Age-dependent requirement for gammadelta T cells in the primary but not secondary protective immune response against an intestinal parasite

Human γδ T cells in diverse tissues exhibit site-specific maturation dynamics across the life span

During ontogeny, γδ T cells emerge from the thymus and directly seed peripheral tissues for in situ immunity. However, their functional role in humans has largely been defined from blood. Here, we analyzed the phenotype, transcriptome, function, and repertoire of human γδ T cells in blood and mucosal and lymphoid tissues from 176 donors across the life span, revealing distinct profiles in children compared with adults. In early life, clonally diverse Vδ1 subsets predominate across blood and tissues, comprising naïve and differentiated effector and tissue repair functions, whereas cytolytic Vδ2 subsets populate blood, spleen, and lungs. With age, Vδ1 and Vδ2 subsets exhibit clonal expansions and elevated cytolytic signatures, which are disseminated across sites. In adults, Vδ2 cells predominate in blood, whereas Vδ1 cells are enriched across tissues and express residency profiles. Thus, antigenic exposures over childhood drive the functional evolution and tissue compartmentalization of γδ T cells, leading to age-dependent roles in immunity.

Surfing on the waves of the human γδ T cell ontogenic sea

While γδ T cells are present virtually in all vertebrates, there is a remarkable lack of conservation of the TRG and TRD loci underlying the generation of the γδ T cell receptor (TCR), which is associated with the generation of species-specific γδ T cells. A prominent example is the human phosphoantigen-reactive Vγ9Vδ2 T cell subset that is absent in mice. Murine γδ thymocyte cells were among the first immune cells identified to follow a wave-based layered development during embryonic and early life, and since this initial observation, in-depth insight has been obtained in their thymic ontogeny. By contrast, less is known about the development of human γδ T cells, especially regarding the generation of γδ thymocyte waves. Here, after providing an overview of thymic γδ wave generation in several vertebrate classes, we review the evidence for γδ waves in the human fetal thymus, where single-cell technologies have allowed the breakdown of human γδ thymocytes into functional waves with important TCR associations. Finally, we discuss the possible mechanisms contributing to the generation of waves of γδ thymocytes and their possible significance in the periphery.

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.

Unconventional MAIT cell responses to bacterial infections

Mucosal Associated Invariant T cells (MAIT) exert potent antimicrobial activity through direct recognition of metabolite-MR1 complexes and indirect activation by inflammatory cytokines. Additionally, via licensing of antigen presenting cells, MAIT cells orchestrate humoral and cellular adaptive immunity. Our recent understanding of molecular mechanisms of MAIT cell activation, and of the signals required to differentiate them in polarised subsets, pave the way for harnessing their functionality through small molecules or adoptive cell therapy.

Identification of distinct functional thymic programming of fetal and pediatric human γδ thymocytes via single-cell analysis

Developmental thymic waves of innate-like and adaptive-like γδ T cells have been described, but the current understanding of γδ T cell development is mainly limited to mouse models. Here, we combine single cell (sc) RNA gene expression and sc γδ T cell receptor (TCR) sequencing on fetal and pediatric γδ thymocytes in order to understand the ontogeny of human γδ T cells. Mature fetal γδ thymocytes (both the Vγ9Vδ2 and nonVγ9Vδ2 subsets) are committed to either a type 1, a type 3 or a type 2-like effector fate displaying a wave-like pattern depending on gestation age, and are enriched for public CDR3 features upon maturation. Strikingly, these effector modules express different CDR3 sequences and follow distinct developmental trajectories. In contrast, the pediatric thymus generates only a small effector subset that is highly biased towards Vγ9Vδ2 TCR usage and shows a mixed type 1/type 3 effector profile. Thus, our combined dataset of gene expression and detailed TCR information at the single-cell level identifies distinct functional thymic programming of γδ T cell immunity in human.

Knowledge about the ontogeny of T cells in the thymus relies heavily on mouse studies because of difficulty to obtain human material. Here the authors perform a single cell analysis of thymocytes from human fetal and paediatric thymic samples to characterise the development of human γδ T cells in the thymus.

γδ T Cells in Brain Homeostasis and Diseases

γδ T cells are a distinct subset of T cells expressing γδ T cell receptor (TCR) rather than αβTCR. Since their discovery, the critical roles of γδ T cells in multiple physiological systems and diseases have been investigated. γδ T cells are preferentially located at mucosal surfaces, such as the gut, although a small subset of γδ T cells can circulate the blood. Additionally, a subset of γδ T cells reside in the meninges in the central nervous system. Recent findings suggest γδ T cells in the meninges have critical roles in brain function and homeostasis. In addition, several lines of evidence have shown γδ T cells can infiltrate the brain parenchyma and regulate inflammatory responses in multiple diseases, including neurodegenerative diseases. Although the importance of γδ T cells in the brain is well established, their roles are still incompletely understood due to the complexity of their biology. Because γδ T cells rapidly respond to changes in brain status and regulate disease progression, understanding the role of γδ T cells in the brain will provide critical information that is essential for interpreting neuroimmune modulation. In this review, we summarize the complex role of γδ T cells in the brain and discuss future directions for research.

Repertoire analysis of γδ T cells in the chicken enables functional annotation of the genomic region revealing highly variable pan-tissue TCR gamma V gene usage as well as identifying public and private repertoires

BACKGROUND

Despite increasing interest in γδ T cells and their non-classical behaviour, most studies focus on animals with low numbers of circulating γδ T cells, such as mice and humans. Arguably, γδ T cell functions might be more prominent in chickens where these cells form a higher proportion of the circulatory T cell compartment. The TCR repertoire defines different subsets of γδ T cells, and such analysis is facilitated by well-annotated TCR loci. γδ T cells are considered at the cusp of innate and adaptive immunity but most functions have been identified in γδ low species. A deeper understanding of TCR repertoire biology in γδ high and γδ low animals is critical for defining the evolution of the function of γδ T cells. Repertoire dynamics will reveal populations that can be classified as innate-like or adaptive-like as well as those that straddle this definition.

