Immune Suppression by γδ T-cells as a Potential Regulatory Mechanism After Cancer Vaccination With IL-12 Secreting Dendritic Cells

Exhausted intratumoral Vδ2- γδ T cells in human kidney cancer retain effector function

Gamma delta (γδ) T cells reside within human tissues including tumors, but their function in mediating antitumor responses to immune checkpoint inhibition is unknown. Here we show that kidney cancers are infiltrated by Vδ2- γδ T cells, with equivalent representation of Vδ1+ and Vδ1- cells, that are distinct from γδ T cells found in normal human tissues. These tumor-resident Vδ2- T cells can express the transcriptional program of exhausted αβ CD8+ T cells as well as canonical markers of terminal T-cell exhaustion including PD-1, TIGIT and TIM-3. Although Vδ2- γδ T cells have reduced IL-2 production, they retain expression of cytolytic effector molecules and co-stimulatory receptors such as 4-1BB. Exhausted Vδ2- γδ T cells are composed of three distinct populations that lack TCF7, are clonally expanded and express cytotoxic molecules and multiple Vδ2- T-cell receptors. Human tumor-derived Vδ2- γδ T cells maintain cytotoxic function and pro-inflammatory cytokine secretion in vitro. The transcriptional program of Vδ2- T cells in pretreatment tumor biopsies was used to predict subsequent clinical responses to PD-1 blockade in patients with cancer. Thus, Vδ2- γδ T cells within the tumor microenvironment can contribute to antitumor efficacy.

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.

γδ T cell costimulatory ligands in antitumor immunity

Antitumor immunity relies on the ability of T cells to recognize and kill tumor targets. γδ T cells are a specialized subset of T cells that predominantly localizes to non-lymphoid tissue such as the skin, gut, and lung where they are actively involved in tumor immunosurveillance. γδ T cells respond to self-stress ligands that are increased on many tumor cells, and these interactions provide costimulatory signals that promote their activation and cytotoxicity. This review will cover costimulatory molecules that are known to be critical for the function of γδ T cells with a specific focus on mouse dendritic epidermal T cells (DETC). DETC are a prototypic tissue-resident γδ T cell population with known roles in antitumor immunity and are therefore useful for identifying mechanisms that may control activation of other γδ T cell subsets within non-lymphoid tissues. This review concludes with a brief discussion on how γδ T cell costimulatory molecules can be targeted for improved cancer immunotherapy.

Harnessing Mechanisms of Immune Tolerance to Improve Outcomes in Solid Organ Transplantation: A Review

Survival after solid organ transplantation (SOT) is limited by chronic rejection as well as the need for lifelong immunosuppression and its associated toxicities. Several preclinical and clinical studies have tested methods designed to induce transplantation tolerance without lifelong immune suppression. The limited success of these strategies has led to the development of clinical protocols that combine SOT with other approaches, such as allogeneic hematopoietic stem cell transplantation (HSCT). HSCT prior to SOT facilitates engraftment of donor cells that can drive immune tolerance. Recent innovations in graft manipulation strategies and post-HSCT immune therapy provide further advances in promoting tolerance and improving clinical outcomes. In this review, we discuss conventional and unconventional immunological mechanisms underlying the development of immune tolerance in SOT recipients and how they can inform clinical advances. Specifically, we review the most recent mechanistic studies elucidating which immune regulatory cells dampen cytotoxic immune reactivity while fostering a tolerogenic environment. We further discuss how this understanding of regulatory cells can shape graft engineering and other therapeutic strategies to improve long-term outcomes for patients receiving HSCT and SOT.

Immune Modulation Properties of Zoledronic Acid on TcRγδ T-Lymphocytes After TcRαβ/CD19-Depleted Haploidentical Stem Cell Transplantation: An analysis on 46 Pediatric Patients Affected by Acute Leukemia

