T regulatory cell separation for clinical application

T cells in obesity-associated inflammation: The devil is in the details

Obesity presents a significant health challenge, affecting 41% of adults and 19.7% of children in the United States. One of the associated health challenges of obesity is chronic low-grade inflammation. In both mice and humans, T cells in circulation and in the adipose tissue play a pivotal role in obesity-associated inflammation. Changes in the numbers and frequency of specific CD4+ Th subsets and their contribution to inflammation through cytokine production indicate declining metabolic health, that is, insulin resistance and T2D. While some Th subset alterations are consistent between mice and humans with obesity, some changes mainly characterize male mice, whereas female mice often resist obesity and inflammation. However, protection from obesity and inflammation is not observed in human females, who can develop obesity-related T-cell inflammation akin to males. The decline in female sex hormones after menopause is also implicated in promoting obesity and inflammation. Age is a second underappreciated factor for defining and regulating obesity-associated inflammation toward translating basic science findings to the clinic. Weight loss in mice and humans, in parallel with these other factors, does not resolve obesity-associated inflammation. Instead, inflammation persists amid modest changes in CD4+ T cell frequencies, highlighting the need for further research into resolving changes in T-cell function after weight loss. How lingering inflammation after weight loss affecting the common struggle to maintain lower weight is unknown. Semaglutide, a newly popular pharmaceutical used for treating T2D and reversing obesity, holds promise for alleviating obesity-associated health complications, yet its impact on T-cell-mediated inflammation remains unexplored. Further work in this area could significantly contribute to the scientific understanding of the impacts of weight loss and sex/hormones in obesity and obesity-associated metabolic decline.

Cellular Strategies for Separating GvHD from GvL in Haploidentical Transplantation

GvHD still remains, despite the continuous improvement of transplantation platforms, a fearful complication of transplantation from allogeneic donors. Being able to separate GvHD from GvL represents the greatest challenge in the allogeneic transplant setting. This may be possible through continuous improvement of cell therapy techniques. In this review, current cell therapies are taken into consideration, which are based on the use of TCR alpha/beta depletion, CD45RA depletion, T regulatory cell enrichment, NK-cell-based immunotherapies, and suicide gene therapies in order to prevent GvHD and maximally amplify the GvL effect in the setting of haploidentical transplantation.

Red blood cell rosetting enables size-based separation of specific lymphocyte subsets from blood in a microfluidic device

The isolation of a specific lymphocyte subset from blood is the required first step in the manufacturing of many novel cellular immunotherapies. Microfluidic size-based separation methods are poised to significantly simplify this process because they require neither centrifugation nor magnetic or fluorescent labeling to operate. Lymphocytes can be separated from red blood cells (RBCs) and platelets as well as monocytes and granulocytes because their size differs from each of these cell types. However, further separation of a specific lymphocyte subset from other unwanted lymphocytes using size-based methods is impossible because all lymphocytes have approximately the same size and can only be distinguished by surface markers. This paper describes a new approach that made it possible for a size-based separation method to isolate a desired subset of lymphocytes by making unwanted lymphocytes as well as other blood cells artificially larger. The separation was enabled by selectively binding multiple RBCs to each unwanted cell to create 'rosettes' with an effective size significantly larger than the diameter of a typical lymphocyte. The desired lymphocytes remained unaffected by rosetting and were separated from the rosettes by passing the mixture through a microfluidic size-based separation device based on controlled incremental filtration (CIF). This new rosette-enabled size-based (RESIZE) separation approach demonstrated recovery of 80-90% for all lymphocyte subsets tested (CD3⁺, CD4⁺, CD56⁺) which was ∼2.5-fold higher than that for the standard immunodensity method (RBC rosetting followed by density gradient centrifugation). The purity of separation was >90% for CD3⁺ cells but declined with increasing cell rarity. Unlike the immunodensity approach, RESIZE required neither centrifugation nor cell washing after the separation and was ∼2.5-fold faster when processing the same sample volume. The results of this study suggest that integration of the RESIZE approach for high-yield isolation of lymphocyte subsets from blood could significantly streamline the manufacturing workflow and thus have a potentially transformative impact on the cost and availability of novel cellular immunotherapies.

The potential for Treg-enhancing therapies in transplantation

Since the discovery of regulatory T cells (Tregs) as crucial regulators of immune tolerance against self-antigens, these cells have become a promising tool for the induction of donor-specific tolerance in transplantation medicine. The therapeutic potential of increasing in vivoTreg numbers for a favorable Treg to Teff cell ratio has already been demonstrated in several sophisticated pre-clinical models and clinical pilot trials. In addition to improving cell quantity, enhancing Treg function utilizing engineering techniques led to encouraging results in models of autoimmunity and transplantation. Here we aim to discuss the most promising approaches for Treg-enhancing therapies, starting with adoptive transfer approaches and ex vivoexpansion cultures (polyclonal vs. antigen specific), followed by selective in vivostimulation methods. Furthermore, we address next generation concepts for Treg function enhancement (CARs, TRUCKs, BARs) as well as the advantages and caveats inherit to each approach. Finally, this review will discuss the clinical experience with Treg therapy in ongoing and already published clinical trials; however, data on long-term results and efficacy are still very limited and many questions that might complicate clinical translation remain open. Here, we discuss the hurdles for clinical translation and elaborate on current Treg-based therapeutic options as well as their potencies for improving long-term graft survival in transplantation.

