Recognition of the antigen-presenting molecule MR1 by a Vδ3+ γδ T cell receptor

Unlike conventional αβ T cells, γδ T cells typically recognize nonpeptide ligands independently of major histocompatibility complex (MHC) restriction. Accordingly, the γδ T cell receptor (TCR) can potentially recognize a wide array of ligands; however, few ligands have been described to date. While there is a growing appreciation of the molecular bases underpinning variable (V)δ1⁺ and Vδ2⁺ γδ TCR-mediated ligand recognition, the mode of Vδ3⁺ TCR ligand engagement is unknown. MHC class I–related protein, MR1, presents vitamin B metabolites to αβ T cells known as mucosal-associated invariant T cells, diverse MR1-restricted T cells, and a subset of human γδ T cells. Here, we identify Vδ1/2⁻ γδ T cells in the blood and duodenal biopsy specimens of children that showed metabolite-independent binding of MR1 tetramers. Characterization of one Vδ3Vγ8 TCR clone showed MR1 reactivity was independent of the presented antigen. Determination of two Vδ3Vγ8 TCR-MR1-antigen complex structures revealed a recognition mechanism by the Vδ3 TCR chain that mediated specific contacts to the side of the MR1 antigen-binding groove, representing a previously uncharacterized MR1 docking topology. The binding of the Vδ3⁺ TCR to MR1 did not involve contacts with the presented antigen, providing a basis for understanding its inherent MR1 autoreactivity. We provide molecular insight into antigen-independent recognition of MR1 by a Vδ3⁺ γδ TCR that strengthens an emerging paradigm of antibody-like ligand engagement by γδ TCRs.

Characterized by both innate and adaptive immune cell functions, γδ T cells are an unconventional T cell subset. While the functional role of γδ T cells is yet to be fully established, they can play a central role in antimicrobial immunity (1), antitumor immunity (2), tissue homeostasis, and mucosal immunity (3). Owing to a lack of clarity on activating ligands and phenotypic markers, γδ T cells are often delineated into subsets based on the expression of T cell receptor (TCR) variable (V) δ gene usage, grouped as Vδ2⁺ or Vδ2⁻.The most abundant peripheral blood γδ T cell subset is an innate-like Vδ2⁺subset that comprises ∼1 to 10% of circulating T cells (4). These cells generally express a Vγ9 chain with a focused repertoire in fetal peripheral blood (5) that diversifies through neonatal and adult life following microbial challenge (6, 7). Indeed, these Vγ9/Vδ2⁺ T cells play a central role in antimicrobial immune response to Mycobacterium tuberculosis (8) and Plasmodium falciparum (9). Vγ9/Vδ2⁺ T cells are reactive to prenyl pyrophosphates that include isopentenyl pyrophosphate and (E)-4-Hydroxy-3-methyl-but-2-enyl pyrophosphate (8) in a butyrophilin 3A1- and BTN2A1-dependent manner (10–13). Alongside the innate-like protection of Vγ9/Vδ2⁺ cells, a Vγ9⁻ population provides adaptive-like immunobiology with clonal expansions that exhibit effector function (14).The Vδ2⁻ population encompasses the remaining γδ T cells but most notably the Vδ1⁺ and Vδ3⁺ populations. Vδ1⁺ γδ T cells are an abundant neonatal lineage that persists as the predominating subset in adult peripheral tissue including the gut and skin (15–18). Vδ1⁺ γδ T cells display potent cytokine production and respond to virally infected and cancerous cells (19). Vδ1⁺ T cells were recently shown to compose a private repertoire that diversifies, from being unfocused to a selected clonal TCR pool upon antigen exposure (20–23). Here, the identification of both Vδ1⁺ T_naive and Vδ1⁺ T_effector subsets and the Vδ1⁺ T_naive to T_effector differentiation following in vivo infection point toward an adaptive phenotype (22).The role of Vδ3⁺ γδ T cells has remained unclear, with a poor understanding of their lineage and functional role. Early insights into Vδ3⁺ γδ T cell immunobiology found infiltration of Vδ3⁺ intraepithelial lymphocytes (IEL) within the gut mucosa of celiac patients (24). More recently it was shown that although Vδ3⁺ γδ T cells represent a prominent γδ T cell component of the gut epithelia and lamina propria in control donors, notwithstanding pediatric epithelium, the expanding population of T cells in celiac disease were Vδ1⁺ (25). Although Vδ3⁺ IELs compose a notable population of gut epithelia and lamina propria T cells (∼3 to 7%), they also formed a discrete population (∼0.2%) of CD4⁻CD8⁻ T cells in peripheral blood (26). These Vδ3⁺ DN γδ T cells are postulated to be innate-like due to the expression of NKG2D, CD56, and CD161 (26). When expanded in vitro, these cells degranulated and killed cells expressing CD1d and displayed a T helper (Th) 1, Th2, and Th17 response in addition to promoting dendritic cell maturation (26). Peripheral Vδ3⁺ γδ T cells frequencies are known to increase in systemic lupus erythematosus patients (27, 28), and upon cytomegalovirus (29) and HIV infection (30), although, our knowledge of their exact role and ligands they recognize remains incomplete.The governing paradigms of antigen reactivity, activation principles, and functional roles of γδ T cells remain unresolved. This is owing partly due to a lack of knowledge of bona fide γδ T cell ligands. Presently, Vδ1⁺ γδ T cells remain the best characterized subset with antigens including Major Histocompatibility Complex (MHC)-I (31), monomorphic MHC-I–like molecules such as CD1b (32), CD1c (33), CD1d (34), and MR1 (35), as well as more diverse antigens such as endothelial protein coupled receptor (EPCR) and phycoerythrin (PE) (36, 37). The molecular determinants of this reactivity were first established for Vδ1⁺ TCRs in complex with CD1d presenting sulfatide (38) and α-galactosylceramide (α-GalCer) (34), which showed an antigen-dependent central focus on the presented lipids and docked over the antigen-binding cleft.In humans, mucosal-associated invariant T (MAIT) cells are an abundant innate-like αβ T cell subset typically characterized by a restricted TCR repertoire (39–43) and reactivity to the monomorphic molecule MR1 presenting vitamin B precursors and drug-like molecules of bacterial origin (41, 44–46). Recently, populations of atypical MR1-restricted T cells have been identified in mice and humans that utilize a more diverse TCR repertoire for MR1-recognition (42, 47, 48). Furthermore, MR1-restricted γδ T cells were identified in blood and tissues including Vδ1⁺, Vδ3⁺, and Vδ5⁺ clones (35). As seen with TRAV 1-2⁻, unconventional MAITs cells the isolated γδ T cells exhibited MR1-autoreactivity with some capacity for antigen discrimination within the responding compartment (35, 48). Structural insight into one such MR1-reactive Vδ1⁺ γδ TCR showed a down-under TCR engagement of MR1 in a manner that is thought to represent a subpopulation of MR1-reactive Vδ1⁺ T cells (35). However, biochemical evidence suggested other MR1-reactive γδ T cell clones would likely employ further unusual docking topologies for MR1 recognition (35).Here, we expanded our understanding of a discrete population of human Vδ3⁺ γδ T cells that display reactivity to MR1. We provide a molecular basis for this Vδ3⁺ γδ T cell reactivity and reveal a side-on docking for MR1 that is distinct from the previously determined Vδ1⁺ γδ TCR-MR1-Ag complex. A Vδ3⁺ γδ TCR does not form contacts with the bound MR1 antigen, and we highlight the importance of non–germ-line Vδ3 residues in driving this MR1 restriction. Accordingly, we have provided key insights into the ability of human γδ TCRs to recognize MR1 in an antigen-independent manner by contrasting mechanisms.     

