Regulatory T-cell therapy for inflammatory bowel disease: more questions than answers

Abstract

T regulatory (Treg) cells are critical for maintaining immune homeostasis and establishing tolerance to foreign, non-pathogenic antigens including those found in commensal bacteria and food. Because of their multiple suppressive mechanisms, Tregs represent a promising strategy for engineering tolerance to self and non-self antigens in chronic inflammatory diseases. Already in clinical trials in the transplantation setting, the question remains whether this therapy would be effective for the treatment of mucosal inflammatory diseases that do not pose an immediate threat to life. In this review we will discuss evidence from both animal models and patients suggesting that Treg therapy would be beneficial in the context of inflammatory bowel disease (IBD). We will examine the role of T-cell versus Treg dysfunction in IBD and discuss the putative antigens that could be potential targets of antigen-directed Treg therapy. Finally, the challenges of using Treg therapy in IBD will be discussed, with a specific emphasis on the role that the microbiota may play in the outcome of this treatment. As Treg therapy becomes a bedside reality in the field of transplantation, there is great hope that it will soon also be deployed in the setting of IBD and ultimately prove more effective than the current non-specific immunosuppressive therapies.

Introduction

T regulatory cells (Tregs) play a critical role in maintaining immune homeostasis and limiting autoimmune responses by modulating cells of both the innate and adaptive immune systems. Considered the primary mediators of peripheral tolerance, Tregs regulate self-reactive lymphocytes via a number of mechanisms including secretion of inhibitory cytokines such as interleukin-10 (IL-10) and transforming growth factor-β (TGF-β), granzyme-mediated cytolysis, CTLA-4 expression, metabolic disruption and dendritic cell targeting (reviewed in refs. 1–3). Classically defined Tregs are found within the CD4⁺ T-cell pool and are identified by their constitutive expression of FoxP3, and, often, the IL-2 receptor α-chain (CD25).⁴ Numerous studies have shown that FoxP3-expressing Tregs can be divided into two distinct subsets: naturally occurring Tregs (nTregs) which develop in the thymus via central tolerance mechanisms and peripherally induced Tregs (iTregs) which differentiate from naive T cells when self or non-self antigen is encountered in the periphery under tolerogenic conditions.^5,6 A third distinct subset of Tregs, referred to as type 1 regulatory (Tr1) cells, do not constitutively express FoxP3 and are induced in the periphery in the presence of IL-10 and/or specialized subsets of antigen-presenting cells.⁷ In contrast to FoxP3⁺ Tregs, there is currently no known lineage-defining transcription factor for Tr1 cells, and they are identified solely on the basis of their cytokine production profile (IL-10⁺ IL-4⁻ interferon-γ^low) as well as their IL-10-dependent suppression of immune responses.⁷

Because of their potent, antigen-specific suppressive capacity, both FoxP3⁺ Tregs and Tr1 cells may be promising candidates for immune therapy in a variety of chronic inflammatory diseases, including inflammatory bowel disease (IBD). The hope is that boosting this natural mechanism of tolerance will offer a replacement for the broad-spectrum immunosuppressive drugs that are often ineffective and carry the risk of promoting cancer or infections. Pioneering studies by Powrie et al.^8,9 demonstrated that the pathology in a mouse model of T-cell-induced colitis, which mimics human IBD, can indeed be prevented by adoptive transfer of FoxP3⁺ Tregs. Furthermore, Tregs can not only prevent but also cure IBD¹⁰ in mouse models. Because IBD varies greatly between mice and humans,¹¹ however, there are many outstanding questions that need to be addressed as this therapy is translated to the clinical setting.

Because of the risks associated with using cells as a therapy to control immune responses, the first clinical trials with Tregs are taking place in the context of allogeneic haematopoietic stem cell transplantation for haematological malignancies. These patients are at high risk for life-threatening graft-versus-host disease so there is a better risk–benefit ratio for experimental therapies than in IBD. Phase I/II clinical trials have already begun to evaluate whether infusion of Tregs might ameliorate graft-versus-host disease following haematopoietic stem cell transplantation,^12–15 and these trials have so far shown that infusion of Tregs is safe and possibly efficacious. These results set the stage for future randomized clinical trials to determine whether Treg therapy is an effective front-line means of establishing immune tolerance upon transplantation of allogeneic cells or tissues.¹⁶

This review will examine evidence that Treg dysfunction contributes to the perpetuation of IBD, and discuss the strengths and limitations of Treg therapy in this setting. Because Treg therapy offers the advantage of antigen specificity and could circumvent the need for global, long-term immunosuppression, the possible antigens that drive mucosal disease will also be examined as putative targets in this strategy.