Regulatory T cells as a therapeutic approach for inflammatory bowel disease

Foxp3+ T regulatory (Treg) cells suppress inflammation and are essential for maintaining tissue homeostasis. A growing appreciation of tissue-specific Treg functions has built interest in leveraging the endogenous suppressive mechanisms of these cells into cellular therapeutics in organ-specific diseases. Notably, Treg cells play a critical role in maintaining the intestinal environment. As a barrier site, the gut requires Treg cells to mediate interactions with the microbiota, support barrier integrity, and regulate the immune system. Without fully functional Treg cells, intestinal inflammation and microbial dysbiosis ensue. Thus, there is a particular interest in developing Treg cellular therapies for intestinal inflammatory disease, such as inflammatory bowel disease (IBD). This article reviews some of the critical pathways that are dysregulated in IBD, Treg cell mechanisms of suppression, and the efforts and approaches in the field to develop these cells as a cellular therapy for IBD.

Regulatory T cell niche in the bone marrow, a new player in Haematopoietic stem cell transplantation

Challenges in haematopoietic stem cell transplantation such as low bone marrow (BM) engraftment, graft versus host disease (GvHD) and the need for long-term immunosuppression could be addressed using T regulatory cells (Tregs) resident in the tissue of interest, in this case, BM Tregs. Controlling the adverse immune response in haematopoietic stem cell transplantation (HSCT) and minimising the associated risks such as infection and secondary cancers due to long-term immunosuppression is a crucial aspect of clinical practice in this field. While systemic immunosuppressive therapy could achieve reasonable GvHD control in most patients, related side effects remain the main limiting factor. Developing more targeted immunosuppressive strategies is an unmet clinical need and is the focus of several ongoing research projects. Tregs are a non-redundant sub-population of CD4⁺ T cells essential for controlling the immune homeostasis. Tregs are known to be reduced in number and function in autoimmune conditions. There is considerable interest in these cells as cell therapy products since they can be expanded in vitro and infused into patients. These trials have found Treg therapy to be safe, well-tolerated, and with some early signs of efficacy. However, Tregs are a heterogeneous subpopulation of T cells, and several novel subpopulations have been identified in recent years beyond the conventional thymic (tTregs) and peripheral (pTregs). There is increasing evidence for the presence of resident and tissue-specific Tregs. Bone marrow (BM) Tregs are one example of tissue-resident Tregs. BM Tregs are enriched within the marrow, serving a dual function of immunosuppression and maintenance of haematopoietic stem cells (HSCs). HSCs maintenance is achieved through direct suppression of HSCs differentiation, maintaining a proliferating pool of HSCs, and promoting the development of functional stromal cells that support HSCs. In this review, we will touch upon the biology of Tregs, focusing on their development and heterogeneity. We will focus on the BM Tregs from their biology to their therapeutic potential, focusing on their use in HSCT.

Challenges and opportunities in achieving effective regulatory T cell therapy in autoimmune liver disease

Autoimmune liver diseases (AILD) include autoimmune hepatitis (AIH), primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC). These immune-mediated liver diseases involve a break down in peripheral self-tolerance with largely unknown aetiology. Regulatory T cells (Treg) are crucial in maintaining immunological tolerance. Hence, Treg immunotherapy is an attractive therapeutic option in AILD. Currently, AILD do not have a curative treatment option and patients take life-long immunosuppression or bile acids to control hepatic or biliary inflammation. Clinical investigations using good manufacturing practice (GMP) Treg in autoimmune liver disease have thus far demonstrated that Treg therapy is safe and that Treg migrate to inflamed liver tissue. For Treg immunotherapy to achieve efficacy in AILD, Treg must be retained within the liver and maintain their suppressive phenotype to dampen ongoing immune responses to hepatocytes and biliary epithelium. Therefore, therapeutic Treg subsets should be selected for tissue residency markers and maximal functionality. Optimisation of dosing regime and understanding longevity of Treg in vivo are critical to successful Treg therapy. It is also essential to consider combination therapy options to complement infused Treg, for instance low-dose interleukin-2 (IL-2) to support pre-existing and infused Treg survival and suppressive function. Understanding the hepatic microenvironment in both early- and late-stage AILD presents significant opportunity to better tailor Treg therapy in different patient groups. Modification of a hostile microenvironment to a more favourable one either prior to or during Treg therapy could enhance the efficacy and longevity of infused GMP-Treg. Applying recent technology to discovery of autoantigen responses in AILD, T cell receptor (TCR) sequencing and use of chimeric antigen receptor (CAR) technology represents the next frontier for disease-specific CAR-Treg therapies. Consideration of all these aspects in future trials and discovery research would position GMP Treg immunotherapy as a viable personalised-medicine treatment option for effective control of autoimmune liver diseases.

