The inter-relatedness and interdependence of mouse T cell receptor γδ+ and αβ+ cells

Biological variability of human intraepithelial lymphocytes throughout the human gastrointestinal tract in health and coeliac disease

BACKGROUND

Intraepithelial lymphocytes are the first line of defence of the human intestinal immune system. Besides, their composition is altered on patients with coeliac disease (CD), so they are considered as biomarkers with utility on their diagnose and/or monitoring. Our aim is to address their variability through the human gastrointestinal tract in health and characterized them in further depth in the coeliac duodenum.

METHODS

Intraepithelial lymphocytes were isolated from human gastric, duodenal, ileal and colonic biopsies, then stained with specific antibodies and acquired by flow cytometry.

RESULTS

Our results confirmed that the profile of Intraepithelial lymphocytes change through the length of the human gastrointestinal tract. Besides and given the central role that Interleukin-15 (IL-15) elicits on CD pathogenesis; we also assessed the expression of its receptor revealing that there was virtually no functional IL-15 receptor on duodenal Intraepithelial lymphocytes. Nevertheless and contrary to our expectations, the active IL-15 receptor was not increased either on Intraepithelial lymphocytes from CD patients.

CONCLUSIONS

IL-15 might require additional stimulus to activate intraepithelial lymphocytes. These findings may provide novel tools to aid on a CD diagnosis and/or monitoring, at the time that provide the bases to perform functional studies in order of getting a deeper insight in the specific function that Intraepithelial lymphocytes elicit on CD pathogenesis.

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.

γδ T cells: origin and fate, subsets, diseases and immunotherapy

The intricacy of diseases, shaped by intrinsic processes like immune system exhaustion and hyperactivation, highlights the potential of immune renormalization as a promising strategy in disease treatment. In recent years, our primary focus has centered on γδ T cell-based immunotherapy, particularly pioneering the use of allogeneic Vδ2+ γδ T cells for treating late-stage solid tumors and tuberculosis patients. However, we recognize untapped potential and optimization opportunities to fully harness γδ T cell effector functions in immunotherapy. This review aims to thoroughly examine γδ T cell immunology and its role in diseases. Initially, we elucidate functional differences between γδ T cells and their αβ T cell counterparts. We also provide an overview of major milestones in γδ T cell research since their discovery in 1984. Furthermore, we delve into the intricate biological processes governing their origin, development, fate decisions, and T cell receptor (TCR) rearrangement within the thymus. By examining the mechanisms underlying the anti-tumor functions of distinct γδ T cell subtypes based on γδTCR structure or cytokine release, we emphasize the importance of accurate subtyping in understanding γδ T cell function. We also explore the microenvironment-dependent functions of γδ T cell subsets, particularly in infectious diseases, autoimmune conditions, hematological malignancies, and solid tumors. Finally, we propose future strategies for utilizing allogeneic γδ T cells in tumor immunotherapy. Through this comprehensive review, we aim to provide readers with a holistic understanding of the molecular fundamentals and translational research frontiers of γδ T cells, ultimately contributing to further advancements in harnessing the therapeutic potential of γδ T cells.

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.

Development of γδ T cells in the thymus - A human perspective

γδ T cells are increasingly emerging as crucial immune regulators that can take on innate and adaptive roles in the defence against pathogens. Although they arise within the thymus from the same hematopoietic precursors as conventional αβ T cells, the development of γδ T cells is less well understood. In this review, we focus on summarising the current state of knowledge about the cellular and molecular processes involved in the generation of γδ T cells in human.

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.

Innate and adaptive γδ T cells: How, when, and why

γδ T cells comprise the third cell lineage of lymphocytes that use, like αβ T cells and B cells, V(D)J gene rearrangement with the potential to generate a highly diverse T cell receptor (TCR) repertoire. There is no obvious conservation of γδ T cell subsets (based on TCR repertoire and/or function) between mice and human, leading to the notion that human and mouse γδ T cells are highly different. In this review, we focus on human γδ T cells, building on recent studies using high-throughput sequencing to analyze the TCR repertoire in various settings. We make then the comparison with mouse γδ T cell subsets highlighting the similarities and differences and describe the remarkable changes during lifespan of innate and adaptive γδ T cells. Finally, we propose mechanisms contributing to the generation of innate versus adaptive γδ T cells. We conclude that key elements related to the generation of the γδ TCR repertoire and γδ T cell activation/development are conserved between human and mice, highlighting the similarities between these two species.

Mechanisms of activation of innate-like intraepithelial T lymphocytes

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

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.

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.

