gammadelta T cell subsets: a link between TCR and function?

SLAM/SAP signaling regulates discrete γδ T cell developmental checkpoints and shapes the innate-like γδ TCR repertoire

During thymic development, most γδ T cells acquire innate-like characteristics that are critical for their function in tumor surveillance, infectious disease, and tissue repair. The mechanisms, however, that regulate γδ T cell developmental programming remain unclear. Recently, we demonstrated that the SLAM-SAP signaling pathway regulates the development and function of multiple innate-like γδ T cell subsets. Here, we used a single-cell proteogenomics approach to identify SAP-dependent developmental checkpoints and to define the SAP-dependent γδ TCR repertoire. SAP deficiency resulted in both a significant loss of an immature Gzma + Blk + Etv5 + Tox2 + γδT17 precursor population, and a significant increase in Cd4 + Cd8 + Rorc + Ptcra + Rag1 + thymic γδ T cells. SAP-dependent diversion of embryonic day 17 thymic γδ T cell clonotypes into the αβ T cell developmental pathway was associated with a decreased frequency of mature clonotypes in neonatal thymus, and an altered γδ TCR repertoire in the periphery. Finally, we identify TRGV4/TRAV13-4(DV7)-expressing T cells as a novel, SAP-dependent Vγ4 γδT1 subset. Together, the data suggest that SAP-dependent γδ/αβ T cell lineage commitment regulates γδ T cell developmental programming and shapes the γδ TCR repertoire.

The roles of immune cells in Behçet's disease

Behçet's disease (BD) is a systemic vasculitis that can affect multiple systems, including the skin, mucous membranes, joints, eyes, gastrointestinal and nervous. However, the pathogenesis of BD remains unclear, and it is believed that immune-inflammatory reactions play a crucial role in its development. Immune cells are a critical component of this process and contribute to the onset and progression of BD. By regulating the function of these immune cells, effective control over the occurrence and development of BD can be achieved, particularly with regards to monocyte activation and aggregation, macrophage differentiation and polarization, as well as T cell subset differentiation. This review provides a brief overview of immune cells and their role in regulating BD progression, which may serve as a theoretical foundation for preventing and treating this disease.

Distinct subpopulations of DN1 thymocytes exhibit preferential γδ T lineage potential

The αβ and γδ T cell lineages both differentiate in the thymus from common uncommitted progenitors. The earliest stage of T cell development is known as CD4-CD8- double negative 1 (DN1), which has previously been shown to be a heterogenous mixture of cells. Of these, only the CD117+ fraction has been proposed to be true T cell progenitors that progress to the DN2 and DN3 thymocyte stages, at which point the development of the αβ and γδ T cell lineages diverge. However, recently, it has been shown that at least some γδ T cells may be derived from a subset of CD117- DN thymocytes. Along with other ambiguities, this suggests that T cell development may not be as straightforward as previously thought. To better understand early T cell development, particularly the heterogeneity of DN1 thymocytes, we performed a single cell RNA sequence (scRNAseq) of mouse DN and γδ thymocytes and show that the various DN stages indeed comprise a transcriptionally diverse subpopulations of cells. We also show that multiple subpopulations of DN1 thymocytes exhibit preferential development towards the γδ lineage. Furthermore, specific γδ-primed DN1 subpopulations preferentially develop into IL-17 or IFNγ-producing γδ T cells. We show that DN1 subpopulations that only give rise to IL-17-producing γδ T cells already express many of the transcription factors associated with type 17 immune cell responses, while the DN1 subpopulations that can give rise to IFNγ-producing γδ T cell already express transcription factors associated with type 1 immune cell responses.

Development of γδ T Cells: Soldiers on the Front Lines of Immune Battles

While the functions of αβ T cells in host resistance to pathogen infection are understood in far more detail than those of γδ lineage T cells, γδ T cells perform critical, essential functions during immune responses that cannot be compensated for by αβ T cells. Accordingly, it is critical to understand how the development of γδ T cells is controlled so that their generation and function might be manipulated in future for therapeutic benefit. This introductory chapter will focus primarily on the basic processes that underlie γδ T cell development in the thymus, as well as the current understanding of how they are controlled.

Glucose metabolism controls human γδ T-cell-mediated tumor immunosurveillance in diabetes

Patients with type 2 diabetes mellitus (T2DM) have an increased risk of cancer. The effect of glucose metabolism on γδ T cells and their impact on tumor surveillance remain unknown. Here, we showed that high glucose induced Warburg effect type of bioenergetic profile in Vγ9Vδ2 T cells, leading to excessive lactate accumulation, which further inhibited lytic granule secretion by impairing the trafficking of cytolytic machinery to the Vγ9Vδ2 T-cell-tumor synapse by suppressing AMPK activation and resulted in the loss of antitumor activity in vitro, in vivo and in patients. Strikingly, activating the AMPK pathway through glucose control or metformin treatment reversed the metabolic abnormalities and restored the antitumor activity of Vγ9Vδ2 T cells. These results suggest that the impaired antitumor activity of Vγ9Vδ2 T cells induced by dysregulated glucose metabolism may contribute to the increased cancer risk in T2DM patients and that metabolic reprogramming by targeting the AMPK pathway with metformin may improve tumor immunosurveillance.

γδT17/γδTreg cell subsets: a new paradigm for asthma treatment

Bronchial asthma (abbreviated as asthma), is a heterogeneous disease characterized by chronic airway inflammation and airway hyperresponsiveness. The main characteristics of asthma include variable reversible airflow limitation and airway remodeling. The pathogenesis of asthma is still unclear. Th1/Th2 imbalance, Th1 deficiency and Th2 hyperfunction are classic pathophysiological mechanisms of asthma. Some studies have shown that the imbalance of the Th1/Th2 cellular immune model and Th17/Treg imbalance play a key role in the occurrence and development of asthma; however, these imbalances do not fully explain the disease. In recent years, studies have shown that γδT and γδT17 cells are involved in the pathogenesis of asthma. γδTreg has a potential immunosuppressive function, but its regulatory mechanisms have not been fully elucidated. In this paper, we reviewed the role of γδT17/γδTreg cells in bronchial asthma, including the mechanisms of their development and activation. Here we propose that γδT17/Treg cell subsets contribute to the occurrence and development of asthma, constituting a novel potential target for asthma treatment.

Characterization of the chicken T cell receptor γ repertoire by high-throughput sequencing

BACKGROUND

As one of "γδ-high" species, chicken is an excellent model for the study of γδ T cells in non-mammalian animals. However, a comprehensive characterization of the TCRγδ repertoire is still missing in chicken. The objective of this study was to characterize the expressed TCRγ repertoire in chicken thymus using high-throughput sequencing.

METHODS

In this study, we first obtained the detailed genomic organization of the TCRγ locus of chicken based on the latest assembly of the red jungle fowl genome sequences (GRCg6a) and then characterized the TCRγ repertoire in the thymus of four chickens by using 5' Rapid Amplification of cDNA Ends (5' RACE) along with high-throughput sequencing (HTS).

