The gut and maintenance of the immune barrier

Inflammatory Bowel Diseases (IBD) represent a condition in which the intestine progressively loses the ability to maintain a stable immune balance. At a microscopic level, the intestinal mucosa becomes the stage for a constant conflict between immune system cells that should cooperate to preserve tissue integrity. Among the key players in this dynamic are T helper 17 cells (Th17, a class of effector T cells) and regulatory T cells (Treg), two populations derived from CD4⁺ T cells with profoundly opposite yet complementary functions: the former promote inflammation, while the latter suppress and control the immune response.

Under physiological conditions, these two forces maintain a dynamic equilibrium that ensures tolerance toward the gut microbiota and prevents autoimmune responses. In IBD, however, this balance is broken: Th17 cells increase excessively, and Treg cells, while retaining their ability to regulate inflammation, prove numerically and locally insufficient to counteract it (Yan et al., 2020) [1].

The inflamed intestinal mucosa shows an increase in CD4⁺ CD25 FOXP3⁺ T helper cells, but this expansion proves inadequate relative to the intensity of the inflammatory response. It is as if a fire brigade, while functional, were too small in number to extinguish a fire that is already out of control. Histological studies have shown that during active phases of the disease, Treg cells decrease in peripheral blood, suggesting their recruitment to sites of intestinal inflammation, where, however, they fail to restore effective homeostasis (Maul et al., 2005) [2].

Beyond intracellular mechanisms, metabolism and diet also profoundly influence this balance. Excessive caloric intake, for example, stimulates the mTOR pathway and the differentiation of Th17 cells, while caloric restriction or the activation of the AMPK pathway (an energy-sensing enzyme present in our cells that is fundamental for energy metabolism) promotes the formation of Treg cells without compromising their function. This suggests that targeted nutritional interventions could complement pharmacological therapies in restoring immune tolerance and reducing intestinal inflammation (Zhang et al., 2024) [3].

In summary, IBD can be viewed as the result of a broken immune balance, in which the inflammatory drive of Th17 cells and the regulatory response of Treg cells lose their harmony; the therapeutic goal is not only to suppress inflammation but to reconcile these two forces, restoring to the intestine its natural ability to distinguish what it must tolerate from what it must fight.

Inflammatory Bowel Diseases (IBD), however, are not limited to affecting the digestive system: they represent a systemic condition capable of profoundly altering the metabolism and functioning of the entire organism. The chronic inflammation that characterizes these pathologies, such as ulcerative colitis and Crohn's disease, does not remain confined to the intestinal mucosa but propagates as a constant signal involving the vascular endothelium (the inner lining of blood vessels), the autonomic nervous system, and even glucose and lipid metabolism. In recent years, numerous studies have highlighted an increasingly close link between IBD and insulin resistance, opening new perspectives on how chronic inflammation can interfere with the body's energy regulation.

In a clinical study conducted on patients with Crohn's disease, it was observed that, even in the absence of diabetes, these subjects exhibit significantly higher plasma insulin levels compared to healthy controls. This hyperinsulinemia, accompanied by an increase in the HOMA-IR index (an index used to estimate insulin resistance and pancreatic β-cell function) and the insulin resistance index, is a phenomenon that forces pancreatic β-cells into more intense compensatory activity to maintain normal glucose tolerance (Bregenzer et al., 2006) [6].

This state of metabolic stress is not limited to the endocrine level but also involves vascular function. Patients with IBD, in fact, show an increase in pulse wave velocity (PWV), a reliable index of arterial stiffness and an early marker of endothelial dysfunction (altered functioning of the inner lining of blood vessels). Even in the absence of traditional cardiovascular risk factors, such as hypertension or dyslipidemia, arterial stiffness is increased and correlates positively with disease duration and plasma levels of insulin and HOMA-IR. This suggests that chronic inflammation and insulin resistance cooperate to compromise vascular elasticity, predisposing patients to premature cardiovascular risk (Korkmaz et al., 2014) [7].

