Synthesis and biological activity of phosphonate analogues and geometric isomers of the highly potent phosphoantigen (E)-1-hydroxy-2-methylbut-2-enyl 4-diphosphate

Synthesis and evaluation of (E)-4-hydroxy-3-methyl-but-2-enyl diphosphate analogs as competitive partial agonists of butyrophilin 3A1

Phosphoantigens (pAgs) induce conformational changes after binding to the intracellular region of BTN3A1 which result in its clustering with BTN2A1, forming an activating ligand for the Vγ9Vδ2 T cell receptor. Here, we designed a small panel of bulky analogs of the prototypical pAg (E)-4-hydroxy-3-methyl-but-2-enyl diphosphate (HMBPP) that contain an aromatic ring attached to the C-3 position in place of methyl group. These compounds bind with high affinity to BTN3A1 but fail to fully support its interaction with BTN2A1 and only partially trigger T cell activation relative to HMBPP. Furthermore, they can compete with HMBPP for cellular binding to BTN3A1 and reduce the cellular response to HMBPP, a classic partial agonist phenotype. Trifluoromethyl analog 6e was the weakest agonist but the strongest inhibitor of HMBPP ELISA response. Our study provides a rationale for the mode of action of pAg-induced γδ T cell activation and provides insights into other naturally occurring BTN proteins and their respective ligands.

Synthetic Methods for Azaheterocyclic Phosphonates and Their Biological Activity: An Update 2004-2024

The increasing importance of azaheterocyclic phosphonates in the agrochemical, synthetic, and medicinal field has provoked an intense search in the development of synthetic routes for obtaining novel members of this family of compounds. This updated review covers methodologies established since 2004, focusing on the synthesis of azaheterocyclic phosphonates, of which the phosphonate moiety is directly substituted onto to the azaheterocyclic structure. Emphasizing recent advances, this review classifies newly developed synthetic approaches according to the ring size and providing information on biological activities whenever available. Furthermore, this review summarizes information on various methods for the formation of C-P bonds, examining sustainable approaches such as the Michaelis-Arbuzov reaction, the Michaelis-Becker reaction, the Pudovik reaction, the Hirao coupling, and the Kabachnik-Fields reaction. After analyzing the biological activities and applications of azaheterocyclic phosphonates investigated in recent years, a predominant focus on the evaluation of these compounds as anticancer agents is evident. Furthermore, emerging applications underline the versatility and potential of these compounds, highlighting the need for continued research on synthetic methods to expand this interesting family.

Diester Prodrugs of a Phosphonate Butyrophilin Ligand Display Improved Cell Potency, Plasma Stability, and Payload Internalization

Activation of Vγ9Vδ2 T cells with butyrophilin 3A1 (BTN3A1) agonists such as (E)-4-hydroxy-3-methyl-but-2-enyl diphosphate (HMBPP) has the potential to boost the immune response. Because HMBPP is highly charged and metabolically unstable, prodrugs may be needed to overcome these liabilities, but the prodrugs themselves may be limited by slow payload release or low plasma stability. To identify effective prodrug forms of a phosphonate agonist of BTN3A1, we have prepared a set of diesters bearing one aryl and one acyloxymethyl group. The compounds were evaluated for their ability to stimulate Vγ9Vδ2 T cell proliferation, increase production of interferon γ, resist plasma metabolism, and internalize into leukemia cells. These bioassays have revealed that varied aryl and acyloxymethyl groups can decouple plasma and cellular metabolism and have a significant impact on bioactivity (>200-fold range) and stability (>10 fold range), including some with subnanomolar potency. Our findings increase the understanding of the structure-activity relationships of mixed aryl/acyloxymethyl phosphonate prodrugs.

Taking Olefin Metathesis to the Limit: Stereocontrolled Synthesis of Trisubstituted Alkenes