RESULTS

The chicken TCRγ locus contains a single Cγ gene, three functional Jγ segments and 44 Vγ segments that could be classified into six subgroups, each containing six, nineteen, nine, four, three and three members. Dot-plot analysis of the chicken TCRγ locus against itself showed that almost all the entire zone containing Vγ segments had arisen through tandem duplication events, and the main homology unit, containing 9 or 10 Vγ gene segments, has tandemly duplicated for four times. For the analysis of chicken TCRγ repertoire, more than 100,000 unique Vγ-region nucleotide sequences were obtained from the thymus of each chicken. After alignment to the germline Vγ and Jγ segments identified above, we found that the four chickens had similar repertoire profile of TCRγ. In brief, four Vγ segments (including Vγ3.7, Vγ2.13, Vγ1.6 and Vγ1.3) and six Vγ-Jγ pairs (including Vγ3.7-Jγ3, Vγ2.13-Jγ1, Vγ2.13-Jγ3, Vγ1.6-Jγ3, Vγ3.7-Jγ1 and Vγ1.6-Jγ1) were preferentially utilized by all four individuals, and vast majority of the unique CDR3γ sequences encoded 4 to 22 amino acids with mean 12.90 amino acids, which exhibits a wider length distribution and/or a longer mean length than CDR3γ of human, mice and other animal species.

CONCLUSIONS

In this study, we present the first in-depth characterization of the TCRγ repertoire in chicken thymus. We believe that these data will facilitate the studies of adaptive immunology in birds.

Preconceptional Immunization Can Modulate Offspring Intrathymic IL-17-Producing γδT Cells with Epigenetic Implications Mediated by microRNAs

The mechanisms through which maternal immunization can modulate offspring thymic maturation of lymphocytes are not fully understood. Here, we aimed to evaluate whether maternal OVA-immunization can inhibit the maturation of IL-17-producing γδT cells in offspring thymus, and if this mechanism has epigenetic implications mediated by microRNAs (miRNAs) expression. Wild-type (WT) C57BL/6 females were immunized with OVA in Alum or Alum alone and were mated with normal WT males. Evaluating their offspring thymus at 3 or 20 days old (d.o.), we observed that maternal OVA immunization could inhibit the thymic frequency of offspring CD27- and IL-17+ γδT cells at the neonatal and until 20 days old. Furthermore, we evaluated the expression of function-related γ and δ variable γδTCR chains (Vγ1, Vγ2, Vγ3, Vδ4, and Vδ6.3), observing that maternal OVA-immunization inhibits Vγ2 chains expression. The small RNAs (sRNAs), particularly miRNAs, and messenger RNAs (mRNA) expression profiles by pools of thymus tissue samples (from 9 to 11 mice) from offspring OVA-immunized or Alum-immunized mothers were analyzed via Illumina sequencing platform and bioinformatics approaches. Using a fold change >4, our results showed that seven miRNAs (mmu-miR-126a-3p, 101a-3p, 744-3p,142-5p, 15a-5p, 532-5p, and 98-5p) were differentially expressed between both groups. Ten target genes were predicted to interact with the seven selected miRNAs. There were no enriched categories of gene ontology functional annotation and pathway enrichment analysis for the target genes. Interestingly, four of the identified miRNAs (mmu-miR-15a, mmu-miR-101 mmu-miR-126, and mmu-miR-142) are related to IL-17 production. Our data is of significance because we demonstrate that maternal immunization can modulate offspring thymic maturation of IL-17-producing γδT cells possibly by an epigenetic mechanism mediated by miRNAs.

Differentiation and functional plasticity of gamma-delta (γδ) T cells under homeostatic and disease conditions

Gamma-delta (γδ) T cells are a heterogeneous population of immune cells, which constitute <5% of total T cells in mice lymphoid tissue and human peripheral blood. However, they comprise a higher proportion of T cells in the epithelial and mucosal barrier, where they perform immune functions, help in tissue repair, and maintaining homeostasis. These tissues resident γδ T cells possess properties of innate and adaptive immune cells which enables them to perform a variety of functions during homeostasis and disease. Emerging data suggest the involvement of γδ T cells during transplant rejection and survival. Interestingly, several functions of γδ T cells can be modulated through their interaction with other immune cells. This review provides an overview of development, differentiation plasticity into regulatory and effector phenotypes of γδ T cells during homeostasis and various diseases.

Deciphering the regulatory landscape of fetal and adult γδ T-cell development at single-cell resolution

γδ T cells with distinct properties develop in the embryonic and adult thymus and have been identified as critical players in a broad range of infections, antitumor surveillance, autoimmune diseases, and tissue homeostasis. Despite their potential value for immunotherapy, differentiation of γδ T cells in the thymus is incompletely understood. Here, we establish a high-resolution map of γδ T-cell differentiation from the fetal and adult thymus using single-cell RNA sequencing. We reveal novel sub-types of immature and mature γδ T cells and identify an unpolarized thymic population which is expanded in the blood and lymph nodes. Our detailed comparative analysis reveals remarkable similarities between the gene networks active during fetal and adult γδ T-cell differentiation. By performing a combined single-cell analysis of Sox13, Maf, and Rorc knockout mice, we demonstrate sequential activation of these factors during IL-17-producing γδ T-cell (γδT17) differentiation. These findings substantially expand our understanding of γδ T-cell ontogeny in fetal and adult life. Our experimental and computational strategy provides a blueprint for comparing immune cell differentiation across developmental stages.

Heterogeneity of Human γδ T Cells and Their Role in Cancer Immunity

The γδ T cells are unconventional lymphocytes that function in both innate and adaptive immune responses against various intracellular and infectious stresses. The γδ T cells can be exploited as cancer-killing effector cells since γδ TCRs recognize MHC-like molecules and growth factor receptors that are upregulated in cancer cells, and γδ T cells can differentiate into cytotoxic effector cells. However, γδ T cells may also promote tumor progression by secreting IL-17 or other cytokines. Therefore, it is essential to understand how the differentiation and homeostasis of γδ T cells are regulated and whether distinct γδ T cell subsets have different functions. Human γδ T cells are classified into Vδ2 and non-Vδ2 γδ T cells. The majority of Vδ2 γδ T cells are Vγ9δ2 T cells that recognize pyrophosphorylated isoprenoids generated by the dysregulated mevalonate pathway. In contrast, Vδ1 T cells expand from initially diverse TCR repertoire in patients with infectious diseases and cancers. The ligands of Vδ1 T cells are diverse and include the growth factor receptors such as endothelial protein C receptor. Both Vδ1 and Vδ2 γδ T cells are implicated to have immunotherapeutic potentials for cancers, but the detailed elucidation of the distinct characteristics of 2 populations will be required to enhance the immunotherapeutic potential of γδ T cells. Here, we summarize recent progress regarding cancer immunology of human γδ T cells, including their development, heterogeneity, and plasticity, the putative mechanisms underlying ligand recognition and activation, and their dual effects on tumor progression in the tumor microenvironment.

