Epithelia Use Butyrophilin-like Molecules to Shape Organ-Specific γδ T Cell Compartments

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

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

BTN3A1 governs antitumor responses by coordinating αβ and γδ T cells

Gamma delta (γδ) T cells infiltrate most human tumors, but current immunotherapies fail to exploit their in situ major histocompatibility complex-independent tumoricidal potential. Activation of γδ T cells can be elicited by butyrophilin and butyrophilin-like molecules that are structurally similar to the immunosuppressive B7 family members, yet how they regulate and coordinate αβ and γδ T cell responses remains unknown. Here, we report that the butyrophilin BTN3A1 inhibits tumor-reactive αβ T cell receptor activation by preventing segregation of N-glycosylated CD45 from the immune synapse. Notably, CD277-specific antibodies elicit coordinated restoration of αβ T cell effector activity and BTN2A1-dependent γδ lymphocyte cytotoxicity against BTN3A1⁺ cancer cells, abrogating malignant progression. Targeting BTN3A1 therefore orchestrates cooperative killing of established tumors by αβ and γδ T cells and may present a treatment strategy for tumors resistant to existing immunotherapies.

β2 Integrins differentially regulate γδ T cell subset thymic development and peripheral maintenance

The γδ T cells reside predominantly at barrier sites and play essential roles in immune protection against infection and cancer. Despite recent advances in the development of γδ T cell immunotherapy, our understanding of the basic biology of these cells, including how their numbers are regulated in vivo, remains poor. This is particularly true for tissue-resident γδ T cells. We have identified the β₂ family of integrins as regulators of γδ T cells. β₂-integrin-deficient mice displayed a striking increase in numbers of IL-17-producing Vγ6Vδ1⁺ γδ T cells in the lungs, uterus, and circulation. Thymic development of this population was normal. However, single-cell RNA sequencing revealed the enrichment of genes associated with T cell survival and proliferation specifically in β₂-integrin-deficient IL-17⁺ cells compared to their wild-type counterparts. Indeed, β₂-integrin-deficient Vγ6⁺ cells from the lungs showed reduced apoptosis ex vivo, suggesting that increased survival contributes to the accumulation of these cells in β₂-integrin-deficient tissues. Furthermore, our data revealed an unexpected role for β₂ integrins in promoting the thymic development of the IFNγ-producing CD27⁺ Vγ4⁺ γδ T cell subset. Together, our data reveal that β₂ integrins are important regulators of γδ T cell homeostasis, inhibiting the survival of IL-17-producing Vγ6Vδ1⁺ cells and promoting the thymic development of the IFNγ-producing Vγ4⁺ subset. Our study introduces unprecedented mechanisms of control for γδ T cell subsets.

HMGB1 Promotes Myeloid Egress and Limits Lymphatic Clearance of Malignant Pleural Effusions

Pleural effusions, when benign, are attributed to cardiac events and suffusion of fluid within the pleural space. When malignant, lymphatic obstruction by tumor and failure to absorb constitutively produced fluid is the predominant formulation. The prevailing view has been challenged recently, namely that the lymphatics are only passive vessels, carrying antigenic fluid to secondary lymphoid sites. Rather, lymphatic vessels can be a selective barrier, efficiently coordinating egress of immune cells and factors within tissues, limiting tumor spread and immune pathology. An alternative explanation, offered here, is that damage associated molecular pattern molecules, released in excess, maintain a local milieu associated with recruitment and retention of immune cells associated with failed lymphatic clearance and functional lymphatic obstruction. We found that levels of high mobility group box 1 (HMGB1) were equally elevated in both benign and malignant pleural effusions (MPEs) and that limited diversity of T cell receptor expressing gamma and delta chain were inversely associated with these levels in MPEs. Acellular fluid from MPEs enhanced γδ T cell proliferation in vitro, while inhibiting cytokine production from γδ T cells and monocytes as well as restricting monocyte chemotaxis. Novel therapeutic strategies, targeting HMGB1 and its neutralization in such effusions as well as direct delivery of immune cells into the pleural space to reconstitute normal physiology should be considered.

Mechanisms of activation of innate-like intraepithelial T lymphocytes

Intraepithelial T lymphocytes (T-IEL) contain subsets of innate-like T cells that evoke innate and adaptive immune responses to provide rapid protection at epithelial barrier sites. In the intestine, T-IEL express variable T cell antigen receptors (TCR), with unknown antigen specificities. Intriguingly, they also express multiple inhibitory receptors, many of which are normally found on exhausted or antigen-experienced T cells. This pattern suggests that T-IEL are antigen-experienced, yet it is not clear where, and in what context, T-IEL encounter TCR ligands. We review recent evidence indicating TCR antigens for intestinal innate-like T-IEL are found on thymic or intestinal epithelium, driving agonist selection of T-IEL. We explore the contributions of the TCR and various co-stimulatory and co-inhibitory receptors in activating T-IEL effector functions. The balance between inhibitory and activating signals may be key to keeping these highly cytotoxic, rapidly activated cells in check, and key to harnessing their immune surveillance potential.

Ligand recognition by the γδ TCR and discrimination between homeostasis and stress conditions

T lymphocytes comprise cells expressing either an αβ or a γδ TCR. The riddle how αβ TCRs are triggered by specific peptides presented in the context of MHC was elucidated some time ago. In contrast, the mechanisms that underlie antigen recognition by γδ TCRs are still baffling the scientific community. It is clear that activation of γδ TCRs does not necessarily depend on MHC antigen presentation. To date, diverse and largely host-cell-derived molecules have been identified as cognate antigens for the γδ TCR. However, for most γδ TCRs, the activating ligand is still unknown and many open questions with regard to physiological relevance and generalizable concepts remain. Especially the question of how γδ T cells can distinguish homeostatic from stress conditions via their TCR remains largely unresolved. Recent discoveries in the field might have paved the way towards a better understanding of antigen recognition by the γδ TCR and have made it conceivable to revise the current knowledge and contextualize the new findings.

