Advances in understanding and managing celiac disease: Pathophysiology and treatment strategies

Abstract

In this editorial, we comment on an article published in the recent issue of the World Journal of Gastroenterology. Celiac disease (CeD) is a disease occurring in genetically susceptible individuals, which is mainly characterized by gluten intolerance in the small intestine and clinical symptoms such as abdominal pain, diarrhea, and malnutrition. Therefore, patients often need a lifelong gluten-free diet, which greatly affects the quality of life and expenses of patients. The gold standard for diagnosis is intestinal mucosal biopsy, combined with serological and genetic tests. At present, the lack of safe, effective, and satisfactory drugs for CeD is mainly due to the complexity of its pathogenesis, and it is difficult to find a perfect target to solve the multi-level needs of patients. In this editorial, we mainly review the pathological mechanism of CeD and describe the current experimental and improved drugs for various pathological aspects.

Key Words: Celiac disease; Gluten-free diet; Pathology; Human leukocyte antigen; Immunotherapy; Treatment; Aspergillus niger prolyl endoprotease

Core Tip: This editorial provides a comprehensive review of the pathophysiological mechanisms underlying celiac disease and explores emerging therapeutic strategies, including the potential role of Aspergillus niger-derived prolyl endopeptidase. The discussion emphasizes the need for new treatments that address the multifaceted nature of the disease, aiming to improve patient quality of life beyond the limitations of a strict gluten-free diet.

INTRODUCTION

Celiac disease (CeD) is mainly characterized by gastrointestinal symptoms, including abdominal pain, diarrhea (fatty diarrhea), and weight loss, as well as more and more attention to extra-intestinal manifestations, such as anemia, osteoporosis, liver damage, and epilepsy[1]. Some patients are complicated with other diseases, such as type 1 diabetes and thyroiditis[2,3]. The main manifestations of CeD are intestinal villus atrophy and secondary nutritional malabsorption, which leads to intestinal mucosal villus atrophy, crypt hyperplasia, and intraepithelial lymphocyte (IEL) increase due to the intake of gluten[4-6]. Surveys and statistics have shown that the global prevalence rate of CeD is about 1.4%[7,8]. When combined with other major diseases, patients with CeD often need a more strict gluten-free but also nutritious diet to meet the needs of the body[9].

At present, genetic factors are considered to be the main cause of CeD[10]. More than 90% of the patients were positive for human leukocyte antigen (HLA)-DQ2 or HLA-DQ8[6,11]. However, the positive target gene does not indicate that the patient must have CeD, and the significance of negative gene test is much clear. It is widely proved that HLA-DQ2 or HLA-DQ8 negativity can basically exclude CeD[7]. The gold standard for diagnosis of CeD is multiple intestinal mucosal biopsies under gastroenteroscopy[12,13]. Serological antibody detection is an important means for screening and diagnosis of CeD. The antibodies with diagnostic value mainly include anti-endomyocardial antibody (EMA), anti-tissue transglutaminase antibody (anti-tTG), anti-deacylated glial protein peptide, and anti-gliadin antibody (AGA)[14,15]. Previous research showed that serum anti-tTG IgA (tTG-IgA) had a sensitivity of 98% and specificity of 75% for CeD diagnosis[16]. As a result, in adults with a strong suspicion of CeD and elevated serum tTG-IgA levels, it may be reasonable to forgo biopsy for diagnosis.

Gluten-free diet (GFD) is the first and main choice for CeD treatment, and is currently the most effective way[15,17]. However, due to the different proportions of gluten in the diet structure of different countries and regional cultures and the fact that the risk of involuntary intake could not be assessed everywhere, GFD would increase the food cost of patients' daily life. Therefore, completely GFD is difficult to achieve and affects the daily life and social interaction of patients[18]. Therefore, there is an urgent need to clarify the comprehensive and accurate pathological mechanisms of CeD and develop new drugs or treatments on this basis to improve the quality of life and reduce the cost of life of CeD patients. This editorial reviews the pathological mechanisms of CeD and the possible effective drugs in the future.