Epithelial IL-15 Is a Critical Regulator of γδ Intraepithelial Lymphocyte Motility within the Intestinal Mucosa

Intraepithelial lymphocytes (IELs) expressing the γδ TCR (γδ IELs) provide continuous surveillance of the intestinal epithelium. However, the mechanisms regulating the basal motility of these cells within the epithelial compartment have not been well defined. We investigated whether IL-15 contributes to γδ IEL localization and migratory behavior in addition to its role in IEL differentiation and survival. Using advanced live cell imaging techniques in mice, we find that compartmentalized overexpression of IL-15 in the lamina propria shifts the distribution of γδ T cells from the epithelial compartment to the lamina propria. This mislocalization could be rescued by epithelial IL-15 overexpression, indicating that epithelial IL-15 is essential for γδ IEL migration into the epithelium. Furthermore, in vitro analyses demonstrated that exogenous IL-15 stimulates γδ IEL migration into cultured epithelial monolayers, and inhibition of IL-2Rβ significantly attenuates the basal motility of these cells. Intravital microscopy showed that impaired IL-2Rβ signaling induced γδ IEL idling within the lateral intercellular space, which resulted in increased early pathogen invasion. Similarly, the redistribution of γδ T cells to the lamina propria due to local IL-15 overproduction also enhanced bacterial translocation. These findings thus reveal a novel role for IL-15 in mediating γδ T cell localization within the intestinal mucosa and regulating γδ IEL motility and patrolling behavior as a critical component of host defense.

Increased TCR signal strength in DN thymocytes promotes development of gut TCRαβ(+)CD8αα(+) intraepithelial lymphocytes

CD4⁽⁺⁾CD8⁽⁺⁾ “double positive” (DP) thymocytes differentiate into diverse αβ T cell sub-types using mechanistically distinct programs. For example, conventional αβ T cells develop from DP cells after partial-agonist T cell receptor (TCR) interactions with self-peptide/MHC, whereas unconventional αβ T cells, such as TCRαβ⁽⁺⁾CD8αα⁽⁺⁾ intraepithelial lymphocytes (IELs), require full-agonist TCR interactions. Despite this, DP cells appear homogeneous, and it remains unclear how distinct TCR signalling instructs distinct developmental outcomes. Moreover, whether TCR signals at earlier stages of development, for example in CD4⁽⁻⁾CD8⁽⁻⁾ double negative (DN) cells, impact on later fate decisions is presently unknown. Here, we assess four strains of mice that display altered TCR signal strength in DN cells, which correlates with altered generation of unconventional TCRαβ⁽⁺⁾CD8αα⁽⁺⁾ IELs. FVB/n mice (compared to C57BL/6 animals) and mice with altered preTCRα (pTα) expression, both displayed weaker TCR signalling in DN cells, an inefficient DN-to-DP transition, and reduced contribution of TCRαβ⁽⁺⁾CD8αα⁽⁺⁾ IELs to gut epithelium. Conversely, TCRαβ⁽⁺⁾CD8αα⁽⁺⁾ IEL development was favoured in mice with increased TCR signal strength in DN cells. Collectively, these data suggest TCR signal strength in DN cells directly impacts on subsequent DP cell differentiation, fundamentally altering the potential of thymocyte progenitors to adopt conventional versus unconventional T cell fates.

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.

Gamma delta T-cell differentiation and effector function programming, TCR signal strength, when and how much?

γδ T-cells boast an impressive functional repertoire that can paint them as either champions or villains depending on the environmental and immunological cues. Understanding the function of the various effector γδ subsets necessitates tracing the developmental program of these subsets, including the point of lineage bifurcation from αβ T-cells. Here, we review the importance of signals from the T-cell receptor (TCR) in determining αβ versus γδ lineage fate, and further discuss how the molecular components of this pathway may influence the developmental programming of γδ T-cells functional subsets. Additionally, we discuss the role of temporal windows in restricting the development of IL-17 producing γδ T-cell subtypes, and explore whether fetal and adult hematopoietic progenitors maintain the same potential for giving rise to this important subset.

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-resident versus recruited γδ T cell response to cutaneous vaccinia virus infection

The study of T cell immunity at barrier surfaces has largely focused on T cells bearing the αβ TCR. However, T cells that express the γδ TCR are disproportionately represented in peripheral tissues of mice and humans, suggesting they too may play an important role responding to external stimuli. In this article, we report that, in a murine model of cutaneous infection with vaccinia virus, dermal γδ T cell numbers increased 10-fold in the infected ear and resulted in a novel γδ T cell population not found in naive skin. Circulating γδ T cells were specifically recruited to the site of inflammation and differentially contributed to dermal populations based on their CD27 expression. Recruited γδ T cells, the majority of which were CD27(+), were granzyme B(+) and made up about half of the dermal population at the peak of the response. In contrast, recruited and resident γδ T cell populations that made IL-17 were CD27(-). Using a double-chimera model that can discriminate between the resident dermal and recruited γδ T cell populations, we demonstrated their divergent functions and contributions to early stages of tissue inflammation. Specifically, the loss of the perinatal thymus-derived resident dermal population resulted in decreased cellularity and collateral damage in the tissue during viral infection. These findings have important implications for our understanding of immune coordination at barrier surfaces and the contribution of innate-like lymphocytes on the front lines of immune defense.