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

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

Human γδ T cells recognize CD1b by two distinct mechanisms

γδ T cells form an abundant part of the human cellular immune system, where they respond to tissue damage, infection, and cancer. The spectrum of known molecular targets recognized by Vδ1-expressing γδ T cells is becoming increasingly diverse. Here we describe human γδ T cells that recognize CD1b, a lipid antigen-presenting molecule, which is inducibly expressed on monocytes and dendritic cells. Using CD1b tetramers to study multiple donors, we found that many CD1b-specific γδ T cells use Vδ1. Despite their common use of Vδ1, three CD1b-specific γδ T cell receptors (TCRs) showed clear differences in the surface of CD1b recognized, the requirement for lipid antigens, and corecognition of butryophilin-like proteins. Several Vγ segments were present among the CD1b-specific TCRs, but chain swap experiments demonstrated that CD1b specificity was mediated by the Vδ1 chain. One of the CD1b-specific Vδ1⁺ TCRs paired with Vγ4 and shows dual reactivity to CD1b and butyrophilin-like proteins. αβ TCRs typically recognize the peptide display platform of MHC proteins. In contrast, our results demonstrate the use of rearranged receptors to mediate diverse modes of recognition across the surface of CD1b in ways that do and do not require carried lipids.

Single-cell RNA sequencing identifies shared differentiation paths of mouse thymic innate T cells

Invariant natural killer T (iNKT), mucosal-associated invariant T (MAIT), and γδ T cells are innate T cells that acquire memory phenotype in the thymus and share similar biological characteristics. However, how their effector differentiation is developmentally regulated is still unclear. Here, we identify analogous effector subsets of these three innate T cell types in the thymus that share transcriptional profiles. Using single-cell RNA sequencing, we show that iNKT, MAIT and γδ T cells mature via shared, branched differentiation rather than linear maturation or TCR-mediated instruction. Simultaneous TCR clonotyping analysis reveals that thymic maturation of all three types is accompanied by clonal selection and expansion. Analyses of mice deficient of TBET, GATA3 or RORγt and additional in vivo experiments corroborate the predicted differentiation paths, while human innate T cells from liver samples display similar features. Collectively, our data indicate that innate T cells share effector differentiation processes in the thymus.

Innate T cells such as iNKT, MAIT and γδ T cells all develop in the thymus, but their differentiation paths are still unclear. Here, the authors show, using single-cell RNA sequencing, that all three cell types develop via shared and branched differentiation paths that are corroborated by additional results from gene-deficient mice and human liver T cells.

T Cell Immunity to Bacterial Pathogens: Mechanisms of Immune Control and Bacterial Evasion

The human body frequently encounters harmful bacterial pathogens and employs immune defense mechanisms designed to counteract such pathogenic assault. In the adaptive immune system, major histocompatibility complex (MHC)-restricted αβ T cells, along with unconventional αβ or γδ T cells, respond to bacterial antigens to orchestrate persisting protective immune responses and generate immunological memory. Research in the past ten years accelerated our knowledge of how T cells recognize bacterial antigens and how many bacterial species have evolved mechanisms to evade host antimicrobial immune responses. Such escape mechanisms act to corrupt the crosstalk between innate and adaptive immunity, potentially tipping the balance of host immune responses toward pathological rather than protective. This review examines the latest developments in our knowledge of how T cell immunity responds to bacterial pathogens and evaluates some of the mechanisms that pathogenic bacteria use to evade such T cell immunosurveillance, to promote virulence and survival in the host.

Butyrophilin-like proteins display combinatorial diversity in selecting and maintaining signature intraepithelial γδ T cell compartments

Butyrophilin-like (Btnl) genes are emerging as major epithelial determinants of tissue-associated γδ T cell compartments. Thus, the development of signature, murine TCRγδ⁺ intraepithelial lymphocytes (IEL) in gut and skin depends on Btnl family members, Btnl1 and Skint1, respectively. In seeking mechanisms underlying these profound effects, we now show that normal gut and skin γδ IEL development additionally requires Btnl6 and Skint2, respectively, and furthermore that different Btnl heteromers can seemingly shape different intestinal γδ⁺ IEL repertoires. This formal genetic evidence for the importance of Btnl heteromers also applied to the steady-state, since sustained Btnl expression is required to maintain the signature TCR.Vγ7⁺ IEL phenotype, including specific responsiveness to Btnl proteins. In sum, Btnl proteins are required to select and to maintain the phenotypes of tissue-protective γδ IEL compartments, with combinatorially diverse heteromers having differential impacts on different IEL subsets.

The contribution of animal models to understanding the role of the immune system in human idiopathic pulmonary fibrosis

Pulmonary fibrosis occurs in a heterogeneous group of lung disorders and is characterised by an excessive deposition of extracellular matrix proteins within the pulmonary interstitium, leading to impaired gas transfer and a loss of lung function. In the past 10 years, there has been a dramatic increase in our understanding of the immune system and how it contributes to fibrogenic processes within the lung. This review will compare some of the models used to investigate the pathogenesis and treatment of pulmonary fibrosis, in particular those used to study immune cell pathogenicity in idiopathic pulmonary fibrosis, highlighting their advantages and disadvantages in dissecting human disease.

Beneficial Effect of Antibiotics and Microbial Metabolites on Expanded Vδ2Vγ9 T Cells in Hepatocellular Carcinoma Immunotherapy

Animal experiments and clinical trials have shown that the gut microbiota modulates host immunity and immune checkpoint-mediated responses to tumor cells. However, it remains unclear whether microbiota can also play a role in the tumor immune response of γδT cells, a kind of cell that targets cancer directly. Here, we report that microbiota dysbiosis induced by antibiotics enhanced γδT cell efficacy during tumor therapy in a mouse model. Further microbiota and metabolite analysis revealed that the alteration of γδT cell cytotoxicity might be closely associated with specific metabolites, which are produced by intestinal bacteria and stimulate γδT cells to release more cytotoxic cytokines, such as granzyme B and perforin. Among the metabolites that we analyzed, 3-indopropionic acid (IPA) showed the highest concentration in antibiotic-treated mice and can improve the cytotoxic ability of γδT cells both in vitro and in vivo. Our research determined how the gut microbiota can influence the antitumor ability of γδT cells and identified potential intermediate molecules that connect the gut microbiota and γδT cells.

