CD4(+)CD25(+) immunoregulatory T Cells: new therapeutics for graft-versus-host disease

Cellular therapies for the prevention and treatment of acute graft-versus-host disease

Acute graft-versus-host disease (aGVHD) is a major complication of allogeneic hematopoietic cell transplantation (allo-HCT) that is caused by donor immune cells attacking and damaging host tissues. Immune suppressive small molecule and protein-based therapeutics targeting donor anti-host immune cells are currently used for GVHD prophylaxis and treatment. Even with these therapies, aGVHD progresses to life-threatening steroid-refractory aGVHD (SR-aGVHD) in up to 50% of cases and is a risk factor for the subsequent development of debilitating chronic GVHD. To improve aGVHD-related outcomes, donor graft engineering techniques and adoptive transfer of immune modulatory cells have been explored. Highly rigorous donor graft T-cell depletion approaches have revealed that mitigation of aGVHD can be accompanied by slow immune recovery post-allo-HCT and reduction in anti-microbial and anti-leukemia responses resulting in increased relapse and infection rates, respectively. Recent T-cell separation techniques allowing for precision graft engineering by selectively eliminating aGVHD-causing T-cells (eg, naïve T-cells) without loss of T-cells with beneficial functions and retaining and/or enriching immune regulatory populations (eg, regulatory T-cells (Tregs) or myeloid-derived suppressor cells) have been tested and will continue to improve. Clinical cell-based regulatory therapies have been employed for targeting SR-aGVHD, particularly mesenchymal stem cells (MSCs) and more recently, Tregs. In this review, we summarize aGVHD pathophysiology, highlight newly discovered aGVHD mechanisms, and discuss current and emerging cellular and graft manipulation approaches for aGVHD prevention and treatment.

Pathophysiology and preclinical relevance of experimental graft-versus-host disease in humanized mice

Graft-versus-host disease (GVHD) is a life-threatening complication of allogeneic hematopoietic cell transplantations (allo-HCT) used for the treatment of hematological malignancies and other blood-related disorders. Until recently, the discovery of actionable molecular targets to treat GVHD and their preclinical testing was almost exclusively based on modeling allo-HCT in mice by transplanting bone marrow and splenocytes from donor mice into MHC-mismatched recipient animals. However, due to fundamental differences between human and mouse immunology, the translation of these molecular targets into the clinic can be limited. Therefore, humanized mouse models of GVHD were developed to circumvent this limitation. In these models, following the transplantation of human peripheral blood mononuclear cells (PBMCs) into immunodeficient mice, T cells recognize and attack mouse organs, inducing GVHD. Thereby, humanized mice provide a platform for the evaluation of the effects of candidate therapies on GVHD mediated by human immune cells in vivo. Understanding the pathophysiology of this xenogeneic GVHD is therefore crucial for the design and interpretation of experiments performed with this model. In this article, we comprehensively review the cellular and molecular mechanisms governing GVHD in the most commonly used model of xenogeneic GVHD: PBMC-engrafted NOD/LtSz-PrkdcscidIL2rγtm1Wjl (NSG) mice. By re-analyzing public sequencing data, we also show that the clonal expansion and the transcriptional program of T cells in humanized mice closely reflect those in humans. Finally, we highlight the strengths and limitations of this model, as well as arguments in favor of its biological relevance for studying T-cell reactions against healthy tissues or cancer cells.

The role of interleukin-2 in graft-versus-host disease pathogenesis, prevention and therapy

Graft-versus-host disease (GVHD) is a significant complication following allogeneic hematopoietic cell transplantation (allo-HCT), posing substantial risks to patient survival. In the late follow-up phase of transplanted patients, GVHD is also a major cause of morbidity and disability, mostly due to low response to first-line steroids and the lack of effective standard therapies in the second line. This review provides a description of GVHD pathogenesis, with a focus on the central role of Interleukin-2 (IL-2). IL-2 is one of the critical mediators in the complex pathogenesis of GVHD, contributing to the intricate balance between regulatory T cells (Tregs) and effector T cells (Teffs). Due to this pivotal role, several studies investigate the potential of IL-2 as a therapeutic option for GVHD management. We discuss the outcomes of low-dose IL-2 therapies and their impact on Treg proliferation and steroid dependency reduction. Additionally, the effects of combining IL-2 with other treatments, such as extracorporeal photopheresis (ECP) and Treg-enriched lymphocyte infusions, are highlighted. Novel approaches, including modified IL-2 complexes and IL-2 receptor blockade, are explored for their potential in selectively enhancing Treg function and limiting Teff activation. The evolving understanding of IL-2's pivotal role in immune regulation presents promising prospects for applying treatment and prevention strategies for GVHD.

Cytomegalovirus infection is associated with thymic dysfunction and chronic graft-versus-host disease after pediatric hematopoietic stem cell transplantation

Pediatric hematopoietic stem cell transplantation (HSCT) is challenged by chronic graft-versus-host disease (cGvHD) significantly affecting survival and long-term morbidity, but underlying mechanisms including the impact of post-HSCT CMV infection are sparsely studied. We first investigated the impact of CMV infection for development of cGvHD in 322 children undergoing standard myeloablative HSCT between 2000 and 2018. Clinically significant CMV infection (n = 61) was an independent risk factor for chronic GvHD in a multivariable Cox regression analysis (HR = 2.17, 95% CI = 1.18-3.97, P = 0.013). We next explored the underlying mechanisms in a subcohort of 39 children. CMV infection was followed by reduced concentration of recent thymic emigrants (17.5 vs. 51.9 × 106/L, P = 0.048) and naïve CD4+ and CD8+ T cells at 6 months post-HSCT (all P < 0.05). Furthermore, CD25highFOXP3+ Tregs tended to be lower in patients with CMV infection (2.9 vs. 9.6 × 106/L, P = 0.055), including Tregs expressing the naivety markers CD45RA and Helios. CD8+ T-cell numbers rose after CMV infection and was dominated by exhausted PD1-expressing cells (66% vs. 39%, P = 0.023). These findings indicate that post-HSCT CMV infection is a main risk factor for development of chronic GvHD after pediatric HSCT and suggest that this effect is caused by reduced thymic function with a persistently impaired production of naïve and regulatory T cells in combination with increased peripheral T-cell exhaustion.

Clinical grade multiparametric cell sorting and gene-marking of regulatory T cells

BACKGROUND AIMS

Regulatory T cells (Tregs) are the main mediators of peripheral tolerance. Treg-directed therapy has shown promising results in preclinical studies of diverse immunopathologies. At present, the clinical applicability of adoptive Treg transfer is limited by difficulties in generating Tregs at sufficient cell dose and purity.

METHODS

We developed a Good Manufacturing Practice (GMP) compliant method based on closed-system multiparametric Fluorescence-Activated Cell Sorting (FACS) to purify Tregs, which are then expanded in vitro and gene-marked with a clinical grade retroviral vector to enable in vivo fate tracking. Following small-scale optimization, we conducted four clinical-scale processing runs.

RESULTS

We showed that Tregs could be enriched to 87- 92% purity following FACS-sorting, and expanded and transduced to yield clinically relevant cell dose of 136-732×106 gene-marked cells, sufficient for a cell dose of at least 2 × 106 cells/kg. The expanded Tregs were highly demethylated in the FOXP3 Treg-specific demethylated region (TSDR), consistent with bona fide natural Tregs. They were suppressive in vitro, but a small percentage could secrete proinflammatory cytokines, including interferon-γ and interleukin-17A.

CONCLUSIONS

This study demonstrated the feasibility of isolating, expanding and gene-marking Tregs in clinical scale, thus paving the way for future phase I trials that will advance knowledge about the in vivo fate of transferred Tregs and its relationship with concomitant Treg-directed pharmacotherapy and clinical response.

