Mast cells suppress murine GVHD in a mechanism independent of CD4+CD25+ regulatory T cells

Mast cells and ocular surface: An update review

Mast cells (MCs), traditionally viewed as key players in IgE-mediated allergic responses, are increasingly recognized for their versatile roles. Situated at critical barrier sites such as the ocular surface, these sentinel cells participate in a broad array of physiological and pathological processes. This review presents a comprehensive update on the immune pathophysiology of MCs, with a particular focus on the mechanisms underlying innate immunity. It highlights their roles at the ocular surface, emphasizing their participation in allergic reactions, maintenance of corneal homeostasis, neovascularization, wound healing, and immune responses in corneal grafts. The review also explores the potential of MCs as therapeutic targets, given their significant contributions to disease pathogenesis and their capacity to modulate immunity. Through a thorough examination of current literature, we aim to elucidate the immune pathophysiology and multifaceted roles of MCs in ocular surface health and disease, suggesting directions for future research and therapeutic innovation.

The novel role of lymphatic vessels in the pathogenesis of ocular diseases

Historically, the eye has been considered as an organ free of lymphatic vessels. In recent years, however, it became evident, that lymphatic vessels or lymphatic-like vessels contribute to several ocular pathologies at various peri- and intraocular locations. The aim of this review is to outline the pathogenetic role of ocular lymphatics, the respective molecular mechanisms and to discuss current and future therapeutic options based thereon. We will give an overview on the vascular anatomy of the healthy ocular surface and the molecular mechanisms contributing to corneal (lymph)angiogenic privilege. In addition, we present (i) current insights into the cellular and molecular mechanisms occurring during pathological neovascularization of the cornea triggered e.g. by inflammation or trauma, (ii) the role of lymphatic vessels in different ocular surface pathologies such as dry eye disease, corneal graft rejection, ocular graft versus host disease, allergy, and pterygium, (iii) the involvement of lymphatic vessels in ocular tumors and metastasis, and (iv) the novel role of the lymphatic-like structure of Schlemm's canal in glaucoma. Identification of the underlying molecular mechanisms and of novel modulators of lymphangiogenesis will contribute to the development of new therapeutic targets for the treatment of ocular diseases associated with pathological lymphangiogenesis in the future. The preclinical data presented here outline novel therapeutic concepts for promoting transplant survival, inhibiting metastasis of ocular tumors, reducing inflammation of the ocular surface, and treating glaucoma. Initial data from clinical trials suggest first success of novel treatment strategies to promote transplant survival based on pretransplant corneal lymphangioregression.

Mast Cell Activation Syndrome Update-A Dermatological Perspective

Mast cells (MCs) are infamous for their role in potentially fatal anaphylaxis reactions. In the last two decades, a more complex picture has emerged, as it has become obvious that MCs are much more than just IgE effectors of anaphylaxis. MCs are defenders against a host of infectious and toxic aggressions (their interactions with other components of the immune system are not yet fully understood) and after the insult has ended, MCs continue to play a role in inflammation regulation and tissue repair. Unfortunately, MC involvement in pathology is also significant. Apart from their role in allergies, MCs can proliferate clonally to produce systemic mastocytosis. They have also been implicated in excessive fibrosis, keloid scaring, graft rejection and chronic inflammation, especially at the level of the skin and gut. In recent years, the term MC activation syndrome (MCAS) was proposed to account for symptoms caused by MC activation, and clear diagnostic criteria have been defined. However, not all authors agree with these criteria, as some find them too restrictive, potentially leaving much of the MC-related pathology unaccounted for. Here, we review the current knowledge on the physiological and pathological roles of MCs, with a dermatological emphasis, and discuss the MCAS classification.

Biomarkers in oral immunotherapy

Food allergy (FA) is a global health problem that affects a large population, and thus effective treatment is highly desirable. Oral immunotherapy (OIT) has been showing reasonable efficacy and favorable safety in most FA subjects. Dependable biomarkers are needed for treatment assessment and outcome prediction during OIT. Several immunological indicators have been used as biomarkers in OIT, such as skin prick tests, basophil and mast cell reactivity, T cell and B cell responses, allergen-specific antibody levels, and cytokines. Other novel indicators also could be potential biomarkers. In this review, we discuss and assess the application of various immunological indicators as biomarkers for OIT.

Utilizing mast cells in a positive manner to overcome inflammatory and allergic diseases

Mast cells (MCs) are immune cells widely distributed in the body, accompanied by diverse phenotypes and functions. Committed mast cell precursors (MCPs) leave the bone marrow and enter the blood circulation, homing to peripheral sites under the control of various molecules from different microenvironments, where they eventually differentiate and mature. Partly attributable to the unique maturation mechanism, MCs display high functional heterogeneity and potentially plastic phenotypes. High plasticity also means that MCs can exhibit different subtypes to cope with different microenvironments, which we call “the peripheral immune education system”. Under the peripheral immune education system, MCs showed a new character from previous cognition in some cases, namely regulation of allergy and inflammation. In this review, we focus on the mucosal tissues, such as the gastrointestinal tract, to gain insights into the mechanism underlying the migration of MCs to the gut or other organs and their heterogeneity, which is driven by different microenvironments. In particular, the immunosuppressive properties of MCs let us consider that positively utilizing MCs may be a new way to overcome inflammatory and allergic disorders.

Roles of Mast Cells in Cutaneous Diseases

Mast cells are present in all vascularized tissues of the body. They are especially abundant in tissues that are in frequent contact with the surrounding environment and act as potential sources of inflammatory and/or regulatory mediators during development of various infections and diseases. Mature mast cells’ cytoplasm contains numerous granules that store a variety of chemical mediators, cytokines, proteoglycans, and proteases. Mast cells are activated via various cell surface receptors, including FcϵRI, toll-like receptors (TLR), Mas-related G-protein-coupled receptor X2 (MRGPRX2), and cytokine receptors. IgE-mediated mast cell activation results in release of histamine and other contents of their granules into the extracellular environment, contributing to host defense against pathogens. TLRs, play a crucial role in host defense against various types of pathogens by recognizing pathogen-associated molecular patterns. On the other hand, excessive/inappropriate mast cell activation can cause various disorders. Here, we review the published literature regarding the known and potential inflammatory and regulatory roles of mast cells in cutaneous inflammation, including atopic dermatitis, psoriasis, and contact dermatitis GVHD, as well as in host defense against pathogens.