Intestinal intraepithelial lymphocytes: Maintainers of intestinal immune tolerance and regulators of intestinal immunity

Intestinal immune tolerance is essential for the immune system, as it prevents abnormal immune responses to large quantities of antigens from the intestinal lumen, such as antigens from commensal microorganisms, and avoids self‐injury. Intestinal intraepithelial lymphocytes (IELs), a special group of mucosal T lymphocytes, play a significant role in intestinal immune tolerance. To accomplish this, IELs exhibit a high threshold of activation and low reactivity to most antigens from the intestinal lumen. In particular, CD8αα⁺TCRαβ⁺ IELs, TCRγδ⁺ IELs, and CD4⁺CD8αα⁺ IELs show great potential for maintaining intestinal immune tolerance and regulating intestinal immunity. However, if the intestinal microenvironment becomes abnormal or intestinal tolerance is broken, IELs may be activated abnormally and become pathogenic.

New Molecular Insights into Immune Cell Development

During development innate lymphoid cells and specialized lymphocyte subsets colonize peripheral tissues, where they contribute to organogenesis and later constitute the first line of protection while maintaining tissue homeostasis. A few of these subsets are produced only during embryonic development and remain in the tissues throughout life. They are generated through a unique developmental program initiated in lympho-myeloid-primed progenitors, which lose myeloid and B cell potential. They either differentiate into innate lymphoid cells or migrate to the thymus to give rise to embryonic T cell receptor-invariant T cells. At later developmental stages, adaptive T lymphocytes are derived from lympho-myeloid progenitors that colonize the thymus, while lymphoid progenitors become specialized in the production of B cells. This sequence of events highlights the requirement for stratification in the establishment of immune functions that determine efficient seeding of peripheral tissues by a limited number of cells.

Analysis of relapse-associated alternative mRNA splicing and construction of a prognostic signature predicting relapse in I-III colon cancer

The literature comprehensively analyzed alternative splicing (AS) events in colon cancer is little and corresponding prognostic signature is still a lack. Based on data of TCGA, the relapse-associated ASs were comprehensively analyzed and a signature was further constructed to predict the relapse in I-III colon cancer. In total 1912 ASs of 1384 mRNA were identified as relapse-associated ASs, protein-protein interactions (PPI) and ASs-splicing factors (SF) interactions network were identified. We finally built a robust signature to predict the relapse of I-III colon cancer with a considerable AUC value in both the training group and the test group. The AUC in the entire set at 1, 3 and 5 year was 0.85, 0.83 and 0.836. Our study provided a profile of relapse-associated ASs in I-III colon cancer and built a robust signature to predict the relapse of I-III colon cancer.

Bibliometrics Analysis of Butyrophilins as Immune Regulators [1992-2019] and Implications for Cancer Prognosis

The butyrophilins (BTNs) represent a unique family of immunoglobulin. They were considered to be involved in milk lactation after their discovery in 1981. With the development of research, an increasing number of research revealed that BTNs play important roles in immune regulation [1992-2019]. Our research aimed to summarize the BTN research status and their relationship with lung cancers and breast cancers by bibliometrics and bioinformatics methods. Our results indicate that the researches on immune-regulatory functions of BTNs gradually developed from 1992 to 2006, whereas they increased quickly after 2007. There are international cooperations among 56 countries, of which the United States is the most active one with the highest number of studies as well as highest citations. By coauthorship and cocitation analysis, we showed that Adrian Hayday, who is active in γδ T-cell field, was an active author in BTN publications with average year of 2015 and led a subfield. By keywords co-occurrence analysis, we found that γδ T cell, which is an important cancer immune regulator, is one important hotspot. Finally, we found that several BTN members' expression levels were significantly correlated with prognosis of lung cancer and breast cancer patients. Thus, these BTNs might play immune regulatory effects and could serve as potential biomarkers for cancer.

An Update on the Molecular Basis of Phosphoantigen Recognition by Vγ9Vδ2 T Cells

About 1-5% of human blood T cells are Vγ9Vδ2 T cells. Their hallmark is the expression of T cell antigen receptors (TCR) whose γ-chains contain a rearrangement of Vγ9 with JP (TRGV9JP or Vγ2Jγ1.2) and are paired with Vδ2 (TRDV2)-containing δ-chains. These TCRs respond to phosphoantigens (PAg) such as (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP), which is found in many pathogens, and isopentenyl pyrophosphate (IPP), which accumulates in certain tumors or cells treated with aminobisphosphonates such as zoledronate. Until recently, these cells were believed to be restricted to primates, while no such cells are found in rodents. The identification of three genes pivotal for PAg recognition encoding for Vγ9, Vδ2, and butyrophilin (BTN) 3 in various non-primate species identified candidate species possessing PAg-reactive Vγ9Vδ2 T cells. Here, we review the current knowledge of the molecular basis of PAg recognition. This not only includes human Vγ9Vδ2 T cells and the recent discovery of BTN2A1 as Vγ9-binding protein mandatory for the PAg response but also insights gained from the identification of functional PAg-reactive Vγ9Vδ2 T cells and BTN3 in the alpaca and phylogenetic comparisons. Finally, we discuss models of the molecular basis of PAg recognition and implications for the development of transgenic mouse models for PAg-reactive Vγ9Vδ2 T cells.

Recent advances in understanding the development and function of γδ T cells

γδ T cells are a subset of T cells with attributes of both the innate and adaptive arms of the immune system. These cells have long been an enigmatic and poorly understood component of the immune system and many have viewed them as having limited importance in host defense. This perspective persisted for some time both because of critical gaps in knowledge regarding how the development of γδ T cells is regulated and because of the lack of effective and sophisticated approaches through which the function of γδ T cells can be manipulated. Here, we discuss the recent advances in both of these areas, which have brought the importance of γδ T cells in both productive and pathologic immune function more sharply into focus.

The Innate Biologies of Adaptive Antigen Receptors

Nonclonal innate immune responses mediated by germ line-encoded receptors, such as Toll-like receptors or natural killer receptors, are commonly contrasted with diverse, clonotypic adaptive responses of lymphocyte antigen receptors generated by somatic recombination. However, the Variable (V) regions of antigen receptors include germ line-encoded motifs unaltered by somatic recombination, and theoretically available to mediate nonclonal, innate responses, that are independent of or largely override clonotypic responses. Recent evidence demonstrates that such responses exist, underpinning the associations of particular γδ T cell receptors (TCRs) with specific anatomical sites. Thus, TCRγδ can make innate and adaptive responses with distinct functional outcomes. Given that αβ T cells and B cells can also make nonclonal responses, we consider that innate responses of antigen receptor V-regions may be more widespread, for example, inducing states of preparedness from which adaptive clones are better selected. We likewise consider that potent, nonclonal T cell responses to microbial superantigens may reflect subversion of physiologic innate responses of TCRα/β chains.

