Development of a novel autoantibody assay for autoimmune gastritis in type 1 diabetic individuals

Update on Serum Biomarkers in Autoimmune Atrophic Gastritis

BACKGROUND

Autoimmune atrophic gastritis (AAG) is a persistent, corpus-restricted immune-mediated destruction of the gastric corpus oxyntic mucosa with reduced gastric acid and intrinsic factor secretion, leading to iron deficiency and pernicious anemia as a consequence of iron and cobalamin malabsorption. Positivity toward parietal cell (PCA) and intrinsic factor (IFA) autoantibodies is very common. AAG may remain asymptomatic for many years, thus making its diagnosis complex and often delayed. Due to the increased risk of gastric neoplasms, a timely diagnosis of AAG is clinically important.

CONTENT

The gold standard for AAG diagnosis is histopathological assessment of gastric biopsies obtained during gastroscopy, but noninvasive, preendoscopic serological screening may be useful in some clinical scenarios. Serum biomarkers for AAG may be divided into 2 groups: gastric autoimmunity-related biomarkers, such as PCA and IFA, and gastric corpus atrophy/reduced gastric acid secretion-related biomarkers, such as serum gastrin and pepsinogens. The present review focuses on the clinical significance and pitfalls of serum biomarkers related to gastric autoimmunity and gastric corpus atrophy, including some discussion of analytical methods.

SUMMARY

Serum assays for PCA, IFA, gastrin, and pepsinogen I show good diagnostic accuracy for noninvasive diagnostic work-up of AAG. Diagnostic performance may increase by combining >1 of these tests, overcoming the problem of seronegative AAG. However, appropriately designed, comparative studies with well-characterized patient cohorts are needed to better define the reliability of these biomarkers in the diagnosis of patients with AAG. Currently, positive serum tests should always be followed by the state-of-art diagnostic test, that is, histopathological assessment of gastric biopsies obtained during gastroscopy to definitively confirm or rule out AAG and eventually neoplastic complications.

Association between islet autoantibodies and the prevalence of autoimmune uveitis

AIM

To evaluate the predictive value of islet autoantibodies for the diagnosis of autoimmune uveitis (AU), as well as to characterize the association bet ween islet autoantibodies and AU.

METHODS

Totally 97 patients with AU and 100 healthy persons without any autoimmune diseases as the control group were recruited. Multiple serum islet autoantibodies were measured using commercial enzyme-linked immunosorbent assay kits (ELISA). A supplementary questionnaire was used to complement the subject's demographics and clinical features. The level of glucose concentrations and white blood cells were measured. Conditional logistic regression was performed to estimate odds ratios (ORs), and 95% confidence intervals (CIs) of AU according to islet autoantibodies and to evaluate the predictive value of islet autoantibodies for AU diagnosis. Autoantibodies subgroups and other variables were included into analysis.

RESULTS

In AU patients, the prevalence of detecting at least one of the autoantibodies was 31.9% (31/97). The most frequent autoantibody was ZnT8A (30.9%), followed by GADA (11.3%), IA-2A (4.1%), ICA (2.1%) and IAA (2.1%). Islet autoantibodies were found to be correlated positively with AU diagnosis [OR (95%CI): 13.86 (3.28, 58.50), P<0.001]. Moreover, Zn-T8A was remarkably correlated with AU diagnosis [OR (95%CI): 6.13 (1.96, 19.17), P<0.001], In contrast, neither GADA nor other islet antibodies (IA-2A, ICA and IAA) showed any association with AU risk under an additive model.

CONCLUSION

The prevalence of islet antibodies, especially ZnT8A, in patients with AU is higher. Islet antibodies as well as novel biomarkers should be included in routine evaluation at AU and is a valuable biological marker to classify newly-diagnosed uveitis.

Measurement of Autoantibodies to Gastric H+,K+-ATPase (ATP4A/B) Using a Luciferase Immunoprecipitation System (LIPS)

The Luciferase Immuno Precipitation System (LIPS) enables the detection of specific serum antibodies by immunoprecipitation of recombinant antigens tagged with a luciferase reporter. Here we describe LIPS assays for the quantification of autoantibodies to the H+, K+-ATPase A (ATP4A) and B (ATP4B) subunits, two serological markers of autoimmune atrophic gastritis and pernicious anemia. In particular, we will describe the expression of luciferase-tagged recombinant ATP4A and ATP4B, their immunoprecipitation with test sera, the recovery and washing of immune-complexes with a protein-A coated resin, and the quantification of autoantibodies by addition of a luciferase substrate and the measurement of the light output from captured luciferase-tagged antigens.

