Advances in the pathophysiology, diagnosis, and management of celiac disease

Trends in global prevalence and incidence of celiac disease

The global prevalence of celiac disease is estimated to be between 0.7% and 1.5% in the general population.123 Prevalence estimates vary by screening method. A meta-analysis of epidemiological studies found that the global prevalence of celiac disease as estimated by serological testing alone was 1.4%, whereas the biopsy based prevalence was 0.7%.3 Prevalence also varies geographically: within Europe and North America, northern latitudes may be associated with higher celiac disease prevalence than southern latitudes, with particularly high rates in Sweden and Finland.520 Interestingly, evidence suggests that seroprevalence of celiac disease may also be higher in northern China than in southern China, but the overall prevalence in the country—and in other parts of Asia—seems to be lower than that in Europe.2122 More robust population level data are needed in Asia, Africa, and Latin America to confirm regional prevalence differences and to elucidate whether they are due to genetic or environmental factors.

The prevalence and incidence of celiac disease has been increasing over the past several decades.352324 One meta-analysis of population based studies found that the incidence of celiac disease has increased by 7.5% per year since the latter half of the 20th century.23 This trend seems to have continued in the past decade: a population based cohort study in Canada found that the incidence of positive celiac disease related serologies increased between 2015 and 2020 despite stable testing rates.24 Although such findings suggest that the rising incidence may be due to environmental effects, improved provider awareness and practice patterns may also play a role in this trend. A nationwide population based cohort study in Sweden, for instance, found that the incidence of small bowel biopsies diagnostic for celiac disease increased in parallel to non-diagnostic small bowel biopsies between 1990 and 2015.25

Environmental effects

The increasing incidence of celiac disease suggests that environmental factors are playing a role, but precise environmental effects have not been identified. Birth cohort studies and RCTs that explored the timing and amount of gluten exposure in early childhood produced mixed results with no strong evidence for a protective effect of specific feeding practices.6262728 Globally, the relation between regional gluten availability and prevalence of celiac disease has shown mixed associations.29 Neither duration of breastfeeding nor mode of birth reliably predicts the development of celiac disease.2630

Infections during childhood and adulthood, particularly those of the gastrointestinal tract, may be linked to a higher risk of celiac disease autoimmunity.3132 Exposure to antibiotics has also been associated with development of celiac disease, but robust data are lacking.3133 Alterations in the gut microbiome, which have been shown in children and adults with celiac disease, have also been hypothesized as a possible trigger for development of celiac disease, but a causal relation has yet to be established.3435

Genetic risk factors

The risk of celiac disease is higher in first degree family members of individuals with celiac disease, presumably owing in part to shared genetic risk factors.636 In one meta-analysis of studies involving 10 016 FDRs of patients with celiac disease, the pooled seroprevalence of celiac disease in FDRs was 11% and the pooled prevalence of biopsy confirmed celiac disease was 7%.7 Human leukocyte antigen (HLA)-DQ2 and/or HLA-DQ8 haplotypes are the most dominant predisposing genetic risk factors for celiac disease and are found in nearly all individuals with celiac disease.37 In a European birth cohort of FDRs with celiac disease and HLA-DQ2 and/or HLA-DQ8, the cumulative incidence of celiac disease was 17.5% by 10 years of age.36 Dozens of non-HLA alleles identified through genome-wide association studies have also been associated with increased risk of celiac disease, which may help to explain the phenotypic variability seen in its presentation.3839

Molecular and genetic basis of celiac disease susceptibility

HLA alleles that confer genetic susceptibility recognize cereal derived peptides and are considered necessary but not sufficient for disease development.37 Mouse models of celiac disease involve both the introduction of HLA-DQ2 or HLA-DQ8 and additional genetic modifications to potentiate intestinal inflammation.41 Several overlapping epitopes for HLA-DQ2.2, HLA-DQ2.5, and HLA-DQ8 are contained within α2-gliadin (residues 57-89, also known as “the 33-mer”), which is rich in proline residues that confer resistance to pepsin and trypsin digestion and rich in glutamine.42 Selective deamidation of the glutamine residues by tissue transglutaminase-2 and elastase introduces negative charges that increase HLA-DQ2 and HLA-DQ8 binding affinity.43 Notably, HLA-DQ8, which is stabilized by glutamate at anchor positions P1 and P9, is less penetrant than HLA-DQ2.5 and HLA-DQ2.2, which have high affinity for peptides with a negative charge at position 3, 4, or 5.4445

Role of tissue transglutaminase

Much of our understanding of the pathophysiology of celiac disease (fig 1) derives from studies of patients with established disease. The multifunctional enzyme tissue transglutaminase-2 plays a central role, as evidenced by the efficacy of the tissue transglutaminase-2 specific inhibitor ZED-1227 in preventing gluten induced duodenal mucosal damage.46 Generation of tissue transglutaminase-2-gliadin complexes enables a hapten carrier phenomenon whereby tissue transglutaminase-2 specific B cells can receive help from gluten specific CD4 positive T cells.47 Thus, most people with celiac disease produce tissue transglutaminase IgA autoantibodies in a gluten dependent manner.