Polyclonally Derived Alloantigen-Specific T Regulatory Cells Exhibit Target-Specific Suppression and Capture MHC Class II from Dendritic Cells

Foxp3+ T regulatory (Treg) cells prevent allograft rejection and graft-versus-host disease. Although polyclonal Tregs have been used both in animal models and in humans, the fine specificity of their suppressive function is poorly defined. We have generated mouse recipient-derived alloantigen-specific Tregs in vitro and explored the fine specificity of their suppressive function and their mechanism of action in vitro and in vivo. In vitro, when alloantigen and peptide Ag were both presented on the same dendritic cell, both responses were suppressed by iTregs specific either for the alloantigen or for the peptide Ag. In vivo, iTreg suppression was limited to the cognate Ag, and no bystander suppression was observed when both allo-antigen and peptide Ag were present on the same dendritic cell. In vitro, alloantigen-specific Tregs captured cognate MHC but failed to capture noncognate MHC. Our results demonstrate that a polyclonal population of iTregs generated from naive T cells can mediate highly specific function in vivo and support the view that Treg therapy, even with unselected polyclonal populations, is likely to be target antigen-specific and that bystander responses to self-antigens or to infectious agents are unlikely.

CD74 supports accumulation and function of regulatory T cells in tumors

Regulatory T cells (Tregs) are plastic cells playing a pivotal role in the maintenance of immune homeostasis. Tregs actively adapt to the microenvironment where they reside; as a consequence, their molecular and functional profiles differ among tissues and pathologies. In tumors, the features acquired by Tregs remains poorly characterized. Here, we observe that human tumor-infiltrating Tregs selectively overexpress CD74, the MHC class II invariant chain. CD74 has been previously described as a regulator of antigen-presenting cell biology, however its function in Tregs remains unknown. CD74 genetic deletion in human primary Tregs reveals that CD74KO Tregs exhibit major defects in the organization of their actin cytoskeleton and intracellular organelles. Additionally, intratumoral CD74KO Tregs show a decreased activation, a drop in Foxp3 expression, a low accumulation in the tumor, and consistently, they are associated with accelerated tumor rejection in preclinical models in female mice. These observations are unique to tumor conditions as, at steady state, CD74KO-Treg phenotype, survival, and suppressive capacity are unaffected in vitro and in vivo. CD74 therefore emerges as a specific regulator of tumor-infiltrating Tregs and as a target to interfere with Treg anti-tumor activity.

Combining Treg Therapy With Donor Bone Marrow Transplantation: Experimental Progress and Clinical Perspective

Donor-specific tolerance remains a goal in transplantation because it could improve graft survival and reduce morbidity. Cotransplantation of donor hematopoietic cells to achieve chimerism is a promising approach for tolerance induction, which was successfully tested in clinical trials. However, current protocols are associated with side effects related to the myelosuppressive recipient conditioning, which makes it difficult to introduce them as standard therapy. More recently, adoptive cell therapy with polyclonal or donor-specific regulatory T cells (Treg) proved safe and feasible in several transplant trials, but it is unclear whether it can induce tolerance on its own. The combination of both approaches-Treg therapy and hematopoietic cell transplantation-leads to chimerism and tolerance without myelosuppressive treatment in murine models. Treg therapy promotes engraftment of allogeneic hematopoietic cells, reducing conditioning requirements and enhancing regulatory mechanisms maintaining tolerance. This review discusses possible modes of action of transferred Treg in experimental chimerism models and describes translational efforts investigating the potent synergy of Treg and chimerism.

Influence of FOXP3 single-nucleotide polymorphism after allogeneic hematopoietic stem cell transplantation

The impact of FOXP3 single-nucleotide polymorphisms (SNP) on clinical outcomes after allogeneic hematopoietic stem cell transplantation (allo-HSCT) remains poorly understood. We investigated the relationship between a FOXP3 SNP (rs3761548) and clinical outcomes in 91 patients with hematological malignancies after allo-HSCT. Multivariate analysis showed that risk of severe chronic graft-versus-host disease (cGVHD) was significantly higher in patients with the FOXP3-3279C/A or FOXP3-3279A/A genotype than those with the FOXP3-3279C/C genotype [hazard ratio (HR), 2.69; 95% confidence interval (CI) 1.14-6.31; p = 0.023]. Therefore, FOXP3 at SNP rs3761548 can be a useful marker for predicting the occurrence of severe cGVHD.

Donor regulatory T cells rapidly adapt to recipient tissues to control murine acute graft-versus-host disease

The adoptive transfer of regulatory T cells is a promising strategy to prevent graft-versus-host disease after allogeneic bone marrow transplantation. Here, we use a major histocompatibility complex-mismatched mouse model to follow the fate of in vitro expanded donor regulatory T cells upon migration to target organs. Employing comprehensive gene expression and repertoire profiling, we show that they retain their suppressive function and plasticity after transfer. Upon entering non-lymphoid tissues, donor regulatory T cells acquire organ-specific gene expression profiles resembling tissue-resident cells and activate hallmark suppressive and cytotoxic pathways, most evidently in the colon, when co-transplanted with graft-versus-host disease-inducing conventional T cells. Dominant T cell receptor clonotypes overlap between organs and across recipients and their relative abundance correlates with protection efficacy. Thus, this study reveals donor regulatory T cell selection and adaptation mechanisms in target organs and highlights protective features of Treg to guide the development of improved graft-versus-host disease prevention strategies.

Engineered Treg cells: The heir to the throne of immunotherapy

Recently, increased interest in the use of Tregs as adoptive cell therapy for the treatment of autoimmune diseases and transplant rejection had led to several advances in the field. However, Treg cell therapies, while constantly advancing, indiscriminately suppress the immune system without the permanent stabilization of certain diseases. Genetically modified Tregs hold great promise towards solving these problems, but, challenges in identifying the most potent Treg subtype, accompanied by the ambiguity involved in identifying the optimal Treg source, along with its expansion and engineering in a clinical-grade setting remain paramount. This review highlights the recent advances in methodologies for the development of genetically engineered Treg cell-based treatments for autoimmune, inflammatory diseases, and organ rejection. Additionally, it provides a systematized guide to all the recent progress in the field and informs the readers of the feasibility and safety of engineered adoptive Treg cell therapy, with the aim to provide a framework for researchers involved in the development of engineered Tregs.

Treg-targeted IL-2/anti-IL-2 complex controls graft-versus-host disease and supports anti-tumor effect in allogeneic hematopoietic stem cell transplantation

Modulating an immune response in opposite directions represents the holy grail in allogeneic hematopoietic stem cell transplantation (allo-HSCT) to avoid insufficient reactivity of donor T cells and hematologic malignancy relapse while controlling the potential development of graft-versus-host disease (GVHD), in which donor T cells attack the recipient's tissues. IL-2/anti-IL-2 complexes (IL-2Cx) represent a therapeutic option to selectively accentuate or dampen the immune response. In dedicated experimental models of allo-HSCT, including also human cells injected in immunodeficient NSG mice, we evaluated side-by-side the therapeutic effect of two IL-2Cx designed either to boost regulatory T cells (Treg) or alternatively to activate effector T cells (Teff), on GVHD occurrence and tumor relapse. We also evaluated the effect of the complexes on the phenotype and function of immune cells in vivo. Unexpectedly, both pro-Treg and pro-Teff IL-2Cx prevented GVHD development. They both induced Treg expansion and reduced CD8+ T-cell numbers, compared to untreated mice. However, only mice treated with the pro-Treg IL-2Cx, showed a dramatic reduction of exhausted CD8+ T cells, consistent with a potent anti-tumor effect. When evaluated on human cells, pro-Treg IL-2Cx also preferentially induced Treg expansion in vitro and in vivo, while allowing the development of a potent anti-tumor effect in NSG mice. Our results demonstrate the clinical relevance of using a pro-Treg, but not a pro-Teff IL2Cx to modulate alloreactivity after HSCT, while promoting a graft-versus-leukemia effect.