Elevated T cell receptor signaling identifies a thymic precursor to the TCRαβ(+)CD4(-)CD8β(-) intraepithelial lymphocyte lineage

The origin and developmental pathway of intestinal T cell receptor αβ(+) CD4(-)CD8β(-) intraepithelial lymphocytes (unconventional iIELs), a major population of innate-like resident cytolytic T cells, have remained elusive. By cloning and expressing several TCRs isolated from unconventional iIELs, we identified immature CD4(lo)CD8(lo)(DP(lo))CD69(hi)PD-1(hi) thymocytes as the earliest postsignaling precursors for these cells. Although these precursors displayed multiple signs of elevated TCR signaling, a sizeable fraction of them escaped deletion to selectively engage in unconventional iIEL differentiation. Conversely, TCRs cloned from DP(lo)CD69(hi)PD-1(hi) thymocytes, a population enriched in autoreactive thymocytes, selectively gave rise to unconventional iIELs upon transgenic expression. Thus, the unconventional iIEL precursor overlaps with the DP(lo) population undergoing negative selection, indicating that, concomitant with the downregulation of both CD4 and CD8 coreceptors, a balance between apoptosis and survival signals results in outcomes as divergent as clonal deletion and differentiation to the unconventional iIEL lineage.

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.

Significant involvement of nuclear factor-κB-inducing kinase in proper differentiation of αβ and γδ T cells

Nuclear factor-κB-inducing kinase (NIK) is known to play a critical role in maintaining proper immune function. This is exemplified in the spontaneous mutant mouse lacking functional NIK, alymphoplasia (aly), which is simultaneously immune-compromised and autoimmune-prone. To investigate the role of NIK in αβ T-cell repertoire formation, we analysed T-cell development in aly/aly mice bearing a transgenic T-cell receptor (TCR). Although there were no apparent abnormalities in the mature αβ T cells of non-transgenic aly/aly mice, the maturation efficiency of idiotype(high+) T cells in the TCR-transgenic mice was lower in aly/aly mice compared with those found in aly/+ mice, suggesting that the mature αβ T-cell repertoire could be altered by the absence of functional NIK. In one strain of TCR-transgenic aly/aly mice with a negatively selecting H-2 background, the proportion of CD8(low+) idiotype(high+) cells, which are thought to potentially represent the γδ lineage of T cells, was markedly decreased. When the γδ T cells in non-transgenic aly/aly mice were investigated, the proportion of γδ T cells in the peripheral organs of aly/aly mice was found to be one-half to one-fifth of those in aly/+ mice. Analyses of bone marrow chimera mice indicated that NIK in host cells, rather than in donor cells was important for generating a normal number of peripheral γδ T cells. Collectively, these results suggest that NIK could be involved in thymic positive selection of some αβ T cells and that NIK in non-haematopoietic cells is important for the optimal development and/or maintenance of γδ T cells.

The origin and fate of γδT cell subsets

Recent experiments indicate that in contrast to αβT cells, γδT cell effector functions are largely preprogrammed in the thymus during fetal life. However the thymus also exports juvenile γδT cells that can mature and be polarized in the periphery. How these developmental pathways are regulated and how much they contribute to the γδT cell effector pool is unclear. Here we discuss recent advances in the understanding of γδT cell subset development, with particular focus on IL-17-producing γδT cells and their beneficial and pathogenic roles in immunity.

Extensive expansion of primary human gamma delta T cells generates cytotoxic effector memory cells that can be labeled with Feraheme for cellular MRI

Gamma delta T cells (GDTc) comprise a small subset of cytolytic T cells shown to kill malignant cells in vitro and in vivo. We have developed a novel protocol to expand GDTc from human blood whereby GDTc were initially expanded in the presence of alpha beta T cells (ABTc) that were then depleted prior to use. We achieved clinically relevant expansions of up to 18,485-fold total GDTc, with 18,849-fold expansion of the Vδ1 GDTc subset over 21 days. ABTc depletion yielded 88.1 ± 4.2 % GDTc purity, and GDTc continued to expand after separation. Immunophenotyping revealed that expanded GDTc were mostly CD27-CD45RA- and CD27-CD45RA+ effector memory cells. GDTc cytotoxicity against PC-3M prostate cancer, U87 glioblastoma and EM-2 leukemia cells was confirmed. Both expanded Vδ1 and Vδ2 GDTc were cytotoxic to PC-3M in a T cell antigen receptor- and CD18-dependent manner. We are the first to label GDTc with ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles for cellular MRI. Using protamine sulfate and magnetofection, we achieved up to 40 % labeling with clinically approved Feraheme (Ferumoxytol), as determined by enumeration of Perls' Prussian blue-stained cytospins. Electron microscopy at 2,800× magnification verified the presence of internalized clusters of iron oxide; however, high iron uptake correlated negatively with cell viability. We found improved USPIO uptake later in culture. MRI of GDTc in agarose phantoms was performed at 3 Tesla. The signal-to-noise ratios for unlabeled and labeled cells were 56 and 21, respectively. Thus, Feraheme-labeled GDTc could be readily detected in vitro via MRI.