Gliadin specific CD4 positive T cells

Tissue transglutaminase-2 and gliadin specific plasma cells in the lamina propria are the dominant antigen presenting cells in established celiac disease.4849 The antigen presenting cell that initially activates CD4 positive T cells when gluten tolerance is first broken has not been identified, and where this interaction occurs is not known. Candidates include CD11c positive, CD103 positive dendritic cells and intestinal epithelial cells.5051 Direct antigen presentation by intestinal epithelial cells is an appealing explanation because it implies that tissue transglutaminase-2 from shed enterocytes interacts with gliadin in the lumen where gluten concentrations are maximal.52 Furthermore, intestinal epithelial cells expressing major histocompatibility complex (MHC) class II molecules have increased expression of CD71,51 a receptor that has been shown to mediate retrotranscytosis of IgA-gliadin complexes.47

Delivery and presentation of immunogenic gluten peptides

Whereas short polypeptides resulting from microbial digestion of gluten may be transported via the paracellular leak pathway,43 passage of larger gluten peptides necessary to elicit a T cell response likely occurs via transcellular pathways such as CD71 mediated retrotranscytosis,51 low density lipoprotein receptor related protein 1 mediated endocytosis of gliadin-tissue transglutaminase-2-α2-microglobulin complexes,47 or persorption of intact gliadin via goblet associated pathways.53 Once activated, gluten reactive CD4 positive T cells produce inflammatory cytokines, including interferon γ, interleukin-21 and interleukin-2. Given the apparent specificity of gluten reactive CD4 positive T cells for celiac disease,54 cytokine release assays are being developed as a disease biomarker.555657

Role of cytotoxic CD8 positive intraepithelial lymphocytes

Activation of gut homing, gluten reactive CD4 positive T cells is a critical event in the pathogenesis of celiac disease, yet the condition has classically been defined by the CD8 positive T cell mediated enteropathy that ensues.58 Cytotoxic CD8 positive T intraepithelial lymphocytes in celiac disease are characterized by up-regulation of NKG2D and CD94/NKG2C receptors which interact with MHC class I chain related protein A (MICA) and HLA-E, respectively.59 These observations support the prevailing view that cytotoxic CD8 positive T intraepithelial lymphocytes are licensed to kill in the context of epithelial stress. Such stress is manifested by up-regulation of interleukin 15, which induces expression of NK (natural killer) receptors, and expression of non-classical MHC class 1 molecules on enterocytes, in addition to interferon-γ, interleukin 2, and other cytokines.6061 Notably, although mucosal damage is ascribed to αβ T intraepithelial lymphocytes, it is γδ T intraepithelial lymphocytes that are consistently and persistently increased in celiac disease irrespective of diet. Neither of these intraepithelial lymphocyte populations has been shown to recognize gluten, even though they are induced by gluten, and they may have redundant roles.62

Role of the innate immune system

Innate immune signaling likely has a role in the pathogenesis of celiac disease and involves a T helper type 1 response. A conserved non-immunogenic α2-gliadin peptide (residues 31-53) activates the innate immune system by a yet-to-be-defined mechanism,6364 whereas an immunogenic deamidated α2-gliadin peptide (residues 57-89) activates toll-like receptor 4.65 Non-gluten proteins (for example, wheat amylase tryptase inhibitors) also have innate immune activity.66 In this context, viruses and bacteria may act as adjuvants to potentiate the adaptive immune response.

Clinical presentation

Classically, celiac disease presented with signs and symptoms of malabsorption such as diarrhea, steatorrhea, or weight loss/growth failure. More recent evidence suggests that although being underweight is more common among people with a diagnosis of celiac disease than the general population, most are of normal weight or overweight at presentation.67 Furthermore, so-called “non-classical” presentations without symptoms of malabsorption have become more prevalent in all age groups.68 Non-classical presentations include extra-intestinal symptoms such as fatigue, joint pain, and headaches, as well as gastrointestinal symptoms such as bloating or abdominal pain. Non-classic gastrointestinal symptoms are non-specific to celiac disease and overlap with disorders of gut-brain interaction.6970 Some people report such symptoms after gluten ingestion but do not meet diagnostic criteria for celiac disease, a condition referred to as non-celiac gluten sensitivity.71

Many extra-intestinal manifestations of celiac disease should prompt serological screening (table 1). Dermatitis herpetiformis, characterized by itchy papulovesicular lesions on the legs, arms, and buttocks, is considered a hallmark extra-intestinal manifestation precipitated by exposure to gluten.72 Oral aphthous stomatitis and tooth enamel defects are also characteristic.73 Gluten ataxia, an idiopathic sporadic ataxia with positive antigliadin antibodies, can occur even in the absence of duodenal enteropathy.71

Many people do not have symptoms of celiac disease but have a diagnosis made during the investigation of common laboratory abnormalities such as iron deficiency anemia or elevated aminotransferases. A meta-analysis of epidemiological studies found that more than 1 in 30 adults with iron deficiency anemia have celiac disease.74 Celiac disease has also been cited as a cause of otherwise unexplained aminotransferase elevation, which may resolve with a GFD.7576

Diagnostic criteria for celiac disease

Antibodies to tissue transglutaminase-2 are the first line screening test for celiac disease, and small intestinal endoscopic biopsy showing characteristic features of intraepithelial lymphocytosis, crypt hyperplasia, and villus atrophy is considered the gold standard confirmatory test.58737778798081 Diagnosis follows a reverse sequence in some patients, whereby a finding of duodenal villus atrophy prompts serologic evaluation.82 These findings are manifestations of gluten induced immune activation. Consequently, normal test results cannot exclude celiac disease in patients on a GFD.