The role of T cells in IBD

The intestinal mucosal tissues pose a unique challenge for the maintenance of immune homeostasis. Representing the largest mucosal surface area in the body, these tissues are in direct contact with the external environment and must simultaneously maintain tolerance to commensal bacteria and food, and the ability to eliminate pathogens.¹⁷ Furthermore, the gut must be permeable, to allow for nutrient absorption, while maintaining a tight barrier against pathogens. The gut has therefore developed a complex immune network that can process and respond to an enormous number of stimuli at one time. This network includes intestinal epithelial cells, macrophages, dendritic cells, conventional T cells, and Tregs, with the latter believed to be critical for the maintenance of intestinal immune homeostasis.¹⁸

Inflammatory bowel disease, an umbrella term for both Crohn’s disease and ulcerative colitis, is thought to be caused by barrier disruption leading to a change in the intestinal flora and a consequent aberrant activation of the mucosal immune system.^19–21 In both diseases, intestinal epithelial cells isolated from patients directly activate CD4⁺ T cells,²² suggesting that non-immune cells directly contribute to the inappropriate immune activation. The resulting chronic inflammatory response is highly dependent on the presence of CD4⁺ T helper (Th) cells: transfer of CD4⁺ T cells from normal mice into immune-deficient mice (which lack Tregs) leads to severe Th1 cell driven colitis.^8,9 Similarly, in humans, correlative data suggest that Crohn’s disease is driven by exaggerated Th1 and Th17 responses, because inflamed lesions contain increased levels of Th1-associated and Th17-associated cytokines including interferon-γ, IL-12, IL-17 and IL-18.^23–27 In contrast, although ulcerative colitis is in the same family of diseases, it is associated with a Th2 cell profile, and patients have high levels of IL-13 in the mucosa compared with Crohn’s disease patients or healthy controls.^19,23,28 Hence, although in most cases T-cell dysfunction is unlikely to be the initiating cause of IBD,²⁹ there is substantial evidence that dysregulated Th cell responses perpetuate the disease and the vicious cycle of chronic inflammation.

Are Tregs dysfunctional in IBD?

Under normal conditions, compared with all other tissues, the intestinal lamina propria has the greatest proportion of CD4⁺ Tregs,³⁰ which are thought to be primarily specific for antigens in food and commensal flora.²⁹ As Crohn’s disease and ulcerative colitis are both T-cell-driven diseases, it logically follows that increasing appropriate Treg activity in the gut should help to restore the balance of suppression in inflamed tissues. However, it is unknown whether the over-abundance of activated T cells in IBD is the result of a numerical lack of Tregs, a defect in their function, resistance of T effector cells to suppression, or a combination of these possibilities. These questions have not been widely studied in animal models, yet they are key to understanding whether restoring/boosting Tregs is likely to have any effect in treating IBD in humans.

There is evidence that simply lacking Tregs leads to IBD. Patients with genetic mutations in FoxP3 who have non-functional or absent Tregs always have severe intestinal inflammation associated with lymphocytic infiltration of the intestinal mucosa.^31,32 Similarly, mice lacking FoxP3⁺ Tregs,³³ or the ability to suppress via Treg-derived cytokines such as IL-10,^34,35 IL-35,³⁶ and in some cases TGF-β,³⁷ develop severe colitis. In the more common forms of IBD, however, there is little evidence to suggest that patients simply lack Tregs in the circulation and/or the affected tissues. Maul et al.³⁸ found that although both Crohn’s disease and ulcerative colitis patients had decreased Treg populations in the peripheral blood during active disease, Treg numbers in intestinal tissue biopsies were not substantially different from those in patients with other inflammatory diseases. Other studies corroborate these results, and in most cases show a consistent expansion of Tregs in both inflamed and non-inflamed sections of the gut in adult and paediatric patients with IBD.^39–41 Notably, the majority of these studies did not take into consideration the possibility that FoxP3 expression can also identify activated effector T cells⁴² or the possibility that FoxP3-expressing cells can trans-differentiate into inflammatory cytokine producing cells that may or may not retain their suppressive function.⁴³ The question of whether or not Tregs are numerically deficient in IBD therefore warrants re-investigation using more comprehensive panels of cell surface markers and cytokines.