RESULTS

Here, a recent discrepancy in the structure of the chicken TCR gamma locus is resolved, demonstrating that tandem duplication events have shaped the evolution of this locus. Importantly, repertoire sequencing revealed large differences in the usage of individual TRGV genes, a pattern conserved across multiple tissues, including thymus, spleen and the gut. A single TRGV gene, TRGV3.3, with a highly diverse private CDR3 repertoire dominated every tissue in all birds. TRGV usage patterns were partly explained by the TRGV-associated recombination signal sequences. Public CDR3 clonotypes represented varying proportions of the repertoire of TCRs utilising different TRGVs, with one TRGV dominated by super-public clones present in all birds.

CONCLUSIONS

The application of repertoire analysis enabled functional annotation of the TCRG locus in a species with a high circulating γδ phenotype. This revealed variable usage of TCRGV genes across multiple tissues, a pattern quite different to that found in γδ low species (human and mouse). Defining the repertoire biology of avian γδ T cells will be key to understanding the evolution and functional diversity of these enigmatic lymphocytes in an animal that is numerically more reliant on them. Practically, this will reveal novel ways in which these cells can be exploited to improve health in medical and veterinary contexts.

Developing the right tools for the job: Lin28 regulation of early life T-cell development and function

T cells comprise a functionally heterogeneous cell population that has important roles in the immune system. While T cells are broadly considered to be a component of the antigen-specific adaptive immune response, certain T-cell subsets display innate-like effector characteristics whereas others perform immunosuppressive functions. These functionally diverse T-cell populations preferentially arise at different stages of ontogeny and are tailored to the immunological priorities of the organism over time. Many differences in early life versus adult T-cell phenotypes can be attributed to the cell-intrinsic properties of the distinct progenitors that seed the thymus throughout development. It is becoming clear that Lin28, an evolutionarily conserved, heterochronic RNA-binding protein that is differentially expressed among early life and adult hematopoietic progenitor cells, plays a substantial role in influencing early T-cell development and function. Here, we discuss the mechanisms by which Lin28 shapes the T-cell landscape to protect the developing fetus and newborn. Manipulation of the Lin28 gene regulatory network is being considered as one means of improving hematopoietic stem cell transplant outcomes; as such, understanding the impact of Lin28 on T-cell function is of clinical relevance.

Malaria and Early Life Immunity: Competence in Context

Childhood vaccines have been the cornerstone tool of public health over the past century. A major barrier to neonatal vaccination is the “immaturity” of the infant immune system and the inefficiency of conventional vaccine approaches at inducing immunity at birth. While much of the literature on fetal and neonatal immunity has focused on the early life propensity toward immune tolerance, recent studies indicate that the fetus is more immunologically capable than previously thought, and can, in some circumstances, mount adaptive B and T cell responses to perinatal pathogens in utero. Although significant hurdles remain before these findings can be translated into vaccines and other protective strategies, they should lend optimism to the prospect that neonatal and even fetal vaccination is achievable. Next steps toward this goal should include efforts to define the conditions for optimal stimulation of infant immune responses, including antigen timing, dose, and route of delivery, as well as antigen presentation pathways and co-stimulatory requirements. A better understanding of these factors will enable optimal deployment of vaccines against malaria and other pathogens to protect infants during their period of greatest vulnerability.

γδ T cells compose a developmentally regulated intrauterine population and protect against vaginal candidiasis

This most comprehensive analysis to date of γδ T cells in the murine uterus reveals them to compose a unique local T-cell compartment. Consistent with earlier reports, most cells expressed a canonical Vγ6Vδ1 TCR, and produced interleukin (IL)-17A upon stimulation. Nonetheless, contrasting with earlier reports, uterine γδ T cells were not obviously intraepithelial, being more akin to sub-epithelial Vγ6Vδ1+ T cells at several other anatomical sites. By contrast to other tissues however, the uterine compartment also included non-Vγ6+, IFN-γ-producing cells; was strikingly enriched in young mice; expressed genes hitherto associated with the uterus, including the progesterone receptor; and did not require microbes for development and/or maintenance. This notwithstanding, γδ T-cell deficiency severely impaired resistance to reproductive tract infection by Candida albicans, associated with decreased responses of IL-17-dependent neutrophils. These findings emphasise tissue-specific complexities of different mucosal γδ cell compartments, and their evident importance in lymphoid stress-surveillance against barrier infection.

γδ T cells in tissue physiology and surveillance

γδ T cells are a unique T cell subpopulation that are rare in secondary lymphoid organs but enriched in many peripheral tissues, such as the skin, intestines and lungs. By rapidly producing large amounts of cytokines, γδ T cells make key contributions to immune responses in these tissues. In addition to their immune surveillance activities, recent reports have unravelled exciting new roles for γδ T cells in steady-state tissue physiology, with functions ranging from the regulation of thermogenesis in adipose tissue to the control of neuronal synaptic plasticity in the central nervous system. Here, we review the roles of γδ T cells in tissue homeostasis and in surveillance of infection, aiming to illustrate their major impact on tissue integrity, tissue repair and immune protection.