Microenvironment in acute myeloid leukemia: focus on senescence mechanisms, therapeutic interactions, and future directions

Acute myeloid leukemia (AML) is a disease with a dismal prognosis, mainly affecting the elderly. In recent years, new drugs have improved life expectancy and quality of life, and a better understanding of the genetic-molecular nature of the disease has shed light on previously unknown aspects of leukemogenesis. In parallel, increasing attention has been attracted to the complex interactions between cells and soluble factors in the bone marrow (BM) environment, collectively known as the microenvironment. In this review, we discuss the central role of the microenvironment in physiologic and pathologic hematopoiesis and the mechanisms of senescence, considered a fundamental protective mechanism against the proliferation of damaged and pretumoral cells. The microenvironment also represents a fertile ground for the development of myeloid malignancies, and the leukemic niche significantly interacts with drugs commonly used in AML treatment. Finally, we focus on the role of the microenvironment in the engraftment and complications of allogeneic hematopoietic stem cell transplantation, the only curative option in a conspicuous proportion of patients.

Clinical Manufacturing of Regulatory T Cell Products For Adoptive Cell Therapy and Strategies to Improve Therapeutic Efficacy

Based on successes in preclinical animal transplant models, adoptive cell therapy (ACT) with regulatory T cells (Tregs) is a promising modality to induce allograft tolerance or reduce the use of immunosuppressive drugs to prevent rejection. Extensive work has been done in optimizing the best approach to manufacture Treg cell products for testing in transplant recipients. Collectively, clinical evaluations have demonstrated that large numbers of Tregs can be expanded ex vivo and infused safely. However, these trials have failed to induce robust drug-free tolerance and/or significantly reduce the level of immunosuppression needed to prevent solid organ transplant (SOTx) rejection. Improving Treg therapy effectiveness may require increasing Treg persistence or orchestrating Treg migration to secondary lymphatic tissues or places of inflammation. In this review, we describe current clinical Treg manufacturing methods used for clinical trials. We also highlight current strategies being implemented to improve delivered Treg ACT persistence and migration in preclinical studies.

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.

The OX40-TRAF6 axis promotes CTLA-4 degradation to augment antitumor CD8+ T-cell immunity

Immune checkpoint blockade (ICB), including anti-cytotoxic T-lymphocyte associated protein 4 (CTLA-4), benefits only a limited number of patients with cancer. Understanding the in-depth regulatory mechanism of CTLA-4 protein stability and its functional significance may help identify ICB resistance mechanisms and assist in the development of novel immunotherapeutic modalities to improve ICB efficacy. Here, we identified that TNF receptor-associated factor 6 (TRAF6) mediates Lys63-linked ubiquitination and subsequent lysosomal degradation of CTLA-4. Moreover, by using TRAF6-deficient mice and retroviral overexpression experiments, we demonstrated that TRAF6 promotes CTLA-4 degradation in a T-cell-intrinsic manner, which is dependent on the RING domain of TRAF6. This intrinsic regulatory mechanism contributes to CD8+ T-cell-mediated antitumor immunity in vivo. Additionally, by using an OX40 agonist, we demonstrated that the OX40-TRAF6 axis is responsible for CTLA-4 degradation, thereby controlling antitumor immunity in both tumor-bearing mice and patients with cancer. Overall, our findings demonstrate that the OX40-TRAF6 axis promotes CTLA-4 degradation and is a potential therapeutic target for the improvement of T-cell-based immunotherapies.

Graft versus Leukemia in 2023

Allogeneic hematopoietic stem cell transplantation (HSCT) is commonly utilized in the management of leukemia across multiple subtypes. Graft versus leukemia (GVL) is a critical component of successful transplantation and involves donor cells eradicating residual leukemia within the recipient. Graft versus host disease (GVHD) by contrast is a common complication of the transplantation process in which donor cells identify the recipient's various organ systems as foreign, thereby leading to a multitude of organ toxicities that can be described as autoimmune in nature. As both GVL and GVHD are mediated by a similar mechanism, these processes are felt to occur in tandem with one another. Here, we review the allogeneic HCT process in the context of GVL.

Regulatory T cells in autoimmune kidney diseases and transplantation

Regulatory T (Treg) cells that express the transcription factor forkhead box protein P3 (FOXP3) are naturally present in the immune system and have roles in the maintenance of immunological self-tolerance and immune system and tissue homeostasis. Treg cells suppress T cell activation, expansion and effector functions by various mechanisms, particularly by controlling the functions of antigen-presenting cells. They can also contribute to tissue repair by suppressing inflammation and facilitating tissue regeneration, for example, via the production of growth factors and the promotion of stem cell differentiation and proliferation. Monogenic anomalies of Treg cells and genetic variations of Treg cell functional molecules can cause or predispose patients to the development of autoimmune diseases and other inflammatory disorders, including kidney diseases. Treg cells can potentially be utilized or targeted to treat immunological diseases and establish transplantation tolerance, for example, by expanding natural Treg cells in vivo using IL-2 or small molecules or by expanding them in vitro for adoptive Treg cell therapy. Efforts are also being made to convert antigen-specific conventional T cells into Treg cells and to generate chimeric antigen receptor Treg cells from natural Treg cells for adoptive Treg cell therapies with the aim of achieving antigen-specific immune suppression and tolerance in the clinic.

Metabolic Disruption Induced by mTOR Signaling Pathway Inhibition in Regulatory T-Cell Expansion for Clinical Application

BACKGROUND

Regulatory T cell (Treg) therapy is considered an alternative approach to induce tolerance in transplantation. If successful, this therapy may have implications on immunosuppression minimization/withdrawal to reduce drug-induced toxicity in patients. The aim of this study was to assess the efficacy of the mTORC1/C2 inhibitor, AZD8055, in the manufacturing of clinically competent Treg cells and compare the effects with those induced by rapamycin (RAPA), another mTOR inhibitor commonly used in Treg expansion protocols.

METHODS

Primary human Treg cells were isolated from leukapheresis product. Cell viability, expansion rates, suppressive function, autophagy, mitochondrial unfolded protein response (mitoUPR), and cell metabolic profile were assessed.

RESULTS

We observed a stronger inhibition of the mTORC2 signaling pathway and downstream events triggered by Interleukin 2 (IL2)-receptor in AZD8055-treated cells compared with those treated with RAPA. AZD8055 induced progressive metabolic changes in mitochondrial respiration and glycolytic pathways that disrupted the long-term expansion and suppressive function of Tregs. Unlike RAPA, AZD8055 treatment impaired autophagy and enhanced the mitoUPR cell stress response pathway.

CONCLUSIONS

A distinct pattern of mTOR inhibition by AZD, compared with RAPA, induced mitochondrial stress response and dysfunction, impaired autophagy, and disrupted cellular bioenergetics, resulting in the loss of proliferative potential and suppressive function of Treg cells.

TCF-1 negatively regulates the suppressive ability of canonical and noncanonical Tregs

Regulatory T cells are suppressive immune cells used in various clinical and therapeutic applications. Canonical regulatory T cells express CD4, FOXP3, and CD25, which are considered definitive markers of their regulatory T-cell status when expressed together. However, a subset of noncanonical regulatory T cells expressing only CD4 and FOXP3 have recently been described in some infection contexts. Using a unique mouse model for the first time demonstrated that the TCF-1 regulation of regulatory T-cell suppressive function is not limited to the thymus during development. Our data showed that TCF-1 also regulated regulatory T cells' suppressive ability in secondary organs and graft-vs-host disease target organs as well as upregulating noncanonical regulatory T cells. Our data demonstrated that TCF-1 regulates the suppressive function of regulatory T cells through critical molecules like GITR and PD-1, specifically by means of noncanonical regulatory T cells. Our in vitro approaches show that TCF-1 regulates the regulatory T-cell effector-phenotype and the molecules critical for regulatory T-cell migration to the site of inflammation. Using in vivo models, we show that both canonical and noncanonical regulatory T cells from TCF-1 cKO mice have a superior suppressive function, as shown by their ability to control conventional T-cell proliferation, avert acute graft-vs-host disease, and limit tissue damage. Thus, for the first time, we provide evidence that TCF-1 negatively regulates the suppressive ability of canonical and noncanonical regulatory T cells. These findings provide evidence that TCF-1 is a novel target for developing strategies to treat alloimmune disorders.

