Photoaffinity antigens for human gammadelta T cells

Critical Roles for Coiled-Coil Dimers of Butyrophilin 3A1 in the Sensing of Prenyl Pyrophosphates by Human Vγ2Vδ2 T Cells

Vγ2Vδ2 T cells play important roles in human immunity to pathogens and tumors. Their TCRs respond to the sensing of isoprenoid metabolites, such as (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate and isopentenyl pyrophosphate, by butyrophilin (BTN) 3A1. BTN3A1 is an Ig superfamily protein with extracellular IgV/IgC domains and intracellular B30.2 domains that bind prenyl pyrophosphates. We have proposed that intracellular α helices form a coiled-coil dimer that functions as a spacer for the B30.2 domains. To test this, five pairs of anchor residues were mutated to glycine to destabilize the coiled-coil dimer. Despite maintaining surface expression, BTN3A1 mutagenesis either abrogated or decreased stimulation by (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate. BTN3A2 and BTN3A3 proteins and orthologs in alpacas and dolphins are also predicted to have similar coiled-coil dimers. A second short coiled-coil region dimerizes the B30.2 domains. Molecular dynamics simulations predict that mutation of a conserved tryptophan residue in this region will destabilize the dimer, explaining the loss of stimulation by BTN3A1 proteins with this mutation. The juxtamembrane regions of other BTN/BTN-like proteins with B30.2 domains are similarly predicted to assume α helices, with many predicted to form coiled-coil dimers. An exon at the end of this region and the exon encoding the dimerization region for B30.2 domains are highly conserved. We propose that coiled-coil dimers function as rod-like helical molecular spacers to position B30.2 domains, as interaction sites for other proteins, and as dimerization regions to allow sensing by B30.2 domains. In these ways, the coiled-coil domains of BTN3A1 play critical roles for its function.

A Photo-Crosslinkable Biotin Derivative of the Phosphoantigen (E)-4-Hydroxy-3-Methylbut-2-Enyl Diphosphate (HMBPP) Activates Vγ9Vδ2 T Cells and Binds to the HMBPP Site of BTN3A1

Vγ9Vδ2 T cells play an important role in the cross talk of the innate and adaptive immune system. For their activation by phosphoantigens (PAgs), both cell surface receptors, the eponymous Vγ9Vδ2 T cell antigen receptors (Vγ9Vδ2 TCRs) on Vγ9Vδ2 T cells and butyrophilin 3A1 (BTN3A1) on the phosphoantigen-"presenting" cell, are mandatory. To find yet undetected but further contributing proteins, a biotinylated, photo-crosslinkable benzophenone probe BioBP-HMBPP (2) was synthesized from a known allyl alcohol in nine steps and overall 16 % yield. 2 is based on the picomolar PAg (E)-4-hydroxy-3-methylbut-2-enyl diphosphate (HMBPP, 1). Laser irradiation of 2 at 308 nm initiated the photo-crosslinking reaction with proteins. When the B30.2 domain of BTN3A1, which contains a positively charged PAg-binding pocket, was exposed to increasing amounts of HMBPP (1), labeling by BioBP-HMBPP (2) was reduced significantly. Because BSA labeling was not impaired, 2 clearly binds to the same site as natural ligand 1. Thus, BioBP-HMBPP (2) is a suitable tool to identify co-ligands or receptors involved in PAg-mediated T cell activation.

The Life of Pi Star: Exploring the Exciting and Forbidden Worlds of the Benzophenone Photophore

The widespread applications of benzophenone (BP) photochemistry in biological chemistry, bioorganic chemistry, and material science have been prominent in both academic and industrial research. BP photophores have unique photochemical properties: upon n-π* excitation at 365 nm, a biradicaloid triplet state is formed reversibly, which can abstract a hydrogen atom from accessible C-H bonds; the radicals subsequently recombine, creating a stable covalent C-C bond. This light-directed covalent attachment process is exploited in many different ways: (i) binding/contact site mapping of ligand (or protein)-protein interactions; (ii) identification of molecular targets and interactome mapping; (iii) proteome profiling; (iv) bioconjugation and site-directed modification of biopolymers; (v) surface grafting and immobilization. BP photochemistry also has many practical advantages, including low reactivity toward water, stability in ambient light, and the convenient excitation at 365 nm. In addition, several BP-containing building blocks and reagents are commercially available. In this review, we explore the "forbidden" (transitions) and excitation-activated world of photoinduced covalent attachment of BP photophores by touring a colorful palette of recent examples. In this exploration, we will see the pros and cons of using BP photophores, and we hope that both novice and expert photolabelers will enjoy and be inspired by the breadth and depth of possibilities.

Sensor Function for Butyrophilin 3A1 in Prenyl Pyrophosphate Stimulation of Human Vγ2Vδ2 T Cells

Vγ2Vδ2 T cells play important roles in human immunity to pathogens and in cancer immunotherapy by responding to isoprenoid metabolites, such as (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate and isopentenyl pyrophosphate. The Ig superfamily protein butyrophilin (BTN)3A1 was shown to be required for prenyl pyrophosphate stimulation. We proposed that the intracellular B30.2 domain of BTN3A1 binds prenyl pyrophosphates, resulting in a change in the extracellular BTN3A1 dimer that is detected by Vγ2Vδ2 TCRs. Such B30.2 binding was demonstrated recently. However, other investigators reported that the extracellular BTN3A1 IgV domain binds prenyl pyrophosphates, leading to the proposal that the Vγ2Vδ2 TCR recognizes the complex. To distinguish between these mechanisms, we mutagenized residues in the two binding sites and tested the mutant BTN3A1 proteins for their ability to mediate prenyl pyrophosphate stimulation of Vγ2Vδ2 T cells to proliferate and secrete TNF-α. Mutagenesis of residues in the IgV site had no effect on Vγ2Vδ2 T cell proliferation or secretion of TNF-α. In contrast, mutagenesis of residues within the basic pocket and surrounding V regions of the B30.2 domain abrogated prenyl pyrophosphate-induced proliferation. Mutations of residues making hydrogen bonds to the pyrophosphate moiety also abrogated TNF-α secretion, as did mutation of aromatic residues making contact with the alkenyl chain. Some mutations further from the B30.2 binding site also diminished stimulation, suggesting that the B30.2 domain may interact with a second protein. These findings support intracellular sensing of prenyl pyrophosphates by BTN3A1 rather than extracellular presentation.

Role of Innate T Cells in Anti-Bacterial Immunity

Innate T cells are a heterogeneous group of αβ and γδ T cells that respond rapidly (<2 h) upon activation. These innate T cells also share a non MHC class I or II restriction requirement for antigen recognition. Three major populations within the innate T cell group are recognized, namely, invariant NKT cells, mucosal associated invariant T cells, and gamma delta T cells. These cells recognize foreign/self-lipid presented by non-classical MHC molecules, such as CD1d, MR1, and CD1a. They are activated during the early stages of bacterial infection and act as a bridge between the innate and adaptive immune systems. In this review, we focus on the functional properties of these three innate T cell populations and how they are purposed for antimicrobial defense. Furthermore, we address the mechanisms through which their effector functions are targeted for bacterial control and compare this in human and murine systems. Lastly, we speculate on future roles of these cell types in therapeutic settings such as vaccination.

