Introduction
Inflammation is a vital biological response that regulates interactions between humans and the environment, with nutrition playing a crucial role. Due to the surge in chronic inflammatory diseases,1 and increasing interest in anti-inflammatory dietary therapy,2,3 the exploration of pro-inflammatory nutrients has become a primary focus for clinical and scientific communities.4 In fact, the understanding of immune system-driven chronic inflammation and its associated chronic diseases are still not well-developed. The contribution of dietary constituents to inflammatory, metabolic, autoimmune, cancerous, allergic, and neurodegenerative diseases remains poorly defined. The frequently consumed Western diet is considered pro-inflammatory,5 while vegetarian, non-processed, and traditional foods are recommended as anti-inflammatory.6,7 Since it is impossible to cover all pro-inflammatory nutrients, this review will focus on the role of gluten/gliadin in celiac disease (CD)-induced inflammation, and explore their potential involvement in other non-celiac chronic inflammatory conditions. Gluten is composed of two main proteins: glutenin and gliadin. Gliadins make up about 70% of the protein in gluten and are the molecules responsible for the harmful immune response that results in intestinal injury in CD. Since the gut is the entry point for gluten and a crossroads for multiple nutrients, food additives, microbes, enzymatic digestion, and absorption, various gluten-affected luminal events irradiate peripherally, inducing remote organ, gluten-related, inflammatory damage.8,9 The luminal content impacts the enteric ecosystem. Certain dietary components, like gluten, breach tight junction integrity, resulting in increased intestinal permeability, and induce changes in the composition and diversity of the microbiome towards disease-specific dysbiosis or pathobiosis. Finally, the enhanced local enzymatic capacity for post-translational modification of proteins can turn naïve peptides to lose their tolerance and become auto-immunogenic ones. The present narrative review expands on the multiple gut-originated axes and their relationship to remote organ autoimmune diseases. Brain, joint, bone, endocrine, liver, kidney, heart, lung, and skin autoimmune diseases are connected to the deregulated events in the intestinal luminal compartment, forming the gut-systemic organ axe. Being a universal pro-inflammatory protein, affecting multiple body compartments, organs and tissue and systemically distributed, gluten peptides should be thoroughly investigated for their potential detrimental effects. If substantiated, traditional gluten dependent diseases should be ruled out and gluten-free Mediterranean diets should be recommended.
Gluten-induced inflammation in celiac disease
The incidence of celiac disease is around 1–2% of the global population, representing a chronic, autoimmune, multisystem, inflammatory, and immune-mediated condition.10 The intestine is the primary target organ; however, extra-intestinal organs are affected as well.8,9 Currently, the only accepted and proven nutrient that induces the disease is gluten, a general name for proteins found in many grains, such as wheat, barley, rye, and partially in oats.11 Interestingly, “gluten” stands for “glue” in Latin, named for its adhesive and viscoelastic properties. The autoantigen associated with CD is tissue transglutaminase (tTG),12,13 and several antibodies are used for its serological diagnosis.14–16
Celiac disease fulfills the criteria of a gluten-induced autoimmune inflammatory condition in the following aspects17–19: histologically, there is epithelial and mucosal inflammatory destruction, presented by villous atrophy and intraepithelial lymphocytosis20,21; immunologically, the adaptive22 and innate immune23 systems are activated24,25; there is a surge in pro-inflammatory cytokines,26 where gliadin peptides induce increased levels of IL-15, IFN-γ, IL-6, tumor necrosis factor (TNF)-α, IL-1β, CCL2, CCL3, and many more27; dysbiosis, a feature of many autoimmune diseases (ADs), exists in the celiac gut lumen, and gliadin can directly induce intestinal flora dysbiosis.28–30 Upon successful gluten withdrawal, all the above-mentioned inflammatory features are significantly ameliorated31,32; refractory CD and celiac crisis respond to steroids or immunosuppressive therapy33; and finally, genetic predisposition is essential for the disease’s development.25 It can be summarized that CD is a hallmark condition where genetics, inflammation, autoimmunity, dysbiosis, and environmental gluten intersect.25,34–38 It is well accepted that gluten-containing cereals contribute to chronic inflammatory conditions and various ADs, primarily by inducing gut dysbiosis, enhancing intestinal permeability, and initiating a pro-inflammatory immune response.6,39–42 Interestingly, in addition to gluten, other wheat components, like lectins and enolase, might also act as proinflammatory molecules.43–46
The place of gluten in human nutrition and as a food additive
Wheat is a major crop grown in most countries. Its annual production is 7.34 × 108 tons, cultivated on an area of 2.14 × 106 km2, which sums up to the size of Greenland.47