Mucosal Mast Cells as Key Effector Cells in Food Allergies

Mucosal mast cells (MMCs) localized in the intestinal mucosa play a key role in the development of IgE-mediated food allergies. Recent advances have revealed that MMCs are a distinctly different population from connective tissue mast cells localized in skin and other connective tissues. MMCs are inducible and transient cells that arise from bone marrow-derived mast cell progenitors, and their numbers increase rapidly during mucosal allergic inflammation. However, the mechanism of the dramatic expansion of MMCs and their cell functions are not well understood. Here, we review recent findings on the mechanisms of MMC differentiation and expansion, and we discuss the potential for the inducers of differentiation and expansion to serve as targets for food allergy therapy. In addition, we also discuss the mechanism by which oral immunotherapy, a promising treatment for food allergy patients, induces unresponsiveness to food allergens and the roles of MMCs in this process. Research focusing on MMCs should provide useful information for understanding the underlying mechanisms of food allergies in order to further advance the treatment of food allergies.

Malaria-induced bacteremia as a consequence of multiple parasite survival strategies

Globally, malaria continues to be an enormous public health burden, with concomitant parasite-induced damage to the gastrointestinal (GI) barrier resulting in bacteremia-associated morbidity and mortality in both adults and children. Infected red blood cells sequester in and can occlude the GI microvasculature, ultimately leading to disruption of the tight and adherens junctions that would normally serve as a physical barrier to translocating enteric bacteria. Mast cell (MC) activation and translocation to the GI during malaria intensifies damage to the physical barrier and weakens the immunological barrier through the release of enzymes and factors that alter the host response to escaped enteric bacteria. In this context, activated MCs release Th2 cytokines, promoting a balanced Th1/Th2 response that increases local and systemic allergic inflammation while protecting the host from overwhelming Th1-mediated immunopathology. Beyond the mammalian host, recent studies in both the lab and field have revealed an association between a Th2-skewed host response and success of parasite transmission to mosquitoes, biology that is evocative of parasite manipulation of the mammalian host. Collectively, these observations suggest that malaria-induced bacteremia may be, in part, an unintended consequence of a Th2-shifted host response that promotes parasite survival and transmission. Future directions of this work include defining the factors and mechanisms that precede the development of bacteremia, which will enable the development of biomarkers to simplify diagnostics, the identification of therapeutic targets to improve patient outcomes and better understanding of the consequences of clinical interventions to transmission blocking strategies.

Physiological Functions of Mcl-1: Insights From Genetic Mouse Models

The ability to regulate the survival and death of a cell is paramount throughout the lifespan of a multicellular organism. Apoptosis, a main physiological form of programmed cell death, is regulated by the Bcl-2 family proteins that are either pro-apoptotic or pro-survival. The in vivo functions of distinct Bcl-2 family members are largely unmasked by genetically engineered murine models. Mcl-1 is one of the two Bcl-2 like pro-survival genes whose germline deletion causes embryonic lethality in mice. Its requisite for the survival of a broad range of cell types has been further unraveled by using conditional and inducible deletion murine model systems in different tissues or cell lineages and at distinct developmental stages. Moreover, genetic mouse cancer models have also demonstrated that Mcl-1 is essential for the survival of multiple tumor types. The MCL-1 locus is commonly amplified across various cancer types in humans. Small molecule inhibitors with high affinity and specificity to human MCL-1 have been developed and explored for the treatment of certain cancers. To facilitate the pre-clinical studies of MCL-1 in cancer and other diseases, transgenic mouse models over-expressing human MCL-1 as well as humanized MCL-1 mouse models have been recently engineered. This review discusses the current advances in understanding the physiological roles of Mcl-1 based on studies using genetic murine models and its critical implications in pathology and treatment of human diseases.

Genetic susceptibility to acute graft versus host disease in pediatric patients undergoing HSCT

The most frequent complication of allogeneic hematopoietic stem cell transplantation is acute Graft versus Host Disease (aGVHD). Proliferation and differentiation of donor T cells initiate inflammatory response affecting the skin, liver, and gastrointestinal tract. Besides recipient-donor HLA disparities, disease type, and the conditioning regimen, variability in the non-HLA genotype have an impact on aGVHD onset, and genetic variability of key cytokines and chemokines was associated with increased risk of aGVHD. To get further insight into the recipient genetic component of aGVHD grades 2-4 in pediatric patients, we performed an exome-wide association study in a discovery cohort (n = 87). Nine loci sustained correction for multiple testing and were analyzed in a validation group (n = 168). Significant associations were replicated for ERC1 rs1046473, PLEK rs3816281, NOP9 rs2332320 and SPRED1 rs11634702 variants through the interaction with non-genetic factors. The ERC1 variant was significant among patients that received the transplant from HLA-matched related individuals (p = 0.03), bone marrow stem cells recipients (p = 0.007), and serotherapy-negative patients (p = 0.004). NOP9, PLEK, and SPRED1 effects were modulated by stem cell source, and serotherapy (p < 0.05). Furthermore, ERC1 and PLEK SNPs correlated with aGVHD 3-4 independently of non-genetic covariates (p = 0.02 and p = 0.003). This study provides additional insight into the genetic component of moderate to severe aGVHD.

Two Sides of the Coin: Mast Cells as a Key Regulator of Allergy and Acute/Chronic Inflammation

It is well known that mast cells (MCs) initiate type I allergic reactions and inflammation in a quick response to the various stimulants, including—but not limited to—allergens, pathogen-associated molecular patterns (PAMPs), and damage-associated molecular patterns (DAMPs). MCs highly express receptors of these ligands and proteases (e.g., tryptase, chymase) and cytokines (TNF), and other granular components (e.g., histamine and serotonin) and aggravate the allergic reaction and inflammation. On the other hand, accumulated evidence has revealed that MCs also possess immune-regulatory functions, suppressing chronic inflammation and allergic reactions on some occasions. IL-2 and IL-10 released from MCs inhibit excessive immune responses. Recently, it has been revealed that allergen immunotherapy modulates the function of MCs from their allergic function to their regulatory function to suppress allergic reactions. This evidence suggests the possibility that manipulation of MCs functions will result in a novel approach to the treatment of various MCs-mediated diseases.