A quarter of patients with type 1 diabetes have co-existing non-islet autoimmunity: the findings of a UK population-based family study

Individuals with type 1 diabetes (T1D) are at increased risk of coeliac disease (CD), autoimmune thyroiditis and autoimmune gastritis, but the absolute risks are unclear. The aim of this study was to investigate the prevalence of autoantibodies to tissue transglutaminase (TGA), thyroid peroxidase (TPOA) and gastric H⁺ /K⁺ -ATPase (ATPA) and their genetic associations in a well-characterized population-based cohort of individuals with T1D from the Bart's-Oxford family study for whom islet autoantibody prevalence data were already available. Autoantibodies in sera from 1072 patients (males/females 604/468; median age 11·8 years, median T1D duration 2·7 months) were measured by radioimmunoassays; HLA class II risk genotype was analysed in 973 (91%) using polymerase chain reaction with sequence specific primers (PCR-SSP). The prevalence of TGA (and/or history of CD), TPOA and ATPA in patients was 9·0, 9·6 and 8·2%, respectively; 3·1% had two or more autoantibodies. Females were at higher risk of multiple autoimmunity; TGA/CD were associated with younger age and TPOA with older age. ATPA were uncommon in patients under 5 years, and more common in older patients. Anti-glutamate decarboxylase autoantibodies were predictive of co-existing TPOA/ATPA. TGA/CD were associated with human leucocyte antigen (HLA) DR3-DQ2, with the DR3-DQ2/DR3-DQ2 genotype conferring the highest risk, followed by DR4-DQ8/DR4-DQ8. ATPA were associated with DR3-DQ2, DRB1*0404 (in males) and the DR3-DQ2/DR4-DQ8 genotype. TPOA were associated with the DR3-DQ2/DR3-DQ2 genotype. Almost one-quarter of patients diagnosed with T1D aged under 21 years have at least one other organ-specific autoantibody. HLA class II genetic profiling may be useful in identifying those at risk of multiple autoimmunity.

ATP4A autoimmunity in pediatric patients with type 1 diabetes and its relationship to blood count, iron metabolism, and vitamin B12

OBJECTIVE

We aimed to assess the prevalence of autoantibodies against the 4A subunit of the gastric proton pump (ATP4A) in pediatric type 1 diabetes (T1D) patients and explore the relationship between ATP4A positivity and blood cell count, iron turnover, and vitamin B12 concentration.

SUBJECTS

The study included 94 (59% female) T1D children (aged 12.5 ± 4.1 years, T1D duration 4.2 ± 3.6 years, HbA1c 7.3 ± 1.5% (57 ± 12.6 mmol/mol) with no other autoimmune diseases.

METHODS

ATP4A antibodies were measured in T1D patients using a radioimmunoprecipitation assay. Blood cell count, iron concentration, total iron binding capacity, ferritin, transferrin, hepcidin, and vitamin B12 concentration were measured in all the study participants.

RESULTS

A total of 16 (17%) children were ATP4A positive. Serum concentrations of ferritin were significantly lower in ATP4A positive than in antibody negative subjects (P = .034). Overall the levels of ATP4A antibodies (ATP4A Index) correlated positively with the age at T1D diagnosis (r = 0.228, P = .026) and negatively with ferritin levels (r = -0.215, P = .037). In ATP4A positive patients, the ATP4A Index correlated positively with age at diagnosis (r = 0.544, P = .032) and negatively with vitamin B12 levels (r = -0.685, P = .004).

CONCLUSIONS

ATP4A antibodies were present in a significant proportion of children with T1D. Higher ATP4A levels in T1D children are associated with lower, yet still fitting within the normal range, levels of vitamin B12, and ferritin. Routine screening of T1D children for gastric autoimmunity (ATP4A) should be considered with follow-up of those positive for vitamin B12 and iron deficiency.

Luminescent Immunoprecipitation System (LIPS) for Detection of Autoantibodies Against ATP4A and ATP4B Subunits of Gastric Proton Pump H+,K+-ATPase in Atrophic Body Gastritis Patients

OBJECTIVES

Circulating autoantibodies targeting the H+/K+-ATPase proton pump of gastric parietal cells are considered markers of autoimmune gastritis, whose diagnostic accuracy in atrophic body gastritis, the pathological lesion of autoimmune gastritis, remains unknown. This study aimed to assess autoantibodies against ATP4A and ATP4B subunits of parietal cells H+, K+-ATPase in atrophic body gastritis patients and controls.