γδ T cells in rheumatic diseases: from fundamental mechanisms to autoimmunity

The innate and adaptive arms of the immune system tightly regulate immune responses in order to maintain homeostasis and host defense. The interaction between those two systems is critical in the activation and suppression of immune responses which if unchecked may lead to chronic inflammation and autoimmunity. γδ T cells are non-conventional lymphocytes, which express T cell receptor (TCR) γδ chains on their surface and straddle between innate and adaptive immunity. Recent advances in of γδ T cell biology have allowed us to expand our understanding of γδ T cell in the dysregulation of immune responses and the development of autoimmune diseases. In this review, we summarize current knowledge on γδ T cells and their roles in skin and joint inflammation as commonly observed in rheumatic diseases.

Innate, innate-like and adaptive lymphocytes in the pathogenesis of MS and EAE

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) in which the immune system damages the protective insulation surrounding the nerve fibers that project from neurons. A hallmark of MS and its animal model, experimental autoimmune encephalomyelitis (EAE), is autoimmunity against proteins of the myelin sheath. Most studies in this field have focused on the roles of CD4⁺ T lymphocytes, which form part of the adaptive immune system as both mediators and regulators in disease pathogenesis. Consequently, the treatments for MS often target the inflammatory CD4⁺ T-cell responses. However, many other lymphocyte subsets contribute to the pathophysiology of MS and EAE, and these subsets include CD8⁺ T cells and B cells of the adaptive immune system, lymphocytes of the innate immune system such as natural killer cells, and subsets of innate-like T and B lymphocytes such as γδ T cells, natural killer T cells, and mucosal-associated invariant T cells. Several of these lymphocyte subsets can act as mediators of CNS inflammation, whereas others exhibit immunoregulatory functions in disease. Importantly, the efficacy of some MS treatments might be mediated in part by effects on lymphocytes other than CD4⁺ T cells. Here we review the contributions of distinct subsets of lymphocytes on the pathogenesis of MS and EAE, with an emphasis on lymphocytes other than CD4⁺ T cells. A better understanding of the distinct lymphocyte subsets that contribute to the pathophysiology of MS and its experimental models will inform the development of novel therapeutic approaches.

T-cell frequencies of CD8+ γδ and CD27+ γδ cells in the stem cell graft predict the outcome after allogeneic hematopoietic cell transplantation

The impact of intra-graft T cells on the clinical outcome after allogeneic hematopoietic cell transplantation has been investigated. Most previous studies have focused on the role of αβ cells while γδ cells have received less attention. It has been an open question whether γδ cells are beneficial or not for patient outcome, especially with regards to graft versus host disease. In this study, graft composition of γδ cell subsets was analyzed and correlated to clinical outcome in 105 recipients who underwent allogeneic hematopoietic cell transplantation between 2013 and 2016. We demonstrate for the first time that grafts containing higher T-cell proportions of CD8⁺γδ cells were associated with increased cumulative incidence of acute graft versus host disease grade II-III (50% vs 22.6%; P = 0.008). Additionally, graft T-cell frequency of CD27⁺γδ cells was inversely correlated with relapse (P = 0.006) and CMV reactivation (P = 0.05). We conclude that clinical outcome after allogeneic hematopoietic cell transplantation is influenced by the proportions of distinct γδ cell subsets in the stem cell graft. We also provide evidence that CD8⁺γδ cells are potentially alloreactive and may play a role in acute graft versus host disease. This study illustrates the importance of better understanding of the role of distinct subsets of γδ cells in allogeneic hematopoietic cell transplantation.

Tissue Adaptations of Memory and Tissue-Resident Gamma Delta T Cells

Epithelial and mucosal barriers are critical interfaces physically separating the body from the outside environment and are the tissues most exposed to microorganisms and potential inflammatory agents. The integrity of these tissues requires fine tuning of the local immune system to enable the efficient elimination of invasive pathogens while simultaneously preserving a beneficial relationship with commensal organisms and preventing autoimmunity. Although they only represent a small fraction of circulating and lymphoid T cells, γδ T cells form a substantial population at barrier sites and even outnumber conventional αβ T cells in some tissues. After their egress from the thymus, several γδ T cell subsets naturally establish residency in predetermined mucosal and epithelial locations, as exemplified by the restricted location of murine Vγ5⁺ and Vγ3Vδ1⁺ T cell subsets to the intestinal epithelium and epidermis, respectively. Because of their preferential location in barrier sites, γδ T cells are often directly or indirectly influenced by the microbiota or the pathogens that invade these sites. More recently, a growing body of studies have shown that γδ T cells form long-lived memory populations upon local inflammation or bacterial infection, some of which permanently populate the affected tissues after pathogen clearance or resolution of inflammation. Natural and induced resident γδ T cells have been implicated in many beneficial processes such as tissue homeostasis and pathogen control, but their presence may also exacerbate local inflammation under certain circumstances. Further understanding of the biology and role of these unconventional resident T cells in homeostasis and disease may shed light on potentially novel vaccines and therapies.

Interleukin-17-producing γδ T (γδ17) cells in inflammatory diseases

Interleukin‐17 (IL‐17) is a pro‐inflammatory cytokine and is involved in the development of many diseases. Recent studies have revealed that IL‐17‐producing γδ T cells (γδ17 cells) in addition to IL‐17‐producing CD4⁺ T cells [T helper type 17 (Th17) cells] are often the main producers of IL‐17 in mouse models of inflammatory diseases. γδ T cells are functionally committed during intra‐thymic differentiation. γδ thymocytes capable of producing IL‐17, which express the transcription factor retinoic‐acid‐receptor‐related orphan receptor γt and the signature cytokine receptor IL‐23R, leave the thymus, and produce IL‐17 rapidly by the stimulation with IL‐1β and IL‐23 in the periphery. Therefore, γδ17 cells play important roles in the early phase of host defence against pathogens and in inflammatory diseases. γδ T cells that can produce IL‐17 are also increased in the skin of patients with psoriasis and in peripheral blood of patients with ankylosing sclerosis. Indeed, the therapy targeting IL‐17 has been approved or is in clinical trials, and proved to be very efficient to treat psoriasis, psoriatic arthritis and ankylosing sclerosis. In this review, we discuss recent knowledge about the pathophysiological function of γδ17 cells in infection and inflammatory diseases and therapeutic advances targeting IL‐17.