Differential Ly49e expression pathways in resting versus TCR-activated intraepithelial γδ T cells

The Ly49 NK receptor family in mice is composed of several members that recognize MHC class I (MHC-I) or MHC-I-related molecules. We and others have shown before that Ly49E is a unique member, with a different expression pattern on NK cells and being triggered by the non-MHC-I-related protein urokinase plasminogen activator. Among the entire Ly49 receptor family, Ly49E is the only Ly49 member expressed by epidermal-localized γδ T cells and their fetal thymic TCRγδ precursors, and it is the most abundantly expressed member on intestinal intraepithelial γδ T cell lymphocytes. In this study, we provide mechanistic insights into the regulation of Ly49e expression in γδ T cells. First, we demonstrate that TCR-mediated activation of intraepithelial γδ T cells significantly increases Ly49E expression. This results from de novo Ly49E expression and is highly selective, because no other Ly49 family members are induced. TCR-mediated Ly49E induction is a conserved feature of skin- and gut-residing intraepithelial-localized γδ T cell subsets, whereas it is not observed in spleen γδ T cells. By investigating Ly49e promoter activities and lymphotoxin (LT) αβ dependency in resting versus TCR-activated intraepithelial γδ T cells, we reveal two separate regulatory pathways for Ly49E expression, as follows: a LTαβ-dependent pathway leading to basal Ly49E expression in resting cells that is induced by Pro2-mediated Ly49e transcription, and a LTαβ-independent pathway leading to elevated, Pro3-driven Ly49E expression in TCR-stimulated cells.

γδ-T cells: an unpolished sword in human anti-infection immunity

γδ-T cells represent a small population of immune cells, but play an indispensable role in host defenses against exogenous pathogens, immune surveillance of endogenous pathogenesis and even homeostasis of the immune system. Activation and expansion of γδ-T cells are generally observed in diverse human infectious diseases and correlate with their progression and prognosis. γδ-T cells have both 'innate' and 'adaptive' characteristics in the immune response, and their anti-infection activities are mediated by multiple pathways that are under elaborate regulation by other immune components. In this review, we summarize the current state of the literature and the recent advancements in γδ-T cell-mediated immune responses against common human infectious pathogens. Although further investigation is needed to improve our understanding of the characteristics of different γδ-T cell subpopulations under specific conditions, γδ-T cell-based therapy has great potential for the treatment of infectious diseases.

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.

Rank signaling links the development of invariant γδ T cell progenitors and Aire(+) medullary epithelium

The thymic medulla provides a specialized microenvironment for the negative selection of T cells, with the presence of autoimmune regulator (Aire)-expressing medullary thymic epithelial cells (mTECs) during the embryonic-neonatal period being both necessary and sufficient to establish long-lasting tolerance. Here we showed that emergence of the first cohorts of Aire(+) mTECs at this key developmental stage, prior to αβ T cell repertoire selection, was jointly directed by Rankl(+) lymphoid tissue inducer cells and invariant Vγ5(+) dendritic epidermal T cell (DETC) progenitors that are the first thymocytes to express the products of gene rearrangement. In turn, generation of Aire(+) mTECs then fostered Skint-1-dependent, but Aire-independent, DETC progenitor maturation and the emergence of an invariant DETC repertoire. Hence, our data attributed a functional importance to the temporal development of Vγ5(+) γδ T cells during thymus medulla formation for αβ T cell tolerance induction and demonstrated a Rank-mediated reciprocal link between DETC and Aire(+) mTEC maturation.

Epithelial barrier biology: good fences make good neighbours

The external surfaces of the body, such as the skin and the gastrointestinal mucosal membrane, are an important line of defence preventing the invasion of microorganisms and their products. Mucosal immune cells, especially intraepithelial lymphocytes, are involved in maintaining the integrity of these epithelial barriers. They contribute towards the tolerance to commensal organisms, which occupy these same sites, and to the immune responses against harmful organisms and their products. The composition of the microbiota is influenced by immune cells as well as external environmental factors, especially the use of antibiotics and diet. There is an increasing appreciation that the microbiota affects systemic immune responses in addition to local immunity. Failure to control the occupancy by microorganisms may result in the disruption of the delicate homeostasis between beneficial and harmful microorganisms and contribute to inflammatory pathologies. This review will discuss some of our current understanding of the impact of immune cells and diet on the microbiota.

An NKG2D-mediated human lymphoid stress surveillance response with high interindividual variation

DNA damage or other physicochemical stresses may increase the expression of major histocompatibility complex class I-related stress antigens, which then activate lymphocytes. This lymphoid stress surveillance (LSS) not only can limit tumor formation but may also promote immunopathology. MICA is a highly polymorphic human stress antigen implicated in tumor surveillance, inflammation, and transplant rejection. However, LSS has not been conclusively demonstrated in humans, and the functional role for MICA polymorphisms remains to be established. We show that MICA coding sequence polymorphisms substantially affected RNA and protein expression. All donors tested showed LSS responses of γδ T and natural killer cells, but unexpectedly, each was individually "tuned." Hence, some responded optimally to highly expressed alleles, whereas others responded better to lower MICA expression, challenging the orthodoxy that higher stress antigen levels promote greater responsiveness. These individual variations in LSS tuning may help explain patient-specific differences in tumor immune surveillance, transplant rejection, and inflammation, as well as provide insight into immune evasion and immunosuppression.