Emerging therapeutic potential of regulatory T (Treg) cells for rheumatoid arthritis: New insights and challenges

Rheumatoid arthritis (RA) is an autoimmune-related disorder characterized by chronic inflammation. Although the etiopathogenesis of RA still remains to be clarified, it is supposed that the breakdown of immune self-tolerance may contribute to the development of RA. Thus, restoring of immune tolerance at the site of inflammation is the ultimate goal of RA treatment. Regulatory T cells (Treg cells) are the main suppressive cells that maintain tolerance and inhibit immunity against auto-antigen. Of note, recent studies demonstrated the efficacy of adoptive transfer of Treg cells in the modulation of the unwanted immune response, which makes them an ideal candidate to maintain immune homeostasis and restore antigen-specific tolerance in the case of RA and other autoimmune diseases. This review intends to submit recent finding of Treg cells-based therapies in RA with a focus on strategies applied to improve the therapeutic value of Treg cells to restore immune tolerance.

A Novel GMP Protocol to Produce High-Quality Treg Cells From the Pediatric Thymic Tissue to Be Employed as Cellular Therapy

Due to their suppressive capacity, the adoptive transfer of regulatory T cells (Treg) has acquired a growing interest in controlling exacerbated inflammatory responses. Limited Treg recovery and reduced quality remain the main obstacles in most current protocols where differentiated Treg are obtained from adult peripheral blood. An alternate Treg source is umbilical cord blood, a promising source of Treg cells due to the higher frequency of naïve Treg and lower frequency of memory T cells present in the fetus’ blood. However, the Treg number isolated from cord blood remains limiting. Human thymuses routinely discarded during pediatric cardiac surgeries to access the retrosternal operative field has been recently proposed as a novel source of Treg for cellular therapy. This strategy overcomes the main limitations of current Treg sources, allowing the obtention of very high numbers of undifferentiated Treg. We have developed a novel good manufacturing practice (GMP) protocol to obtain large Treg amounts, with very high purity and suppressive capacity, from the pediatric thymus (named hereafter thyTreg). The total amount of thyTreg obtained at the end of the procedure, after a short-term culture of 7 days, reach an average of 1,757 x10⁶ (range 50 x 10⁶ – 13,649 x 10⁶) cells from a single thymus. The thyTreg product obtained with our protocol shows very high viability (mean 93.25%; range 83.35% – 97.97%), very high purity (mean 92.89%; range 70.10% – 98.41% of CD25⁺FOXP3⁺ cells), stability under proinflammatory conditions and a very high suppressive capacity (inhibiting in more than 75% the proliferation of activated CD4⁺ and CD8⁺ T cells in vitro at a thyTreg:responder cells ratio of 1:1). Our thyTreg product has been approved by the Spanish Drug Agency (AEMPS) to be administered as cell therapy. We are recruiting patients in the first-in-human phase I/II clinical trial worldwide that evaluates the safety, feasibility, and efficacy of autologous thyTreg administration in children undergoing heart transplantation (NCT04924491). The high quality and amount of thyTreg and the differential features of the final product obtained with our protocol allow preparing hundreds of doses from a single thymus with improved therapeutic properties, which can be cryopreserved and could open the possibility of an “off-the-shelf” allogeneic use in another individual.

Process development and validation of expanded regulatory T cells for prospective applications: an example of manufacturing a personalized advanced therapy medicinal product

BACKGROUND

A growing number of clinical trials have shown that regulatory T (Treg) cell transfer may have a favorable effect on the maintenance of self-tolerance and immune homeostasis in different conditions such as graft-versus-host disease (GvHD), solid organ transplantation, type 1 diabetes, and others. In this context, the availability of a robust manufacturing protocol that is able to produce a sufficient number of functional Treg cells represents a fundamental prerequisite for the success of a cell therapy clinical protocol. However, extended workflow guidelines for nonprofit manufacturers are currently lacking. Despite the fact that different successful manufacturing procedures and cell products with excellent safety profiles have been reported from early clinical trials, the selection and expansion protocols for Treg cells vary a lot. The objective of this study was to validate a Good Manufacturing Practice (GMP)-compliant protocol for the production of Treg cells that approaches the whole process with a risk-management methodology, from process design to completion of final product development. High emphasis was given to the description of the quality control (QC) methodologies used for the in-process and release tests (sterility, endotoxin test, mycoplasma, and immunophenotype).

RESULTS

The GMP-compliant protocol defined in this work allows at least 4.11 × 109 Treg cells to be obtained with an average purity of 95.75 ± 4.38% and can be used in different clinical settings to exploit Treg cell immunomodulatory function.

CONCLUSIONS

These results could be of great use for facilities implementing GMP-compliant cell therapy protocols of these cells for different conditions aimed at restoring the Treg cell number and function, which may slow the progression of certain diseases.

