Tonic-signaling chimeric antigen receptors drive human regulatory T cell exhaustion

Protocol for the simultaneous activation and lentiviral transduction of primary human T cells with artificial T cell receptors

T cell-based therapies hold immense promise, but their production remains time-consuming and technically complex. Here, we present a protocol that streamlines the activation and lentiviral transduction of primary human T cells with artificial receptors. We describe steps for T cell isolation, lentivirus production, and the simultaneous activation/transduction of T cells. By eliminating magnetic T cell activation beads, sequential steps, and lengthy spinoculation, this protocol significantly enhances efficiency, scalability, and accessibility for research and therapeutic applications.

Discovery and preclinical development of a SdAb-based CAR-T technology for targeting CD33 in AML

Chimeric antigen receptor T cell (CAR-T) therapies have revolutionized cancer immunotherapy. Traditional single-chain variable fragments (ScFvs) used as CAR recognition moieties face challenges such as high tonic signaling, compromised binding epitopes, and suboptimal affinity. Single-domain antibodies (SdAbs) offer an attractive alternative due to their smaller size, stability, and reduced immunogenicity. In this work, we developed an SdAb-CAR-T cell discovery platform integrating generation, characterization, and selection of SdAbs based on various properties. This approach was demonstrated by developing CAR-T cells with SdAbs against CD33, a target for acute myeloid leukemia (AML). We identified diverse SdAbs against CD33, with affinities ranging from 3.9-115 nM, and characterized their binding kinetics and epitope recognition. Using SdAb-based second-generation CARs, we assessed tonic signaling, T cell phenotypes, cytotoxicity and cytokine release in vitro, resulting in reduced tonic signaling and increased cytokine production. In vivo, SdAb-based CAR-T cells exhibited enhanced efficacy at lower doses, in a xenograft AML mouse model, demonstrating advantages over ScFv-based CD33 CAR-T cells.

IL23R-Specific CAR Tregs for the Treatment of Crohn's Disease

BACKGROUND AND AIMS

Regulatory T cells (Tregs) are key regulators in maintaining tissue homeostasis. Disrupted immune homeostasis is associated with Crohn's disease (CD) pathogenesis. Thus, Treg therapy represents a promising long-acting treatment to restore immune balance in the diseased intestine. Chimeric antigen receptor (CAR) T-cell therapy has revolutionized cancer treatment. This innovative approach also provides the opportunity to improve therapy for CD. By targeting a disease-relevant protein, interleukin-23 receptor (IL23R), we engineered Tregs expressing IL23R-CAR for treating active CD.

METHODS

Intestinal IL23R expression from active CD was verified by immunohistochemical analysis. Phenotypic and functional characteristics of IL23R-CAR Tregs were assessed using in vitro assays and their migration capacity was monitored in a xenograft tumor model. Transcriptomic and proteomic analyses were performed to associate molecular profiles with IL23R-CAR Treg activation against colon biopsy-derived cells from active CD patients.

RESULTS

Our study showed that IL23R-CAR displayed negligible tonic signaling and a strong signal-to-noise ratio. IL23R-CAR Tregs maintained regulatory phenotype during in vitro expansion, even when chronically exposed to proinflammatory cytokines and target antigen. IL23R engagement on IL23R-CAR Tregs triggered CAR-specific activation and significantly enhanced their suppressive activity. Also, IL23R-CAR Tregs migrated to IL23R-expressing tissue in humanized mice. Finally, IL23R-CAR Tregs elicited a specific activation against colon biopsy-derived cells from active CD, suggesting an efficient CAR engagement in active CD. Molecular profiling of CD patient biopsies also revealed transcriptomic and proteomic patterns associated with IL23R-CAR activation.

CONCLUSIONS

Overall, our results demonstrate that IL23R-CAR Tregs represent a promising therapy for active CD.