Improving the estimation of celiac disease sibling risk by non-HLA genes

Celiac Disease (CD) is a polygenic trait, and HLA genes explain less than half of the genetic variation. Through large GWAs more than 40 associated non-HLA genes were identified, but they give a small contribution to the heritability of the disease. The aim of this study is to improve the estimate of the CD risk in siblings, by adding to HLA a small set of non-HLA genes. One-hundred fifty-seven Italian families with a confirmed CD case and at least one other sib and both parents were recruited. Among 249 sibs, 29 developed CD in a 6 year follow-up period. All individuals were typed for HLA and 10 SNPs in non-HLA genes: CCR1/CCR3 (rs6441961), IL12A/SCHIP1 and IL12A (rs17810546 and rs9811792), TAGAP (rs1738074), RGS1 (rs2816316), LPP (rs1464510), OLIG3 (rs2327832), REL (rs842647), IL2/IL21 (rs6822844), SH2B3 (rs3184504). Three associated SNPs (in LPP, REL, and RGS1 genes) were identified through the Transmission Disequilibrium Test and a Bayesian approach was used to assign a score (BS) to each detected HLA+SNPs genotype combination. We then classified CD sibs as at low or at high risk if their BS was respectively < or ≥ median BS value within each HLA risk group. A larger number (72%) of CD sibs showed a BS ≥ the median value and had a more than two fold higher OR than CD sibs with a BS value < the median (O.R = 2.53, p = 0.047). Our HLA+SNPs genotype classification, showed both a higher predictive negative value (95% vs 91%) and diagnostic sensitivity (79% vs 45%) than the HLA only. In conclusion, the estimate of the CD risk by HLA+SNPs approach, even if not applicable to prevention, could be a precious tool to improve the prediction of the disease in a cohort of first degree relatives, particularly in the low HLA risk groups.

Dynamics of the interaction of γδ T cells with their neighbors in vivo

γδ T cells are a diverse component of the immune system in humans and mice with presumably important but still largely unknown functions. Understanding the dynamic interaction of γδ T cells with their neighbors should help to understand their physiological role. This review addresses recent advances and strategies to visualize the dynamic interactions of γδ T cells with their neighbors in vivo. Current knowledge regarding the dynamic contacts of tissue resident γδ T cells and epithelial cells, but also of the communication between circulating γδ T cells and DCs, monocytes and FoxP3(+) regulatory T cells is revisited with emphasis on the role of γδ T cell motility.

CD4 T cells play important roles in maintaining IL-17-producing γδ T-cell subsets in naive animals

A proportional balance between αβ and γδ T-cell subsets in the periphery is exceedingly well maintained by a homeostatic mechanism. However, a cellular mechanism underlying the regulation remains undefined. We recently reported that a subset of developing γδ T cells spontaneously acquires interleukin (IL)-17-producing capacity even within naive animals through a transforming growth factor (TGF)β1-dependent mechanism, thus considered 'innate' IL-17-producing cells. Here, we report that γδ T cells generated within αβ T cell (or CD4 T cell)-deficient environments displayed altered cytokine profiles; particularly, 'innate' IL-17 expression was significantly impaired compared with those in wild-type mice. Impaired IL-17 production in γδ T cells was directly related to CD4 T-cell deficiency, because depletion of CD4 T cells in wild-type mice diminished and adoptive CD4 T-cell transfer into T-cell receptor β-/- mice restored IL-17 expression in γδ T cells. CD4 T cell-mediated IL-17 expression required TGFβ1. Moreover, Th17 but not Th1 or Th2 effector CD4 T cells were highly efficient in enhancing γδ T-cell IL-17 expression. Taken together, our results highlight a novel CD4 T cell-dependent mechanism that shapes the generation of IL-17+ γδ T cells in naive settings.

Molecular aspects of epithelial γδ T cell regulation

γδ T cells lie at the interface between innate and adaptive immunity, sharing features with both arms of the immune system. The vast majority of γδ T cells reside in epithelial layers of tissues such as skin, gut, lung, tongue and reproductive tract where they provide a first line of defense against environmental attack. The existence of epithelium-resident γδ T cells has been known for over 20 years but our understanding of the molecular events regulating development and function of these cells is incomplete. We review recent advances in the field, with particular emphasis on the γδ T cell population resident in mouse epidermis. These studies have enhanced our knowledge and understanding of the life cycle of this enigmatic population of cells.