TcRαβ/CD19-cell depleted HLA-haploidentical hematopoietic stem cell transplantation (haplo-HSCT) represents a promising new platform for children affected by acute leukemia in need of an allograft and lacking a matched donor, disease recurrence being the main cause of treatment failure. The use of zoledronic acid to enhance TcRγδ+ lymphocyte function after TcRαβ/CD19-cell depleted haplo-HSCT was tested in an open-label, feasibility, proof-of-principle study. Forty-six children affected by high-risk acute leukemia underwent haplo-HSCT after removal of TcRαβ+ and CD19+ B lymphocytes. No post-transplant pharmacological graft-versus-host disease (GvHD) prophylaxis was given. Zoledronic acid was administered monthly at a dose of 0.05 mg/kg/dose (maximum dose 4 mg), starting from day +20 after transplantation. A total of 139 infusions were administered, with a mean of 3 infusions per patient. No severe adverse event was observed. Common side effects were represented by asymptomatic hypocalcemia and acute phase reactions (including fever, chills, malaise, and/or arthralgia) within 24–48 h from zoledronic acid infusion. The cumulative incidence of acute and chronic GvHD was 17.3% (all grade I-II) and 4.8% (all limited), respectively. Patients given 3 or more infusions of zoledronic acid had a lower incidence of both acute GvHD (8.8 vs. 41.6%, p = 0.015) and chronic GvHD (0 vs. 22.2%, p = 0.006). Transplant-related mortality (TRM) and relapse incidence at 3 years were 4.3 and 30.4%, respectively. Patients receiving repeated infusions of zoledronic acid had a lower TRM as compared to those receiving 1 or 2 administration of the drug (0 vs. 16.7%, p = 0.01). Five-year overall survival (OS) and disease-free survival (DFS) for the whole cohort were 67.2 and 65.2%, respectively, with a trend toward a better OS for patients receiving 3 or more infusions (73.1 vs. 50.0%, p = 0.05). The probability of GvHD/relapse-free survival was significantly worse in patients receiving 1–2 infusions of zoledonic acid than in those given ≥3 infusions (33.3 vs. 70.6%, respectively, p = 0.006). Multivariable analysis showed an independent positive effect on outcome given by repeated infusions of zoledronic acid (HR 0.27, p = 0.03). These data indicate that the use of zoledronic acid after TcRαβ/CD19-cell depleted haploHSCT is safe and may result in a lower incidence of acute GvHD, chronic GvHD, and TRM.

Suppressive activity of Vδ2+ γδ T cells on αβ T cells is licensed by TCR signaling and correlates with signal strength

Despite recent progress in the understanding of γδ T cells' roles and functions, their interaction with αβ T cells still remains to be elucidated. In this study, we sought to clarify what precisely endows peripheral Vδ2+ T cells with immunosuppressive function on autologous αβ T cells. We found that negatively freshly isolated Vδ2+ T cells do not exhibit suppressive behavior, even after stimulation with IL-12/IL-18/IL-15 or the sheer contact with butyrophilin-3A1-expressing tumor cell lines (U251 or SK-Mel-28). On the other hand, Vδ2+ T cells positively isolated through TCR crosslinking or after prolonged stimulation with isopentenyl pyrophosphate (IPP) mediate strong inhibitory effects on αβ T cell proliferation. Stimulation with IPP in the presence of IL-15 induces the most robust suppressive phenotype of Vδ2+ T cells. This indicates that Vδ2+ T cells' suppressive activity is dependent on a TCR signal and that the degree of suppression correlates with its strength. Vδ2+ T cell immunosuppression does not correlate with their Foxp3 expression but rather with their PD-L1 protein expression, evidenced by the massive reduction of suppressive activity when using a blocking antibody. In conclusion, pharmacologic stimulation of Vδ2+ T cells via the Vδ2 TCR for activation and expansion induces Vδ2+ T cells' potent killer activity while simultaneously licensing them to suppress αβ T cell responses. Taken together, the study is a further step to understand-in more detail-the suppressive activity of Vδ2+ γδ T cells.

Vγ9Vδ2 T Cells: Can We Re-Purpose a Potent Anti-Infection Mechanism for Cancer Therapy?

Cancer therapies based on in vivo stimulation, or on adoptive T cell transfer of Vγ9Vδ2 T cells, have been tested in the past decades but have failed to provide consistent clinical efficacy. New, promising concepts such as γδ Chimeric Antigen Receptor (CAR) -T cells and γδ T-cell engagers are currently under preclinical evaluation. Since the impact of factors, such as the relatively low abundance of γδ T cells within tumor tissue is still under investigation, it remains to be shown whether these effector T cells can provide significant efficacy against solid tumors. Here, we highlight key learnings from the natural role of Vγ9Vδ2 T cells in the elimination of host cells bearing intracellular bacterial agents and we translate these into the setting of tumor therapy. We discuss the availability and relevance of preclinical models as well as currently available tools and knowledge from a drug development perspective. Finally, we compare advantages and disadvantages of existing therapeutic concepts and propose a role for Vγ9Vδ2 T cells in immune-oncology next to Cluster of Differentiation (CD) 3 activating therapies.

In the Absence of a TCR Signal IL-2/IL-12/18-Stimulated γδ T Cells Demonstrate Potent Anti-Tumoral Function Through Direct Killing and Senescence Induction in Cancer Cells