Tissue mast cell counts may be associated with decreased severity of gastrointestinal acute GVHD and nonrelapse mortality

The functions of mast cells in human graft-versus-host disease (GVHD) are unknown. We studied 56 patients who had an allogeneic hematopoietic cell transplantation (alloHCT) with a biopsy for diagnosis of gastrointestinal tract (GIT) GVHD before any treatment (including steroids): 35 with GIT GVHD, 21 HCT recipients whose biopsies did not confirm GVHD, and 9 with a new diagnosis of inflammatory bowel disease (IBD) as a comparison. The median number of mast cells (mean of CD117+ cells, counted in 3 selected spots under 40× magnification) was similar between patients with GVHD (59 cells) and those without GVHD (60 cells). However, the median number of mast cells was significantly associated with maximum clinical stage of GIT GVHD; the lowest counts of mast cells were observed in the highest clinical stage of GIT GVHD (stage 1, 80; stage 2, 69; stage 3, 54; stage 4, 26; P = .01). Moreover, every decrease by 10 mast cells was associated with increased nonrelapse mortality through 1 year (hazard ratio, 0.77; 95% confidence interval, 0.59-1.00; P = .05). AlloHCT recipients all had significantly fewer mast cells, even those without GVHD compared with those with IBD (median, 59 vs 119; P < .01). The median number of GIT mast cells was also significantly lower in patients who received myeloablative conditioning (61.5 cells) than in those who received reduced intensity conditioning (78 cells) in the entire study population (P = .02). We conclude that GIT mast cells are depleted in all alloHCT patients, more prominently in those receiving myeloablative conditioning and those with severe GIT GVHD. Our novel findings warrant further investigation into the biological effects of mast cells in GIT GVHD.

IL-10 in Mast Cell-Mediated Immune Responses: Anti-Inflammatory and Proinflammatory Roles

Mast cells (MCs) play critical roles in Th2 immune responses, including the defense against parasitic infections and the initiation of type I allergic reactions. In addition, MCs are involved in several immune-related responses, including those in bacterial infections, autoimmune diseases, inflammatory bowel diseases, cancers, allograft rejections, and lifestyle diseases. Whereas antigen-specific IgE is a well-known activator of MCs, which express FcεRI on the cell surface, other receptors for cytokines, growth factors, pathogen-associated molecular patterns, and damage-associated molecular patterns also function as triggers of MC stimulation, resulting in the release of chemical mediators, eicosanoids, and various cytokines. In this review, we focus on the role of interleukin (IL)-10, an anti-inflammatory cytokine, in MC-mediated immune responses, in which MCs play roles not only as initiators of the immune response but also as suppressors of excessive inflammation. IL-10 exhibits diverse effects on the proliferation, differentiation, survival, and activation of MCs in vivo and in vitro. Furthermore, IL-10 derived from MCs exerts beneficial and detrimental effects on the maintenance of tissue homeostasis and in several immune-related diseases including contact hypersensitivity, auto-immune diseases, and infections. This review introduces the effects of IL-10 on various events in MCs, and the roles of MCs in IL-10-related immune responses and as a source of IL-10.

Mast Cells in the Skin: Defenders of Integrity or Offenders in Inflammation?

Mast cells (MCs) are best-known as key effector cells of immediate-type allergic reactions that may even culminate in life-threatening anaphylactic shock syndromes. However, strategically positioned at the host–environment interfaces and equipped with a plethora of receptors, MCs also play an important role in the first-line defense against pathogens. Their main characteristic, the huge amount of preformed proinflammatory mediators embedded in secretory granules, allows for a rapid response and initiation of further immune effector cell recruitment. The same mechanism, however, may account for detrimental overshooting responses. MCs are not only detrimental in MC-driven diseases but also responsible for disease exacerbation in other inflammatory disorders. Focusing on the skin as the largest immune organ, we herein review both beneficial and detrimental functions of skin MCs, from skin barrier integrity via host defense mechanisms to MC-driven inflammatory skin disorders. Moreover, we emphasize the importance of IgE-independent pathways of MC activation and their role in sustained chronic skin inflammation and disease exacerbation.

Mast Cell Involvement in Fibrosis in Chronic Graft-Versus-Host Disease

Allogeneic hematopoietic stem cell transplantation (HSCT) is most commonly a treatment for inborn defects of hematopoiesis or acute leukemias. Widespread use of HSCT, a potentially curative therapy, is hampered by onset of graft-versus-host disease (GVHD), classified as either acute or chronic GVHD. While the pathology of acute GVHD is better understood, factors driving GVHD at the cellular and molecular level are less clear. Mast cells are an arm of the immune system that are known for atopic disease. However, studies have demonstrated that they can play important roles in tissue homeostasis and wound healing, and mast cell dysregulation can lead to fibrotic disease. Interestingly, in chronic GVHD, aberrant wound healing mechanisms lead to pathological fibrosis, but the cellular etiology driving this is not well-understood, although some studies have implicated mast cells. Given this novel role, we here review the literature for studies of mast cell involvement in the context of chronic GVHD. While there are few publications on this topic, the papers excellently characterized a niche for mast cells in chronic GVHD. These findings may be extended to other fibrosing diseases in order to better target mast cells or their mediators for treatment of fibrotic disease.

Resident Innate Immune Cells in the Cornea

The cornea is a special interface between the internal ocular tissue and the external environment that provides a powerful chemical, physical, and biological barrier against the invasion of harmful substances and pathogenic microbes. This protective effect is determined by the unique anatomical structure and cellular composition of the cornea, especially its locally resident innate immune cells, such as Langerhans cells (LCs), mast cells (MCs), macrophages, γδ T lymphocytes, and innate lymphoid cells. Recent studies have demonstrated the importance of these immune cells in terms of producing different cytokines and other growth factors in corneal homeostasis and its pathologic conditions. This review paper briefly describes the latest information on these resident immune cells by specifically analyzing research from our laboratory.

Orally desensitized mast cells form a regulatory network with Treg cells for the control of food allergy

Oral immunotherapy (OIT) is an effective approach to controlling food allergy. Although the detailed molecular and cellular mechanisms of OIT are unknown currently, they must be understood to advance the treatment of allergic diseases in general. To elucidate the mechanisms of OIT, especially during the immunological transition from desensitization to allergy regulation, we generated a clinical OIT murine model and used it to examine immunological events of OIT. We found that in mice that completed OIT successfully, desensitized mast cells (MCs) showed functionally beneficial alterations, such as increased induction of regulatory cytokines and enhanced expansion of regulatory T cells. Importantly, these regulatory-T-cell-mediated inhibitions of allergic responses were dramatically decreased in mice lacking OIT-induced desensitized MC. Collectively, these findings show that the desensitization process modulates the activation of MCs, leading directly to enhanced induction of regulatory-T-cell expansion and promotion of clinical allergic unresponsiveness. Our results suggest that efficiently inducing regulatory MCs is a novel strategy for the treatment of allergic disease.