ConspectusIn this Account, we share the story of the development of catalytic olefin metathesis processes that efficiently deliver a wide range of acyclic and macrocyclic E- or Z-trisubstituted alkenes. The tale starts with us unveiling, in collaboration with Richard Schrock and his team, the blueprint in 2009 for the design of kinetically controlled Z-selective olefin metathesis reactions. This paved the way for the development of Mo-, W-, and Ru-based catalysts and strategies for synthesizing countless linear and macrocyclic Z-olefins. Six years later, in 2015, we found that abundant Z-alkene feedstocks, such as oleic acid, can be directly transformed to high-value and more difficult-to-access alkenes through a cross-metathesis reaction promoted by a Ru-catechothiolate complex that we had developed; the approach, later coined stereoretentive olefin metathesis, was extended to the synthesis of E-alkenes.It was all about disubstituted alkenes until when in 2017 we addressed the challenge of accessing stereodefined Z- and E-trisubstituted alkenes, key to medicine and materials research. These transformations can be most effectively catalyzed by Mo monoaryloxides pyrrolide (MAP) and chloride (MAC) complexes. A central aspect of the advance is the merging of olefin metathesis, which delivered trisubstituted alkenyl fluorides, chlorides, and bromides with cross-coupling. These catalytic and stereoretentive transformations can be used in various combinations, thereby enabling access to assorted Z- or E-trisubstituted alkene. Ensuing work led to the emergence of other transformations involving substrates that can be purchased with high stereoisomeric purity, notably E- and Z-trihalo alkenes. Trisubstituted olefins, Z or E, bearing a chemoselectively and stereoretentively alterable F,Cl-terminus or B(pin),Cl-terminus may, thus, be easily and reliably synthesized. Methods for stereoretentive preparation of other alkenyl bromide regioisomers and α,β-unsaturated carboxylic and thiol esters, nitriles, and acid fluorides followed, along with stereoretentive ring-closing metathesis reactions that afford macrocyclic trisubstituted olefins. Z- and E-Macrocyclic trisubstituted olefins, including those that contain little or no entropic support for cyclization (minimally functionalized) and/or are disfavored under substrate-controlled conditions, can now be synthesized. The utility of this latest chapter in the history of olefin metathesis has been highlighted by applications to the synthesis of several biologically active compounds, as well as their analogues, such as those marked by one or more site-specifically incorporated fluorine atoms or more active but higher energy and otherwise unobtainable conformers.The investigations discussed here, which represent every stereoretentive method that has been reported thus far for preparing a trisubstituted olefin, underscore the inimitable power of Mo-based catalysts. This Account also showcases a variety of mechanistic attributes─some for the first time, and each instrumental in solving a problem. Extensive knowledge of mechanistic nuances will be needed if we are to address successfully the next challenging problem, namely, the development of catalysts and strategies that may be used to synthesize a wide range of tetrasubstituted alkenes, especially those that are readily modifiable, with high stereoisomeric purity.

The Multifaceted MEP Pathway: Towards New Therapeutic Perspectives

Isoprenoids, a diverse class of natural products, are present in all living organisms. Their two universal building blocks are synthesized via two independent pathways: the mevalonate pathway and the 2-C-methyl-ᴅ-erythritol 4-phosphate (MEP) pathway. The presence of the latter in pathogenic bacteria and its absence in humans make all its enzymes suitable targets for the development of novel antibacterial drugs. (E)-4-Hydroxy-3-methyl-but-2-enyl diphosphate (HMBPP), the last intermediate of this pathway, is a natural ligand for the human Vγ9Vδ2 T cells and the most potent natural phosphoantigen known to date. Moreover, 5-hydroxypentane-2,3-dione, a metabolite produced by Escherichia coli 1-deoxy-ᴅ-xylulose 5-phosphate synthase (DXS), the first enzyme of the MEP pathway, structurally resembles (S)-4,5-dihydroxy-2,3-pentanedione, a signal molecule implied in bacterial cell communication. In this review, we shed light on the diversity of potential uses of the MEP pathway in antibacterial therapies, starting with an overview of the antibacterials developed for each of its enzymes. Then, we provide insight into HMBPP, its synthetic analogs, and their prodrugs. Finally, we discuss the potential contribution of the MEP pathway to quorum sensing mechanisms. The MEP pathway, providing simultaneously antibacterial drug targets and potent immunostimulants, coupled with its potential role in bacterial cell-cell communication, opens new therapeutic perspectives.

Efficiency of bis-amidate phosphonate prodrugs

Bis-amidate derivatives have been viewed as attractive phosphonate prodrug forms because of their straightforward synthesis, lack of phosphorus stereochemistry, plasma stability and nontoxic amino acid metabolites. However, the efficiency of bis-amidate prodrug forms is unclear, as prior studies on this class of prodrugs have not evaluated their activation kinetics. Here, we synthetized a small panel of bis-amidate prodrugs of butyrophilin ligands as potential immunotherapy agents. These compounds were examined relative to other prodrug forms delivering the same payload for their stability in plasma and cell lysate, their ability to stimulate T cell proliferation in human PBMCs, and their activation kinetics in a leukemia co-culture model of T cell cytokine production. The bis-amidate prodrugs demonstrate high plasma stability and improved cellular phosphoantigen activity relative to the free phosphonic acid. However, the efficiency of bis-amidate activation is low relative to other prodrugs that contain at least one ester such as aryl-amidate, aryl-acyloxyalkyl ester, and bis-acyloxyalkyl ester forms. Therefore, bis-amidate prodrugs do not drive rapid cellular payload accumulation and they would be more useful for payloads in which slower, sustained-release kinetics are preferred.