METHODS

One-hundred and four cases with atrophic body gastritis and 205 controls were assessed for serological autoantibodies specific for ATP4A or ATP4B subunits using luminescent immunoprecipitation system (LIPS). Recombinant luciferase-reporter-fused-antigens were expressed by in vitro transcription-translation (ATP4A) or after transfection in Expi293F cells (ATP4B), incubated with test sera, and immune complexes recovered using protein-A-sepharose. LIPS assays were compared with a commercial enzyme immunoassay (EIA) for parietal cell autoantibodies.

RESULTS

ATP4A and ATP4B autoantibody titers were higher in cases compared to controls (P<0.0001). The area under the receiver-operating characteristic curve was 0.98 (95% CI 0.965-0.996) for ATP4A, and 0.99 (95% CI 0.979-1.000) for ATP4B, both higher as compared with that of EIA: 0.86 (95% CI 0.809-0.896), P<0.0001. Sensitivity-specificity were 100-89% for ATP4A and 100-90% for ATP4B assay. Compared with LIPS, EIA for parietal cell autoantibodies showed a lower sensitivity (72%, P<0.0001) at a similar specificity (92%, P=0.558).

CONCLUSIONS

Positivity to both, ATP4A and ATP4B autoantibodies is closely associated with atrophic body gastritis. Both assays had the highest sensitivity, at the cost of diagnostic accuracy (89 and 90% specificity), outperforming traditional EIA. Once validated, these LIPS assays should be valuable screening tools for detecting biomarkers of damaged atrophic oxyntic mucosa.

Anti-parietal cell antibodies - diagnostic significance

Anti-parietal cell antibodies (APCA) are an advantageous tool for screening for autoimmune atrophic gastritis (AAG) and pernicious anemia (PA). The target for APCA is the H+/K+ ATP-ase. It has been demonstrated, that APCA target both, the alpha, and beta subunits of the proton pump, although the major antigen is the alpha subunit. Circulating serum APCA can be detected by means of immunofluorescence, enzyme-linked immunosorbent assay - currently the most commonly used method, and radioimmunoprecipitation assay (RIA) - the 4A subunit has been optimized as a molecular-specific antigen probe. RIA is the most accurate method of antibody assessment, characterized by highest sensitivity. APCA can be found in 85-90% of patients with PA. Their presence is not sufficient for diagnosis, because they are not specific for PA as they are also found in the circulation of individuals with other diseases. APCA are more prevalent in the serum of patients with T1D, autoimmune thyroid diseases, vitiligo, celiac disease. People with autoimmune diseases should be closely screened for AAG/PA. The anemia develops longitudinally over many years in APCA-positive patients, symptomless, slowly promotes atrophy of the gastric mucosa and parietal cells. APCA are present in 7.8-19.5% of the general healthy adult population. A fraction of these sero-positive people, will never develop AAG or PA. An interesting and not fully explained question is whether APCA presence is related to Helicobacter pylori infection. APCA are found in up to 20.7% of these patients. H. pylori is implicated as one of the candidates causing AAG.

Autoimmune gastritis: Pathologist’s viewpoint

Western countries are seeing a constant decline in the incidence of Helicobacter pylori-associated gastritis, coupled with a rising epidemiological and clinical impact of autoimmune gastritis. This latter gastropathy is due to autoimmune aggression targeting parietal cells through a complex interaction of auto-antibodies against the parietal cell proton pump and intrinsic factor, and sensitized T cells. Given the specific target of this aggression, autoimmune gastritis is typically restricted to the gastric corpus-fundus mucosa. In advanced cases, the oxyntic epithelia are replaced by atrophic (and metaplastic) mucosa, creating the phenotypic background in which both gastric neuroendocrine tumors and (intestinal-type) adenocarcinomas may develop. Despite improvements in our understanding of the phenotypic changes or cascades occurring in this autoimmune setting, no reliable biomarkers are available for identifying patients at higher risk of developing a gastric neoplasm. The standardization of autoimmune gastritis histology reports and classifications in diagnostic practice is a prerequisite for implementing definitive secondary prevention strategies based on multidisciplinary diagnostic approaches integrating endoscopy, serology, histology and molecular profiling.