Advances in the Pathogenesis and Treatment Strategies for Type 1 Diabetes Mellitus

Type 1 diabetes (T1D) is a complex autoimmune disorder distinguished by the infiltration of immune cells into pancreatic islets, primarily resulting in damage to pancreatic β-cells. Despite extensive research, the precise pathogenesis of T1D remains elusive, with its etiology linked to a complex interplay of genetic, immune, and environmental factors. While genetic predispositions, such as HLA and other susceptibility genes, are necessary, they do not fully account for disease development. Environmental influences such as viral infections and dietary factors may contribute to the disease by affecting the immune system and epigenetic modifications. Additionally, endogenous retroviruses (ERVs) might play a role in T1D pathogenesis. Current therapeutic approaches, including insulin replacement therapy, immune omodulatory therapy, autoantigen immunotherapy, organ transplantation, and genetic modification, offer potential to alter disease progression but are still constrained by limitations. This review presents updated knowledge on T1D, with a focus on risk factors, predisposing hypotheses, and recent advancements in therapeutic strategies.

Harnessing the biology of regulatory T cells to treat disease

Regulatory T (Treg) cells are a suppressive subset of CD4+ T cells that maintain immune homeostasis and restrain inflammation. Three decades after their discovery, the promise of strategies to harness Treg cells for therapy has never been stronger. Multiple clinical trials seeking to enhance endogenous Treg cells or deliver them as a cell-based therapy have been performed and hint at signs of success, as well as to important limitations and unanswered questions. Strategies to deplete Treg cells in cancer are also in active clinical testing. Furthermore, multi-dimensional methods to interrogate the biology of Treg cells are leading to a refined understanding of Treg cell biology and new approaches to harness tissue-specific functions for therapy. A new generation of Treg cell clinical trials is now being fuelled by advances in nanomedicine and synthetic biology, seeking more precise ways to tailor Treg cell function. This Review will discuss recent advances in our understanding of human Treg cell biology, with a focus on mechanisms of action and strategies to assess outcomes of Treg cell-targeted therapies. It highlights results from recent clinical trials aiming to enhance or inhibit Treg cell activity in a variety of diseases, including allergy, transplantation, autoimmunity and cancer, and discusses ongoing strategies to refine these approaches.

Integrating binding affinity and tonic signaling enables a rational CAR design for augmented T cell function

BACKGROUND

The success of chimeric antigen receptor (CAR) T cell therapy for hematological malignancies has not yet translated into long-term elimination of solid tumors indicating the need for adequately tuning CAR T cell functionality.

METHODS

We leveraged a translational pipeline including biophysical characterization and structural prediction of the CAR binding moiety, evaluation of cellular avidity, synapse formation, T cell motility, and functional capacities under repetitive target challenge and in sustained tumor control.

RESULTS

As an example of clinical relevance, we derived a panel of anti-Her2 CARs covering a 4-log affinity range, all expected to target the same Her2 epitope. The same scFv mutations increased both antigen-specific affinity, cellular avidity, and antigen-independent "tonic" signaling; above a minimum threshold, raise in affinity translated into functional avidity in a non-linear fashion. In this case, replacement by amino acids of higher hydrophobicity within the scFv coincidentally augmented affinity, non-specific binding, spontaneous CAR clustering, and tonic signaling, all together relating to T cell functionality in an integrated fashion.

CONCLUSIONS

Data emphasize that tonic signaling is not always due to the positive charge but can be driven by hydrophobic interactions of the scFv. CAR binding affinity above the threshold and tonic signaling are required for sustained T cell functionality in antigen rechallenge and long-term tumor control.