Fetal public Vγ9Vδ2 T cells expand and gain potent cytotoxic functions early after birth

Vγ9Vδ2 T cells are a major human blood γδ T cell population that respond in a T cell receptor (TCR)-dependent manner to phosphoantigens which are generated by a variety of microorganisms. It is not clear how Vγ9Vδ2 T cells react toward the sudden microbial exposure early after birth. We found that human Vγ9Vδ2 T cells with a public/shared fetal-derived TCR repertoire expanded within 10 wk postpartum. Such an expansion was not observed in non-Vγ9Vδ2 γδ T cells, which possessed a private TCR repertoire. Furthermore, only the Vγ9Vδ2 T cells differentiated into potent cytotoxic effector cells by 10 wk of age, despite their fetal origin. Both the expansion of public fetal Vγ9Vδ2 T cells and their functional differentiation were not affected by newborn vaccination with the phosphoantigen-containing bacillus Calmette-Guérin (BCG) vaccine. These findings suggest a strong and early priming of the public fetal-derived Vγ9Vδ2 T cells promptly after birth, likely upon environmental phosphoantigen exposure.

Type 1 Treg cells promote the generation of CD8+ tissue-resident memory T cells

Tissue-resident memory T (T_RM) cells, functionally distinct from circulating memory T cells, have a critical role in protective immunity in tissues, are more efficacious when elicited after vaccination and yield more effective antitumor immunity, yet the signals that direct development of T_RM cells are incompletely understood. Here we show that type 1 regulatory T (T_reg) cells, which express the transcription factor T-bet, promote the generation of CD8⁺ T_RM cells. The absence of T-bet-expressing type 1 T_reg cells reduces the presence of T_RM cells in multiple tissues and increases pathogen burden upon infectious challenge. Using infection models, we show that type 1 T_reg cells are specifically recruited to local inflammatory sites via the chemokine receptor CXCR3. Close proximity with effector CD8⁺ T cells and T_reg cell expression of integrin-β8 endows the bioavailability of transforming growth factor-β in the microenvironment, thereby promoting the generation of CD8⁺ T_RM cells.

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.

Perinatal inflammation influences but does not arrest rapid immune development in preterm babies

Infection and infection-related complications are important causes of death and morbidity following preterm birth. Despite this risk, there is limited understanding of the development of the immune system in those born prematurely, and of how this development is influenced by perinatal factors. Here we prospectively and longitudinally follow a cohort of babies born before 32 weeks of gestation. We demonstrate that preterm babies, including those born extremely prematurely (<28 weeks), are capable of rapidly acquiring some adult levels of immune functionality, in which immune maturation occurs independently of the developing heterogeneous microbiome. By contrast, we observe a reduced percentage of CXCL8-producing T cells, but comparable levels of TNF-producing T cells, from babies exposed to in utero or postnatal infection, which precedes an unstable post-natal clinical course. These data show that rapid immune development is possible in preterm babies, but distinct identifiable differences in functionality may predict subsequent infection mediated outcomes.

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.

The immune response to malaria in utero

Malaria causes tremendous early childhood morbidity and mortality, providing an urgent impetus for the development of a vaccine that is effective in neonates. However, the infant immune response to malaria may be influenced by events that occur well before birth. Placental malaria infection complicates one quarter of all pregnancies in Africa and frequently results in exposure of the fetus to malaria antigens in utero, while the immune system is still developing. Some data suggest that in utero exposure to malaria may induce immunologic tolerance that interferes with the development of protective immunity during childhood. More recently, however, a growing body of evidence suggests that fetal malaria exposure can prime highly functional malaria-specific T- and B-cells, which may contribute to postnatal protection from malaria. In utero exposure to malaria also impacts the activation and maturation of fetal antigen presenting cells and innate lymphocytes, which could have implications for global immunity in the infant. Here, we review recent advances in our understanding of how various components of the fetal immune system are altered by in utero exposure to malaria, discuss factors that may tilt the critical balance between tolerance and adaptive immunity, and consider the implications of these findings for malaria prevention strategies.

Parasite infections in a social carnivore: Evidence of their fitness consequences and factors modulating infection load

There are substantial individual differences in parasite composition and infection load in wildlife populations. Few studies have investigated the factors shaping this heterogeneity in large wild mammals or the impact of parasite infections on Darwinian fitness, particularly in juveniles. A host's parasite composition and infection load can be shaped by factors that determine contact with infective parasite stages and those that determine the host's resistance to infection, such as abiotic and social environmental factors, and age. Host-parasite interactions and synergies between coinfecting parasites may also be important. We test predictions derived from these different processes to investigate factors shaping infection loads (fecal egg/oocyte load) of two energetically costly gastrointestinal parasites: the hookworm Ancylostoma and the intracellular Cystoisospora, in juvenile spotted hyenas (Crocuta crocuta) in the Serengeti National Park, in Tanzania. We also assess whether parasite infections curtail survival to adulthood and longevity. Ancylostoma and Cystoisospora infection loads declined as the number of adult clan members increased, a result consistent with an encounter-reduction effect whereby adults reduced encounters between juveniles and infective larvae, but were not affected by the number of juveniles in a clan. Infection loads decreased with age, possibly because active immune responses to infection improved with age. Differences in parasite load between clans possibly indicate variation in abiotic environmental factors between clan den sites. The survival of juveniles (<365 days old) to adulthood decreased with Ancylostoma load, increased with age, and was modulated by maternal social status. High-ranking individuals with low Ancylostoma loads had a higher survivorship during the first 4 years of life than high-ranking individuals with high Ancylostoma loads. These findings suggest that high infection loads with energetically costly parasites such as hookworms during early life can have negative fitness consequences.

Mitochondria maintain controlled activation state of epithelial-resident T lymphocytes

Epithelial-resident T lymphocytes, such as intraepithelial lymphocytes (IELs) located at the intestinal barrier, can offer swift protection against invading pathogens. Lymphocyte activation is strictly regulated because of its potential harmful nature and metabolic cost, and most lymphocytes are maintained in a quiescent state. However, IELs are kept in a heightened state of activation resembling effector T cells but without cytokine production or clonal proliferation. We show that this controlled activation state correlates with alterations in the IEL mitochondrial membrane, especially the cardiolipin composition. Upon inflammation, the cardiolipin composition is altered to support IEL proliferation and effector function. Furthermore, we show that cardiolipin makeup can particularly restrict swift IEL proliferation and effector functions, reducing microbial containment capability. These findings uncover an alternative mechanism to control cellular activity, special to epithelial-resident T cells, and a novel role for mitochondria, maintaining cells in a metabolically poised state while enabling rapid progression to full functionality.