Abundant IFN-γ secretion, potent cytotoxicity, and major histocompatibility complex-independent targeting of a large spectrum of tumors make γδ T cells attractive candidates for cancer immunotherapy. Upon tumor recognition through the T-cell receptor (TCR), NK-receptors, or NKG2D, γδ T cells generate the pro-inflammatory cytokines TNF-α and IFN-γ, or granzymes and perforin that mediate cellular apoptosis. Despite these favorable potentials, most clinical trials testing the adoptive transfer of pharmacologically TCR-targeted and expanded γδ T cells resulted in a limited response. Recently, the TCR-independent activation of γδ T cells was identified. However, the modulation of γδ T cell’s effector functions solely by cytokines remains to be elucidated. In the present study, we systematically analyzed the impact of IL-2, IL-12, and IL-18 in parallel with TCR stimulation on proliferation, cytokine production, and anti-tumor activity of γδ T cells. Our results demonstrate that IL-12 and IL-18, when combined, constitute the most potent stimulus to enhance anti-tumor activity and induce proliferation and IFN-γ production by γδ T cells in the absence of TCR signaling. Intriguingly, stimulation with IL-12 and IL-18 without TCR stimulus induces a comparable degree of anti-tumor activity in γδ T cells to TCR crosslinking by killing tumor cells and driving cancer cells into senescence. These findings approve the use of IL-12/IL-18-stimulated γδ T cells for adoptive cell therapy to boost anti-tumor activity by γδ T cells.

IL-12 Expands and Differentiates Human Vγ2Vδ2 T Effector Cells Producing Antimicrobial Cytokines and Inhibiting Intracellular Mycobacterial Growth

While IL-12 plays a key role in differentiation of protective CD4⁺ Th1 response, little is known about mechanisms whereby IL-12 differentiates other T-cell populations. Published studies suggest that predominant Vγ2Vδ2 T cells in humans/nonhuman primates (NHP) are a fast-acting T-cell subset, with capacities to rapidly expand and produce Th1 and cytotoxic cytokines in response to phosphoantigen (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP) produced by Mycobacterium tuberculosis (Mtb) or others. However, whether IL-12 signaling pathway mediates fast-acting and Th1 or anti-microbial features of Vγ2Vδ2 T cells remains poorly defined. Here, we show that IL-12, but not other IL-12 family members IL-27/IL-35, apparently expanded HMBPP-activated Vγ2Vδ2 T cells. Although IL-12 and IL-2 similarly expanded HMBPP-activated Vγ2Vδ2 T-cell clones, the IL-12-induced expansion did not require endogenous IL-2 or IL-2 co-signaling during HMBPP + IL-12 co-treatment. IL-12-induced expansion of Vγ2Vδ2 T cells required the PI3K/AKT and STAT4 activation pathways and endogenous TNF-α signaling but did not involve p38/MAPK or IFN-γ signals. IL-12-expanded Vγ2Vδ2 T cells exhibited central/effector memory phenotypes and differentiated into polyfunctional effector cell subtypes which expressed TBX21/T-bet, antimicrobial cytokines IFN-γ, TNF-α, GM-CSF, and cytotoxic granule molecules. Furthermore, the IL-12-expanded Vγ2Vδ2 T cells inhibited the growth of intracellular mycobacteria in IFN-γ- or TNF-α-dependent fashion. Our findings support the concept that IL-12 drives early development of fast-acting Vγ2Vδ2 T effector cells in antimicrobial immune responses.

Vγ9Vδ2 T Cells in the Bone Marrow of Myeloma Patients: A Paradigm of Microenvironment-Induced Immune Suppression

Vγ9Vδ2 T cells are non-conventional T cells with a natural inclination to recognize and kill cancer cells. Malignant B cells, including myeloma cells, are privileged targets of Vγ9Vδ2 T cells in vitro. However, this inclination is often lost in vivo due to multiple mechanisms mediated by tumor cells and local microenvironment. Multiple myeloma (MM) is a paradigm disease in which antitumor immunity is selectively impaired at the tumor site. By interrogating the immune reactivity of bone marrow (BM) Vγ9Vδ2 T cells to phosphoantigens, we have revealed a very early and long-lasting impairment of Vγ9Vδ2 T-cell immune functions which is already detectable in monoclonal gammopathy of undetermined significance (MGUS) and not fully reverted even in clinical remission after autologous stem cell transplantation. Multiple cell subsets [MM cells, myeloid-derived suppressor cells, regulatory T cells, and BM-derived stromal cells (BMSC)] are involved in Vγ9Vδ2 T-cell inhibition via several immune suppressive mechanisms including the redundant expression of multiple immune checkpoints (ICPs). This review will address some aspects related to the dynamics of ICP expression in the BM of MM patients in relationship to the disease status (MGUS, diagnosis, remission, and relapse) and how this multifaceted ICP expression impairs Vγ9Vδ2 T-cell function. We will also provide some suggestions how to rescue Vγ9Vδ2 T cells from the immune suppression operated by ICP and to recover their antimyeloma immune effector functions at the tumor site.

Regulatory functions of γδ T cells

γδ T cells share characteristics of innate and adaptive immune cells and are involved in a broad spectrum of pro-inflammatory functions. Nonetheless, there is accumulating evidence that γδ T cells also exhibit regulatory functions. In this review, we describe the different phenotypes of regulatory γδ T cells in correlation with the identified mechanisms of suppression.