Enhancing Regulatory T Cells to Treat Inflammatory and Autoimmune Diseases

Regulatory T cells (Tregs) control immune responses and are essential to maintain immune homeostasis and self-tolerance. Hence, it is no coincidence that autoimmune and chronic inflammatory disorders are associated with defects in Tregs. These diseases have currently no cure and are treated with palliative drugs such as immunosuppressant and immunomodulatory agents. Thereby, there is a great interest in developing medical interventions against these diseases based on enhancing Treg cell function and numbers. Here, we give an overview of Treg cell ontogeny and function, paying particular attention to mucosal Tregs. We review some notable approaches to enhance immunomodulation by Tregs with therapeutic purposes including adoptive Treg cell transfer therapy and discuss relevant clinical trials for inflammatory bowel disease. We next introduce ways to expand mucosal Tregs in vivo using microbiota and dietary products that have been the focus of clinical trials in various autoimmune and chronic-inflammatory diseases.

Donor Genetic Predisposition to High Interleukin-10 Production Appears Protective against Acute Graft-Versus-Host Disease

The persistence of graft-versus-host disease (GVHD) as the principal complication of allogeneic hematopoietic cell transplantation (HCT) demonstrates that HLA matching alone is insufficient to prevent alloreactivity. We performed molecular and functional characterization of 22 candidate cytokine genes for their potential to improve matching in 315 myeloablative, 10/10 HLA-matched donor−recipient pairs. Recipients of a graft carrying the -1082GG IL10 gene promoter region variant had a three-fold lower incidence of grade II−IV acute GVHD compared to IL10-1082AA graft recipients (SHR = 0.25, p = 0.005). This was most evident in matched unrelated donor (MUD) transplants, where the greatest alloreactivity is expected. IL10-1082GG transplants did not experience an increased incidence of relapse, and, consequently, overall survival was two-fold higher in IL10-1082GG MUD transplants (HR = 0.17, p = 0.023). Longitudinal post-transplant measurements demonstrated that -1082GG is a high-IL10-producing and -expressing genotype with attenuated CD8+ T-cell reconstitution. High post-transplant donor chimerism in T- and myeloid-cells (>95%) confirmed a predominant donor, rather than recipient, genotype effect on immune function and aGVHD. To date, this is the first study to report corroborating genome-to-cellular evidence for a non-HLA donor immunogenetic variant that appears to be protective against GVHD. The incorporation of IL10 variants in donor selection criteria and clinical-management decisions has the potential to improve patient outcomes.

Milestones in acute GVHD pathophysiology

In the past 65 years, over 25 000 referenced articles have been published on graft-versus-host disease (GVHD). Although this included clinically orientated papers or publications on chronic GVHD, the conservative estimate of scientific publications still contains several thousands of documents on the pathophysiology of acute GVHD. Thus, summarizing what we believe are prominent publications that can be considered milestones in our knowledge of this disease is a challenging and inherently biased task. Here we review from a historical perspective what can be regarded as publications that have made the field move forward. We also included several references of reviews on aspects we could not cover in detail.

Forced Fox-P3 expression can improve the safety and antigen-specific function of engineered regulatory T cells

Regulatory T cells (Treg) are potent inhibitors of autoreactive T cells. The intracellular transcription factor FoxP3 controls the expression levels of a diverse set of genes and plays a critical role in programming functional Tregs. Although, antigen-specific Tregs are more potent than polyclonal Tregs in treating ongoing autoimmunity, phenotype plasticity associated with loss of FoxP3 expression in Tregs can lead to the conversion into antigen-specific effector T cells which might exacerbate autoimmune pathology. In this study, we designed a retroviral vector driving the expression of FoxP3 and a human HLA-DR-restricted TCR from the same promoter. Transduction of purified human Tregs revealed that all TCR-positive cells had elevated levels of FoxP3 expression, increased CD25 and CTLA4 expression and potent suppressive function. Elevated FoxP3 expression did not impair the in vitro expansion of engineered Tregs. Adoptive transfer into HLA-DR transgenic mice revealed that FoxP3+TCR engineered Tregs showed long-term persistence with stable FoxP3 and TCR expression. In contrast, adoptive transfer of Tregs engineered with TCR only resulted in the accumulation of TCR-positive, FoxP3-negative T cells which displayed antigen-specific effector function when stimulated with the TCR-recognised peptides. Our data indicate that forced expression of FoxP3 can prevent accumulation of antigen-specific effector T cells without impairing the engraftment and persistence of engineered Tregs.

Murine CAR19 Tregs suppress acute graft-versus-host disease and maintain graft-versus-tumor responses

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) efficacy is complicated by graft-versus-host disease (GVHD), a leading cause of morbidity and mortality. Regulatory T cells (Tregs) have shown efficacy in preventing GVHD. However, high Treg doses are often required, necessitating substantial ex vivo or in vivo expansion that may diminish suppressor function. To enhance in vivo suppressor function, murine Tregs were transduced to express an anti-human CD19 chimeric antigen receptor (hCAR19) and infused into lethally irradiated, hCD19-transgenic recipients for allo-HSCT. Compared with recipients receiving control transduced Tregs, those receiving hCAR19 Tregs had a marked decrease in acute GVHD lethality. Recipient hCD19 B cells and murine hCD19 TBL12-luciferase (TBL12luc) lymphoma cells were both cleared by allogeneic hCAR19 Tregs, which was indicative of graft-versus-tumor (GVT) maintenance and potentiation. Mechanistically, hCAR19 Tregs killed syngeneic hCD19+ but not hCD19- murine TBL12luc cells in vitro in a perforin-dependent, granzyme B-independent manner. Importantly, cyclophosphamide-treated, hCD19-transgenic mice given hCAR19 cytotoxic T lymphocytes without allo-HSCT experienced rapid lethality due to systemic toxicity that has been associated with proinflammatory cytokine release; in contrast, hCAR19 Treg suppressor function enabled avoidance of this severe complication. In conclusion, hCAR19 Tregs are a potentially novel and effective strategy to suppress GVHD without loss of GVT responses.

Different subpopulations of regulatory T cells in human autoimmune disease, transplantation, and tumor immunity

CD4⁺CD25⁺ regulatory T cells (Tregs), a subpopulation of naturally CD4⁺ T cells that characteristically express transcription factor Forkhead box P3 (FOXP3), play a pivotal role in the maintenance of immune homeostasis and the prevention of autoimmunity. With the development of biological technology, the understanding of plasticity and stability of Tregs has been further developed. Recent studies have suggested that human Tregs are functionally and phenotypically diverse. The functions and mechanisms of different phenotypes of Tregs in different disease settings, such as tumor microenvironment, autoimmune diseases, and transplantation, have gradually become hot spots of immunology research that arouse extensive attention. Among the complex functions, CD4⁺CD25⁺FOXP3⁺ Tregs possess a potent immunosuppressive capacity and can produce various cytokines, such as IL‐2, IL‐10, and TGF‐β, to regulate immune homeostasis. They can alleviate the progression of diseases by resisting inflammatory immune responses, whereas promoting the poor prognosis of diseases by helping cells evade immune surveillance or suppressing effector T cells activity. Therefore, methods for targeting Tregs to regulate their functions in the immune microenvironment, such as depleting them to strengthen tumor immunity or expanding them to treat immunological diseases, need to be developed. Here, we discuss that different subpopulations of Tregs are essential for the development of immunotherapeutic strategies involving Tregs in human diseases.