There is also little evidence to support the possibility that intestinal Tregs are dysfunctional in IBD because Tregs isolated from the intestinal mucosa of patients with IBD are suppressive in vitro.^38,40 On the other hand, there is evidence that Tregs from inflamed colonic tissue undergo apoptosis more readily than Tregs found in non-inflamed tissue, possibly rendering the Tregs less effective.⁴⁴ It is important to note, however, that the functional Treg assays in these studies were performed using non-specific antigen stimulation in conditions lacking many of the cytokines that would be found in the inflamed intestinal environment. Moreover, to date only suppression of T-cell responses has been examined, and the possibility that Tregs from IBD patients may lack the ability to suppress other cell types, such as antigen-presenting cells or B cells, has yet to be investigated. Hence whether or not the inflamed mucosal environment renders Tregs dysfunctional remains unknown, as does what would happen to Tregs – i.e. would they remain suppressive – if they were administered as a cellular therapy.

If the inflamed intestine has a normal number of Tregs which, at least in vitro, appear to be functional, then why are they unable to block inflammation? In other autoimmune diseases, including type 1 diabetes and multiple sclerosis, there is extensive evidence suggesting that the defect in immune regulation lies within the effector cell/inflammatory environment and not the Tregs themselves.⁴⁵ In IBD the question of whether effector T cells show abnormal resistance to suppression in IBD has not yet been comprehensively studied but there are some studies suggesting that this may be the case. In colitic mice and humans effector T cells can be resistant to Tregs if they become insensitive to TGF-β-mediated suppression.^46,47 How the inflamed intestinal environment affects the result of Treg activity is a major outstanding question: addition of more Treg cells to a setting that is resistant to their effects may be futile.

Treg subsets as cellular therapy in IBD

All Tregs are ultimately defined by their ability to suppress immune responses; however, nTregs, iTregs and Tr1 cells may differ in the suppressive mechanisms they employ and so have distinct advantages as therapies in mucosal diseases. nTregs are the best-studied type of Tregs and have already been successfully used in humans,^12–15 but as these cells are primarily thought to be specific for self-antigens⁴⁸ they may lack potency towards immune responses directed to the foreign antigens present in the gut. It may be possible to expand nTregs specific for intestinal antigens using antigen-expressing intestinal epithelial cells,⁴⁹ but the clinical utility of such cells remains to be validated in humans given the significant technical challenges of maintaining nTreg purity during rapid expansion from small cell numbers.¹⁶ An alternative approach to expansion of nTregs in vitro may be to use biological therapies such as anti-tumour necrosis factor-α antibodies so as to maximize the function of nTregs in vivo.^9,50

The development of iTregs for clinical applications might provide a superior alternative in IBD. In mouse models, iTregs are known to prevent T-cell driven colitis,³⁷ and it may be easier to generate cells specific for relevant antigens using this approach. In addition to differentiation using compounds such as TGF-β and rapamycin,⁵¹ iTregs can be generated when naive T cells are stimulated in vitro by tolerogenic dendritic cells, which are from the intestine and induce antigen-specific FoxP3⁺ Tregs in a TGF-β and retinoic acid dependent manner.^52–55 A slight variation on this strategy would be to use vitamin A or its derivative, retinoic acid, to directly enhance tolerance and the generation of iTregs in the intestine in vivo.^21,56 Antigens could also be targeted to tolerogenic intestinal dendritic cells in vivo using a single-chain antibody specific for unique cell surface makers as a delivery system.⁵⁷ This latter strategy is thought to mimic the natural process of oral tolerance where antigens are presented by tolerogenic dendritic cells⁵⁸ and so may generate more effective and stable populations of antigen-specific iTregs in comparison with in vitro-derived cells.