Mast Cell Functions Linking Innate Sensing to Adaptive Immunity

Although mast cells (MCs) are known as key drivers of type I allergic reactions, there is increasing evidence for their critical role in host defense. MCs not only play an important role in initiating innate immune responses, but also influence the onset, kinetics, and amplitude of the adaptive arm of immunity or fine-tune the mode of the adaptive reaction. Intriguingly, MCs have been shown to affect T-cell activation by direct interaction or indirectly, by modifying the properties of antigen-presenting cells, and can even modulate lymph node-borne adaptive responses remotely from the periphery. In this review, we provide a summary of recent findings that explain how MCs act as a link between the innate and adaptive immunity, all the way from sensing inflammatory insult to orchestrating the final outcome of the immune response.

Immunomodulatory role of Interleukin-33 in large vessel vasculitis

The mechanisms regulating inflammation in large vessels vasculitis (LVV) are poorly understood. Interleukin 33 (IL-33) has been shown to license innate and adaptive immunity by enhancing Th2 cytokines production. We aimed to examine the role of IL-33 in the immunomodulation of T cell activation in LVV. T cell homeostasis and cytokines production were determined in peripheral blood from 52 patients with giant cell arteritis (GCA) and 50 healthy donors (HD), using Luminex assay, flow cytometry, quantitative RT-PCR and by immunofluorescence analysis in inflammatory aorta lesions. We found increased level of IL-33 and its receptor ST2/IL-1R4 in the serum of patient with LVV. Endothelial cells were the main source of IL-33, whereas Th2 cells, Tregs and mast cells (MC) express ST2 in LVV vessels. IL-33 had a direct immunomodulatory impact by increasing Th2 and Tregs. IL-33 and MC further enhanced Th2 and regulatory responses by inducing a 6.1 fold increased proportion of Tregs (p = 0.008). Stimulation of MC by IL-33 increased indoleamine 2 3-dioxygenase (IDO) activity and IL-2 secretion. IL-33 mRNA expression was significantly correlated with the expression of IL-10 and TGF-β within aorta inflammatory lesions. To conclude, our findings suggest that IL-33 may exert a critical immunoregulatory role in promoting Tregs and Th2 cells in LVV.

Tissue immune profiles supporting response to mesenchymal stromal cell therapy in acute graft-versus-host disease-a gut feeling

Acute graft-versus-host disease (aGvHD), post-allogeneic hematopoietic stem cell transplantation, is associated with high mortality rates in patients not responding to standard line care with steroids. Adoptive mesenchymal stromal cell (MSC) therapy has been established in some countries as a second-line treatment.

Limitations in our understanding as to MSC mode of action and what segregates patient responders from non-responders to MSC therapy remain. The principal aim of this study was to evaluate the immune cell profile in gut biopsies of patients diagnosed with aGvHD and establish differences in baseline cellular composition between responders and non-responders to subsequent MSC therapy.

Our findings indicate that a pro-inflammatory immune profile within the gut at the point of MSC treatment may impede their therapeutic potential. These findings support the need for further validation in a larger cohort of patients and the development of improved biomarkers in predicting responsiveness to MSC therapy.

Hematopoietic stem cell transplantation using single UM171-expanded cord blood: a single-arm, phase 1-2 safety and feasibility study

BACKGROUND

Benefits of cord blood transplantation include low rates of relapse and chronic graft-versus-host disease (GVHD). However, the use of cord blood is rapidly declining because of the high incidence of infections, severe acute GVHD, and transplant-related mortality. UM171, a haematopoietic stem cell self-renewal agonist, has been shown to expand cord blood stem cells and enhance multilineage blood cell reconstitution in mice. We aimed to investigate the safety and feasibility of single UM171-expanded cord blood transplantation in patients with haematological malignancies who do not have a suitable HLA-matched donor.

METHODS

This single-arm, open-label, phase 1-2 safety and feasibility study was done at two hospitals in Canada. The study had two parts. In part 1, patients received two cord blood units (one expanded with UM171 and one unmanipulated cord blood) until UM171-expanded cord blood demonstrated engraftment. Once engraftment was documented we initiated part 2, reported here, in which patients received a single UM171-expanded cord blood unit with a dose de-escalation design to determine the minimal cord blood unit cell dose that achieved prompt engraftment. Eligible patients were aged 3-64 years, weighed 12 kg or more, had a haematological malignancy with an indication for allogeneic hematopoietic stem cell transplant and did not have a suitable HLA-matched donor, and a had a Karnofsky performance status score of 70% or more. Five clinical sites were planned to participate in the study; however, only two study sites opened, both of which only treated adult patients, thus no paediatric patients (aged <18 years) were recruited. Patients aged younger than 50 years without comorbidities received a myeloablative conditioning regimen (cyclophosphamide 120 mg/kg, fludarabine 75 mg/m², and 12 Gy total body irradiation) and patients aged older than 50 years and those with comorbidities received a less myeloablative conditioning regimen (cyclophosphamide 50 mg/kg, thiotepa 10 mg/kg, fludarabine 150 mg/m², and 4 Gy total body irradiation). Patients were infused with the 7-day UM171-expanded CD34-positive cells and the lymphocyte-containing CD34-negative fraction. The primary endpoints were feasibility of UM171 expansion, safety of the transplant, kinetics of hematopoietic reconstitution (time to neutrophil and platelet engraftment) of UM171-expanded cord blood, and minimal pre-expansion cord blood unit cell dose that achieved prompt engraftment. We analysed feasibility in all enrolled patients and all other primary outcomes were analysed per protocol, in all patients who received single UM171-expanded cord blood transplantation. This trial has been completed and was registered with ClinicalTrials.gov, NCT02668315.

FINDINGS

Between Feb 17, 2016, and Nov 11, 2018, we enrolled 27 patients, four of whom received two cord blood units for safety purposes in part 1 of the study. 23 patients were subsequently enrolled in part 2 to receive a single UM171-expanded cord blood transplant and 22 patients received a single UM171-expanded cord blood transplantation. At data cutoff (Dec 31, 2018), median follow-up was 18 months (IQR 12-22). The minimal cord blood unit cell dose at thaw that achieved prompt engraftment as a single cord transplant after UM171 expansion was 0·52 × 10⁵ CD34-positive cells. We successfully expanded 26 (96%) of 27 cord blood units with UM171. Among the 22 patients who received single UM171-expanded cord blood transplantation, median time to engraftment of 100 neutrophils per μL was 9·5 days (IQR 8-12), median time to engraftment of 500 neutrophils per μL was 18 days (12·5-20·0), and no graft failure occurred. Median time to platelet recovery was 42 days (IQR 35-47). The most common non-haematological adverse events were grade 3 febrile neutropenia (16 [73%] of 22 patients) and bacteraemia (nine [41%]). No unexpected adverse events were observed. One (5%) of 22 patients died due to treatment-related diffuse alveolar haemorrhage.