TNFR2 blockade of regulatory T cells unleashes an antitumor immune response after hematopoietic stem-cell transplantation

Background

Targeting immune checkpoints that inhibit antitumor immune responses has emerged as a powerful new approach to treat cancer. We recently showed that blocking the tumor necrosis factor receptor-type 2 (TNFR2) pathway induces the complete loss of the protective function of regulatory T cells (Tregs) in a model of graft-versus-host disease (GVHD) prevention that relies on Treg-based cell therapy. Here, we tested the possibility of amplifying the antitumor response by targeting TNFR2 in a model of tumor relapse following hematopoietic stem-cell transplantation, a clinical situation for which the need for efficient therapeutic options is still unmet.

Method

We developed appropriate experimental conditions that mimic patients that relapsed from their initial hematological malignancy after hematopoietic stem-cell transplantation. This consisted of defining in allogeneic bone marrow transplantation models developed in mice, the maximum number of required tumor cells and T cells to infuse into recipient mice to develop a model of tumor relapse without inducing GVHD. We next evaluated whether anti-TNFR2 treatment could trigger alloreactivity and consequently antitumor immune response. In parallel, we also studied the differential expression of TNFR2 on T cells including Treg from patients in post-transplant leukemia relapse and in patients developing GVHD.

Results

Using experimental conditions in which neither donor T cells nor TNFR2-blocking antibody per se have any effect on tumor relapse, we observed that the coadministration of a suboptimal number of T cells and an anti-TNFR2 treatment can trigger alloreactivity and subsequently induce a significant antitumor effect. This was associated with a reduced percentage of activated CD4⁺ and CD8⁺ Tregs. Importantly, human Tregs over-expressed TNFR2 relative to conventional T cells in healthy donors and in patients experiencing leukemia relapse or cortico-resistant GVHD after hematopoietic stem cell transplantation.

Conclusions

These results highlight TNFR2 as a new target molecule for the development of immunotherapies to treat blood malignancy relapse, used either directly in grafted patients or to enhance donor lymphocyte infusion strategies. More widely, they open the door for new perspectives to amplify antitumor responses against solid cancers by directly targeting Tregs through their TNFR2 expression.

Immune-Related Genes for Predicting Future Kidney Graft Loss: A Study Based on GEO Database

Objective

We aimed to identify feature immune-related genes that correlated with graft rejection and to develop a prognostic model based on immune-related genes in kidney transplantation.

Methods

Gene expression profiles were obtained from the GEO database. The GSE36059 dataset was used as a discovery cohort. Then, differential expression analysis and a machine learning method were performed to select feature immune-related genes. After that, univariate and multivariate Cox regression analyses were used to identify prognosis-related genes. A novel Riskscore model was built based on the results of multivariate regression. The levels of these feature genes were also confirmed in an independent single-cell dataset and other GEO datasets.

Results

15 immune-related genes were expressed differently between non-rejection and rejection kidney allografts. Those differentially expressed immune-related genes (DE-IRGs) were mainly associated with immune-related biological processes and pathways. Subsequently, a 5-immune-gene signature was constructed and showed favorable predictive results in the GSE21374 dataset. Recipients were divided into the high-risk and low-risk groups according to the median value of RiskScore. The GO and KEGG analysis indicated that the differentially expressed genes (DEGs) between high-risk and low-risk groups were mainly involved in inflammatory pathways, chemokine-related pathways, and rejection-related pathways. Immune infiltration analysis demonstrated that RiskScore was potentially related to immune infiltration. Kaplan-Meier survival analysis suggested that recipients in the high-risk group had poor graft survival. AUC values of 1- and 3-year graft survival were 0.804 and 0.793, respectively.

Conclusion

Our data suggest that this immune-related prognostic model had good sensitivity and specificity in predicting the 1- and 3-year kidney graft survival and might act as a useful tool for predicting kidney graft loss.

T Helper Cell Lineage-Defining Transcription Factors: Potent Targets for Specific GVHD Therapy?

Allogenic hematopoietic stem cell transplantation (allo-HSCT) represents a potent and potentially curative treatment for many hematopoietic malignancies and hematologic disorders in adults and children. The donor-derived immunity, elicited by the stem cell transplant, can prevent disease relapse but is also responsible for the induction of graft-versus-host disease (GVHD). The pathophysiology of acute GVHD is not completely understood yet. In general, acute GVHD is driven by the inflammatory and cytotoxic effect of alloreactive donor T cells. Since several experimental approaches indicate that CD4 T cells play an important role in initiation and progression of acute GVHD, the contribution of the different CD4 T helper (Th) cell subtypes in the pathomechanism and regulation of the disease is a central point of current research. Th lineages derive from naïve CD4 T cell progenitors and lineage commitment is initiated by the surrounding cytokine milieu and subsequent changes in the transcription factor (TF) profile. Each T cell subtype has its own effector characteristics, immunologic function, and lineage specific cytokine profile, leading to the association with different immune responses and diseases. Acute GVHD is thought to be mainly driven by the Th1/Th17 axis, whereas Treg cells are attributed to attenuate GVHD effects. As the differentiation of each Th subset highly depends on the specific composition of activating and repressing TFs, these present a potent target to alter the Th cell landscape towards a GVHD-ameliorating direction, e.g. by inhibiting Th1 and Th17 differentiation. The finding, that targeting of Th1 and Th17 differentiation appears more effective for GVHD-prevention than a strategy to inhibit Th1 and Th17 cytokines supports this concept. In this review, we shed light on the current advances of potent TF inhibitors to alter Th cell differentiation and consecutively attenuate GVHD. We will focus especially on preclinical studies and outcomes of TF inhibition in murine GVHD models. Finally, we will point out the possible impact of a Th cell subset-specific immune modulation in context of GVHD.

Interleukin-2-inducible T-cell kinase (Itk) signaling regulates potent noncanonical regulatory T cells

Regulatory T cells (Tregs) play an important role in controlling autoimmunity and limiting tissue damage and inflammation. IL2-inducible T cell kinase (Itk) is part of the Tec family of tyrosine kinases and is a critical component of T cell receptor mediated signaling. Here, we showed that either genetic ablation of Itk signaling or inhibition of Itk signaling pathways resulted in increased frequency of "noncanonical" CD4+ CD25- FOXP3+ Tregs (ncTregs), as well as of "canonical" CD4+ CD25+ FOXP3+ Tregs (canTregs). Using in vivo models, we showed that ncTregs can avert the formation of acute graft-versus-host disease (GVHD), in part by reducing conventional T cell proliferation, proinflammatory cytokine production, and tissue damage. This reduction in GVHD occurred without disruption of graft-versus-leukaemia (GVL) effects. RNA sequencing revealed that a number of effector, cell adhesion, and migration molecules were upregulated in Itk-/- ncTregs. Furthermore, disrupting the SLP76: ITK interaction using a specific peptide inhibitor led to enhanced Treg development in both mouse and primary human cells. This peptide inhibitor also significantly reduced inflammatory cytokine production in primary GVHD patient samples and mouse T cells without causing cell death or apoptosis. We provide evidence that specifically targeting Itk signaling could be a therapeutic strategy to treat autoimmune disorders.