Other studies have highlighted that the metabolic impact of IBD also reflects on cardiac physiology: in patients with ulcerative colitis and Crohn's disease, a longer corrected QT interval dispersion (QTcd) has been found compared to healthy subjects; in simple terms, this is an anomaly of the electrical processes occurring in biological cells and tissues, particularly in the heart, which increases the risk of ventricular arrhythmias. This prolongation of the QTcd is associated with higher values of insulin and HOMA-IR, indicating that insulin resistance may also contribute to modulating the heart's electrical function through alterations in systemic inflammatory processes (Yorulmaz et al., 2013) [8].

At an epidemiological level, the relationship between insulin dysfunction and the risk of developing IBD has been further confirmed by a recent prospective study conducted on the UK Biobank, which involved over 327,000 participants: the triglyceride-glucose (TyG) index, a surrogate biomarker of insulin resistance, was found to be significantly associated with an increased risk of ulcerative colitis and Crohn's disease; for every unit increase in the TyG index, the risk of developing IBD increased by 14.3% and 13.8%, respectively. Individuals with the highest TyG values had a risk of disease up to 28% higher than subjects with normal values (Zhang et al., 2025) [9]. This association remained even after adjusting the data for genetic and lifestyle factors, suggesting that metabolic alterations may independently contribute to the etiology of IBD.

Understanding these studies means moving beyond the traditional view of IBD as exclusively gastrointestinal diseases. It means recognizing that, behind abdominal pain and diarrhea, there lies a complex network of inflammatory, hormonal, and metabolic signals that involves the entire organism. And perhaps, intervening in this network by acting on the metabolic factors that determine an alteration of the insulin pathway could represent one of the most promising keys to restoring the lost balance.

It is precisely within this network of metabolic and inflammatory signals that one of the main points of convergence between alterations in the insulin pathway and the immune dysregulation characteristic of IBD emerges: mTOR. Located downstream of the PI3K/AKT pathway, mTOR is not merely an effector of insulin signaling, but a true metabolic sensor capable of integrating nutrient availability, growth signals, cytokines, and stress conditions. Its activity can therefore translate a metabolic alteration into a different cellular and immune response, creating a direct link between the previously described state of insulin resistance and the mechanisms that sustain intestinal inflammation.

In 2024, researchers described mTOR as an important metabolic regulator capable of directing the behavior of immune cells based on the nutrients and cytokines present in the microenvironment. Its activity contributes in particular to determining the fate of T lymphocytes, influencing the balance between effector and regulatory populations. In IBD, an alteration of this control can therefore favor a more pro-inflammatory response, supporting the differentiation and activity of Th17 cells at the expense of the regulatory mechanisms necessary to contain intestinal mucosal inflammation (Zhang et al., 2024) [3].

This relationship becomes particularly important considering that Th17 cells produce cytokines such as IL-17, IL-21, and IL-22, which are capable of profoundly modifying the intestinal inflammatory microenvironment. When mTOR activity favors metabolic programs associated with the effector response, naïve T cells (T lymphocytes not yet activated by their specific antigen) can be more oriented toward pro-inflammatory phenotypes; conversely, the reduction of mTOR signaling can favor the development of Treg cells, which are responsible for controlling the immune response and maintaining tolerance.

An experimental demonstration of this balance emerges from the work of some researchers, in which the use of the mTOR inhibitor INK128 in animal models of colitis resulted in a significant attenuation of intestinal inflammation. The treatment was associated with a decrease in Th1 and Th17 populations and, simultaneously, an increase in the Treg component. The treated animals also showed better colon integrity and less weight loss, indicating how intervention in mTOR signaling can translate into concrete changes in both the immune response and intestinal damage (Shi et al., 2019) [4].

Consistent results were also reported in 2016, evaluating the effects of mTOR inhibition using AZD8055 (a substance that directly inhibits mTOR) in experimental models of colitis. In this case as well, the treatment reduced inflammation and the presence of Th1 and Th17 populations, while simultaneously favoring the expansion of Tregs and the increase of IL-10, an important anti-inflammatory cytokine associated with the regulatory response. mTOR inhibition therefore shifted the immune balance from a predominantly effector and pro-inflammatory condition toward a more regulatory profile (Hu et al., 2016) [5].