MOG-specific CAR Tregs: a novel approach to treat multiple sclerosis

Multiple sclerosis (MS) is an autoimmune disease affecting the central nervous system (CNS) with the immune system attacking myelin sheaths leading to neuronal death. While several disease-modifying therapies are available to treat MS, these therapies are not universally effective and do not stop disease progression. More personalized long-term treatment options that target specific aspects of the disease, such as reducing relapse frequency, delaying disability accumulation, and addressing symptoms that impact daily functioning, as well as therapies that can promote neuroprotection and repair are needed. Chimeric Antigen Receptor (CAR) Tcell therapies have revolutionized cancer treatment by intravenously (IV) administering a defined dose of T cells with high specificity provided by the CAR. An autologous CAR T cell therapy using suppressive regulatory T cells (Tregs) inducing long-lasting tolerance would be the ideal treatment for patients. Hence, we expanded the application of CAR-T cells by introducing a CAR into Tregs to treat MS patients. We developed a myelin oligodendrocyte glycoprotein (MOG)-specific CAR Treg cell therapy for patients with MS. MOG is expressed on the outer membrane of the myelin sheath, the insulating layer the forms around nerves, making it an ideal target for CAR Treg therapy. Our lead candidate is a 2nd generation CAR, composed of an anti-MOG scFv screened from a large human library. In vitro, we demonstrated CAR-dependent functionality and showed efficacy in vivo using a passive EAE mouse model. Additionally, the MOG-CAR Tregs have very low tonic signaling with a desirable signal-to-noise ratio resulting in a highly potent CAR. In summary our data suggest that MOG-CAR Tregs are a promising MS treatment option with the potential to induce long-lasting tolerance in patients.

CD8+ T cell exhaustion and its regulatory mechanisms in the tumor microenvironment: key to the success of immunotherapy

A steady dysfunctional state caused by chronic antigen stimulation in the tumor microenvironment (TME) is known as CD8+ T cell exhaustion. Exhausted-like CD8+ T cells (CD8+ Tex) displayed decreased effector and proliferative capabilities, elevated co-inhibitory receptor generation, decreased cytotoxicity, and changes in metabolism and transcription. TME induces T cell exhaustion through long-term antigen stimulation, upregulation of immune checkpoints, recruitment of immunosuppressive cells, and secretion of immunosuppressive cytokines. CD8+ Tex may be both the reflection of cancer progression and the reason for poor cancer control. The successful outcome of the current cancer immunotherapies, which include immune checkpoint blockade and adoptive cell treatment, depends on CD8+ Tex. In this review, we are interested in the intercellular signaling network of immune cells interacting with CD8+ Tex. These findings provide a unique and detailed perspective, which is helpful in changing this completely unpopular state of hypofunction and intensifying the effect of immunotherapy.

An autoimmune transcriptional circuit drives FOXP3+ regulatory T cell dysfunction

Autoimmune diseases, among the most common disorders of young adults, are mediated by genetic and environmental factors. Although CD4+FOXP3+ regulatory T cells (Tregs) play a central role in preventing autoimmunity, the molecular mechanism underlying their dysfunction is unknown. Here, we performed comprehensive transcriptomic and epigenomic profiling of Tregs in the autoimmune disease multiple sclerosis (MS) to identify critical transcriptional programs regulating human autoimmunity. We found that up-regulation of a primate-specific short isoform of PR domain zinc finger protein 1 (PRDM1-S) induces expression of serum and glucocorticoid-regulated kinase 1 (SGK1) independent from the evolutionarily conserved long PRDM1, which led to destabilization of forkhead box P3 (FOXP3) and Treg dysfunction. This aberrant PRDM1-S/SGK1 axis is shared among other autoimmune diseases. Furthermore, the chromatin landscape profiling in Tregs from individuals with MS revealed enriched activating protein-1 (AP-1)/interferon regulatory factor (IRF) transcription factor binding as candidate upstream regulators of PRDM1-S expression and Treg dysfunction. Our study uncovers a mechanistic model where the evolutionary emergence of PRDM1-S and epigenetic priming of AP-1/IRF may be key drivers of dysfunctional Tregs in autoimmune diseases.