Innate Lymphocyte Mechanisms in Skin Diseases

Innate lymphocyte populations are emerging as key effectors in tissue homeostasis, microbial defense, and inflammatory skin disease. The cells are evolutionarily ancient and carry conserved principles of function, which can be achieved through shared or unique specific mechanisms. Recent technological and treatment advances have provided insight into heterogeneity within and between individuals and species. Similar pathways can extend through to adaptive lymphocytes, which softens the margins with innate lymphocyte populations and allows investigation of nonredundant pathways of immunity and inflammation that might be amenable to therapeutic intervention. Here, we review advances in understanding of innate lymphocyte biology with a focus on skin disease and the roles of commensal and pathogen responses and tissue homeostasis.

MAIT cells are imprinted by the microbiota in early life and promote tissue repair

How early-life colonization and subsequent exposure to the microbiota affect long-term tissue immunity remains poorly understood. Here, we show that the development of mucosal-associated invariant T (MAIT) cells relies on a specific temporal window, after which MAIT cell development is permanently impaired. This imprinting depends on early-life exposure to defined microbes that synthesize riboflavin-derived antigens. In adults, cutaneous MAIT cells are a dominant population of interleukin-17A (IL-17A)-producing lymphocytes, which display a distinct transcriptional signature and can subsequently respond to skin commensals in an IL-1-, IL-18-, and antigen-dependent manner. Consequently, local activation of cutaneous MAIT cells promotes wound healing. Together, our work uncovers a privileged interaction between defined members of the microbiota and MAIT cells, which sequentially controls both tissue-imprinting and subsequent responses to injury.

Human γδ TCR Repertoires in Health and Disease

The T cell receptor (TCR) repertoires of γδ T cells are very different to those of αβ T cells. While the theoretical TCR repertoire diversity of γδ T cells is estimated to exceed the diversity of αβ T cells by far, γδ T cells are still understood as more invariant T cells that only use a limited set of γδ TCRs. Most of our current knowledge of human γδ T cell receptor diversity builds on specific monoclonal antibodies that discriminate between the two major subsets, namely Vδ2⁺ and Vδ1⁺ T cells. Of those two subsets, Vδ2⁺ T cells seem to better fit into a role of innate T cells with semi-invariant TCR usage, as compared to an adaptive-like biology of some Vδ1⁺ subsets. Yet, this distinction into innate-like Vδ2⁺ and adaptive-like Vδ1⁺ γδ T cells does not quite recapitulate the full diversity of γδ T cell subsets, ligands and interaction modes. Here, we review how the recent introduction of high-throughput TCR repertoire sequencing has boosted our knowledge of γδ T cell repertoire diversity beyond Vδ2⁺ and Vδ1⁺ T cells. We discuss the current understanding of clonal composition and the dynamics of human γδ TCR repertoires in health and disease.

Functional Phenotypes of Human Vγ9Vδ2 T Cells in Lymphoid Stress Surveillance

Butyrophilin and butyrophilin-like proteins select γδ T cells and direct the migration of γδ T cell subsets to distinct anatomical sites. γδ T cells expressing Vδ2 paired with Vγ9 (Vγ9Vδ2 T cells) are the predominant γδ T cell type in human peripheral blood. Vγ9Vδ2 T cells, which cannot be studied easily in vivo because they do not exist in rodents, are often referred to as innate-like T cells. The genetically recombined γδ T cell receptor (TCR) that responds to isoprenoid-derived pyrophosphates (phosphoantigens) produced by infected and malignant cells in a butyrophilin-dependent manner qualifies them as therapeutically relevant components of the adaptive immune system. On the other hand, cell-surface proteins such as the C-type lectin CD161 mark a functional phenotype of Vγ9Vδ2 T cells that mediates TCR-independent innate-like responses. Moreover, CD56 (neural cell adhesion molecule, NCAM) and the G protein-coupled receptor GPR56 define Vγ9Vδ2 T cells with increased cytolytic potential and, like CD161, may also be expressed by dendritic cells, principally facilitating the generation of an innate-like immunological synapse. In this review, we summarise current knowledge of Vγ9Vδ2 T cell functional phenotypes that are critical to lymphoid stress surveillance.

Revisiting the Interaction of γδ T-Cells and B-Cells

Right after the discovery of γδ T-cells in 1984, people started asking how γδ T-cells interact with other immune cells such as B-cells. Early reports showed that γδ T-cells are able to help B-cells to produce antibodies and to sustain the production of germinal centers. Interestingly, the presence of γδ T-cells seems to promote the generation of antibodies against "self" and less against challenging pathogens. More recently, these hypotheses were supported using γδ T-cell-deficient mouse strains, in different mouse models of systemic lupus erythematous, and after induction of epithelial cell damage. Together, these studies suggest that the link between γδ T-cells and the production of autoantibodies may be more relevant for the development of autoimmune diseases than generally acknowledged and thus targeting γδ T-cells could represent a new therapeutic strategy. In this review, we focus on what is known about the communication between γδ T-cells and B-cells, and we discuss the importance of this interaction in the context of autoimmunity.

Butyrophilin-2A1 Directly Binds Germline-Encoded Regions of the Vγ9Vδ2 TCR and Is Essential for Phosphoantigen Sensing

Vγ9Vδ2 T cells respond in a TCR-dependent fashion to both microbial and host-derived pyrophosphate compounds (phosphoantigens, or P-Ag). Butyrophilin-3A1 (BTN3A1), a protein structurally related to the B7 family of costimulatory molecules, is necessary but insufficient for this process. We performed radiation hybrid screens to uncover direct TCR ligands and cofactors that potentiate BTN3A1's P-Ag sensing function. These experiments identified butyrophilin-2A1 (BTN2A1) as essential to Vγ9Vδ2 T cell recognition. BTN2A1 synergised with BTN3A1 in sensitizing P-Ag-exposed cells for Vγ9Vδ2 TCR-mediated responses. Surface plasmon resonance experiments established Vγ9Vδ2 TCRs used germline-encoded Vγ9 regions to directly bind the BTN2A1 CFG-IgV domain surface. Notably, somatically recombined CDR3 loops implicated in P-Ag recognition were uninvolved. Immunoprecipitations demonstrated close cell-surface BTN2A1-BTN3A1 association independent of P-Ag stimulation. Thus, BTN2A1 is a BTN3A1-linked co-factor critical to Vγ9Vδ2 TCR recognition. Furthermore, these results suggest a composite-ligand model of P-Ag sensing wherein the Vγ9Vδ2 TCR directly interacts with both BTN2A1 and an additional ligand recognized in a CDR3-dependent manner.