NK Cells in Chronic Lymphocytic Leukemia and Their Therapeutic Implications

Key features of chronic lymphocytic leukemia (CLL) are defects in the immune system and the ability of leukemic cells to evade immune defenses and induce immunosuppression, resulting in increased susceptibility to infections and disease progression. Several immune effectors are impaired in CLL, including T and natural killer (NK) cells. The role of T cells in defense against CLL and in CLL progression and immunotherapy has been extensively studied. Less is known about the role of NK cells in this leukemia, and data on NK cell alterations in CLL are contrasting. Besides studies showing that NK cells have intrinsic defects in CLL, there is a large body of evidence indicating that NK cell dysfunctions in CLL mainly depend on the escape mechanisms employed by leukemic cells. In keeping, it has been shown that NK cell functions, including antibody-dependent cellular cytotoxicity (ADCC), can be retained and/or restored after adequate stimulation. Therefore, due to their preserved ADCC function and the reversibility of CLL-related dysfunctions, NK cells are an attractive source for novel immunotherapeutic strategies in this disease, including chimeric antigen receptor (CAR) therapy. Recently, satisfying clinical responses have been obtained in CLL patients using cord blood-derived CAR-NK cells, opening new possibilities for further exploring NK cells in the immunotherapy of CLL. However, notwithstanding the promising results of this clinical trial, more evidence is needed to fully understand whether and in which CLL cases NK cell-based immunotherapy may represent a valid, alternative/additional therapeutic option for this leukemia. In this review, we provide an overview of the current knowledge about phenotypic and functional alterations of NK cells in CLL and the mechanisms by which CLL cells circumvent NK cell-mediated immunosurveillance. Additionally, we discuss the potential relevance of using NK cells in CLL immunotherapy.

Clinical and Basic Research Progress on Treg-Induced Immune Tolerance in Liver Transplantation

Rejection after organ transplantation is a cause of graft failure. Effectively reducing rejection and inducing tolerance is a challenge in the field of transplantation immunology. The liver, as an immunologically privileged organ, has high rates of spontaneous and operational tolerance after transplantation, allowing it to maintain its normal function for long periods. Although modern immunosuppression regimens have serious toxicity and side effects, it is very risky to discontinue immunosuppression regimens blindly. A more effective treatment to induce immune tolerance is the most sought-after goal in transplant medicine. Tregs have been shown to play a pivotal role in the regulation of immune balance, and infusion of Tregs can also effectively prevent rejection and cure autoimmune diseases without significant side effects. Given the immune characteristics of the liver, the correct use of Tregs can more effectively induce the occurrence of operational tolerance for liver transplants than for other organ transplants. This review mainly summarizes the latest research advances regarding the characteristics of the hepatic immune microenvironment, operational tolerance, Treg generation in vitro, and the application of Tregs in liver transplantation. It is hoped that this review will provide a deeper understanding of Tregs as the most effective treatment to induce and maintain operational tolerance after liver transplantation.

Cell therapy in transplantation: A comprehensive review of the current applications of cell therapy in transplant patients with the focus on Tregs, CAR Tregs, and Mesenchymal stem cells

Organ transplantation is a practical treatment for patients with end-stage organ failure. Despite the advances in short-term graft survival, long-term graft survival remains the main challenge considering the increased mortality and morbidity associated with chronic rejection and the toxicity of immunosuppressive drugs. Since a novel therapeutic strategy to induce allograft tolerance seems urgent, focusing on developing novel and safe approaches to prolong graft survival is one of the main goals of transplant investigators. Researchers in the field of organ transplantation are interested in suppressing or optimizing the immune responses by focusing on immune cells including mesenchymal stem cells (MSCs), polyclonal regulatory Tcells (Tregs), and antigen-specific Tregs engineered with chimeric antigen receptors (CAR Tregs). We review the mechanistic pathways, phenotypic and functional characteristics of these cells, and their promising application in organ transplantation.

Lessons, Challenges and Future Therapeutic Opportunities for PI3K Inhibition in CLL

Simple Summary

The phosphoinositide 3-kinase (PI3K) is a family of kinases that play a key role in the biology of chronic lymphocytic leukemia (CLL). Inhibitors of PI3K demonstrated efficacy in the treatment of CLL, associated with significant adverse events that limited the clinical use of this drugs. In this review, we underlined the relevance of PI3K inhibitors in CLL, we collected recent data about the use of these molecules in clinical practice and in clinical trial discussing strategies for the management of adverse events, which could help to improve the use of these therapies in the treatment of CLL.

Abstract

Chronic lymphocytic leukemia (CLL) shows constitutive phosphatidylinositol 3-kinase (PI3K) activation resulting from aberrant regulation of the B-cell receptor (BCR) signaling. PI3K inhibitors have been evaluated in CLL therapy, bringing a new treatment opportunity for patients with this disease. Despite the proven therapeutic efficacy, the use of approved PI3K inhibitors is limited by severe immune-mediated toxicities and given the availability of other more tolerable agents. This article reviews the relevance of PI3K signaling and pharmacologic inhibition in CLL. Data on efficacy and toxicity of PI3K inhibitors are also presented, as well as strategies for overcoming barriers for their clinical use in CLL treatment.