Revolutionizing Immunotherapy: Unveiling New Horizons, Confronting Challenges, and Navigating Therapeutic Frontiers in CAR-T Cell-Based Gene Therapies

The CAR-T cell therapy has marked the dawn of new era in the cancer therapeutics and cell engineering techniques. The review emphasizes on the challenges that obstruct the therapeutic efficiency caused by cell toxicities, immunosuppressive tumor environment, and decreased T cell infiltration. In the interest of achieving the overall survival (OS) and event-free survival (EFS) of patients, the conceptual background of potential target selection and various CAR-T cell design techniques are described which can minimize the off-target effects, reduce toxicity, and thus increase the resilience of CAR-T cell treatment in the haematological malignancies as well as in solid tumors. Furthermore, it delves into cutting-edge technologies like gene editing and synthetic biology, providing new opportunities to enhance the functionality of CAR-T cells and overcome mechanisms of immune evasion. This review provides a comprehensive understanding of the complex and diverse aspects of CAR-T cell-based gene treatments, including both scientific and clinical aspects. By effectively addressing the obstacles and utilizing the capabilities of cutting-edge technology, CAR-T cell therapy shows potential in fundamentally changing immunotherapy and reshaping the approach to cancer treatment.

Role of TIM-3 in ovarian cancer: the forsaken cop or a new noble

T cell immunoglobulin and mucin domain-3 (TIM-3), a crucial immune checkpoint following PD1 and CTLA4, is widely found in several immune cells. Nonetheless, its performance in recent clinical trials appears disappointing. Ovarian cancer (OC), a malignant tumor with a high mortality rate in gynecology, faces significant hurdles in immunotherapy. The broad presence of TIM-3 offers a new opportunity for immunotherapy in OC. This study reviews the role of TIM-3 in OC and assesses its potential as a target for immunotherapy. The regulatory effects of TIM-3 on the immune microenvironment in OC are discussed, with a focus on preclinical studies that demonstrate TIM-3's modulation of various immune cells in OC. Additionally, the potential therapeutic advantages and challenges of targeting TIM-3 in OC are examined.

Unlocking T cell exhaustion: Insights and implications for CAR-T cell therapy

Chimeric antigen receptor T (CAR-T) cell therapy as a form of adoptive cell therapy (ACT) has shown significant promise in cancer treatment, demonstrated by the FDA-approved CAR-T cell therapies targeting CD19 or B cell maturation antigen (BCMA) for hematological malignancies, albeit with moderate outcomes in solid tumors. However, despite these advancements, the efficacy of CAR-T therapy is often compromised by T cell exhaustion, a phenomenon that impedes the persistence and effector function of CAR-T cells, leading to a relapse rate of up to 75% in patients treated with CD19 or CD22 CAR-T cells for hematological malignancies. Strategies to overcome CAR-T exhaustion employ state-of-the-art genomic engineering tools and single-cell sequencing technologies. In this review, we provide a comprehensive understanding of the latest mechanistic insights into T cell exhaustion and their implications for the current efforts to optimize CAR-T cell therapy. These insights, combined with lessons learned from benchmarking CAR-T based products in recent clinical trials, aim to address the challenges posed by T cell exhaustion, potentially setting the stage for the development of tailored next-generation approaches to cancer treatment.

The regulation and differentiation of regulatory T cells and their dysfunction in autoimmune diseases

The discovery of FOXP3+ regulatory T (Treg) cells as a distinct cell lineage with a central role in regulating immune responses provided a deeper understanding of self-tolerance. The transcription factor FOXP3 serves a key role in Treg cell lineage determination and maintenance, but is not sufficient to enable the full potential of Treg cell suppression, indicating that other factors orchestrate the fine-tuning of Treg cell function. Moreover, FOXP3-independent mechanisms have recently been shown to contribute to Treg cell dysfunction. FOXP3 mutations in humans cause lethal fulminant systemic autoinflammation (IPEX syndrome). However, it remains unclear to what degree Treg cell dysfunction is contributing to the pathophysiology of common autoimmune diseases. In this Review, we discuss the origins of Treg cells in the periphery and the multilayered mechanisms by which Treg cells are induced, as well as the FOXP3-dependent and FOXP3-independent cellular programmes that maintain the suppressive function of Treg cells in humans and mice. Further, we examine evidence for Treg cell dysfunction in the context of common autoimmune diseases such as multiple sclerosis, inflammatory bowel disease, systemic lupus erythematosus and rheumatoid arthritis.