Bibliometric Analysis of Dendritic Epidermal T Cell (DETC) Research From 1983 to 2019

Dendritic epidermal T cells (DETC) are a group of immune cells expressing canonical γδ TCR in the murine epidermis. Similar to γδ T cells in the human epidermis, DETC serve an important barrier cell in the skin and participate in skin immune surveillance, immune regulation, skin homeostasis, tissue protection, and other activities. Since its discovery in 1983, research on DETC has grown rapidly and unevenly. To evaluate DETC research trends and map the DETC knowledge structure, we have applied bibliometric methods and techniques. A total of 384 DETC-related articles obtained from the Scopus database published between 1983 and 2019 were analyzed using indicators of publication and citation metrics, country and international cooperation, author and co-authorship, and keyword co-occurrence cluster. The present research status, the emerging global trends and the future development direction are also visualized and discussed. In summary, this study provides novel and useful data for the DETC research scientific community, and will help researchers explore DETC more intuitively and effectively.

Alpaca (Vicugna pacos), the first nonprimate species with a phosphoantigen-reactive Vγ9Vδ2 T cell subset

Vγ9Vδ2 T cells are a major γδ T cell population in the human blood expressing a characteristic Vγ9JP rearrangement paired with Vδ2. This cell subset is activated in a TCR-dependent and MHC-unrestricted fashion by so-called phosphoantigens (PAgs). PAgs can be microbial [(E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate, HMBPP] or endogenous (isopentenyl pyrophosphate, IPP) and PAg sensing depends on the expression of B7-like butyrophilin (BTN3A, CD277) molecules. IPP increases in some transformed or aminobisphosphonate-treated cells, rendering those cells a target for Vγ9Vδ2 T cells in immunotherapy. Yet, functional Vγ9Vδ2 T cells have only been described in humans and higher primates. Using a genome-based study, we showed in silico translatable genes encoding Vγ9, Vδ2, and BTN3 in a few nonprimate mammalian species. Here, with the help of new monoclonal antibodies, we directly identified a T cell population in the alpaca (Vicugna pacos), which responds to PAgs in a BTN3-dependent fashion and shows typical TRGV9- and TRDV2-like rearrangements. T cell receptor (TCR) transductants and BTN3-deficient human 293T cells reconstituted with alpaca or human BTN3 or alpaca/human BTN3 chimeras showed that alpaca Vγ9Vδ2 TCRs recognize PAg in the context of human and alpaca BTN3. Furthermore, alpaca BTN3 mediates PAg recognition much better than human BTN3A1 alone and this improved functionality mapped to the transmembrane/cytoplasmic part of alpaca BTN3. In summary, we found remarkable similarities but also instructive differences of PAg-recognition by human and alpaca, which help in better understanding the molecular mechanisms controlling the activation of this prominent population of γδ T cells.

Single-cell TCR sequencing of gut intraepithelial γδ T cells reveals a vast and diverse repertoire in celiac disease

A hallmark of celiac disease (CeD), a chronic condition driven by cereal gluten exposure, is increase of gut intraepithelial γδ T cells. This may indicate pathogenic involvement of γδ T cells and existence of disease-specific γδ T-cell receptors (TCRs) recognizing defined antigen(s). We performed high-throughput and paired γδ TCR sequencing of single intraepithelial γδ T cells of untreated CeD patients (n = 8; 1821 cells), CeD patients treated with a gluten-free diet (n = 5; 436 cells) and controls (n = 7; 1068 cells). We found that CeD patients, both untreated and treated, had larger and more diverse γδ TCR repertoires, more frequent usage of TRDV1 and TRDV3 and different patterns of TCRγ/TCRδ-pairing compared with controls. Although we observed no public CDR3δ sequences, there were several public CDR3γ sequences-many of which were shared by not only the CeD patients, but also by the controls. These public CDR3s were characterized by few N/P nucleotide insertions with germline and near-germline configuration, hence being easy to generate. Previous findings of CeD-specific CDR3 motifs were not replicated. Thus, being unable to raise evidence for CeD-specific γδ TCRs in this first large, paired γδ TCR single-cell sequencing study, we project challenges for identification of CeD-relevant γδ TCR ligands.

Development and Function of γδT Cells in the Oral Mucosa

To successfully withstand a wide variety of microbial and mechanical challenges, the immune system of the oral mucosa is composed of tissue-resident and specially recruited leukocytes. These leukocytes facilitate the establishment and maintenance of local homeostasis but are also capable to cause oral pathologies when are unrestrained. γδT cells represent an important tissue-resident innate T-cell population in various mucosal and nonmucosal barrier tissues, in which they are ideally located to assist in immunosurveillance, tissue repair, and homeostasis. Whereas most works studying γδT cells were focused on tissues such as the skin and intestine, these cells in the oral mucosa were only recently thoroughly studied. The findings obtained by those studies appear to be both complementary and contradicting, likely reflecting differences in the experimental settings and the type of transgenic mouse modalities employed by each study. Nevertheless, oral γδT cells were shown to consist of developmentally distinct tissue-resident Vγ6 cells and circulating Vγ1 and Vγ4 subsets that are independently maintained in the oral mucosa. In the gingiva, a particularly challenging barrier tissue due to its proximity to the dental plaque, γδT cells are strategically positioned close to the plaque and represent the major source of IL-17. While this suggests that γδT cells might be involved in controlling the dental biofilm, conflicting data were reported in this regard. In vivo studies have shown that γδT cells either play a protective role during age-associated bone loss or, alternatively, have no impact in this process. Also, recent reports suggested opposing data concerning the impact of γδT cells in experimental periodontitis based on the ligature model. This review summarizes and discusses the most up-to-date literature on oral γδT cells, providing a balanced perspective regarding our current understanding on the development of oral γδT cells and their role under physiologic conditions and certain oral pathologies.