ATPase4A Autoreactivity and Its Association With Autoimmune Phenotypes in the Type 1 Diabetes Genetics Consortium Study

Autoantibodies targeting the H+/K+-ATPase proton pump of the gastric parietal cell (parietal cell antibodies [PCA]) are diagnostic of atrophic body gastritis (ABG) leading to pernicious anemia (PA). PCA, ABG, and PA occur in increased frequency in patients with type 1 diabetes and their relatives and are considered "minor" components of forms of autoimmune polyglandular syndrome (APS). A customized radioimmunoprecipitation assay was applied to 6,749 samples from the Type 1 Diabetes Genetics Consortium to measure ATP4A autoreactivity. Autoantibody prevalence was correlated with variants in HLA class II, PTPN22, and CTLA4 genes. With an ATP4A radioimmunoprecipitation assay, PCA were detected in sera from 20.9% of affected individuals. PCA prevalence increased with age and was greater in females (25.3%) than males (16.5%) and among Hispanics (36.3%) and blacks (26.2%) compared with non-Hispanic whites (20.8%) and Asians (16.7%). PCA and other organ-specific autoantibodies GAD65, IA-2, thyroid peroxidase (TPO), 21-hydroxylase (21-OH), and transglutaminase (TG) clustered within families with heritability estimates from 71 to 95%. PCA clustered with TPO, 21-OH, and persistent GAD65 autoantibodies but not with celiac (TG) or IA-2 autoantibodies. PCA-positive subjects showed an increased frequency of DRB1*0404, DPB1*0201, and PTPN22 R620W (rs2476601-T) and a decreased frequency of DRB1*0101, DPB1*0301, and CTLA4 CT60 (rs3087243-T). Genetic variants accounted for 4-5% of the heritable risk for PCA. The same alleles were associated with other autoantibody phenotypes in a consistent pattern. Whereas most of the heritable risk for PCA and other antibodies reflects genetic effects that are tissue specific, parietal cell autoimmunity is a major pathogenetic contributor in APS2.

Changes in Zinc Transporter 8 Autoantibodies Following Type 1 Diabetes Onset: The Type 1 Diabetes Genetics Consortium Autoantibody Workshop

Zinc transporter 8 autoantibodies (ZnT8A) were analyzed in sera from 1,504 subjects as part of the Type 1 Diabetes Genetics Consortium (T1DGC) Autoantibody Workshop. For these participants with type 1 diabetes (T1D), samples were collected within 3 years of T1D diagnosis. ZnT8A were detected in 862 subjects (57.3%), with the highest frequencies and median titers being associated with the shortest duration of disease. ZnT8A were present at similar frequencies in non-Hispanic whites, non-Hispanic blacks, and Hispanics, but significantly less prevalent in those of Asian ancestry. Sera containing ZnT8A selectively recognizing at least one of the SLC30A8 single nucleotide polymorphisms (encoding ZnT8A) were detected in all populations; however, Trp-specific sera were much less frequent in non-Hispanic blacks, consistent with the anticipated lower frequency of the SLC30A8 rs13266634 T allele in African American populations. ZnT8A positivity was associated with HLA-DQ8, but this was primarily due to the DRB1*0404-DQ8 haplotype. This was in contrast to autoantibodies to IA-2 that were strongly associated with DRB1*0401-DQ8. These effects appeared essentially independent of racial or ethnic background. The DRB1*0401-DQ8 and DRB1*0404-DQ8 haplotypes were associated with T1D subjects positive for GAD65, IA-2, and ZnT8A. In contrast to DRB1*0401-DQ8, there was no significant association of DRB1*0404-DQ8 with single or dual autoantibody positivity. The DRB1*0404-DQ8 haplotype was also associated with T1D subjects whose sera recognized both polymorphic variants of zinc transporter 8, an effect not seen for DRB1*0401-DQ8.

ATP4A autoimmunity and Helicobacter pylori infection in children with type 1 diabetes

Persistent presence of ATP4A autoantibodies (ATP4AA) directed towards parietal cells is typical for atrophic body gastritis (ABG), an autoimmune disease associated with type 1 diabetes. We assessed whether Helicobacter pylori (Hp) infection might be associated with positivity for ATP4AA in children with type 1 diabetes. Sera were collected from 70 (38♀) type 1 diabetes children [aged 13·2 ± 4·5 years, age at diagnosis 8·8 ± 4·3 years, diabetes duration 4·5 ± 3·8 years, mean HbA1c 7·8 ± 1·6% (62 ± 17·5 mmol/mol)] seen at the regional diabetes clinic in Katowice, Poland. Patients were tested concurrently for Hp infection by means of a 13C urea breath test. ATP4AA were measured using a novel radioimmunoprecipitation assay developed at the Barbara Davies Center for Childhood Diabetes, University of Colorado. ATP4AA were present in 21 [30%, 95% confidence interval (CI) = 19-41%] and Hp infection was detected in 23 (33%, 95% CI = 22-44%) children. There was no statistically significant association between ATP4AA presence and Hp status. ATP4AA presence was not associated with current age, age at type 1 diabetes diagnosis, diabetes duration or current HbA1c. ATP4AA were more prevalent in females [42% (26-58%)] than males [16% (3-28%)], P = 0·016. ATP4A are found in nearly one-third of children with type 1 diabetes and more common among females. In this cross-sectional analysis, Hp infection was not associated with autoimmunity against parietal cells.

