Selective regulation of IFN-γ and IL-4 co-producing unconventional T cells by purinergic signaling

Unconventional T cells, including mucosal-associated invariant T (MAIT), natural killer T (NKT), and gamma-delta T (γδT) cells, comprise distinct T-bet+, IFN-γ+ and RORγt+, IL-17+ subsets which play differential roles in health and disease. NKT1 cells are susceptible to ARTC2-mediated P2X7 receptor (P2RX7) activation, but the effects on other unconventional T-cell types are unknown. Here, we show that MAIT, γδT, and NKT cells express P2RX7 and are sensitive to P2RX7-mediated cell death. Mouse peripheral T-bet+ MAIT1, γδT1, and NKT1 cells, especially in liver, co-express ARTC2 and P2RX7. These markers could be further upregulated upon exposure to retinoic acid. Blocking ARTC2 or inhibiting P2RX7 protected MAIT1, γδT1, and NKT1 cells from cell death, enhanced their survival in vivo, and increased the number of IFN-γ-secreting cells without affecting IL-17 production. Importantly, this revealed the existence of IFN-γ and IL-4 co-producing unconventional T-cell populations normally lost upon isolation due to ARTC2/P2RX7-induced death. Administering extracellular NAD in vivo activated this pathway, depleting P2RX7-sensitive unconventional T cells. Our study reveals ARTC2/P2RX7 as a common regulatory axis modulating the unconventional T-cell compartment, affecting the viability of IFN-γ- and IL-4-producing T cells, offering important insights to facilitate future studies into how these cells can be regulated in health and disease.

Long-lived central memory γδ T cells confer protection against murine cytomegalovirus reinfection

The involvement of γδ TCR-bearing lymphocytes in immunological memory has gained increasing interest due to their functional duality between adaptive and innate immunity. γδ T effector memory (TEM) and central memory (TCM) subsets have been identified, but their respective roles in memory responses are poorly understood. In the present study, we used subsequent mouse cytomegalovirus (MCMV) infections of αβ T cell deficient mice in order to analyze the memory potential of γδ T cells. As for CMV-specific αβ T cells, MCMV induced the accumulation of cytolytic, KLRG1+CX3CR1+ γδ TEM that principally localized in infected organ vasculature. Typifying T cell memory, γδ T cell expansion in organs and blood was higher after secondary viral challenge than after primary infection. Viral control upon MCMV reinfection was prevented when masking γδ T-cell receptor, and was associated with a preferential amplification of private and unfocused TCR δ chain repertoire composed of a combination of clonotypes expanded post-primary infection and, more unexpectedly, of novel expanded clonotypes. Finally, long-term-primed γδ TCM cells, but not γδ TEM cells, protected T cell-deficient hosts against MCMV-induced death upon adoptive transfer, probably through their ability to survive and to generate TEM in the recipient host. This better survival potential of TCM cells was confirmed by a detailed scRNASeq analysis of the two γδ T cell memory subsets which also revealed their similarity to classically adaptive αβ CD8 T cells. Overall, our study uncovered memory properties of long-lived TCM γδ T cells that confer protection in a chronic infection, highlighting the interest of this T cell subset in vaccination approaches.

Perinatal thymic-derived CD8αβ-expressing γδ T cells are innate IFN-γ producers that expand in IL-7R-STAT5B-driven neoplasms

The contribution of γδ T cells to immune responses is associated with rapid secretion of interferon-γ (IFN-γ). Here, we show a perinatal thymic wave of innate IFN-γ-producing γδ T cells that express CD8αβ heterodimers and expand in preclinical models of infection and cancer. Optimal CD8αβ+ γδ T cell development is directed by low T cell receptor signaling and through provision of interleukin (IL)-4 and IL-7. This population is pathologically relevant as overactive, or constitutive, IL-7R-STAT5B signaling promotes a supraphysiological accumulation of CD8αβ+ γδ T cells in the thymus and peripheral lymphoid organs in two mouse models of T cell neoplasia. Likewise, CD8αβ+ γδ T cells define a distinct subset of human T cell acute lymphoblastic leukemia pediatric patients. This work characterizes the normal and malignant development of CD8αβ+ γδ T cells that are enriched in early life and contribute to innate IFN-γ responses to infection and cancer.