Bovine gamma delta T cells and the function of gamma delta T cell specific WC1 co-receptors

The study of γδ T cells in ruminants dates to the discovery of the γδ TCR in humans and mice. It is important since cattle offer an alternative model to the mouse for evaluating the role of γδ T cells in zoonotic disease research and for control of disease reservoirs in non-human animals. In addition, maintaining the health of cattle and other members of the order Artiodactyla is critical to meet the global human need for animal-source protein. In this review, we examine the bovine γδ T cell responses to Mycobacteria, which infects a third of the human population, and bovine γ and δ TCR diversity and the relationship to the TCR of human mycobacteria-responsive γδ T cells. We review the utilization of the γδ T cell specific scavenger receptor cysteine-rich (SRCR) glycoproteins known as WC1, and that are part of the CD163 family, which function as both γδ T cell activating co-receptors and pattern recognition receptors (PRR) for bovine γδ T cells and highlight the presence and evolution of this multigenic array, with potential for the same function, in birds, reptiles, jawless and bony fishes, and prototherian and eutherian mammals.

Molecules and Mechanisms Implicated in the Peculiar Antigenic Activation Process of Human Vγ9Vδ2 T Cells

In human beings, as well as in most non-human primates, the major peripheral γδ T cell subset, which accounts several percent of the whole lymphoid cells pool in adults, carries an heterodimeric TCR composed of Vγ9 and Vδ2 chains. Vγ9Vδ2 T cells are specifically and strongly activated by small organic pyrophosphate molecules termed phosphoantigens (phosphoAg). These low molecular weight compounds are metabolites that are produced by either microbes or endogenously, as intermediates of the mammalian mevalonate pathway, and can accumulate intracellularly during cell stress like transformation or infection. Despite the characterization of numerous natural and synthetic phosphoAg, the mechanism(s) underlying the unique and specific antigenic activation process induced by these compounds remains poorly understood. Activation is both TCR- and cell-to-cell contact-dependent, and results of previous studies have also strongly suggested a key contribution of membrane-associated molecules of primate origin expressed on target cells. The recent identification of B7-related butyrophilin (BTN) molecules CD277/BTN3A, and more precisely their BTN3A1 isoforms, as mandatory molecules in the phosphoAg-induced recognition of target cells by Vγ9Vδ2 T cells opens important opportunities for research and applications in this field. Here, we review the unusual and complex antigenic reactivity of human Vγ9Vδ2 T cells. We highlight the recent advances in our understanding of this process, and propose a model that integrates the type I glycoprotein BTN3A1 and its intracellular B30.2 domain as a physical intermediate implicated in the detection of dysregulated intracellular levels of phosphoAg and the sensing of cell stress by Vγ9Vδ2T cells. A better understanding of this mechanism will help optimize novel immunotherapeutical approaches that utilize the unique functional potential of this major γδ T cell subset.

The Vγ9Vδ2 T Cell Antigen Receptor and Butyrophilin-3 A1: Models of Interaction, the Possibility of Co-Evolution, and the Case of Dendritic Epidermal T Cells

Most circulating human gamma delta T cells are Vγ9Vδ2 T cells. Their hallmark is the expression of T cell antigen receptors (TCR) whose γ-chains show a Vγ9-JP (Vγ2-Jγ1.2) rearrangement and are paired with Vδ2-containing δ-chains, a dominant TCR configuration, which until recently seemed to occur in primates only. Vγ9Vδ2 T cells respond to phosphoantigens (PAg) such as (E)-4-Hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP), which is produced by many pathogens and isopentenyl pyrophosphate (IPP), which accumulates in certain tumors or cells treated with aminobisphosphonates such as zoledronate. A prerequisite for PAg-induced activation is the contact of Vγ9Vδ2 T cells with cells expressing butyrophilin-3 A1 (BTN3A1). We will first critically review models of how BTN3 might act in PAg-mediated Vγ9Vδ2 T cell activation and then address putative co-evolution of Vγ9, Vδ2, and BTN3 genes. In those rodent and lagomorphs used as animal models, all three genes are lost but a data-base analysis showed that they emerged together with placental mammals. A strong concomitant conservation of functional Vγ9, Vδ2, and BTN3 genes in other species suggests co-evolution of these three genes. A detailed analysis was performed for the new world camelid alpaca (Vicugna pacos). It provides an excellent candidate for a non-primate species with presumably functional Vγ9Vδ2 T cells since TCR rearrangements share features characteristic for PAg-reactive primate Vγ9Vδ2 TCR and proposed PAg-binding sites of BTN3A1 have been conserved. Finally, we analyze the possible functional relationship between the butyrophilin-family member Skint1 and the γδ TCR-V genes used by murine dendritic epithelial T cells (DETC). Among placental mammals, we identify five rodents, the cow, a bat, and the cape golden mole as the only species concomitantly possessing potentially functional homologs of murine Vγ3, Vδ4 genes, and Skint1 gene and suggest to search for DETC like cells in these species.

Metabolic engineering of Salmonella vaccine bacteria to boost human Vγ2Vδ2 T cell immunity

Human Vγ2Vδ2 T cells monitor isoprenoid metabolism by recognizing foreign (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP), a metabolite in the 2-C-methyl-D-erythritol-4-phosphate pathway used by most eubacteria and apicomplexan parasites, and self isopentenyl pyrophosphate, a metabolite in the mevalonate pathway used by humans. Whereas microbial infections elicit prolonged expansion of memory Vγ2Vδ2 T cells, immunization with prenyl pyrophosphates or aminobisphosphonates elicit short-term Vγ2Vδ2 expansion with rapid anergy and deletion upon subsequent immunizations. We hypothesized that a live, attenuated bacterial vaccine that overproduces HMBPP would elicit long-lasting Vγ2Vδ2 T cell immunity by mimicking a natural infection. Therefore, we metabolically engineered the avirulent aroA(-) Salmonella enterica serovar Typhimurium SL7207 strain by deleting the gene for LytB (the downstream enzyme from HMBPP) and functionally complementing for this loss with genes encoding mevalonate pathway enzymes. LytB(-) Salmonella SL7207 had high HMBPP levels, infected human cells as efficiently as did the wild-type bacteria, and stimulated large ex vivo expansions of Vγ2Vδ2 T cells from human donors. Importantly, vaccination of a rhesus monkey with live lytB(-) Salmonella SL7207 stimulated a prolonged expansion of Vγ2Vδ2 T cells without significant side effects or anergy induction. These studies provide proof-of-principle that metabolic engineering can be used to derive live bacterial vaccines that boost Vγ2Vδ2 T cell immunity. Similar engineering of metabolic pathways to produce lipid Ags or B vitamin metabolite Ags could be used to derive live bacterial vaccine for other unconventional T cells that recognize nonpeptide Ags.