In addition to FoxP3⁺ Tregs, Tr1 cells are also candidates for cellular therapy in mucosal diseases. The intestinal environment naturally relies on IL-10 for the maintenance of immune homeostasis; in mouse models, IL-10 secretion by myeloid intestinal cells is required to maintain Treg suppressive capacity,⁵⁹ and Tregs themselves must secrete IL-10 to prevent colitis.^18,34,35 In a therapeutic setting, subcutaneous delivery of human recombinant IL-10 produced disappointing clinical results, but this was probably the result of protein degradation and a suboptimal route of delivery.^7,60 An alternative strategy, delivering IL-10 to the target environment using genetically modified bacteria, is currently being tested in humans.⁶¹ Tr1-mediated delivery of IL-10, however, should offer a therapeutic advantage over direct protein delivery because of the possibility of delivering antigen-specific suppression. Following studies in mice showing that ovalbumin (OVA)-specific Tr1 cells prevent colitis following transfer of polyclonal T cells, a Phase I/II clinical trial was initiated to test if OVA-specific Tr1 cell clones could also treat refractory Crohn’s disease.⁶² The trial is based on the concept that Tregs mediate bystander suppression and infectious tolerance,⁶³ so the hope is that upon antigen recognition in vivo, the OVA-specific Tr1 cells will secrete IL-10 and tolerize all T cells, irrespective of their antigen-specificity, in the local environment. Tr1 cell clone administration was tolerated and showed dose-dependent efficacy in patients suffering from severe disease.⁶² These data represent the first bench-to-bedside test of Tregs as a therapy for IBD and set the stage for more comprehensive trials.

Intestinal antigen-specific Treg cell therapy

Recent work in the field of transplantation and autoimmunity has shown that antigen-specific Tregs are much more effective at preventing graft rejection or diabetes than are polyclonal populations;¹⁶ significantly fewer antigen-specific Tregs are required to mediate potent suppression, and the delivery of antigen-specific cells decreases the risk of global immunosuppression and the possibility of increased risk of infection and cancer. Notably, antigen-specific Tregs can prevent colitis, as demonstrated by the adoptive transfer of OVA-specific Tregs⁶⁴ or Tr1 cells,⁶⁵ but because OVA is unlikely to be a disease-driving antigen in IBD, the question of whether OVA-specific Tregs would be effective at suppressing established effector responses directed at pathogenic antigens remains outstanding.

To develop antigen-specific Treg therapy at least some of the dominant antigens that perpetuate effector T-cell responses in the intestine need to be identified. Using T-cell clones isolated from IBD patients, Duchmann et al.⁶⁶ found that many of the clones were specific for commensal gut flora, including species of Enterobacteriaceae, Bacteroides and Bifidobacterium. Corroborating these data, Cong et al.⁶⁷ found that T cells specific for enteric bacterial flora drive disease in spontaneously colitic C3H/HeJBir mice. It was subsequently demonstrated that bacterial flagellin, a protein present on all flagellated bacteria including commensal species found in the gut, is a dominant antigen in these mice. In addition, flagellin expressed by a Clostridium species, known as CBir, is targeted by antibodies in colitic mice and humans,⁶⁸ and transfer of CBir-specific CD4⁺ T-cell lines into immunodeficient mice causes severe colitis.⁶⁸ Further evidence that T cells that recognize flagellin are relevant in colitis comes from studies with Escherichia coli-derived flagellin, the delivery of which exacerbates dextran sodium sulphate-induced colitis in a TLR5-independent manner.^69,70 Although this is a new and rapidly evolving field, these data collectively suggest bacterial flagellin as a candidate antigen to target for Treg cellular therapy of IBD.

Although dominant antigens that drive IBD are still being discovered, and there are likely to be many different disease-relevant antigens, antigen-directed Treg therapy could currently be tested in a chronic inflammatory gastrointestinal disease that shares similar defects in immune regulation to IBD. Coeliac disease is a chronic immune-mediated inflammatory disorder initiated by wheat gliadin and related proteins in barley and rye.⁷¹ This disease occurs primarily in genetically predisposed individuals,⁷¹ and, like IBD, the immunopathogenesis involves both innate and adaptive immune mechanisms.⁷² Also similar to IBD, patients suffering from untreated coeliac disease have increased numbers of FoxP3⁺ Tregs and IL-10-producing Tr1 cells in the intestine,^73–77 the latter known to be gliadin specific.⁷⁸ The failure of Tregs to control inflammation in this disease may therefore be a consequence of their functional impairment or target resistance. Circulating FoxP3⁺ CD4⁺ T cells from patients with active coeliac disease do not efficiently inhibit autologous effector T cells, but they are functional when co-cultured with T cells from healthy donors.⁷⁷ Moreover, Tregs from healthy adults fail to suppress effector T cells isolated from coeliac patients.⁷⁷ Analogous to the data from IBD studies, these data suggest that in coeliac disease the immune defect is not intrinsic to the Tregs, but rather is related to the resistance of effector T cells to suppression. Coeliac disease therefore represents an ideal setting in which to test whether antigen-specific Treg cell therapy can reverse established mucosal disease. Not only is the antigen well-defined, but it could also be administered and removed as necessary. The availability of tetramers to track gliadin-specific T-cell responses would also allow quantitative monitoring of crucial components of the response to therapy in these patients.⁷⁹