A monoclonal Trd chain supports the development of the complete set of functional γδ T cell lineages

The clonal selection theory describes key features of adaptive immune responses of B and T cells. For αβ T cells and B cells, antigen recognition and selection principles are known at a detailed molecular level. The precise role of the antigen receptor in γδ T cells remains less well understood. To better understand the role of the γδ T cell receptor (TCR), we generate an orthotopic TCRδ transgenic mouse model. We demonstrate a multi-layered functionality of γδ TCRs and diverse roles of CDR3δ-mediated selection during γδ T cell development. Whereas epithelial populations using Vγ5 or Vγ7 chains are almost unaffected in their biology in the presence of the transgenic TCRδ chain, pairing with Vγ1 positively selects γδ T cell subpopulations with distinct programs in several organs, thereby distorting the repertoire. In conclusion, our data support dictation of developmental tropism together with adaptive-like recognition principles in a single antigen receptor.

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.

The role of γδ T17 cells in cardiovascular disease

Due to the ability of γδ T cells to bridge adaptive and innate immunity, γδ T cells can respond to a variety of molecular cues and acquire the ability to induce a variety of cytokines such as IL-17 family, IFN-γ, IL-4, and IL-10. IL-17⁺ γδ T cells (γδ T17 cells) populations have recently received considerable interest as they are the major early source of IL-17A in many immune response models. However, the exact mechanism of γδ T17 cells is still poorly understood, especially in the context of cardiovascular disease (CVD). CVD is the leading cause of death in the world, and it tends to be younger. Here, we offer a review of the cardiovascular inflammatory and immune functions of γδ T17 cells in order to understand their role in CVD, which may be the key to developing new clinical applications.

A Distinctive γδ T Cell Repertoire in NOD Mice Weakens Immune Regulation and Favors Diabetic Disease

Previous studies in mice and humans suggesting that γδ T cells play a role in the development of type 1 diabetes have been inconsistent and contradictory. We attempted to resolve this for the type 1 diabetes-prone NOD mice by characterizing their γδ T cell populations, and by investigating the functional contributions of particular γδ T cells subsets, using Vγ-gene targeted NOD mice. We found evidence that NOD Vγ4+ γδ T cells inhibit the development of diabetes, and that the process by which they do so involves IL-17 production and/or promotion of regulatory CD4+ αβ T cells (Tregs) in the pancreatic lymph nodes. In contrast, the NOD Vγ1+ cells promote diabetes development. Enhanced Vγ1+ cell numbers in NOD mice, in particular those biased to produce IFNγ, appear to favor diabetic disease. Within NOD mice deficient in particular γδ T cell subsets, we noted that changes in the abundance of non-targeted T cell types also occurred, which varied depending upon the γδ T cells that were missing. Our results indicate that while certain γδ T cell subsets inhibit the development of spontaneous type 1 diabetes, others exacerbate it, and they may do so via mechanisms that include altering the levels of other T cells.

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.

Gamma Delta T Cells and Their Pathogenic Role in Psoriasis

γδT cells are an unconventional population of T lymphocytes that play an indispensable role in host defense, immune surveillance, and homeostasis of the immune system. They display unique developmental, distributional, and functional patterns and rapidly respond to various insults and contribute to diverse diseases. Although γδT cells make up only a small portion of the total T cell pool, emerging evidence suggest that aberrantly activated γδT cells may play a role in the pathogenesis of psoriasis. Dermal γδT cells are the major IL-17-producing cells in the skin that respond to IL-23 stimulation. Furthermore, γδT cells exhibit memory-cell-like characteristics that mediate repeated episodes of psoriatic inflammation. This review discusses the differentiation, development, distribution, and biological function of γδT cells and the mechanisms by which they contribute to psoriasis. Potential therapeutic approaches targeting these cells in psoriasis have also been detailed.

Recognition of synthetic polyanionic ligands underlies "spontaneous" reactivity of Vγ1 γδTCRs

Although γδTCRs were discovered more than 30 yr ago, principles of antigen recognition by these receptors remain unclear and the nature of these antigens is largely elusive. Numerous studies reported that T cell hybridomas expressing several Vγ1-containing TCRs, including the Vγ1Vδ6 TCR of γδNKT cells, spontaneously secrete cytokines. This property was interpreted as recognition of a self-ligand expressed on the hybridoma cells themselves. Here, we revisited this finding using a recently developed reporter system and live single cell imaging. We confirmed strong spontaneous signaling by Vγ1Vδ6 and related TCRs, but not by TCRs from several other γδ or innate-like αβ T cells, and demonstrated that both γ and δ chains contributed to this reactivity. Unexpectedly, live single cell imaging showed that activation of this signaling did not require any interaction between cells. Further investigation revealed that the signaling is instead activated by interaction with negatively charged surfaces abundantly present under regular cell culture conditions and was abrogated when noncharged cell culture vessels were used. This mode of TCR signaling activation was not restricted to the reporter cell lines, as interaction with negatively charged surfaces also triggered TCR signaling in ex vivo Vγ1 γδ T cells. Taken together, these results explain long-standing observations on the spontaneous reactivity of Vγ1Vδ6 TCR and demonstrate an unexpected antigen presentation-independent mode of TCR activation by a spectrum of chemically unrelated polyanionic ligands.