CD28 Superagonist D665-mediated activation of mouse regulatory T cells maintains their phenotype without loss of suppressive quality

Regulatory T cells (Tregs) maintain immune homeostasis by regulating the activation of other immune cells. Preclinical studies show that the infusion of Tregs can promote immunological tolerance to allografts and prevent or cure multiple autoimmune diseases. However, Treg therapy is limited by high numbers of cells required to induce tolerance. In this study, we aimed at improving the in vitro expansion of sort purified mouse Tregs using the CD28 Superagonist (CD28-SA) D665 and comparing it to the conventional expansion using anti-CD3/anti-CD28 Dynabeads®. CD28-SA-stimulated Tregs expanded more than Dynabead®-stimulated Tregs while maintaining their phenotype by expressing the same level of CD4, CD25 and Foxp3. CD28-SA-expanded Tregs produced comparable amounts of IL-10 and TGFβ while showing a slightly superior suppressive capacity compared to Dynabead®-stimulated Tregs. Thus, stimulating murine Tregs with the CD28-SA is a promising alternative since it maintains their suppressive capacity without altering their phenotype and yields a higher fold expansion within 14 days.

Regulatory T Cell Therapy of Graft-versus-Host Disease: Advances and Challenges

Graft-versus-host disease (GVHD) is the leading cause of morbidity and mortality after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Immunomodulation using regulatory T cells (Tregs) offers an exciting option to prevent and/or treat GVHD as these cells naturally function to maintain immune homeostasis, can induce tolerance following HSCT, and have a tissue reparative function. Studies to date have established a clinical safety profile for polyclonal Tregs. Functional enhancement through genetic engineering offers the possibility of improved potency, specificity, and persistence. In this review, we provide the most up to date preclinical and clinical data on Treg cell therapy with a particular focus on GVHD. We discuss the different Treg subtypes and highlight the pharmacological and genetic approaches under investigation to enhance the application of Tregs in allo-HSCT. Lastly, we discuss the remaining challenges for optimal clinical translation and provide insights as to future directions of the field.

The Pathophysiology and Treatment of Graft-Versus-Host Disease: Lessons Learnt From Animal Models

Allogeneic hematopoietic cell transplantation (HCT) is a curative treatment for hematologic malignancies, bone marrow failure syndromes, and inherited immunodeficiencies and metabolic diseases. Graft-versus-host disease (GVHD) is the major life-threatening complication after allogeneic HCT. New insights into the pathophysiology of GVHD garnered from our understanding of the immunological pathways within animal models have been pivotal in driving new therapeutic paradigms in the clinic. Successful clinical translations include histocompatibility matching, GVHD prophylaxis using cyclosporine and methotrexate, posttransplant cyclophosphamide, and the use of broad kinase inhibitors that inhibit cytokine signaling (e.g. ruxolitinib). New approaches focus on naïve T cell depletion, targeted cytokine modulation and the inhibition of co-stimulation. This review highlights the use of animal transplantation models to guide new therapeutic principles.

Exploiting Single-Cell Tools in Gene and Cell Therapy

Single-cell molecular tools have been developed at an incredible pace over the last five years as sequencing costs continue to drop and numerous molecular assays have been coupled to sequencing readouts. This rapid period of technological development has facilitated the delineation of individual molecular characteristics including the genome, transcriptome, epigenome, and proteome of individual cells, leading to an unprecedented resolution of the molecular networks governing complex biological systems. The immense power of single-cell molecular screens has been particularly highlighted through work in systems where cellular heterogeneity is a key feature, such as stem cell biology, immunology, and tumor cell biology. Single-cell-omics technologies have already contributed to the identification of novel disease biomarkers, cellular subsets, therapeutic targets and diagnostics, many of which would have been undetectable by bulk sequencing approaches. More recently, efforts to integrate single-cell multi-omics with single cell functional output and/or physical location have been challenging but have led to substantial advances. Perhaps most excitingly, there are emerging opportunities to reach beyond the description of static cellular states with recent advances in modulation of cells through CRISPR technology, in particular with the development of base editors which greatly raises the prospect of cell and gene therapies. In this review, we provide a brief overview of emerging single-cell technologies and discuss current developments in integrating single-cell molecular screens and performing single-cell multi-omics for clinical applications. We also discuss how single-cell molecular assays can be usefully combined with functional data to unpick the mechanism of cellular decision-making. Finally, we reflect upon the introduction of spatial transcriptomics and proteomics, its complementary role with single-cell RNA sequencing (scRNA-seq) and potential application in cellular and gene therapy.

Regulatory T Cells in GVHD Therapy

Graft versus host disease (GVHD) is a common complication and the leading cause of morbidity and mortality after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Pharmacological immunosuppression used in GVHD prophylaxis and treatment lacks specificity and can increase the likelihood of infection and relapse. Regulatory T lymphocytes (Tregs) play a vital role in restraining excessive immune responses and inducing peripheral immune tolerance. In particular, clinical trials have demonstrated that Tregs can prevent and treat GVHD, without increasing the risk of relapse and infection. Hence, adoptive transfer of Tregs to control GVHD using their immunosuppressive properties represents a promising therapeutic approach. To optimally apply Tregs for control of GVHD, a thorough understanding of their biology is necessary. In this review, we describe the biological characteristics of Tregs, including how the stability of FOXP3 expression can be maintained. We will also discuss the mechanisms underlying Tregs-mediated modulation of GVHD and approaches to effectively increase Tregs’ numbers. Finally, we will examine the developing trends in the use of Tregs for clinical therapy.

High proportion of terminally differentiated regulatory T cells after allogeneic hematopoietic stem cell transplantation

It is now well-established that regulatory T cells (Treg) represent a heterogeneous group of CD4⁺ T cells. Previous studies have demonstrated that Treg homeostasis was impacted by allogeneic hematopoietic cell transplantation (allo-HCT) and particularly so in patients with chronic graft-versus-host disease (GVHD). Here, we first assessed the ability of various Treg subsets to phosphorylate STAT5 in response to IL-2 or IL-7 stimulation in vitro. We then compared the frequencies of different Treg subtypes in healthy controls as well as in allo-HCT patients with or without chronic GVHD. The highest phosphorylated STAT5 (pSTAT5) signal in response to IL-2 was observed in the CD45RO⁺CD26^-CD39⁺HLA-DR⁺ Treg fraction. In contrast, naive Treg were mostly less susceptible to IL-2 stimulation in vitro. Following IL-7 stimulation, most Treg subpopulations upregulated pSTAT5 expression but to a lesser extent than conventional T cells. Compared to healthy controls, allo-HCT patients had lower frequencies of the naive CD45RA^brightCD26⁺ Treg subpopulation but higher frequencies of the most differentiated memory CD45RO⁺CD26^-CD39⁺ Treg subpopulations. Further, unbiased analysis revealed that six Treg clusters characterized by high expression of CD25, HLA-DR, and ICOS were significantly more frequent in patients with no or with limited chronic GVHD than in those with moderate/severe chronic GVHD.

Mobilized Multipotent Hematopoietic Progenitors Promote Expansion and Survival of Allogeneic Tregs and Protect Against Graft Versus Host Disease

Allogeneic Hematopoietic Stem Cell Transplantation (Allo-HSCT) is routinely performed with peripheral blood stem cells (PBSCs) mobilized by injection of G-CSF, a growth factor which not only modulates normal hematopoiesis but also induces diverse immature regulatory cells. Based on our previous evidence that G-CSF-mobilized multipotent hematopoietic progenitors (MPP) can increase survival and proliferation of natural regulatory T cells (Tregs) in autoimmune disorders, we addressed the question how these cells come into play in mice and humans in an alloimmune setting. Using a C57BL/6 mouse model, we demonstrate that mobilized MPP enhance the immunosuppressant effect exerted by Tregs, against alloreactive T lymphocytes, both in vitro and in vivo. They do so by migrating to sites of allopriming, interacting with donor Tregs and increasing their numbers, thus reducing the lethality of graft-versus-host disease (GVHD). Protection correlates likewise with increased allospecific Treg counts. Furthermore, we provide evidence for a phenotypically similar MPP population in humans, where it shares the capacity to promote selective Treg expansion in vitro. We postulate that G-CSF-mobilized MPPs might become a valuable cellular therapy to expand donor Tregs in vivo and prevent GVHD, thereby making allo-HSCT safer for the treatment of leukemia patients.