TOP CAR with TMIGD2 as a safe and effective costimulatory domain in CAR cells treating human solid tumors

Chimeric antigen receptor (CAR)-T cell therapy shows impressive efficacy treating hematologic malignancies but requires further optimization in solid tumors. Here, we developed a TMIGD2 optimized potent/persistent (TOP) CAR that incorporated the costimulatory domain of TMIGD2, a T and NK cell costimulator, and monoclonal antibodies targeting the IgV domain of B7-H3, an immune checkpoint expressed on solid tumors and tumor vasculature. Comparing second- and third-generation B7-H3 CARs containing TMIGD2, CD28, and/or 4-1BB costimulatory domains revealed superior antitumor responses in B7-H3.TMIGD2 and B7-H3.CD28.4-1BB CAR-T cells in vitro. Comparing these two constructs using in vivo orthotopic human cancer models demonstrated that B7-H3.TMIGD2 CAR-T cells had equivalent or superior antitumor activity, survival, expansion, and persistence. Mechanistically, B7-H3.TMIGD2 CAR-T cells maintained mitochondrial metabolism; produced less cytokines; and established fewer exhausted cells, more central memory cells, and a larger CD8/CD4 T cell ratio. These studies demonstrate that the TOP CAR with TMIGD2 costimulation offered distinct benefits from CD28.41BB costimulation and is effective against solid tumors.

CRISPR-Cas9 applications in T cells and adoptive T cell therapies

T cell immunity is central to contemporary cancer and autoimmune therapies, encompassing immune checkpoint blockade and adoptive T cell therapies. Their diverse characteristics can be reprogrammed by different immune challenges dependent on antigen stimulation levels, metabolic conditions, and the degree of inflammation. T cell-based therapeutic strategies are gaining widespread adoption in oncology and treating inflammatory conditions. Emerging researches reveal that clustered regularly interspaced palindromic repeats-associated protein 9 (CRISPR-Cas9) genome editing has enabled T cells to be more adaptable to specific microenvironments, opening the door to advanced T cell therapies in preclinical and clinical trials. CRISPR-Cas9 can edit both primary T cells and engineered T cells, including CAR-T and TCR-T, in vivo and in vitro to regulate T cell differentiation and activation states. This review first provides a comprehensive summary of the role of CRISPR-Cas9 in T cells and its applications in preclinical and clinical studies for T cell-based therapies. We also explore the application of CRISPR screen high-throughput technology in editing T cells and anticipate the current limitations of CRISPR-Cas9, including off-target effects and delivery challenges, and envisioned improvements in related technologies for disease screening, diagnosis, and treatment.

Immune modulation in transplant medicine: a comprehensive review of cell therapy applications and future directions

Balancing the immune response after solid organ transplantation (SOT) and vascularized composite allotransplantation (VCA) remains an ongoing clinical challenge. While immunosuppressants can effectively reduce acute rejection rates following transplant surgery, some patients still experience recurrent acute rejection episodes, which in turn may progress to chronic rejection. Furthermore, these immunosuppressive regimens are associated with an increased risk of malignancies and metabolic disorders. Despite significant advancements in the field, these IS related side effects persist as clinical hurdles, emphasizing the need for innovative therapeutic strategies to improve transplant survival and longevity. Cellular therapy, a novel therapeutic approach, has emerged as a potential pathway to promote immune tolerance while minimizing systemic side-effects of standard IS regiments. Various cell types, including chimeric antigen receptor T cells (CAR-T), mesenchymal stromal cells (MSCs), regulatory myeloid cells (RMCs) and regulatory T cells (Tregs), offer unique immunomodulatory properties that may help achieve improved outcomes in transplant patients. This review aims to elucidate the role of cellular therapies, particularly MSCs, T cells, Tregs, RMCs, macrophages, and dendritic cells in SOT and VCA. We explore the immunological features of each cell type, their capacity for immune regulation, and the prospective advantages and obstacles linked to their application in transplant patients. An in-depth outline of the current state of the technology may help SOT and VCA providers refine their perioperative treatment strategies while laying the foundation for further trials that investigate cellular therapeutics in transplantation surgery.