Dermal Vγ4+T cells enhance the IMQ-induced psoriasis-like skin inflammatidon in re-challenged mice

To investigate the role of dermal Vγ₄⁺γδ T cells in psoriasis-like skin inflammation induced by a re-challenge with imiquimod (IMQ), we compared the development of dermatitis induced by topical application of IMQ in primary challenged mice and re-challenged mice. We also compared the development of dermatitis induced by IMQ between re-challenged control mice and Vγ₄^- depleted re-challenged mice that had been initially subjected to IMQ-induced dermatitis 30 prior. We found that the IMQ-induced dermatitis was exacerbated in the re-challenged group compared with the primary challenged group and the Vγ₄^- depleted re-challenged group. In addition, the Vγ₄⁺γδ T cells increased in number and secreted more IL-17A, γ-IFN and IL-22 in the re-challenged control group compared with the primary challenged group. However, in the Vγ₄^- depleted re-challenged group, the Vγ₄^-γδ T cells increased in number and produced more IL-17A and IL-22 compared with re-challenged control mice. These findings suggest that dermal Vγ₄⁺γδ T cells enhance relapsing psoriasis-like skin inflammation induced by IMQ in C57BL/6 mice by secreting IL-17A and γ-IFN.

A Retrospective Analysis of γδ T Cell Expression in Chronic Rhinosinusitis and Its Association with Recurrence of Nasal Polyps

BACKGROUND/AIMS

To examine whether γδ T cell is expressed in the nasal mucosa of chronic rhinosinusitis (CRS) patients and its potential association with recurrence of nasal polyps.

METHODS

Thirty-six patients with CRS with nasal polyps (CRSwNP) and 25 patients with CRS without nasal polyps (CRSsNP) were recruited. Twenty-six patients with other nasal diseases served as controls. The CRSwNP group was divided into the eosinophilic CRSwNP and noneosinophilic CRSwNP groups. The expression of γδ T cells was detected by immunohistochemistry. The expression of each subtype of γδ T cells was detected by using qRT-PCR. All patients underwent nasal endoscopy, and postoperative follow-up lasted over 12 months. CRS patients were evaluated by preoperative VAS scores of symptoms and nasal endoscopy Lund-Kennedy scores.

RESULTS

The expression of γδ T cells in the CRSwNP groups was stronger than in the CRSsNP and the control group (p < 0.05, p < 0.05). The expression of Vγ1+γδ T cells in the eosinophilic CRSwNP group was higher than that in the CRSsNP group and the control group (p < 0.05, p < 0.05). The expression of γδ T cells was associated with high rate of recurrence, tissue eosinophil infiltration, worse symptom score of nasal obstruction, and higher Lund-Kennedy score (all p < 0.05).

CONCLUSIONS

Increased expression of γδ T cells in CRSwNP may be associated with recurrence of nasal polyps.

Choroidal γδ T cells in protection against retinal pigment epithelium and retinal injury

γδ T cells located near the epithelial barrier are integral components of local inflammatory and innate immune responses. We have previously reported the presence of choroidal γδ T cells in a model of chronic degeneration of the retinal pigment epithelium (RPE). The goals of the current study were to further define the functions of choroidal γδ T cells and to explore the underlying mechanisms of their action. Our data demonstrate that choroidal γδ T cells are activated by RPE injury in response to NaIO₃ treatment, and that they express genes that encode immunosuppressive cytokines, such as IL-4 and IL-10. γδ-T-cell-deficient mice developed profound RPE and retinal damage at doses that caused minimal effects in wild-type mice, and adoptive transfer of γδ T cells prevented sensitization. Intravitreal injection of IL-4 and IL-10 ameliorated RPE toxicity that was induced by NaIO₃Ex vivo coculture of γδ T cells with RPE explants activated the production of anti-inflammatory cytokines via an aryl hydrocarbon receptor (AhR)-dependent mechanism. AhR deficiency abolished the protective effects of γδ T cells after adoptive transfer. Collectively, these findings define important roles for choroid γδ T cells in maintaining tissue homeostasis in the outer retina.-Zhao, Z., Liang, Y., Liu, Y., Xu, P., Flamme-Wiese, M. J., Sun, D., Sun, J., Mullins, R. F., Chen, Y., Cai, J. Choroidal γδ T cells in protection against retinal pigment epithelium and retinal injury.

The major diversification of Vγ1.1+ and Vγ2+ thymocytes in mice occurs after commitment to the γδ T-cell lineage

γδ T cells are a heterogeneous cell population with different subsets playing specialized and often opposing roles during immune responses. A key question is whether γδ thymocytes are determined for their effector function already at an early stage, before their commitment to the γδ T-cell lineage, or are instructed during their later development. Here, we show that the adult Vγ1.1⁺ and Vγ2⁺ γδ T-cell subsets both go through a CD73⁺ CD24⁺ development stage, and that the gene regulation involved in lineage commitment is shared by both subsets. We demonstrate that the major subset diversification first occurs after the cells have committed to the γδ T-cell lineage, strongly supporting an instructive model for functional programming of γδ T cells. In conclusion, we show that the two major adult γδ T-cell subsets in mice develop through a shared pathway utilizing similar cellular machinery and that they diverge after the CD24⁺ CD73⁺ maturity stage.

Development of γδ T Cells, the Special-Force Soldiers of the Immune System

While the functions of αβ T cells in host resistance to pathogen infection are understood in far more detail than those of γδ lineage T cells, γδ T cells perform critical, essential functions during immune responses that cannot be compensated by αβ T cells. Accordingly, it is essential to understand how the development of γδ T cells is controlled so that their generation and function might be manipulated in future for therapeutic benefit. This introductory chapter will cover the basic processes that underlie γδ T cell development in the thymus, as well as the current understanding of how they are controlled.

The burgeoning family of unconventional T cells

While most studies of T lymphocytes have focused on T cells reactive to complexes of peptide and major histocompatibility complex (MHC) proteins, many other types of T cells do not fit this paradigm. These include CD1-restricted T cells, MR1-restricted mucosal associated invariant T cells (MAIT cells), MHC class Ib-reactive T cells, and γδ T cells. Collectively, these T cells are considered 'unconventional', in part because they can recognize lipids, small-molecule metabolites and specially modified peptides. Unlike MHC-reactive T cells, these apparently disparate T cell types generally show simplified patterns of T cell antigen receptor (TCR) expression, rapid effector responses and 'public' antigen specificities. Here we review evidence showing that unconventional T cells are an abundant component of the human immune system and discuss the immunotherapeutic potential of these cells and their antigenic targets.

TCR signal strength controls thymic differentiation of discrete proinflammatory γδ T cell subsets

The murine thymus produces discrete γδ T cell subsets making either interferon-γ (IFN--γ) or interleukin 17 (IL-17), but the role of the TCR in this developmental process remains controversial. Here we show that mice haploinsufficient for both Cd3g and Cd3d (CD3DH, for CD3 double haploinsufficient) have reduced TCR expression and signaling strength selectively on γδ T cells. CD3DH mice had normal numbers and phenotype of αβ thymocyte subsets but impaired differentiation of fetal Vγ6⁺ (but not Vγ4⁺) IL-17-producing γδ T cells and a marked depletion of IFN-γ-producing CD122⁺ NK1.1⁺ γδ T cells throughout ontogeny. Adult CD3DH mice showed reduced peripheral IFN-γ⁺ γδ T cells and were resistant to experimental cerebral malaria. Thus, TCR signal strength within specific thymic developmental windows is a major determinant of the generation of proinflammatory γδ T cell subsets and their impact on pathophysiology.