αβ versus γδ fate choice: counting the T-cell lineages at the branch point

Both αβ and γδ T cells develop in the thymus from a common progenitor. Historically distinguished by their T-cell receptor (TCR), these lineages are now defined on the basis of distinct molecular programs. Intriguingly, in many transgenic and knockout systems these programs are mismatched with the TCR type, leading to the development of γδ lineage cells driven by αβTCR and vice versa. These puzzling observations were recently explained by the demonstration that TCR signal strength, rather than TCR type per se, instructs lineage fate, with stronger TCR signal favoring γδ and weaker signal favoring αβ lineage fates. These studies also highlighted the ERK (extracellular signal regulated kinase)-Egr (early growth response)-Id3 (inhibitor of differentiation 3) axis as a potential molecular switch downstream of TCR that determines lineage choice. Indeed, removal of Id3 was sufficient to redirect TCRγδ transgenic cells to the αβ lineage, even in the presence of strong TCR signal. However, in TCR non-transgenic Id3 knockout mice the overall number of γδ lineage cells was increased due to an outgrowth of a Vγ1Vδ6.3 subset, suggesting that not all γδ T cells depend on this molecular switch for lineage commitment. Thus, the γδ lineage may in fact be a collection of two or more lineages not sharing a common molecular program and thus equipollent to the αβ lineage. TCR signaling is not the only factor that is required for development of αβ and γδ lineage cells; other pathways, such as signaling from Notch and CXCR4 receptors, cooperate with the TCR in this process.

Mouse and human intestinal immunity: same ballpark, different players; different rules, same score

The study of animal immune physiology and animal models of human disease have accelerated many aspects of translational research by allowing direct, definitive investigations. In particular, the use of mice has allowed genetic manipulation, adoptive transfer, immunization, and focused cell and tissue sampling, which would obviously be unthinkable for studies in humans. However, the disease relevance of some animal models may be uncertain and difficulties in interpretation may occur as a consequence of immunological differences between the two species. In this review, we will consider general differences in the structure and development of human and mouse mucosal lymphoid microenvironments and then discuss species differences in mucosal B- and T-cell biology that relate to the current concepts of intestinal immune function.

Unexpected role for the B cell-specific Src family kinase B lymphoid kinase in the development of IL-17-producing γδ T cells

The Ag receptors on αβ and γδ T cells differ not only in the nature of the ligands that they recognize but also in their signaling potential. We hypothesized that the differences in αβ- and γδTCR signal transduction were due to differences in the intracellular signaling pathways coupled to these two TCRs. To investigate this, we used transcriptional profiling to identify genes encoding signaling molecules that are differentially expressed in mature αβ and γδ T cell populations. Unexpectedly, we found that B lymphoid kinase (Blk), a Src family kinase expressed primarily in B cells, is expressed in γδ T cells but not in αβ T cells. Analysis of Blk-deficient mice revealed that Blk is required for the development of IL-17-producing γδ T cells. Furthermore, Blk is expressed in lymphoid precursors and, in this capacity, plays a role in regulating thymus cellularity during ontogeny.

Current progress in γδ T-cell biology

T lymphocytes bearing γ- and δ-chain T-cell receptor heterodimers are named γδ T cells. Interestingly, γδ and αβ T cells share the same progenitors, and they undergo a fate decision in the thymus. Functional differentiation of γδ T cells occurs both inside and outside the thymus. Antigen recognition of γδ T-cell receptors is very unique, and the responses frequently exhibit innate characteristics. Nevertheless, peripheral γδ T cells exert a number of effector and regulatory functions. γδ T cells rapidly produce cytokines like interferon (IFN)-γ and IL-17 and promote inflammation, partly due to the inherent epigenetic and transcriptional programs, which facilitates a quick and extensive response. Moreover, γδ T cells lyse target cells directly, and this is necessary for pathogen or tumor clearance. γδ T cells can even serve as regulatory cells, and may contribute to immune suppression. Orchestration of γδ T-cell and other immune cell interactions may be critical for host defense and immune regulation. Recently, γδ T cells have been used for immunotherapy for infectious diseases and malignancy. In this review, we summarize the abstracts presented at the recent γδ T cell Conference held from 19 to 21 May 2010, in Kiel, Germany (please see the website for details: http://www.gammadelta-conference.uni-kiel.de/index.html).

Determining γδ versus αß T cell development

The thymus produces several types of functionally distinct T cell subsets. However, at a more fundamental level only two genetically distinct T cell lineages exist: the γδ and αß T cell lineages. Precisely how these two T cell lineages are generated from common thymocyte progenitor cells remains to be fully elucidated and is under intense investigation. Here, we highlight recent findings that have helped to provide important clues to the mechanisms that underpin the generation of γδ T cells in the mouse thymus.

Towards a molecular understanding of the differential signals regulating alphabeta/gammadelta T lineage choice

While insights into the molecular processes that specify adoption of the alphabeta and gammadelta fates are beginning to emerge, the basis for control of specification remains highly controversial. This review highlights the current models attempting to explain T lineage commitment. Recent observations support the hypothesis that the T cell receptor (TCR) provides instructive cues through differences in TCR signaling intensity and/or longevity. Accordingly, we review evidence addressing the importance of differences in signal strength/longevity, how signals differing in intensity/longevity may be generated, and finally how such signals modulate the activity of downstream effectors to promote the opposing developmental fates.