Engineered antigen-specific regulatory T cells for autoimmune skin conditions

Regulatory T cells (Tregs) are a subset of T cells responsible for the regulation of immune responses, thereby maintaining immune homeostasis and providing immune tolerance to both self and non-self-antigens. An increasing number of studies revealed Treg numbers and functions in a variety of autoimmune diseases. Treg deficiency can cause the development of several autoimmune skin diseases including vitiligo, alopecia areata, pemphigoid and pemphigus, psoriasis, and systemic sclerosis. Many clinical trials have been performed for autoimmune conditions using polyclonal Tregs, but efficiency can be significantly improved using antigen-specific Tregs engineered using T cell receptor (TCR) or chimeric antigen receptor (CAR) constructs. In this review, we systematically reviewed altered frequencies, impaired functions, and phenotypic features of Tregs in autoimmune skin conditions. We also summarized new advances in TCR and CAR based antigen-specific Tregs tested both in animal models and in clinics. The advantages and limitations of each approach were carefully discussed emphasizing possible clinical relevance to patients with autoimmune skin diseases. Moreover, we have reviewed potential approaches for engineering antigen-specific Tregs, and strategies for overcoming possible hurdles in clinical applications. Thereby, antigen-specific Tregs can be infused using autologous adoptive cell transfer to restore Treg numbers and to provide local immune tolerance for autoimmune skin disorders.

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.

Adoptive Treg cell-based immunotherapy: Frontier therapeutic aspects in rheumatoid arthritis

The major current focus on treating rheumatoid arthritis is to put an end to long-term treatments and instead, specifically block widespread immunosuppression by developing antigen-specific tolerance, while also permitting an intact immune response toward other antigens to occur. There have been promising preclinical findings regarding adoptive Treg cells immunotherapy with a critically responsible function in the prevention of autoimmunity, tissue repair and regeneration, which make them an attractive candidate to develop effective therapeutic approaches to achieve this interesting concept in many human immune-mediated diseases, such as rheumatoid arthritis. Ex vivo or invivo manipulation protocols are not only utilized to correct Treg cells defect, but also to benefit from their specific immunosuppressive properties by identifying specific antigens that are expressed in the inflamedjoint. The methods able to address these deficiencies can be considered as a target for immunity interventions to restore appropriate immune function.

Cell Therapy in Solid Organ Transplantation

Transplantation is the only cure for end-stage organ failure. Current immunosuppressive drugs have two major limitations: 1) non antigen specificity, which increases the risk of cancer and infection diseases, and 2) chronic toxicity. Cell therapy appears to be an innovative and promising strategy to minimize the use of immunosuppression in transplantation and to improve long-term graft survival. Preclinical studies have shown efficacy and safety of using various suppressor cells, such as regulatory T cells, regulatory B cells and tolerogenic dendritic cells. Recent clinical trials using cellbased therapies in solid organ transplantation also hold out the promise of improving efficacy. In this review, we will briefly go over the rejection process, current immunosuppressive drugs, and the potential therapeutic use of regulatory cells in transplantation.

Past, Present, and Future of Regulatory T Cell Therapy in Transplantation and Autoimmunity

Regulatory T cells (Tregs) are important for the induction and maintenance of peripheral tolerance therefore, they are key in preventing excessive immune responses and autoimmunity. In the last decades, several reports have been focussed on understanding the biology of Tregs and their mechanisms of action. Preclinical studies have demonstrated the ability of Tregs to delay/prevent graft rejection and to control autoimmune responses following adoptive transfer in vivo. Due to these promising results, Tregs have been extensively studied as a potential new tool for the prevention of graft rejection and/or the treatment of autoimmune diseases. Currently, solid organ transplantation remains the treatment of choice for end-stage organ failure. However, chronic rejection and the ensuing side effects of immunosuppressants represent the main limiting factors for organ acceptance and patient survival. Autoimmune disorders are chronic diseases caused by the breakdown of tolerance against self-antigens. This is triggered either by a numerical or functional Treg defect, or by the resistance of effector T cells to suppression. In this scenario, patients receiving high doses of immunosuppressant are left susceptible to life-threatening opportunistic infections and have increased risk of malignancies. In the last 10 years, a few phase I clinical trials aiming to investigate safety and feasibility of Treg-based therapy have been completed and published, whilst an increasing numbers of trials are still ongoing. The first results showed safety and feasibility of Treg therapy and phase II clinical trials are already enrolling. In this review, we describe our understanding of Tregs focussing primarily on their ontogenesis, mechanisms of action and methods used in the clinic for isolation and expansion. Furthermore, we will describe the ongoing studies and the results from the first clinical trials with Tregs in the setting of solid organ transplantation and autoimmune disorders. Finally, we will discuss strategies to further improve the success of Treg therapy.