γδ T Cell-Dependent Regulatory T Cells Prevent the Development of Autoimmune Keratitis

To prevent potentially damaging inflammatory responses, the eye actively promotes local immune tolerance via a variety of mechanisms. Owing to trauma, infection, or other ongoing autoimmunity, these mechanisms sometimes fail, and an autoimmune disorder may develop in the eye. In mice of the C57BL/10 (B10) background, autoimmune keratitis often develops spontaneously, particularly in the females. Its incidence is greatly elevated in the absence of γδ T cells, such that ∼80% of female B10.TCRδ(-/-) mice develop keratitis by 18 wk of age. In this article, we show that CD8(+) αβ T cells are the drivers of this disease, because adoptive transfer of CD8(+), but not CD4(+), T cells to keratitis-resistant B10.TCRβ/δ(-/-) hosts induced a high incidence of keratitis. This finding was unexpected because in other autoimmune diseases, more often CD4(+) αβ T cells, or both CD4(+) and CD8(+) αβ T cells, mediate the disease. Compared with wild-type B10 mice, B10.TCRδ(-/-) mice also show increased percentages of peripheral memory phenotype CD8(+) αβ T cells, along with an elevated frequency of CD8(+) αβ T cells biased to produce inflammatory cytokines. In addition, B10.TCRδ-/- mice have fewer peripheral CD4(+) CD25(+) Foxp3(+) αβ regulatory T cells (Tregs), which express lower levels of receptors needed for Treg development and function. Together, these observations suggest that in B10 background mice, γδ T cells are required to generate adequate numbers of CD4(+) CD25(+) Foxp3(+) Tregs, and that in B10.TCRδ(-/-) mice a Treg deficiency allows dysregulated effector or memory CD8(+) αβ T cells to infiltrate the cornea and provoke an autoimmune attack.

γδ T cells in cancer

With the promise of T cell-based therapy for cancer finally becoming reality, this Review focuses on the less-studied γδ T cell lineage and its diverse responses to tumours. γδ T cells have well-established protective roles in cancer, largely on the basis of their potent cytotoxicity and interferon-γ production. Besides this, recent studies have revealed a series of tumour-promoting functions that are linked to interleukin-17-producing γδ T cells. Here, we integrate the current knowledge from both human and mouse studies to highlight the potential of γδ T cell modulation to improve cancer immunotherapy.

PLZF Controls the Development of Fetal-Derived IL-17+Vγ6+ γδ T Cells

Expression of promyelocytic leukemia zinc finger (PLZF) protein directs the effector differentiation of invariant NKT (iNKT) cells and IL-4(+) γδ NKT cells. In this study, we show that PLZF is also required for the development and function of IL-17(+) γδ T cells. We observed that PLZF is expressed in fetal-derived invariant Vγ5(+) and Vγ6(+) γδ T cells, which secrete IFN-γ and IL-17, respectively. PLZF deficiency specifically affected the effector differentiation of Vγ6(+) cells, leading to reduced numbers of mature CD27(-)CD44(+) phenotype capable of secreting IL-17. Although PLZF was not required for Vγ5(+) γδ T cells to develop, when these cells were reprogrammed into IL-17-secreting cells in Skint-1 mutant mice, they required PLZF for their effector maturation, similarly to Vγ6(+) γδ T cells. The impaired effector differentiation of PLZF-deficient Vγ6(+) γδ T cells was not due to increased apoptosis and it was related to reduced proliferation of immature CD27(+)CD44(-) Vγ6(+) γδ T cells, which was required for their differentiation into mature CD27(-)CD44(+) IL-17-secreting cells. Thus, the present study identifies that PLZF function is not restricted to NKT or IL-4(+) T cells, but it also controls the development of IL-17(+) γδ T cells.

IL-1 receptor antagonist-deficient mice develop autoimmune arthritis due to intrinsic activation of IL-17-producing CCR2(+)Vγ6(+)γδ T cells

Interleukin-17 (IL-17)-producing γδ T (γδ17) cells have been implicated in inflammatory diseases, but the underlying pathogenic mechanisms remain unclear. Here, we show that both CD4⁺ and γδ17 cells are required for the development of autoimmune arthritis in IL-1 receptor antagonist (IL-1Ra)-deficient mice. Specifically, activated CD4⁺ T cells direct γδ T-cell infiltration by inducing CCL2 expression in joints. Furthermore, IL-17 reporter mice reveal that the Vγ6⁺ subset of CCR2⁺ γδ T cells preferentially produces IL-17 in inflamed joints. Importantly, because IL-1Ra normally suppresses IL-1R expression on γδ T cells, IL-1Ra-deficient mice exhibit elevated IL-1R expression on Vγ6⁺ cells, which play a critical role in inducing them to produce IL-17. Our findings demonstrate a pathogenic mechanism in which adaptive and innate immunity induce an autoimmune disease in a coordinated manner.

Control of γδ T-cell activation remains incompletely understood. Here the authors show that during autoimmune arthritis development αβ CD4⁺ T cells recruit a subset of IL-17-producing γδ T cells to the joints, and that both components are essential to cause pathology in a mouse model of the disease.

Role of the intestinal cytokine microenvironment in shaping the intraepithelial lymphocyte repertoire

Local resident IELs are composed of distinct subsets of T cells with potent cytolytic and immunoregulatory capacities. As IELs are located within this unique interface between the core of the body and the outside environment, the specific development and function of intestinal IELs must be tightly regulated. To accomplish this, the cytokine microenvironment of the intestine has evolved sophisticated mechanisms that modulate the phenotype, ontogeny, and function of these cells. In this review, we summarize the evidence demonstrating the origin of certain intestinal cytokines, including IL-7, IL-15, IL-2, TGF-β, and SCF and discuss what influence such cytokines may have on IELs. Moreover, we review data suggesting that the abnormal expression of cytokines that leads to the heightened activation of IELs may also contribute to immunopathological responses or exacerbate inflammatory diseases, such as IBD and celiac disease, or promote cancer development and progression.

Origins of γδ T cell effector subsets: a riddle wrapped in an enigma

αβ and γδ T cells are thought to arise from a common precursor in the thymus but play distinct roles in pathogen resistance. Although conventional αβ T cells exit the thymus in a naive state and acquire effector function in the periphery, the effector fate of many γδ T cells is specified in the thymus and exhibits limited plasticity thereafter. This review describes the current models that have been proposed to explain the acquisition of effector fate by γδ T cells, as well as the apparent linkage to Vγ gene usage. The two predominant models are the predetermination model, which suggests that effector fate is determined prior to TCR expression, perhaps in association with the developmental timing of Vγ rearrangement, and the TCR-dependence model, which proposes that the nature of the TCR signal, particularly its intensity or duration, plays an important role in influencing effector fate.