γδ T cells and their potential for immunotherapy

Vγ9Vδ2 (also termed Vγ2Vδ2) T cells, a major human peripheral blood γδ T cell subset, recognize microbial (E)-4-hydroxy-3-methylbut-2-enyl diphosphate and endogenous isopentenyl diphosphate in a TCR-dependent manner. The recognition does not require specific accessory cells, antigen uptake, antigen processing, or MHC class I, class II, or class Ib expression. This subset of T cells plays important roles in mediating innate immunity against a wide variety of infections and displays potent and broad cytotoxic activity against human tumor cells. Because γδT cells express both natural killer receptors such as NKG2D and γδ T cell receptors, they are considered to represent a link between innate and adaptive immunity. In addition, activated γδ T cells express a high level of antigen-presenting cell-related molecules and can present peptide antigens derived from destructed cells to αβ T cells. Utilizing these antimicrobial and anti-tumor properties of γδ T cells, preclinical and clinical trials have been conducted to develop novel immunotherapies for infections and malignancies. Here, we review the immunological properties of γδ T cells including the underlying recognition mechanism of nonpeptitde antigens and summarize the results of γδ T cell-based therapies so far performed. Based on the results of the reported trials, γδ T cells appear to be a promising tool for novel immunotherapies against certain types of diseases.

The tumor targeted superantigen ABR-217620 selectively engages TRBV7-9 and exploits TCR-pMHC affinity mimicry in mediating T cell cytotoxicity

The T lymphocytes are the most important effector cells in immunotherapy of cancer. The conceptual objective for developing the tumor targeted superantigen (TTS) ABR-217620 (naptumomab estafenatox, 5T4Fab-SEA/E-120), now in phase 3 studies for advanced renal cell cancer, was to selectively coat tumor cells with cytotoxic T lymphocytes (CTL) target structures functionally similar to natural CTL pMHC target molecules. Here we present data showing that the molecular basis for the anti-tumor activity by ABR-217620 resides in the distinct interaction between the T cell receptor β variable (TRBV) 7-9 and the engineered superantigen (Sag) SEA/E-120 in the fusion protein bound to the 5T4 antigen on tumor cells. Multimeric but not monomeric ABR-217620 selectively stains TRBV7-9 expressing T lymphocytes from human peripheral blood similar to antigen specific staining of T cells with pMHC tetramers. SEA/E-120 selectively activates TRBV7-9 expressing T lymphocytes resulting in expansion of the subset. ABR-217620 selectively triggers TRBV7-9 expressing cytotoxic T lymphocytes to kill 5T4 positive tumor cells. Furthermore, ABR-217620 activates TRBV7-9 expressing T cell line cells in the presence of cell- and bead-bound 5T4 tumor antigen. Surface plasmon resonance analysis revealed that ABR-217620 binds to 5T4 with high affinity, to TRBV7-9 with low affinity and to MHC class II with very low affinity. The T lymphocyte engagement by ABR-217620 is constituted by displaying high affinity binding to the tumor cells (K_D approximately 1 nM) and with the mimicry of natural productive immune TCR-pMHC contact using affinities of around 1 µM. This difference in kinetics between the two components of the ABR-217620 fusion protein will bias the binding towards the 5T4 target antigen, efficiently activating T-cells via SEA/E-120 only when presented by the tumor cells.

Butyrophilin 3A1 binds phosphorylated antigens and stimulates human γδ T cells

Human T cells that express a T cell antigen receptor (TCR) containing γ-chain variable region 9 and δ-chain variable region 2 (Vγ9Vδ2) recognize phosphorylated prenyl metabolites as antigens in the presence of antigen-presenting cells but independently of major histocompatibility complex (MHC), the MHC class I-related molecule MR1 and antigen-presenting CD1 molecules. Here we used genetic approaches to identify the molecule that binds and presents phosphorylated antigens. We found that the butyrophilin BTN3A1 bound phosphorylated antigens with low affinity, at a stoichiometry of 1:1, and stimulated mouse T cells with transgenic expression of a human Vγ9Vδ2 TCR. The structures of the BTN3A1 distal domain in complex with host- or microbe-derived phosphorylated antigens had an immunoglobulin-like fold in which the antigens bound in a shallow pocket. Soluble Vγ9Vδ2 TCR interacted specifically with BTN3A1-antigen complexes. Accordingly, BTN3A1 represents an antigen-presenting molecule required for the activation of Vγ9Vδ2 T cells.

Butyrophilin 3A1 plays an essential role in prenyl pyrophosphate stimulation of human Vγ2Vδ2 T cells

Most human γδ T cells express Vγ2Vδ2 TCRs and play important roles in microbial and tumor immunity. Vγ2Vδ2 T cells are stimulated by self- and foreign prenyl pyrophosphate intermediates in isoprenoid synthesis. However, little is known about the molecular basis for this stimulation. We find that a mAb specific for butyrophilin 3 (BTN3)/CD277 Ig superfamily proteins mimics prenyl pyrophosphates. The 20.1 mAb stimulated Vγ2Vδ2 T cell clones regardless of their functional phenotype or developmental origin and selectively expanded blood Vγ2Vδ2 T cells. The γδ TCR mediates 20.1 mAb stimulation because IL-2 is released by β(-) Jurkat cells transfected with Vγ2Vδ2 TCRs. 20.1 stimulation was not due to isopentenyl pyrophosphate (IPP) accumulation because 20.1 treatment of APC did not increase IPP levels. In addition, stimulation was not inhibited by statin treatment, which blocks IPP production. Importantly, small interfering RNA knockdown of BTN3A1 abolished stimulation by IPP that could be restored by re-expression of BTN3A1 but not by BTN3A2 or BTN3A3. Rhesus monkey and baboon APC presented HMBPP and 20.1 to human Vγ2Vδ2 T cells despite amino acid differences in BTN3A1 that localize to its outer surface. This suggests that the conserved inner and/or top surfaces of BTN3A1 interact with its counterreceptor. Although no binding site exists on the BTN3A1 extracellular domains, a model of the intracellular B30.2 domain predicts a basic pocket on its binding surface. However, BTN3A1 did not preferentially bind a photoaffinity prenyl pyrophosphate. Thus, BTN3A1 is required for stimulation by prenyl pyrophosphates but does not bind the intermediates with high affinity.

Tripartite immune cell co-operation in the Bacillus Calmette Guérin-induced activation of γδ T cells

γδ T cells contribute to immunosurveillance of pathogenic infections and malignant transformations; however, mechanisms of activation have yet to be fully defined. In this study we demonstrate a novel mechanism by which human Vδ2(+) γδ T cells are activated by the model pathogen Bacillus Calmette Guérin (BCG). We show in vitro that Vδ2 cell cytokine production and cytotoxic activity in response to BCG are dependent on both dendritic cells (DCs) and memory CD4(+) αβ T cells (CD4 T cells). We found that Vδ2 cells are indirectly activated by BCG in an interleukin (IL)-12p70-dependent manner, and that DC production of the IL-12p70 responsible for Vδ2 cell activation requires Toll-like receptor 2/4 ligands from BCG and interferon (IFN)-γ from memory CD4 T cells. Our data suggest that Vδ2 cell responses to BCG are dependent on the activation of IFN-γ-producing memory CD4 T cells, and provide novel insight into the complex interplay between cells of the innate and adaptive immune response.