Wheat consumption surpasses all other crops combined, making it the world’s most favored staple food. The annual increase in gluten usage as a processed food additive over the last four to six decades is estimated at 1.8 ± 0.4%.6 The global vital wheat gluten market, valued at $2 billion in 2019, is projected to reach $2.74 billion by 2027, representing a compound annual growth rate of 4% during the forecast period.48
Discovered in the Fertile Crescent nearly 14,000 years ago,49 wheat-based foods became a staple after domestication. Wheat is a major source of protein, with gluten making up 80% of its total protein content.6,50
Gluten is a protein naturally found in certain prolamins, including wheat, barley, and rye. Although oats are considered gluten-free, they are often cross-contaminated with gluten.11,51 Besides prolamins, gluten is found in numerous non-nutritional products, such as medications, toothpaste, and cosmetics. It acts as a binder, holding substances together and adding a “stretchy” quality. When omitted in baking, the resultant dough tears easily.52
In Western countries, wheat contributes significantly to health, providing dietary fiber, B vitamins, and mineral micronutrients, notably selenium, iron, and zinc.53 There are two perspectives on wheat and gluten: public perception and reality.50,53 Recently, the roles and functions of wheat and gluten have been scrutinized in unproven or pseudoscientific publications and popular media reports, giving the impression that wheat or gluten consumption has a deleterious and addictive effect on human health.50,53 The common claim that gluten-free foods are inherently healthy is not well-supported by scientifically accepted controlled studies. Consequently, wheat/gluten withdrawal fashionistas have impacted multiple Western societies, changing their dietary habits.50,53 While gluten itself provides no essential nutrients and its consumption can be avoided without compromising human well-being.54 However, the only scientifically defined indication to withdraw gluten is in well-proven gluten-dependent conditions, namely, CD, dermatitis herpetiformis, gluten ataxia, and gluten/wheat allergy.31,32,41,53,55–57 Despite the significant interest in non-celiac gluten sensitivity, much remains to be explored about its pathogenesis, and its potential nutritional triggers are still controversial.55,58–61 A combined gluten-free and Mediterranean diet might be an attractive alternative to address the unhealthy aspects of a gluten-free diet (GFD).53,62–64
The side effects of gluten
The more gluten is explored, the more side effects are disclosed. This topic has recently been reviewed by several groups.6,8,9,41,42,65–67 There are various adverse effects of gluten that might impact health. These harmful effects are delivered through immunological and toxic pathways, leading to gut dysfunction or inadequacy.
Pro-inflammatory
As mentioned above, gluten acts as a pro-inflammatory molecule.8,9,25–27,34–39,68,69 The gliadin p31-43 peptide induces cellular stress, activates proliferative mechanisms of cryptic epithelial cells, drives enterocyte stress, induces a Ca2+ surge (thus activating the CD autoantigen tTG), triggers a local pro-inflammatory storm, activates the NFκB signaling pathway, inhibits CFTR (cystic fibrosis transmembrane conductance regulator, an ion channel protein), alters vesicular trafficking, activates the inflammasome platform, and reduces autophagy.68,70,71 Gliadin peptides are pro-oxidative, induce DNA damage, and are pro-apoptotic in in-vitro and ex-vivo studies.72
Alters the gut microbiome and increases intestinal permeability
Gluten decreases the microbiome/dysbiome ratio composition and diversity, suppressing the beneficial metabolome toward inflammation.28–30,73–77 Intestinal tight junction functional integrity is one of the most conserved protective mechanisms for human survival and is crucial for maintaining intestinal homeostasis. When disrupted, foreign molecules enter the epithelial barrier, come into contact with the subepithelial dense immune systems, and initiate chronic inflammation and autoimmunity. Increased intestinal permeability is a common feature of many of these conditions, including CD,6,35–38,41,65–68,74,75 where gluten is a major disruptor of tight junction protective function.6,78–81 Several observations strengthen the gluten-zonulin-increased permeability axis. Zonulin is a measurable blood protein that reflects tight junction functionality. Increased zonulin blood level is considered a marker of an impaired intestinal barrier.6,78–81 This active axis has been shown to operate in active and non-active CD patients and even in normal controls.82 Larazotide acetate (AT-1001), a small peptide derived from Vibrio cholerae toxin, is one of the potential non-nutritional pharmacological strategies to treat CD patients.56 Acting as a modulator of tight junction integrity with its anti-zonulin activity, it has been shown to be superior to placebo in improving gut symptoms in active CD patients.83 The drug shows promise for counteracting enhanced intestinal permeability in some other chronic systemic diseases,84–86 including severe COVID-19,87,88 although it is not yet approved.