Epigenetic and transcriptional regulation of γδ T cell differentiation: Programming cells for responses in time and space

γδ T cells are major providers of the pro-inflammatory cytokines interferon-γ (IFNγ) and interleukin-17 (IL-17) in protective or pathogenic immune responses. Notably, murine γδ T cells commit to either IFNγ or IL-17 production during development in the thymus, before any subsequent activation in the periphery. Here we discuss the molecular networks that underlie thymic γδ T cell differentiation, as well as the mechanisms that sustain or modify their functional properties in the periphery. We concentrate on recent findings on lymphoid and tissue-resident γδ T cell subpopulations, with an emphasis on genome-wide studies and their added value to elucidate the regulation of γδ T cell differentiation at the transcriptional and epigenetic (chromatin) levels.

Dermal γδ T cells--What have we learned?

Over the last several years, a number of papers have called attention to a distinct population of γδ T cells preferentially found in the dermis of the skin of normal mice. These cells appear to play an important role in promoting the development of psoriasis, but also are critical for host resistance to particular pathogens. They are characterized by the expression of a limited subset of γδ T cell receptors and a strong propensity to secrete IL-17. Perhaps most importantly, humans appear to carry an equivalent dermal γδ T cell population, likewise biased to secrete IL-17 and also implicated as playing a pathogenic role in psoriasis. This review will attempt to summarize and reconcile recent findings concerning the dermal γδ T cells.

Five Layers of Receptor Signaling in γδ T-Cell Differentiation and Activation

The contributions of γδ T-cells to immunity to infection or tumors critically depend on their activation and differentiation into effectors capable of secreting cytokines and killing infected or transformed cells. These processes are molecularly controlled by surface receptors that capture key extracellular cues and convey downstream intracellular signals that regulate γδ T-cell physiology. The understanding of how environmental signals are integrated by γδ T-cells is critical for their manipulation in clinical settings. Here, we discuss how different classes of surface receptors impact on human and murine γδ T-cell differentiation, activation, and expansion. In particular, we review the role of five receptor types: the T-cell receptor (TCR), costimulatory receptors, cytokine receptors, NK receptors, and inhibitory receptors. Some of the key players are the costimulatory receptors CD27 and CD28, which differentially impact on pro-inflammatory subsets of γδ T-cells; the cytokine receptors IL-2R, IL-7R, and IL-15R, which drive functional differentiation and expansion of γδ T-cells; the NK receptor NKG2D and its contribution to γδ T-cell cytotoxicity; and the inhibitory receptors PD-1 and BTLA that control γδ T-cell homeostasis. We discuss these and other receptors in the context of a five-step model of receptor signaling in γδ T-cell differentiation and activation, and discuss its implications for the manipulation of γδ T-cells in immunotherapy.

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.

Global characterization of differential gene expression profiles in mouse Vγ1+ and Vγ4+ γδ T cells

Peripheral γδ T cells in mice are classified into two major subpopulations, Vγ1⁺ and Vγ4⁺, based on the composition of T cell receptors. However, their intrinsic differences remain unclear. In this study, we analyzed gene expression profiles of the two subsets using Illumina HiSeq 2000 Sequencer. We identified 1995 transcripts related to the activation of Vγ1⁺ γδ T cells, and 2158 transcripts related to the activation of Vγ4⁺ γδ T cells. We identified 24 transcripts differentially expressed between the two subsets in resting condition, and 20 after PMA/Ionomycin treatment. We found that both cell types maintained phenotypes producing IFN-γ, TNF-α, TGF-β and IL-10. However, Vγ1⁺ γδ T cells produced more Th2 type cytokines, such as IL-4 and IL-5, while Vγ4⁺ γδ T cells preferentially produced IL-17. Our study provides a comprehensive gene expression profile of mouse peripheral Vγ1⁺ and Vγ4⁺ γδ T cells that describes the inherent differences between them.

CD3bright signals on γδ T cells identify IL-17A-producing Vγ6Vδ1+ T cells

Interleukin-17A (IL-17A) is a pro-inflammatory cytokine that has an important role at mucosal sites in a wide range of immune responses including infection, allergy and auto-immunity. γδ T cells are recognized as IL-17 producers, but based on the level of CD3 expression, we now define the remarkable ability of a CD3(bright) γδ T-cell subset with an effector memory phenotype to rapidly produce IL-17A, but not interferon-γ. CD3(bright) γδ T cells uniformly express the canonical germline encoded Vγ6/Vδ1(+) T-cell receptor. They are widely distributed with a preferential representation in the lungs and skin are negatively impacted in the absence of retinoic acid receptor-related orphan receptor gammat expression or endogenous flora. This population responded rapidly to various stimuli in a mechanism involving IL-23 and NOD-like receptor family, pyrin domain containing 3 (NLRP3)-inflammasome-dependent IL-1β. Finally, we demonstrated that IL-17-producing CD3(bright) γδ T cells responded promptly and strongly to pneumococcal infection and during skin inflammation. Here, we propose a new way to specifically analyze IL-17-producing Vγ6/Vδ1(+) T cells based on the level of CD3 signals. Using this gating strategy, our data reinforce the crucial role of this γδ T-cell subset in respiratory and skin disorders.

Cytotoxic T-cell and NK-cell lymphomas: current questions and controversies

The cytotoxic T-cell and natural killer (NK)-cell lymphomas and related disorders are important but relatively rare lymphoid neoplasms that frequently are a challenge for practicing pathologists. This selective review, based on a meeting of the International Lymphoma Study Group, briefly reviews T-cell and NK-cell development and addresses questions related to the importance of precise cell lineage (αβ-type T cell, γδ T cell, or NK cell), the implications of Epstein-Barr virus infection, the significance of anatomic location including nodal disease, and the question of further categorization of enteropathy-associated T-cell lymphomas. Finally, developments subsequent to the 2008 World Health Organization Classification, including the recognition of indolent NK-cell and T-cell disorders of the gastrointestinal tract are presented.

Staphylococcus aureus infection of mice expands a population of memory γδ T cells that are protective against subsequent infection

The development of vaccines against Staphylococcus aureus has consistently failed in clinical trials, likely due to inefficient induction of cellular immunity. T cell-derived IL-17 is one of the few known correlates of antistaphylococcoal immunity, conferring protection against S. aureus infections through its ability to promote phagocytic cell effector functions. A comprehensive understanding of the discrete T cell subsets critical for site-specific IL-17-mediated bacterial clearance will therefore be necessary to inform the development of vaccines that efficiently target cellular immunity. In this study, we have identified a population of CD44+ CD27- memory γδ T cells, expanded upon infection of C57BL/6 mice with S. aureus, which produce high levels of IL-17 and mediate enhanced bacterial clearance upon reinfection with the bacterium. These cells are comprised largely of the Vγ4+ subset and accumulate at the site of infection subsequent to an initial Vγ1.1+ and Vγ2+ T cell response. Moreover, these Vγ4+ T cells are retained in the peritoneum and draining mediastinal lymph nodes for a prolonged period following bacterial clearance. In contrast to its critical requirement for γδ T cell activation during the primary infection, IL-1 signaling was dispensable for activation and expansion of memory γδ T cells upon re-exposure to S. aureus. Our findings demonstrate that a γδ T cell memory response can be induced upon exposure to S. aureus, in a fashion analogous to that associated with classical αβ T cells, and suggest that induction of IL-17-expressing γδ T cells may be an important property of a protective vaccine against S. aureus.

