Review articleRole of orally induced regulatory T cells in immunotherapy and tolerance
Graphical abstract
Introduction
The small and large intestine are continuously exposed to a large variety of foreign antigens derived from food as well as commensal bacteria. Nevertheless, the intestinal immune system does not necessarily mount cellular or humoral immune responses to non-self-antigens due to regulatory mechanisms. One such mechanism is termed “oral tolerance”, referring to the natural development of induced tolerance to orally ingested stimuli in the gut-associated lymphoid tissue (GALT) [1]. Failure to establish oral tolerance leads to food allergy or the development of intestinal inflammatory diseases [2], [3]. While the colon is thought to respond to the massive saturation stimulation by bacterial or bacterially induced antigens, the immune system of the small intestine is distinct; it processes orally delivered antigens as well as systemic antigens, contains specialized lymphoid structures such as Peyer’s patches, has a distinct microbiome, distinct mechanisms of immune regulation, and uniquely interacts with the mesenteric lymph node (MLN) [4], [5], [6], [7], [8], [9], [10]. Hence oral tolerance mechanisms, including oral induction of regulatory T cells (Treg), take place in the small intestine.
Recently, the term oral immunotherapy (OIT) was coined to define the oral delivery of antigen with the objective to suppress immune responses. Delivering a specific target antigen or antibody that affects T cell function have been the two main methods used to achieve OIT. Alternatives routes of OIT include nasal, sublingual, subcutaneous and epicutaneous administration [11], [12], [13], [14], [15].
Oral tolerance was first demonstrated by Wells and Osbourne more than a century ago [16]. These investigators found that guinea pigs fed with corn-containing diet, but not corn-free diet, failed to show anaphylactic reactions against zein, a major protein of corn. In 1946, another study reported that prior feeding of certain allergenic compounds to non-sensitive subjects induced a state of immunological tolerance against subsequent experimental dermal sensitization with the same compounds [17]. Subsequent studies confirmed the existence of an oral tolerance mechanism. For instance, rats developed tolerance to horse serum or pollen extract when fed with these antigens prior to non-oral exposure [18]. This was also demonstrated in other animal models that were fed bovine serum albumin [19] or sheep red blood cells [20]. However, induction of oral tolerance in humans was only demonstrated in the early 1990s, when adults fed with keyhole limpet hemocyanin followed by subcutaneous immunization with the same antigen were prevented from developing a subsequent delayed type hypersensitivity response [21]. Currently, OIT has been applied toward tolerance induction in autoimmunity and inflammatory diseases such as peanut allergy [22], [23], allergic asthma [24], [25], pollen allergy [26], hepatitis C infection [27], nonalcoholic steatohepatitis (NASH) [28], Pompe disease [29], rheumatoid arthritis [30], [31], type I diabetes [32], [33], hemophilia A and B [34], [35], [36], [37] in clinical as well as preclinical studies.
The development of oral tolerance is thought to take place in both the small and large gastrointestinal (GI) tract, where the GALT plays a key role in regulating responses to ingested antigens [38]. Antigen can be acquired directly by phagocytes or can be delivered through goblet cell associated passages prior to capture by dendritic cells (DCs) in lamina propria (LP) [39]. Antigen uptake by a subset of regulatory DCs expressing CD103, which migrate from the gut mucosa to the MLN, concomitant secretory IgA production [40], forkhead box protein P3 (FoxP3) expressing Treg that produce transforming growth factor β (TGF-β) and IL-10, and expression of indoleamine 2,3-dioxygenase (IDO) or the vitamin A metabolite retinoic acid (RA) [41] are all implicated in this process.
For the last three decades, the role of T cells in oral tolerance induction has been studied in increasing detail. It was found that depletion of CD4+ T cells abolished oral tolerance development [42], and that oral tolerance could be transferred from one animal to another by adoptive CD4+ T cell transfer [43]. Moreover, a population of TGF-β secreting CD4+ T cells termed Th3 were found to play a key role in oral tolerance [44], supporting the role of Treg in the induction of oral tolerance. Another conceptual advance in the oral tolerance field was the discovery of CD4+ T cells expressing a membrane-bound form of TGF-β that contains latency-associated peptide (LAP) [1], [45], [46]. LAP+CD4+ T cells were found to have important immune regulatory functions in oral tolerance. Interestingly, TGF-β is required for the induction of both FoxP3+ Treg and LAP+ Treg [47], [48].