PATHOGENESIS

Genetic factors

The majority of CeD patients are positive for the susceptibility gene HLA-DQ2 or HLA-DQ8, which is the most important part of the pathogenesis of most CeD[6,11]. HLA class I and class II genes are located on the 6^th of chromosomes of major histocompatibility complexes[19]. We summarize the information of possible HLA genes and single nucleotide polymorphisms associated with CeD in Table 1 and Table 2. These genes encode HLA-DQ2/DQ8 on the surface of antigen-presenting cells (APCs), and they bind to gluten and are degraded in the small intestine to form HLA polypeptide complexes, which are presented to CD4⁺ T cells[20,21]. APCs secrete interleukin (IL)-15 to promote the proliferation of intraepithelial lymphocytes (IELs) interspersed between or beneath epithelial cells, express natural killer cell receptor (NKG2D) on the cell surface, and induce epithelial cells to express the NKG2D ligand major histocompatibility complex related gene A (MICA). NK cells that express NKG2D recognize MICA and kill the intestinal epithelial cells (IECs) that express MICA on the surface, thus increasing intestinal permeability[22-24]. On the one hand, T cells activate lymphoid B cells to differentiate into plasma cells, which secrete antibodies against gluten and transglutaminase 2[25,26]. On the other hand, T cells secrete pro-inflammatory factors such as interferon-γ (IFN-γ) and IL-21, which cause damage to the intestinal epithelium and enhance its permeability[27].

Table 1 Information of human leukocyte antigen genes related with celiac disease.

HLA gene name	Encoding alleles	Chromosome location	Association with celiac disease
HLA-DQ2.5[78]	DQA1*05, DQB1*02	6p21.3	Strongly associated with celiac disease; present in 90%-95% of patients
HLA-DQ8[79]	DQA1*03, DQB1*03: 02	6p21.3	Associated with celiac disease; present in 5%-10% of patients
HLA-DQ7[80]	DQA1*05, DQB1*03: 01	6p21.3	Possibly associated with celiac disease; lower risk, often requires combination with other high-risk genes
HLA-DR3[81]	DRB1*03	6p21.3	Typically coexists with HLA-DQ2.5, indirectly increases celiac disease risk
HLA-DR4[82]	DRB1*04	6p21.3	Risk with celiac disease is low, but in some cases may lead to the disease

HLA: Human leukocyte antigen.

Table 2 Key single nucleotide polymorphisms associated with celiac disease.

Single nucleotide polymorphism	Gene	Location	Variation(s)	Association with celiac disease
rs2187668[83]	HLA-DQA1	chr6: 32638107	C>T	Strongly associated with celiac disease; often used as a marker for the HLA-DQ2.5 haplotype
rs7775228[84]	HLA-DQB1	chr6: 32690302	T>A, T>C	Associated with the HLA-DQ8 haplotype, another strong genetic risk factor for celiac disease
rs3135388[85]	HLA-DRB1	chr6: 32445274	A>C, A>G, A>T	Linked to the HLA-DR3 haplotype, which may indirectly increase celiac disease risk when co-inherited with DQ2.5
rs13015714[86]	IL18R1	chr2: 102355405	G>A, G>T	Associated with increased risk of celiac disease and severe symptoms due to its role in immune response regulation
rs1738074[87]	TAGAP	chr6: 159044945	T>C	Linked to immune function and associated with increased susceptibility to celiac disease
rs3087243[88]	CTLA4	chr2: 203874196	G>A, G>T	Rare variant associated with autoimmune conditions; may exacerbate celiac disease symptoms when present
rs3184504[85]	SH2B3	chr12: 111446804	T>A, T>C, T>G	Involved in immune response, linked to increased risk of autoimmune diseases, including celiac disease
rs13031237[89]	REL	chr2: 60908994	G>T	Associated with celiac disease and other autoimmune disorders, may affect the severity of the disease

HLA: Human leukocyte antigen.

Gluten

Gluten is the main protein component in a variety of grains (such as wheat, barley, and rye)[28]. It can be broken down into amino acids and peptides by enzymes in the lumen and on the brush edge of the small intestine. Gluten mainly includes gliadin and glutenin, in which gliadin rich in glutamine and proline, is the main antigen protein leading to CeD, and its decomposition products are difficult to be digested by digestive enzymes in intestinal lumen[13,29].