The intestinal microbiota as a consideration in Treg therapy

Inflammatory bowel disease is thought to be a multi-step process involving an initial barrier injury, leading to a shift in the normal intestinal microbiota,^20,80 increasing numbers of Enterobacteriaceae and reducing the species thought to protect from IBD, such as Faecalibacterium and Roseburia.²⁰ The microbiota facilitate post-thymic education of the immune system and are important for tolerance to microbial antigens,⁸¹ so changes in the gut flora in IBD may be a driving force for effector T-cell responses against commensal bacteria and must therefore be considered in the context of cellular therapy. Indeed, in mice, colitis does not occur unless microbial antigens are present to drive activation and differentiation of T cells.⁸²

The intestinal microbiota also plays an important role in modulating Tregs. For example, certain species of commensal bacteria specifically promote FoxP3⁺ Tregs in the colon,^83,84 and some species of bacteria induce tolerance by signalling through TLR2 on Tregs.⁸⁵ Hence, depending on the balance of species, microbial communities may either drive pathogenic T-cell responses or induce Tregs in a normal homeostatic environment. It follows that for Treg cellular therapy to be effective in IBD, microbial communities may need to be shifted towards a balance of species that is more permissive of tolerance. One way that the microbiome could be manipulated is by administration of probiotics.⁸⁶ This is an area of intense research and the possibility that co-delivery of probiotics with Tregs may optimally promote long-lasting peaceful immune homeostasis warrants investigation.

Challenges and outstanding questions for Treg therapy in IBD

A number of major questions must be answered before Treg therapy can be contemplated in the context of IBD. If a polyclonal, systemic approach is pursued, would such Treg therapy be any better than current immunosuppressant regimens? If a targeted approach is taken, on the other hand, how would the resultant sudden increase in suppressive mechanisms at the tissue–environment interface affect the risk of infection while preserving a normal balance of commensal flora?

Another caveat is the potential for infused Tregs to transdifferentiate and lose their suppressive function. Although expanded Tregs may be suppressive in vitro, the environmental milieu of inflamed mucosal tissues could substantially alter the in vivo function of these cells. For example, in the presence of activated effector T cells secreting inflammatory cytokines, mucosal tissues could preferentially shift Tregs towards Th17-like cells.⁸⁷ The delivery of Tregs generated in the presence of retinoic acid may minimize this risk, because this procedure is reported to lead to stable Tregs that are less likely to switch to a Th17 cell in vivo.⁵³ Other reports suggest that the microbiome determines the balance between Treg and Th17 cells,⁸⁸ supporting the possibility mentioned above, that Treg therapy may only be effective in conjunction with microbiota-altering factors.

Notably, although Tregs may acquire the ability to make effector cytokines in vivo, their suppressive capacity may nevertheless be maintained, circumventing the need to avoid ‘Th17 conversion’in vivo. Indeed, although Crohn’s disease patients have increased levels of FoxP3⁺ IL-17⁺ T cells in their inflamed mucosal tissues, these cells retain potent suppressive capacity.⁸⁹ Similarly in mice, transfer of FoxP3⁺ Tregs that recognize microbial antigens into immune-deficient animals results in the conversion of these cells into interferon-γ producers, but both their regulatory activity and FoxP3 expression are maintained.⁹⁰ In the context of cellular therapy, these latter studies are promising, because they suggest that regardless of the inflammatory environment they encounter, and any transient effector cytokine production, Tregs will remain suppressive.

How to ensure that therapeutic Tregs travel to the site(s) at which they could be maximally effective? It is currently unclear whether relevant suppression might occur in the local lymph nodes or in the intestinal tissue itself. On the one hand, Tregs could be targeted to the intestinal environment by engineering them to express chemokine receptors that attract them to specific tissues.⁹¹ On the other hand, it is possible that antigen-specific Tregs would in any case traffic appropriately to the sites where the relevant antigen is concentrated.