NOTCH and Graft-Versus-Host Disease

In allogeneic hematopoietic stem cell transplantation, which is the major curative therapy for hematological malignancies, T cells play a key role in the development of graft-versus-host disease (GvHD). NOTCH pathway is a conserved signal transduction system that regulates T cell development and differentiation. The present review analyses the role of the NOTCH signaling as a new regulator of acute GvHD. NOTCH signaling could also represent a new therapeutic target for GvHD.

Expanded Regulatory T Cells Induce Alternatively Activated Monocytes With a Reduced Capacity to Expand T Helper-17 Cells

Regulatory T cells (Tregs) are essential in maintaining peripheral immunological tolerance by modulating several subsets of the immune system including monocytes. Under inflammatory conditions, monocytes migrate into the tissues, where they differentiate into dendritic cells or tissue-resident macrophages. As a result of their context-dependent plasticity, monocytes have been implicated in the development/progression of graft-vs-host disease (GvHD), autoimmune diseases and allograft rejection. In the last decade, Tregs have been exploited for their use in cell therapy with the aim to induce tolerance after solid organ transplantation and for the treatment of autoimmune diseases and GvHD. To date, safety and feasibility of Treg infusion has been demonstrated; however, many questions of how these cells induce tolerance have been raised and need to be answered. As monocytes constitute the major cellular component in inflamed tissues, we have developed an in vitro model to test how Tregs modulate their phenotype and function. We demonstrated that expanded Tregs can drive monocytes toward an alternatively activated state more efficiently than freshly isolated Tregs. The effect of expanded Tregs on monocytes led to a reduced production of pro-inflammatory cytokines (IL-6 and tumor necrosis factor-α) and NF-κB activation. Furthermore, monocytes co-cultured with expanded Tregs downregulated the expression of co-stimulatory and MHC-class II molecules with a concomitant upregulation of M2 macrophage specific markers, CD206, heme oxygenase-1, and increased interleukin-10 production. Importantly, monocytes co-cultured with expanded Tregs showed a reduced capacity to expand IL-17-producing T cells compared with monocyte cultured with freshly isolated Tregs and conventional T cells. The capacity to decrease the expansion of pro-inflammatory Th-17 was not cytokine mediated but the consequence of their lower expression of the co-stimulatory molecule CD86. Our data suggest that expanded Tregs have the capacity to induce phenotypical and functional changes in monocytes that might be crucial for tolerance induction in transplantation and the prevention/treatment of GvHD and autoimmune diseases.

Utilization of leukapheresis and CD4 positive selection in Treg isolation and the ex-vivo expansion for a clinical application in transplantation and autoimmune disorders

Adoptive transfer of T regulatory cells (Tregs) is of great interest as a novel immunosuppressive therapy in autoimmune disorders and transplantation. Obtaining a sufficient number of stable and functional Tregs generated according to current Good Manufacturing Practice (cGMP) requirements has been a major challenge in introducing Tregs as a clinical therapy. Here, we present a protocol involving leukapheresis and CD4⁺ cell pre-enrichment prior to Treg sorting, which allows a sufficient number of Tregs for a clinical application to be obtained. With this method there is a decreased requirement for ex-vivo expansion. The protocol was validated in cGMP conditions. Our final Treg product passed all release criteria set for clinical applications. Moreover, during expansion Tregs presented their stable phenotype: percentage of CD4⁺CD25^hiCD127⁻ and CD4⁺FoxP3⁺ Tregs was > 95% and > 80%, respectively, and Tregs maintained proper immune suppressive function in vitro. Our results suggest that utilization of leukapheresis and CD4 positive selection during Treg isolation improves the likelihood of obtaining a sufficient number of high quality Treg cells during subsequent ex-vivo expansion and they can be applied clinically.

Cell banking for regulatory T cell-based therapy: strategies to overcome the impact of cryopreservation on the Treg viability and phenotype

The first clinical trials with adoptive Treg therapy have shown safety and potential efficacy. Feasibility of such therapy could be improved if cells are cryopreserved and stored until optimal timing for infusion. Herein, we report the evaluation of two cell-banking strategies for Treg therapy: 1) cryopreservation of CD4+ cells for subsequent Treg isolation/expansion and 2) cryopreservation of ex-vivo expanded Tregs (CD4+CD25hiCD127lo/- cells). First, we checked how cryopreservation affects cell viability and Treg markers expression. Then, we performed Treg isolation/expansion with the final products release testing. We observed substantial decrease in cell number recovery after thawing and overnight culture. This observation might be explained by the high percentage of necrotic and apoptotic cells found just after thawing. Furthermore, we noticed fluctuations in percentage of CD4+CD25hiCD127- and CD4+FoxP3+ cells obtained from cryopreserved CD4+ as well as Treg cells. However, after re-stimulation Tregs expanded well, presented a stable phenotype and fulfilled the release criteria at the end of expansions. Cryopreservation of CD4+ cells for subsequent Treg isolation/expansion and cryopreservation of expanded Tregs with re-stimulation and expansion after thawing, are promising solutions to overcome detrimental effects of cryopreservation. Both of these cell-banking strategies for Treg therapy can be applied when designing new clinical trials.