Serology

Antibodies to tissue transglutaminase-2, specifically tissue transglutaminase IgA and anti-endomysial antibody (EMA), are the essential screening tools for celiac disease. Both tissue transglutaminase IgA and EMA recognize the same autoantigen, so results are highly correlated.5878 EMA uses a tissue based fluorescence assay that is more labor intensive and operator dependent than automated recombinant tissue transglutaminase fluorescent assays,78 so tissue transglutaminase IgA is generally preferred. In a large international prospective cohort study, tissue transglutaminase IgA had a high sensitivity (98.0%) and specificity (75.0%) for the detection of duodenal villus atrophy in adults with suspected celiac disease.83 Tissue transglutaminase IgA antibodies are also highly sensitive and specific in pediatric populations, although the addition of anti-deamidated gliadin peptide (DGP) IgG may increase diagnostic sensitivity in children under the age of 2.84 When interpreting negative tissue transglutaminase IgA results, ruling out IgA deficiency and recognizing that different tissue transglutaminase IgA assays may perform differently across populations are important.8385

Given that development of autoantibodies to tissue transglutaminase-2 is exceedingly rare in people without HLA-DQ2 and/or HLA-DQ8 haplotypes, HLA genotyping may be useful for ruling out celiac disease in individuals in whom the diagnosis is unclear, including those who have already adopted a GFD.86

Histology

Upper endoscopy with multiple duodenal biopsies (including at least four from the distal duodenum and one to two from the duodenal bulb) is the gold standard diagnostic tool for celiac disease in all age groups.73 A large retrospective cohort study found that more duodenal biopsies increase the probability of a diagnosis of celiac disease,87 likely owing to variability in quality of biopsies and the patchy distribution of mucosal injury. Sampling of the duodenal bulb in addition to the more distal duodenum increases diagnostic yield and can identify individuals with ultra-short celiac disease,88 an increasingly recognized phenotype in those with positive serologies and villus atrophy confined to the first portion of the duodenum.89

Mucosal changes associated with celiac disease can be classified according to Marsh-Oberhuber criteria, with higher grades corresponding to villus atrophy, crypt hyperplasia, and presence of intraepithelial lymphocytes.9091 Complete villus atrophy (Marsh-Oberhuber grade III) has a high specificity for celiac disease. However, clinicians should rule out other causes of duodenal villus atrophy, including common variable immunodeficiency, infection (for example, tropical sprue, giardia), drug induced changes (for example, angiotensin receptor blockers, especially olmesartan), and inflammatory bowel disease.8292 This is especially important in people with negative celiac disease serologies and/or without intraepithelial lymphocytosis. Of note, some patients with positive celiac disease serologies may have architecturally normal duodenal mucosa (Marsh-Oberhuber grade 0) on initial biopsy, referred to as potential celiac disease.71 These patients are at increased risk of developing villus atrophy in the future: in one cohort of 31 children with potential celiac disease but no symptoms, 14 developed celiac disease during a mean follow-up period of nearly six years.93

Alternative confirmatory tests

In the past decade, some international societies have advocated using a second serologic test in lieu of small intestinal biopsy to confirm a diagnosis of celiac disease. In 2012 the European Society for Pediatric Gastroenterology, Hepatology and Nutrition (ESPGHAN) endorsed use of EMA IgA in a second blood sample and permissive HLA genotype as an alternative to biopsy for diagnosis of celiac disease in children with malabsorptive symptoms and a tissue transglutaminase IgA >10 times the upper limit of normal.94 Although recommendations were initially based on consensus, subsequent prospective data have shown high positive predictive values for this approach in European children with celiac disease.95 These criteria have been relaxed in the more recent ESPGHAN 2020 guidelines, with elimination of the requirement for genetic testing and shared decision making recommended for children without symptoms.58 In 2023 the American College of Gastroenterology offered a conditional recommendation for this approach.73 Children with type 1 diabetes were explicitly excluded from the ESPGHAN 2020 recommendation for serologic diagnosis owing to lack of data and concerns related to an increased false positive rate in this population. Notably, only a minority of children (and adults) have tissue transglutaminase IgA >10 times the upper limit of normal, so most still need biopsy for diagnostic confirmation.