Incorporation of a FRET pair within a phosphonate diester

Cell-cleavable protecting groups are an effective tactic for construction of biological probes because such compounds can improve problems with instability, solubility, and cellular uptake. Incorporation of fluorescent groups in the protecting groups may afford useful probes of cellular functions, especially for payloads containing phosphonates that would be highly charged if not protected, but little is known about the steric or electronic factors that impede release of the payload. In this report we present a strategy for the synthesis of a coumarin fluorophore and a 4-((4-(dimethylamino)phenyl)diazenyl)benzoic acid (DABCYL) ester chromophore incorporated as a FRET pair within a single phosphonate. Such compounds were designed to deliver a BTN3A1 ligand payload to its intracellular receptor. Both final products and some synthetic intermediates were evaluated for their ability to undergo metabolic activation in γδ T cell functional assays, and for their photophysical properties by spectrophotometry. One phosphonate bearing a DABCYL acyloxyester and a novel tyramine-linked coumarin fluorophore exhibited strong, rapid, and potent cellular activity for γδ T cell stimulation and also showed FRET interactions. This strategy demonstrates that bioactivatable phosphonates containing FRET pairs can be utilized to develop probes to monitor cellular uptake of otherwise charged payloads.

Synthesis and Metabolism of BTN3A1 Ligands: Studies on Modifications of the Allylic Alcohol

(E)-4-Hydroxy-3-methyl-but-2-enyl diphosphate (HMBPP) and its phosphonate analogs are potent phosphoantigens. HMBPP contains an (E)-allylic alcohol which interacts with the molecular target BTN3A1 giving an antigenic signal to activate Vγ9Vδ2 T cells. As probes of BTN3A1 function, we prepared prodrug derivatives of the HMBPP analog C-HMBP that lack the (E)-allylic alcohol or have modified it to an aldehyde or aldoxime and evaluated their biological activity. Removal of the alcohol completely abrogates phosphoantigenicity in these compounds while the aldoxime modification decreases potency relative to the (E)-allylic alcohol form. However, homoprenyl derivatives oxidized to an aldehyde stimulate Vγ9Vδ2 T cells at nanomolar concentrations. Selection of phosphonate protecting groups (i.e., prodrug forms) impacts the potency of phosphoantigen aldehydes, with mixed aryl acyloxyalkyl forms exhibiting superior activity relative to aryl amidate forms. The activity correlates with the cellular reduction of the aldehyde to the alcohol form. Thus, the functionality on this ligand framework can be altered concurrently with phosphonate protection to promote cellular transformation to highly potent phosphoantigens.

Asymmetric synthesis of the two enantiomers of β-phosphorus-containing α-amino acids via hydrophosphinylation and hydrophosphonylation of chiral Ni(ii)-complexes

A new approach for the synthesis of the two enantiomers of β-phosphorus-containing α-amino acids was developed via Michael addition of secondary phosphine oxides and dialkyl phosphites to chiral Ni(ii)-complexes of a dehydroalanine-Schiff base.

Diversity in recognition and function of human γδ T cells

As interest increases in harnessing the potential power of tissue-resident cells for human health and disease, γδ T cells have been thrust into the limelight due to their prevalence in peripheral tissues, their sentinel-like phenotypes, and their unique antigen recognition capabilities. This review focuses primarily on human γδ T cells, highlighting their distinctive characteristics including antigen recognition, function, and development, with an emphasis on where they differ from their αβ T cell comparators, as well as from γδ T cell populations in the mouse. We review the antigens that have been identified thus far to regulate members of the human Vδ1 population and discuss what players are involved in transducing phosphoantigen-mediated signals to human Vγ9Vδ2 T cells. We also briefly review distinguishing features of these cells in terms of TCR signaling, use of coreceptor and costimulatory molecules and their development. These cells have great potential to be harnessed in a clinical setting, but caution must be taken to understand their unique capabilities and how they differ from the populations to which they are commonly compared.

Structure-Activity Relationships of Butyrophilin 3 Ligands

Phosphoantigens (pAgs) are small phosphorus-containing molecules that stimulate Vγ9Vδ2 T cells with sub-nanomolar cellular potency. Recent work has revealed that these compounds work through binding to the transmembrane immunoglobulin butyrophilin 3A1 (BTN3A1) within its intracellular B30.2 domain. Engagement of BTN3A1 is critical to the formation of an immune synapse between cells that contain pAgs and the Vγ9Vδ2 T cells. This minireview summarizes the structure-activity relationships of pAgs and their implications to the mechanisms of butyrophilin 3 activation leading to Vγ9Vδ2 T cell response.