Novel Concept of CD4-Mediated Activation of Regulatory T Cells for the Treatment of Graft-Versus-Host Disease

Allogeneic hematopoietic stem cell transplantation is the only curative treatment option for several hematological malignancies and immune deficiency syndromes. Nevertheless, the development of a graft-versus-host disease (GvHD) after transplantation is a high risk and a severe complication with high morbidity and mortality causing therapeutic challenges. Current pharmacological therapies of GvHD lead to generalized immunosuppression followed by severe adverse side effects including infections and relapse of leukemia. Several novel cell-based immunomodulatory strategies for treatment or prevention of GvHD have been developed. Herein, thymus-derived regulatory T cells (tTreg), essential for the maintenance of peripheral immunologic tolerance, are in the focus of investigation. However, due to the limited number of tTreg in the peripheral blood, a complex, time- and cost-intensive in vitro expansion protocol is necessary for the production of an efficient cellular therapeutic. We demonstrated that activation of tTreg using the CD4-binding human immunodeficiency virus-1 protein gp120 leads to a substantially increased suppressor activity of tTreg without the need for additional expansion. Gp120-activated tTreg prevent GvHD development in a preclinical humanized mouse model. In addition, gp120 is not only effective in prevention but also in therapy of GvHD by suppressing all clinical symptoms and improving survival of treated mice. These data indicate that tTreg activation by gp120 is a feasible and potent strategy for significant functional improvement of tTreg as cellular therapeutic for GvHD treatment without the need of complicated, time-intensive, and expensive in vitro expansion of isolated tTreg.

Clinical-Grade-Expanded Regulatory T Cells Prevent Graft-versus-Host Disease While Allowing a Powerful T Cell-Dependent Graft-versus-Leukemia Effect in Murine Models

We developed a good manufacturing practices-compatible expansion protocol to improve number and purity of regulatory T cells (Tregs) available for clinical trials. Six clinical-grade separation procedures were performed, followed by expansion with high-dose interleukin (IL)-2, anti-CD3/anti-CD28 TCR stimulation, and rapamycin for 19 days achieving a median of 8.5-fold (range, 6.25 to 13.7) expansion. FOXP3 expression was stably maintained over the culture period, while the percentage of CD127 was significantly reduced. The in vitro suppression assay showed a strong Mixed Lymphocytes Reaction inhibition. In vitro amplification did not induce any karyotypic modification. To evaluate the graft-versus-host disease (GVHD)/graft-versus-leukemia (GVL) bifunctional axis, expanded Tregs and conventional T cells (Tcons) were tested in NOD/SCID/IL2Rgnull mice injected with primary acute myeloid leukemia (AML) cells, AML cell line, acute lymphoid leukemia Philadelphia cell line, or Burkitt-like lymphoma cell line. All mice that received leukemia cells together with expanded Tregs and Tcons were rescued from leukemia and survived without GVHD, showing that Treg expansion procedure did not compromise GVHD control and the strong Tcon-mediated GVL activity. This report might represent the basis for treating high-risk leukemia and/or relapsed/refractory leukemia patients with high-dose Treg/Tcons.

Treg therapy in transplantation: a general overview

Solid organ transplantation remains the treatment of choice for end-stage organ failure. Whilst the short-term outcomes post-transplant have improved in the last decades, chronic rejection and immunosuppressant side effects remain an ongoing concern. Hematopoietic stem cell transplantation is a well-established procedure for the treatment of patients with haematological disorders. However, donor T cells are continually primed and activated to react against the host causing graft-versus-host disease (GvHD) that leads to tissue damages and death. Regulatory T cells (Tregs) play an essential role in maintaining tolerance to self-antigens, preventing excessive immune responses and abrogating autoimmunity. Due to their suppressive properties, Tregs have been extensively studied for their use as a cellular therapy aiming to treat GvHD and limit immune responses responsible for graft rejection. Several clinical trials have been conducted or are currently ongoing to investigate safety and feasibility of Treg-based therapy. This review summarizes the general understanding of Treg biology and presents the methods used to isolate and expand Tregs. Furthermore, we describe data from the first clinical trials using Tregs, explaining the limitations and future application of these cells.