Current advances in experimental and computational approaches to enhance CAR T cell manufacturing protocols and improve clinical efficacy

Since the FDA's approval of chimeric antigen receptor (CAR) T cells in 2017, significant improvements have been made in the design of chimeric antigen receptor constructs and in the manufacturing of CAR T cell therapies resulting in increased in vivo CAR T cell persistence and improved clinical outcome in certain hematological malignancies. Despite the remarkable clinical response seen in some patients, challenges remain in achieving durable long-term tumor-free survival, reducing therapy associated malignancies and toxicities, and expanding on the types of cancers that can be treated with this therapeutic modality. Careful analysis of the biological factors demarcating efficacious from suboptimal CAR T cell responses will be of paramount importance to address these shortcomings. With the ever-expanding toolbox of experimental approaches, single-cell technologies, and computational resources, there is renowned interest in discovering new ways to streamline the development and validation of new CAR T cell products. Better and more accurate prognostic and predictive models can be developed to help guide and inform clinical decision making by incorporating these approaches into translational and clinical workflows. In this review, we provide a brief overview of recent advancements in CAR T cell manufacturing and describe the strategies used to selectively expand specific phenotypic subsets. Additionally, we review experimental approaches to assess CAR T cell functionality and summarize current in silico methods which have the potential to improve CAR T cell manufacturing and predict clinical outcomes.

Beyond FOXP3: a 20-year journey unravelling human regulatory T-cell heterogeneity

The initial idea of a distinct group of T-cells responsible for suppressing immune responses was first postulated half a century ago. However, it is only in the last three decades that we have identified what we now term regulatory T-cells (Tregs), and subsequently elucidated and crystallized our understanding of them. Human Tregs have emerged as essential to immune tolerance and the prevention of autoimmune diseases and are typically contemporaneously characterized by their CD3+CD4+CD25high CD127lowFOXP3+ phenotype. It is important to note that FOXP3+ Tregs exhibit substantial diversity in their origin, phenotypic characteristics, and function. Identifying reliable markers is crucial to the accurate identification, quantification, and assessment of Tregs in health and disease, as well as the enrichment and expansion of viable cells for adoptive cell therapy. In our comprehensive review, we address the contributions of various markers identified in the last two decades since the master transcriptional factor FOXP3 was identified in establishing and enriching purity, lineage stability, tissue homing and suppressive proficiency in CD4+ Tregs. Additionally, our review delves into recent breakthroughs in innovative Treg-based therapies, underscoring the significance of distinct markers in their therapeutic utilization. Understanding Treg subsets holds the key to effectively harnessing human Tregs for immunotherapeutic approaches.

Applications of Genome-Editing Technologies for Type 1 Diabetes

Type 1 diabetes (T1D) is a chronic autoimmune disease characterized by the destruction of insulin-producing pancreatic β-cells by the immune system. Although conventional therapeutic modalities, such as insulin injection, remain a mainstay, recent years have witnessed the emergence of novel treatment approaches encompassing immunomodulatory therapies, such as stem cell and β-cell transplantation, along with revolutionary gene-editing techniques. Notably, recent research endeavors have enabled the reshaping of the T-cell repertoire, leading to the prevention of T1D development. Furthermore, CRISPR-Cas9 technology has demonstrated remarkable potential in targeting endogenous gene activation, ushering in a promising avenue for the precise guidance of mesenchymal stem cells (MSCs) toward differentiation into insulin-producing cells. This innovative approach holds substantial promise for the treatment of T1D. In this review, we focus on studies that have developed T1D models and treatments using gene-editing systems.