The activation of intestinal immune response by gluten is mainly through two aspects: Innate immunity and adaptive immunity. Regarding innate immunity, the non-immunogen dominant peptide (P31-P43) in gluten could directly promote the proliferation and differentiation of epithelial lymphocytes, the proliferation of CD8⁺ T cells and NK cells, and the production of IFN-γ[29]. In addition, it induces the expression of NKG2D receptor ligands in the intestinal epithelium and enhances the killing effect of NK cells on normal IECs, resulting in increased intestinal epithelial permeability[22-24]. With regard to adaptive immunity, due to the increase of intestinal mucosal epithelial permeability caused by infection or innate immunity, some larger peptides in gluten (such as 33mer) could enter the intestinal lamina propria through the intestinal mucosal barrier[30]. After deamidation by tTG, it is easier to be captured by APCs and presented to CD4⁺ T cells[31]. 33-mer peptide could promote the proliferation and differentiation of B cells to produce antibodies such as AGA, anti-EMA, and anti-tTG actibody[32,33]. Because the damaged intestinal cells can express CD71 transporter on the apical side, the secretory IgA-gliadin complex enhances the transport of gluten from the lumen to the lamina propria by reversing endocytosis[34,35], and that is also the reason why gluten immunogenic peptides (GIPs) could cross the epithelial cells into blood stream and be tested out in urine[36,37]. Finally, the interaction between CD4⁺ T cells and gliadin in the lamina propria of the intestinal tract induces their activation and proliferation, and induces crypt proliferation by producing pro-inflammatory cytokines which also causes chemotaxis of immune cells[38], and villi would become blunt after IEC death[39].

Intestinal bacterial populations

Intestinal bacterial populations can be categorized into three groups based on their interaction with the host: Probiotics (e.g., Lactobacillus and Bifidobacterium), pathogenic bacteria (e.g., Clostridium and Enterococcus faecalis), and opportunistic pathogens. There was a disorder of digestive tract bacterial populations in patients with CeD, and the abundance and diversity of beneficial bacteria decreased while those of pathogenic bacteria increased[40]. The human body lacks proteases that could fully digest gluten, and many genera of bacteria in the intestine have been found to be related to gluten metabolism (such as Lactobacillus, Streptococcus, Staphylococcus, Clostridium, and Bifidobacterium). These bacteria are mainly colonized in the large intestine and partly distributed in the small intestine[41,42]. The sensitivity of different CeD patients to different intake of gluten may also be related to the disorder of intestinal bacterial populations[43,44].

PRESENT TREATMENTS IN TRIALS

Peptidase therapy

Aspergillus niger-derived prolyl endoprotease: Aspergillus niger-derived prolyl endoprotease (AN-PEP), a kind of PEP extracted from Aspergillus niger, can effectively degrade the antigen protein in gluten, thus reducing its immunogenicity and avoiding intestinal damage[37]. The advantage of this kind of PEP is that it is an oral enzyme with activity between pH 2-8 and 4-5, so it can resist pepsin digestion without any special treatment[45]. Previous studies have shown that oral AN-PEP can enhance the degradation and digestion of gluten in healthy people, and similar result has been obtained in patients with gluten sensitivity[46]. Oral AN-PEP has also been found to degrade most of the gluten collagen before the chyme reaches the small intestine[47,48]. In in vitro model experiments, it was found that dietary composition affected the degradation efficiency of AN-PEP for gluten. For example, the addition of carbonated beverage strongly enhanced the activity of AN-PEP due to its acidification, fat did not affect the degradation of glutenin by AN-PEP, but the presence of food protein slowed down the detoxification of glutenin[49]. In this study, compared with the placebo group, AN-PEP treatment did not significantly reduce the total fecal concentration of GIPs, and there was no statistical difference in serological tests, but the clinical information system questionnaire showed that the rate of patients with severe symptoms in the AN-PEP group was significantly lower, suggesting that the auxiliary effect of AN-PEP on the GFD is still worthy of further exploration[50].