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.Like conventional αβ T cells and B cells, γδ T cells use V(D)J gene rearrangement with the potential to generate a set of highly diverse receptors to recognize antigens. This diversity is generated mainly in the complementary-determining region 3 (CDR3) of the T-cell antigen receptor (TCR) or B-cell antigen receptor (1–3). The tripartite subdivision of lymphocytes possessing rearranged receptors into B cells, αβ T cells, and γδ T cells has been conserved since the emergence of jawed vertebrates more than 450 Mya (1). Recently, a similar division of variable lymphocyte receptor A (VLRA)⁺, VLRB⁺, and VLRC⁺ cells, resembling αβ T cells, B cells, and γδ T cells, respectively, has been found in jawless vertebrates (e.g., lamprey), showing the same basic principle of lymphocyte differentiation along two distinct T-cell–like lineages and one B-cell–like lineage (4). These evolutionary data highlight the importance of both γδ T cells and αβ T cells. A major difference between αβ T cells and γδ T cells is the way they recognize antigens. In contrast to conventional αβ T cells, γδ T cells are not dependent on classical MHC molecules presenting peptides. Based on the ligands that have been identified, it appears that some γδ TCRs can recognize antigens in an antibody-like fashion, whereas the TCRs of other γδ T-cell subsets can bind to nonclassical MHC-I or MHC-like proteins (2, 5–11). Although there are common characteristics among γδ T cells, some of which are shared with VLRC⁺ cells (4), it is clear that γδ T cells do not represent a homogenous population of cells with a single physiological role (12). γδ T cells expressing the TCR containing the γ-chain variable region 9 and the δ-chain variable region 2 (Vγ9Vδ2 T cells) are activated by microbe- and host-derived phosphorylated prenyl metabolites (phosphorylated antigens or “phosphoantigens”) derived from the isoprenoid metabolic pathway, the most active of which are microbial (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMB-PP), produced by the 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway, and host isopentenyl pyrophosphate (IPP) (13). These phosphoantigens recently have been shown to be presented or to be sensed by the butyrophilin BTN3A1 (14–16). Although phosphoantigen-reactive Vγ9Vδ2 T cells were thought to be restricted to primates, there is recent evidence that Vγ9, Vδ2, and BTN3A1 genes are coconserved across a variety of placental mammals including primates, alpaca, armadillo, sloth, dolphin, dromedary, and orca, but not rodents (17). The recognition of phosphoantigens allows Vγ9Vδ2 T cells to develop potent antimicrobial immune responses or to promote the killing of transformed host cells that up-regulate IPP production (18, 19). Also, treatment of cells with the aminobisphosphonate family of drugs, of which zoledronate (Zometa) is the most potent member, leads to endogenous IPP accumulation (18). This feature has been used to develop clinical trials targeting Vγ9Vδ2 T cells of patients with leukemia or solid cancers (19, 20). Vγ9Vδ2 T cells represent the main population of γδ T cells in adult human peripheral blood: About 50–90% of γδ T cells in the circulation express this combination of Vγ and Vδ chains because of postnatal expansion (21). In contrast, γδ T cells expressing the Vδ1 chain, which can pair with a variety of Vγ chains, are enriched in adult tissues such as the gut (21).Instead of being regarded as just an immature version of the adult immune system, the immune system in early life increasingly is being recognized as different, with a bias toward the induction of a Th2 response or of immune tolerance (22–26). Indeed, one of the last cytokines to reach adult levels after birth is the Th1-promoting cytokine IL-12 (IL-12p70) (27). Originally proposed as a hypothesis by Adrian Hayday (1), there is increasing evidence, including our own results, that γδ T cells are important in early life (28–33). Although in humans circulating T cells can be detected as early as 12.5 wk gestation, most information on T cells in early life, including γδ T cells, is derived from studies on cord blood at term delivery (>37 wk gestation) (34). We hypothesized that the human fetus could produce particular fetal type of γδ T cells, as has been well established in the mouse model (35–37). Furthermore, it has been reported recently that fetal and adult hematopoietic stem cells can give rise to distinct T-cell lineages in humans, with a bias toward immune tolerance in the fetus (24).Here we found that, unexpectedly, fetal blood around midgestation (before 30 wk) contained high levels of Vγ9Vδ2 T cells. These lymphocytes expressed a semi-invariant TCR, were phosphoantigen reactive, and showed a preprogrammed effector potential, suggesting that these γδ T cells may fulfill an important role in the immunosurveillance of fetal tissues.     