Identification of a new tuberculosis antigen recognized by γδ T cell receptor

The immune protection initiated by γδ T cells plays an important role in mycobacterial infection. The γδ T cells activated by Mycobacterium tuberculosis-derived nonpeptidic, phosphorylated biometabolites (phosphoantigens) provide only partial immune protection against mycobacterium, while evidence has suggested that protein antigen-activated γδ T cells elicit effective protective immune responses. To date, only a few distinct mycobacterial protein antigens have been identified. In the present study, we screened protein antigens recognized by γδ T cells using cells transfected with the predominant pulmonary tuberculosis γδ T cell receptor (TCR) CDR3 fragment. We identified two peptides, TP1 and TP2, which not only bind to the pulmonary tuberculosis predominant γδ TCR but also effectively activate γδ T cells isolated from pulmonary tuberculosis patients. Moreover, 1-deoxy-d-xylulose 5-phosphate synthase 2 (DXS2), the TP1-matched mycobacterial protein, was confirmed as a ligand for the γδ TCR and was found to activate γδ T cells from pulmonary tuberculosis patients. The extracellular region (extracellular peptide [EP]) of Rv2272, a TP2-matched mycobacterial transmembrane protein, was also shown to activate γδ T cells from pulmonary tuberculosis patients. Both DXS2- and EP-expanded γδ T cells from pulmonary tuberculosis patients could secrete gamma interferon (IFN-γ) and monocyte chemoattractant protein 1 (MCP-1), which play important roles in mediating cytotoxicity against mycobacterium and stimulating monocyte chemotaxis toward the site of infection. In conclusion, our study identified novel mycobacterial protein antigens recognized by γδ TCR cells that could be candidates for the development of vaccines or adjuvants against mycobacterium infection.

Diversity of γδ T-cell antigens

In the last two decades, it has become clear that γδ T cells recognize a diverse array of antigens including self and foreign, large and small, and peptidic and non-peptidic molecules. In this respect, γδ antigens as a whole resemble more the antigens recognized by antibodies than those recognized by αβ T cells. Because of this antigenic diversity, no single mechanism-such as the major histocompatibility complex (MHC) restriction of αβ T cells-is likely to provide a basis for all observed T-cell antigen receptor (TCR)-dependent γδ T-cell responses. Furthermore, available evidence suggests that many individual γδ T cells are poly-specific, probably using different modes of ligand recognition in their responses to unrelated antigens. While posing a unique challenge in the maintenance of self-tolerance, this broad reactivity pattern might enable multiple overlapping uses of γδ T-cell populations, and thus generate a more efficient immune response.

Human γδ T-cell responses in infection and immunotherapy: common mechanisms, common mediators?

Upon receiving the Nobel Prize in Physiology or Medicine in 1987, Susumu Tonegawa referred to the then recent discovery of the γδ T-cell receptor and stated that "while the function of the T cells bearing this receptor is currently unknown (…) these T cells may be involved in an entirely new aspect of immunity". [Tonegawa, S., Scand. J. Immunol. 1993. 38: 303-319]. Twenty-five years of intense research later this ambivalent view still holds true. Immunologists now appreciate that γδ T cells indeed represent a highly intriguing "new aspect of immunity" that is unique and distinct from conventional lymphocytes, yet even scientists in the field still struggle to understand the molecular basis of γδ T-cell responses, especially with respect to the enigmatic mode of antigen recognition. Here, we portray the peculiar responsiveness of human Vγ9/Vδ2 T cells to microorganisms, tumor cells and aminobisphosphonates, in an attempt to integrate the corresponding - and at times confusing - findings into a "theory of everything" that may help explain how such diverse stimuli result in similar γδ T-cell responses via the recognition of soluble low molecular weight phosphoantigens.

Multifunctional immune responses of HMBPP-specific Vγ2Vδ2 T cells in M. tuberculosis and other infections

Vγ2Vδ2 T (also known as Vγ9Vδ2 T) cells exist only in primates, and in humans represent a major γδ T-cell sub-population in the total population of circulating γδ T cells. Results from recent studies suggest that while (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP) phosphoantigen from Mycobacterium tuberculosis (Mtb) and other microbes activates and expands primate Vγ2Vδ2 T cells, the Vγ2Vδ2 T-cell receptor (TCR) recognizes and binds to HMBPP on antigen-presenting cells (APC). In response to HMBPP stimulus, Vγ2Vδ2 TCRs array to form signaling-related nanoclusters or nanodomains during the activation of Vγ2Vδ2 T cells. Primary infections with HMBPP-producing pathogens drive the evolution of multieffector functional responses in Vγ2Vδ2 T cells, although Vγ2Vδ2 T cells display different patterns of responses during the acute and chronic phases of Mtb infection and in other infections. Expanded Vγ2Vδ2 T cells in primary Mtb infection can exhibit a broader TCR repertoire and a greater clonal response than previously assumed, with different distribution patterns of Vγ2Vδ2 T-cell clones in lymphoid and non-lymphoid compartments. Emerging in vivo data suggest that HMBPP activation of Vγ2Vδ2 T cells appears to impact other immune cells during infection.

Primary MHC-class II(+) cells are necessary to promote resting Vδ2 cell expansion in response to (E)-4-hydroxy-3-methyl-but-2-enyl-pyrophosphate and isopentenyl pyrophosphate

Human Vγ9δ2 (Vδ2) T cells represent a unique effector T cell population in humans and primates detecting nonpeptid phosphoantigens, playing an important role in antimicrobial and antitumor immunity. Currently, it is believed that various leukocyte subsets can promote phosphoantigen-driven Vδ2 cell expansion, but the essential cell type required remains elusive. We have used high purity cell sorting to analyze the cellular requirements for (E)-4-hydroxy-3-methyl-but-2-enyl-pyrophosphate (HMBPP)-driven Vδ2 cell expansion. To our knowledge, we show for the first time that primary human MHC-class II(+) cells are indispensable for HMBPP- and isopentenylpyrophosphate-driven Vδ2 cell expansion. In contrast, MHC-class II(-) cells are unable to promote Vδ2 cell expansion. Moreover, purified primary human TCRαβ(+) T cells, CD4(+), or CD8(+) T cells also failed to promote HMBPP-mediated Vδ2 expansion. Depletion of CD4(+)CD25(+) T cells demonstrated that inability of TCRαβ(+) cells to expand Vδ2 cells was not related to the presence of regulatory T cells. Separation of MHC-class II(+) cells into dendritic cells, monocytes, and B cells revealed that dendritic cells were the most potent Vδ2 expanders. Pulsing experiments demonstrated that HMBPP transforms MHC-class II(+) but not negative cells into Vδ2 expanders. MHC-class II-blocking experiments with mAbs and secondary MHC-class II induction on CD4(+) T cells after CD3/CD28 costimulation indicated that MHC-class II is necessary, but not sufficient to promote Vδ2 expansion. Our results provide novel insight into the primary cell-specific requirements for human Vδ2 expansion.

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.