Beyond alphabeta/gammadelta lineage commitment: TCR signal strength regulates gammadelta T cell maturation and effector fate

Signaling by the gammadelta T cell receptor (TCR) is required not only for alphabeta/gammadelta lineage commitment but also to activate and elicit effector functions in mature gammadelta T cells. Notably, at both of these stages, the signal delivered by the gammadeltaTCR is more robust than the one delivered by either the preTCR or the alphabetaTCR. Recent studies now provide evidence that signaling by the gammadeltaTCR is also required at other stages during gammadelta T cell development. Remarkably, the strength of the gammadeltaTCR signal also plays a role at these other stages, as evidenced by the findings that genetic manipulation of gammadeltaTCR signal strength affects gammadelta T cell maturation and effector fate. In this review, we discuss how a strong TCR signal is a recurring theme in gammadelta T cell development and activation.

Fine-scale staging of T cell lineage commitment in adult mouse thymus

T cell development is marked by the loss of alternative lineage choices accompanying specification and commitment to the T cell lineage. Commitment occurs between the CD4 and CD8 double-negative (DN) 2 and DN3 stages in mouse early T cells. To determine the gene regulatory changes that accompany commitment, we sought to distinguish and characterize the earliest committed wild-type DN adult thymocytes. A transitional cell population, defined by the first downregulation of surface c-Kit expression, was found to have lost the ability to differentiate into dendritic cells and NK cells when cultured without Notch-Delta signals. In the presence of Notch signaling, this subset generates T lineage descendants in an ordered precursor-product relationship between DN2, with the highest levels of surface c-Kit, and c-Kit-low DN3 cells. These earliest committed cells show only a few differences in regulatory gene expression, compared with uncommitted DN2 cells. They have not yet established the full expression of Notch-related and T cell differentiation genes characteristic of DN3 cells before beta selection. Instead, the downregulation of select stem cell and non-T lineage genes appears to be key to the extinction of alternative lineage choices.

gammadelta and alphabeta T cell lineage choice: resolution by a stronger sense of being

A common bipotent thymocyte precursor gives rise to both lineages of T cells, alphabeta and gammadelta. However, the cell intrinsic and extrinsic factors that influence alphabeta- versus gammadelta-lineage bifurcation remain controversial. gammadelta T cells play a unique and vital role in host defense, from maintaining integrity at epithelial and mucosal barriers to their newly defined role as an important innate source of interleukin-17. Although a T cell receptor (TCR)-independent fate choice may take place, emerging data supports a model in which the differential signaling capacity of alphabeta and gammadeltaTCRs play an instructional role in specifying lineage fate, with strength of signal measured by the amount of ERK/MAPK pathway activation. Here we discuss how the interplay between intrinsic TCR signals and cell extrinsic signals provided by Notch and TCR ligands help to assign and support a final lineage fate decision.

Disorderly conduct in gammadelta versus alphabeta T cell lineage commitment

The mechanism of T cell precursor commitment to the gammadelta or alphabeta T cell lineage remains unclear. While TCR signal strength has emerged as a key factor in lineage commitment based on TCR transgenic models, the entire TCR repertoire may not possess the same discriminatory power. A counterbalance to the TCR as the lineage determinant is the pre-existing heterogeneity in gene expression among precursors, which suggests that single precursors are unlikely to respond homogeneously to a given instructive signal.

Repertoire development and the control of cytotoxic/effector function in human gammadelta T cells

T cells develop into two major populations distinguished by their T cell receptor (TCR) chains. Cells with the alphabeta TCR generally express CD4 or CD8 lineage markers and mostly fall into helper or cytotoxic/effector subsets. Cells expressing the alternate gammadelta TCR in humans generally do not express lineage markers, do not require MHC for antigen presentation, and recognize nonpeptidic antigens. We are interested in the dominant Vgamma2Vdelta2+ T cell subset in human peripheral blood and the control of effector function in this population. We review the literature on gammadelta T cell generation and repertoire selection, along with recent work on CD56 expression and defining a cytotoxic/effector lineage within the phosphoantigen-reactive Vgamma2Vdelta2 cells. A unique mechanism for MHC-independent repertoire selection is linked to the control of effector function that is vital to the role for gammadelta T cells in tumor surveillance. Better understanding of these mechanisms will improve our ability to exploit this population for tumor immunotherapy.

gammadelta T cells: the overlooked T-cell subset in demyelinating disease

gammadelta T cells represent a small subpopulation of T cells expressing a restricted repertoire of T-cell receptors and, unlike alphabeta T cells, function more as cells of the innate immune system. These cells are found in skin and mucosal sites as well as secondary lymphoid tissues and frequently act as first line of defense sentinels. gammadelta T cells have been implicated in the pathogenesis of demyelinating disease, although little was known regarding their trafficking and effector functions. In this Mini-Review, we highlight recent studies demonstrating that gammadelta T cells migrate rapidly to the CNS during experimental autoimmune encephalomyelitis (EAE), the animal model for multiple sclerosis. gammadelta T-cell trafficking to the CNS is independent of beta(2)-integrins and occurs well before onset of clinical signs of disease, peaking early during the acute phase of disease. gammadelta T-cell-mediated production of inflammatory cytokines, including interferon-gamma and tumor necrosis factor-alpha, appears critical for EAE development, suggesting that these cells may set the stage for activation of other subsets of infiltrating effector cells. These data suggest that gammadelta T cells or subsets of gammadelta T cells may represent a new therapeutic target in demeylinating disease.

