New therapeutic perspectives in Type 1 Diabetes: dietary interventions prevent β cell-autoimmunity by modifying the gut metabolic environment

The gut microbiota plays a key role in the pathogenesis of autoimmune diseases such as Type 1 Diabetes (T1D), but the mechanism underlying this modulation is yet to be defined. This paper highlights the crucial role of the microbiota-induced gut metabolic profile, rather than the presence of single bacterial strains, in T1D modulation. Most importantly, it provides the first evidence that dietary interventions can modulate the autoimmune pathogenesis of T1D by altering the gut metabolic environment.

Substantial environmental changes and lifestyle modifications have been held responsible for the dramatic peak in the incidence of autoimmune diseases like T1D that occurred in the last half century in developed countries, the so-called ‘autoimmune epidemic’.¹ Many environmental modifications such as changes in dietary habits, higher hygiene conditions and extensive use of antibiotics might have increased incidence of autoimmune diseases by altering the human intestinal microbiota. In fact, the gut microbiota has a strong impact in the immune system not only at the intestinal level but also systemically and modulate extra-intestinal autoimmune diseases like T1D.² Effector T cells migrate from the gut to peripheral lymphoid organs and tissues³ and chronic inflammation and altered Teff/Treg cell ratio in the intestinal mucosa can underlie the pathogenesis of extra-intestinal autoimmune diseases like T1D as demonstrated in humans⁴ and pre-clinical models.⁵ Those lines of evidence support the notion that extra-intestinal autoimmune diseases and T1D can be prevented by promoting a beneficial gut microbiota profile and restoring a correct immune homeostasis at the intestinal level through administration of probiotics.⁶ However, it is now clear that it is not the presence of specific bacterial strains that modulates the immune system but rather the microbiota-associated metabolic pathways induced at the intestinal level by different microbial consortia. In a study performed in atopic children, immune dysregulation and allergy were associated with activation of microbial genes linked to specific metabolic pathways.⁷ For example, short chain fatty acids (SCFAs), that is, butyrate, propionate and acetate, are metabolites produced by bacterial communities that use xylane, xylose, and carboxymethylcellulose for their metabolism and are crucial to maintain gut barrier integrity⁸ and intestinal immune homeostasis.⁹ A SCFA-enriched gut metabolic environment promotes immune tolerance by enhancing FoxP3⁺ Treg cell expansion.¹⁰ This led to the hypothesis that a SCFA-enriched gut metabolic environment could prevent extra-intestinal autoimmune diseases like T1D by restoring immune tolerance at the intestinal and systemic level.

The article recently published by Mariño et al. establishes a causative link between the presence of a tolerogenic gut metabolic environment enriched in SCFA metabolites and prevention of autoimmune diabetes.⁹ First, the Authors demonstrated that diabetes progression in SPF-hosted NOD mice is associated with reduced presence of the SCFAs acetate and butyrate in portal vein blood compared to NOD. Myd88^-/- mice that host a protective gut microbiota. Then, they specifically asked whether restoration of a SCFA-rich gut metabolic environment in NOD mice can halt diabetes progression. Oral administration of SCFA does not properly recapitulate metabolite physiological absorption from the colon through bacterial fermentation of fibers. Here the Authors exploited an elegant approach to induce a SCFA-rich metabolic environment in the gut through the administration of specialized diets capable of releasing high amounts of acetate or butyrate upon bacterial fermentation and tested its effect on T1D.

Short-time administration of either acetate or butyrate yelding diets at the time of onset of inflammatory events associated with progression from insulitis to clinical diabetes (from 5 to 15 weeks of age) was sufficient to protect NOD mice from T1D. Importantly, combination of the two diets showed an additive effect thus suggesting that these two metabolites protect through different mechanisms. The observation that the diet-mediated protection from T1D (particularly that induced by the acetate-enriching diet) was associated with a significant reduction of the islet-reactive autoimmune T cell repertoire clearly indicated that the two metabolites have an immunomodulatory effect. To further explore how the SCFAs modulate islet autoimmunity, the Authors analyzed different immune cell functions in mice fed on different diets. First, the Authors demonstrated that the acetate-releasing diet selectively impairs the B cell antigen presenting cell (APC) function (Figure 1). Acetate and butyrate directly affect the APC function of B cells through an epigenetic-driven mechanism leading to reduced differential expression of 14 genes encoding products involved in important B cell functions such as antigen presentation, signaling via the BCR, cell metabolism and activation of cytotoxic T cells. Since previous studies demonstrated that B cells are crucial APCs to spread islet-specific T cell responses to various self epitopes and expand the autoimmune T cell repertoire in pre-clinical models of T1D,¹¹ the SCFA effect on B cells could explain the protection observed in mice receiving an SCFA-enriching diet.

Figure 1.

