing number of preclinical and clinical trials focused on immunotherapy for the treatment of malignant gliomas, the prognosis for this disease remains grim. Cancer immunotherapy seeks to recruit an effective immune response to eliminate tumor cells. To date, cancer vaccines have shown only limited effectiveness because of our incomplete understanding of the necessary effector cells and mechanisms that yield efficient tumor clearance. CD8+ T cell cytotoxic activity has long been proposed as the primary effector function necessary for tumor regression. However, there is increasing evidence that indicates that components of the immune system other than CD8+ T cells play important roles in tumor eradication and control. The following review should provide an understanding of the mechanisms involved in an effective antitumor response to guide future therapeutic designs. The information provided suggests an alternate means of effective tumor clearance in malignant glioma to the canonical CD8+ cytotoxic T cell mechanism.

Clinical applications of gamma delta T cells with multivalent immunity

γδ T cells hold promise for adoptive immunotherapy because of their reactivity to bacteria, viruses, and tumors. However, these cells represent a small fraction (1–5%) of the peripheral T-cell pool and require activation and propagation to achieve clinical benefit. Aminobisphosphonates specifically expand the Vγ9Vδ2 subset of γδ T cells and have been used in clinical trials of cancer where objective responses were detected. The Vγ9Vδ2 T cell receptor (TCR) heterodimer binds multiple ligands and results in a multivalent attack by a monoclonal T cell population. Alternatively, populations of γδ T cells with oligoclonal or polyclonal TCR repertoire could be infused for broad-range specificity. However, this goal has been restricted by a lack of applicable expansion protocols for non-Vγ9Vδ2 cells. Recent advances using immobilized antigens, agonistic monoclonal antibodies (mAbs), tumor-derived artificial antigen presenting cells (aAPC), or combinations of activating mAbs and aAPC have been successful in expanding gamma delta T cells with oligoclonal or polyclonal TCR repertoires. Immobilized major histocompatibility complex Class-I chain-related A was a stimulus for γδ T cells expressing TCRδ1 isotypes, and plate-bound activating antibodies have expanded Vδ1 and Vδ2 cells ex vivo. Clinically sufficient quantities of TCRδ1, TCRδ2, and TCRδ1^negTCRδ2^neg have been produced following co-culture on aAPC, and these subsets displayed differences in memory phenotype and reactivity to tumors in vitro and in vivo. Gamma delta T cells are also amenable to genetic modification as evidenced by introduction of αβ TCRs, chimeric antigen receptors, and drug-resistance genes. This represents a promising future for the clinical application of oligoclonal or polyclonal γδ T cells in autologous and allogeneic settings that builds on current trials testing the safety and efficacy of Vγ9Vδ2 T cells.

Neuroblastoma killing properties of Vδ2 and Vδ2-negative γδT cells following expansion by artificial antigen-presenting cells

PURPOSE

The majority of circulating human γδT lymphocytes are of the Vγ9Vδ2 lineage, and have T-cell receptor (TCR) specificity for nonpeptide phosphoantigens. Previous attempts to stimulate and expand these cells have therefore focused on stimulation using ligands of the Vγ9Vδ2 receptor, whereas relatively little is known about variant blood γδT subsets and their potential role in cancer immunotherapy.

EXPERIMENTAL DESIGN

To expand the full repertoire of γδT without bias toward specific TCRs, we made use of artificial antigen-presenting cells loaded with an anti γδTCR antibody that promoted unbiased expansion of the γδT repertoire. Expanded cells from adult blood donors were sorted into 3 populations expressing respectively Vδ2 TCR chains (Vδ2(+)), Vδ1 chains (Vδ1(+)), and TCR of other δ chain subtypes (Vδ1(neg)Vδ2(neg)).

RESULTS

Both freshly isolated and expanded cells showed heterogeneity of differentiation markers, with a less differentiated phenotype in the Vδ1 and Vδ1(neg)Vδ2(neg) populations. Expanded cells were largely of an effector memory phenotype, although there were higher numbers of less differentiated cells in the Vδ1(+) and Vδ1(neg)Vδ2(neg) populations. Using neuroblastoma tumor cells and the anti-GD2 therapeutic mAb ch14.18 as a model system, all three populations showed clinically relevant cytotoxicity. Although killing by expanded Vδ2 cells was predominantly antibody dependent and proportionate to upregulated CD16, Vδ1 cells killed by antibody-independent mechanisms.