The contribution of mTOR is not limited to adaptive immunity: the colon epithelium (the tissue that lines and protects the body's surfaces), far from being a passive barrier, "talks" to the immune system by modulating the tissue's cytokine profile based on its differentiation state, a process that depends precisely on mTOR signaling. When this circuit is altered, the epithelium promotes hyperplasia and pro-inflammatory recruitment, fueling colitis from within the tissue itself (Lyons et al., 2018) [13].

This same metabolic "control center" governs the language choice of T cells: when mTOR is active, naive T cells take the effector pathways (Th1/Th2/Th17); when the signal is dampened, the regulatory pathway prevails and Tregs and Foxp3⁺ cells appear, which are better suited to restoring tolerance. It is an immunological switch that transforms a discordant chorus into more orderly music (Delgoffe et al., 2009) [11].

In another study from 2013, mTOR was defined as a metabolic sensor that integrates nutrients, cytokines, and stress and decides the fate of cells in the intestine and immune system. When its natural brake, PTEN (a protein that slows the PI3K/AKT pathway), is lowered, the PI3K/AKT/mTOR insulin pathway becomes hyperactivated in T cells and mucosal lymphocytes in Crohn's disease, supporting proliferation, survival, and inflammatory mediators as if one were keeping their foot on the accelerator on a road already crowded with inflammation (Long et al., 2013) [10].

It is no coincidence that, under controlled ex vivo conditions, rapamycin (a drug that primarily inhibits mTOR activity) allows for the expansion of human Tregs while maintaining their phenotype and suppressive function, a practical clue as to how modulating mTOR can rebalance the regulatory arm in contexts of chronic hyper-activation (Battaglia et al., 2012) [12].

It is not surprising, therefore, that pharmacological strategies against mTOR are emerging, as we will see in the next paragraph, as concrete avenues in IBD: preclinical data show reductions in cytokines, restoration of the barrier, and Th17/Treg rebalancing, while clinical practice still requires larger studies to define long-term efficacy and safety (Lashgari et al., 2021) [14]; (Afzal et al., 2022) [15].

Overall, the evidence analyzed outlines a close interaction in IBD between metabolic alterations, the insulin pathway, and immune dysregulation. In this scenario, mTOR emerges as a central node capable of integrating signals from the PI3K/AKT pathway with the metabolic programs that regulate T lymphocyte activity. Its excessive activation can favor proliferation, pro-inflammatory responses, and effector polarization, contributing to the Th17/Treg imbalance and the persistence of intestinal inflammation. Conversely, proper modulation of mTOR can support autophagy, the regulatory function of Tregs, and epithelial homeostasis, highlighting the importance of the balance of this network in the pathophysiology of IBD.

mTOR inhibition currently represents one of the most promising avenues in therapeutic research for inflammatory bowel diseases (IBD). While in the early stages the study of mTOR was limited to its role as a regulator of cell growth, we now know that this metabolic sensor is a true “immune switch”: it controls T-cell differentiation, cytokine production, and the autophagy processes that maintain the health of the intestinal mucosa. Modulating mTOR does not mean suppressing the immune system, but rather restoring its balance, bringing it back from a state of pathological overactivation to physiological homeostasis.

One of the first drugs evaluated in this context was everolimus (a drug derived from rapamycin), a second-generation selective mTORC1 inhibitor. In an IL-10^-/^- murine model of chronic colitis, which reproduces the immunopathological characteristics of human Crohn's disease, oral administration of everolimus significantly reduced the severity of colitis. The treatment resulted in a decrease in CD4⁺ T cells in the lamina propria (the layer of tissue beneath the mucosa) and reduced production of interferon-γ (a protein naturally produced by cells to stimulate the immune response), attenuating the aberrant Th1 response typical of the disease (Matsuda et al., 2007) [16].

Its structural analog, sirolimus (rapamycin), has also shown extraordinary results in experimental models and early human clinical cases: in a murine study of TNBS-induced colitis (a chemical compound used in the laboratory as a reagent), sirolimus reduced the levels of TNF-α, IL-6, and IL-17 (pro-inflammatory immune system communication molecules) while simultaneously increasing TGF-β (a signaling molecule that regulates the immune response) and regulatory T cells, thereby correcting the Th17/Treg imbalance that characterizes the inflamed mucosa (Yin et al., 2013) [17].