Promises and Pitfalls of Next-Generation Treg Adoptive Immunotherapy

Regulatory T cells (Tregs) are fundamental to maintaining immune homeostasis by inhibiting immune responses to self-antigens and preventing the excessive activation of the immune system. Their functions extend beyond immune surveillance and subpopulations of tissue-resident Treg cells can also facilitate tissue repair and homeostasis. The unique ability to regulate aberrant immune responses has generated the concept of harnessing Tregs as a new cellular immunotherapy approach for reshaping undesired immune reactions in autoimmune diseases and allo-responses in transplantation to ultimately re-establish tolerance. However, a number of issues limit the broad clinical applicability of Treg adoptive immunotherapy, including the lack of antigen specificity, heterogeneity within the Treg population, poor persistence, functional Treg impairment in disease states, and in vivo plasticity that results in the loss of suppressive function. Although the early-phase clinical trials of Treg cell therapy have shown the feasibility and tolerability of the approach in several conditions, its efficacy has remained questionable. Leveraging the smart tools and platforms that have been successfully developed for primary T cell engineering in cancer, the field has now shifted towards "next-generation" adoptive Treg immunotherapy, where genetically modified Treg products with improved characteristics are being generated, as regards antigen specificity, function, persistence, and immunogenicity. Here, we review the state of the art on Treg adoptive immunotherapy and progress beyond it, while critically evaluating the hurdles and opportunities towards the materialization of Tregs as a living drug therapy for various inflammation states and the broad clinical translation of Treg therapeutics.

Eras of designer Tregs: Harnessing synthetic biology for immune suppression

Since their discovery, CD4+ CD25hi FOXP3hi regulatory T cells (Tregs) have been firmly established as a critical cell type for regulating immune homeostasis through a plethora of mechanisms. Due to their immunoregulatory power, delivery of polyclonal Tregs has been explored as a therapy to dampen inflammation in the settings of transplantation and autoimmunity. Evidence shows that Treg therapy is safe and well-tolerated, but efficacy remains undefined and could be limited by poor persistence in vivo and lack of antigen specificity. With the advent of new genetic engineering tools, it is now possible to create bespoke "designer" Tregs that not only overcome possible limitations of polyclonal Tregs but also introduce new features. Here, we review the development of designer Tregs through the perspective of three 'eras': (1) the era of FOXP3 engineering, in which breakthroughs in the biological understanding of this transcription factor enabled the conversion of conventional T cells to Tregs; (2) the antigen-specificity era, in which transgenic T-cell receptors and chimeric antigen receptors were introduced to create more potent and directed Treg therapies; and (3) the current era, which is harnessing advanced genome-editing techniques to introduce and refine existing and new engineering approaches. The year 2022 marked the entry of "designer" Tregs into the clinic, with exciting potential for application and efficacy in a wide variety of immune-mediated diseases.

Unlocking the potential of Tregs: innovations in CAR technology

Regulatory T cells (Tregs) adoptive immunotherapy is emerging as a viable treatment option for both autoimmune and alloimmune diseases. However, numerous challenges remain, including limitations related to cell number, availability of target-specific cells, stability, purity, homing ability, and safety concerns. To address these challenges, cell engineering strategies have emerged as promising solutions. Indeed, it has become feasible to increase Treg numbers or enhance their stability through Foxp3 overexpression, post-translational modifications, or demethylation of the Treg-specific demethylated region (TSDR). Specificity can be engineered by the addition of chimeric antigen receptors (CARs), with new techniques designed to fine-tune specificity (tandem chimeric antigen receptors, universal chimeric antigen receptors, synNotch chimeric antigen receptors). The introduction of B-cell targeting antibody receptor (BAR) Tregs has paved the way for effective regulation of B cells and plasma cells. In addition, other constructs have emerged to enhance Tregs activation and function, such as optimized chimeric antigen receptors constructs and the use of armour proteins. Chimeric antigen receptor expression can also be better regulated to limit tonic signaling. Furthermore, various opportunities exist for enhancing the homing capabilities of CAR-Tregs to improve therapy outcomes. Many of these genetic modifications have already been explored for conventional CAR-T therapy but need to be further considered for CAR-Tregs therapies. This review highlights innovative CAR-engineering strategies that have the potential to precisely and efficiently manage immune responses in autoimmune diseases and improve transplant outcomes. As these strategies are further explored and optimized, CAR-Treg therapies may emerge as powerful tools for immune intervention.