CONCLUSIONS

In conclusion, we have demonstrated that polyclonal-expanded populations of γδT cells are capable of both antibody-dependent and -independent effector functions in neuroblastoma.

Large-scale expansion of γδ T cells and peptide-specific cytotoxic T cells using zoledronate for adoptive immunotherapy

Specific cellular immunotherapy for cancer requires efficient generation and expansion of cytotoxic T lymphocytes (CTLs) that recognize tumor-associated antigens. However, it is difficult to isolate and expand functionally active T-cells ex vivo. In this study, we investigated the efficacy of a new method to induce expansion of antigen-specific CTLs for adoptive immunotherapy. We used tumor-associated antigen glypican-3 (GPC3)-derived peptide and cytomegalovirus (CMV)-derived peptide as antigens. Treatment of human peripheral blood mononuclear cells (PBMCs) with zoledronate is a method that enables large-scale γδ T-cell expansion. To induce expansion of γδ T cells and antigen-specific CTLs, the PBMCs of healthy volunteers or patients vaccinated with GPC3 peptide were cultured with both peptide and zoledronate for 14 days. The expansion of γδ T cells and peptide-specific CTLs from a few PBMCs using zoledronate yields cell numbers sufficient for adoptive transfer. The rate of increase of GPC3-specific CTLs was approximately 24- to 170,000-fold. These CD8⁺ cells, including CTLs, showed GPC3-specific cytotoxicity against SK-Hep-1/hGPC3 and T2 pulsed with GPC3 peptide, but not against SK-Hep-1/vec and T2 pulsed with human immunodeficiency virus peptide. On the other hand, CD8⁻ cells, including γδ T cells, showed cytotoxicity against SK-Hep-1/hGPC3 and SK-Hep-1/vec, but did not show GPC3 specificity. Furthermore, adoptive cell transfer of CD8⁺ cells, CD8⁻ cells, and total cells after expansion significantly inhibited tumor growth in an NOD/SCID mouse model. This study indicates that simultaneous expansion of γδ T cells and peptide-specific CTLs using zoledronate is useful for adoptive immunotherapy.

Insights into the Relationship between Toll Like Receptors and Gamma Delta T Cell Responses

The tumor microenvironment is an important aspect of cancer biology that contributes to tumor initiation, tumor progression and responses to therapy. The composition and characteristics of the tumor microenvironment vary widely and are important in determining the anti-tumor immune response. Successful immunization requires activation of both innate and adaptive immunity. Generally, immune system is compromised in patients with cancer due to immune suppression, loss of tumor antigen expression and dysfunction of antigen presenting cells (APC). Thus, therapeutic immunization leading to cancer regression remains a significant challenge. Certain cells of the immune system, including dendritic cells (DCs) and gamma delta (γδ) T cells are capable of driving potent anti-tumor responses. The property of MHC-unrestricted cytotoxicity, high potential of cytokine release, tissue tropism and early activation in infections and malignant disease makes γδ T cells as an emerging candidate for immunotherapy. Various strategies are being developed to enhance anti-tumor immune responses of γδ T cells and DCs one of them is the use of novel adjuvants like toll like receptors (TLR) agonists, which enhance γδ T cell function directly or through DC activation, which has ability to prime γδ T cells. TLR agonists are being used clinically either alone or in combination with tumor antigens and has shown initial success in both enhancing immune responses and eliciting anti-tumor activity. TLR activated γδ T cells and DCs nurture each other's activation. This provides a potent base for first line of defense and manipulation of the adaptive response against pathogens and cancer. The available data provides a strong rationale for initiating combinatorial therapy for the treatment of diseases and this review will summarize the application of adjuvants (TLRs) for boosting immune response of γδ T cells to treat cancer and infectious diseases and their use in combinatorial therapy.