Immunogenicity
One of the major unwanted effects of gluten is its immunological impact, closely connected to the inflammatory response. Gluten is an immunogenic protein that elicits anti-gluten/gliadin antibodies, even in non-CD patients and normal controls.89–93 In addition, microbial transglutaminase (mTG)-treated gluten peptides in patients with CD are immunogenic.94 When gliadin is cross-linked to tissue or mTG, transforming the naïve protein into an immunogenic one, CD patients mount substantial levels of neo-epitope tTG and mTG antibodies, respectively.8,9,14–16,41,66,94–99 Intriguingly, in the presence of tTG and mTG-assisted gliadin docking, gluten/gliadin loses its human body tolerance, resulting in corresponding antibody secretion, representing a classical post-translation modification of proteins.29,30,100 When tested in CD patients, the neo-epitope tTG and mTG exhibit higher immunogenic activity compared to gliadin undocked enzymes. Furthermore, the tTG neo-epitope IgA+IgG isotypes show higher optical density activity, better reflect intestinal injury, and expose higher specificity and sensitivity by targeting different autoantigens compared to the conventional tTG isotypes.98,99 The same was found for the mTG neo-epitope.14,16,95 However, the list of gluten’s immunological adverse effects is much more extensive.41
In vitro studies have shown that gluten induces macrophages to produce proinflammatory cytokines and nitric oxide (NO).101–103 Upregulated MHCII, co-stimulatory molecules, TRLs, cytokine, and chemokine production were observed in dendritic cells.104,105 Higher expression of NKG2D and CD71 on NKp46(+) cells has been shown in lymphoid organs.106 Finally, increased permeability and the production of TNFα and IL-1β were detected when gluten was applied to the Caco-2 cell line.107
In vivo studies on rats and mice have shown the following compared to controls: cytokine surge in TH1 intestinal and mesenteric lymph nodes, TH1 cytokine pattern in islet infiltrate, and increased number of intestinal pathogenic intraepithelial cells.108–110 Studies on non-obese diabetic (NOD) mice revealed increased activated intestinal CD4+ T cells, changes in TH1/TH2 intestinal cytokine ratios associated with activated dendritic and TH17 cells, increased natural killer cell cytotoxicity, and cytokine secretion of IFN-γ and IL-6.106,111–113 NKG2D is a proinflammatory, auto-immunogenic co-stimulatory molecule; it is an activating receptor mostly expressed on cells of the cytotoxic arm of the immune system. Gluten withdrawal lowered NKG2D and its ligand expression in NOD and BALB/c mice, attesting to gluten’s impact on the co-stimulatory interplay between tolerance and immune inflammation.114
When exposed to gliadin/gluten, BALB/c mice showed proportional changes in regulatory T-cell subsets, increased numbers of TH17 in peripheral lymph nodes, proinflammatory cytokine patterns in FOX3− and FOXP3+ T cells, and robust activation of innate immune and TH17 cells.115–117Ex vivo and mice studies showed gluten-induced dendritic cells’ production of IL-1β and, interestingly, enhanced neutrophil migration towards gliadin peptides.118,119
Cellular dysfunction and cytotoxicity
At the cellular level, gliadin has been found to drive cytotoxicity, decrease cell viability and differentiation, induce LDH secretion, promote apoptosis, and decrease RNA, DNA, and glycoprotein synthesis when applied to HCT116 cells.41 In 1976, Hudson et al.120 documented growth inhibition and phenotypic changes in various human cell lines induced by gliadin exposure. Several in vitro studies point to the cytotoxicity of this molecule. Gliadin induced agglutination in K562 cells, decreased F-actin content in enteric 407 cell lines, suppressed cell growth and viability, induced apoptosis, and altered redox equilibrium in Caco-2 cells and cell morphology in LoVo, two- and three-dimensional cell culture while causing rearrangement of the cytoskeleton through the zonulin molecular structure. This results in the loss of tight junction functionality in IEC-6 cells.121
Disturbance of oxidative equilibrium