Serologic confirmation has not yet been widely recommended for adults.77787980 Exceptions include Finland, where a serologic diagnosis has been offered to all patients since 2018.96 The American College of Gastroenterology guidelines suggest that a combination of high concentrations of tissue transglutaminase IgA and positive EMA in a second blood sample may be used as an “after the fact” diagnosis of likely celiac disease in adults already on a gluten-free diet or who are unwilling or unable to undergo intestinal biopsy.73 Evidence supporting a serology based diagnosis in adults first surfaced nearly three decades ago97 and has been validated in more recent prospective cohorts, including the use of DGP IgG rather than EMA as a confirmatory test.8398 A recent analysis found that tissue transglutaminase IgA concentrations ≥10 times the upper limit of normal had a positive predictive value of 98% for identifying individuals with celiac disease, suggesting that serologic diagnosis may have a role in adults at moderate-to-high risk with a high pre-test probability of celiac disease.99

Screening people without symptoms

Serological screening is recommended in certain individuals without symptoms who are at high risk for developing celiac disease. This includes FDRs of patients with celiac disease, who have approximately seven times the risk of developing celiac disease as the general population.587379100 Screening is also appropriate in people with strongly associated diseases or genetic conditions (see table 1). In some contexts, the association is strong enough that serological screening can be considered at multiple time points: screening can be considered at two and five years after diagnosis of type 1 diabetes, for instance, or at an increased frequency (for example, every two years) among genetically predisposed children with affected FDRs.36

Mass screening for celiac disease in people with no symptoms and at average risk is not routine in most countries, but it is an area of active debate. Although the US Preventive Services Task Force concluded that insufficient evidence exists to recommend for or against screening people without symptoms regardless of baseline risk of celiac disease,100 multiple lines of evidence support the notion of general population screening. Underdiagnosis of celiac disease seems to be common in part because symptom based screening, the default practice in the absence of mass screening, is limited by the heterogeneous and non-specific clinical presentation of celiac disease.101 Undiagnosed celiac disease has been associated with impaired growth and behavioral problems in children and with osteoporosis, cardiovascular disease, cancer, and mortality in adults.4101102103 Although the relative benefit of early diagnosis remains unknown, a cost effectiveness model in the Netherlands suggested that screening all children with tissue transglutaminase IgA at age 3 would be cost effective over time.104 Italy has recently implemented mass screening of children for both celiac disease and type 1 diabetes, which will provide much needed empirical data for this approach.105

Sociodemographic disparities in diagnosis of celiac disease

Access to healthcare resources may affect testing and diagnosis rates for celiac disease. In Sweden, where access to healthcare is universally available, socioeconomic status was not related to risk of celiac disease.106 In the US, however, a diagnosis of celiac disease was positively associated with higher income and proximity to a celiac disease specialty center and negatively associated with social deprivation.107 A recent global survey found that low income countries had decreased awareness of celiac disease among general physicians, fewer celiac disease trained dietitians, and fewer celiac disease patient support groups than did high income countries.108 Race may also be a factor: in the US, black patients with signs or symptoms of celiac disease are less likely to undergo duodenal biopsy than white patients.109 Seronegative celiac disease may also be more common in black people, which would make diagnosis and screening more difficult.110

Morbidity

People with celiac disease have an increased risk of developing other autoimmune diseases, presumably owing to a shared genetic predisposition and/or immune activation driven by exposure to gluten. The most well established association is among patients with type 1 diabetes, in whom the prevalence of celiac disease is between 5.1% and 6.0%.111 Multiple population based studies have shown that celiac disease is also positively associated with autoimmune thyroid disease, juvenile idiopathic arthritis, rheumatoid arthritis, psoriasis, and inflammatory bowel disease, among other conditions.112113114115 Whether undiagnosed (and therefore untreated) celiac disease further increases the risk of autoimmunity remains uncertain. Such an increased risk would support early diagnosis and treatment of celiac disease in people without symptoms, but existing epidemiological data are conflicting.116117

Celiac disease is also associated with liver diseases including autoimmune hepatitis, primary biliary cholangitis, and primary sclerosing cholangitis.118119 People with celiac disease also seem to be at increased risk of fatty liver disease associated with metabolic dysfunction,75120 perhaps related to a GFD, which can be high in sugar and total fat.121 A meta-analysis of cross sectional, case-control, and prospective cohort studies showed that individuals with cryptogenic cirrhosis had an increased prevalence of celiac disease (4.6%), higher than that of the general population.122

Celiac disease is associated with other systemic disease states, although the causal pathway is not entirely clear. For instance, population based studies have shown that individuals with celiac disease have increased risk of developing and dying from cardiovascular disease than do matched controls.8123 Older adults with celiac disease also have increased rates of end stage renal disease and frailty than do matched comparators.124125

Cancer and mortality

A nationwide cohort study found that celiac disease was associated with an overall increased risk of cancer relative to the general population (hazard ratio 1.11).9 A strong association exists between celiac disease and non-Hodgkin’s lymphoma, particularly enteropathy associated T cell lymphoma.9126127 In a nationwide case-control cohort study in France, people with celiac disease were four times more likely to develop non-Hodgkin’s lymphoma than were matched controls.127 Of note, the risk of lymphoproliferative disorders is increased in patients with celiac disease who have persistent villus atrophy on follow-up biopsy compared with those who have mucosal healing.128 Studies have also associated celiac disease with small bowel, hepatobiliary, pancreatic, esophageal, gastric, and non-melanoma skin malignancies but a decreased risk of breast cancer.9127129