The efficacy of sirolimus has also been confirmed in pediatric populations: in a cohort of children with refractory IBD—that is, those who do not respond to conventional medical therapies—nearly half of the patients with ulcerative colitis and all those with Crohn's achieved clinical remission or mucosal healing, with a reduction in the use of corticosteroids and biologics and without serious adverse events (Mutalib et al., 2014) [18]. These results, although preliminary, indicate that sirolimus can act as a rescue therapy in resistant forms, selectively modulating lymphocyte proliferation and promoting protective autophagy.

Regarding natural molecules, several plant-derived substances show the ability to inhibit the PI3K/AKT/mTOR pathway, restoring immune homeostasis and epithelial integrity. Rhein, an organic compound extracted from Rheum palmatum (a medicinal plant known as Chinese rhubarb), reduces the pro-inflammatory cytokines TNF-α, IL-6, and IL-1β and suppresses the activation of PI3K, AKT, and mTOR, while simultaneously modulating the gut microbiota and promoting the expansion of beneficial bacteria (Dong et al., 2022) [19]. A similar effect was observed for glaucocalyxin A, a compound derived from Isodon japonicus (an Asian plant of the Lamiaceae family), which attenuates ulcerative colitis by inhibiting AKT and mTOR, with a reduction in inflammatory mediators and histological improvement (Zhou et al., 2025) [20].

Another compound of interest is moringine, an organic compound extracted from Moringa oleifera (a plant used as food and in herbal tradition), which is capable of acting simultaneously on multiple pathways: it activates antioxidants, suppresses the PI3K/AKT/mTOR insulin pathway, and protects the mucosal barrier from oxidative inflammation (Zhang et al., 2024) [21]. Triptolide, a compound derived from Tripterygium wilfordii (a medicinal plant native to East Asia), also acts on the mTOR pathway by promoting the repolarization of macrophages from the pro-inflammatory M1 phenotype to the reparative M2 phenotype, thereby reducing the production of TNF-α and IL-6 and promoting IL-10 (Zhu et al., 2022) [22].

All these approaches, both pharmacological and natural, converge on a key concept: mTOR is the meeting point between metabolism, immunity, and tissue regeneration. Its targeted inhibition does not act as suppression, but as a form of cellular “re-education.” By restoring autophagy, reducing effector polarization, and supporting the epithelial barrier, mTOR inhibitors offer an integrated therapeutic perspective capable of extinguishing chronic inflammation at its root, rather than merely containing the symptoms. In this sense, mTOR becomes not only a pharmacological target but the symbol of a paradigm shift in the treatment of IBD: from damage control to the reconstruction of balance.

In chronic inflammatory bowel diseases, and particularly in ulcerative colitis, the activation of mTOR can support the expression of HIF-1α, profoundly transforming the metabolism of immune cells. mTOR functions as a sensor of energy and nutritional availability and, when it remains persistently active, it promotes metabolic programs oriented toward glycolysis. HIF-1α represents one of the central effectors of this response: by increasing the expression of glycolytic transporters and enzymes, it allows activated immune cells to rapidly produce energy and metabolic intermediates necessary for the inflammatory response. In the intestinal context, this axis can promote the polarization of macrophages toward the M1 phenotype and modify the balance between Th17 and Treg lymphocytes, contributing to the maintenance of mucosal inflammation.

This vision fits into a broader framework of immunometabolism: effector Th cells and inflammatory innate cells thrive on high glycolysis (a fundamental metabolic process that breaks down glucose to produce energy) and mTORC1/HIF-1α; Treg cells, on the other hand, prefer lipid oxidation and metabolic conditions that reduce the cell's ability to activate growth processes, orienting it toward a state of conservation. Intervening on these metabolic nodes, mTOR first and foremost, allows for the precise remodeling of the response, going beyond immunosuppression that broadly affects all components of the immune response, both innate and adaptive, and moving toward therapies targeted at cellular metabolism (Bittencourt et al., 2021) [23].