Resolving the mystery-How TCR transgenic mouse models shed light on the elusive case of gamma delta T cells

Cutting-edge questions in αβ T cell biology were addressed by investigating a range of different genetically modified mouse models. In comparison, the γδ T cell field lacks behind on the availability of such models. Nevertheless, transgenic mouse models proved useful for the investigation of γδ T cell biology and their stepwise development in the thymus. In general, animal models and especially mouse models give access to a wide range of opportunities of modulating γδ T cells, which is unachievable in human beings. Because of their complex biology and specific tissue tropism, it is especially challenging to investigate γδ T cells in in vitro experiments since they might not reliably reflect their behavior and phenotype under physiologic conditions. This review aims to provide a comprehensive historical overview about how different transgenic mouse models contributed in regards of the understanding of γδ T cell biology, whereby a special focus is set on studies including the elusive role of the γδTCR. Furthermore, evolutionary and translational remarks are discussed under the aspect of future implications for the field. The ultimate full understanding of γδ T cells will pave the way for their usage as a powerful new tool in immunotherapy.

γδ T Cell Update: Adaptate Orchestrators of Immune Surveillance

As interest in γδ T cells grows rapidly, what key points are emerging, and where is caution warranted? γδ T cells fulfill critical functions, as reflected in associations with vaccine responsiveness and cancer survival in humans and ever more phenotypes of γδ T cell-deficient mice, including basic physiological deficiencies. Such phenotypes reflect activities of distinct γδ T cell subsets, whose origins offer interesting insights into lymphocyte development but whose variable evolutionary conservation can obfuscate translation of knowledge from mice to humans. By contrast, an emerging and conserved feature of γδ T cells is their "adaptate" biology: an integration of adaptive clonally-restricted specificities, innate tissue-sensing, and unconventional recall responses that collectively strengthen host resistance to myriad challenges. Central to adaptate biology are butyrophilins and other γδ cell regulators, the study of which should greatly enhance our understanding of tissue immunogenicity and immunosurveillance and guide intensifying clinical interest in γδ cells and other unconventional lymphocytes.

Regulation of γδ T Cell Effector Diversification in the Thymus

γδ T cells are the first T cell lineage to develop in the thymus and take up residence in a wide variety of tissues where they can provide fast, innate-like sources of effector cytokines for barrier defense. In contrast to conventional αβ T cells that egress the thymus as naïve cells, γδ T cells can be programmed for effector function during development in the thymus. Understanding the molecular mechanisms that determine γδ T cell effector fate is of great interest due to the wide-spread tissue distribution of γδ T cells and their roles in pathogen clearance, immunosurveillance, cancer, and autoimmune diseases. In this review, we will integrate the current understanding of the role of the T cell receptor, environmental signals, and transcription factor networks in controlling mouse innate-like γδ T cell effector commitment.

Butyrophilin 2A1 is essential for phosphoantigen reactivity by γδ T cells

Gamma delta (γδ) T cells are essential to protective immunity. In humans, most γδ T cells express Vγ9Vδ2⁺ T cell receptors (TCRs) that respond to phosphoantigens (pAgs) produced by cellular pathogens and overexpressed by cancers. However, the molecular targets recognized by these γδTCRs are unknown. Here, we identify butyrophilin 2A1 (BTN2A1) as a key ligand that binds to the Vγ9⁺ TCR γ chain. BTN2A1 associates with another butyrophilin, BTN3A1, and these act together to initiate responses to pAg. Furthermore, binding of a second ligand, possibly BTN3A1, to a separate TCR domain incorporating Vδ2 is also required. This distinctive mode of Ag-dependent T cell activation advances our understanding of diseases involving pAg recognition and creates opportunities for the development of γδ T cell-based immunotherapies.

Characterizing Innate Lymphoid Cell Phenotype and Function in Human Inflammatory Bowel Disease

Innate lymphoid cells (ILCs) are emerging as important effectors of innate immunity and play a critical role in maintaining intestinal immune homeostasis. They are tissue-residing immune cells that can be subdivided based on master transcription factor and cytokine expression, bearing striking resemblance to their CD4⁺ T helper (Th) cell counterparts. ILCs are increasingly recognized as potential mediators of inflammatory bowel disease (IBD) providing a need to explore their functional and phenotypic differences in health vs. disease. In this chapter we outline protocols for the characterization of human ILCs and intracellular cytokine expression using flow cytometry. We include protocols for isolating human peripheral blood and colonic lamina propria mononuclear cells essential for evaluating human IBD specimens.

The immunology of Plasmodium vivax malaria

Plasmodium vivax infection, the predominant cause of malaria in Asia and Latin America, affects ~14 million individuals annually, with considerable adverse effects on wellbeing and socioeconomic development. A clinical hallmark of Plasmodium infection, the paroxysm, is driven by pyrogenic cytokines produced during the immune response. Here, we review studies on the role of specific immune cell types, cognate innate immune receptors, and inflammatory cytokines on parasite control and disease symptoms. This review also summarizes studies on recurrent infections in individuals living in endemic regions as well as asymptomatic infections, a serious barrier to eliminating this disease. We propose potential mechanisms behind these repeated and subclinical infections, such as poor induction of immunological memory cells and inefficient T effector cells. We address the role of antibody-mediated resistance to P. vivax infection and discuss current progress in vaccine development. Finally, we review immunoregulatory mechanisms, such as inhibitory receptors, T regulatory cells, and the anti-inflammatory cytokine, IL-10, that antagonizes both innate and acquired immune responses, interfering with the development of protective immunity and parasite clearance. These studies provide new insights for the clinical management of symptomatic as well as asymptomatic individuals and the development of an efficacious vaccine for vivax malaria.