ALV003: ALV003, also known as latiglutinase, is an experimental drug composed of two recombinant proteases administered orally: ALV001 and ALV002[51]. While ALV001 is recognized as a cysteine endoprotease B-isoform, and ALV002 as a prolyl endopeptidase, previous clinical studies have shown that ALV003 has higher safety. No serious adverse reactions and consequences were found after a single intake of 1800 mg of ALV003, and the symptoms and quality of life of patients were improved at the same time[52]. Recent studies have shown that compared with the placebo group, ALV003 could effectively protect the intestinal mucosa and improve symptoms, and lower the levels of GIPs in urine[53,54]. It is reasonable to infer that most gluten decomposes in the gastrointestinal tract due to the action of ALV003 and fails to enter the intestinal lamina propria.

AGA: AGA is a polyclonal IgY antibody against gluten. Because of its cross-reactivity, it can neutralize all CeD-inducing prolamins. IgY was collected from the yolks of highly immunized hens, sprayed and dried, and then encapsulated with 50% mannitol[55]. In a clinical trial of patients with a GFD, 1 g of AGA was given per meal, and 10 patients had fewer celiac symptoms (especially tiredness, headache, and bloating), improved quality of life, lowered antibodies, and lactulose/mannitol excretion ratio when compared to the run-in period[56].

TAK-062: TAK-062, derived from the enzyme kumamolisin of Alicyclobacillus sendaiensis and engineered to enhance its proteolytic activity, specifically targets proline-glutamine (P-Q) dipeptide motifs and is designed to maintain its proteolytic function even in the varying pH levels and presence of proteases in the gastrointestinal tract[57,58]. TAK-062 is expected to work irrespectively of meal composition. In vitro, TAK-062 at a dose of 900 mg was well-tolerated and degraded more than 99% of gluten (3 g and 9 g) within 10 min[58].

TG2 inhibition: ZED1227 is a small molecule tTG2 inhibitor. The compound selectively binds to the active state of tTG2, forming a stable covalent bond with the cysteine in its catalytic center[59]. Clinical trials have confirmed that ZED1227 has high biological safety, and it is still safe and effective when the dose is up to 500 mg. In a proof-of-concept trial, it was confirmed by intestinal mucosal biopsy that ZED1227 could effectively reduce intestinal mucosal damage with the increase of drug dose (10 mg, 50 mg, and 100 mg)[60].

IL-15 antibody: The pro-inflammatory cytokine IL-15 has been identified as the main pathophysiological mediator of CeD[61]. IL-15 is produced by APCs and epithelial cells in the small intestine and is a necessary factor for the activation and proliferation of IELs[61,62]. IELs, mainly CD8⁺ T cells, destroy IECs and cause villus atrophy, which is characteristic of CeD. AMG714 is the first anti-IL-15 monoclonal antibody to be investigated for the treatment of CeD. In a clinical trial, patients with CeD who received a long-term GFD were treated with subcutaneous injection of AMG714 twice every 2 wk for 10 wk (a total of 6 doses) and gluten stimulation (2-4 g per day) from 2 to 12 wk. Compared with the placebo group, AMG714 did not prevent intestinal mucosal damage caused by gluten exposure at 150 mg and 300 mg doses[63]. However, at 300 mg, the increase of IELs in the treatment group was less significant and the symptoms were milder. In addition, another clinical trial found no difference in the main end points of abnormal IELs between patients with refractory type 2 CeD who received AMG714 or placebo for 10 wk[64].

Intestinal barrier repairing

Larazotide acetate (also known as AT-1001) is a synthesis of occlusive zone toxin produced by Vibrio cholerae[65]. Zonulin is the main regulatory element of paracellular junction. The main role of AT-1001 is to act as an anti-zonulin receptor inhibitor to reduce the increase of intestinal barrier permeability mediated by zonulin[66]. Existing clinical trials have confirmed its good biological safety, and the improvement of intestinal symptoms is better in patients treated with AT-1001 combined with a GFD than the GFD alone, but there is no statistical difference in lactulose-to-mannitol ratio when compared with the placebo group[67,68]. Whether there are other mechanisms for the effect of AT-1001 on intestinal permeability and how to apply it in clinical practice need further research.

Inducing immune tolerance

Gluten enters the lamina propria of the mucosa and is deacylated by TG2 and then presented to CD4⁺ T cells by APCs, which then activates a series of immune responses, resulting in intestinal mucosal damage. Therefore, it is expected to become a target for the treatment of CeD by interfering with antigen presentation and activating T cells, inducing immune tolerance, and reducing the activation of CD4⁺ T cells.