A particularly direct demonstration was provided by some researchers in 2024: using a co-culture consisting of HT-29 intestinal epithelial cells and human CD4⁺ T lymphocytes exposed to TNF-α, the authors directly manipulated mTOR expression. The increase in mTOR led to an increase in HIF-1α, glycolysis, and the proportion of Th17 cells, accompanied by a reduction in Tregs and an increase in pro-inflammatory mediators. Conversely, silencing mTOR reduced HIF-1α, glycolysis, and Th17 differentiation, rebalancing the response toward a more regulatory phenotype. The study therefore directly demonstrates the sequence mTOR → HIF-1α → glycolysis → Th17/Treg imbalance → inflammation in ulcerative colitis (Wang et al., 2024) [24].

A second particularly significant contribution was provided in 2021, studying macrophage polarization in DSS-induced colitis (a substance used experimentally to induce colitis). Treatment with dioscin (a natural compound belonging to steroidal saponins) inhibited the mTORC1/HIF-1α axis, reducing aerobic glycolysis and the polarization of macrophages toward the pro-inflammatory M1 phenotype; simultaneously, oxidative metabolism associated with M2 macrophages was favored. The causal importance of mTORC1 was confirmed using leucine, an mTORC1 agonist, which attenuated the effects of dioscin and reduced its benefit on colitis. This work therefore identifies the mTORC1 → HIF-1α → glycolysis → M1 axis as an important immunometabolic mechanism of intestinal disease (Wu et al., 2021) [25].

Further evidence was obtained in 2025 by studying the metabolism of regulatory T cells in a mouse model of DSS-induced ulcerative colitis. In mice with colitis, both glycolytic metabolism and the distribution of Treg populations were altered. Treatment with Astragalus polysaccharides (a medicinal plant of the Fabaceae family) reduced the activation of the mTOR/HIF-1α pathway, decreased glycolysis, and favored the recovery of regulatory T cells, accompanied by a reduction in pro-inflammatory cytokines. The work therefore suggests that the persistent activation of this pathway can compromise Treg homeostasis through pathological glycolytic reprogramming (Liu et al., 2025) [26].

Even more recent confirmation comes from a study published in 2026, which analyzed the role of artesunate (a drug derived from artemisinin used primarily against malaria) in DSS-induced colitis. The authors demonstrated that the drug reduces M1 macrophage polarization and macrophage infiltration through the inhibition of mTORC1/HIF-1α signaling. The use of leucine as an mTORC1 agonist allowed for the functional confirmation of the pathway's involvement. Furthermore, target identification experiments indicated INSR as an upstream component: artesunate modulated INSR and consequently reduced mTORC1, HIF-1α, and the glycolysis associated with the M1 phenotype. The sequence INSR → mTORC1 → HIF-1α → glycolysis → M1 macrophage → intestinal inflammation is thus outlined (Huang et al., 2026) [27].

Finally, another 2026 study further strengthened this model using an in vivo and in vitro approach (two different ways of conducting scientific research): in the colon of mice with DSS-induced colitis, PI3K, AKT, mTOR, and HIF-1α were found to be increased; the same profile was observed in RAW264.7 macrophages (cells similar to mouse macrophages used as a model) stimulated with LPS (a component of Gram-negative bacteria capable of activating an inflammatory response). The experimental treatment progressively reduced the activation of the PI3K/AKT/mTOR pathway and, in parallel, the expression of HIF-1α, decreasing M1 macrophage markers and increasing M2 markers. The authors therefore identified HIF-1α as a downstream target of the PI3K–AKT–mTOR cascade, linking the activation of this pathway to the pro-inflammatory polarization of macrophages in ulcerative colitis (Li et al., 2026) [28].

Taken together, these findings outline a coherent model in which the persistent activation of mTOR supports HIF-1α and glycolysis, modifying the behavior of the main immune populations involved in IBD. In macrophages, the mTOR–HIF-1α axis favors M1 polarization and the production of pro-inflammatory mediators; in T lymphocytes, on the other hand, it can shift the balance toward Th17 cells at the expense of Tregs. In this way, an originally metabolic signal is converted into an immune program capable of maintaining intestinal inflammation. The overall sequence can therefore be summarized as PI3K/AKT/mTORC1 activation → increase in HIF-1α → increase in glycolysis → M1 polarization → production of pro-inflammatory cytokines → Th17 and relative reduction of Tregs → persistence of intestinal mucosal inflammation.