In Acute Dengue Infection,High TIM-3 Expression May Contribute to the Impairment of IFNγ Production by Circulating Vδ2 T Cells

γδ T cells are innate cells able to quickly eliminate pathogens or infected/tumoral cells by their antiviral and adjuvancy activities. The role of γδ T cells during Dengue Viral Infection (DENV) infection is not fully elucidated. Nevertheless, human primary γδ T cells have been shown to kill in vitro DENV-infected cells, thus highlighting their possible antiviral function. The aim of this work was to characterize the phenotype and function of Vδ2 T cells in DENV patients. Fifteen DENV patients were enrolled for this study and peripheral blood mononuclear cells (PBMC) were used to analyze Vδ2-T-cell frequency, differentiation profile, activation/exhaustion status, and functionality by multiparametric flow cytometry. Our data demonstrated that DENV infection was able to significantly reduce Vδ2-T-cell frequency and to increase their activation (CD38 and HLA-DR) and exhaustion markers (PD-1 and TIM-3). Furthermore, Vδ2 T cells showed a reduced capability to produce IFN-γ after phosphoantigenic stimulation that can be associated to TIM-3 expression. Several studies are needed to depict the possible clinical impact of γδ-T-cell impairment on disease severity and to define the antiviral and immunoregulatory activities of γδ T cells in the first phases of infection.     

The Role of γδ T Cells as a Line of Defense in Viral Infections after Allogeneic Stem Cell Transplantation: Opportunities and Challenges

In the complex interplay between inflammation and graft-versus-host disease (GVHD) after allogeneic stem cell transplantation (allo-HSCT), viral reactivations are often observed and cause substantial morbidity and mortality. As toxicity after allo-HSCT within the context of viral reactivations is mainly driven by αβ T cells, we describe that by delaying αβ T cell reconstitution through defined transplantation techniques, we can harvest the full potential of early reconstituting γδ T cells to control viral reactivations. We summarize evidence of how the γδ T cell repertoire is shaped by CMV and EBV reactivations after allo-HSCT, and their potential role in controlling the most important, but not all, viral reactivations. As most γδ T cells recognize their targets in an MHC-independent manner, γδ T cells not only have the potential to control viral reactivations but also to impact the underlying hematological malignancies. We also highlight the recently re-discovered ability to recognize classical HLA-molecules through a γδ T cell receptor, which also surprisingly do not associate with GVHD. Finally, we discuss the therapeutic potential of γδ T cells and their receptors within and outside the context of allo-HSCT, as well as the opportunities and challenges for developers and for payers.     

Revisiting the Role of γδ T Cells in Anti-CMV Immune Response after Transplantation

Gamma delta (γδ) T cells form an unconventional subset of T lymphocytes that express a T cell receptor (TCR) consisting of γ and δ chains. Unlike conventional αβ T cells, γδ T cells share the immune signature of both the innate and the adaptive immunity. These features allow γδ T cells to act in front-line defense against infections and tumors, rendering them an attractive target for immunotherapy. The role of γδ T cells in the immune response to cytomegalovirus (CMV) has been the focus of intense research for several years, particularly in the context of transplantation, as CMV reactivation remains a major cause of transplant-related morbidity and mortality. Therefore, a better understanding of the mechanisms that underlie CMV immune responses could enable the design of novel γδ T cell-based therapeutic approaches. In this regard, the advent of next-generation sequencing (NGS) and single-cell TCR sequencing have allowed in-depth characterization of CMV-induced TCR repertoire changes. In this review, we try to shed light on recent findings addressing the adaptive role of γδ T cells in CMV immunosurveillance and revisit CMV-induced TCR reshaping in the era of NGS. Finally, we will demonstrate the favorable and unfavorable effects of CMV reactive γδ T cells post-transplantation.     