Selective Synthesis of Z-Cinnamyl Ethers and Cinnamyl Alcohols through Visible Light-Promoted Photocatalytic E to Z Isomerization

A photocatalytic E to Z isomerization of alkenes using an iridium photosensitizer under mild reaction conditions is disclosed. This method provides scalable and efficient access to Z-cinnamyl ether and allylic alcohol derivatives in high yields with excellent stereoselectivity. Importantly, this method also provides a powerful strategy for the selective synthesis of Z-magnolol and honokiol derivatives possessing potential biological activity.

Synthesis and Bioactivity of the Alanyl Phosphonamidate Stereoisomers Derived from a Butyrophilin Ligand

Aryloxy phosphonamidate derivatives of a butyrophilin 3A1 ligand are stimulants of Vγ9 Vδ2 T cells. However, when bonded to an aryl ester and an amine, the phosphorus is stereogenic, and past compounds were studied as racemates. To determine the impact of stereochemistry on the activity, we now have prepared phosphonate derivatives of l- and d-alanine ethyl ester, separated the diastereomers, and evaluated their biological activity as single stereoisomers. The results demonstrate that phosphonamidates substituted with l-alanine stimulate Vγ9 Vδ2 T cells at lower concentrations than the racemic glycine counterpart, while those derived from d-alanine require higher concentrations. All four diastereomers are more active than charged phosphoantigens such as HMBPP. Surprisingly, only a 2-fold difference was observed between the l-alanine phosphorus isomers, with the R _P isomer more potent. This suggests that the small phosphoantigen scaffold reduces but does not eliminate dependence upon phosphorus stereochemistry for cellular activity.

Probing the Ligand-Binding Pocket of BTN3A1

Small-molecule phosphoantigens such as (E)-4-hydroxy-3-methyl-but-2-enyl diphosphate stimulate human Vγ9Vδ2 T cells after binding to the intracellular B30.2 domain of the immune receptor butyrophilin 3 isoform A1 (BTN3A1). To understand the ligand-target interaction in greater detail, we performed molecular docking. Based on the docking results, we synthesized the novel ligand (E)-(7-hydroxy-6-methylhept-5-en-1-yl)phosphonate and mutated proposed binding site residues. We evaluated the impact on butyrophilin binding of existing and novel ligands using a newly developed high-throughput fluorescence polarization assay. We also evaluated the ability of the compounds to stimulate proliferation and interferon-γ production of Vγ9Vδ2 T cells. Mutation of H381 fully blocked ligand binding, whereas mutations to charged surface residues impacted diphosphate interactions. Monophosphonate analogs bind similarly to BTN3A1, although they differ in their antigenicity, demonstrating that binding and efficacy are not linearly correlated. These results further define the structure-activity relationships underlying BTN3A1 ligand binding and antigenicity and support further structure-guided drug design.

A power law function describes the time- and dose-dependency of Vγ9Vδ2 T cell activation by phosphoantigens

Phosphoantigens stimulate Vγ9Vδ2 T cells after binding to BTN3A1 in target cells and cell-cell contact. We evaluated phosphoantigens including diphosphates, bisphosphonates, and prodrugs for ability to induce leukemia cells to stimulate Vγ9Vδ2 T cell interferon-γ secretion. Most compounds displayed time-dependent activity at exposure times between 15 and 240 min. Potency (EC₅₀ values) ranged between 8.4 nM and >100 µM. The diphosphate C-HMBPP displayed a shallow dose-response slope (Hill slope = 0.71), while the bisphosphonate slopes were steep (Hill slopes > 2), and the prodrugs intermediate. The bis-acyloxyalkyl POM₂-C-HMBP showed low nanomolar potency even at an exposure time of 1 min. Mixed aryl-POM prodrugs also retained excellent potency at 15 min, while aryl-amidates were time dependent below 240 min. The sum of the dose and time logarithms is often constant, while a power law function fits most compounds. Collectively, these findings illustrate the exquisite activity of prodrugs relative to diphosphates and bisphosphonates.