TAK-101: TAK-101 is a drug in which gluten (a complex subcomponent of natural wheat gliadin) is encapsulated in negatively charged polylactic acid-glycolic acid nanoparticles[69]. In a mouse model of CeD, it has been verified that the drug is captured by immune cells in the liver and spleen and immune tolerance is induced by intravenous administration[70]. Existing clinical trials of gluten exposure show that the level of IFN-γ in the treatment group is lower than that in the placebo group, suggesting that the therapeutic effect of the drug is definite[71]. However, the drug does not seem to reduce symptoms such as nausea and vomiting, and further trials are needed.

KAN-101: KAN-101 is also a drug that induces immune tolerance in the liver by intravenous injection. It consists of a synthetic liver-targeted glycosylated polymer that is connected to a synthetic immunodominant deaminated gliadin peptide (KAN0009) that binds to HLA-DQ2.5[68]. The possible mechanism is to inhibit the immune response of CD4⁺ T cells by activating Treg cells[72]. Its biosafety has been confirmed in clinical trials but needs further clinical verification.

Nexvax2: Nexvax2 is composed of three synthetic peptides and contains six HLA-DQ2.5 restricted immune dominant T cell epitopes, which is similar to the induction of bystander inhibition to play the role of immune tolerance[73]. Unfortunately, the follow-up clinical trial was terminated because the symptom improvement and serological tests did not meet the requirements.

Intestinal microbial therapy

VSL#3 probiotic preparation has the capacity to hydrolyze gliadin polypeptides responsible for celiac sprue, which was testified in vitro[74]. However, in a multicenter study, the probiotic preparation VSL#3 was tested with a GFD in 45 adults with symptoms of CeD, but there was no significant improvement in symptom severity and fecal microbiota[75]. A clinical trial showed that the treatment combined with lactic acid bacteria could improve gastrointestinal symptoms and increase the possible presence of lactic acid bacteria, staphylococci, and bifidobacteria in stool[76]. In addition, another trial explored the effects of Bifidobacterium on 22 untreated adult patients with active CeD. The results showed that the probiotics improved gastrointestinal symptoms and normalized immune markers, including a reduction in the number of α-defensins and Paneth cells in duodenal biopsy samples. At the end of the study, all patients were diagnosed with CeD by duodenal biopsy[68,77].

CONCLUSION

The pathological mechanism of CeD is complex (Figure 1). It is most important to find the key points in the pathogenesis of CeD and develop targeted drugs to interrupt the process of immune response. Glutenin and drugs that can degrade or bind gluten are expected to improve patients' quality of life and symptoms when their diet is contaminated with gluten. Immunosuppressants for pathological processes are expected to completely save patients from the suffering of a strict GFD. In addition, in many clinical trials, there is a phenomenon that symptoms are not completely consistent with serological examination and intestinal mucosal biopsy results. CeD patients often experience subjective symptoms such as abdominal pain and diarrhea. It is important to explore whether there is a temporal or spatial discrepancy between these symptoms and medical examination findings, and how they may be linked to the patients' emotional state, the brain-gut axis, and intestinal flora. Further research is needed to better understand these relationships.

Figure 1 Pathophysiological mechanisms of celiac disease. Gluten or gliadin crosses the epithelial cells, and after the deamidation by transglutaminase-2 (TG2), they become easier to be recognized by antigen presenting cells (APCs) expressing HLA-DQ2/8. APCs could secrete interleukin (IL)-15 which causes damage to epithelial cells directly or through activation of intraepithelial lymphocytes expressing natural killer cell receptor. Also, APCs present the deamidated polypeptides complex to CD4⁺ T cells which further secrete interferon-γ and IL-21 to cause damage to epithelial cells. In another way, CD4⁺ T cells activate B cells to differentiate into plasma cells with secretion of antibodies against gluten, TG2, and deamidated polypeptides complex. TG2: Transglutaminase-2; IL: Interleukin; HLA: Human leukocyte antigen; APC: Antigen presenting cells; TCR: T cell receptor; IELs: Intraepithelial lymphocytes; IFN-γ: Interferon-γ; NKG2D: Natural killer cell receptor.