Prolonged antigen survival and cytosolic export in cross-presenting human γδ T cells

Human blood Vγ9Vδ2 T cells respond to signals from microbes and tumors and subsequently differentiate into professional antigen-presenting cells (γδ T-APCs) for induction of CD4⁺ and CD8⁺ T cell responses. γδ T-APCs readily take up and degrade exogenous soluble protein for peptide loading on MHC I, in a process termed antigen cross-presentation. The mechanisms underlying antigen cross-presentation are ill-defined, most notably in human dendritic cells (DCs), and no study has addressed this process in γδ T-APCs. Here we show that intracellular protein degradation and endosomal acidification were significantly delayed in γδ T-APCs compared with human monocyte-derived DCs (moDCs). Such conditions are known to favor antigen cross-presentation. In both γδ T-APCs and moDCs, internalized antigen was transported across insulin-regulated aminopeptidase (IRAP)–positive early and late endosomes; however, and in contrast to various human DC subsets, γδ T-APCs efficiently translocated soluble antigen into the cytosol for processing via the cytosolic proteasome-dependent cross-presentation pathway. Of note, γδ T-APCs cross-presented influenza antigen derived from virus-infected cells and from free virus particles. The robust cross-presentation capability appears to be a hallmark of γδ T-APCs and underscores their potential application in cellular immunotherapy.     

The Unknown Unknowns: Recovering Gamma-Delta T Cells for Control of Human Immunodeficiency Virus (HIV)

Recent advances in γδ T cell biology have focused on the unique attributes of these cells and their role in regulating innate and adaptive immunity, promoting tissue homeostasis, and providing resistance to various disorders. Numerous bacterial and viral pathogens, including human immunodeficiency virus-1 (HIV), greatly alter the composition of γδ T cells in vivo. Despite the effectiveness of antiretroviral therapy (ART) in controlling HIV and restoring health in those affected, γδ T cells are dramatically impacted during HIV infection and fail to reconstitute to normal levels in HIV-infected individuals during ART for reasons that are not clearly understood. Importantly, their role in controlling HIV infection, and the implications of their failure to rebound during ART are also largely unknown and understudied. Here, we review important aspects of human γδ T cell biology, the effector and immunomodulatory properties of these cells, their prevalence and function in HIV, and their immunotherapeutic potential.     

Kinetics of T cell receptor β, γ, and δ rearrangements during adult thymic development: T cell receptor rearrangements are present in CD44+CD25+ Pro-T thymocytes

We performed a comprehensive analysis of T cell receptor (TCR) γ rearrangements in T cell precursors of the mouse adult thymus. Using a sensitive quantitative PCR method, we show that TCRγ rearrangements are present in CD44⁺CD25⁺ Pro-T thymocytes much earlier than expected. TCRγ rearrangements increase significantly from the Pro-T to the CD44⁻CD25⁺ Pre-T cell transition, and follow different patterns depending on each Vγ gene segment, suggesting that ordered waves of TCRγ rearrangement exist in the adult mouse thymus as has been described in the fetal mouse thymus. Recombinations of TCRγ genes occur concurrently with TCRδ and D-Jβ rearrangements, but before Vβ gene assembly. Productive TCRγ rearrangements do not increase significantly before the Pre-T cell stage and are depleted in CD4⁺CD8⁺ double-positive cells from normal mice. In contrast, double-positive thymocytes from TCRδ−/− mice display random proportions of TCRγ rearranged alleles, supporting a role for functional TCRγ/δ rearrangements in the γδ divergence process.     

Effects of aging on human leukocytes (part II): immunophenotyping of adaptive immune B and T cell subsets

Immunosenescence results from a continuous deterioration of immune responses resulting in a decreased response to vaccines. A well-described age-related alteration of the immune system is the decrease of de novo generation of T and B cells. In addition, the accumulation of memory cells and loss of diversity in antigen specificities resulting from a lifetime of exposure to pathogens has also been described. However, the effect of aging on subsets of γδTCR⁺ T cells and Tregs has been poorly described, and the efficacy of the recall response to common persistent infections in the elderly remains obscure. Here, we investigated alterations in the subpopulations of the B and T cells among 24 healthy young (aged 19–30) and 26 healthy elderly (aged 53–67) individuals. The analysis was performed by flow cytometry using freshly collected peripheral blood. γδTCR⁺ T cells were overall decreased, while CD4⁺CD8⁻ cells among γδTCR⁺ T cells were increased in the elderly. Helios⁺Foxp3⁺ and Helios⁻Foxp3⁺ Treg cells were unaffected with age. Recent thymic emigrants, based on CD31 expression, were decreased among the Helios⁺Foxp3⁺, but not the Helios⁻Foxp3⁺ cell populations. We observed a decrease in Adenovirus-specific CD4⁺ and CD8⁺ T cells and an increase in CMV-specific CD4⁺ T cells in the elderly. Similarly, INFγ⁺TNFα⁺ double-positive cells were decreased among activated T cells after Adenovirus stimulation but increased after CMV stimulation. The data presented here indicate that γδTCR⁺ T cells might stabilize B cells, and functional senescence might dominate at higher ages than those studied here.