Butyrophilin-like 3 Directly Binds a Human Vγ4+ T Cell Receptor Using a Modality Distinct from Clonally-Restricted Antigen

Butyrophilin (BTN) and butyrophilin-like (BTNL/Btnl) heteromers are major regulators of human and mouse γδ T cell subsets, but considerable contention surrounds whether they represent direct γδ T cell receptor (TCR) ligands. We demonstrate that the BTNL3 IgV domain binds directly and specifically to a human Vγ4⁺ TCR, “LES” with an affinity (∼15–25 μM) comparable to many αβ TCR-peptide major histocompatibility complex interactions. Mutations in germline-encoded Vγ4 CDR2 and HV4 loops, but not in somatically recombined CDR3 loops, drastically diminished binding and T cell responsiveness to BTNL3-BTNL8-expressing cells. Conversely, CDR3γ and CDR3δ loops mediated LES TCR binding to endothelial protein C receptor, a clonally restricted autoantigen, with minimal CDR1, CDR2, or HV4 contributions. Thus, the γδ TCR can employ two discrete binding modalities: a non-clonotypic, superantigen-like interaction mediating subset-specific regulation by BTNL/BTN molecules and CDR3-dependent, antibody-like interactions mediating adaptive γδ T cell biology. How these findings might broadly apply to γδ T cell regulation is also examined.

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BTNL3 binds directly and specifically to Vγ4⁺ TCRs via its IgV domain

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The superantigen-like binding mode focuses on germline-encoded TCR regions

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In contrast, γδ TCR binding to a clonally restricted antigen is CDR3-mediated

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Mutagenesis indicates parallels with BTN3A1-mediated activation of Vγ9Vδ2 T cells

An innate-like Vδ1+ γδ T cell compartment in the human breast is associated with remission in triple-negative breast cancer

Innate-like tissue-resident γδ T cell compartments capable of protecting against carcinogenesis are well established in mice. Conversely, the degree to which they exist in humans, their potential properties, and their contributions to host benefit are mostly unresolved. Here, we demonstrate that healthy human breast harbors a distinct γδ T cell compartment, primarily expressing T cell receptor (TCR) Vδ1 chains, by comparison to Vδ2 chains that predominate in peripheral blood. Breast-resident Vδ1+ cells were functionally skewed toward cytolysis and IFN-γ production, but not IL-17, which has been linked with inflammatory pathologies. Breast-resident Vδ1+ cells could be activated innately via the NKG2D receptor, whereas neighboring CD8+ αβ T cells required TCR signaling. A comparable population of Vδ1+ cells was found in human breast tumors, and when paired tumor and nonmalignant samples from 11 patients with triple-negative breast cancer were analyzed, progression-free and overall survival correlated with Vδ1+ cell representation, but not with either total γδ T cells or Vδ2+ T cells. As expected, progression-free survival also correlated with αβ TCRs. However, whereas in most cases TCRαβ repertoires focused, typical of antigen-specific responses, this was not observed for Vδ1+ cells, consistent with their innate-like responsiveness. Thus, maximal patient benefit may accrue from the collaboration of innate-like responses mounted by tissue-resident Vδ1+ compartments and adaptive responses mounted by αβ T cells.

Novel molecules mediate specialized functions of human regulatory macrophages

PURPOSE OF REVIEW

Now that adoptive transfer of regulatory macrophages (Mregs) is clinically practicable, we ask whether this approach could be used to achieve self-sustaining peripheral regulation and what mechanisms may be involved.

RECENT FINDINGS

Dehydrogenase/reductase 9 (DHRS9)-expressing Mregs are a specialized subset of monocyte-derived macrophages that are currently being investigated as a tolerogenic cell-based therapy. Human Mregs are defined by their capacity to convert naïve CD4 T cells to IL-10-secreting FoxP3 regulatory T cells (Tregs) through an activation-dependent process involving signals mediated by TGF-β, retinoic acid, indoleamine 2,3-dioxygenase activity, notch and progestagen associated endometrial protein (PAEP). Mreg-induced iTregs (miTregs) are a phenotypically distinct type of in-vitro-derived human iTreg that expresses butyrophilin-like protein 8 (BTNL8) and T cell immunoreceptor with Ig and ITIM domains (TIGIT). miTregs are nonspecifically suppressive of mitogen-stimulated bystander T cell proliferation and inhibit TNFα-induced maturation of monocyte-derived dendritic cells. Preclinical and clinical studies find that intravenous infusion of allogeneic Mregs leads to enrichment of circulating TIGIT Tregs.

SUMMARY

These results suggest a feed-forward mechanism by which Mreg treatment could promote solid organ transplant acceptance through rapid induction of direct pathway Tregs.

Human γδ T-cell receptor repertoire is shaped by influenza viruses, age and tissue compartmentalisation

Background

Although γδ T cells comprise up to 10% of human peripheral blood T cells, questions remain regarding their role in disease states and T‐cell receptor (TCR) clonal expansions. We dissected anti‐viral functions of human γδ T cells towards influenza viruses and defined influenza‐reactive γδ TCRs in the context of γδ‐TCRs across the human lifespan.

Methods

We performed ⁵¹Cr‐killing assay and single‐cell time‐lapse live video microscopy to define mechanisms underlying γδ T‐cell‐mediated killing of influenza‐infected targets. We assessed cytotoxic profiles of γδ T cells in influenza‐infected patients and IFN‐γ production towards influenza‐infected lung epithelial cells. Using single‐cell RT‐PCR, we characterised paired TCRγδ clonotypes for influenza‐reactive γδ T cells in comparison with TCRs from healthy neonates, adults, elderly donors and tissues.

Results

We provide the first visual evidence of γδ T‐cell‐mediated killing of influenza‐infected targets and show distinct features to those reported for CD8⁺ T cells. γδ T cells displayed poly‐cytotoxic profiles in influenza‐infected patients and produced IFN‐γ towards influenza‐infected cells. These IFN‐γ‐producing γδ T cells were skewed towards the γ9δ2 TCRs, particularly expressing the public GV9‐TCRγ, capable of pairing with numerous TCR‐δ chains, suggesting their significant role in γδ T‐cell immunity. Neonatal γδ T cells displayed extensive non‐overlapping TCRγδ repertoires, while adults had enriched γ9δ2‐pairings with diverse CDR3γδ regions. Conversely, the elderly showed distinct γδ‐pairings characterised by large clonal expansions, a profile also prominent in adult tissues.

Conclusion

Human TCRγδ repertoire is shaped by age, tissue compartmentalisation and the individual's history of infection, suggesting that these somewhat enigmatic γδ T cells indeed respond to antigen challenge.