People with celiac disease have an increased risk of overall mortality, driven in large part by malignancy. In a large population based cohort study from Sweden, people with celiac disease had a small but significant increased risk of mortality relative to controls (hazard ratio 1.21), with increased risk of death from cancer, cardiovascular disease, and respiratory disease.8 In a meta-analysis of 25 studies reporting mortality in celiac disease compared with the general population, although people with celiac disease had increased all cause mortality and increased mortality due to malignancy (particularly non-Hodgkin’s lymphoma) and respiratory disease, the mortality risk has been attenuated over time.10

Gluten-free diet

Lifelong avoidance of dietary wheat, barley, and rye—referred to as a GFD—is the recommended treatment for celiac disease. Intestinal and extra-intestinal symptoms are expected to improve within two to four weeks of starting a GFD.130 Serologies can normalize within the first year of starting a GFD and often decline significantly within months of starting.131132 To provide education about a GFD, multiple international guidelines recommend evaluation by a dietitian, preferably with expertise in celiac disease, soon after diagnosis.7377133 Better knowledge of GFDs by patients or their parents has been linked to improved adherence to a GFD and higher general and celiac disease specific quality of life.1314133134

Limitations of gluten-free diet

Adherence to a GFD varies widely despite best efforts by care givers and patients. In a systematic review of 49 studies of children with celiac disease, rates of “adequate” adherence ranged from 23% to 98%.14 Surveys of adults have found similarly high rates of non-adherence.1213

Inadvertent exposure to gluten seems to be common among patients on a GFD. Among 18 adults with reportedly good adherence to a GFD, two thirds had detectable amounts of gluten immunogenic peptides (GIP) in stool or urine over just 10 days of observation15; in most exposures, the gluten source could not be identified and was unsuspected.16 “Gluten-free” food labeling, defined by the Codex Alimentarius Commission as food containing <20 ppm gluten,135 is critical to GFD adherence and is supported by evidence that gluten doses above 50 mg/day (which may occur on ingesting sufficient quantities of foods with >20 ppm gluten) can result in intestinal damage in people with celiac disease.136 Significant inter-individual variability in gluten responsiveness was observed in this study and is a barrier to establishing a universal threshold for the maximum tolerable amount of gluten for a patient with celiac disease.136 Inadvertent gluten exposure may also occur from introduction of gluten during food processing, as with conventional oats, and speaks to the pervasiveness of gluten in contemporary western diets.137

The difficulty of GFD adherence can lead to hypervigilance among people with celiac disease. One cross sectional study of 80 teenagers and adults with celiac disease found that hypervigilance was associated with significantly lower quality of life scores.17 The burden of maintaining a GFD can be particularly difficult and socially isolating for children and adolescents.18 Specialty gluten-free products are often more expensive than gluten containing equivalents, creating an economic burden for patients with celiac disease that is rarely covered by health insurance.19

Persistent symptoms on GFD

Although celiac disease related symptoms generally improve within weeks of starting a GFD, 50-60% of patients experience persistent symptoms.130138 Inadvertent gluten exposure is the most common cause of ongoing symptoms, which manifests as persistent mucosal damage.139 In a multicenter study of patients with celiac disease and persistent villus atrophy on a GFD, ongoing gluten exposure or slow response to the GFD was implicated in nearly 80% of cases.140 As such, identifying and eliminating sources of gluten in a GFD is critical in patients with ongoing symptoms.

When gastrointestinal symptoms persist despite strict GFD adherence and normalization of villus crypt architecture, comorbid gastrointestinal disorders such as irritable bowel syndrome are an important consideration. An RCT of 70 patients with celiac disease and persistent intestinal symptoms found that a GFD low in fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAP) significantly reduced symptom burden relative to a GFD alone (mean difference in Gastrointestinal Symptom Rating Scale for intervention versus control: −10.8, 95% confidence interval −14.8 to −6.8).141

Evaluation of dietary adherence

Given that the most common cause of persistent celiac disease activity is ongoing gluten ingestion, consistent long term follow-up with a dietitian is paramount.7377133 This enables close monitoring of the nutritional balance of a GFD, which can be low in essential vitamins and fiber and high in processed or fatty foods.121143 Studies have linked GFDs to weight gain and cardiovascular disease.144145 Validated patient questionnaires, such as the Biagi Score and the Celiac Disease Compliance Assessment Test, are useful tools in the evaluation of persistent symptoms of celiac disease and may be superior to subjective self-reporting in monitoring adherence to a GFD.133146147