INTERPRETATION

Our preliminary findings suggest that UM171 cord blood stem cell expansion is feasible, safe, and allows for the use of small single cords without compromising engraftment. UM171-expanded cord blood might have the potential to overcome the disadvantages of other cord blood transplants while maintaining the benefits of low risk of chronic GVHD and relapse, and warrants further investigation in randomised trials.

FUNDING

Canadian Institutes of Health Research, Canadian Cancer Society and Stem Cell Network.

Mast Cells Are Mediators of Fibrosis and Effector Cell Recruitment in Dermal Chronic Graft-vs.-Host Disease

Allogeneic hematopoietic stem cell transplant (allo-HSCT) is often used to treat acute leukemia or defects of hematopoiesis. Its widespread use is hampered by graft-vs.-host disease (GVHD), which has high morbidity and mortality in both acute and chronic subtypes. Chronic GVHD (cGVHD) occurs most frequently in skin and often is characterized by pathogenic fibrosis. Mast cells (MCs) are known to be involved in the pathogenesis of other fibrotic diseases. In a murine model of cGVHD after allo-HSCT, C57BL/6J recipients of allogeneic LP/J donor cells develop sclerodermatous dermal cGVHD which is significantly decreased in mast cell-deficient B6.Cg-Kit^W-sh/HNihrJaeBsmGlliJ recipients. The presence of MCs is associated with fibrosis, chemokine production, and recruitment of GVHD effector cells to the skin. Chemokine production by MCs is blocked by drugs used to treat cGVHD. The importance of MCs in skin cGVHD is mirrored by increased MCs in the skin of patients with dermal cGVHD. We show for the first time a role for MCs in skin cGVHD that may be targetable for preventive and therapeutic intervention in this disease.

Mast cells contribute to the induction of ocular mucosal alloimmunity

Beyond their historical role as the effector cells in allergic disorders, mast cells have been implicated in regulating both innate and adaptive immune responses. Possessing considerable functional plasticity, mast cells are abundant at mucosal surfaces, where the host and external environments interface. The purpose of this study was to evaluate the contribution of mast cells to allograft rejection at the ocular surface. Using a well-characterized murine model of corneal transplantation, we report that mast cells promote allosensitization. Our data show mast cell frequencies and activation are increased following transplantation. We demonstrate that mast cell inhibition (a) limits the infiltration of inflammatory cells and APC maturation at the graft site; (b) reduces allosensitization and the generation of Th1 cells in draining lymphoid tissues; (c) decreases graft infiltration of alloimmune-inflammatory cells; and (d) prolongs allograft survival. Our data demonstrate a novel function of mast cells in promoting allosensitization at the ocular surface.

Mast cells as sources of cytokines, chemokines, and growth factors

Mast cells are hematopoietic cells that reside in virtually all vascularized tissues and that represent potential sources of a wide variety of biologically active secreted products, including diverse cytokines and growth factors. There is strong evidence for important non-redundant roles of mast cells in many types of innate or adaptive immune responses, including making important contributions to immediate and chronic IgE-associated allergic disorders and enhancing host resistance to certain venoms and parasites. However, mast cells have been proposed to influence many other biological processes, including responses to bacteria and virus, angiogenesis, wound healing, fibrosis, autoimmune and metabolic disorders, and cancer. The potential functions of mast cells in many of these settings is thought to reflect their ability to secrete, upon appropriate activation by a range of immune or non-immune stimuli, a broad spectrum of cytokines (including many chemokines) and growth factors, with potential autocrine, paracrine, local, and systemic effects. In this review, we summarize the evidence indicating which cytokines and growth factors can be produced by various populations of rodent and human mast cells in response to particular immune or non-immune stimuli, and comment on the proven or potential roles of such mast cell products in health and disease.

Mast cells participate in allograft rejection: can IL-37 play an inhibitory role?

OBJECTIVE

The aim of this study was to evaluate the role of mast cells (MCs) in allograft rejection, eventually inhibited by IL-37. Immune cells including MCs participate in allograft rejection by generating IL-1, IL-33, TNF and other cytokines.

METHODS

We evaluated allograft rejection on the experience of our experimental data and using the relevant literature.

RESULTS

MCs are involved in initiation and regulation of innate and adaptive immune responses-pathways. MCs are important pro-inflammatory cells which express high-affinity receptor FceRI and can be activated by IgE and some pro-inflammatory cytokines, such as IL-1 and IL-33. The cross-linkage of high affinity IgE receptor on MCs by antigen ligation has a crucial role in allergy, asthma, anaphylaxis, cancer and allograft rejection. MCs mediate immunity in organ transplant, leading to the activation of allospecific T cells implicated in the rejection and generate pro-inflammatory cytokines/chemokines. IL-1 pro-inflammatory cytokine family members released by MCs mediate allograft rejection and inflammation. IL-37 is also an IL-1 family member generated by macrophage cell line in small amounts, which binds to IL-18Rα and produces an anti-inflammatory effect. IL-37 provokes the inhibition of TLR signaling, TLR-induced mTOR and (MyD88)-mediated responses, suppressing pro-inflammatory IL-1 family members and increasing IL-10.

CONCLUSION

IL-37 inhibition offers the opportunity to immunologically modulate MCs, by suppressing their production of IL-1 family members and reducing the risk of allograft rejection, resulting as a potential good therapeutic new cytokine. Here, we report the relationship between inflammatory MCs, allograft rejection and pro-inflammatory and anti-inflammatory IL-37.