Selection of the best candidates for Treg therapy presents a further problem, because symptom presentation, onset, severity, and treatment response all vary. In mouse models, Tregs are more effective at preventing disease then curing it.¹⁰ Treg therapy would probably be most effective in the early stages of disease, but because these patients have many other therapeutic options, it may be difficult to find cohorts in which testing of this therapy can be justified. Furthermore, IBD is a heterogeneous disease and each individual is likely to have distinct disease aetiology, microbiota composition, and relevant antigens. It may therefore be challenging to determine standard dosing and delivery schedules, as well as to monitor outcomes.

Animal models of Treg therapy for IBD have relied on transfer of cells into T-cell-deficient animals. Will a similar conditioning step be necessary in IBD to make space for the Tregs to engraft and allow their expansion through homeostatic expansion mechanisms? As IBD is not usually a life-threatening disease, would such a pre-conditioning regimen be ethical? Here we will be able to learn from the results of a trial in type 1 diabetes, which is currently enrolling patients, where Tregs will be infused into immunocompetent individuals (http://www.clinicaltrials.gov/ct2/show/NCT01210664).

Once Treg therapy is administered, what parameters will determine the extent to which treatment has been effective? In contrast to the scenario of transplantation,^92,93 there are currently no known effective biomarkers of relevant immune status in IBD, and apart from monitoring disease symptoms and crude analysis of T cells from biopsies, there is no way to test if the therapy has re-set immune homeostasis. The efficacy of current therapeutic agents such as anti-tumour necrosis factor-α antibodies will be likely to set the bar high for Treg therapy, possibly requiring life-long cure with minimal side-effects.

Conclusions

Although there are still many unknowns and theoretical risks (Fig. 1), it is the hope that delivery of Tregs will indeed be able to reset intestinal immunity that justifies the study of these approaches. Current treatment strategies for IBD rely on the use of non-specific immunosuppressive agents such as steroids and anti-cytokine antibodies; these treatments are not effective in all patients, are non-specific, and never provide a cure. Antigen-specific Treg cellular therapy would, in contrast, offer a cure through specific and potent targeting of the response to disease-driving antigens at the site of inflammation. Because evidence to date suggests that Tregs are indeed functional in IBD patients, expansion of autologous cells is likely to be a feasible approach. In the context of haematopoietic stem cell transplantation, a major concern has been the purity of such expanded autologous Tregs, because contaminating effector T cells could theoretically cause graft-versus-host disease.⁹⁴ Several groups have worked to identify markers that can be used in conjunction with CD25 to improve the purity of the expanded cells.^95–97 In the setting of IBD the purity of infused Treg cells may not be as crucial, because an inflammatory response mounted by contaminating T effector cells would be unlikely to lead to immediate organ failure and death.

Figure 1.

Open questions in regulatory T cell (Treg) therapy for inflammatory bowel disease (IBD). The use of Tregs as a therapy for IBD has many advantages, but there are also many outstanding questions. Shown is a schematic diagram illustrating that Treg therapy could involve enhancing the function of endogenous cells in vivo, or delivering cells that had been manipulated in vitro. As the intestine is inherently tolerogenic and capable of rapid regeneration, Treg therapy could be very effective in this environment. There are many questions that need to be answered in experimental models of disease while translational strategies are developed.

Many animal models have shown that the long-lasting effects of a short dose of Treg cells relies on infectious tolerance – that is, the in vivo generation of new Tregs which ultimately maintain tolerance.⁶³ Compared with solid organs, the gut is rife with tolerance inducing factors, including TGF-β and retinoic acid.³⁷ Indeed, Treg-derived TGF-β has already been shown to mediate infectious tolerance in models of colitis.⁹⁸ Therefore the gut may be the optimal site to which to target Tregs with the expectation of inducing a life-long therapeutic effect. In addition, the gut’s capacity for regeneration supports the hope of return to normal homeostasis when chronic inflammation is relieved.

With phase I clinical trials using Treg therapy for the treatment of type 1 diabetes currently enrolling participants, Treg cellular therapy for IBD is eagerly anticipated. Major concerns specific to this disease, however, must first be addressed. Chief among these are concerns relating to diversity of the mucosal environment, the desirability of the antigen-specific approach, the significant influence of the microbiota, and the means of determining treatment efficacy. In all likelihood, such an approach will need to be highly individualized to abrogate the need for immunosuppressive drugs, provide relief from inflammatory symptoms and ultimately, long-lasting immune homeostasis.

Disclosures

The authors have no conflicts of interest to disclose.