New Insights Into the Regulation of γδ T Cells by BTN3A and Other BTN/BTNL in Tumor Immunity

Recent findings in the immunology field have pointed out the emergent role of butyrophilins/butyrophilin-like molecules (BTN/BTNL in human, Btn/Btnl in mouse) in the modulation of γδ T cells. As long as the field develops exponentially, new relationships between certain γδ T cell subsets, on one hand, and their BTN/BTNL counterparts mainly present on epithelial and tumor cells, on the other, are described in the scientific literature. Btnl1/Btnl6 in mice and BTNL3/BTNL8 in humans regulate the homing and maturation of Vγ7+ and Vγ4+ T cells to the gut epithelium. Similarly, Skint-1 has shown to shape the dendritic epidermal T cells repertoire and their activation levels in mice. We and others have identified BTN3A proteins are the key mediators of phosphoantigen sensing by human Vγ9Vδ2 T cells. Here, we first synthesize the modulation of specific γδ T cell subsets by related BTN/BTNL molecules, in human and mice. Then, we focus on the role of BTN3A in the activation of Vγ9Vδ2 T cells, and we highlight the recent advances in the understanding of the expression, regulation, and function of BTN3A in tumor immunity. Hence, recent studies demonstrated that several signals induced by cancer cells or their microenvironment can regulate the expression of BTN3A. Moreover, antibodies targeting BTN3A have shown in vitro and in vivo efficacy in human tumors such as acute myeloid leukemia or pancreatic cancer. We thus finally discuss how these findings could help develop novel γδ T cell-based immunotherapeutical approaches.

Vγ9Vδ2 T cell activation by strongly agonistic nucleotidic phosphoantigens

Human Vγ9Vδ2 T cells can sense through their TCR tumor cells producing the weak endogenous phosphorylated antigen isopentenyl pyrophosphate (IPP), or bacterially infected cells producing the strong agonist hydroxyl dimethylallyl pyrophosphate (HDMAPP). The recognition of the phosphoantigen is dependent on its binding to the intracellular B30.2 domain of butyrophilin BTN3A1. Most studies have focused on pyrophosphate phosphoantigens. As triphosphate nucleotide derivatives are naturally co-produced with IPP and HDMAPP, we analyzed their specific properties using synthetic nucleotides derived from HDMAPP. The adenylated, thymidylated and uridylated triphosphate derivatives were found to activate directly Vγ9Vδ2 cell lines as efficiently as HDMAPP in the absence of accessory cells. These antigens were inherently resistant to terminal phosphatases, but apyrase, when added during a direct stimulation of Vγ9Vδ2 cells, abrogated their stimulating activity, indicating that their activity required transformation into strong pyrophosphate agonists by a nucleotide pyrophosphatase activity which is present in serum. Tumor cells can be sensitized with nucleotide phosphoantigens in the presence of apyrase to become stimulatory, showing that this can occur before their hydrolysis into pyrophosphates. Whereas tumors sensitized with HDMAPP rapidly lost their stimulatory activity, sensitization with nucleotide derivatives, in particular with the thymidine derivative, induced long-lasting stimulating ability. Using isothermal titration calorimetry, binding of some nucleotide derivatives to BTN3A1 intracellular domain was found to occur with an affinity similar to that of IPP, but much lower than that of HDMAPP. Thus, nucleotide phosphoantigens are precursors of pyrophosphate antigens which can deliver strong agonists intracellularly resulting in prolonged and strengthened activity.

BTN3A1 Discriminates γδ T Cell Phosphoantigens from Nonantigenic Small Molecules via a Conformational Sensor in Its B30.2 Domain

Human Vγ9/Vδ2 T-cells detect tumor cells and microbial infections by recognizing small phosphorylated prenyl metabolites termed phosphoantigens (P-Ag). The type-1 transmembrane protein Butyrophilin 3A1 (BTN3A1) is critical to the P-Ag-mediated activation of Vγ9/Vδ2 T-cells; however, the molecular mechanisms involved in BTN3A1-mediated metabolite sensing are unclear, including how P-Ag's are discriminated from nonantigenic small molecules. Here, we utilized NMR and X-ray crystallography to probe P-Ag sensing by BTN3A1. Whereas the BTN3A1 immunoglobulin variable domain failed to bind P-Ag, the intracellular B30.2 domain bound a range of negatively charged small molecules, including P-Ag, in a positively charged surface pocket. However, NMR chemical shift perturbations indicated BTN3A1 discriminated P-Ag from nonantigenic small molecules by their ability to induce a specific conformational change in the B30.2 domain that propagated from the P-Ag binding site to distal parts of the domain. These results suggest BTN3A1 selectively detects P-Ag intracellularly via a conformational antigenic sensor in its B30.2 domain and have implications for rational design of antigens for Vγ9/Vδ2-based T-cell immunotherapies.

Mixed Aryl Phosphonate Prodrugs of a Butyrophilin Ligand

Studies of aryl phosphonate derivatives of a butyrophilin 3A1 ligand have resulted in identification of a potent stimulant of Vγ9 Vδ2 T cells. This compound, a mixed ester bearing one pivaloyloxymethyl substituent and one 1-naphthyl ester displayed an EC₅₀ of 0.79 nM as a stimulant of T cell proliferation, and a 9.0 nM EC₅₀ in an assay designed to measure interferon gamma production. In both assays, this is the most potent butyrophilin ligand prodrug yet reported, and thus it should be a valuable tool for studies of T cell function. Furthermore, mixed aryl/acyloxyalkyl esters may represent a new class of phosphonate prodrugs with high efficacy.