A single strain of Bacteroides fragilis protects gut integrity and reduces GVHD

Graft-versus-host disease (GVHD) is a pathological process caused by an exaggerated donor lymphocyte response to host antigens after allogeneic hematopoietic cell transplantation (allo-HCT). Donor T cells undergo extensive clonal expansion and differentiation, which culminate in damage to recipient target organs. Damage to the gastrointestinal tract is a main contributor to morbidity and mortality. The loss of diversity among intestinal bacteria caused by pretransplant conditioning regimens leads to an outgrowth of opportunistic pathogens and exacerbated GVHD after allo-HCT. Using murine models of allo-HCT, we found that an increase of Bacteroides in the intestinal microbiota of the recipients was associated with reduced GVHD in mice given fecal microbial transplantation. Administration of Bacteroides fragilis through oral gavage increased gut microbiota diversity and beneficial commensal bacteria and significantly ameliorated acute and chronic GVHD development. Preservation of gut integrity following B. fragilis exposure was likely attributed to increased short chain fatty acids, IL-22, and regulatory T cells, which in turn improved gut tight junction integrity and reduced inflammatory cytokine production of pathogenic T cells. The current study provides a proof of concept that a single strain of commensal bacteria can be a safe and effective means to protect gut integrity and ameliorate GVHD after allo-HCT.

Maturation and Phenotypic Heterogeneity of Human CD4+ Regulatory T Cells From Birth to Adulthood and After Allogeneic Stem Cell Transplantation

CD4+ Regulatory T cells (Treg) play a critical role in maintaining immune homeostasis. Various Treg subsets have been identified, however the heterogeneity of Treg subpopulations during development remains uncharacterized. Using mass cytometry we obtained single cell data on expression of 35 functional markers to examine the heterogeneity of Treg cells at birth and in adults. Unsupervised clustering algorithms FlowSOM and ACCENSE were used to quantify Treg heterogeneity. As expected, Treg in umbilical cord blood were predominately naïve while Treg in adult blood were predominately central memory and effector memory cells. Although umbilical cord blood Treg are mostly naïve cells, we observed multiple phenotypic Treg subsets in cord blood. Nevertheless, peripheral blood in adults contained higher percentages of Treg and the heterogeneity of Treg was significantly increased in adults. We also studied Treg heterogeneity throughout a 2-year period after allogeneic hematopoietic stem cell transplantation (alloHSCT) and in patients with chronic graft-versus-host disease (cGVHD). Treg heterogeneity recovered rapidly after alloHSCT and gradually increased in the first two years post-transplant. However, patients with cGVHD had significantly fewer distinct Treg subpopulations, proposing a correlation between a disrupted Treg heterogeneity and cGVHD. Our study is the first to compare human Treg heterogeneity at birth, in healthy adults and in patients after alloHSCT with and without cGVHD. This approach to characterize Treg heterogeneity based on expression of a large panel of functional markers may enable future studies to identify specific Treg defects that contribute to immune dysfunction.

Regulatory T Cells in the Tumor Microenvironment

Regulatory T cells (T_regs) are an immunosuppressive subpopulation of CD4⁺ T cells that are endowed with potent suppressive activity and function to limit immune activation and maintain homeostasis. These cells are identified by the hallmark transcription factor FOXP3 and the high-affinity interleukin-2 (IL-2) receptor chain CD25. T_regs can be recruited to and persist within the tumor microenvironment (TME), acting as a potent barrier to effective antitumor immunity. This chapter will discuss [i] the history and hallmarks of T_regs; [ii] the recruitment, development, and persistence of T_regs within the TME; [iii] T_reg function within TME; asnd [iv] the therapeutic targeting of T_regs in the clinic. This chapter will conclude with a discussion of likely trends and future directions.

Current Preventions and Treatments of aGVHD: From Pharmacological Prophylaxis to Innovative Therapies

Graft versus host disease (GVHD) is one of the main causes of mortality and the reason for up to 50% of morbidity after hematopoietic stem cell transplantations (HSCT) which is the treatment of choice for many blood malignancies. Thanks to years of research and exploration, we have acquired a profound understanding of the pathophysiology and immunopathology of these disorders. This led to the proposition and development of many therapeutic approaches during the last decades, some of them with very promising results. In this review, we have focused on the recent GVHD treatments from classical chemical and pharmacological prophylaxis to more innovative treatments including gene therapy and cell therapy, most commonly based on the application of a variety of immunomodulatory cells. Furthermore, we have discussed the advantages and potentials of cell-free therapy as a newly emerging approach to treat GVHD. Among them, we have particularly focused on the implication of the TNFα-TNFR2 axis as a new immune checkpoint signaling pathway controlling different aspects of many immunoregulatory cells.

CD4+CD8+ T-Lymphocytes in Xenogeneic and Human Graft-versus-Host Disease

Mechanisms driving acute graft-versus-host disease (aGVHD) onset in patients undergoing allogeneic hematopoietic stem cell transplantation (allo-HSCT) are still poorly understood. To provide a detailed characterization of tissue-infiltrating T lymphocytes (TL) and search for eventual site-specific specificities, we developed a xenogeneic model of aGVHD in immunodeficient mice. Phenotypic characterization of xenoreactive T lymphocytes (TL) in diseased mice disclosed a massive infiltration of GVHD target organs by an original CD4⁺CD8⁺ TL subset. Immunophenotypic and transcriptional profiling shows that CD4⁺CD8⁺ TL comprise a major PD1⁺CD62L^−/+ transitional memory subset (>60%) characterized by low level expression of cytotoxicity-related transcripts. CD4⁺CD8⁺ TL produce high IL-10 and IL-13 levels, and low IL-2 and IFN-γ, suggestive of regulatory function. In vivo tracking of genetically labeled CD4⁺ or CD8⁺ TL subsequently found that CD4⁺CD8⁺ TL mainly originate from chronically activated cytotoxic TL (CTL). On the other hand, phenotypic profiling of CD3⁺ TL from blood, duodenum or rectal mucosa in a cohort of allo-HSCT patients failed to disclose abnormal expansion of CD4⁺CD8⁺ TL independent of aGVHD development. Collectively, our results show that acquisition of surface CD4 by xenoreactive CD8⁺ CTL is associated with functional diversion toward a regulatory phenotype, but rule out a central role of this subset in the pathogenesis of aGVHD in allo-HSCT patients.

Downregulation of p53 by Insufficient CTCF in CD4+ T Cells Is an Important Factor Inducing Acute Graft-Versus-Host Disease

Recent evidence indicates that p53 plays a protective role against various systemic autoimmune diseases by suppressing pro-inflammatory cytokine production and reducing the number of pathogenic T cells. However, whether abnormal p53 expression participates in the development of acute graft-versus-host disease (aGVHD) remains unclear. In this study, we demonstrated that p53 was downregulated in CD4⁺ T cells from patients with aGVHD compared with the non-aGVHD group. Furthermore, we confirmed that low expression of CCCTC-binding factor (CTCF) in CD4⁺ T cells from aGVHD cases is an important factor affecting histone H3K9/K14 hypoacetylation in the p53 promoter and p53 downregulation. Restoring CTCF expression in CD4⁺ T cells from aGVHD patients increased p53 amounts and corrected the imbalance of Th17 cells/Tregs. Taken together, these results provide novel insights into p53 downregulation in CD4⁺ T cells from aGVHD patients.