Genetic models reveal origin, persistence and non-redundant functions of IL-17-producing γδ T cells

The authors present a genetic mouse model for conditional depletion of γδ T cells, confirming the fetal origin and persistence of Tγδ17 cells. They show differential phenotypes after acute depletion versus constitutive γδ T cell deficiency in imiquimod-induced psoriasis.

γδ T cells are highly conserved in jawed vertebrates, suggesting an essential role in the immune system. However, γδ T cell–deficient Tcrd^−/− mice display surprisingly mild phenotypes. We hypothesized that the lack of γδ T cells in constitutive Tcrd^−/− mice is functionally compensated by other lymphocytes taking over genuine γδ T cell functions. To test this, we generated a knock-in model for diphtheria toxin–mediated conditional γδ T cell depletion. In contrast to IFN-γ–producing γδ T cells, IL-17–producing γδ T cells (Tγδ17 cells) recovered inefficiently after depletion, and their niches were filled by expanding Th17 cells and ILC3s. Complementary genetic fate mapping further demonstrated that Tγδ17 cells are long-lived and persisting lymphocytes. Investigating the function of γδ T cells, conditional depletion but not constitutive deficiency protected from imiquimod-induced psoriasis. Together, we clarify that fetal thymus-derived Tγδ17 cells are nonredundant local effector cells in IL-17–driven skin pathology.

Policing the intestinal epithelial barrier: Innate immune functions of intraepithelial lymphocytes

PURPOSE OF REVIEW

This review will explore the contribution of IELs to mucosal innate immunity and highlight the similarities in IEL functional responses to bacteria, viruses and protozoan parasite invasion.

RECENT FINDINGS

IELs rapidly respond to microbial invasion by activating host defense responses, including the production of mucus and antimicrobial peptides to prevent microbes from reaching the epithelial surface. During active infection, IELs promote epithelial cytolysis, cytokine and chemokine production to limit pathogen invasion, replication and dissemination. Commensal-induced priming of IEL effector function or continuous surveillance of the epithelium may be important contributing factors to the rapidity of response.

SUMMARY

Impaired microbial recognition, dysregulated innate immune signaling or microbial dysbiosis may limit the protective function of IELs and increase susceptibility to disease. Further understanding of the mechanisms regulating IEL surveillance and sentinel function may provide insight into the development of more effective targeted therapies designed to reinforce the mucosal barrier.

Eimeria vermiformis Infection Model of Murine Small Intestine

Eimeria vermiformis is a tissue specific, intracellular protozoan that infects the murine small intestinal epithelia, which has been widely used as a coccidian model to study mucosal immunology. This mouse infection model is valuable to investigate the mechanisms of host protection against primary and secondary infection in the small intestine. Here, we describe the generation of an E. vermiformis stock solution, preparation of sporulated E. vermiformis to infect mice and determination of oocysts burden. This protocol should help to establish a highly reproducible natural infection challenge model to study immunity in the small intestine. The information obtained from using this mouse model can reveal fundamental mechanisms of interaction between the pathogen and the immune response, e.g., provided by intraepithelial lymphocytes (IEL) at the basolateral site of epithelial cells but also a variety of other immune cell populations present in the gut.

CD73 expression identifies a subset of IgM+ antigen-experienced cells with memory attributes that is T cell and CD40 signalling dependent

B-cell memory was long characterized as isotype-switched, somatically mutated and germinal centre (GC)-derived. However, it is now clear that the memory pool is a complex mixture that includes unswitched and unmutated cells. Further, expression of CD73, CD80 and CD273 has allowed the categorization of B-cell memory into multiple subsets, with combinatorial expression of the markers increasing with GC progression, isotype-switching and acquisition of somatic mutations. We have extended these findings to determine whether these markers can be used to identify IgM memory phenotypically as arising from T-dependent versus T-independent responses. We report that CD73 expression identifies a subset of antigen-experienced IgM⁺ cells that share attributes of functional B-cell memory. This subset is reduced in the spleens of T-cell-deficient and CD40-deficient mice and in mixed marrow chimeras made with mutant and wild-type marrow, the proportion of CD73⁺ IgM memory is restored in the T-cell-deficient donor compartment but not in the CD40-deficient donor compartment, indicating that CD40 ligation is involved in its generation. We also report that CD40 signalling supports optimal expression of CD73 on splenic T cells and age-associated B cells (ABCs), but not on other immune cells such as neutrophils, marginal zone B cells, peritoneal cavity B-1 B cells and regulatory T and B cells. Our data indicate that in addition to promoting GC-associated memory generation during B-cell differentiation, CD40-signalling can influence the composition of the unswitched memory B-cell pool. They also raise the possibility that a fraction of ABCs may represent T-cell-dependent IgM memory.