Regulatory γδ T cells induced by G-CSF participate in acute graft-versus-host disease regulation in G-CSF-mobilized allogeneic peripheral blood stem cell transplantation

Background

The immunomodulatory effects of granulocyte colony-stimulating factor (G-CSF) on T cells result in a low incidence of acute graft-versus-host disease (aGVHD) in G-CSF-mobilized allogeneic peripheral blood stem cell transplantation (G-PBSCT). However, the exact mechanism remains unclear. Regulatory γδ T cells (γδTregs), characterized by the presence of TCRγδ and Foxp3, have aroused great concern in the maintenance of immune tolerance. We hypothesized that γδTregs might involve in the immunomodulatory effects of G-CSF mobilization.

Methods

The expression and immunomodulatory function of γδTreg subsets in peripheral blood of donors before and after G-CSF treatment in vivo and in vitro were evaluated by flow cytometry and CFSE assays. To investigate the effects of γδTregs on aGVHD, the association between γδTreg subsets in grafts and aGVHD in recipients was estimated.

Results

The proportions of Vδ1Tregs, CD27⁺Vδ1Tregs and CD25⁺Vδ1Tregs were significantly increased in peripheral blood after G-CSF treatment in vivo. γδTregs could be generated in vitro by stimulating with anti-TCRγδ in the presence of G-CSF. The immune phenotype, proliferation suppression function, and cytokine secretion of G-CSF-induced γδTregs were similar to that of transforming growth factor-β (TGF-β)-induced γδTregs. The clinical data demonstrated that the proportion of CD27⁺Vδ1Tregs in grafts was significantly lower in the patients who experienced aGVHD than in those who did not develop aGVHD (P = 0.028), and the proportions of other γδTreg subsets in grafts did not differ significantly between the two groups. The best cutoff value for CD27⁺Vδ1Treg proportion in grafts in prediction of aGVHD was 0.33%, with an area under the curve value of 0.725 (P = 0.043). Eight patients (26.7%) were classified as the low-CD27⁺Vδ1Treg group (< 0.33%), and 22 patients (73.3%) as the high-CD27⁺Vδ1Treg group (≥ 0.33%). The incidence of aGVHD was higher in the low-CD27⁺Vδ1Treg group than in the high-CD27⁺Vδ1Treg group (75.0% versus 22.7%, P = 0.028).

Conclusions

G-CSF could induce the generation of γδTregs in vivo and in vitro, and γδTregs might participate in aGVHD regulation in G-PBSCT.

γδ T Cells: Unexpected Regulators of Cancer Development and Progression

Accumulating evidence suggests a role for gamma delta (γδ) T cells as unexpected drivers of tumor development and progression. These protumoral γδ T cells are abundant in the tumor microenvironment in both mouse and human. They promote tumor progression by: (i) inducing an immunosuppressive tumor microenvironment and angiogenesis via cytokine production; (ii) functioning as regulatory T (Treg)/T helper 2 (Th2)-like cells; (iii) interfering with dendritic cell (DC) effector function; and (iv) inhibiting antitumor adaptive T cell immunity via the programmed death-1 (PD-1)-programmed death ligand-1 (PD-L1) pathway. Understanding how these cells are regulated and what their specific role in cancer is will provide insight for the development of approaches that specifically target these cells and can thereby improve the efficacy of cancer immunotherapies.

Prospects for chimeric antigen receptor (CAR) γδ T cells: A potential game changer for adoptive T cell cancer immunotherapy

Excitement is growing for therapies that harness the power of patients' immune systems to combat their diseases. One approach to immunotherapy involves engineering patients' own T cells to express a chimeric antigen receptor (CAR) to treat advanced cancers, particularly those refractory to conventional therapeutic agents. Although these engineered immune cells have made remarkable strides in the treatment of patients with certain hematologic malignancies, success with solid tumors has been limited, probably due to immunosuppressive mechanisms in the tumor niche. In nearly all studies to date, T cells bearing αβ receptors have been used to generate CAR T cells. In this review, we highlight biological characteristics of γδ T cells that are distinct from those of αβ T cells, including homing to epithelial and mucosal tissues and unique functions such as direct antigen recognition, lack of alloreactivity, and ability to present antigens. We offer our perspective that these features make γδ T cells promising for use in cellular therapy against several types of solid tumors, including melanoma and gastrointestinal cancers. Engineered γδ T cells should be considered as a new platform for adoptive T cell cancer therapy for mucosal tumors.

Adoptive Cellular Therapy (ACT) for Cancer Treatment

Adoptive cellular therapy (ACT) with various lymphocytes or antigen-presenting cells is one stone in the pillar of cancer immunotherapy, which relies on the tumor-specific T cell. The transfusion of bulk T-cell population into patients is an effective treatment for regression of cancer. In this chapter, we summarize the development of various strategies in ACT for cancer immunotherapy and discuss some of the latest progress and obstacles in technical, safety, and even regulatory aspects to translate these technologies to the clinic. ACT is becoming a potentially powerful approach to cancer treatment. Further experiments and clinical trials are needed to optimize this strategy.