Key implication of CD277/butyrophilin-3 (BTN3A) in cellular stress sensing by a major human γδ T-cell subset

Human peripheral Vγ9Vδ2 T cells are activated by phosphorylated metabolites (phosphoagonists [PAg]) of the mammalian mevalonate or the microbial desoxyxylulose-phosphate pathways accumulated by infected or metabolically distressed cells. The underlying mechanisms are unknown. We show that treatment of nonsusceptible target cells with antibody 20.1 against CD277, a member of the extended B7 superfamily related to butyrophilin, mimics PAg-induced Vγ9Vδ2 T-cell activation and that the Vγ9Vδ2 T-cell receptor is implicated in this effect. Vγ9Vδ2 T-cell activation can be abrogated by exposing susceptible cells (tumor and mycobacteria-infected cells, or aminobisphosphonate-treated cells with up-regulated PAg levels) to antibody 103.2 against CD277. CD277 knockdown and domain-shuffling approaches confirm the key implication of the CD277 isoform BTN3A1 in PAg sensing by Vγ9Vδ2 T cells. Fluorescence recovery after photobleaching (FRAP) experiments support a causal link between intracellular PAg accumulation, decreased BTN3A1 membrane mobility, and ensuing Vγ9Vδ2 T-cell activation. This study demonstrates a novel role played by B7-like molecules in human γδ T-cell antigenic activation and paves the way for new strategies to improve the efficiency of immunotherapies using Vγ9Vδ2 T cells.

Target specificity of an autoreactive pathogenic human γδ-T cell receptor in myositis

In polymyositis and inclusion body myositis, muscle fibers are surrounded and invaded by CD8-positive cytotoxic T cells expressing the αβ-T cell receptor (αβ-TCR) for antigen. In a rare variant of myositis, muscle fibers are similarly attacked by CD8-negative T cells expressing the γδ-TCR (γδ-T cell-mediated myositis). We investigated the antigen specificity of a human γδ-TCR previously identified in an autoimmune tissue lesion of γδ-T cell-mediated myositis. We show that this Vγ1.3Vδ2-TCR, termed M88, recognizes various proteins from different species. Several of these proteins belong to the translational apparatus, including some bacterial and human aminoacyl-tRNA synthetases (AA-RS). Specifically, M88 recognizes histidyl-tRNA synthetase, an antigen known to be also targeted by autoantibodies called anti-Jo-1. The M88 target epitope is strictly conformational, independent of post-translational modification, and exposed on the surface of the respective antigenic protein. Extensive mutagenesis of the translation initiation factor-1 from Escherichia coli (EcIF1), which served as a paradigm antigen with known structure, showed that a short α-helical loop around amino acids 39 to 42 of EcIF1 is a major part of the M88 epitope. Mutagenesis of M88 showed that the complementarity determining regions 3 of both γδ-TCR chains contribute to antigen recognition. M88 is the only known example of a molecularly characterized γδ-TCR expressed by autoaggressive T cells in tissue. The observation that AA-RS are targeted by a γδ-T cell and by autoantibodies reveals an unexpected link between T cell and antibody responses in autoimmune myositis.

Synthesis and immunological evaluation of the 4-β-glucoside of HMBPP

HMBPP ((E)-4-hydroxy-3-methyl-2-butenyl pyrophosphate) is a highly potent innate immunogen that stimulates human γδ T cells expressing the Vγ2Vδ2 T cell antigen receptor. To determine if glycoside conjugates of HMBPP retain activity, the 4-β-glucoside and its acetylated homolog were synthesized and tested for their ability to stimulate γδ T cells. The glycoside HMBPP conjugate stimulated human γδ T cells with an EC(50) of 78nM. The tetraacetyl glycoside HMBPP conjugate was also active (EC(50)=360nM). The two isomeric mono-β-glucosides of the parent (E)-2-methylbut-2-ene-1,4-diol, however, were not active. Thus, HMBPP glycosylated at the 4-OH position stimulates γδ T cells as long as the pyrophosphate moiety is present.

Deep sequencing of the human TCRγ and TCRβ repertoires suggests that TCRβ rearranges after αβ and γδ T cell commitment

T lymphocytes respond to a broad array of pathogens with the combinatorial diversity of the T cell receptor (TCR). This adaptive response is possible because of the unique structure of the TCR, which is composed of two chains, either αβ or γδ, that undergo genetic rearrangement in the thymus. αβ and γδ T cells are functionally distinct within the host but are derived from a common multipotent precursor. The canonical model for T cell lineage commitment assumes that the γ, δ, and β chains rearrange before αβ or γδ T cell commitment. To test the standard model in humans, we used high-throughput sequencing to catalog millions of TCRγ and TCRβ chains from peripheral blood αβ and γδ T cells from three unrelated individuals. Almost all sampled αβ and γδ T cells had rearranged TCRγ sequences. Although sampled αβ T cells had a diverse repertoire of rearranged TCRβ chains, less than 4% of γδ T cells in peripheral blood had a rearranged TCRβ chain. Our data suggest that TCRγ rearranges in all T lymphocytes, consistent with TCRγ rearranging before T cell lineage commitment. However, rearrangement of the TCRβ locus appears to be restricted after T cell precursors commit to the αβ T cell lineage. Indeed, in T cell leukemias and lymphomas, TCRγ is almost always rearranged and TCRβ is only rearranged in a subset of cancers. Because high-throughput sequencing of TCRs is translated into the clinic for monitoring minimal residual for leukemia/lymphoma, our data suggest the sequencing target should be TCRγ.

Indirect stimulation of human Vγ2Vδ2 T cells through alterations in isoprenoid metabolism

Human Vγ2Vδ2 T cells monitor isoprenoid metabolism by recognizing (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP), an intermediate in the 2-C-methyl-d-erythritol-4-phosphate pathway used by microbes, and isopentenyl pyrophosphate (IPP), an intermediate in the mevalonate pathway used by humans. Aminobisphosphonates and alkylamines indirectly stimulate Vγ2Vδ2 cells by inhibiting farnesyl diphosphate synthase (FDPS) in the mevalonate pathway, thereby increasing IPP/triphosphoric acid 1-adenosin-5'-yl ester 3-(3-methylbut-3-enyl) ester that directly stimulate. In this study, we further characterize stimulation by these compounds and define pathways used by new classes of compounds. Consistent with FDPS inhibition, stimulation of Vγ2Vδ2 cells by aminobisphosphonates and alkylamines was much more sensitive to statin inhibition than stimulation by prenyl pyrophosphates; however, the continuous presence of aminobisphosphonates was toxic for T cells and blocked their proliferation. Aminobisphosphonate stimulation was rapid and prolonged, independent of known Ag-presenting molecules, and resistant to fixation. New classes of stimulatory compounds-mevalonate, the alcohol of HMBPP, and alkenyl phosphonates-likely stimulate differently. Mevalonate, a rate-limiting metabolite, appears to enter cells to increase IPP levels, whereas the alcohol of HMBPP and alkenyl phosphonates are directly recognized. The critical chemical feature of bisphosphonates is the amino moiety, because its loss switched aminobisphosphonates to direct Ags. Transfection of APCs with small interfering RNA downregulating FDPS rendered them stimulatory for Vγ2Vδ2 cells and increased cellular IPP. Small interfering RNAs for isopentenyl diphosphate isomerase functioned similarly. Our results show that a variety of manipulations affecting isoprenoid metabolism lead to stimulation of Vγ2Vδ2 T cells and that pulsing aminobisphosphonates would be more effective for the ex vivo expansion of Vγ2Vδ2 T cells for adoptive cancer immunotherapy.