Electronic supplementary material

The online version of this article (doi:10.1007/s11357-015-9829-2) contains supplementary material, which is available to authorized users.     

TGF-β from the Porcine Intestinal Cell Line IPEC-J2 Induced by Porcine Circovirus 2 Increases the Frequency of Treg Cells via the Activation of ERK (in CD4+ T Cells) and NF-κB (in IPEC-J2)

Porcine circovirus 2 (PCV2) causes immunosuppression. Piglets infected with PCV2 can develop enteritis. Given that the gut is the largest immune organ, however, the response of the gut’s immune system to PCV2 is still unclear. Here, IPEC-J2 cells with different treatments were co-cultured with PBMC or CD4⁺ T cells (Transwell). Flow cytometry and Western blotting revealed that PCV2-infected IPEC-J2 increased the frequency of CD4⁺ T cells among piglets’ peripheral blood mononuclear cells (PBMCs) and caused CD4⁺ T cells to undergo a transformation into Foxp3⁺ regulatory T cells (Treg cells) via activating CD4⁺ T ERK. Cytokines production and an inhibitor assay showed that the induction of Tregs by PCV2-infected IPEC-J2 was dependent on TGF-β induced by PCV2 in IPEC-J2, which was associated with the activation of NF-κB. Taken together, PCV2-infected IPEC-J2 activated NF-κB to stimulate the synthesis of TGF-β, which enhanced the differentiation of CD4⁺ T cells into Treg cells through the activation of ERK in CD4⁺ T cells. This information sheds light on PCV2′s function in the intestinal immune system and suggests a potential immunosuppressive mechanism for PCV2 infection.     

T cell vaccination induces T cell receptor Vβ-specific Qa-1-restricted regulatory CD8+ T cells

Vaccination of mice with activated autoantigen-reactive CD4⁺ T cells (T cell vaccination, TCV) has been shown to induce protection from the subsequent induction of a variety of experimental autoimmune diseases, including experimental allergic encephalomyelitis (EAE). Although the mechanisms involved in TCV-mediated protection are not completely known, there is some evidence that TCV induces CD8⁺ regulatory T cells that are specific for pathogenic CD4⁺ T cells. Previously, we demonstrated that, after superantigen administration in vivo, CD8⁺ T cells emerge that preferentially lyse and regulate activated autologous CD4⁺ T cells in a T cell receptor (TCR) Vβ-specific manner. This TCR Vβ-specific regulation is not observed in β₂-microglobulin-deficient mice and is inhibited, in vitro, by antibody to Qa-1. We now show that similar Vβ8-specific Qa-1-restricted CD8⁺ T cells are also induced by TCV with activated CD4⁺ Vβ8⁺ T cells. These CD8⁺ T cells specifically lyse murine or human transfectants coexpressing Qa-1 and murine TCR Vβ8. Further, CD8⁺ T cell hybridoma clones generated from B10.PL mice vaccinated with a myelin basic protein-specific CD4⁺Vβ8⁺ T cell clone specifically recognize other CD4⁺ T cells and T cell tumors that express Vβ8 and the syngeneic Qa-1^a but not the allogeneic Qa-1^b molecule. Thus, Vβ-specific Qa-1-restricted CD8⁺ T cells are induced by activated CD4⁺ T cells. We suggest that these CD8⁺ T cells may function to specifically regulate activated CD4⁺ T cells during immune responses.     

IL-15 receptor α signaling constrains the development of IL-17–producing γδ T cells