Translating gammadelta (γδ) T cells and their receptors into cancer cell therapies

Clinical responses to checkpoint inhibitors used for cancer immunotherapy seemingly require the presence of αβT cells that recognize tumour neoantigens, and are therefore primarily restricted to tumours with high mutational load. Approaches that could address this limitation by engineering αβT cells, such as chimeric antigen receptor T (CAR T) cells, are being investigated intensively, but these approaches have other issues, such as a scarcity of appropriate targets for CAR T cells in solid tumours. Consequently, there is renewed interest among translational researchers and commercial partners in the therapeutic use of γδT cells and their receptors. Overall, γδT cells display potent cytotoxicity, which usually does not depend on tumour-associated (neo)antigens, towards a large array of haematological and solid tumours, while preserving normal tissues. However, the precise mechanisms of tumour-specific γδT cells, as well as the mechanisms for self-recognition, remain poorly understood. In this Review, we discuss the challenges and opportunities for the clinical implementation of cancer immunotherapies based on γδT cells and their receptors.

Diverse developmental pathways of intestinal intraepithelial lymphocytes

The intestinal epithelial barrier is patrolled by resident intraepithelial lymphocytes (IELs) that are involved in host defence against pathogens, wound repair and homeostatic interactions with the epithelium, microbiota and nutrients. Intestinal IELs are one of the largest populations of lymphocytes in the body and comprise several distinct subsets, the identity and lineage relationships of which have long remained elusive. Here, we review advances in unravelling the complexity of intestinal IEL populations, which comprise conventional αβ T cell receptor (TCRαβ)⁺ subsets, unconventional TCRαβ⁺ and TCRγδ⁺ subsets, group 1 innate lymphoid cells (ILC1s) and ILC1-like cells. Although these intestinal IEL lineages have partially overlapping effector programmes and recognition properties, they have strikingly different developmental pathways. We suggest that evolutionary pressure has driven the recurrent generation of cytolytic effector lymphocytes to protect the intestinal epithelial layer, but they may also precipitate intestinal inflammatory disorders, such as coeliac disease.

Mice lacking all of the Skint family genes

γδT cells develop in the thymus and play important roles in protection against infection and tumor development, but the mechanisms by which the thymic microenvironment supports γδT cell differentiation remain largely unclear. Skint1, a B7-related protein expressed in thymic epithelial cells, was shown to be essential for the development of mouse Vγ5Vδ1 γδT cells. The Skint family in mouse consists of 11 members, Skint1-11. Here we generated mutant mice lacking the entire genomic region that contains all of the Skint genes. These mice exhibited a marked reduction of Vγ5Vδ1 γδT cells in the thymus and skin, but surprisingly, had normal development of other γδT cell subsets and leukocytes including αβT, B and myeloid cells. This phenotype is essentially identical to that of Skint1-deficient mice. These results indicate that the Skint family exerts an exclusive function in regulating the development of Vγ5Vδ1 γδT cells and is dispensable for development of other leukocytes.

Analysis of macaque BTN3A genes and transcripts in the extended MHC: conserved orthologs of human γδ T cell modulators

Butyrophilins (BTN), specifically BTN3A, play a central role in the modulation of γδ T cells, which are mainly present in gut and mucosal tissues. BTN3A1 is known, for example, to activate Vγ9Vδ2 T cells by means of a phosphoantigen interaction. In the extended HLA region, three genes are located, designated BTN3A1, BTN3A2 and BTN3A3, which were also defined in rhesus macaques. In contrast to humans, rhesus monkeys have an additional gene, BTN3A3Like, which has the features of a pseudogene. cDNA analysis of 32 Indian rhesus and 16 cynomolgus macaques originating from multiple-generation families revealed that all three genes are oligomorphic, and the deduced amino acids display limited variation. The macaque BTN3A alleles segregated together with MHC alleles, proving their location in the extended (Major Histocompatibility Complex) MHC. BTN3A nearly full-length transcripts of macaques and humans cluster tightly together in the phylogenetic tree, suggesting that the genes represent true orthologs of each other. Despite the limited level of polymorphism, 15 Mamu- and 14 Mafa-BTN3A haplotypes were defined, and, as in humans, all three BTN3A genes are transcribed in PBMCs and colon tissues. In addition to regular full-length transcripts, a high number of various alternative splicing (AS) products were observed for all BTN3A alleles, which may result in different isoforms. The comparable function of certain subsets of γδ T cells in human and non-human primates in concert with high levels of sequence conservation observed for the BTN3A transcripts presents the opportunity to study these not yet well understood molecules in macaques as a model species.

Sonic Hedgehog Is a Determinant of γδ T-Cell Differentiation in the Thymus

Here we investigate the function of Hedgehog (Hh) signaling in thymic γδ T-cell maturation and subset differentiation. Analysis of Hh mutants showed that Hh signaling promotes γδ T-cell development in the thymus and influences γδ T-cell effector subset distribution. Hh-mediated transcription in thymic γδ cells increased γδ T-cell number, and promoted their maturation and increased the γδNKT subset, whereas inhibition of Hh-mediated transcription reduced the thymic γδ T-cell population and increased expression of many genes that are normally down-regulated during γδ T-cell maturation. These changes were also evident in spleen, where increased Hh signaling increased γδNKT cells, but reduced CD27-CD44+ and Vγ2+ populations. Systemic in vivo pharmacological Smoothened-inhibition reduced γδ T-cell and γδNKT cells in the thymus, and also reduced splenic γδ T-cell and γδNKT populations, indicating that Hh signaling also influences homeostasis of peripheral γδ T-cell populations. Taken together our data indicate that Sonic Hedgehog is an important determinant of γδ T-cell effector subset differentiation.

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.

γδ T cells: pleiotropic immune effectors with therapeutic potential in cancer

The potential of cancer immunotherapy relies on the mobilization of immune cells capable of producing antitumour cytokines and effectively killing tumour cells. These are major attributes of γδ T cells, a lymphoid lineage that is often underestimated despite its major role in tumour immune surveillance, which has been established in a variety of preclinical cancer models. This situation notwithstanding, in particular instances the tumour microenvironment seemingly mobilizes γδ T cells with immunosuppressive or tumour-promoting functions, thus emphasizing the importance of regulating γδ T cell responses in order to realize their translation into effective cancer immunotherapies. In this Review we outline both seminal work and recent advances in our understanding of how γδ T cells participate in tumour immunity and how their functions are regulated in experimental models of cancer. We also discuss the current strategies aimed at maximizing the therapeutic potential of human γδ T cells, on the eve of their exploration in cancer clinical trials that may position them as key players in cancer immunotherapy.