Tumor-infiltrating γδT cells predict prognosis and adjuvant chemotherapeutic benefit in patients with gastric cancer

Purpose : Tumor-infiltrating γδT cells (γδTILs) have different prognostic value and functions among various cancers. The aim of the present study was to evaluate the effect of γδTILs in gastric cancer. Patients and methods : A discovery set (n = 190) and a validation set (n = 273) were involved in this study. Patients with TNM II and III disease were used to predict response to 5-fluorouracil (5-FU)-based adjuvant chemotherapy (ACT) in both sets. γδTILs were defined as intense (γδT cells≥ 5/HPF) versus nonintense (γδT cells<5/HPF). Kaplan-Meier curve was plotted to analysis survival. Hazard ratio (HR) and 95%CI associated with γδTILs were evaluated by multivariable Cox models. Findings : The prognostic value of γδTILs in the discovery set (HR, 0.193; 95%CI, 0.097-0.383; P<0.001) was confirmed in the validation set (HR, 0.442; 95%CI, 0.251-0.779; P = 0.005) for overall survival (OS). Patients whose tumors with γδT cells≥ 5/HPF could benefit from ACT, with a reduced risk of compromised survival compared with those with γδT cells<5/HPF (HR, 0.086; 95%CI, 0.023-0.327; P<0.001 in discovery set; and HR, 0.077; 95%CI, 0.023-0.256; P<0.001 in validation set). Conclusion : The present study shows that intense γδT cells infiltration is an independent prognostic factor in patients with gastric cancer and is predictive of a survival benefit from adjuvant chemotherapy in patients with TNM II and III disease.

Clonal selection in the human Vδ1 T cell repertoire indicates γδ TCR-dependent adaptive immune surveillance

γδ T cells are considered to be innate-like lymphocytes that respond rapidly to stress without clonal selection and differentiation. Here we use next-generation sequencing to probe how this paradigm relates to human Vδ2^neg T cells, implicated in responses to viral infection and cancer. The prevalent Vδ1 T cell receptor (TCR) repertoire is private and initially unfocused in cord blood, typically becoming strongly focused on a few high-frequency clonotypes by adulthood. Clonal expansions have differentiated from a naive to effector phenotype associated with CD27 downregulation, retaining proliferative capacity and TCR sensitivity, displaying increased cytotoxic markers and altered homing capabilities, and remaining relatively stable over time. Contrastingly, Vδ2⁺ T cells express semi-invariant TCRs, which are present at birth and shared between individuals. Human Vδ1⁺ T cells have therefore evolved a distinct biology from the Vδ2⁺ subset, involving a central, personalized role for the γδ TCR in directing a highly adaptive yet unconventional form of immune surveillance.

γδ T cells are generally considered innate-like lymphocytes. Here the authors sequence human γδ T cell receptors (TCR) to show focusing of the private Vδ1 TCR repertoire, suggesting that, unlike Vδ2 T cells, the Vδ1 T cell compartment has adaptive attributes.

Gamma delta T cells are early responders to Mycobacterium avium ssp. paratuberculosis in colostrum-replete Holstein calves

Peripheral blood mononuclear cells (PBMC) and mesenteric node lymphocytes (MNL) were obtained from 30 calves that were assigned randomly at birth to 1 of 6 treatment groups with 5 calves per treatment in a 14-d study: (1) colostrum-deprived (CD), no vitamins; (2) colostrum-replacer (CR), no vitamins; (3) CR, vitamin A; (4) CR, vitamin D₃; (5) CR, vitamin E; (6) CR, vitamins A, D₃, E. Calves were injected with appropriate vitamin supplements and fed pasteurized whole milk (CD calves) or fractionated colostrum replacer (CR calves) at birth. Thereafter, all calves were fed pasteurized whole milk fortified with vitamins according to treatment group. Calves were orally inoculated with 10⁸ cfu of Mycobacterium avium ssp. paratuberculosis (MAP) on d 1 and 3. The PBMC and MNL harvested on d 13 were analyzed by flow cytometry as fresh cells, after 3-d culture with phytohemagglutinin (PHA), and after 6-d culture with a whole-cell sonicate of MAP (MPS). Peripheral γδ T cells were a predominant lymphocyte subset in neonatal calves, with a decreased percentage noted in CD calves compared with CR calves. As well, CD25 expression was higher in γδ T cells compared with other cell subsets, regardless of treatment group. Stimulation of PBMC with PHA resulted in increased CD4⁺ and CD8⁺ subsets, whereas MNL response was dominated by expansion of B-cell subpopulations. Stimulation with PHA and MPS decreased the relative abundance of PBMC γδ T cells, but MNL γδ T cells increased upon stimulation with MPS. These results identify γδ T cells as key early responders to intracellular infection in neonatal calves and suggest that colostrum may be an important mediator of this response.

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.

Cystoisospora suis - A Model of Mammalian Cystoisosporosis

Cystoisospora suis is a coccidian species that typically affects suckling piglets. Infections occur by oral uptake of oocysts and are characterized by non-hemorrhagic transient diarrhea, resulting in poor weight gain. Apparently, primary immune responses to C. suis cannot readily be mounted by neonates, which contributes to the establishment and rapid development of the parasite, while in older pigs age-resistance prevents disease development. However, the presence of extraintestinal stages, although not unequivocally demonstrated, is suspected to enable parasite persistence together with the induction and maintenance of immune response in older pigs, which in turn may facilitate the transfer of C. suis-specific factors from sow to offspring. It is assumed that neonates are particularly prone to clinical disease because infections with C. suis interfere with the establishment of the gut microbiome. Clostridia have been especially inferred to profit from the altered intestinal environment during parasite infection. New tools, particularly in the area of genomics, might illustrate the interactions between C. suis and its host and pave the way for the development of new control methods not only for porcine cystoisosporosis but also for other mammalian Cystoisospora infections. The first reference genome for C. suis is under way and will be a fertile ground to discover new drugs and vaccines. At the same time, the establishment and refinement of an in vivo model and an in vitro culture system, supporting the complete life cycle of C. suis, will underpin the functional characterization of the parasite and shed light on its biology and control.