The development and homeostasis of γδ T cells is highly dependent on distinct cytokine networks. Here we examine the role of IL-15 and its unique receptor, IL-15Rα, in the development of IL-17–producing γδ (γδ-17) T cells. Phenotypic analysis has shown that CD44^high γδ-17 cells express IL-15Rα and the common gamma chain (CD132), yet lack the IL-2/15Rβ chain (CD122). Surprisingly, we found an enlarged population of γδ-17 cells in the peripheral and mesenteric lymph nodes of adult IL-15Rα KO mice, but not of IL-15 KO mice. The generation of mixed chimeras from neonatal thymocytes indicated that cell-intrinsic IL-15Rα expression was required to limit IL-17 production by γδ T cells. γδ-17 cells also were increased in the peripheral lymph nodes of transgenic knock-in mice, where the IL-15Rα intracellular signaling domain was replaced with the intracellular portion of the IL-2Rα chain (that lacks signaling capacity). Finally, an analysis of neonatal thymi revealed that the CD44^lo/int precursors of γδ-17 cells, which also expressed IL-15Rα, were increased in newborn mice deficient in IL-15Rα signaling, but not in IL-15 itself. Thus, these findings demonstrate that signaling through IL-15Rα regulates the development of γδ-17 cells early in ontogeny, with long-term effects on their peripheral homeostasis in the adult.Both αβ and γδ T cells rely heavily on cytokine signaling for their development and survival. Many of these cytokines belong to the IL-2 cytokine family, whose receptors all share a common γ receptor chain (γ_c; CD132). Among these, IL-15 has a unique, nonredundant role in both T-cell and natural killer (NK)-cell biology, such that CD8⁺ memory T cells and NK cells are absent in IL-15–deficient environments (1, 2). The predominant mechanism through which IL-15 functions is termed transpresentation, whereby IL-15 is preassociated with its specific α-chain (IL-15Rα) inside the cell and presented at the cell surface in trans to a responding cell expressing γ_c and IL-2/15Rβ (CD122) (3–6). IL-15 is unique among its family members owing to its ability to act either in cis or in trans. Whether or not direct signaling (in cis) via IL-15Rα plays a significant biological role in immunobiology has not been resolved (7–11).Certain populations of γδ T cells are known to be sensitive to the availability of IL-15 for their development and/or survival. A population of specialized γδ T cells called dendritic epidermal T cells are absent from the skin of IL-15 knockout (KO) and IL-15Rα KO mice (12, 13). The CD8αα⁺ γδ T-cell receptor (TCR) intraepithelial lymphocytes are also decreased in these two KO mouse strains (1, 2). In addition, IL-15 KO mice have a reduced population of IFN-γ⁺ γδ T cells in the peritoneum (14). All of these populations express CD122, suggesting they can receive IL-15–dependent signals via transpresentation.Recently, γδ T cells have emerged as important contributors to the generation of immune responses. The innate-like γδ T cells that produce IL-17 (γδ-17 cells) have been implicated in immune responses generated during bacterial and fungal infections, experimental autoimmune encephalomyelytois (EAE), psoriasis, and anticancer immunity (reviewed in ref. 15). The γδ-17 subset of γδ T cells has a restricted TCR use that is largely limited to Vγ2 and Vγ4 expression (Garmin nomenclature; Vγ4 and Vγ6 in Tonegawa nomenclature) and a molecularly distinct gene expression profile (16). Considering the strictly regulated developmental progression of γδ T-cell subsets (17), the vast majority of γδ-17 cells are known to emerge during a limited window early in ontogeny, ∼E16.5 up to shortly after birth (18). The exogenous signals that impact γδ-17–cell development during this window remain unclear. Here we identify the IL-15Rα chain as a critical determinant through which γδ-17–cell development is regulated. Furthermore, in contrast to IL-15Rα’s predominant role in the immune system via IL-15 transpresentation, we found IL-15Rα–dependent changes in both neonatal thymic development and peripheral homeostasis in adulthood, suggesting that γδ-17 cells are dependent on cell-intrinsic signals received through IL-15Rα in cis.     

Enforcement of γδ-lineage commitment by the pre–T-cell receptor in precursors with weak γδ-TCR signals