Mast Cells Exert Anti-Inflammatory Effects in an IL10-/- Model of Spontaneous Colitis

Mast cells are well established as divergent modulators of inflammation and immunosuppression, but their role in inflammatory bowel disease (IBD) remains to be fully defined. While previous studies have demonstrated a proinflammatory role for mast cells in acute models of chemical colitis, more recent investigations have shown that mast cell deficiency can exacerbate inflammation in spontaneous colitis models, thus suggesting a potential anti-inflammatory role of mast cells in IBD. Here, we tested the hypothesis that in chronic, spontaneous colitis, mast cells are protective. We compared colitis and intestinal barrier function in IL10^−/− mice to mast cell deficient/IL10^−/− (double knockout (DKO): Kit^Wsh/Wsh × IL10^−/−) mice. Compared with IL10^−/− mice, DKO mice exhibited more severe colitis as assessed by increased colitis scores, mucosal hypertrophy, intestinal permeability, and colonic cytokine production. PCR array analyses demonstrated enhanced expression of numerous cytokine and chemokine genes and downregulation of anti-inflammatory genes (e.g., Tgfb2, Bmp2, Bmp4, Bmp6, and Bmp7) in the colonic mucosa of DKO mice. Systemic reconstitution of DKO mice with bone marrow-derived mast cells resulted in significant amelioration of IL10^−/−-mediated colitis and intestinal barrier injury. Together, the results presented here demonstrate that mast cells exert anti-inflammatory properties in an established model of chronic, spontaneous IBD. Given the previously established proinflammatory role of mast cells in acute chemical colitis models, the present findings provide new insight into the divergent roles of mast cells in modulating inflammation during different stages of colitis. Further investigation of the mechanism of the anti-inflammatory role of the mast cells may elucidate novel therapies.

Studies of Mast Cells: Adventures in Serendipity

Like many of us who had the great fortune to work with Bill Paul, my science life was immeasurably altered by my interactions with him. Although intimidating at first because of his stature in the immunology world, it was soon clear that he not only truly cared about the specific research we were doing together, but he wished to convey to his trainees an approach to science that was open, always questioning, and infinitely fun. His enthusiasm was infectious and after my training with him, despite stresses due to funding and publishing hurdles, I never regretted the path I took. My research took a sharp turn from the studies of adaptive immunity I had planned on pursuing after my fellowship with Bill to a life long quest to understand the wonders of the mast cell, a relatively rare innate immune cell. This came about because Bill’s curiosity and expectation of the unexpected allowed him to view, in retrospect, a rather mundane observation we made together involving a non-physiological transformed mast cell line as something that might be really interesting. I have never forgotten that lesson: Look at the data with an eye on the big picture. Sometimes the unexpected is more interesting than predicted results. His example in this regard was incredibly important when as an independent investigator a mistake in mouse sex determination led to unexpected and very confusing data. Yet, these data ultimately revealed a role for mast cells in male-specific protection in experimental autoimmune encephalomyelitis, the mouse model of multiple sclerosis. Bill’s influence in immunology is far-reaching and will continue to be felt as those of us who train our own students and post-doctoral fellows pass on his wisdom and approach to scientific research.

AllergoOncology: Opposite outcomes of immune tolerance in allergy and cancer

While desired for the cure of allergy, regulatory immune cell subsets and nonclassical Th2-biased inflammatory mediators in the tumour microenvironment can contribute to immune suppression and escape of tumours from immunological detection and clearance. A key aim in the cancer field is therefore to design interventions that can break immunological tolerance and halt cancer progression, whereas on the contrary allergen immunotherapy exactly aims to induce tolerance. In this position paper, we review insights on immune tolerance derived from allergy and from cancer inflammation, focusing on what is known about the roles of key immune cells and mediators. We propose that research in the field of AllergoOncology that aims to delineate these immunological mechanisms with juxtaposed clinical consequences in allergy and cancer may point to novel avenues for therapeutic interventions that stand to benefit both disciplines.

Human mast cells present antigen to autologous CD4+ T cells

BACKGROUND

Mast cells (MCs), the primary effector cell of the atopic response, participate in immune defense at host/environment interfaces, yet the mechanisms by which they interact with CD4⁺ T cells has been controversial.

OBJECTIVE

We used in situ-matured primary human MCs and matched CD4⁺ T cells to diligently assess the ability of MCs to act as antigen-presenting cells.

METHODS

We examined mature human skin-derived MCs using flow cytometry for expression of antigen-presenting molecules, for their ability to stimulate CD4⁺ T cells to express CD25 and proliferate when exposed to superantigen or to cytomegalovirus (CMV) antigen using matched T cells and MCs from CMV-seropositive or CMV-seronegative donors, and for antigen uptake. Subcellular localization of antigen, HLA molecules, and tryptase was analyzed by using structured illumination microscopy.

RESULTS

Our data show that IFN-γ induces HLA class II, HLA-DM, CD80, and CD40 expression on MCs, whereas MCs take up soluble and particulate antigens in an IFN-γ-independent manner. IFN-γ-primed MCs guide activation of T cells by Staphylococcus aureus superantigen and, when preincubated with CMV antigens, induce a recall CD4⁺ T_H1 proliferation response only in CMV-seropositive donors. MCs co-opt their secretory granules for antigen processing and presentation. Consequently, MC degranulation increases surface delivery of HLA class II/peptide, further enhancing stimulation of T-cell proliferation.

CONCLUSIONS

IFN-γ primes human MCs to activate T cells through superantigen and to present CMV antigen to T_H1 cells, co-opting MC secretory granules for antigen processing and presentation and creating a feed-forward loop of T-cell-MC cross-activation.

Mast cells in rheumatoid arthritis: friends or foes?

Mast cells are tissue-resident cells of the innate immunity, implicated in the pathogenesis of many autoimmune diseases, including rheumatoid arthritis (RA). They are present in synovia and their activation has been linked to the potentiation of inflammation in the course of RA. However, recent investigations questioned the role of mast cells in arthritis. In particular, animal models generated conflicting results, so that many of their pro-inflammatory, i.e. pro-arthritogenic functions, even though supported by robust experimental evidence, have been labelled as redundant. At the same time, a growing body of evidence suggests that mast cells can act as tunable immunomodulatory cells. These characteristics, not yet fully understood in the context of RA, could partially explain the inconsistent results obtained with experimental models, which do not account for the pro- and anti-inflammatory functions exerted in more chronic heterogeneous conditions such as RA. Here we present an overview of the current knowledge on mast cell involvement in RA, including the intriguing hypothesis of mast cells acting as subtle immunomodulatory cells and the emerging concept of synovial mast cells as potential biomarkers for patient stratification.

The gastrointestinal tract: properties and role in allogeneic hematopoietic stem cell transplantation

INTRODUCTION

The GI-tract is a major target for both the intensive chemo and/or radiotherapy conditioning as well as for GVHD and therefore is closely associated with transplant outcome. Apart from being a target, the GI-tract is also a mediator and therefore is also a key player of the pathogenetic process following allogeneic transplantation. Areas covered: The intestinal homeostasis is regulated through complicated interactions between the key players of this process which are the intestinal epithelium, the intestinal immune system, and the intestinal microbiota. A brief description of these elements, based on published english-language articles in PubMed, as well as their role during the process of allo-HSCT is discussed in this review. Expert commentary: Data on GI-tract properties suggest a central role for the intestine in regulation of immunity, both in healthy - steady state conditions and in pathological states such as during allo-HSCT. Given the fact that in the allogeneic transplant setting severe complications such as infections and GVHD are limiting this treatment modality, understanding the mechanisms that mediate intestinal homeostasis could lead to new preventive methods and improved outcomes.