The MAPK-Activated Kinase MK2 Attenuates Dendritic Cell-Mediated Th1 Differentiation and Autoimmune Encephalomyelitis

Dendritic cell (DC)-mediated inflammation induced via TLRs is promoted by MAPK-activated protein kinase (MK)-2, a substrate of p38 MAPK. In this study we show an opposing role of MK2, by which it consolidates immune regulatory functions in DCs through modulation of p38, ERK1/2-MAPK, and STAT3 signaling. During primary TLR/p38 signaling, MK2 mediates the inhibition of p38 activation and positively cross-regulates ERK1/2 activity, leading to a reduction of IL-12 and IL-1α/β secretion. Consequently, MK2 impairs secondary autocrine IL-1α signaling in DCs, which further decreases the IL-1α/p38 but increases the anti-inflammatory IL-10/STAT3 signaling route. Therefore, the blockade of MK2 activity enables human and murine DCs to strengthen proinflammatory effector mechanisms by promoting IL-1α-mediated Th1 effector functions in vitro. Furthermore, MK2-deficient DCs trigger Th1 differentiation and Ag-specific cytotoxicity in vivo. Finally, wild-type mice immunized with LPS in the presence of an MK2 inhibitor strongly accumulate Th1 cells in their lymph nodes. These observations correlate with a severe clinical course in DC-specific MK2 knockout mice compared with wild-type littermates upon induction of experimental autoimmune encephalitis. Our data suggest that MK2 exerts a profound anti-inflammatory effect that prevents DCs from prolonging excessive Th1 effector T cell functions and autoimmunity.

Empowering gamma delta T cells with antitumor immunity by dendritic cell-based immunotherapy

Gamma delta (γδ) T cells are the all-rounders of our immune-system with their major histocompatibility complex-unrestricted cytotoxicity, capacity to secrete immunosti-mulatory cytokines and ability to promote the generation of tumor antigen-specific CD8⁺ and CD4⁺ T cell responses. Dendritic cell (DC)-based vaccine therapy has the prospective to harness these unique features of the γδ T cells in the fight against cancer. In this review, we will discuss our current knowledge on DC-mediated γδ T cell activation and related opportunities for tumor immunologists.

HIV-1-induced impairment of dendritic cell cross talk with γδ T lymphocytes

The interplay between dendritic cells (DC) and γδ T lymphocytes represents a network of paracrine and cell contact interactions important for an integrated immune response to pathogens. HIV-1 infection dramatically affects the number and functions of both cell populations, and DC/γδ T cell cross talk may represent a target of virus-induced immune escape. We investigated whether HIV-exposed DC could deliver aberrant signals to interacting γδ T cells. Here we report that the interaction of human γδ T lymphocytes with HIV-1-exposed autologous monocyte-derived DC, but not direct exposure to the virus, impairs lymphocyte expansion and gamma interferon (IFN-γ) production in response to phosphoantigens. This effect is independent of virus strain and occurred in 55% of the donors analyzed. The donor-dependent variation observed relies on the responsiveness of DC to HIV-1 and is strictly related to the capacity of the virus to suppress the maturation-induced expression of interleukin 12 (IL-12). In fact, γδ T cell response to phosphoantigens is almost completely recovered when this cytokine is exogenously added to the DC/lymphocyte cocultures. Interestingly, we show that γδ T lymphocytes are recruited by HIV-1-exposed DC through a CCR5-mediated mechanism and exert a CCL4-mediated control on virus dissemination within DC and susceptible CD4(+) T lymphocytes. These results demonstrate an association between HIV-induced DC dysfunction and alterations of γδ T cell responses. The aberrant cross talk between these two cell populations may contribute to the pathogenesis of HIV infection by further reducing the strength of antiviral immune response.

IMPORTANCE

This study provides new evidence on the mechanisms exploited by HIV-1 to evade the host immune response. We report that HIV-1 impairs the cross talk between DC and γδ T lymphocytes, by reducing the capacity of DC to promote functional γδ T cell activation. Interestingly, the virus does not per se interfere with γδ T cell activation, thus highlighting the key role of early DC-HIV-1 interaction in this phenomenon. Furthermore, the results obtained unravel the novel role of γδ T cells in controlling HIV-1 dissemination within the DC population as well as virus transfer to susceptible CD4(+) T lymphocytes. The interactions of DC with innate lymphocytes represent a major control mechanism for an integrated immune response to infection. Understanding how HIV-1 harnesses these pathways may provide important insights on the pathogenesis of disease and offer new opportunities for therapeutic interventions.