A SCFA-enriched gut metabolic environment prevents autoimmune diabetes. Specialized diets that trigger release of high amounts of acetate or butyrate upon bacterial fermentation prevent autoimmune diabetes. Acetate predominantly act on B cells by reducing their antigen-presenting function and capacity to spread autoimmune T cell responses to various self epitopes of islet antigens. Butyrate expands the FoxP3⁺ Treg cell subset that is crucial to maintain self tolerance and prevents autoimmune diabetes. Both immunomodulatory effects are mediated by epigenetic mechanisms that involve histone modifications: increase of histone-deacetylase (HDAC) activity in B cells and FoxP3 locus acetylation in FoxP3⁺ Treg cells.

An additional immune-mediated mechanism of diabetes protection by SCFAs was demonstrated. Specifically, the Authors showed that SCFA-enriching diet expanded the FoxP3⁺ Treg cell population not only in the colonic mucosa, as previously demonstrated,¹⁰ but also systemically in the spleen and pancreatic lymph nodes (Figure 1). Since FoxP3⁺ Treg cells are crucial in maintaining immune tolerance towards self antigens in humans and pre-clinical models of T1D,¹² the effect of acetate and butyrate on the FoxP3⁺ Treg cell repertoire can be another mechanism through which SCFA-enriching diets prevent T1D in NOD mice. Although both SCFAs expanded the FoxP3⁺ Treg cell repertoire, only the butyrate-yelding diet was directly capable to promote conversion of adoptively transferred T cells into FoxP3⁺ Treg cells. This is consistent with the improved acetylation, and therefore transcriptional activity, of the FoxP3 locus and other Treg cell function-related genes observed in mice receiving a butyrate-releasing diet.

Interestingly, both immunomodulatory effects of SCFAs on B cells and FoxP3⁺ Treg cells were mediated by epigenetic mechanisms that involve histone modifications. Although previous studies already suggested the ability of SCFAs to regulate histone-deacetylase (HDAC) activity, this study shed light on the variety of effects that single SCFAs, through increase of HDAC level in B cells for acetate and histone acetylation in FoxP3⁺ Treg cells for butyrate, can have on transcriptional regulation of different immune cell subsets, ultimately resulting in modulation of autoimmune responses.

The complex picture delineated so far would not have been complete without a closer look at the ability of SCFA-yielding diets to modify the metabolic environment and the microbial ecology in the gut (Figure 2). In line with the previously demonstrated role of SCFAs in preserving gut barrier integrity and dampening tissue inflammation,^{8, 9} the Authors showed that NOD mice fed with SCFA-releasing diets have less bacterial translocation (reduced serum level of LPS) and reduced markers of intestinal inflammation IL-21 and higher expression of markers of epithelial barrier integrity, that is, IL-22. Also, both acetate and butyrate-yelding diets were able to promote the establishment of a protective microbiota composition enriched in bacteria of the Bacteroides genus, which was sufficient to transfer T1D protection in faecal transplantation experiments.

Figure 2.

Diet and other environmental factors modulate autoimmune diabetes by modifying the gut metabolic environment. Specific dietary components such as soluble fibers and omega-3 fat but also antibiotics and hygenic conditions have a strong impact on the microbiota composition. Those factors modulate the relative abundance of SCFA-producing microbial species and have a strong impact on the gut metabolic environment thus affecting the immune regulatory mechanisms that are controlled at the intestinal level.

The concept of diet-mediated immune modulation of T1D is very fascinating. Although epidemiological studies linked some dietary components (e.g., gluten, cow milk’s proteins, etc.) with differential risk of developing T1D,^{13, 14} it is yet to be determined whether and how diet modulates autoimmunity in extra-intestinal organs such as pancreatic islets in T1D. The work by Mariño at al. provides the first evidence that diet can directly affect the autoimmune pathogenesis of T1D by favoring a tolerogenic gut metabolic environment enriched in SCFAs. However, although the effects of the SCFA-enriching diets on the gut metabolic environment and microbiota composition were clear, it is yet to be demonstrated that modifications at the intestinal level are important for the immune modulatory effect of the SCFA-enriching diets on T1D. In fact, the administered diets increased the SCFAs level in the intestine but also in the portal blood, thus suggesting that the SCFA-mediated effects on B cell function and FoxP3⁺ Treg cell expansion could take place systemically rather than locally in the intestinal mucosa. Microbial metabolites can pass from the gut into the systemic circulation and affect immune tolerance, that is, FoxP3⁺ Treg cell differentiation, at sites distal from the intestine.¹⁵ A previous report showed that intestinal inflammation and alteration of gut immune homeostasis with defective local differentiation of FoxP3⁺ Treg cells are present in humans affected by T1D.⁴ Although that finding suggests that intestinal FoxP3⁺ Treg cell expansion is defective in the intestine of T1D patients, further studies are necessary to clarify whether the gut metabolic environment counter-regulates the pathogenesis of human T1D by altering the autoimmune T cell repertoire locally at the intestinal level.

Altogether the data presented by Mariño et al. provide a fresh demonstration of the possibility to re-educate the (auto)immune response at the mucosal interface on at least two different levels: dietary and microbial composition (Figure 2). Importantly, those results represent a solid rationale to start proof-of-concept clinical studies in humans aimed at preventing extra-intestinal autoimmune diseases through nutritional interventions that increase the relative abundance of SCFA-producing bacteria and promote a tolerogenic gut metabolic environment.