Peptide antigens for gamma/delta T cells

γδ T cells express adaptive antigen receptors encoded by rearranging genes. Their diversity is highest in the small region of TCR V-J junctions, especially in the δ chain, which should enable the γδ TCRs to distinguish differences in small epitopes. Indeed, recognition of small molecules, and of an epitope on a larger protein has been reported. Responses to small non-peptides known as phospho-antigens are multi-clonal yet limited to a single γδ T cell subset in humans and non-human primates. Responses to small peptides are multi-clonal or oligo-clonal, include more than one subset of γδ T cells, and occur in rodents and primates. However, less effort has been devoted to investigate the peptide responses. To settle the questions of whether peptides can be ligands for the γδ TCRs, and whether responses to small peptides might occur normally, peptide binding will have to be demonstrated, and natural peptide ligands identified.

γδ T-APCs: a novel tool for immunotherapy?

The series of seminal articles in this book clearly illustrate the multi-functional nature of γδ T cells. Some of the functions correlate with the tissue tropism of distinct γδ T cell subsets whereas others appear to result from oligoclonal selection. Here, we discuss the antigen-presenting cell (APC) function of the major subset of circulating γδ T cells, Vγ9/Vδ2 T cells, present in human blood. During tissue culture, Vγ9/Vδ2 T cells uniformly respond to a class of non-peptide antigens, so-called prenyl pyrophosphates, derived from stressed host cells or from microbes. It is this feature that distinguishes human (and primate) Vγ9/Vδ2 T cells from αβ and γδ T cells of all other species and that forms the basis for detailed studies of human Vγ9/Vδ2 T cells. One of the consequences of Vγ9/Vδ2 T cell activation is the rapid acquisition of APC characteristics (γδ T-APCs) reminiscent of mature dendritic cells (DCs). In the following discussion, we will discriminate between the potential use of γδ T-APCs as a cellular vaccine in immunotherapy and their role in anti-microbial immunity. Exploiting the APC function in γδ T-APCs represents a true novelty in current immunotherapy research and may lead to effective, anti-tumor immunity in cancer patients.

Immune biology of Ag-specific γδ T cells in infections

Accumulating evidence suggests that human γδ T cells act as non-classical T cells and contribute to both innate and adaptive immune responses in infections. Vγ2 Vδ2 T (also termed Vγ9 Vδ2 T) cells exist only in primates, and in humans represent a dominant circulating γδ T-cell subset. Primate Vγ2 Vδ2 T cells are the only γδ T cell subset capable of recognizing microbial phosphoantigen. Since nonhuman primate Vγ2 Vδ2 T cells resemble their human counterparts, in-depth studies have been undertaken in macaques to understand the biology and function of human Vγ2 Vδ2 T cells. This article reviews the recent progress for immune biology of Vγ2 Vδ2 T cells in infections.

Human neutrophil clearance of bacterial pathogens triggers anti-microbial γδ T cell responses in early infection

Human blood Vγ9/Vδ2 T cells, monocytes and neutrophils share a responsiveness toward inflammatory chemokines and are rapidly recruited to sites of infection. Studying their interaction in vitro and relating these findings to in vivo observations in patients may therefore provide crucial insight into inflammatory events. Our present data demonstrate that Vγ9/Vδ2 T cells provide potent survival signals resulting in neutrophil activation and the release of the neutrophil chemoattractant CXCL8 (IL-8). In turn, Vγ9/Vδ2 T cells readily respond to neutrophils harboring phagocytosed bacteria, as evidenced by expression of CD69, interferon (IFN)-γ and tumor necrosis factor (TNF)-α. This response is dependent on the ability of these bacteria to produce the microbial metabolite (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMB-PP), requires cell-cell contact of Vγ9/Vδ2 T cells with accessory monocytes through lymphocyte function-associated antigen-1 (LFA-1), and results in a TNF-α dependent proliferation of Vγ9/Vδ2 T cells. The antibiotic fosmidomycin, which targets the HMB-PP biosynthesis pathway, not only has a direct antibacterial effect on most HMB-PP producing bacteria but also possesses rapid anti-inflammatory properties by inhibiting γδ T cell responses in vitro. Patients with acute peritoneal-dialysis (PD)-associated bacterial peritonitis – characterized by an excessive influx of neutrophils and monocytes into the peritoneal cavity – show a selective activation of local Vγ9/Vδ2 T cells by HMB-PP producing but not by HMB-PP deficient bacterial pathogens. The γδ T cell-driven perpetuation of inflammatory responses during acute peritonitis is associated with elevated peritoneal levels of γδ T cells and TNF-α and detrimental clinical outcomes in infections caused by HMB-PP positive microorganisms. Taken together, our findings indicate a direct link between invading pathogens, neutrophils, monocytes and microbe-responsive γδ T cells in early infection and suggest novel diagnostic and therapeutic approaches.

The immune system of all jawed vertebrates harbors three distinct lymphocyte populations – αβ T cells, γδ T cells and B cells – yet only higher primates including humans possess so-called Vγ9/Vδ2 T cells, an enigmatic γδ T cell subset that uniformly responds to the majority of bacterial pathogens. For reasons that are not understood, this responsiveness is absent in all other animals although they too are constantly exposed to a plethora of potentially harmful micro-organisms. We here investigated how Vγ9/Vδ2 T cells respond to live microbes by mimicking physiological conditions in acute disease. Our experiments demonstrate that Vγ9/Vδ2 T cells recognize a small common molecule released when invading bacteria become ingested and killed by other white blood cells. The stimulation of Vγ9/Vδ2 T cells at the site of infection amplifies the inflammatory response and has important consequences for pathogen clearance and the development of microbe-specific immunity. However, if triggered at the wrong time or the wrong place, this rapid reaction toward bacteria may also lead to inflammation-related damage. These findings improve our insight into the complex cellular interactions in early infection, identify novel biomarkers of diagnostic and predictive value and highlight new avenues for therapeutic intervention.

γδ T cell receptor ligands and modes of antigen recognition

T lymphocytes expressing the γδ-type of T cell receptors (TCRs) for antigens contribute to all aspects of immune responses, including defenses against viruses, bacteria, parasites and tumors, allergy and autoimmunity. Multiple subsets have been individualized in humans as well as in mice and they appear to recognize in a TCR-dependent manner antigens as diverse as small non-peptidic molecules, soluble or membrane-anchored polypeptides and molecules related to MHC antigens on cell surfaces, implying diverse modes of antigen recognition. We review here the γδ TCR ligands which have been identified along the years and their characteristics, with emphasis on a few systems which have been extensively studied such as human γδ T cells responding to phosphoantigens or murine γδ T cells activated by allogeneic MHC antigens. We discuss a speculative model of antigen recognition involving simultaneous TCR recognition of MHC-like and non-MHC ligands which could fit with most available data and shares many similarities with the classical model of MHC-restricted antigen recognition for peptides or lipids by T cells subsets with αβ-type TCRs.

gammadelta T cells in cancer immunotherapy: current status and future prospects

gammadelta T lymphocytes are a distinct T-cell subset that display unique features with respect to T-cell receptor (TCR) gene usage, tissue tropism and antigen recognition. Phosphoantigens contributed by a dysregulated mevalonate pathway or the bacterial nonmevalonate pathway and aminobisphosphonates are capable of activating Vgamma9Vdelta2 T cells. With the aid of synthetic phosphoantigens, large-scale expansion of gammadelta T cells and their adoptive transfer into human hosts is now possible. The present review summarizes triumphs and tribulations of clinical trials using gammadelta T-cell immunotherapy. Adoptive transfer of phosphoantigen-activated gammadelta T cells or coadministration with aminobisphosphonates/cytokines/monoclonal antibodies appear to be promising approaches for cancer immunotherapy. It can be predicted that a comprehensive understanding of the molecular interactions of this unique T-cell subset with other key immune regulators (dendritic cells and regulatory T cells) will provide an impetus to bring this modality of treatment from bench to bedside.