Developing thymocytes bifurcate from a bipotent precursor into αβ- or γδ-lineage T cells. Considering this common origin and the fact that the T-cell receptor (TCR) β-, γ-, and δ-chains simultaneously rearrange at the double negative (DN) stage of development, the possibility exists that a given DN cell can express and transmit signals through both the pre-TCR and γδ-TCR. Here, we tested this scenario by defining the differentiation outcomes and criteria for lineage choice when both TCR-β and γδ-TCR are simultaneously expressed in Rag2^−/− DN cells via retroviral transduction. Our results showed that Rag2^−/− DN cells expressing both TCRs developed along the γδ-lineage, down-regulated CD24 expression, and up-regulated CD73 expression, showed a γδ-biased gene-expression profile, and produced IFN-γ in response to stimulation. However, in the absence of Inhibitor of DNA-binding 3 expression and strong γδ-TCR ligand, γδ-expressing cells showed a lower propensity to differentiate along the γδ-lineage. Importantly, differentiation along the γδ-lineage was restored by pre-TCR coexpression, which induced greater down-regulation of CD24, higher levels of CD73, Nr4a2, and Rgs1, and recovery of functional competence to produce IFN-γ. These results confirm a requirement for a strong γδ-TCR ligand engagement to promote maturation along the γδ T-cell lineage, whereas additional signals from the pre-TCR can serve to enforce a γδ-lineage choice in the case of weaker γδ-TCR signals. Taken together, these findings further cement the view that the cumulative signal strength sensed by developing DN cells serves to dictate its lineage choice.T cells can differentiate along distinct αβ- or γδ-cell lineages, but bifurcate from a common bipotent precursor (1, 2). In mice, the earliest subset of T cells contains CD4⁻ CD8⁻ or double-negative (DN) thymocytes, and this can further be divided into four subgroups (DN1–4) based on the expression of CD25 and CD44 (3, 4). Single-cell progenitor analyses have identified the DN3 stage as the point of T-lineage commitment, and also the final stage at which a DN cell specifies its lineage fate as αβ or γδ (1, 5). The αβ- or γδ-lineage choice decision is governed by several factors. Two competing models have been proposed for this process: the stochastic and instructional models (2). Although evidence exists to support either model, a version of the instructional model posits that the strength of signal transduced by the T-cell receptor (TCR) expressed by the DN3 cell dictates its lineage specification (6, 7).The apparent connection between lineage choice and the TCR expressed by the cell can be severed by manipulations of TCR signal strength. We previously noted that stimulating stronger signals via expression of the ERK/MAPK-induced Inhibitor of DNA-binding 3 (Id3) appears to promote the γδ-lineage fate in developing DN3 cells in the absence of TCR expression (8), suggesting a critical role for Id3 in mediating αβ- versus γδ-lineage decisions at this developmental checkpoint. Nevertheless, absence of Id3 also appears to favor the emergence of innate-like Vγ1.1/Vδ6.3 γδ-TCR–bearing T cells from the thymus over other γδ-TCR subsets (9, 10).Several studies have shown that ligand engagement highly influences the αβ- versus γδ-lineage decision because of its effects on γδ-TCR signal strength (6, 7, 9, 11, 12). γδ-TCR–expressing DN3 cells develop along the αβ-lineage and become CD4⁺ CD8⁺ (double-positive, DP) cells in the absence of ligand engagement (7), whereas provision of the ligand, or the use of antibodies to mimic ligand engagement (11), allows these cells to adopt the γδ-lineage fate, remain DN, and down-regulate expression of CD24. Additional signals, such as those mediated by Notch, can also influence αβ- versus γδ-lineage fate outcomes (1, 13–16). We showed that γδ-TCR–bearing thymocytes adopting the γδ-lineage do not require concurrent signals from Notch to mature past the DN3 stage, whereas their pre-TCR–expressing counterparts are completely dependent upon Notch signaling to facilitate their pre-TCR–dependent differentiation to the DP stage (1, 17).Considering the common origin of αβ- and γδ-lineage cells, it is possible for a bipotent DN3 cell to simultaneously express and transmit signals through a functional pre-TCR and a functional γδ-TCR, especially considering that TCR-β, -γ, and -δ genes complete their rearrangements at the DN3 stage. Additionally, γδ-T cells have been shown to contain TCR-β rearrangements (18) and αβ-lineage cells show evidence of both TCR-γ and -δ rearrangements (19–21). In a previous study looking to address the consequences of simultaneously expressing a TCR-β and γδ-TCR in vivo using transgenic (Tg) mice, the numbers of αβ- and γδ-lineage cells in TCR-β/γδ–expressing cells were both high, and comparable to TCR-β- and γδ-TCR-Tg mice, respectively (22). In this case, however, the TCR chains were expressed earlier than physiological for T-cell development, and premature expression of αβ-TCR transgene can lead to aberrant developmental progression (23, 24).Here, we attempt to definitively answer the question of lineage choice by simultaneously expressing TCR-β and γδ-TCR in Rag2^−/− DN3 cells via retroviral transduction followed by in vitro coculture, including limiting dilution and clonal analyses. We now find that Rag2^−/− DN3 cells expressing both pre-TCR and γδ-TCR mature along the γδ-lineage into functionally competent cells that produce IFN-γ in response to stimulation. However, in the absence of Id3 expression and strong γδ-TCR ligand, γδ-expressing cells show a lower propensity to differentiate along the γδ-lineage, but when expressing both pre- and γδ-TCRs, these cells showed increased γδ-lineage differentiation and recover functional competence to produce IFN-γ, indicating that the pre-TCR can serve to enforce to a γδ-lineage choice in the case of weaker γδ-TCR signals. Taken together, these findings further cement the view that the cumulative signal strength sensed by developing DN cells dictates its lineage choice.     

Cross-α/β polymorphism of PSMα3 fibrils

The formation of ordered cross-β amyloid protein aggregates is associated with a variety of human disorders. While conventional infrared methods serve as sensitive reporters of the presence of these amyloids, the recently discovered amyloid secondary structure of cross-α fibrils presents new questions and challenges. Herein, we report results using Fourier transform infrared spectroscopy and two-dimensional infrared spectroscopy to monitor the aggregation of one such cross-α–forming peptide, phenol soluble modulin alpha 3 (PSMα3). Phenol soluble modulins (PSMs) are involved in the formation and stabilization of Staphylococcus aureus biofilms, making sensitive methods of detecting and characterizing these fibrils a pressing need. Our experimental data coupled with spectroscopic simulations reveals the simultaneous presence of cross-α and cross-β polymorphs within samples of PSMα3 fibrils. We also report a new spectroscopic feature indicative of cross-α fibrils.