Oxidative equilibrium plays an essential role in cell homeostasis, and its imbalance is involved in many chronic inflammatory diseases. Gliadin-induced oxidative stress was reported extensively on various cell lines, including Caco-2, HT29, SH-SY5Y, T84, and LoVo, and reviewed in depth.121–123 For example, the content of glutathione was reduced (−20% vs. controls), and the activity of related enzymes was inhibited.121 The dysfunctional antioxidant machinery can result in inflamed CD intestinal mucosa, making it more vulnerable to further oxidative stress and hindering mucosal recovery.123
Induce apoptosis
Intestinal homeostasis relies heavily on enterocyte viability and death to maintain the high cellular turnover necessary to cope with a hostile environment. Programmed cell death is pivotal for this equilibrium but can be detrimental in pathological conditions. The apoptotic pathway is over-activated in CD patients and plays a key role in inducing gut inflammation.73,124 Inflammatory response and enteric damage induced by gliadin p31-43 drive multiple programmed cell death pathways in the small intestine of mice.124
Gluten impacts epigenetics
The HLA-DQ2 and HLA-DQ8 haplotypes are widely associated with CD, but some people without these genes still develop the disease. Genetic predisposition can be regulated or affected by epigenetic modifications and cannot account for all reported CD cases. Environmental epigenetics adds substantial understanding to the disease’s evolution and its multi-faceted phenotypic presentations.8,125,126 The main epigenetic pathways include histone modifications, DNA methylation, non-coding RNAs, and RNA methylation, where microRNAs might be used to characterize various classes of CD patients.125 Gluten affects gene expression by changing methylation status.122,127 The impact of gliadin on epigenetics has been observed in CD and non-CD MH-SY4Y and Caco-2 cell lines.122 Wheat-derived peptide epigenetic alterations might be important during the postnatal nutritional transition from maternal breastfeeding or infant formula to complementary gluten consumption.122,127
Gluten affects cellular metabolism
Being pro-inflammatory, cytotoxic, oxidative, apoptotic, and highly immunogenic, gluten peptides can alter fundamental cellular metabolic networks. Wheat-derived peptides induce 50% inhibition in cellular proliferation, 20% suppression of colony-forming ability, and significantly lower alkaline phosphatase activity during Caco-2 cell line differentiation.128 Moreover, the peptic-tryptic digestion of wheat inhibited more than half of DNA and RNA synthesis, glycoprotein synthesis, and altered mitochondrial functions in Caco-2 cells.129,130 Wheat and gluten peptides are important in nutrigenomics and nutrigenetics, revealing various interplays between diet, specific nutritional components, and gene expression.127
Gluten affects mental health
A plethora of peripheral and central neurological manifestations affect the celiac population65,131–135 indicating that gluten consumption can also impact psychiatric behavior and mental health. Anti-neuronal antibodies such as transglutaminase 6, GAD-65, GAD-67, cerebellar peptide, and myelin-associated glycoprotein are part of the CD-associated autoantibodies.136 During intestinal digestion, resulting gluten fragments have strong opioid activity.137 These morphine-like substances, called gluten exorphins, have proven opioid effects that might affect mental health.138 Opioid receptors are scattered throughout the body, including in the gut, brain, and peripheral nervous system. Facing intestinal and blood-brain barrier disruption139,140 caused by microbes, stress, dietary components, pollutants, alcohol, or over-the-counter drugs, gluten-originated exorphins can impact mental functions.6,41,42,65,66,131,137 Cognitive impairment and “brain fog” might be associated with CD,141,142 and responsiveness to gluten withdrawal has been reported.143–145
In a more holistic view, the association between oxidative stress, gene expression, dysbiome and its mobilome, impaired gut and brain permeability146 and gut inflammation associated with gluten-derived peptides, is interrelated and interconnected during the autoimmune cascade evolution in CD.