The origin of DCs and capacity for immunologic tolerance in central and peripheral tissues

Dendritic cells (DCs) are specialized immune sentinels that play key role in maintaining immune homeostasis by efficiently regulating the delicate balance between protective immunity and tolerance to self. Although DCs respond to maturation signals present in the surrounding milieu, multiple layers of suppression also co-exist that reduce the infringement of tolerance against self-antigens. These tolerance inducing properties of DCs are governed by their origin and a range of other factors including distribution, cytokines, growth factors, and transcriptional programing, that collectively impart suppressive functions to these cells. DCs directing tolerance secrete anti-inflammatory cytokines and induce naïve T cells or B cells to differentiate into regulatory T cells (Tregs) or B cells. In this review, we provide a detailed outlook on the molecular mechanisms that induce functional specialization to govern central or peripheral tolerance. The tolerance-inducing nature of DCs can be exploited to overcome autoimmunity and rejection in graft transplantation.

ST2 blockade reduces sST2-producing T cells while maintaining protective mST2-expressing T cells during graft-versus-host disease

Graft-versus-host disease (GVHD) remains a devastating complication after allogeneic hematopoietic cell transplantation (HCT). We previously identified high plasma soluble suppression of tumorigenicity 2 (sST2) as a biomarker of the development of GVHD and death. sST2 sequesters interleukin-33 (IL-33), limiting its availability to T cells expressing membrane-bound ST2 (mST2) [T helper 2 (TH2) cells and ST2(+)FoxP3(+) regulatory T cells]. We report that blockade of sST2 in the peritransplant period with a neutralizing monoclonal antibody (anti-ST2 mAb) reduced GVHD severity and mortality. We identified intestinal stromal cells and T cells as major sources of sST2 during GVHD. ST2 blockade decreased systemic interferon-γ, IL-17, and IL-23 but increased IL-10 and IL-33 plasma levels. ST2 blockade also reduced sST2 production by IL-17-producing T cells while maintaining protective mST2-expressing T cells, increasing the frequency of intestinal myeloid-derived suppressor cells, and decreasing the frequency of intestinal CD103 dendritic cells. Finally, ST2 blockade preserved graft-versus-leukemia activity in a model of green fluorescent protein (GFP)-positive MLL-AF9 acute myeloid leukemia. Our findings suggest that ST2 is a therapeutic target for severe GVHD and that the ST2/IL-33 pathway could be investigated in other T cell-mediated immune disorders with loss of tolerance.

Regulatory roles of mast cells in immune responses

Mast cells are important immune cells for host defense through activation of innate immunity (via toll-like receptors or complement receptors) and acquired immunity (via FcεRI). Conversely, mast cells also act as effector cells that exacerbate development of allergic or autoimmune disorders. Yet, several lines of evidence show that mast cells act as regulatory cells to suppress certain inflammatory diseases. Here, we review the mechanisms by which mast cells suppress diseases.

Tolerogenic IDO(+) Dendritic Cells Are Induced by PD-1-Expressing Mast Cells

Mast cells (MCs) are tissue resident cells, rich in inflammatory mediators, involved in allergic reactions, and with an increasingly recognized role in immunomodulation. Dendritic cells (DCs), on the other hand, are central to the determination of immune response patterns, being highly efficient antigen-presenting cells that respond promptly to changes in their microenvironment. Here, we show that direct cell contact between immature monocyte-derived DCs (iDCs) and MC bends DCs toward tolerance induction. DCs that had direct contact with MC (MC-iDC) decreased HLA-DR but increased PD-L1 expression and stimulated regulatory T lymphocytes, which expresses FoxP3(+), secrete TGF-β and IL-10, and suppress the proliferation of mitogen-stimulated naïve T lymphocytes. Furthermore, MC-iDC expressed higher levels of indoleamine-2,3-deoxigenase (IDO), a phenomenon that was blocked by treatment of MC with anti-PD-1 or by the treatment of DCs with anti-PD-L1 or anti-PD-L2, but not by blocking of H1 and H2 histamine receptors on DCs. Contact with MC also increased phosphorylated STAT-3 levels in iDCs. When a STAT-3 inhibitor, JSI-124, was added to the DCs before contact with MC, the MC-iDC recovered their ability to induce allogeneic T cell proliferation and did not increase their IDO expression.

An Interleukin-33-Mast Cell-Interleukin-2 Axis Suppresses Papain-Induced Allergic Inflammation by Promoting Regulatory T Cell Numbers

House dust mite-derived proteases contribute to allergic disorders in part by disrupting epithelial barrier function. Interleukin-33 (IL-33), produced by lung cells after exposure to protease allergens, can induce innate-type airway eosinophilia by activating natural helper (NH) cells, a member of group 2 innate lymphoid cells (ILC2), to secrete Th2 type-cytokines. Because IL-33 also can induce mast cells (MCs) to secrete Th2 type-cytokines, MCs are thought to cooperate with NH cells in enhancing protease or IL-33-mediated innate-type airway eosinophilia. However, we found that MC-deficient Kit(W-sh/W-sh) mice exhibited exacerbated protease-induced lung inflammation associated with reduced numbers of regulatory T (Treg) cells. Moreover, IL-2 produced by IL-33-stimulated MCs promoted expansion of numbers of Treg cells, thereby suppressing development of papain- or IL-33-induced airway eosinophilia. We have thus identified a unique anti-inflammatory pathway that can limit induction of innate-type allergic airway inflammation mediated by NH cells.

Potential effector and immunoregulatory functions of mast cells in mucosal immunity

Mast cells (MCs) are cells of hematopoietic origin that normally reside in mucosal tissues, often near epithelial cells, glands, smooth muscle cells, and nerves. Best known for their contributions to pathology during IgE-associated disorders such as food allergy, asthma, and anaphylaxis, MCs are also thought to mediate IgE-associated effector functions during certain parasite infections. However, various MC populations also can be activated to express functional programs--such as secreting preformed and/or newly synthesized biologically active products--in response to encounters with products derived from diverse pathogens, other host cells (including leukocytes and structural cells), damaged tissue, or the activation of the complement or coagulation systems, as well as by signals derived from the external environment (including animal toxins, plant products, and physical agents). In this review, we will discuss evidence suggesting that MCs can perform diverse effector and immunoregulatory roles that contribute to homeostasis or pathology in mucosal tissues.