Cancer Immunotherapy Using γδT Cells: Dealing with Diversity

The broad and potent tumor-reactivity of innate-like γδT cells makes them valuable additions to current cancer immunotherapeutic concepts based on adaptive immunity, such as monoclonal antibodies and αβT cells. However, clinical success using γδT cells to treat cancer has so far fallen short. Efforts of recent years have revealed a striking diversity in γδT cell functions and immunobiology, putting these cells forward as true “swiss army knives” of immunity. At the same time, however, this heterogeneity poses new challenges to the design of γδT cell-based therapeutic concepts and could explain their rather limited clinical efficacy in cancer patients. This review outlines the recent new insights into the different levels of γδT cell diversity, including the myriad of γδT cell-mediated immune functions, the diversity of specificities and affinities within the γδT cell repertoire, and the multitude of complex molecular requirements for γδT cell activation. A careful consideration of the diversity of antibodies and αβT cells has delivered great progress to their clinical success; addressing also the extraordinary diversity in γδT cells will therefore hold the key to more effective immunotherapeutic strategies with γδT cells as additional and valuable tools to battle cancer.

Human gamma delta T regulatory cells in cancer: fact or fiction?

While gamma delta T cell (γδTc) anticancer immunotherapies are being developed, recent reports suggest a regulatory role for γδTc tumor-infiltrating lymphocytes. This mini-review surveys available evidence, determines strengths and weaknesses thereof and suggest directions for further exploration. We focus on human γδTc, as mouse and human γδTc repertoires differ. Regulatory γδTc are defined and compared to conventional Tregs and their roles in health and disease (focusing in on cancer) are discussed. We contrast the suggested regulatory roles for γδTc in breast and colorectal cancer with their cytotoxic capabilities in other malignancies, emphasizing the context dependence of γδTc functional plasticity. Since γδTc can be induced to exhibit regulatory properties (in some cases reversible), we carefully scrutinize experimental procedures in published reports. As γδTc garner increasing interest for their therapeutic potential, it is critical that we appreciate the full extent of their role(s) and interactions with other cell types in both the circulation and the tumor microenvironment. A comprehensive understanding will enable manipulation of γδTc to improve anti-tumor efficacy and patient outcomes.

Cytotoxic and regulatory properties of circulating Vδ1+ γδ T cells: a new player on the cell therapy field?

Exploration of cancer immunotherapy strategies that incorporate γδ T cells as primary mediators of antitumor immunity are just beginning to be explored and with a primary focus on the use of manufactured phosphoantigen-stimulated Vγ9Vδ2 T cells. Increasing evidence, however, supports a critical role for Vδ1+ γδ T cells, a minor subset in peripheral blood with distinct innate recognition properties that possess powerful tumoricidal activity. They are activated by a host of ligands including stress-induced self-antigens, glycolipids presented by CD1c/d, and potentially many others that currently remain unidentified. In contrast to Vγ9Vδ2 T cells, tumor-reactive Vδ1+ T cells are not as susceptible to activation-induced cell death and can persist in the circulation for many years, potentially offering durable immunity to some cancers. In addition, specific populations of Vδ1+ T cells can also exhibit immunosuppressive and regulatory properties, a function that can also be exploited for therapeutic purposes. This review explores the biology, function, manufacturing strategies, and potential therapeutic role of Vδ1+ T cells. We also discuss clinical experience with Vδ1+ T cells in the setting of cancer, as well as the potential of and barriers to the development of Vδ1+ T cell-based adoptive cell therapy strategies.

Phenotype and regulation of immunosuppressive Vδ2-expressing γδ T cells

The proliferation and interleukin-2 production of CD4(+)CD25(-) αβ T cells were inhibited in a cell-contact manner by Vδ2 γδ T cells. The transcription factor Helios was constitutively expressed in about one-third of circulating γδ T cells and was upregulated by CD28-signaling. Our data suggest that Helios could serve as a marker for differential activation status rather than for regulatory T cells (Treg). Our findings also indicate that the interaction of CD86 on activated Vδ2 T cells and cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) on activated αβ T cells mediated the suppression because the suppressive effect was abolished by blocking the CD86:CTLA-4 interaction. Pre-treatment of Vδ2 T cells with Toll-like receptor 2 ligands enhanced phosphorylation of MAPKs, Akt, and NF-κB and partially abrogated the suppressive capacity, whereas on co-cultured responder T cells inhibitory molecules were downregulated and Akt and NF-κB phosphorylation was restored. Our results suggest that the regulation of αβ T cell proliferation by activated Vδ2 T cells might contribute to fine-tuning of αβ T cell responses.