Gluten peptides are systemically distributed
Prolamins containing gluten are main nutritional staples, and processed food gluten is heavily consumed.6,41,42,65,66,94 Consequently, gluten is widespread in the environment, in the gut lumen, and in contact with the epithelial monolayer and mucosal immune systems. CD is considered a gradually developing, mostly hypo-symptomatic or even asymptomatic chronic enteric inflammatory condition. In reality, it can abruptly erupt as an acute, symptomatic, sometimes life-threatening event involving the gut and extra-intestinal peripheral organs.8,147 Many in vivo/ex vivo or in vitro models involving CD duodenal biopsies, intestinal cell lines, or incidental gluten intake have reported acute effects within 48 h of incubation or ingestion, which were recently summarized. These acute phenotypic, cellular, and laboratory events demonstrate the potential ability of gluten peptides to impact the entire human body.8,9,147,148 A major question is whether gluten/gliadin peptides pass the protective mechanical or immunological intestinal barriers to penetrate inside the body and reach remote compartments and organs. Several observations support the systemic distribution of these peptides, and suggested mechanisms include:
Transepithelial passage of gluten peptides
The discussion on how gliadin peptides pass the gut epithelial monolayer is ongoing, but it is known that both paracellular and transcellular pathways are involved.67 There are three methods of transporting molecules through a cell: endocytosis,67,149 endoplasmic reticulum-assisted transcytosis,150 and secretory IgA-transferrin receptor-assisted translocation of intact gluten peptides below the epithelium.151 Paracellularly, following gliadin digest binding to its CXCR3 receptor, increased zonulin levels compromise tight junction function by activating the EGFR-PAR2-MyD88-mediated signaling pathways, resulting in increased intestinal permeability.152
Most recently, Stricker S. et al. visualized gliadin peptide transport into CD enterocytes using intestinal biopsies and the RACE (Rapid uptake of Antigen into the Cytosol of Enterocytes) cell line.150 The nutrition-originated peptides were transported through the endoplasmic reticulum and deposited below the enterocyte monolayer. This deposition proves that luminal gluten peptides penetrate the epithelial barrier, hence, facing the mucosal and submucosal immune networks.
In fact, gluten-dependent subepithelial deposits involving IgA-tTG are among the hallmark markers for early CD, even in seronegative patients and before histological damage occurs.153 The cohabitation of gluten peptides with these specific IgA-tTG deposits in the subepithelial space reinforces the transepithelial transport of gluten peptides.13,154 Additionally, the immunogenic CD supra-molecule, a 33-residue peptide from alpha-2 gliadin, was directly visualized in gluten-sensitive macaques,155 and gluten-stimulated CD-specific enteric T cells were shown to increase the transepithelial flux of gluten peptides.156 TTG-gluten polymeric complexes are potent antigens for tTG-specific mucosal B cells, supported by diverse subepithelial gluten-specific T cells.157 Finally, gluten peptides can be presented by subepithelial dendritic cells.158 Thus, isolated or complexed gluten/gliadin peptides located in the lamina propria are presented by local antigen-presenting cells, activating the adaptive and innate mucosal networks and inducing CD-specific autoantibodies.
Gluten metabolites are found in human body fluids
Physio-anatomical logic indicates that urinary secretion of a peptide most likely originates from the bloodstream. Recently, Upadhyay D et al. reported that gluten sensitivity expresses itself in a potential CD at the metabolic level before any intestinal damage.159 Decreased levels of histidine, tyrosine, glycine, and tryptophan, and altered levels of another six metabolites were detected in the mucosa or plasma of potential CD patients compared to active CD patients and healthy controls. Intriguingly, raising the topic of gluten addiction and mental health, gluten metabolites, namely, exorphin B4 and B5, are found in normal human blood.160,161 A hypothetical mechanism for gluten masking its own toxicity by these gluten-originated exorphins has been suggested.138 Additionally, multiple gluten-dependent circulating miRNAs that appear before IgA-tTG positivity and are responsive to gluten withdrawal have been characterized.162,163
Urinary gluten metabolites have been extensively reported. Gluten dose escalation, gluten-free diet adherence assessment, urinary gluten intake-dependent miRNAs, urine peptidomics analysis, and urinary metabolic alterations have all been documented.159,163–166
Tissue transglutaminase whole body distribution and biological functions
Before describing gluten peptide distribution in tissues and organs, it is important to remember that tTG is ubiquitous in the human body.13 It is the autoantigen in CD,12 its prime substrate is gluten,94 and the enzyme induces posttranslational modification of gluten,29 making gluten immunogenic in several gluten-dependent conditions.13,66 Cellular-wise, the enzyme spans all intracellular organelles and compartments, including transmembrane areas, and is secreted extracellularly.13,167 Tissue-wise, it is ubiquitously expressed in most human tissues.13,167 Due to its enzymatic biochemical activities, tTG is involved in multiple human biological events and diseases.13,167,168 Since gluten peptides circulate systemically, the chances of tTG encountering them are high. The missing part of the tTG-blood-gluten triangle is the localization of gluten/gliadin metabolites in remote extra-intestinal organs.