Amyloids are elongated fibers of proteins or peptides typically composed of stacked cross β-sheets (1, 2). Self-assembling amyloids are notorious for their involvement in human neurodegenerative diseases such as Alzheimer’s and Parkinson’s diseases (1, 2). Phenol soluble modulins (PSMs) are amyloid peptides secreted by the bacteria Staphylococcus aureus (S. aureus) (3–5). Of the PSM family, PSMα3 is of recent interest due to its unique secondary structure upon fibrillation. Whereas other PSM variants undergo conformational changes with aggregation, the α-helical PSMα3 peptide retains its secondary structure while stacking in a manner reminiscent of β-sheets, forming what has been termed cross-α fibrils (3, 4, 6). Although “α-sheet” amyloid fibrils have been previously observed in two-dimensional infrared (2DIR) (7) and associated with PSMs (8), the novel cross-α fibril is distinct from that class of structures. To avoid confusion between these two similarly named but distinct secondary structures, a comparison between the α-sheet domain in cytosolic phosphatase A2 (9) (Protein Data Bank [PDB] identification:1rlw) (10) and cross-α fibrils adopted by PSMα3 (PDB ID:5i55) (3) has been highlighted in SI Appendix, Fig. S1. Interestingly, shorter terminations of PSMα3 have been shown to exhibit β-sheet polymorphs (11). The proposed cross-α fibril structure of the full-length PSMα3 peptide has been confirmed with X-ray diffraction and circular dichroism (4). The present study aims to further characterize these fibrils with linear and nonlinear infrared spectroscopies.S. aureus is an infectious human pathogen with the ability to form communities of microorganisms called biofilms that hinder traditional treatment methods (12–14). PSMs contribute to inflammatory response and play a crucial role in structuring and detaching biofilms (11, 12, 14). While biofilm growth requires the presence of multiple PSMs (14, 15), Andreasen and Zaman have demonstrated that PSMα3 acts as a scaffold, seeding the amyloid formation of other PSMs (5). To effectively inhibit S. aureus biofilm growth, a better understanding of PSMα3 aggregation is needed.The α-helical structure of PSMα3 (12) presents a challenge for probing the vibrational modes and secondary structure of both the monomer and the fibrils. While IR spectroscopy has been used extensively to characterize β-sheets (16–19), the spectral features associated with α-helices are difficult to distinguish from those of the random coil secondary structure (20, 21). This limitation has left researchers to date with an incomplete picture of the spectroscopic features unique to cross-α fibers. The present work combines a variety of 2DIR methods to remove these barriers and probe the active infrared vibrational modes of cross-α fibers.The full-length, 22-residue PSMα3 peptide was synthesized and prepared for aggregation studies following reported methods (3, 4, 11). A total of 10 mM PSMα3 was incubated in D₂O at room temperature over 7 d. These data were compared to the monomer treated under similar conditions. Monomeric samples were prepared at a significantly lower concentration of 0.5 mM to prevent aggregation. Fiber formation was confirmed by transmission electron microscopy (see SI Appendix, Fig. S2 for details). Fourier transform infrared (FTIR) spectra were taken for both the fibrils in solution as well as the low concentration monomers. Spectroscopic simulations of the PSMα3 monomer and fibers were performed on previously reported PDB structures (PDB identification: 5i55) (3) (Fig. 1).Open in a separate windowFig. 1.PDB structures of PSMα3 (A) monomers and (B) cross-α fibers extended along the screw axis. (C) FTIR spectra of 0.5 mM monomeric PSMα3 (blue) compared to the 10 mM PSMα3 fibril (red) in D₂O upon aggregation.     

Pilot Study on Interferon-γ-producing T Cell Subsets after the Protective Vaccination with Radiation-attenuated Cercaria of Schistosoma japonicum in the Miniature Pig Model

CLAWN miniature pig has been shown to serve as a suitable host for the experimental infection of Schistosoma japonicum. In this study, we found that radiation-attenuated cercaria (RAC) vaccine gave CLAWN miniature pigs protective immunity against subsequent challenge infection with S. japonicum cercaria. To characterize the protective immune response of the pig model vaccinated by attenuated cercaria, flow cytometric analysis of the reactive T cell subsets was performed. The intracellular interferon (IFN)-γ and the cell surface markers revealed the peripheral blood CD3+ T-lymphocytes produced significant amounts of IFN-γ during the immunization period and after the challenge infection. CD4+ αβ-T cells as well as CD4+/CD8α^mid double positive and/or CD8α^high αβ-T cells were the major IFN-γ-producing CD3+ T cells. On the contrary, γδ T cells did not produce intracellular IFN-γ. Our results suggested that RAC-vaccinated miniature pigs showed effective protective immunity through the activation of αβ T cells bearing antigen specific T-cell receptors but not through the activation of γδ T cells.     

Multivalent structure of an αβT cell receptor

Whether there is one or multiple αβT cell antigen receptor (TCR) recognition modules in a given TCR/CD3 complex is a long-standing controversy in immunology. We show that T cells from transgenic mice that coexpress comparable amounts of two distinct TCRβ chains incorporate at least two αβTCRs in a single TCR/CD3 complex. Evidence for bispecific αβTCRs was obtained by immunoprecipitation and immunoblotting and confirmed on the surface of living cells both by fluorescence resonance energy transfer and comodulation assays by using antibodies specific for TCRβ-variable regions. Such (αβ)₂TCR/CD3 or higher-order complexes were evident in T cells studied either ex vivo or after expansion in vitro. T cell activation is thought by many, but not all, to require TCR cross-linking by its antigen/major histocompatibility complex ligand. The implications of a multivalent (αβ)₂TCR/CD3 complex stoichiometry for the ordered docking of specific antigen/major histocompatibility complex, CD4, or CD8 coreceptors and additional TCRs are discussed.