F1-adenosine triphosphatase displays properties characteristic of an antigen presentation molecule for Vgamma9Vdelta2 T cells

Human Vgamma9Vdelta2 T lymphocytes are activated by phosphoantigens provided exogenously or produced by tumors and infected cells. Activation requires a contact between Vgamma9Vdelta2 cells and neighboring cells. We previously reported a role for cell surface F1-adenosine triphosphatase (ATPase) in T cell activation by tumors and specific interactions between Vgamma9Vdelta2 TCRs and purified F1-ATPase. 721.221 cells do not express surface F1-ATPase and do not support phosphoantigen responses unless they are rendered apoptotic by high doses of zoledronate, a treatment that promotes F1-expression as well as endogenous phosphoantigen production. By monitoring calcium flux in single cells, we show in this study that contact of T cells with F1-ATPase on polystyrene beads can partially replace the cell-cell contact stimulus during phosphoantigen responses. Triphosphoric acid 1-adenosin-5'-yl ester 3-(3-methylbut-3-enyl) ester, an adenylated derivative of isopentenyl pyrophosphate, can stably bind to F1-ATPase-coated beads and promotes TCR aggregation, lymphokine secretion, and activation of the cytolytic process provided that nucleotide pyrophosphatase activity is present. It also acts as an allosteric activator of F1-ATPase. In the absence of Vgamma9Vdelta2 cells, triphosphoric acid 1-adenosin-5'-yl ester 3-(3-methylbut-3-enyl) ester immobilized on F1-ATPase is protected from nucleotide pyrophosphatase activity, as is the antigenic activity of stimulatory target cells. Our experiments support the notion that Vgamma9Vdelta2 T cells are dedicated to the recognition of phosphoantigens on cell membranes in the form of nucleotide derivatives that can bind to F1-ATPase acting as a presentation molecule.

Vgamma2Vdelta2 T Cell Receptor recognition of prenyl pyrophosphates is dependent on all CDRs

gammadelta T cells differ from alphabeta T cells in the Ags they recognize and their functions in immunity. Although most alphabeta TCRs recognize peptides presented by MHC class I or II, human gammadelta T cells expressing Vgamma2Vdelta2 TCRs recognize nonpeptide prenyl pyrophosphates. To define the molecular basis for this recognition, the effect of mutations in the TCR CDR was assessed. Mutations in all CDR loops altered recognition and cover a large footprint. Unlike murine gammadelta TCR recognition of the MHC class Ib T22 protein, there was no CDR3delta motif required for recognition because only one residue is required. Instead, the length and sequence of CDR3gamma was key. Although a prenyl pyrophosphate-binding site was defined by Lys109 in Jgamma1.2 and Arg51 in CDR2delta, the area outlined by critical mutations is much larger. These results show that prenyl pyrophosphate recognition is primarily by germline-encoded regions of the gammadelta TCR, allowing a high proportion of Vgamma2Vdelta2 TCRs to respond. This underscores its parallels to innate immune receptors. Our results also provide strong evidence for the existence of an Ag-presenting molecule for prenyl pyrophosphates.

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.

Monocytes and gammadelta T cells: close encounters in microbial infection

gammadelta T cells comprise an evolutionarily conserved yet poorly understood subset of T cells. Numerous features place these unconventional lymphocytes at the branching point between antigen-presenting cells and natural killer cells of the innate immune system and major-histocompatibility-complex-restricted alphabeta T cells of the adaptive immune system. We propose a role for human Vgamma9/Vdelta2 T cells in the generation of monocyte-derived inflammatory dendritic cells during infection. Our model incorporates the peculiar innate-like specificity of Vgamma9/Vdelta2 T cells for the microbial metabolite (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMB-PP), co-recruitment of monocytes and Vgamma9/Vdelta2 T cells to sites of infection, and their crosstalk, with profound consequences for the initiation of antigen-specific alphabeta T-cell responses. Vgamma9/Vdelta2 T cells act thus as a cellular switch between innate and adaptive defence mechanisms.

Gammadelta T cells and the lymphoid stress-surveillance response

The investigation of gammadelta T cells has identified a rapid lymphoid stress-surveillance response to microbial and nonmicrobial tissue perturbation. In addition to providing local protection, this response provides an immediate source of cytokines, chemokines, and other functions that can substantially affect downstream, adaptive immunity. Recent studies have identified striking mechanisms by which gammadelta cells meet the requirements of stress surveillance. For example, high response frequencies can reflect a unique nature of antigen engagement by the T cell receptor (TCR), developmental focusing of the repertoire by selection events, or the use of nonclonotypic receptors to initiate responses. Likewise, rapid functional deployment can be facilitated by the preprogramming of gammadelta cells during development. Additionally, gammadelta cells can directly influence adaptive immunity by functioning as antigen-presenting cells. With lymphoid stress surveillance likely to underpin numerous aspects of inflammation, tumor immunology, infectious disease, and autoimmunity, this perspective considers its properties and its emerging potential for clinical manipulation.

Novel approaches for identifying target antigens of autoreactive human B and T cells

Antigen-specific immune responses in multiple sclerosis have been studied for decades, but the target antigens of the putatively autoaggressive B and T cells still remain elusive. Here, we summarize recent strategies which are based on the direct analysis of biopsy or autopsy specimens from patients. Since this material is extremely scarce, the experimental methods need to be exceptionally sensitive. We describe technologies to distinguish (auto) aggressive T cells from irrelevant bystander lymphocytes by analyzing clonal expansions in relation to the morphological location of the cells in the tissue lesions. We then discuss approaches to clone matching α- and β-chains of the antigen-specific T cell receptor (TCR) molecules from single T cells. This is necessary because usually, several clones are expanded and are diluted by many irrelevant cells. The matching TCR chains from individual T cells can be resurrected in hybridoma cells which may then be used for antigen searches. We discuss strategies to identify antigens of γδ- and αβ-TCR molecules, such as biochemical methods, candidate antigens, human leukocyte antigen requirements, synthetic peptide, and cDNA libraries. These strategies are tailored to characterize the antigens of the membrane-anchored, low-affinity TCR molecules. The strategies to identify (auto) reactive B cells or immunoglobulin (Ig) molecules are fundamentally different, because Ig molecules are water-soluble and have high affinities. We further discuss proteome-based approaches, techniques that analyze Ig-chains from single B cells, and a repertoire-based method that compares Ig-proteomes and Ig-transcriptomes. The first method detects Ig antigens directly, whereas the latter two methods allow reconstruction of Ig molecules, which can be used for antigen searches.