Roles of the Src tyrosine kinases Lck and Fyn in regulating gammadeltaTCR signal strength

Lck and Fyn, members of the Src family of tyrosine kinases, are key components of the αβTCR-coupled signaling pathway. While it is generally accepted that both Lck and Fyn positively regulate signal transduction by the αβTCR, recent studies have shown that Lck and Fyn have distinct functions in this signaling pathway, with Lck being a positive regulator and Fyn being a negative regulator of αβTCR signal transduction. To determine whether Lck and Fyn also differentially regulate γδTCR signal transduction, we analyzed γδ T cell development and function in mice with reduced Lck or Fyn expression levels. We found that reducing Lck or Fyn levels altered the strength of the γδTCR signaling response, with low levels of Lck weakening γδTCR signal strength and low levels of Fyn augmenting γδTCR signal strength. These alterations in γδTCR signal strength had profound effects not only on αβ/γδ lineage choice, but also on γδ thymocyte maturation and γδ T cell effector function. These results indicate that the cellular levels of Lck and Fyn play a role in regulating the strength of the γδTCR signaling response at different stages in the life of the γδ T cell.

Alphabeta versus gammadelta lineage choice at the first TCR-controlled checkpoint

Alphabeta and gammadelta T cells develop in the thymus from a common precursor. Although lineages initially were defined by the type of TCR they express, it soon became clear that the TCR type per se does not play a deterministic role in the lineage decision, since in various transgenic and knockout models, as well as in a small fraction of cells in wt mice, the TCRgammadelta can drive the differentiation of alphabeta lineage cells and the TCRalphabeta can drive differentiation of gammadelta lineage cells. Thus until recently it was unclear what determines lineage choice and at which stage the two lineages diverge. Recent observations suggest that TCR signal strength determines lineage fate and that lineage choice is made at or shortly after the first TCR-controlled checkpoint. While it is clear that the decision between alphabeta and gammadelta lineages is made at the first TCR-controlled checkpoint and the alphabeta sublineages split off later, it is less clear whether gammadelta sublineages divert already at the first TCR-controlled checkpoint or later. Recent experiments support the former view.

Development of promyelocytic zinc finger and ThPOK-expressing innate gamma delta T cells is controlled by strength of TCR signaling and Id3

The broad-complex tramtrack and bric a brac-zinc finger transcriptional regulator (BTB-ZF), promyelocytic leukemia zinc finger (PLZF), was recently shown to control the development of the characteristic innate T cell phenotype and effector functions of NK T cells. Interestingly, the ectopic expression of PLZF was shown to push conventional T cells into an activated state that seems to be proinflammatory. The factors that control the normal expression of PLZF in lymphocytes are unknown. In this study, we show that PLZF expression is not restricted to NK T cells but is also expressed by a subset of gammadelta T cells, functionally defining distinct subsets of this innate T cell population. A second BTB-ZF gene, ThPOK, is important for the phenotype of the PLZF-expressing gammadelta T cells. Most importantly, TCR signal strength and expression of inhibitor of differentiation gene 3 control the frequency of PLZF-expressing gammadelta T cells. This study defines the factors that control the propensity of the immune system to produce potentially disease-causing T cell subsets.

Marked induction of the helix-loop-helix protein Id3 promotes the gammadelta T cell fate and renders their functional maturation Notch independent

alphabeta and gammadelta T cells arise from a common thymocyte progenitor during development in the thymus. Emerging evidence suggests that the pre-T cell receptor (pre-TCR) and gammadelta T cell receptor (gammadeltaTCR) play instructional roles in specifying the alphabeta and gammadelta T-lineage fates, respectively. Nevertheless, the signaling pathways differentially engaged to specify fate and promote the development of these lineages remain poorly understood. Here, we show that differential activation of the extracellular signal-related kinase (ERK)-early growth response gene (Egr)-inhibitor of DNA binding 3 (Id3) pathway plays a defining role in this process. In particular, Id3 expression served to regulate adoption of the gammadelta fate. Moreover, Id3 was both necessary and sufficient to enable gammadelta-lineage cells to differentiate independently of Notch signaling and become competent IFNgamma-producing effectors. Taken together, these findings identify Id3 as a central player that controls both adoption of the gammadelta fate and its maturation in the thymus.