The TCR ligand-inducible expression of CD73 marks γδ lineage commitment and a metastable intermediate in effector specification

CD73 expression is induced in response to TCR ligation and identifies a population of thymocytes that are committed to the γδ T cell fate.

Numerous studies indicate that γδ T cell receptor (γδTCR) expression alone does not reliably mark commitment of early thymic progenitors to the γδ fate. This raises the possibility that the γδTCR is unable to intrinsically specify fate and instead requires additional environmental factors, including TCR–ligand engagement. We use single cell progenitor assays to reveal that ligand acts instructionally to direct adoption of the γδ fate. Moreover, we identify CD73 as a TCR ligand-induced cell surface protein that distinguishes γδTCR-expressing CD4⁻CD8⁻ progenitors that have committed to the γδ fate from those that have not yet done so. Indeed, unlike CD73⁻ γδTCR⁺ progenitors, which largely adopt the αβ fate upon separation from the intrathymic selecting environment, those that express CD73 remain CD4⁻CD8⁻ and committed to the γδ fate. CD73 is expressed by >90% of peripheral γδ cells, suggesting this is a common occurrence during development. Moreover, CD73 induction appears to mark a metastable intermediate stage before acquisition of effector function, suggesting that γδ lineage and effector fate are specified sequentially. These findings have important implications for the role of ligand in γδ lineage commitment and its relationship to the specification of effector fate.

Unique T cells with unconventional cytokine profiles induced in the livers of mice during Schistosoma mansoni infection

During infection with Schistosoma, serious hepatic disorders are induced in the host. The liver possesses unique immune systems composed of specialized cells that differ from those of other immune competent organs or tissues. Host immune responses change dramatically during Schistosoma mansoni infection; in the early phase, Th1-related responses are induced, whereas during the late phase Th2 reactions dominate. Here, we describe unique T cell populations induced in the liver of mice during the period between Th1- and Th2-phases, which we term the transition phase. During this phase, varieties of immune cells including T lymphocytes increase in the liver. Subsets of CD4⁺ T cells exhibit unique cytokine production profiles, simultaneously producing both IFN-γ and IL-13 or both IFN-γ and IL-4. Furthermore, cells triply positive for IFN-γ, IL-13 and IL-4 also expand in the S. mansoni-infected liver. The induction of these unique cell populations does not occur in the spleen, indicating it is a phenomenon specific to the liver. In single hepatic CD4⁺ T cells showing the unique cytokine profiles, both T-bet and GATA-3 are expressed. Thus, our studies show that S. mansoni infection triggers the induction of hepatic T cell subsets with unique cytokine profiles.

SerpinB1 regulates homeostatic expansion of IL-17+ γδ and CD4+ Th17 cells

SerpinB1 is an endogenous inhibitor of serine proteases recognized for its anti-inflammatory and host-protective properties. Although loss of serpinB1 in mice does not result in gross immune deregulation, serpinb1a(-/-) mice display increased mortality and inflammation-associated morbidity upon challenge with influenza virus. Here, we show that IL-17A(+) γδ and CD4(+) Th17 cells are already expanded in the lungs of serpinb1a(-/-) mice at steady-state. Both γδ and αβ(+) CD4(+) CCR6(+) T cells isolated from the lungs of naive serpinb1a(-/-) mice displayed a skewed transcriptional profile relative to WT cells, including increased Th17 signature transcripts [Il17a, l17f, and Rorc (RORγt)] and decreased Th1 signature transcripts [Ifng, Cxcr3, and Tbx21 (T-bet)] in γδ T cells. In addition to the lung, IL-17A(+) γδ and CD4(+) Th17 cells were increased in the spleen of naive serpinb1a(-/-) mice, despite normal αβ and γδ T cell development in the thymus. Within the γδ T cell compartment, loss of serpinb1a prompted selective expansion of Vγ4(+) and Vγ6/Vδ1(+) cells, which also displayed elevated expression of the proliferating cell nuclear antigen, Ki-67, and IL-17A. Given that serpinb1a is preferentially expressed in WT IL-17A(+) γδ and CD4(+) Th17 cell subsets vis-à-vis other T cell lineages, our findings reveal a novel function of serpinB1 in limiting untoward expansion of lymphocytes with a Th17 phenotype.

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.

Differentiation and activation of γδ T Lymphocytes: Focus on CD27 and CD28 costimulatory receptors

γδ T lymphocytes are major providers of the pro-inflammatory cytokines interferon-γ (IFN-γ) and interleukin-17 (IL-17) at early stages of (auto)immune responses. We and others have recently described the phenotype and differentiation requirements of two distinct murine γδ T cell subsets producing either IFN-γ or IL-17. Here we summarize our current understanding of the molecular mechanisms that control γδ T cell differentiation, which is programmed in the thymus, and peripheral activation upon infection. We focus on the costimulatory receptors CD27 and CD28, which play independent and non-redundant roles in the physiology of γδ T cells in mice and in humans.

Shared and distinct transcriptional programs underlie the hybrid nature of iNKT cells

Invariant natural killer T cells (iNKT cells) are innate-like T lymphocytes that act as critical regulators of the immune response. To better characterize this population, we profiled gene expression in iNKT cells during ontogeny and in peripheral subsets as part of the Immunological Genome Project. High-resolution comparative transcriptional analyses defined developmental and subset-specific programs of gene expression by iNKT cells. In addition, we found that iNKT cells shared an extensive transcriptional program with NK cells, similar in magnitude to that shared with major histocompatibility complex (MHC)-restricted T cells. Notably, the program shared by NK cells and iNKT cells also operated constitutively in γδ T cells and in adaptive T cells after activation. Together our findings highlight a core effector program regulated distinctly in innate and adaptive lymphocytes.

Intrathymic programming of effector fates in three molecularly distinct γδ T cell subtypes

Innate γδ T cells function in the early phase of immune responses. Although innate γδ T cells have often been studied as one homogenous population, they can be functionally classified into effector subsets on the basis of the production of signature cytokines, analogous to adaptive helper T cell subsets. However, unlike the function of adaptive T cells, γδ effector T cell function correlates with genomically encoded T cell antigen receptor (TCR) chains, which suggests that clonal TCR selection is not the main determinant of the differentiation of γδ effector cells. A high-resolution transcriptome analysis of all emergent γδ thymocyte subsets segregated on the basis of use of the TCR γ-chain or δ-chain indicated the existence of three separate subtypes of γδ effector cells in the thymus. The immature γδ subsets were distinguished by unique transcription-factor modules that program effector function.