Gluten metabolites are found in human organs
Because CD is a multifaceted condition with a plethora of extra-intestinal phenotypes, patients with the disease are at risk of developing remote organ pathologies.8,9,30,41,42,55,65,66 The enzyme tTG can cross-link numerous protein substrates, and the resulting aggregates can be deposited in various organs.169 Below are the main organs where tTG and gluten might orchestrate or be involved in local pathologies:
Gluten deposits in the cerebellum
Gluten ataxia is an autoimmune ataxia and an integral part of gluten-dependent ADs. Brain IgA-tTG2 and tTG6 deposits have been reported in patients, primarily in the cerebellum, pons, and medulla.170,171 Intense perivascular deposition and inflammation might allow circulating gluten peptide and IgA-tTG2/tTG6 antibody entry through a leaky blood-brain barrier, depositing in the central nervous system.172 Indeed, when serum from gluten ataxia patients was injected into the ventricles of mice, ataxia developed within 3 h post-injection.172,173 Moreover, cross-reactive antibodies are shared between gluten peptides and Purkinje cell epitopes, suggesting a potential molecular mimicry pathway to cerebellar autoimmunogenesis.174 Despite these findings, the characterization of gluten-IgA-tTG2/tTG6 deposits in the patient’s cerebellum requires further evaluation.
Gluten peptides impact chronic inflammatory brain conditions
The gut-brain axis was recently described,131,135,175,176 but the role of nutrients in activating these pathways is not clearly defined.6,8,9,29,30,177 Recent papers strengthen the potential relationship between gluten consumption and neurodegenerative, neuroinflammatory, and central ADs in susceptible individuals.65,66,134,178 It appears that gluten peptides contribute to neurodegeneration and chronic brain inflammatory diseases.65,66,178 By specific dysbiosis, enhanced gut permeability, many cross-reactive antibodies with sequence similarity to human brain epitopes, and multiple adverse effects described above, the circulating repertoire of gluten peptides is involved in neurodegeneration.65,66,134 The combined cross-reactivity and sequence similarity suggest molecular mimicry and allude to autoimmune mechanisms resulting in gluten-related brain conditions.
Gluten deposit in the thyroid
Hashimoto’s thyroiditis and CD often overlap, sharing genetic, environmental, symptomatic, immunogenic, pathological, hormonal, and even serological aspects.179 Both entities are integral parts of polyendocrinopathy syndrome.180 There is a continuous debate concerning whether and when to screen patients for CD serology and if there is a place for a gluten-free diet in autoimmune thyroid diseases.179,181–183 The tTG enzyme resides in the thyroid follicles and extracellular matrix.184,185 In CD patients, tTG antibodies bind to these thyroid regions, and their levels correlate with thyroid peroxidase antibody activity.185 The findings suggest that CD-associated antibodies could be involved in thyroid dysfunction, thus reinforcing the gut-thyroid axis. Supporting this, Vojdani et al. applied affinity-purified antibodies made against wheat, alpha-gliadin peptide, and wheat germ agglutinin to various human tissue antigens, finding moderate to strong reactions with thyroid peroxidase and many other autoantigens.44,186
Gluten deposits in the skin
Dermatitis herpetiformis is a dermatological gluten-dependent disease characterized by cutaneous anti-tTG3 IgA deposits, where tTG3c is the autoantigen of the disease.187 Interestingly, similar aggregates can be detected in the skin of CD patients.188 tTG3 is a member of the tTG family and can cross-link with its favorable gluten/gliadin peptides. The skin anti-tTG3 IgA deposits in dermatitis herpetiformis mirror the subepithelial IgA-tTG deposits in CD. The herpetiform dermatological eruption is only one of many gluten-dependent autoimmune manifestations of the skin.189
Gluten deposits in the pancreas
Type 1 diabetes mellitus is highly associated with CD,8,136,180,190 but the role of gluten in inducing insulitis remains controversial.191 Some researchers claim that gluten-containing cereals are associated with an increased risk of pancreatic islet autoimmunity,79,192–194 while others see no such connection.195 Notably, gluten peptides have been shown to localize in the pancreatic islets, enhancing beta-cell hyperactivity, increasing the expression of beta-cell antigens, and resulting in pancreatic autoimmunity.196 Additionally, the posttranslational modifications of human islet antigens induced by local tTG increase the affinity to HLA-DQ, improving presentation to the adjacent pro-inflammatory T cells and initiating the autoimmune cascade.197 Thus, gluten intake can provoke type 1 diabetes.198
Finally, applying a GFD to diabetes-prone animals reduced tTG activity in the pancreatic islets, reduced insulitis, and delayed or reduced diabetes incidence.196 Recently, Hansen et al. substantiated these beneficial effects of gluten withdrawal in NOD mice across three generations by modulating the systemic immune system in a microbiota-independent manner, probably through epigenetic modifications.199 Additionally, a GFD was shown to modulate inflammation in the salivary glands and pancreatic islets in NOD mice.200
The role of the pancreatic gluten-tTG axis in human type 1 diabetes mellitus requires further investigation.