The butyrophilin 3A1 intracellular domain undergoes a conformational change involving the juxtamembrane region

Small isoprenoid diphosphates, such as (E)-4-hydroxy-3-methyl-but-2-enyl diphosphate (HMBPP), are ligands of the internal domain of BTN3A1. Ligand binding in target cells promotes activation of Vγ9Vδ2 T cells. We demonstrate by small-angle X-ray scattering (SAXS) that HMBPP binding to the internal domain of BTN3A1 induces a conformational change in the position of the B30.2 domain relative to the juxtamembrane (JM) region. To better understand the molecular details of this conformational rearrangement, NMR spectroscopy was used to discover that the JM region interacts with HMBPP, specifically at the diphosphate. The spectral location of the affected amide peaks, partial NMR assignments, and JM mutants (ST²⁹⁶AA or T³⁰⁴A) investigated, confirm that the backbone amide of at least one Thr (Thr³⁰⁴), adjacent to conserved Ser, comes close to the HMBPP diphosphate, whereas double mutation of nonconserved residues (Ser/Thr^296/297) may perturb the local fold. Cellular mutation of either of the identified Thr residues reduces the activation of Vγ9Vδ2 T cells by HMBPP, zoledronate, and POM₂-C-HMBP, but not by a partial agonist BTN3 antibody. Taken together, our results show that ligand binding to BTN3A1 induces a conformational change within the intracellular domain that involves the JM region and is required for full activation.-Nguyen, K., Li, J., Puthenveetil, R., Lin, X., Poe, M. M., Hsiao, C.-H. C., Vinogradova, O., Wiemer, A. J. The butyrophilin 3A1 intracellular domain undergoes a conformational change involving the juxtamembrane region.

Phosphinophosphonates and Their Tris-pivaloyloxymethyl Prodrugs Reveal a Negatively Cooperative Butyrophilin Activation Mechanism

Butyrophilin 3A1 (BTN3A1) binds small phosphorus-containing molecules, which initiates transmembrane signaling and activates butyrophilin-responsive cells. We synthesized several phosphinophosphonates and their corresponding tris-pivaloyloxymethyl (tris-POM) prodrugs and examined their effects on BTN3A1. An analog of (E)-4-hydroxy-3-methyl-but-2-enyl diphosphate (HMBPP) bound to BTN3A1 with intermediate affinity, which was enthalpy-driven. Docking studies revealed binding to the basic surface pocket and interactions between the allylic hydroxyl group and the BTN3A1 backbone. The phosphinophosphonate stimulated proliferation of Vγ9Vδ2 T cells with moderate activity (EC₅₀ = 26 μM). Cellular potency was enhanced >600-fold in the tris-POM prodrug (EC₅₀ = 0.041 μM). The novel prodrug also induced T cell mediated leukemia cell lysis. Analysis of dose-response data reveals HMBPP-induced Hill coefficients of 0.69 for target cell lysis and 0.68 in interferon secretion. Together, tris-POM prodrugs enhance the cellular activity of phosphinophosphonates, reveal structure-activity relationships of butyrophilin ligands, and support a negatively cooperative model of cellular butyrophilin activation.

Sensing of Pyrophosphate Metabolites by Vγ9Vδ2 T Cells

The predominant population of γδ T cells in human blood express a T cell receptor (TCR) composed of a Vγ9 (Vγ2 in an alternate nomenclature) and Vδ2 domains. These cells came into the limelight when it was discovered they can respond to certain microbial infections and tumorigenic cells through the detection of small, pyrophosphate containing organic molecules collectively called “phosphoantigens” or “pAgs.” These molecules are intermediates in both eukaryotic and prokaryotic metabolic pathways. Chemical variants of these intermediates have been used in the clinic to treat a range of different cancers, however, directed optimization of these molecules requires a full understanding of their mechanism of action on target cells. We and others have identified a subclass of butyrophilin-related molecules (BTN3A1-3) that are directly involved in pAg sensing in the target cell, leading to engagement and activation of the T cell through the TCR. Our data and that of others support the pAg binding site to be the intracellular B30.2 domain of BTN3A1, which is the only isoform capable of mediating pAg-dependent stimulation of Vγ9Vδ2 T cells. Here, we review the data demonstrating pAg binding to the B30.2 domain and our studies of the structural conformations of the BTN3A extracellular domains. Finally, we synthesize a model linking binding of pAg to the intracellular domain with T cell detection via the extracellular domains in an “inside-out” signaling mechanism of the type characterized first for integrin molecule signaling. We also explore the role of Vγ9Vδ2 TCR variability in the CDR3 γ and δ loops and how this may modulate Vγ9Vδ2 cells as a population in surveillance of human health and disease.