IL-7: the global builder of the innate lymphoid network and beyond, one niche at a time

The development and homeostasis of adaptive and innate lymphocytes is dependent on the stromal cytokine IL-7. The initial priming of immune responses to pathogenic challenges is executed by innate lymphoid cells (ILCs) with programmed capacity to rapidly secrete effector cytokines. How ILCs are controlled by IL-7 in distinct anatomical locale has evolved into a more complex problem as IL-7 receptor is not only expressed on ILCs, but also on surrounding neighbors, including vascular endothelium and mesenchymal cells that compete for limiting IL-7. For the generation of γδ T and B cells IL-7 is required for the production of antigen receptors, and it is likely that IL-7 performs critical function in facilitating ILC effector programming in addition to its regulatory actions on cell survival and proliferation. Most of our current understanding of the highly calibrated regulatory circuits of IL-7 function and IL-7 receptor signaling has derived from studies of adaptive, conventional lymphocytes. Here we highlight recent advances in mapping the gene circuits and cellular interactions that regulate temporospatial activities of IL-7 in diverse macro and micro niches that have direct relevance to deciphering the sphere of impact of IL-7 on ILC differentiation.

Role of γδ T cells in α-galactosylceramide-mediated immunity

Attempts to harness mouse type I NKT cells in different therapeutic settings including cancer, infection, and autoimmunity have proven fruitful using the CD1d-binding glycolipid α-galactosylceramide (α-GalCer). In these different models, the effects of α-GalCer mainly relied on the establishment of a type I NKT cell-dependent immune cascade involving dendritic cell, NK cell, B cell, or conventional CD4(+) and CD8(+) T cell activation/regulation as well as immunomodulatory cytokine production. In this study, we showed that γδ T cells, another population of innate-like T lymphocytes, displayed a phenotype of activated cells (cytokine production and cytotoxic properties) and were required to achieve an optimal α-GalCer-induced immune response. Using gene-targeted mice and recombinant cytokines, a critical need for IL-12 and IL-18 has been shown in the α-GalCer-induced IFN-γ production by γδ T cells. Moreover, this cytokine production occurred downstream of type I NKT cell response, suggesting their bystander effect on γδ T cells. In line with this, γδ T cells failed to directly recognize the CD1d/α-GalCer complex. We also provided evidence that γδ T cells increase their cytotoxic properties after α-GalCer injection, resulting in an increase in killing of tumor cell targets. Moreover, using cancer models, we demonstrated that γδ T cells were required for an optimal α-GalCer-mediated anti-tumor activity. Finally, we reported that immunization of wild-type mice with α-GalCer enhanced the adaptive immune response elicited by OVA, and this effect was strongly mediated by γδ T cells. We conclude that γδ T cells amplify the innate and acquired response to α-GalCer, with possibly important outcomes for the therapeutic effects of this compound.

Distribution of immune cells and expression of interleukin receptors in ileal Peyer's patches of calves

Newborn calves lack a mature immune system. The immune system develops with age, but the role of the expression of cytokine receptors in the development of immune cells of Peyer's patches (PPs) in the intestines of calves in the first 2 months has not yet been elucidated. In this study, the distribution of immune cells and the expression of interleukin (IL) receptors (R) in the ileal PPs of newborn and 2-month-old calves were investigated immunohistochemically with monoclonal antibodies against bovine CD4, CD8, IgM, γδTCR, T19, WC3, WC5, and WC6 antigens. The expression of ILRs was examined with antibodies against CD25 (IL-2Rα), IL-2Rγ, IL-4R, IL-6R, IL-10R, and IL-13R antigens. CD4(+), CD8(+), γδTCR(+), T19(+), and WC6(+) cells were found to be more widely distributed in the ileal PPs of 2-month-old calves than in those of newborn calves. Moreover, the expression of CD25 (IL-2Rα), IL-4R, and IL-13R in the ileal PPs of 2-month-old calves was more prominent than that in newborn calves. These data suggest that the immune system of calves at 2 months of age is developed by reactions to foreign antigens and aging.

αβ TCR⁺ T cells, but not B cells, promote autoimmune keratitis in b10 mice lacking γδ T cells

PURPOSE

To investigate additional factors in the spontaneous development of keratitis previously reported in B10.TCRδ⁻/⁻ female mice.

METHODS

The study tested whether susceptible B10.TCRδ⁻/⁻ mice have dry eyes compared with resistant B6.TCRδ⁻/⁻ females and also rederived the B10.TCRδ⁻/⁻ strain to test for the role of an infectious agent. Also assessed was whether adoptive transfer of αβ T cells from autoimmune mice induced keratitis in resistant mice. In addition, a potential role was examined for B cells or autoantibodies by B-cell inactivation, and the role of female hormones was tested by ovariectomy. Finally, the study investigated whether adoptive transfer of Vγ1⁺ γδ T cells confers protection.

RESULTS

Tear production in B10.TCRδ⁻/⁻ females was actually higher than in B6.TCRδ⁻/⁻ controls. Rederived B10.TCRδ⁻/⁻ mice still developed keratitis. Keratitis was induced in resistant mice after adoptive transfer of αβ T cells from keratitic donors. Inactivation of B cells from susceptible mice had no effect on the development of keratitis. Ovariectomy did not significantly reduce disease in B10.TCRδ⁻/⁻ females. Adoptive transfer of Vγ1⁺ cells from wild-type donors reduced keratitis in B10.TCRδ⁻/⁻ females.

CONCLUSIONS

Neither low tear levels nor ovarian hormones contribute to spontaneous keratitis in B10.TCRδ⁻/⁻ female mice, nor does it appear to depend on an infectious agent carried vertically in this strain. However, αβ T cells from keratitic hosts are sufficient to induce disease in the resistant B10.TCRβ⁻/⁻δ⁻/⁻ strain. Autoaggressive αβ T cells in the absence of Vγ1⁺ T cells in B10.TCRδ⁻/⁻ mice may be insufficiently checked to prevent disease.

Skint-1 identifies a common molecular mechanism for the development of interferon-γ-secreting versus interleukin-17-secreting γδ T cells

Murine T cell development begins with the generation of a unique Vγ5(+)Vδ1(+) epidermal γδ T cell compartment and a unique, more broadly distributed Vγ6(+)Vδ1(+) subset that is an important source of interleukin-17 (IL-17). This study showed that these respective functional programs were determined by Skint-1, a thymic epithelial cell determinant. By engaging Skint-1(+) cells, Vγ5(+)Vδ1(+) thymocytes induced an Egr3-mediated pathway, provoking differentiation and the potential to produce IFN-γ while suppressing the γδ T cell lineage factor, Sox13, and a RORγt transcription factor-associated IL-17-producing capacity. Hence, the functions of the earliest T cells are substantially preprogrammed in the thymus. Additionally, the phenotype of Skint-1-selected fetal thymocytes permitted identification in the adult thymus of an analogous gene regulatory network regulated by the γδ T cell receptor. Hence, these observations describe a molecular pathway by which distinct stress-responsive lymphocyte repertoires may emerge throughout ontogeny and offer parallels with emerging perspectives on the functional selection of other lymphoid cells.