Intestinal intraepithelial lymphocyte activation promotes innate antiviral resistance

Unrelenting environmental challenges to the gut epithelium place particular demands on the local immune system. In this context, intestinal intraepithelial lymphocytes (IEL) compose a large, highly conserved T cell compartment, hypothesized to provide a first line of defence via cytolysis of dysregulated intestinal epithelial cells (IEC) and cytokine-mediated re-growth of healthy IEC. Here we show that one of the most conspicuous impacts of activated IEL on IEC is the functional upregulation of antiviral interferon (IFN)-responsive genes, mediated by the collective actions of IFNs with other cytokines. Indeed, IEL activation in vivo rapidly provoked type I/III IFN receptor-dependent upregulation of IFN-responsive genes in the villus epithelium. Consistent with this, activated IEL mediators protected cells against virus infection in vitro, and pre-activation of IEL in vivo profoundly limited norovirus infection. Hence, intraepithelial T cell activation offers an overt means to promote the innate antiviral potential of the intestinal epithelium.

γδ T cells confer protection against murine cytomegalovirus (MCMV)

Cytomegalovirus (CMV) is a leading infectious cause of morbidity in immune-compromised patients. γδ T cells have been involved in the response to CMV but their role in protection has not been firmly established and their dependency on other lymphocytes has not been addressed. Using C57BL/6 αβ and/or γδ T cell-deficient mice, we here show that γδ T cells are as competent as αβ T cells to protect mice from CMV-induced death. γδ T cell-mediated protection involved control of viral load and prevented organ damage. γδ T cell recovery by bone marrow transplant or adoptive transfer experiments rescued CD3ε^−/− mice from CMV-induced death confirming the protective antiviral role of γδ T cells. As observed in humans, different γδ T cell subsets were induced upon CMV challenge, which differentiated into effector memory cells. This response was observed in the liver and lungs and implicated both CD27⁺ and CD27⁻ γδ T cells. NK cells were the largely preponderant producers of IFNγ and cytotoxic granules throughout the infection, suggesting that the protective role of γδ T cells did not principally rely on either of these two functions. Finally, γδ T cells were strikingly sufficient to fully protect Rag^−/−γc^−/− mice from death, demonstrating that they can act in the absence of B and NK cells. Altogether our results uncover an autonomous protective antiviral function of γδ T cells, and open new perspectives for the characterization of a non classical mode of action which should foster the design of new γδ T cell based therapies, especially useful in αβ T cell compromised patients.

γδ T cells are unconventional T lymphocytes that play a unique role in host protection against pathogens. Human Cytomegalovirus (HCMV) is a widespread virus that can cause severe organ disease such as hepatitis and pneumonitis in immune-compromised patients. Our decade-long study conveys compelling evidence for the implication of human γδ T cells in the immune response against HCMV, but their protective role could not be formally demonstrated in humans. In the present study we use the murine model of CMV infection which allows the spatial and temporal analysis of viral spread and anti-viral immune responses. We show that, in the absence of αβ T cells, γδ T cells control MCMV-induced hepatitis, pneumonitis and death by restricting viral load in the liver, lungs and spleen. γδ T cells expand in these organs and display memory features that could be further incorporated into vaccination strategies. In conclusion, γδ T cells represent an important arm in the immune response against CMV infection that could be particularly important in the context of αβ T cell immune-suppression.

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.

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.

Effector Vγ9Vδ2 T cells dominate the human fetal γδ T-cell repertoire

γδ T cells are unconventional T cells recognizing antigens via their γδ T-cell receptor (TCR) in a way that is fundamentally different from conventional αβ T cells. γδ T cells usually are divided into subsets according the type of Vγ and/or Vδ chain they express in their TCR. T cells expressing the TCR containing the γ-chain variable region 9 and the δ-chain variable region 2 (Vγ9Vδ2 T cells) are the predominant γδ T-cell subset in human adult peripheral blood. The current thought is that this predominance is the result of the postnatal expansion of cells expressing particular complementary-determining region 3 (CDR3) in response to encounters with microbes, especially those generating phosphoantigens derived from the 2-C-methyl-d-erythritol 4-phosphate pathway of isoprenoid synthesis. However, here we show that, rather than requiring postnatal microbial exposure, Vγ9Vδ2 T cells are the predominant blood subset in the second-trimester fetus, whereas Vδ1(+) and Vδ3(+) γδ T cells are present only at low frequencies at this gestational time. Fetal blood Vγ9Vδ2 T cells are phosphoantigen responsive and display very limited diversity in the CDR3 of the Vγ9 chain gene, where a germline-encoded sequence accounts for >50% of all sequences, in association with a prototypic CDR3δ2. Furthermore, these fetal blood Vγ9Vδ2 T cells are functionally preprogrammed (e.g., IFN-γ and granzymes-A/K), with properties of rapidly activatable innatelike T cells. Thus, enrichment for phosphoantigen-responsive effector T cells has occurred within the fetus before postnatal microbial exposure. These various characteristics have been linked in the mouse to the action of selecting elements and would establish a much stronger parallel between human and murine γδ T cells than is usually articulated.

γδ T Cell-Mediated Immune Responses in Disease and Therapy

The role of γδ T cells in immunotherapy has gained specific importance in the recent years because of their prominent function involving directly or indirectly in the rehabilitation of the diseases. γδ T cells represent a minor population of T cells that express a distinct T cell receptor (TCR) composed of γδ chains instead of αβ chains. Unlike αβ T cells, γδ T cells display a restricted TCR repertoire and recognize mostly unknown non-peptide antigens. γδ T cells act as a link between innate and adaptive immunity, because they lack precise major histocompatibility complex (MHC) restriction and seize the ability to recognize ligands that are generated during affliction. Skin epidermal γδ T cells recognize antigen expressed by damaged or stressed keratinocytes and play an indispensable role in tissue homeostasis and repair through secretion of distinct growth factors. γδ T cell based immunotherapy strategies possess great prominence in the treatment because of the property of their MHC-independent cytotoxicity, copious amount of cytokine release, and a immediate response in infections. Understanding the role of γδ T cells in pathogenic infections, wound healing, autoimmune diseases, and cancer might provide knowledge for the successful treatment of these diseases using γδ T cell based immunotherapy. Enhancing the human Vγ9Vδ2 T cells functions by administration of aminobisphosphonates like zoledronate, pamidronate, and bromohydrin pyrophosphate along with cytokines and monoclonal antibodies shows a hopeful approach for treatment of tumors and infections. The current review summarizes the role of γδ T cells in various human diseases and immunotherapeutic approaches using γδ T cells.