Serological monitoring

Monitoring serologies is recommended after initiation of a GFD in patients with celiac disease.133 Serologies that remain persistently positive or do not decline after a GFD is started may suggest ongoing exposure to gluten over time.131148 Of note, complete normalization of serology often takes longer (more than a year) in children with more severe disease at the time of diagnosis.149 Although tissue transglutaminase IgA is recommended for serological monitoring,133 recent evidence suggests that DGP IgA titers may correlate better with recent gluten exposure.150 Importantly, however, serological monitoring does not reliably predict either symptoms attributed to celiac disease or persistent mucosal injury. A meta-analysis of 26 studies comparing serologies with duodenal biopsy found that the sensitivity of tissue transglutaminase IgA and EMA for predicting persistent villus atrophy was just 0.50 and 0.45, respectively.151

Role of follow-up endoscopy with biopsy to assess end organ damage

High quality evidence is lacking on the routine performance of follow-up duodenal biopsy in patients with asymptomatic celiac disease without concerning clinical features. One cross sectional study (n=760) found that patients who had a repeat upper endoscopy at least one year after diagnosis of celiac disease had similar prognosis (over an average follow-up of eight years) to those who did not receive a follow-up endoscopy.152 Recent consensus based guidelines recommend against a routine re-biopsy strategy, but if this is done they advise waiting at least 12-24 months after diagnosis to allow time for mucosal healing on a GFD.133 Prospective data are needed to determine the optimal timing and frequency of endoscopy during long term follow-up.

Uncertainties about the role of routine follow-up endoscopy for patients with asymptomatic celiac disease contrast with widespread consensus that small intestinal biopsy is indicated in GFD adherent patients with ongoing symptoms, persistently positive serologies, worsening anemia, or signs of ongoing malabsorption.737779133 The location and number of follow-up biopsies should mirror recommendations for diagnostic sampling (that is, one to two from the duodenal bulb and four from the more distal duodenum).

A primary goal of follow-up endoscopy in patients with celiac disease is to identify individuals with persistent duodenal villus atrophy. Persistent villus atrophy has been found in 23-53% of patients on follow-up biopsy, and predicting it non-invasively can be difficult because it is not associated with symptoms or celiac disease specific serologies.140151152153154155156 Cohort studies have found that risk factors for persistent villus atrophy include poor adherence to a GFD, severe disease at diagnosis, older age at diagnosis, and lower educational attainment.140153154155156

Persistent villus atrophy on follow-up biopsy may be linked to worse long term outcomes. A large population based cohort of patients with celiac disease in Sweden found that persistent villus atrophy on follow-up biopsy was associated with an increased risk of developing lymphoproliferative disorders and osteoporotic fractures.128156 The association of persistent villus atrophy with mortality is less clear; a population based study found that persistent villus atrophy did not predict mortality,8 but two retrospective studies found a positive association.140157

Urinary and fecal gluten immunogenic peptides

Measurement of GIP concentration in the urine or stool is a promising tool for monitoring adherence to a GFD and detecting inadvertent gluten exposure. Following gluten ingestion, GIPs can be detected in urine for up to 24-48 hours and in stool for up to seven days, and GIP concentration seems to correlate with the amount of gluten ingested.158159160 Multiple prospective studies have shown that measurement of GIP, particularly fecal GIP, is more sensitive than standardized questionnaires, food diaries, symptom assessment, and serologies in detecting exposure to gluten in patients with celiac disease.16161162

Evidence suggests that the clinical utility of GIP testing is in the negative predictive value of multiple tests over an extended monitoring period, rather than a single GIP test at one point in time. In a prospective study of 94 patients with celiac disease on a GFD followed for 12 months, a negative urinary GIP measure at at least two distinct time points was associated with the absence of duodenal enteropathy on follow-up biopsy.161 In a separate prospective study of 58 patients with celiac disease, 89% of those with normal villus crypt architecture had no detectable urinary GIP throughout the course of the study.158

The routine use of urinary and fecal GIP detection devices in clinical practice has limitations, and further research is needed. Some high sensitivity assays, for instance, detect gluten concentrations that may not be clinically significant for most people with celiac disease.163 Furthermore, isolated GIP concentrations can be difficult to interpret owing to individual variability in gluten metabolism.16159 Prospective evidence is needed to optimize timing and frequency of GIP monitoring.

Refractory celiac disease

Refractory celiac disease (RCD) is diagnosed in patients with ongoing malabsorptive symptoms and persistent enteropathy on follow-up biopsy despite strict adherence to a GFD for at least a year.71 A diagnosis of RCD is rare, being estimated to occur in just 0.3-0.5% of patients with celiac disease, although prevalence estimates are limited by a lack of robust population based data.164165 Symptoms such as diarrhea, nutritional deficiencies, weight loss, anemia, and hypoalbuminemia predominate.166