New insights into the basic biology of acute graft-versus-host-disease

Although allogeneic hematopoietic stem cell transplantation is an important therapy for many hematological and non-hematological diseases, acute graft-versus-host-disease (aGVHD) is a major obstacle to its success. The pathogenesis of aGVHD is divided into three distinct phases which occur largely as the result of interactions between infused donor T cells and numerous cell types of both hematopoietic and non-hematopoietic origin. In light of the disease's immensely complex biology, epigenetics has emerged as a framework with which to examine aGVHD. This review focuses on new findings that clarify the roles specific epigenetic regulators play in T cell-mediated aGVHD development and discusses how their modulation could disrupt that process to beneficial effects. DNA methyltransferases, histone methyltransferases and histone deacetylases are the most closely studied regulators across aGVHD priming, induction and effector phases and have been manipulated using drugs and other methods in both murine models and clinical trials to varying degrees of success. Antigen-presenting cells, effector T cells and memory T cells, among others, are targeted and affected by these regulators in different ways. Finally, our review highlights new directions for study and potential novel targets for modulation to abrogate aGVHD.

How to Make an Immune System and a Foreign Host Quickly Cohabit in Peace? The Challenge of Acute Graft-Versus-Host Disease Prevention After Allogeneic Hematopoietic Cell Transplantation

Allogeneic hematopoietic cell transplantation (alloHCT) has been used as cellular immunotherapy against hematological cancers for more than six decades. Its therapeutic efficacy relies on the cytoreductive effects of the conditioning regimen but also on potent graft-versus-tumor (GVT) reactions mediated by donor-derived immune cells. However, beneficial GVT effects may be counterbalanced by acute GVHD (aGVHD), a systemic syndrome in which donor immune cells attack healthy tissues of the recipient, resulting in severe inflammatory lesions mainly of the skin, gut, and liver. Despite standard prophylaxis regimens, aGVHD still occurs in approximately 20–50% of alloHCT recipients and remains a leading cause of transplant-related mortality. Over the past two decades, advances in the understanding its pathophysiology have helped to redefine aGVHD reactions and clinical presentations as well as developing novel strategies to optimize its prevention. In this review, we provide a brief overview of current knowledge on aGVHD immunopathology and discuss current approaches and novel strategies being developed and evaluated in clinical trials for aGVHD prevention. Optimal prophylaxis of aGVHD would prevent the development of clinically significant aGVHD, while preserving sufficient immune responsiveness to maintain beneficial GVT effects and immune defenses against pathogens.

Clinical-Grade Expanded Regulatory T Cells Are Enriched with Highly Suppressive Cells Producing IL-10, Granzyme B, and IL-35

In the setting of T cell-depleted, full-haplotype mismatched transplantation, adoptive immunotherapy with regulatory T cells (Tregs) and conventional T cells (Tcons) can prevent graft-versus-host disease (GVHD) and improve post-transplantation immunologic reconstitution and is associated with a powerful graft-versus-leukemia effect. To improve the purity and the quantity of the infused Tregs, good manufacturing practices (GMP)-compatible expansion protocols are needed. Here we expanded Tregs using an automated, clinical-grade protocol. Cells were extensively characterized in vitro, and their efficiency was tested in vivo in a mouse model. Tregs were selected by CliniMacs (CD4⁺CD25⁺, 94.5 ± 6.3%; FoxP3⁺, 63.7 ± 11.5%; CD127⁺, 20 ± 3%; suppressive activity, 60 ± 7%), and an aliquot of 100 × 10⁶ was expanded for 14 days using the CliniMACS Prodigy System, obtaining 684 ± 279 × 10⁶ cells (CD4⁺CD25⁺, 99.6 ± 0.2%; FoxP3⁺, 82 ± 8%; CD127⁺, 1.1 ± 0.8%; suppressive activity, 75 ± 12%). CD39 and CTLA4 expression levels increased from 22.4 ± 12% to 58.1 ± 13.3% (P < .05) and from 20.4 ± 6.7% to 85.4 ± 9.8% (P < .01), respectively. TIM3 levels increased from .4 ± .05% to 29 ± 16% (P < .05). Memory Tregs were the prevalent population, whereas naive Tregs almost disappeared at the end of the culture. mRNA analysis displayed significant increases in CD39, IL-10, granzyme B, and IL-35 levels at the end of culture period (P < .05). Conversely, IFNγ expression decreased significantly by day +14. Expanded Tregs were sorted according to TIM3, CD39, and CD62L expression levels (purity >95%). When sorted populations were analyzed, TIM3⁺ cells showed significant increases in IL-10 and granzyme B (P < .01) .When expanded Tregs were infused in an NSG murine model, mice that received Tcons only died of GVHD, whereas mice that received both Tcons and Tregs survived without GVHD. GMP grade expanded cells that display phenotypic and functional Treg characteristics can be obtained using a fully automated system. Treg suppression is mediated by multiple overlapping mechanisms (eg, CTLA-4, CD39, IL-10, IL-35, TGF-β, granzyme B). TIM3⁺ cells emerge as a potentially highly suppressive population. © 2020 American Society for Transplantation and Cellular Therapy. Published by Elsevier Inc.

Immune regulation in hematopoietic cell transplantation

Immune regulation is critically important in health and disease. These immune effects have direct relevance in the setting of allogeneic hematopoietic cell transplantation (HCT), principally for the control of aberrant immune reactions, such as graft-versus-host disease (GVHD). Murine models have been critically important to evaluate the potential of two of the most potent immune regulatory cells CD4⁺CD25⁺FoxP⁺ regulatory T cells (T_reg) and invariant natural killer T cells (iNKT cells). These cells have been shown to be remarkably effective in murine models to control GVHD and allow for the maintenance of graft-versus-tumor (GVT) effects. Interestingly, there are critical interactions between these different cell populations. Future studies are aimed at exploring the biology of these important regulatory cells and to translate these concepts to the clinic that holds promise for controlling some of the major challenges of allogeneic HCT.

Regulatory T cell therapy: Current and future design perspectives

Regulatory T cells (Tregs) maintain immune equilibrium by suppressing immune responses through various multistep contact dependent and independent mechanisms. Cellular therapy using polyclonal Tregs in transplantation and autoimmune diseases has shown promise in preclinical models and clinical trials. Although novel approaches have been developed to improve specificity and efficacy of antigen specific Treg based therapies, widespread application is currently restricted. To date, design-based approaches to improve the potency and persistence of engineered chimeric antigen receptor (CAR) Tregs are limited. Here, we describe currently available Treg based therapies, their advantages and limitations for implementation in clinical studies. We also examine various strategies for improving CAR T cell design that can potentially be applied to CAR Tregs, such as identifying co-stimulatory signalling domains that enhance suppressive ability, determining optimal scFv affinity/avidity, and co-expression of accessory molecules. Finally, we discuss the importance of tailoring CAR Treg design to suit the individual disease.

Induction of CD4+CD25+ Regulatory T Cells from In Vitro Grown Human Mononuclear Cells by Sparteine Sulfate and Harpagoside

Regulatory T cells (Tregs) are key players in the regulation of inflammatory responses. In this study, two natural molecules, namely, sparteine sulfate (SS) and harpagoside (Harp), were investigated for their ability to induce Tregs in human peripheral blood mononuclear cells (PBMCs). PBMCs were isolated from healthy volunteers and grown in the presence or absence of ConA, with TGF-beta, SS or Harp. Expression of the mRNA of FoxP3, TGF-beta, IL-10 and GAPDH was assessed via q-PCR. The expression of Treg markers including CD4, CD25, CD127 and FoxP3 was measured via flow cytometry. The secretion of IL-10 and TGF-beta by cultured cells was assessed by ELISA. Furthermore, the suppressive role of SS and Harp on PBMCs in vitro was tested via allogeneic mixed lymphocyte reaction (MLR). Data obtained show that both compounds increased the expression of FoxP3, TGF-beta and IL-10 mRNA in resting PBMCs but to a lesser extent in activated cells. Moreover, they significantly increased the percent of CD4+CD25+FoxP3+CD127- Tregs in activated and naïve PBMCs. Functionally, both compounds caused a significant reduction in the stimulation index in allogeneic MLR. Together, our data demonstrate for the first time that SS and Harp can induce human Tregs in vitro and therefore have great potential as anti-inflammatory agents.