Immunity to cytomegalovirus in early life

Cytomegalovirus (CMV) is the most common congenital infection and is the leading non-genetic cause of neurological defects. CMV infection in early life is also associated with intense and prolonged viral excretion, indicating limited control of viral replication. This review summarizes our current understanding of the innate and adaptive immune responses to CMV infection during fetal life and infancy. It illustrates the fact that studies of congenital CMV infection have provided a proof of principle that the human fetus can develop anti-viral innate and adaptive immune responses, indicating that such responses should be inducible by vaccination in early life. The review also emphasizes the fact that our understanding of the mechanisms involved in symptomatic congenital CMV infection remains limited.

γδ T cells recognize the insulin B:9-23 peptide antigen when it is dimerized through thiol oxidation

The insulin peptide B:9-23 is a natural antigen in the non-obese diabetic (NOD) mouse model of type 1 diabetes (T1D). In addition to αβ T cells and B cells, γδ T cells recognize the peptide and infiltrate the pancreatic islets where the peptide is produced within β cells. The peptide contains a cysteine in position 19 (Cys19), which is required for the γδ but not the αβ T cell response, and a tyrosine in position 16 (Tyr16), which is required for both. A peptide-specific mAb, tested along with the T cells, required neither of the two amino acids to bind the B:9-23 peptide. We found that γδ T cells require Cys19 because they recognize the peptide antigen in an oxidized state, in which the Cys19 thiols of two peptide molecules form a disulfide bond, creating a soluble homo-dimer. In contrast, αβ T cells recognize the peptide antigen as a reduced monomer, in complex with the MHCII molecule I-A(g7). Unlike the unstructured monomeric B:9-23 peptide, the γδ-stimulatory homo-dimer adopts a distinct secondary structure in solution, which differs from the secondary structure of the corresponding portion of the native insulin molecule. Tyr16 is required for this adopted structure of the dimerized insulin peptide as well as for the γδ response to it. This observation is consistent with the notion that γδ T cell recognition depends on the secondary structure of the dimerized insulin B:9-23 antigen.

γδ T cells: first line of defense and beyond

γδ T cells, αβ T cells, and B cells are present together in all but the most primitive vertebrates, suggesting that each population contributes to host immune competence uniquely and that all three are necessary for maintaining immune competence. Functional and molecular analyses indicate that in infections, γδ T cells respond earlier than αβ T cells do and that they emerge late after pathogen numbers start to decline. Thus, these cells may be involved in both establishing and regulating the inflammatory response. Moreover, γδ T cells and αβ T cells are clearly distinct in their antigen recognition and activation requirements as well as in the development of their antigen-specific repertoire and effector function. These aspects allow γδ T cells to occupy unique temporal and functional niches in host immune defense. We review these and other advances in γδ T cell biology in the context of their being the major initial IL-17 producers in acute infection.

Exploring the RNA world in hematopoietic cells through the lens of RNA-binding proteins

The discovery of microRNAs has renewed interest in posttranscriptional modes of regulation, fueling an emerging view of a rich RNA world within our cells that deserves further exploration. Much work has gone into elucidating genetic regulatory networks that orchestrate gene expression programs and direct cell fate decisions in the hematopoietic system. However, the focus has been to elucidate signaling pathways and transcriptional programs. To bring us one step closer to reverse engineering the molecular logic of cellular differentiation, it will be necessary to map posttranscriptional circuits as well and integrate them in the context of existing network models. In this regard, RNA-binding proteins (RBPs) may rival transcription factors as important regulators of cell fates and represent a tractable opportunity to connect the RNA world to the proteome. ChIP-seq has greatly facilitated genome-wide localization of DNA-binding proteins, helping us to understand genomic regulation at a systems level. Similarly, technological advances such as CLIP-seq allow transcriptome-wide mapping of RBP binding sites, aiding us to unravel posttranscriptional networks. Here, we review RBP-mediated posttranscriptional regulation, paying special attention to findings relevant to the immune system. As a prime example, we highlight the RBP Lin28B, which acts as a heterochronic switch between fetal and adult lymphopoiesis.

The aryl hydrocarbon receptor in innate T cell immunity

Recent studies highlight an important role of the aryl hydrocarbon receptor (AhR) at mucosal barriers. Surprisingly, activation of the AhR, required for the maintenance of lymphocytes as well as lymphoid architecture, can be achieved via cues derived from the external environment. This environment contains both beneficial and harmful microorganisms as well as a diverse array of compounds, and the epithelia must offer very sophisticated levels of defence. This is achieved via multifaceted immune recognition diversity and cellular complexity. Mucosal associated tissues, particularly in the gastrointestinal tract, constitute a complex immune organ for local lymphocytes and contain highly organised lymphoid structures. We will discuss the recent observations concerning the AhR in relation to the function and maintenance of innate T cells, with focus on γδ T cells found enriched at epithelial barriers.

The gut mucosal immune system in the neonatal period

Invasive sepsis in the newborn period is a major cause of childhood morbidity and mortality worldwide. The infant immune system undoubtedly differs intrinsically from the mature adult immune system. Current understanding is that the newborn infant immune system displays a range of competencies and is developing rather than deficient. The infant gut mucosal immune system is complex and displays a plethora of phenotypic and functional irregularities that may be clinically important. Various factors affect and modulate the infant gut mucosal immune system: components of the intestinal barrier, the infant gut microbiome, nutrition and the maternal-infant hybrid immune system. Elucidation of the phenotypic distribution of immune cells, their functional significance and the mucosa-specific pathways used by these cells is essential to the future of research in the field of infant immunology.