RCD is categorized on the basis of the immunophenotype and T cell receptor clonality of duodenal intraepithelial lymphocytes as measured by flow cytometry. RCD type I is characterized by immunophenotypically normal intraepithelial lymphocytes with polyclonal expansion, which is indistinguishable from celiac disease. Distinguishing RCD type I from celiac disease with ongoing gluten exposure can be challenging and requires careful assessment by an expert dietitian and corroboration with celiac disease serologies, which are usually normal in RCD. The much rarer RCD type II is characterized by clonal expansion of aberrant CD8 positive intraepithelial lymphocytes that make up at least 20% of the total intraepithelial lymphocyte population.167 Cohort studies suggest that RCD type II is typically more severe at presentation and has a significantly worse prognosis than RCD type I, with more severe malnutrition and more frequent progression to enteropathy associated T cell lymphoma.168169 RCD 1, RCD 2, and enteropathy associated T cell lymphoma have recently been proposed to define a spectrum of gluten independent immune activation associated with increasing degrees of pathogenic mutations in immunoregulatory pathways.167

Given its rarity and poor prognosis, patients with RCD should be treated at referral centers with expertise in celiac disease. Corticosteroids, most commonly open capsule budesonide, are the first line treatment for both RCD type I and type II.166 A case series (n=57) found histological improvement with budesonide in 89% of patients with RCD type I and 92% of those with RCD type II.170 Subsequent studies have shown the importance of opening the capsule for budesonide to be delivered to and effective in the proximal small intestine.171 A definitive second line therapy for RCD, particularly for RCD type II, has not yet been determined. The predominance of JAK1 and/or STAT3 gain-of-function mutations in RCD type II suggests that newer therapies targeting these pathways, such as the Janus kinase inhibitor tofacitinib, may be more effective than chemotherapeutic agents that have traditionally been used.172 Small case series of autologous stem cell transplantation for therapy of RCD type II have also shown varying degrees of clinical remission.173174

Degradation of intraluminal gluten

Peptidases that degrade intraluminal gluten and prevent the formation of immunogenic gliadin peptides may be able to prevent downstream immune activation from inadvertent gluten exposure. Latiglutenase, an orally administered mixture of two gluten specific proteases, has shown promise in early phase clinical trials.176 In a double blind, randomized controlled phase 2b trial, 43 patients with celiac disease on a GFD with mucosal healing on baseline biopsy underwent a daily gluten challenge for six weeks while receiving either latiglutenase or placebo daily.177 On repeat duodenal biopsy after gluten challenge, a trend toward reduced mucosal damage was seen, as measured by change in villus height to crypt depth (Vh:Cd) ratio, in the latiglutenase group relative to placebo (−0.04 v −0.35; P=0.06). The latiglutenase group also had significantly fewer intraepithelial lymphocytes and reduced symptom severity relative to placebo.

Another drug in this class, TAK-062, is a synthetic gliadin peptidase activated in the acidic environment of the stomach that degraded 97-99% of gluten within 65 minutes of ingestion in phase 1 studies178; a phase 2 trial was recently completed. A separate enzyme, an Aspergillus niger derived prolyl endoprotease (AN-PEP), decreased the prevalence of severe symptoms of celiac disease but did not significantly reduce stool gluten excretion relative to placebo in patients on a GFD.179

Transglutaminase inhibition

Blocking gliadin deamidation with tissue transglutaminase inhibitors, which may reduce immune activation by decreasing availability of modified gluten peptides that bind HLA-DQ2/DQ8, is also being explored. One drug in this class, ZED 1227/TAK-227, seems to inhibit tissue transglutaminase both in the lamina propria and on the enterocyte surface.180 In a phase 2 randomized, placebo controlled trial of 163 patients on a GFD who underwent a gluten challenge, ZED1227 attenuated mucosal damage at three different dosages compared with placebo (for highest dose: 0.48 (95% confidence interval 0.20 to 0.77) difference from placebo in change in mean Vh:Cd from baseline).46 The highest dose of the drug also improved symptom and quality of life scores relative to placebo, with minimal serious adverse events.46

Immune tolerance induction

Several strategies to induce immune tolerance are in clinical trials for celiac disease and other immune mediated conditions with known driver antigens. The first attempted tolerogenic therapy in celiac disease, Nexvax2, a subcutaneous injection of deamidated synthetic gluten peptides designed to block the interaction between HLA-DQ2.5 and CD4 positive T cells, was discontinued for futility during a placebo controlled phase 2 randomized trial. An interim analysis of 67 patients with celiac disease found that Nexvax2 did not change gastrointestinal symptoms relative to placebo following a masked gluten challenge (P=0.43).181

TAK-101 is an intravenously delivered nanoparticle containing encapsulated gluten proteins that is taken up by antigen presenting cells in the liver and spleen, thereby reducing presentation and subsequent activation of CD4 positive T cells.182 In a placebo controlled phase 2a trial of 33 patients with celiac disease on a GFD who received a 14 day gluten challenge, TAK-101 reduced the production of gluten specific T cell derived interferon-γ forming units by 88% relative to placebo (2.01 v 17.58; P=0.006).182 Minimal adverse events were reported and a non-significant trend toward attenuating mucosal damage was seen with TAK-101; a phase 2 study is ongoing. Another drug in this class, KAN-101, is a liver targeting glycopolymer conjugated to a gluten peptide designed to induce immune tolerance in the liver by binding to HLA-DQ2.5.183 In phase 1 trials, KAN-101 attenuated the rise in circulating interleukin-2 following oral gluten challenge, and results of a recently completed phase 2 trial are awaited.183