Ex vivo expanded human regulatory T cells delay islet allograft rejection via inhibiting islet-derived monocyte chemoattractant protein-1 production in CD34+ stem cells-reconstituted NOD-scid IL2rγnull mice

Type 1 diabetes mellitus (T1DM) is an autoimmune disease caused by immune-mediated destruction of insulin-secreting β cells of the pancreas. Near complete dependence on exogenous insulin makes T1DM very difficult to control, with the result that patients are exposed to high blood glucose and risk of diabetic complications and/or intermittent low blood glucose that can cause unconsciousness, fits and even death. Allograft transplantation of pancreatic islets restores normoglycemia with a low risk of surgical complications. However, although successful immediately after transplantation, islets are progressively lost, with most of the patients requiring exogenous insulin within 2 years post-transplant. Therefore, there is an urgent requirement for the development of new strategies to prevent islet rejection. In this study, we explored the importance of human regulatory T cells in the control of islets allograft rejection. We developed a pre-clinical model of human islet transplantation by reconstituting NOD-scid IL2rγ^null mice with cord blood-derived human CD34⁺ stem cells and demonstrated that although the engrafted human immune system mediated the rejection of human islets, their survival was significantly prolonged following adoptive transfer of ex vivo expanded human T_regs. Mechanistically, T_regs inhibited the infiltration of innate immune cells and CD4⁺ T cells into the graft by down-regulating the islet graft-derived monocyte chemoattractant protein-1. Our findings might contribute to the development of clinical strategies for T_reg therapy to control human islet rejection. We also show for the first time that CD34⁺ cells-reconstituted NOD-scid IL2rγ^null mouse model could be beneficial for investigating human innate immunity in vivo.

Importance of regulatory T cells in the pathogenesis of psoriasis: review of the literature

Psoriasis is a common chronic relapsing inflammatory cutaneous disease; the main role in the inflammation of this condition is played by lymphocyte Th1, Th17 and their cytokines. The activity of these cells is modulated by a particular kind of T cells recently described: the T regulatory cells (Treg). These are able to inhibit the immunological response and to maintain the cutaneous immunological homeostasis, thus preventing autoimmunity against self antigens. Few data are available in the literature as to Treg in psoriasis; several studies demonstrate that the function of these cells is impaired in this condition and treatments for psoriasis may increase the number and activity of Treg. The role of these cells in the pathogenesis of psoriasis is very important to understand how they may contribute to the development of this cutaneous disorder. In the near future it would be possible to target therapies at these defects, improving the activity of these cells and maintaining cutaneous homeostasis, preventing psoriasis or other inflammatory cutaneous conditions.

Mesenchymal stem cells (MSCs) from scleroderma patients (SSc) preserve their immunomodulatory properties although senescent and normally induce T regulatory cells (Tregs) with a functional phenotype: implications for cellular-based therapy

Systemic sclerosis (SSc) is a chronic disease, with early activation of the immune system. The aim of our work was to address how SSc-mesenchymal stem cells (MSCs), although senescent, might preserve specific immunomodulatory abilities during SSc. MSCs were obtained from 10 SSc patients and 10 healthy controls (HC). Senescence was evaluated by assessing cell cycle, β-galactosidase (β-Gal) activity, p21 and p53 expression; doxorubicin was used as acute senescence stimulus to evaluate their ability to react in stressed conditions. Immunomodulatory abilities were studied co-culturing MSCs with peripheral blood mononuclear cells (PBMCs) and CD4(+) cells, in order to establish both their ability to block proliferation in mixed lymphocyte reaction and in regulatory T cells (Tregs) induction. SSc-MSC showed an increase of senescence biomarkers. Eighty per cent of MSCs were in G0-G1 phase, without significant differences between SSc and HC. SSc-MSCs showed an increased positive β-Gal staining and higher p21 transcript level compared to HC cells. After doxorubicin, β-Gal staining increased significantly in SSc-MSCs. On the contrary, doxorubicin abolished p21 activation and elicited p53 induction both in SSc- and HC-MSCs. Interleukin (IL)-6 and transforming growth factor (TGF)-β-related transcripts and their protein levels were significantly higher in SSc-MSCs. The latter maintained their immunosuppressive effect on lymphocyte proliferation and induced a functionally regulatory phenotype on T cells, increasing surface expression of CD69 and restoring the regulatory function which is impaired in SSc. Increased activation of the IL-6 pathway observed in our cells might represent an adaptive mechanism to senescence, but preserving some specific cellular functions, including immunosuppression.

Notch1 modulates mesenchymal stem cells mediated regulatory T-cell induction

Notch1 signaling is involved in regulatory T (Treg)-cell differentiation. We previously demonstrated that, when cocultured with CD3(+) cells, mesenchymal stem cells (MSCs) induced a T-cell population with a regulatory phenotype. Here, we investigated the molecular mechanism underlying MSC induction of human Treg cells. We show that the Notch1 pathway is activated in CD4(+) T cells cocultured with MSCs. Inhibition of Notch1 signaling through GSI-I or the Notch1 neutralizing antibody reduced expression of HES1 (the Notch1 downstream target) and the percentage of MSC-induced CD4(+) CD25(high) FOXP3(+) cells in vitro. Moreover, we demonstrate that FOXP3 is a downstream target of Notch signaling in human cells. No crosstalk between Notch1 and TGF-β signaling pathways was observed in our experimental system. Together, these findings indicate that activation of the Notch1 pathway is a novel mechanism in the human Treg-cell induction mediated by MSCs.