Here we review current understanding of oral tolerance mechanisms, applications of OIT in allergy, autoimmune disease and in inducing tolerance to protein replacement therapies for monogenic disorders. We highlight key regulatory cells that are induced by orally delivered antigen, circumstances leading to their induction and the suppressive mechanisms exerted by them. Understanding these mechanisms are critical to identify new strategies for modulating tolerance.
Section snippets
Subsets of orally induced regulatory T cells
In the intestine, crosstalk among several cells occurs in order to induce a naturally tolerogenic environment. Unlike thymus derived (t)Treg in other organs, which have a self-antigen TCR repertoire, intestinal Treg display a peripheral (p) Treg TCR repertoire responsive to resident and non-resident microbiota and dietary antigens, thus playing a crucial role in controlling pro-inflammatory responses [49], [50], [51], [52]. Both tTreg and pTreg are located in the intestine, but it appears that
Regulatory CD8+ T cells
The majority of cells involved in oral tolerance are thought to be CD4+ T cells, but these may not be the only immune regulatory cells involved in OIT. For example, it has been reported that CD8+ T cells with regulatory activity may be induced upon interaction with intestinal epithelial cells [126]. Regulatory CD8+ T cells express lower levels of FoxP3 compared to CD4+ Treg in mice, rats and humans [127]. In mice, surface markers such as CD122(+) or CD28(−) have been used to identify regulatory
RORγt-expressing Treg
The abundant microbial community found in the intestine plays a prominent role in the regulation of oral tolerance. Retinoic acid-related orphan receptor gamma t (RORγt)-expressing Treg (RORγt+ Treg) represent a substantial number among Treg in the intestine and are dependent on the microbiota for maintenance and thus might be sensitive to microbiota shifts [3], [53], [158], [159]. Clostridia and Helicobacter species are examples of bacteria that may induce RORγt+ Treg differentiation in the
Mechanisms of oral Treg induction
Orally induced Treg cells have a crucial role in maintaining tolerance to microbiota, diet and other harmless antigens [50]. Here, we describe mechanisms that mediate Treg differentiation during OIT.
Mechanisms of suppression
Treg preserve homeostasis in the intestinal tract through multiple suppressive mechanisms: production of inhibitory cytokines IL-10, IL-35 and TGF-β; apoptosis or anergy of effector T cells, perforin and granzyme-dependent cytolysis of target cells; suppression of DC maturation and function through expression of PD-1, CTLA4, LAG3; and apoptosis of effector T cells by deprivation of IL-2. Of these, inhibitory cytokines have been studied the most in intestinal Treg.
Examples of OIT-induced Treg in treatment of disease
OIT can be used to alleviate inflammatory disease in an antigen-nonspecific fashion by non-specific enhancement of Treg [1], [97], [216]. This can be accomplished by supply of IL-2, which is critical for Treg development, expansion and maintenance of suppressive function [49], [217], [218]. Several studies have shown that low dose subcutaneous injections of IL-2 leads to an increase in number and function of Treg, thereby ameliorating autoimmune disease in patients [12], [15]. Bonnet and
Conclusions and future perspectives
In the past decade, there has been an increase in knowledge regarding OIT, as well as mechanisms that mediate the development of this process. Three distinct Treg subsets have been shown to majorly contribute to OIT: FoxP3+ Treg, LAP+ Treg and Tr1 cells. These cells share several key characteristics and functions, such as the ability to produce IL-10 and TGF-β, which are critical mediators of immune suppression in OIT. TGF-β is also a key cytokine for FoxP3+ and LAP+ Treg differentiation. CD103+
Acknowledgments
This work was supported by NIH grants R01 HL133191 and R01 HL107904 to HD and RWH; R01 HL131093 to RWH and CT; and U54 HL142012 to RWH. Figures were generated using BioRender.
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