Tight junction modulators

Drugs that decrease mucosal permeability by modulating tight junctions in the intestinal epithelium have been studied with mixed results. One such drug, larazotide acetate, seemed to reduce celiac disease related symptoms in a phase 2 trial,184 but a phase 3 trial was subsequently discontinued owing to lack of significant benefit at interim analysis. More recently, a small molecule modulator, IMU-856, showed attenuation of mucosal damage relative to placebo during a gluten challenge in 43 patients with celiac disease (mean decrease in villus height: −22.5 (standard deviation 51.1) μm in IMU-856 160 mg group and −60.3 (52.2) μm in placebo group).185

Monoclonal antibodies

Biologic therapy with monoclonal antibodies has shown success in other autoimmune diseases and multiple food allergies and is being studied for celiac disease. Monoclonal antibodies against interleukin 15, a cytokine involved in cytotoxic T cell activation, may decrease intraepithelial lymphocyte expansion after gluten exposure in celiac disease, but early studies have yet to show attenuation of mucosal injury after a gluten challenge.186 DONQ52, a monoclonal antibody designed to bind to HLA-DQ2.5 molecules and block gliadin presentation to T cells, has shown promise in pre-clinical trials.187 Investigators are also exploring an anti-tumor necrosis factor monoclonal antibody against OX-40, a receptor involved in the activation of celiac disease specific intestinal CD4 positive T cells.188 Phase 2 trials are also ongoing to assess whether drugs approved for other indications can be repurposed for celiac disease, including ritlecitinib (an oral combined JAK3/TEC kinase inhibitor) and amlitelimab (which targets OX-40L thereby preventing activation of CD4 positive T cells).

Other therapeutic strategies

Many other therapeutic strategies are under investigation. For instance, genetically modified gluten may allow patients with celiac disease to safely consume low immunogenic wheat products.189 Sequestering agents that bind gluten in the intestinal lumen and prevent uptake in the gut also hold promise.190 Manipulating the microbiome, which is altered in people with celiac disease, is another potential therapeutic strategy in the future.

Standardizing therapeutic clinical trials in celiac disease

Given the variability in the methods used in drug trials in celiac disease in the past decade, better standardization is needed to optimize drug development. To this end, both the US Food and Drug Administration and an international panel of experts have published guidance on inclusion criteria, gluten challenges, and outcome measures to use in RCTs of drugs in celiac disease.191 Histological outcomes are recommended as an endpoint in RCTs that use a gluten challenge, given that patient reported outcomes are subject to individual variability and are not reliable markers of acute gluten mediated immune response.192 Recommended histological outcomes include Vh:Cd ratio and intraepithelial lymphocyte density, quantitative metrics that are more objective than traditional measures of celiac disease related mucosal injury such as the Marsh-Oberhuber score.191193 A composite Vh:Cd and intraepithelial lymphocyte scale may be an even better measure of mucosal injury than either component alone, as evidenced in a post hoc analysis of the phase 2 latiglutenase trial.177194

Biomarkers of disease activity in celiac disease

Antibodies used in clinical practice, such as tissue transglutaminase IgA, are essential diagnostic tools but imperfect markers of celiac disease severity and disease activity. Advances in our understanding of the immunopathogenesis of celiac disease have identified T cell derived cytokines that may better identify people with celiac disease. Cytokine release assays for interleukin 2 and interferon-γ, for instance, are in development and have already been used as surrogate endpoints in clinical drug trials.555657195 Serum proteomic biomarkers are also an area of active study, including markers to predict the development of villus atrophy in children with potential celiac disease.93 Tissue biomarkers, including transcriptomic signatures and proteome scores, are also being explored to glean more detailed information about disease activity than can be obtained from conventional histology.196197

Software assisted histological analysis

Digitization of pathology slides enables automated and software assisted histological assessment of intestinal tissue. This is of particular interest given current limitations in histological analysis of duodenal tissue, which include poorly oriented slides and inter-observer variability in Marsh classification.95198 Investigators have developed a mathematical morphological approach to duodenal biopsies, which is scale and staining agnostic, to accurately predict Vh:Cd ratios.199 Neural network and machine learning techniques have also been trained by pathologists to quantitate and score mucosal injury in celiac disease.200 Such approaches have potential not only to increase the sensitivity of celiac disease diagnoses but also to disseminate celiac disease specific histological analysis to regions that lack expert pathologists.

Clinical questions after approval of drug therapies for celiac disease

Regulatory approval of one or more of the many drug therapies under development would introduce a host of clinical questions (table 3). Initial registration trials will likely focus on adults who have not had an adequate response to the GFD. How this will be reflected in the drug label and affect access is unclear. Patients and clinicians are interested in the use of safe and effective therapies for a broader population, including those without symptoms who have intestinal damage and those hoping to liberalize their GFD. Given that celiac disease often develops in childhood, including children and adolescents in later phase drug development programs is essential.