Gluten deposits in the Kidney
The CD is associated with several renal abnormalities,201 with IgA nephropathy, also known as Berger’s disease, being highly affected by gluten.202 It is evident that interactions between tTG and gluten peptides occur in the kidneys of IgA nephropathy patients. Indeed, tTG is overexpressed in the gut of IgA nephropathy mice, and gliadin peptides participate in the disease pathology.203 The immune mechanism of gluten-induced nephropathy involves transferrin receptors, IgA1, gliadin peptides, and soluble CD89.202 As proof of concept, several CD patients on GFDs showed resolution of their autoimmune kidney disease.42,204 The systemic circulation of gluten peptides excreted via urine further strengthens the concept of the enteric gluten-kidney axis, where gluten withdrawal might be beneficial.
Gluten deposits in the liver
The gut-liver axis operates in CD, with several liver conditions associated with hepatic disease, ranging from isolated transaminasemia to autoimmune hepatitis.8,9,30,41,42,205,206 Recently, a causal relationship was demonstrated for hepatic IgA-tTG deposits in CD patients, showing 100% sensitivity and 85% positive predictive value, establishing the association between gluten consumption, liver IgA-tTG aggregates, and liver pathology in CD.207 Not surprisingly, the resolution of liver injury and disappearance of these colocalized deposits were demonstrated with GFD in patients.207
Gluten deposits in the heart
IgG and IgA immune deposition can be detected in the pericardium of individuals with gluten-dependent dermatitis herpetiformis suffering from recurrent pericarditis.208 However, the gut-heart axis presents multiple clinical phenotypes in CD, including atrial fibrillation, dilated cardiomyopathy, pericarditis, myocarditis, angina pectoris, myocardial infarction, and even death from anoxic heart disease.209–211 Some of these manifestations arise from hypercoagulability in CD, which is caused by multifactorial mechanisms involving nutritional deficiencies and autoantibodies.212–214 Alpha-enolase, a glycolytic enzyme expressed in most tissues that plays a role in many cell functions, has been identified as an autoantigen in Hashimoto’s encephalopathy. Recently, alpha-enolase has been suggested to play a role in the cardiac manifestations of CD.46 Regarding cardiac involvement, CD IgA-tTG antibodies have shown strong fluorescence when applied to heart structures.215,216
Gluten and neurodegenerative diseases
The well-established gut-brain axis has been thoroughly reported and reviewed.131,176,217 However, the topic of gluten involvement in neurodegenerative conditions has recently generated scientific and clinical interest.8,65,66,134,217,218 The current hypothesis is that indigestible luminal immunogenic gluten peptides are transported transcellularly and enter paracellularly. After crossing the blood-brain barrier, gluten peptides, cross-linked gluten complexes, gluten-induced antibodies, or gut-originated gluten-restricted CD4 T cells initiate and maintain proinflammatory cytokines, driving neurodegenerative diseases.65,66,131,134,217,218 Most recently, cross-reactive antibodies between gluten and human brain epitopes have been described.44,45,219–222 Specifically, cross-reactive antibodies between tTG, mTG, and amyloid-beta 1-42 have been identified,223 potentially contributing to intraneuronal deposition of A-beta-P-42 in Alzheimer’s disease Similarly, cross-reactivity between tTG, mTG, wheat proteins, and alpha-synuclein has been reported,220 reinforcing the role of gluten-tTG-mTG interactions in both Alzheimer’s and Parkinson’s diseases.
Both human endogenous and microbial exogenous transglutaminases are heavily involved in CD evolution. The tTG is the autoantigen in CD,12,13 while mTG, a heavily consumed processed food additive, is described as a potential driver in CD66,94,134,224–229 and systemic autoimmunity. Gluten is a prime substrate for both enzymes.66,94 Gliadin-cross-linked complexes formed by these enzymes elicit high antibody levels in untreated CD children,13–16,66,96,98,99 and the corresponding serological markers are very reliable for diagnosing gluten-sensitive enteropathy.14–16,97,99 Finally, sequence similarity between gluten and brain epitopes was recently detected for Parkinson’s disease and other neurodegenerative conditions.65,220 Both mechanisms—cross-reactivity and sequence similarity between gluten peptides—contribute to molecular mimicry, which may result in neurodegenerative, neuroinflammatory, and neuropsychiatric conditions.