Commensal microbiome effects on mucosal immune system development in the ruminant gastrointestinal tract

Commensal microflora play many roles within the mammalian gastrointestinal tract (GIT) that benefit host physiology by way of direct or indirect interactions with mucosal surfaces. Commensal flora comprises members across all microbial phyla, although predominantly bacterial, with population dynamics varying with host species, genotype, and environmental factors. Little is known, however, about the complex mechanisms regulating host-commensal interactions that underlie this mutually beneficial relationship and how alterations in the microbiome may influence host development and susceptibility to infection. Research into the gut microbiome has intensified as it becomes increasingly evident that symbiont-host interactions have a significant impact on mucosal immunity and health. Furthermore, evidence that microbial populations vary significantly throughout the GIT suggest that regional differences in the microbiome may also influence immune function within distinct compartments of the GIT. Postpartum colonization of the GIT has been shown to have a direct effect on mucosal immune system development, but information is limited regarding regional effects of the microbiome on the development, activation, and maturation of the mucosal immune system. This review discusses factors influencing the colonization and establishment of the microbiome throughout the GIT of newborn calves and the evidence that regional differences in the microbiome influence mucosal immune system development and maturation. The implications of this complex interaction are also discussed in terms of possible effects on responses to enteric pathogens and vaccines.

Six-of-the-best: unique contributions of γδ T cells to immunology

γδ T cells are a unique and conserved population of lymphocytes that have been the subject of a recent explosion of interest owing to their essential contributions to many types of immune response and immunopathology. But what does the integration of recent and long-established studies really tell us about these cells and their place in immunology? The time is ripe to consider the evidence for their unique and crucial functions. We conclude that whereas B cells and αβ T cells are commonly thought to contribute primarily to the antigen-specific effector and memory phases of immunity, γδ T cells are distinct in that they combine conventional adaptive features (inherent in their T cell receptors and pleiotropic effector functions) with rapid, innate-like responses that can place them in the initiation phase of immune reactions. This underpins a revised perspective on lymphocyte biology and the regulation of immunogenicity.

Comparative biology of γδ T cell function in humans, mice, and domestic animals

γδ T cells are a functionally heterogeneous population and contribute to many early immune responses. The majority of their activity is described in humans and mice, but the immune systems of all jawed vertebrates include the γδ T cell lineage. Although some aspects of γδ T cells vary between species, critical roles in early immune responses are often conserved. Common features of γδ T cells include innate receptor expression, antigen presentation, cytotoxicity, and cytokine production. Herein we compare studies describing these conserved γδ T cell functions and other, potentially unique, functions. γδ T cells are well documented for their potential immunotherapeutic properties; however, these proposed therapies are often focused on human diseases and the mouse models thereof. This review consolidates some of these studies with those in other animals to provide a consensus for the current understanding of γδ T cell function across species.

Interleukin 7 (IL-7) selectively promotes mouse and human IL-17-producing γδ cells

IL-17-producing CD27(-) γδ cells (γδ(27-) cells) are widely viewed as innate immune cells that make critical contributions to host protection and autoimmunity. However, factors that promote them over IFN-γ-producing γδ(27+) cells are poorly elucidated. Moreover, although human IL-17-producing γδ cells are commonly implicated in inflammation, such cells themselves have proved difficult to isolate and characterize. Here, murine γδ(27-) T cells and thymocytes are shown to be rapidly and substantially expanded by IL-7 in vitro and in vivo. This selectivity owes in substantial part to the capacity of IL-7 to activate STAT3 in such cells. Additionally, IL-7 promotes strong responses of IL-17-producing γδ cells to TCR agonists, thus reemphasizing the cells' adaptive and innate potentials. Moreover, human IL-17-producing γδ cells are also substantially expanded by IL-7 plus TCR agonists. Hence, IL-7 has a conserved potential to preferentially regulate IL-17-producing γδ cells, with both biological and clinical implications.

Understanding the complexity of γδ T-cell subsets in mouse and human

γδ T cells are increasingly recognized as having important functional roles in a range of disease scenarios such as infection, allergy, autoimmunity and cancer. With this has come realization that γδ cells are not a homogeneous population of cells with a single physiological role. Instead, ever increasing complexity in both phenotype and function is being ascribed to γδ cell subsets from various tissues and locations, and in both mouse and human. Here, we review this complexity by describing how diverse γδ cell subsets are generated in the murine thymus, and how these events relate to subsequent γδ subset function in the periphery. We then review the two major γδ cell populations in human, highlighting the several similarities of Vδ1(+) cells to certain murine γδ subsets, and describing the remarkable functional plasticity of human Vδ2(+) cells. A better understanding of this spectrum of γδ cell phenotypes should facilitate more targeted approaches to utilise their tremendous functional potential in the clinic.

Close link between development and function of gamma-delta T cells

Murine γδ T cells develop as the first T-cell lineage within the fetal thymus and disproportionately localize in mucosal tissues such as lung, skin, uterus, and intestine of adult mice. These unique developmental features and distribution patterns of γδ T cells enable rapid functioning against various insults from pathogens. γδ T cells are also able to respond to local inflammation and consequently regulate the pathogenesis of autoimmune disorders and development of tumors in mice and humans. Hence, it is clinically important to understand the mechanisms that regulate γδ T cell functions. Recent evidence has shown that generations of effector γδ T cell subsets producing IFN-γ, IL-4, and IL-17 are programmed in the murine thymus before their migration to peripheral tissues. This review outlines our current understanding of the development and function of γδ T cells as they influence both innate and acquired immunity.

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.