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.

Crystal structure of 2-(4-chloro-phen-yl)-3-(4-meth-oxy-phen-yl)-3-(methyl-sulfanyl)-acrylo-nitrile

In the title compound, C₁₇H₁₄ClNOS, the aromatic rings are inclined to one another by 64.22 (9)°. The acrylo­nitrile group (C=C—C N) is planar to within 0.003 (2) Å, with the S atom and the methyl C atom displaced from this plane by 0.2317 (6) and −0.637 (2) Å, respectively. In the crystal, mol­ecules are linked via pairs of C—H⋯π inter­actions, forming inversion dimers. There are no other significant inter­molecular inter­actions present.

The intracellular B30.2 domain of butyrophilin 3A1 binds phosphoantigens to mediate activation of human Vγ9Vδ2 T cells

In humans, Vγ9Vδ2 T cells detect tumor cells and microbial infections, including Mycobacterium tuberculosis, through recognition of small pyrophosphate containing organic molecules known as phosphoantigens (pAgs). Key to pAg-mediated activation of Vγ9Vδ2 T cells is the butyrophilin 3A1 (BTN3A1) protein that contains an intracellular B30.2 domain critical to pAg reactivity. Here, we have demonstrated through structural, biophysical, and functional approaches that the intracellular B30.2 domain of BTN3A1 directly binds pAg through a positively charged surface pocket. Charge reversal of pocket residues abrogates binding and Vγ9Vδ2 T cell activation. We have also identified a gain-of-function mutation within this pocket that, when introduced into the B30.2 domain of the nonstimulatory BTN3A3 isoform, transfers pAg binding ability and Vγ9Vδ2 T cell activation. These studies demonstrate that internal sensing of changes in pAg metabolite concentrations by BTN3A1 molecules is a critical step in Vγ9Vδ2 T cell detection of infection and tumorigenesis.

In vivo manipulation of γ9(+) T cells in the common marmoset (Callithrix Jacchus) with phosphoantigen and effect on the progression of respiratory melioidosis

Burkholderia pseudomallei is a dangerous human pathogen. Phosphoantigens specifically the target primate specific γ9(+)δ2(+) T cells subset and some have been developed as potential immunotherapeutics. Previously, we demonstrated that, when stimulated with the phosphoantigen CHDMAPP, γ9(+)δ2(+) T cells aid in the killing of intracellular B. pseudomallei bacteria. Moreover, we found that common marmoset (Callithrix Jacchus) γ9(+) T cells increase in frequency and respond to the phosphoantigen CHDMAPP and/or B. pseudomallei, in combination with IL-2, in a similar manner to human γ9(+)δ2(+) T cells. Here we evaluate the efficacy of the phosphoantigen CHDMAPP, in combination with IL-2, as a therapy against B. pseudomallei infection, in vivo. We found that the previous studies predicted the in vivo responsiveness of γ9(+) T cells to the CHDMAPP+IL-2 treatment and significant expansion of the numbers of peripheral and splenic γ9(+) T cells were observed. This effect was similar to those reported in other primate species treated with phosphoantigen. Furthermore, splenocytes were retrieved 7 days post onset of treatment, restimulated with CHDMAPP or heat-killed B. pseudomallei and the cultured γ9(+) T cells demonstrated no reduction in IFN-γ response when CHDMAPP+IL-2 animals were compared to IL-2 only treated animals. Using an established model of B. pseudomallei infection in the marmoset, we assessed the potential for using phosphoantigen as a novel immunotherapy. The CHDMAPP treatment regime had no effect on the progression of respiratory melioidosis and this was despite the presence of elevated numbers of γ9(+) T cells in the spleen, liver and lung and an increased proportion of IFN-γ(+) cells in response to infection. We therefore report that the common marmoset has proven a good model for studying the effect in vivo of γ9(+) T cell stimulation; however, γ9(+) T cells have little or no effect on the progression of lethal, respiratory B. pseudomallei infection.