γδ T Cell Update: Adaptate Orchestrators of Immune Surveillance

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

Regulation of γδ T Cell Effector Diversification in the Thymus

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

Styk1 is specifically expressed in NK1.1+ lymphocytes including NK, γδ T, and iNKT cells in mice, but is dispensable for their ontogeny and function

Innate T cells, NK cells, and innate-like lymphocytes (ILCs) share transcriptional signatures that translate into overlapping developmental and functional programs. A prominent example for genes that are highly expressed in NK cells but not in ILCs is serine-threonine-tyrosine kinase 1 (Styk1 encoded by Styk1). We found Styk1 to be specifically expressed in lymphocytes positive for Killer cell lectin-like receptor subfamily B, member 1, also known as CD161 or NK1.1, i.e. in NK cell, αβ iNKT, and γδ NKT cell lineages. To investigate the role of Styk1 in the development and function of NK1.1⁺ innate T-cell subsets, we generated and analyzed a novel Styk1^null mutant mouse line. Furthermore, we validated Styk1 expression in γδ NKT cells and in thymic, but not in peripheral invariant αβ iNKT cells through ex vivo analysis of a concomitantly generated transgenic Styk1 reporter mouse line. Despite the very specific expression of Styk1 in NK cells, γδ NKT cells, and thymic αβ iNKT, its absence did not alter homeostasis and function of these lineages. Thus, Styk1 expression is specific for NK cells and selected NK-like innate T-cell subsets, but dispensable for their development and function.

Developmental origins of murine γδ T-cell subsets

Murine γδ T cells display diverse responses to pathogens and tumours through early provision of pro-inflammatory cytokines such as interleukin-17A (IL-17) and interferon-γ (IFN-γ). Although it is now clear that acquisition of these cytokine-secreting effector fates is to a great extent developmentally pre-programmed in the thymus, the stages through which γδ progenitor cells transition, and the underlying mechanistic processes that govern these commitment events, are still largely unclear. Here, we review recent progress in the field, with particular consideration of how TCR-γδ signalling impacts on developmental programmes initiated before TCR-γδ expression.

Id3 Restricts γδ NKT Cell Expansion by Controlling Egr2 and c-Myc Activity

γδ NKT cells are neonatal-derived γδ T lymphocytes that are grouped together with invariant NKT cells based on their shared innate-like developmental program characterized by the transcription factor PLZF (promyelocytic leukemia zinc finger). Previous studies have demonstrated that the population size of γδ NKT cells is tightly controlled by Id3-mediated inhibition of E-protein activity in mice. However, how E proteins promote γδ NKT cell development and expansion remains to be determined. In this study, we report that the transcription factor Egr2, which also activates PLZF expression in invariant NKT cells, is essential for regulating γδ NKT cell expansion. We observed a higher expression of Egr family genes in γδ NKT cells compared with the conventional γδ T cell population. Loss of function of Id3 caused an expansion of γδ NKT cells, which is accompanied by further upregulation of Egr family genes as well as PLZF. Deletion of Egr2 in Id3-deficient γδ NKT cells prevented cell expansion and blocked PLZF upregulation. We further show that this Egr2-mediated γδ NKT cell expansion is dependent on c-Myc. c-Myc knockdown attenuated the proliferation of Id3-deficient γδ NKT cells, whereas c-Myc overexpression enhanced the proliferation of Id3/Egr2-double-deficient γδ NKT cells. Therefore, our data reveal a regulatory circuit involving Egr2-Id3-E2A, which normally restricts the population size of γδ NKT cells by adjusting Egr2 dosage and c-Myc expression.

Thymic Determinants of γδ T Cell Differentiation

γd T cells have emerged as major sources of the proinflammatory cytokines interleukin-17 (IL-17) and interferon-γ (IFNγ) in multiple models of infection, cancer and autoimmune disease. However, unlike their αβ T cell counterparts that require peripheral activation for effector cell differentiation, γδ T cells instead can be 'developmentally programmed' in the thymus to generate discrete γδ T cell effector subsets with distinctive molecular signatures. Nonetheless, recent studies have presented conflicting viewpoints on the signals involved in thymic γδ T cell development and differentiation, namely on the role of both T cell receptor (TCR)-dependent and TCR-independent factors. Here we review the current data and the ongoing controversies.