Specific recruitment of γδ regulatory T cells in human breast cancer

Understanding the role of different subtypes of tumor-infiltrating lymphocytes (TIL) in the immunosuppressive tumor microenvironment is essential for improving cancer treatment. Enriched γδ1 T-cell populations in TILs suppress T-cell responses and dendritic cell maturation in breast cancer, where their presence is correlated negatively with clinical outcomes. However, mechanism(s) that explain the increase in this class of regulatory T cells (γδ Treg) in patients with breast cancer have yet to be elucidated. In this study, we show that IP-10 secreted by breast cancer cells attracted γδ Tregs. Using neutralizing antibodies against chemokines secreted by breast cancer cells, we found that IP-10 was the only functional chemokine that causes γδ Tregs to migrate toward breast cancer cells. In a humanized NOD-scid IL-2Rγ(null) (NSG) mouse model, human breast cancer cells attracted γδ Tregs as revealed by a live cell imaging system. IP-10 neutralization in vivo inhibited migration and trafficking of γδ Tregs into breast tumor sites, enhancing tumor immunity mediated by tumor-specific T cells. Together, our studies show how γδ Tregs accumulate in breast tumors, providing a rationale for their immunologic targeting to relieve immunosuppression in the tumor microenvironment.

Gammadelta T cells: innately adaptive immune cells?

T cells employ a cell surface heterodimeric molecule, the T cell receptor (TCR), to recognize specific antigens (Ags) presented by major histocompatibility complex (MHC) molecules and carry out adaptive immune responses. Most T cells possess a TCR with an α and a β chain. However, a TCR constituted by a γ and a δ chain has been described, defining a novel subset of T cells. γδ TCRs specific for a wide variety of ligands, including bacterial phosphoantigens, nonclassical MHC-I molecules and unprocessed proteins, have been found, greatly expanding the horizons of T cell immune recognition. This review aims to provide background in γδ T cell history and function in mouse and man, as well as to provide a critical view of some of the latest developments on this still enigmatic class of immune cells.

Fast dendritic cells stimulated with alternative maturation mixtures induce polyfunctional and long-lasting activation of innate and adaptive effector cells with tumor-killing capabilities

The clinical usage of dendritic cells (DC) for tumor immunotherapy still requires improvements. In this study, three alternative maturation mixtures were compared with the cytokine-based gold standard, and the overall interaction of the resulting DC with effector cells from the innate as well as the adaptive immunity was evaluated in healthy donors. Stimulation with the TLR-4 ligand monophosphoryl lipid A together with IFN-γ (alt-2 DC) resulted in DC with the highest levels of costimulatory molecule expression and IL-12p70/IL-10 ratio. Whereas all alternative DC were able to induce NK and γδ T cells to acquire cytotoxic properties and secrete type 1 and proinflammatory cytokines, after both short (20-h)- and long (5-8 d)-time coculture, secretion of IFN-γ by the innate populations was induced in response to alt-2 and alt-1 DC (TNF-α, IFN-α, IFN-γ, IL-1β, poly IC), but not to alt-3 DC (TNF-α, IFN-γ, IL-1β, CL097). Regarding CD8(+) T cell-mediated Ag-specific immune responses, a heterogeneous pattern of responses was obtained among the healthy donors, suggesting rather a competition than a synergy among the different effector cells. Our data promote further evaluation of alt-2 fast DC for translatability into clinical immunotherapy trials, while also fostering the need to identify biomarkers for immune cell responsiveness and tumor susceptibility to be able to select for each patient the best possible DC-based therapy.

Regulatory functions of γδ T cells

γδ T cells account for approximately 5% of peripheral blood T cells but are more abundant in mucosal tissue. Based on the recognized ligands and their general lack of MHC restriction, γδ T cells are considered as unconventional T cells that link innate and adaptive immunity. γδ T cells produce a diverse range of cytokines, exert cytotoxic effector function, can act as antigen-presenting cells, and display regulatory activity. Here we review the current knowledge on the regulatory functions of murine and human γδ T cells. Some γδ T cells produce inhibitory cytokines such as transforming growth factor-β but γδ T cells can utilize additional regulatory mechanisms. By subverting regulatory T cells (Treg) through induction of Treg apoptosis or cytokine-dependent reversal of Treg activity, however, γδ T cells can also enhance effector T cell activity and thereby contribute to autoimmunity. A more precise understanding of the plasticity of regulatory γδ T cells is required to specifically identify strategies for intentional modulation of their beneficial or detrimental regulatory activity.