In IBD, vitamin D acts like a silent "orchestra conductor": it makes no noise, but when it is missing, the entire immune symphony becomes discordant. Low levels of this vitamin are more frequently associated with active disease and flare-ups: in a large review of over eight thousand patients, those with vitamin D deficiency had a higher probability of clinical activity, mucosal inflammation, and relapses, as well as a poorer quality of life. In simple terms: when the tank is empty, the intestine loses part of its ability to "tolerate" and becomes inflamed more easily (Gubatan et al., 2019) [29].

This link is not theoretical: in real-world clinical cohorts, deficiency is very frequent and correlates with activity indices and inflammatory markers: in a hospital case series, a cut-off of approximately 27.5 ng/mL (limit value) best discriminated patients in remission from those in the active phase, suggesting a pragmatic threshold level for clinical monitoring; those who received supplementation showed significantly higher serum values and a more favorable clinical course (Hausmann et al., 2019) [30].

In parallel, a prospective cohort of Crohn's patients followed for three years showed an inverse association between vitamin D and C-reactive protein (a protein produced by the liver in response to inflammatory processes): the lower the vitamin D, the higher the systemic inflammation and the greater the probability of clinically active disease (Alrefai et al., 2017) [31].

Meta-analyses confirm the picture: in patients with Crohn's, vitamin D levels are on average lower than in healthy individuals, more than half present with vitamin D deficiency, and as clinical severity increases, values drop further: it is like watching the reserve fuel gauge go down just when the engine—the intestine—is under the most strain (Sadeghian et al., 2016) [32].

A meta-analysis conducted on 18 randomized clinical trials, comprising a total of 908 patients with IBD, evaluated the efficacy of vitamin D supplementation: the treatment significantly increased circulating levels of 25(OH)D₃ and resulted in a reduction in the relapse rate compared to controls, without significantly increasing adverse events. The authors concluded that vitamin D may represent a useful adjunctive treatment in IBD, especially for correcting vitamin deficiency and contributing to the control of relapses (Li et al., 2018) [33].

The mechanisms are consistent with what has been observed clinically: through its receptor (VDR), vitamin D reduces IL-6/IL-17/TNF-α, promotes IL-10, supports Tregs, and strengthens the "zippers" of tight junctions (connections between intestinal cells that regulate the passage of substances and protect the intestinal barrier), such as the protein occludin, limiting bacterial translocation and inflammatory reactivation. In other words, it improves both the "border control" of the barrier and the education of immune cells, with repercussions on the gut microbiota and the response to therapies (Wu et al., 2022) [34].

On a practical level, small interventional studies suggest a tangible clinical benefit: in a pilot study on mild-to-moderate Crohn's, supplementing vitamin D up to 5,000 IU/day for 24 weeks brought its values from ~16 to ~45 ng/mL and reduced the CDAI (Crohn's Disease Activity Index) by over one hundred points, with a perceived improvement in quality of life; in daily terms, refilling the tank coincided with a "quieter" intestine (Yang et al., 2013) [35].

Consistently, in a Chinese cross-sectional study, vitamin D levels dropped progressively from remission to severe forms in both ulcerative colitis and Crohn's disease, reinforcing the rationale for measuring and correcting it throughout the course of care (Zhao et al., 2019) [36].

Vitamin D, therefore, in simple words, helps the intestine "recognize friends," keeps the barrier firm, and dampens the excesses of the immune system. Monitoring it and correcting its deficiency does not replace underlying therapies, but it can make the environment less favorable to flare-ups and more receptive to treatments.

In conclusion, this vitamin acts as a universal mediator of immune and metabolic balance; in fact, as we saw in chapter 6 dedicated to the immune system, through the modulation of mTOR, it reduces the production of inflammatory mediators and limits the abnormal proliferation of effector T cells and stimulates the production of regulatory T cells, processes that are central to the pathogenesis of the disease. This regulatory effect promotes better control of the immune response, preventing autoimmune aggression against tissues.