MicroRNA-181a/b-1 Is Not Required for Innate γδ NKT Effector Cell Development

Thymic development of αβ T lymphocytes into invariant natural killer (NK) T cells depends on their selection via agonistic lipid antigen presented by CD1d. If successful, newly selected NKT cells gain effector functions already in the thymus. Some γδ T cell subsets also acquire effector functions in the thymus. However, it is not clear whether agonistic TCR stimulation is involved in thymic γδ T cell selection and development. Here we combine two genetic models to address this question. MiR-181a/b-1^–/–mice, which show impaired agonistic T cell selection of invariant αβ NKT cells, were crossed to Tcrd-H2BeGFP reporter mice to monitor selection, intra-thymic expansion and differentiation of γδ T cells. We found that miR-181a/b-1-deficiency had no effect on numbers of thymic γδ T cell or on their differentiation towards an IL-17- or IFN-γ-producing effector phenotype. Also, the composition of peripheral lymph node γδ T cells was not affected by miR-181a/b-1-deficiency. Dendritic epidermal γδ T cells were normally present in knock-out animals. However, we observed elevated frequencies and numbers of γδ NKT cells in the liver, possibly because γδ NKT cells can expand and replace missing αβ NKT cells in peripheral niches. In summary, we investigated the role of miR-181a/b-1 for selection, intrathymic development and homeostasis of γδ T cells. We conclude that miR-181a/b-1-dependent modulation of T cell selection is not critically required for innate development of γδ NKT cells or of any other γδ T cell subtypes.

Differential dependence on nuclear factor-κB-inducing kinase among natural killer T-cell subsets in their development

Natural killer T cells (NKT cells) are comprised of several subsets. However, the possible differences in their developmental mechanisms have not been fully investigated. To evaluate the dependence of some NKT subpopulations on nuclear factor-κB-inducing kinase (NIK) for their generation, we analysed the differentiation of NKT cells, dividing them into subsets in various tissues of alymphoplasia (aly/aly), a mutant mouse strain that lacks functional NIK. The results indicated that the efficient differentiation of both invariant NKT (iNKT) and non-iNKT cells relied on NIK expression in non-haematopoietic cells; however, the dependence of non-iNKT cells was lower than that of iNKT cells. Especially, the differentiation of CD8(+) non-iNKT cells was markedly resistant to the aly mutation. The proportion of two other NKT cell subsets, NK1.1(+) γδ T cells and NK1.1(-) iNKT cells, was also significantly reduced in aly/aly mice, and this defect in their development was reversed in wild-type host mice given aly/aly bone marrow cells. In exerting effector functions, NIK in NKT-αβ cells appeared dispensable, as NIK-deficient NKT-αβ cells could secrete interleukin-4 or interferon-γ and exhibit cytolytic activity at a level comparable to that of aly/+ NKT-αβ cells. Collectively, these results imply that the NIK in thymic stroma may be critically involved in the differentiation of most NKT cell subsets (although the level of NIK dependence may vary among the subsets), and also that NIK in NKT-αβ cells may be dispensable for their effector function.

IFN-γ-producing and IL-17-producing γδ T cells differentiate at distinct developmental stages in murine fetal thymus

γδ T cells develop at the double-negative (DN) 2 and DN3 stages and acquire functions to produce IL-17 and IFN-γ in fetal thymus. However, the relationship between differentiation stages and their functions was unclear. In this study, we found that, although IFN-γ-producing and IL-17-producing γδ T cells developed from DN2 cells, only IFN-γ-producing γδ T cells developed from DN3 cells, indicating the direct generation of IL-17-producing γδ T cells from the DN2 stage, not through the DN3 stage. Single-cell analysis revealed that DN2 cells contained heterogeneous γδ T cell precursors with or without an ability to develop IL-17 producers. Inactivation of B cell leukemia/lymphoma 11b, a zinc finger transcription factor responsible for transition from early to late stages of DN2 cells, completely abrogated the development of IL-17-producing γδ T cells, although a unique subset of IFN-γ-producing γδ T cells expressing a high level of promyelocytic leukemia zinc finger was able to develop. Thus, our results reveal that γδ T cells are functionally differentiated to IFN-γ and IL-17 producers at different developmental stages in fetal thymus.