An assessment of common marmoset (Callithrix jacchus) γ9(+) T cells and their response to phosphoantigen in vitro

γ9δ2 T cells are a primate-specific γδ T cell subtype that expand and become activated during infection, responding directly to phosphoantigens which are by-products of essential metabolic pathways in both bacteria and mammals. Analogues of natural phosphoantigens have been developed as potential immunotherapeutics for treatment of tumours and infectious diseases. Several non-human primate models have been used in preclinical studies, however, little is known about marmoset γ9δ2 T cell responses. We identified γ9(+) T cells in various tissues in the marmoset and determined that these cells respond to phosphoantigen in a similar manner to human γ9δ2 T cells in vitro. Both human γ9δ2 T cells and marmoset γ9(+) T cells were able to reduce growth of the intracellular bacterium Burkholderia pseudomallei in vitro following expansion with phosphoantigen. This suggests that the marmoset is an appropriate model for examining the immunotherapeutic potential of compounds which target γ9δ2 T cells.

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.

Role of gamma-delta T-cells in cancer: another opening door to immunotherapy

The gamma-delta (γδ) T-cells are a subset of T-lymphocytes characterized by the presence of a surface antigen recognition complex type 2. Those γδ T-cells represent 2-5 % of peripheral T-cells only, but they are common in organs and mucosae, acting as a first defense system in the entries to the organism. The γδ T-cells take part on immune response by direct cytolysis, development of memory phenotypes, and modulation of immune cells, and they have been implied in autoimmune disorders, immune deficiencies, infections, and tumor diseases. We reported the role of γδ T-cells in oncology, focusing in their potential applications for cancer treatment. Experimental designs and clinical trials in the treatment of solid malignancies are extensively reviewed.

Synthesis of phosphoantigens: scalable accesses to enantiomers of BrHPP and studies on N-HDMAPP synthesis

Phosphoantigens enable the access to a new anti-tumoral and anti-infectious therapeutic pathway, based on innate immunity through the selective activation of Tγ9δ2 lymphocytes. The first proof of concept of this new immunotherapy approach was demonstrated with the synthetic phosphoantigen named bromohydrin pyrophosphate (BrHPP, IPH 1101) which was administrated in racemic form to about 200 patients in six clinical trials with good safety and promising early signals of efficacy in type C viral hepatitis and follicular non-Hodgkin's lymphoma. Enantiopure samples of BrHPP in gram scale are required for further studies on structure-bioactivity relationship. Thus we developed two complementary synthetic pathways, the first using transformation of a chiral compound and the second involving asymmetric synthesis starting from a prochiral building-block. The synthesis of a second-generation phosphoantigen, N-HDMAPP, which bears a phosphoramidate moiety, was also investigated.

(Z)-3-o-Tolyl-3-(p-tol-yloxy)acrylonitrile

The title compound, C₁₇H₁₅NO, exists in a Z conformation. The dihedral angle between the O-bonded benzene ring and the vinyl plane is 80.97 (18)° while the dihedral angle between the rings is 80.06 (10)°. In the crystal structure, no classical hydrogen bonds occur.

(E)-2-(4-tert-Butylphenyl)-2-cyano-1-(3-methyl-1-vinyl-1H-pyrazol-5-yl)vinyl 2,2-dimethylpropanoate

In the title compound, C₂₄H₂₉N₃O₂, the dihedral angle between the benzene and pyrazole rings is 80.55 (7)°. The mol­ecule contains an acrylonitrile moiety and exists in an E conformation. Bioassay tests showed that the title compound exhibited higher acaricidal activity than its Z isomer.

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.

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.

Stimulated γδ T cells increase the in vivo efficacy of trastuzumab in HER-2+ breast cancer

One fourth of women with HER-2(+) metastatic breast carcinoma are treated with a combination regimen with trastuzumab, but the frequent resistance to this Ab requires definition of new means to improve its bioactivity. The mechanisms of action of trastuzumab involve several pathways including Ab-dependent cellular cytotoxicity. Because human γδ T lymphocytes mediate Ab-dependent cellular cytotoxicity and can be activated further by phosphoantigens, these cells are prone to improve the efficacy of Abs, as recently demonstrated for CD20(+) B cell lymphomas. Whether this concept applies as well with carcinomas remained to be demonstrated in vivo, however. In this study, we asked whether a combination of trastuzumab and phosphoantigen-stimulated γδ lymphocytes increases the efficacy of trastuzumab against HER-2(+) breast carcinoma cell lines in vivo. We report that repeated infusions of this combination had a better efficacy than that of trastuzumab alone against HER-2(+) mammary carcinoma xenografts in mice. In these models, reduction of tumor growth was observed together with trastuzumab opsonization of HER-2(+) cells and tumor infiltration by γδ lymphocytes. In addition in humans, the mammary carcinomas of 27 of 30 patients showed significant γδ T cell infiltrates. Altogether, these findings indicate that combination of trastuzumab and stimulated γδ cells represents a new strategy to improve the efficacy of Herceptin (trastuzumab) in HER-2(+) breast cancer.

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.