A GFD might be beneficial in many non-celiac autoimmune diseases
If gluten is a proinflammatory and auto-immunogenic nutrient and is the offending toxic inflammatory molecule in gluten-dependent ADs, a major question arises considering its beneficial curative effects when withdrawn: Might a GFD be helpful for patients affected by non-celiac ADs? This topic was recently reviewed and summarized.41,42,55,230 In a recent systematic review summarizing 83 publications, we found that 911/1,408 AD-affected patients showed improvement on a GFD. Abstaining from gluten intake was found to be efficient in 80% of the publications and clinically beneficial to 65% of the patients.42 The following ADs were screened: rheumatoid arthritis, antiphospholipid syndrome, dermatomyositis, undifferentiated connective tissue disease, Raynaud’s phenomenon, spondylarthritis, psoriasis, vitiligo, pemphigus, erythema elevatum diutinum, inflammatory bowel disease, Crohn’s disease, ulcerative colitis, autoimmune hepatitis, primary biliary cholangitis, primary sclerosing cholangitis, autoimmune pancreatitis, autoimmune enteropathy, thyroiditis, Graves’ disease, Hashimoto’s disease, type 1 diabetes, Addison’s disease, autoimmune hypopituitarism, multiple sclerosis, myasthenia gravis, autoimmune myocarditis, autoimmune pericarditis, IgA nephropathy, uveitis, idiopathic thrombocytopenic purpura, and idiopathic dilated cardiomyopathy.42 We concluded that a GFD might be beneficial for some patients affected by ADs. We suggested screening autoimmune patients for CD-associated antibodies, and only those who test positive should consider gluten withdrawal.42 Overall, there is insufficient evidence to support a GFD for all AD patients, and official guidelines for patient selection have not yet been issued.42,55,230,231 The topic is still controversial, and some studies oppose gluten withdrawal in non-celiac ADs.232–234
Logically, a GFD might counteract the harmful effects of gluten consumption. Before detailing the potential pathways and mechanisms by which gluten withdrawal might alleviate the clinical phenotype, evolution, and behavior of ADs, the following is a summary of gluten’s side effects.
A challenging puzzle is the pathophysiological pathways and mechanisms by which gluten peptides induce inflammatory pathologies in various organs. Clarifying these mechanisms will improve our understanding of the beneficial effects of a GFD. The following summary (Table 1) is based on past and recent publications.6,8,9,14–16,25–30,34–39,41,42,55,65-68,70–82,89–119,121–130,134,224-230,235 It should be stressed that most studies were done ex vivo or on animal models. Substantiation of all of them in vivo, on humans, is highly encouraged.
Table 1Harmful effects and pathogenic mechanisms of gluten peptide-induced inflammation and cellular damage
| Detrimental effects and mechanisms of gluten peptide-induced pathology | References |
|---|
| Pro-inflammatory | 8,9,25–27,34–39,42,68,235 |
| Drive: cytotoxicity, apoptosis, LDH; Suppress: cell viability, differentiation, RNA, DNA and glycoprotein synthesis | 41,121,128–130 |
| Cellular stress induction | 42,68,70,71 |
| Induce zonulin production | 78,79,121 |
| Pro-oxidative | 42,72,121–123 |
| Epigenetics impact | 8,122,125–127 |
| Pro-apoptotic | 73,124 |
| Impact nutrigenomics, nutrigenetics, gene expression | 127 |
| Induce dysbiosis | 28–30,73–77 |
| Increase macrophage’s proinflammatory cytokine | 101–103 |
| Increase intestinal permeability | 6,78–82,121 |
| Enhance NO production | 102 |
| Immunogenic and induced antibodies | 89–93 |
| Upregulate MHCII, co-stimulatory molecules, TRLs, cytokine and chemokine production | 104,105 |
| Cross-linked to mTG immunogenicity | 8,9,14–16,29,30,41,66,94,96–100 |
| Stimulate TH1 cytokine profile | 108–110 |
| Enhance NO production | 102 |
| Increase intraepithelial lymphocyte and intestinal damage | 110 |
| Upregulate MHCII, co-stimulatory molecules, TLRs, cytokine and chemokine production | 104,105 |
| Activated intestinal CD4+ T cells, dendritic and TH17 cells, natural killer cell cytotoxicity | 106,111–113 |
| Increase expression of NKG2D and CD71 | 106,114 |
| overproduction of IL-17 | 112,115–117 |
| Induce TNFα and IL-1β | 107 |
| Increase IL-1β, activate NLRP3 inflammasome | 118 |
| Enhance neutrophil migration | 119 |