Toll-like receptor 4 engagement drives differentiation of human and murine dendritic cells from a pro- into an anti-inflammatory mode

The dendritic cell (DC) coordinates innate and adaptive immunity to fight infections and cancer. Our observations reveal that DCs exposed to the microbial danger signal lipopolysaccharide (LPS) in the presence of interferon-γ (IFN-γ) acquire a continuously changing activation/maturation phenotype. The DCs’ initial mode of action is pro-inflammatory via up-regulation among others of the signaling molecule interleukin (IL) 12, which polarizes IFN-γ secreting type 1 helper T-cells (Th1). Within 24 hours the same DC switches from the pro- into an anti-inflammatory phenotype. This is mediated by autocrine IL-10 release and secretion of soluble IL-2 receptor alpha (sIL-2RA) molecules. T-cells, when contacted with DCs during their anti-inflammatory phase loose their proliferative capacity and develop regulatory T-cell (Treg) -like anti-inflammatory functions indicated by IL-10 secretion and elevated FoxP3 levels. Studying the kinetics of IL-12 and IL-10 expression from LPS/IFN-γ activated myeloid DCs on a single cell level confirmed these observations. When T-cells are separated from DCs within 24 hours, they are spared from the anti-inflammatory DC activity. We conclude that, in addition to differentiation of DCs into distinct subsets, the observed sequential functional phases of DC differentiation permit the fine-tuning of an immune response. A better understanding of time-kinetic DC features is required for optimally exploiting the therapeutic capacity of DCs in cancer immune therapy.

The multifunctionality of human Vγ9Vδ2 γδ T cells: clonal plasticity or distinct subsets?

The dominant subset of γδ T cells in human peripheral blood expresses Vγ9 paired with Vδ2 as variable TCR elements. Vγ9Vδ2 T cells recognize pyrophosphates derived from the microbial non-mevalonate isoprenoid biosynthesis pathway at pico- to nanomolar concentrations. Structurally related pyrophosphates are generated in eukaryotic cells through the mevalonate pathway involved in protein prenylation and cholesterol synthesis. However, micromolar concentrations of endogenous pyrophosphates are required to be recognized by Vγ9Vδ2 T cells. Such concentrations are not produced by normal cells but can accumulate upon cellular stress and transformation. Therefore, many tumour cells are susceptible to γδ T cell-mediated lysis owing to the overproduction of endogenous pyrophosphates. This explains why Vγ9Vδ2 T cells contribute to both anti-infective and anti-tumour immunity. Ex vivo analysed Vγ9Vδ2 T cells can be subdivided on the basis of additional surface markers, including chemokine receptors and markers for naïve and memory T cells. At the functional level, Vγ9Vδ2 T cells produce a broad range of cytokines, display potent cytotoxic activity, regulate αβ T cell responses, and - quite surprisingly - can act as professional antigen-presenting cells. Thus, an exceptional range of effector functions has been assigned to a population of T cells, which all recognize invariant exogenous or endogenous pyrophosphates that are not seen by any other immune cell. Here, we discuss whether this plethora of effector functions reflects the plasticity of individual Vγ9Vδ2 T cells or can be assigned to distinct subsets.

Optimizing dendritic cell-based immunotherapy: tackling the complexity of different arms of the immune system

Earlier investigations have revealed a surprising complexity and variety in the range of interaction between cells of the innate and adaptive immune system. Our understanding of the specialized roles of dendritic cell (DC) subsets in innate and adaptive immune responses has been significantly advanced over the years. Because of their immunoregulatory capacities and because very small numbers of activated DC are highly efficient at generating immune responses against antigens, DCs have been vigorously used in clinical trials in order to elicit or amplify immune responses against cancer and chronic infectious diseases. A better insight in DC immunobiology and function has stimulated many new ideas regarding the potential ways forward to improve DC therapy in a more fundamental way. Here, we discuss the continuous search for optimal in vitro conditions in order to generate clinical-grade DC with a potent immunogenic potential. For this, we explore the molecular and cellular mechanisms underlying adequate immune responses and focus on most favourable DC culture regimens and activation stimuli in humans. We envisage that by combining each of the features outlined in the current paper into a unified strategy, DC-based vaccines may advance to a higher level of effectiveness.

Interleukin-12p70 expression by dendritic cells of HIV-1-infected patients fails to stimulate gag-specific immune responses

A variety of immune-based therapies has been developed in order to boost or induce protective CD8⁺ T cell responses in order to control HIV replication. Since dendritic cells (DCs) are professional antigen-presenting cells (APCs) with the unique capability to stimulate naïve T cells into effector T cells, their use for the induction of HIV-specific immune responses has been studied intensively. In the present study we investigated whether modulation of the activation state of DCs electroporated with consensus codon-optimized HxB2 gag mRNA enhances their capacity to induce HIV gag-specific T cell responses. To this end, mature DCs were (i) co-electroporated with mRNA encoding interleukin (IL)-12p70 mRNA, or (ii) activated with a cytokine cocktail consisting of R848 and interferon (IFN)-γ. Our results confirm the ability of HxB2 gag-expressing DCs to expand functional HIV-specific CD8⁺ T cells. However, although most of the patients had detectable gag-specific CD8⁺ T cell responses, no significant differences in the level of expansion of functional CD8⁺ T cells could be demonstrated when comparing conventional or immune-modulated DCs expressing IL-12p70. This result which goes against expectation may lead to a re-evaluation of the need for IL-12 expression by DCs in order to improve T-cell responses in HIV-1-infected individuals.