Tregs in transplantation tolerance: role and therapeutic potential

CD4+ Foxp3+ regulatory T cells (Tregs) are indispensable for preventing autoimmunity, and they play a role in cancer and transplantation settings by restraining immune responses. In this review, we describe evidence for the importance of Tregs in the induction versus maintenance of transplantation tolerance, discussing insights into mechanisms of Treg control of the alloimmune response. Further, we address the therapeutic potential of Tregs as a clinical intervention after transplantation, highlighting engineered CAR-Tregs as well as expansion of donor and host Tregs.

Regulatory T-cell dysfunction and its implication for cell therapy

Regulatory T cells (Tregs) are a subtype of CD4+ T cells that can mediate immune tolerance by a multitude of immunomodulatory mechanisms. Treg-based adoptive immunotherapy is currently being tested in multiple phases I and II clinical trials in transplantation and autoimmune diseases. We have learned from the work done on conventional T cells that distinct mechanistic states can define their dysfunctions, such as exhaustion, senescence, and anergy. All three can negatively impact the therapeutic effectiveness of T-cell-based therapies. However, whether Tregs are susceptible to such dysfunctional states is not well studied, and results are sometimes found to be controversial. In addition, Treg instability and loss of FOXP3 expression is another Treg-specific dysfunction that can decreasein their suppressive potential. A better understanding of Treg biology and pathological states will be needed to compare and interpret the results of the different clinical and preclinical trials. We will review herein Tregs' mechanisms of action, describe different T-cell dysfunction subtypes and how and if they apply to Tregs (exhaustion, senescence, anergy, and instability), and finally how this knowledge should be taken into consideration when designing and interpreting Treg adoptive immunotherapy trials.

Engineered Treg cells as putative therapeutics against inflammatory diseases and beyond

Regulatory T (Treg) cells ensure tolerance against self-antigens, limit excessive inflammation, and support tissue repair processes. Therefore, Treg cells are currently attractive candidates for the treatment of certain inflammatory diseases, autoimmune disorders, or transplant rejection. Early clinical trials have proved the safety and efficacy of certain Treg cell therapies in inflammatory diseases. We summarize recent advances in engineering Treg cells, including the concept of biosensors for inflammation. We assess Treg cell engineering possibilities for novel functional units, including Treg cell modifications influencing stability, migration, and tissue adaptation. Finally, we outline perspectives of engineered Treg cells going beyond inflammatory diseases by using custom-designed receptors and read-out systems, aiming to use Treg cells as in vivo diagnostic tools and drug delivery vehicles.

Tonic-signaling chimeric antigen receptors drive human regulatory T cell exhaustion

Regulatory T cell (Treg) therapy is a promising approach to improve outcomes in transplantation and autoimmunity. In conventional T cell therapy, chronic stimulation can result in poor in vivo function, a phenomenon termed exhaustion. Whether or not Tregs are also susceptible to exhaustion, and if so, if this would limit their therapeutic effect, was unknown. To "benchmark" exhaustion in human Tregs, we used a method known to induce exhaustion in conventional T cells: expression of a tonic-signaling chimeric antigen receptor (TS-CAR). We found that TS-CAR-expressing Tregs rapidly acquired a phenotype that resembled exhaustion and had major changes in their transcriptome, metabolism, and epigenome. Similar to conventional T cells, TS-CAR Tregs upregulated expression of inhibitory receptors and transcription factors such as PD-1, TIM3, TOX and BLIMP1, and displayed a global increase in chromatin accessibility-enriched AP-1 family transcription factor binding sites. However, they also displayed Treg-specific changes such as high expression of 4-1BB, LAP, and GARP. DNA methylation analysis and comparison to a CD8+ T cell-based multipotency index showed that Tregs naturally exist in a relatively differentiated state, with further TS-CAR-induced changes. Functionally, TS-CAR Tregs remained stable and suppressive in vitro but were nonfunctional in vivo, as tested in a model of xenogeneic graft-versus-host disease. These data are the first comprehensive investigation of exhaustion in Tregs and reveal key similarities and differences with exhausted conventional T cells. The finding that human Tregs are susceptible to chronic stimulation-driven dysfunction has important implications for the design of CAR Treg adoptive immunotherapy strategies.