Emerging functions of amphiregulin in orchestrating immunity, inflammation, and tissue repair

Type 2 inflammatory responses can be elicited by diverse stimuli, including toxins, venoms, allergens, and infectious agents, and play critical roles in resistance and tolerance associated with infection, wound healing, tissue repair, and tumor development. Emerging data suggest that in addition to characteristic type 2-associated cytokines, the epidermal growth factor (EGF)-like molecule Amphiregulin (AREG) might be a critical component of type 2-mediated resistance and tolerance. Notably, numerous studies demonstrate that in addition to the established role of epithelial- and mesenchymal-derived AREG, multiple leukocyte populations including mast cells, basophils, group 2 innate lymphoid cells (ILC2s), and a subset of tissue-resident regulatory CD4(+) T cells can express AREG. In this review, we discuss recent advances in our understanding of the AREG-EGF receptor pathway and its involvement in infection and inflammation and propose a model for the function of this pathway in the context of resistance and tissue tolerance.

Mast cells: innate attractors recruiting protective CD8 T cells to sites of cytomegalovirus infection

Reactivation of latent cytomegalovirus (CMV) in the transient immunocompromised state after hematoablative treatment is a major concern in patients undergoing hematopoietic cell transplantation (HCT) as a therapy of hematopoietic malignancies. Timely reconstitution of antiviral CD8 T cells and their efficient recruitment to the lungs is crucial for preventing interstitial pneumonia, the most severe disease manifestation of CMV in HCT recipients. Here, we review recent work in a murine model, implicating mast cells (MC) in the control of pulmonary infection. Murine CMV (mCMV) productively infects MC in vivo and triggers their degranulation, resulting in the release of the CC chemokine ligand 5 (CCL5) that attracts CD8 T cells to infiltrate infected tissues. Comparing infection of MC-sufficient C57BL/6 mice and congenic MC-deficient Kit (W-sh/W-sh) "sash" mutants revealed an inverse relation between the number of lung-infiltrating CD8 T cells and viral burden in the lungs. Specifically, reduced lung infiltration by CD8 T cells in "sash" mutants was associated with an impaired infection control. The causal, though indirect, involvement of MC in antiviral control was confirmed by reversion of the deficiency phenotype in "sash" mutants reconstituted with MC. These recent findings predict that efficient MC reconstitution facilitates the control of CMV infection also in immunocompromised HCT recipients.

Mast cell plasticity and sphingosine-1-phosphate in immunity, inflammation and cancer

Mast cells (MC) are found in all vascularized tissues at homeostasis and, until recently, were viewed only as effector cells of allergic reactions via degranulation, the canonical process through which MC release mediators, including histamine and pre-formed proteases and cytokines such as TNF. Cross-linking of IgE bound to surface high affinity receptors for IgE (FcɛRI) by a specific antigen (Ag) triggers signaling events leading to degranulation. We and others have reported the concomitant production and export of an influential multifaceted sphingolipid mediator, sphingosine-1-phosphate (S1P) transported outside of MC by ATP-binding cassettes (ABC) transporters, i.e., independently of degranulation. Indeed, the MC horizon expanded by the discovery of their unique ability to selectively release mediators depending upon the stimulus and receptors involved. Aside from degranulation and transporter usage, MC are also endowed with piecemeal degranulation, a slower process during which mediator release occurs with minor morphological changes. The broad spectrum of pro- and anti-inflammatory bioactive substances MC produce and release, their amounts and delivery pace render these cells bona fide fine-tuners of the immune response. In this viewpoint article, MC developmental, phenotypic and functional plasticity, its modulation by microRNAs and its relevance to immunity, inflammation and cancer will be discussed.

HPV16-E7 expression in squamous epithelium creates a local immune suppressive environment via CCL2- and CCL5- mediated recruitment of mast cells

Human Papillomavirus (HPV) 16 E7 protein promotes the transformation of HPV infected epithelium to malignancy. Here, we use a murine model in which the E7 protein of HPV16 is expressed as a transgene in epithelium to show that mast cells are recruited to the basal layer of E7-expressing epithelium, and that this recruitment is dependent on the epithelial hyperproliferation induced by E7 by inactivating Rb dependent cell cycle regulation. E7 induced epithelial hyperplasia is associated with increased epidermal secretion of CCL2 and CCL5 chemokines, which attract mast cells to the skin. Mast cells in E7 transgenic skin, in contrast to those in non-transgenic skin, exhibit degranulation. Notably, we found that resident mast cells in E7 transgenic skin cause local immune suppression as evidenced by tolerance of E7 transgenic skin grafts when mast cells are present compared to the rejection of mast cell-deficient E7 grafts in otherwise competent hosts. Thus, our findings suggest that mast cells, recruited towards CCL2 and CCL5 expressed by epithelium induced to proliferate by E7, may contribute to an immunosuppressive environment that enables the persistence of HPV E7 protein induced pre-cancerous lesions.

Tr1 cells and the counter-regulation of immunity: natural mechanisms and therapeutic applications

T regulatory Type 1 (Tr1) cells are adaptive T regulatory cells characterized by the ability to secrete high levels of IL-10 and minimal amounts of IL-4 and IL-17. Recently, CD49b and LAG-3 have been identified as Tr1-cell-specific biomarkers in mice and humans. Tr1 cells suppress T-cell- and antigen-presenting cell- (APC) responses primarily via the secretion of IL-10 and TGF-β. In addition, Tr1 cells release granzyme B and perforin and kill myeloid cells. Tr1 cells inhibit T cell responses also via cell-contact dependent mechanisms mediated by CTLA-4 or PD-1, and by disrupting the metabolic state of T effector cells via the production of the ectoenzymes CD39 and CD73. Tr1 cells were first described in peripheral blood of patients who developed tolerance after HLA-mismatched fetal liver hematopoietic stem cell transplant. Since their discovery, Tr1 cells have been proven to be important in maintaining immunological homeostasis and preventing T-cell-mediated diseases. Furthermore, the possibility to generate and expand Tr1 cells in vitro has led to their utilization as cellular therapy in humans. In this chapter we summarize the unique and distinctive biological properties of Tr1 cells, the well-known and newly discovered Tr1-cell biomarkers, and the different methods to induce Tr1 cells in vitro and in vivo. We also address the role of Tr1 cells in infectious diseases, autoimmunity, and transplant rejection in different pre-clinical disease models and in patients. Finally, we highlight the pathological settings in which Tr1 cells can be beneficial to prevent or to cure the disease.