Involvement of Notch in activation and effector functions of γδ T cells

Notch signaling plays a pivotal role in cell fate decision and lineage commitment of lymphocytes. Although the role of Notch in CD4(+) and CD8(+) αβ T cells is well documented, there are no reports on how Notch signaling regulates effector functions of γδ T cells. γδ T cells are a minor fraction in the peripheral blood but are known to play a major role in defense against pathogens and tumors. In this study, we show that Notch receptors (mRNA and protein) are expressed in peripheral γδ T cells. Inhibition of Notch signaling by γ-secretase inhibitor inhibited the proliferation and IFN-γ secretion of γδ T cells in response to stimulation with phosphoantigens and anti-CD3 mAb. In the presence of γ-secretase inhibitor, the antitumor cytolytic ability of γδ T cells was inhibited with a decreased CD107a expression. Knockdown of Notch1 and Notch2 genes in γδ T cells using small interfering RNA inhibited their antitumor cytotoxic potential. Our study describes for the first time, to our knowledge, the role of Notch as an additional signal contributing to Ag-specific effector functions of γδ T cells.

γδ T cells for cancer immunotherapy: A systematic review of clinical trials

γδ T cells contribute to the front line of lymphoid antitumor surveillance and bridge the gap between innate and adaptive immunity. They can be readily expanded to high numbers in vivo and in vitro, starting from the blood of cancer patients, and a number of Phase I trials have demonstrated that these cells can be employed in cancer immunotherapy. Sufficient patients have received γδ T cell-based immunotherapies in the context of clinical trials to evaluate their utility, and to inform the direction of new trials. A systematic approach was used to identify Phase I, Phase II, and feasibility studies testing γδ T cell-based immunotherapy in cancer patients. Studies were excluded from further analysis if they did not provide patient-specific data. Data were compiled to evaluate efficacy, with stratification by treatment approach. When possible, comparisons were made with the efficacy of second-line conventional therapeutic approaches for the same malignancy. Twelve eligible studies were identified, providing information on 157 patients who had received γδ T cell-based immunotherapy. The comparison of objective response data suggests that γδ T cell-based immunotherapy is superior to current second-line therapies for advanced renal cell carcinoma and prostate cancer, but not for non-small cell lung carcinoma. An evaluation of pooled data from 132 published in vitro experiments shows a consistent improvement in the cytotoxicity of γδ T cells in the presence of antitumor antibodies. Immunotherapy using γδ T cells alone shows promising clinical activity, but there is a strong preclinical rationale for combining this treatment modality with cancer-targeting antibodies to augment its efficacy.

γδ T cells and their potential for immunotherapy

Vγ9Vδ2 (also termed Vγ2Vδ2) T cells, a major human peripheral blood γδ T cell subset, recognize microbial (E)-4-hydroxy-3-methylbut-2-enyl diphosphate and endogenous isopentenyl diphosphate in a TCR-dependent manner. The recognition does not require specific accessory cells, antigen uptake, antigen processing, or MHC class I, class II, or class Ib expression. This subset of T cells plays important roles in mediating innate immunity against a wide variety of infections and displays potent and broad cytotoxic activity against human tumor cells. Because γδT cells express both natural killer receptors such as NKG2D and γδ T cell receptors, they are considered to represent a link between innate and adaptive immunity. In addition, activated γδ T cells express a high level of antigen-presenting cell-related molecules and can present peptide antigens derived from destructed cells to αβ T cells. Utilizing these antimicrobial and anti-tumor properties of γδ T cells, preclinical and clinical trials have been conducted to develop novel immunotherapies for infections and malignancies. Here, we review the immunological properties of γδ T cells including the underlying recognition mechanism of nonpeptitde antigens and summarize the results of γδ T cell-based therapies so far performed. Based on the results of the reported trials, γδ T cells appear to be a promising tool for novel immunotherapies against certain types of diseases.