CD3 expression distinguishes two gammadeltaT cell receptor subsets with different phenotype and effector function in tuberculous pleurisy

Tuberculous pleurisy is a naturally occurring site of Mycobacterium tuberculosis (Mtb) infection. Herein, we describe the expression of activation, natural killer (NK) and cell migration markers, as well as effector functions from gammadeltaT cells in peripheral blood (PB) and pleural effusion (PE) from tuberculosis patients (TB). We observed a decreased percentage of circulating gammadeltaT from TB patients and differential expression of NK as well as of chemokine receptors on PB and PE. Two subsets of gammadeltaT cells were differentiated by the CD3/gammadeltaT cell receptor (gammadeltaTCR) complex. The gammadeltaTCR(low) subset had a higher CD3 to TCR ratio and was enriched in Vdelta2(+) cells, whereas most Vdelta1(+) cells belonged to the gammadeltaTCR(high) subset. In PB from TB, most gammadeltaTCR(high) were CD45RA(+)CCR7(-) and gammadeltaTCR(low) were CD45RA(+/-)CCR7(+)CXCR3(+). In the pleural space the proportion of CD45RA(-)CCR7(+)CXCR3(+) cells was higher. Neither spontaneous nor Mtb-induced interferon (IFN)-gamma production was observed in PB-gammadeltaT cells from TB; however, PE-gammadeltaT cells showed a strong response. Both PB- and PE-gammadelta T cells expressed surface CD107a upon stimulation with Mtb. Notably, PE-gammadeltaTCR(low) cells were the most potent effector cells. Thus, gammadeltaT cells from PB would acquire a further activated phenotype within the site of Mtb infection and exert full effector functions. As gammadeltaT cells produce IFN-gamma within the pleural space, they would be expected to play a beneficial role in tuberculous pleurisy by helping to maintain a T helper type 1 profile.

Specific requirements for Vgamma9Vdelta2 T cell stimulation by a natural adenylated phosphoantigen

Human Vgamma9Vdelta2 T lymphocytes recognize phosphorylated alkyl Ags. Isopentenyl pyrophosphate (IPP) was previously proposed as the main Ag responsible for Vgamma9Vdelta2 T cell activation by cancer cells. However, triphosphoric acid 1-adenosin-5'-yl ester 3-(3-methylbut-3-enyl) ester (ApppI), a metabolite in which the isopentenyl moiety is linked to ATP, was reported in cells activated with aminobisphosphonates. The contribution of this compound to tumor-stimulatory activity was thus examined. ApppI induces selective expansion of Vgamma9Vdelta2 T cells from PBMCs. In the absence of APCs, however, ApppI has little stimulatory activity on Vgamma9Vdelta2 T cells, and optimal activation with ApppI requires addition of a nucleotide pyrophosphatase releasing IPP plus AMP. Thus, ApppI has no intrinsic stimulatory activity. Nevertheless, stimulation by ApppI is strengthened by the presence of APCs. Moreover, in contrast to IPP, ApppI can be efficiently pulsed on dendritic cells as well as on nonprofessional APCs. Pulsed APCs display stable and phosphatase-resistant stimulatory activity, indicative of Ag modification. HPLC analysis of tumor cell extracts indicates that latent phosphoantigenic activity is stored intracellularly in the Vgamma9Vdelta2 cell-sensitive tumor Daudi and can be activated by a nucleotide pyrophosphatase activity. The presence of ApppI in Daudi cell extracts was demonstrated by mass spectrometry. Nucleotidic Ags such as ApppI are thus a storage form of phosphoantigen which may represent a major source of phosphoantigenic activity in tumor cells. The unique properties of ApppI may be important for the design of Ags used in anticancer immunotherapeutic protocols using Vgamma9Vdelta2 cells.

Antigen-restricted gammadelta T-cell receptors?

After more than two decades of investigation, the biological role of the gammadelta T-cell receptors (TCRs) remains elusive. In fact, a theory of ligand recognition is still lacking that accounts for their adaptable structure, their peripheral selection, and the observed responses of gammadelta T cells, which do not require immunization but only include cells sharing germline-encoded components of the TCR. Assuming that all gammadelta T cells recognize ligands by a common mechanism, we now propose that germline-encoded components of the gammadelta TCRs provide for the specific recognition of a select set of antigenic determinants (Ags) which appear on the cell surface in various molecular associations. Furthermore, we hypothesize that the adaptivity of the gammadelta TCRs serves to increase affinity for the molecules with which these Ags associate rather than for the Ags themselves. Here we outline this hypothetical mechanism and discuss its possible implications for thymic selection and potential for complementing known innate and adaptive mechanisms of immune defense.

A rapid crosstalk of human gammadelta T cells and monocytes drives the acute inflammation in bacterial infections

Vγ9/Vδ2 T cells are a minor subset of T cells in human blood and differ from other T cells by their immediate responsiveness to microbes. We previously demonstrated that the primary target for Vγ9/Vδ2 T cells is (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMB-PP), an essential metabolite produced by a large range of pathogens. Here we wished to study the consequence of this unique responsiveness in microbial infection. The majority of peripheral Vγ9/Vδ2 T cells shares migration properties with circulating monocytes, which explains the presence of these two distinct blood cell types in the inflammatory infiltrate at sites of infection and suggests that they synergize in anti-microbial immune responses. Our present findings demonstrate a rapid and HMB-PP-dependent crosstalk between Vγ9/Vδ2 T cells and autologous monocytes that results in the immediate production of inflammatory mediators including the cytokines interleukin (IL)-6, interferon (IFN)-γ, tumor necrosis factor (TNF)-α, and oncostatin M (OSM); the chemokines CCL2, CXCL8, and CXCL10; and TNF-related apoptosis-inducing ligand (TRAIL). Moreover, under these co-culture conditions monocytes differentiate within 18 hours into inflammatory dendritic cells (DCs) with antigen-presenting functions. Addition of further microbial stimuli (lipopolysaccharide, peptidoglycan) induces CCR7 and enables these inflammatory DCs to trigger the generation of CD4⁺ effector αβ T cells expressing IFN-γ and/or IL-17. Importantly, our in vitro model replicates the responsiveness to microbes of effluent cells from peritoneal dialysis (PD) patients and translates directly to episodes of acute PD-associated bacterial peritonitis, where Vγ9/Vδ2 T cell numbers and soluble inflammatory mediators are elevated in patients infected with HMB-PP-producing pathogens. Collectively, these findings suggest a direct link between invading pathogens, microbe-responsive γδ T cells, and monocytes in the inflammatory infiltrate, which plays a crucial role in the early response and the generation of microbe-specific immunity.

As antibiotic resistance is spreading and posing a significant threat in many bacterial diseases, there is a need for a better understanding of host responses to infection. The precise role of an enigmatic subset of human immune cells, so-called Vγ9/Vδ2 T cells, in early infection still remains to be unveiled. These cells respond to a common molecule shared by the majority of bacterial pathogens and appear to be quickly drawn to sites of acute inflammation, where they will encounter invading microbes in the context of other immune cells, mainly granulocytes and monocytes. We here observed an unexpected interplay between microbe-activated Vγ9/Vδ2 T cells and monocytes that attracts further effector cells, enhances the activity of scavenger cells, and promotes the development of microbe-specific immunity. These findings not only improve our insight into the complex cellular interactions in early infection but may also suggest new therapies by modulating immune responses to improve host defenses and to resolve inflammatory activities.