Human Vγ2Vδ2 T cells limit breast cancer growth by modulating cell survival-, apoptosis-related molecules and microenvironment in tumors

Innate immune system has been known to play an important role in inhibiting the malignant transformation, tumor progression and invasion. However, the mechanistic basis remains ambiguous. Despite polyclonality of human γδ T cells, Vγ2Vδ2 T cell subset was shown to recognize and limit the growth of various tumors at various degrees. The differential recognition of the tumor cells by Vγ2Vδ2 T cells are yet to be defined. Our study reveals that γδ T cells limit in vitro growth of most breast tumor cells, such as SkBr7 (HER2+), MCF7 (ER+) and MDA-MB-231 (ER-) by inhibiting their survival and inducing apoptosis, except BrCa-MZ01 (PR+) cells. To investigate detail mechanisms of antineoplastic effects, we found that cell death was associated with the surface expression levels of MICA/B and ICAM1. Molecular signaling analysis demonstrated that inhibition of cell growth by γδ T cells was associated with the lower expression levels of cell survival-related molecules such as AKT, ERK and concomitant upregulation of apoptosis-related molecules, such as PARP, cleaved caspase 3 and tumor suppressor genes PTEN and P53. However, opposite molecular signaling was observed in the resistant cell line after coculture with γδ T cells. In vivo, antineoplastic effects of γδ T cells were also documented, where tumor growth was inhibited due to the downregulation of survival signals, strong induction of apoptotic molecules, disruption of microvasculature and increased infiltration of tumor associated macrophages. These findings reveal that a complex molecular signaling is involved in γδ T cell-mediated antineoplastic effects.

γδ-T cells: an unpolished sword in human anti-infection immunity

γδ-T cells represent a small population of immune cells, but play an indispensable role in host defenses against exogenous pathogens, immune surveillance of endogenous pathogenesis and even homeostasis of the immune system. Activation and expansion of γδ-T cells are generally observed in diverse human infectious diseases and correlate with their progression and prognosis. γδ-T cells have both 'innate' and 'adaptive' characteristics in the immune response, and their anti-infection activities are mediated by multiple pathways that are under elaborate regulation by other immune components. In this review, we summarize the current state of the literature and the recent advancements in γδ-T cell-mediated immune responses against common human infectious pathogens. Although further investigation is needed to improve our understanding of the characteristics of different γδ-T cell subpopulations under specific conditions, γδ-T cell-based therapy has great potential for the treatment of infectious diseases.

The gene expression profile of phosphoantigen-specific human γδ T lymphocytes is a blend of αβ T-cell and NK-cell signatures

Global transcriptional technologies have revolutionised the study of lymphoid cell populations, but human γδ T lymphocytes specific for phosphoantigens remain far less deeply characterised by these methods despite the great therapeutic potential of these cells. Here we analyse the transcriptome of circulating TCRVγ(+) γδ T cells isolated from healthy individuals, and their relation with those from other lymphoid cell subsets. We report that the gene signature of phosphoantigen-specific TCRVγ(+) γδ T cells is a hybrid of those from αβ T and NK cells, with more 'NK-cell' genes than αβ T cells have and more 'T-cell' genes than NK cells. The expression profile of TCRVγ(+) γδ T cells stimulated with phosphoantigen recapitulates their immediate physiological functions: Th1 cytokine, chemokine and cytotoxic activities reflect their high mitotic activity at later time points and do not indicate antigen-presenting functions. Finally, such hallmarks make the transcriptome of γδ T cells, whether resting or clonally expanding, clearly distinctive from that of NK/T or peripheral T-cell lymphomas of the γδ subtype.

Targeting γδ T lymphocytes for cancer immunotherapy: from novel mechanistic insight to clinical application

Abundant interferon-γ secretion, potent cytotoxicity, and major histocompatibility complex-independent targeting of a large spectrum of tumors make γδ T cells attractive mediators of cancer immunotherapy. However, a better understanding of the molecular mechanisms involved in tumor cell recognition and γδ T-cell activation is required to improve the limited success of γδ T-cell-mediated treatments. Here, we review key advances in basic knowledge made over the past 3 years, and summarize the results of γδ T-cell-based clinical trials concluded to date. We also highlight new research directions on the basis of the modulation of receptors that control the function of γδ T cells.