Novel diabetes autoantibodies and prediction of type 1 diabetes

Autoantibodies are currently the most robust biomarkers of type 1 diabetes and are frequently used to establish entry criteria for the participation of genetically at-risk individuals in secondary prevention/intervention clinical trials. Since their original description almost 40 years ago, considerable efforts have been devoted toward identifying the precise molecular targets that are recognized. Such information can have significant benefit for developing improved metrics for identifying/stratifying of at-risk subjects, developing potential therapeutic targets, and advancing understanding of the pathophysiology of the disease. Currently, four major molecular targets ([pro]insulin, GAD65, IA-2, and ZnT8) have been confirmed, with approximately 94% of all subjects with a clinical diagnosis of type 1 diabetes expressing autoantibodies to at least one of these molecules at clinical onset. In this review, we summarize some of the salient properties of these targets that might contribute to their autoantigenicity and methods that have been used in attempts to identify new components of the humoral autoresponse.

Autoimmune polyendocrine syndromes

Autoimmune polyendocrine syndromes (APS), also called polyglandular autoimmune syndromes (PGAS), are a heterogeneous group of rare diseases characterized by autoimmune activity against more than one endocrine organs, although non-endocrine organs can be affected. The two major autoimmune polyendocrine syndromes, (type1-type2/APS-1 and APS-2), both have Addison's disease as a prominent component. Further autoimmune polyendocrine syndromes include APS3 and APS4. The major autoimmune polyendocrine syndromes have a strong genetic component with the type 2 syndrome occurring in multiple generations and the type I syndrome in siblings. It is well recognized that more than 20years may elapse between the onset on one endocrinopathy and the diagnosis of the next, for example, almost 40-50% of subjects with Addison's disease will develop an associated endocrinopathy. The discovery of the polyendocrine autoimmune syndromes offered the possibility to understand autoimmune disorders with particular interest for type 1A diabetes and the neuroendocrine immunology (NEI) is further contributing to understand the links.

Autoimmunity against INS-IGF2 protein expressed in human pancreatic islets

Insulin is a major autoantigen in islet autoimmunity and progression to type 1 diabetes. It has been suggested that the insulin B-chain may be critical to insulin autoimmunity in type 1 diabetes. INS-IGF2 consists of the preproinsulin signal peptide, the insulin B-chain, and eight amino acids of the C-peptide in addition to 138 amino acids from the IGF2 gene. We aimed to determine the expression of INS-IGF2 in human pancreatic islets and autoantibodies in newly diagnosed children with type 1 diabetes and controls. INS-IGF2, expressed primarily in beta cells, showed higher levels of expression in islets from normal compared with donors with either type 2 diabetes (p = 0.006) or high HbA1c levels (p < 0.001). INS-IGF2 autoantibody levels were increased in newly diagnosed patients with type 1 diabetes (n = 304) compared with healthy controls (n = 355; p < 0.001). Displacement with cold insulin and INS-IGF2 revealed that more patients than controls had doubly reactive insulin-INS-IGF2 autoantibodies. These data suggest that INS-IGF2, which contains the preproinsulin signal peptide, the B-chain, and eight amino acids of the C-peptide may be an autoantigen in type 1 diabetes. INS-IGF2 and insulin may share autoantibody-binding sites, thus complicating the notion that insulin is the primary autoantigen in type 1 diabetes.

Extrapancreatic autoantibody profiles in type I diabetes

Type I diabetes (T1D) is an autoimmune disease characterized by destruction of insulin-producing β-cells in the pancreas. Although several islet cell autoantigens are known, the breadth and spectrum of autoantibody targets has not been fully explored. Here the luciferase immunoprecipitation systems (LIPS) antibody profiling technology was used to study islet and other organ-specific autoantibody responses in parallel. Examination of an initial cohort of 93 controls and 50 T1D subjects revealed that 16% of the diabetic subjects showed anti-gastric ATPase autoantibodies which did not correlate with autoantibodies against GAD65, IA2, or IA2-β. A more detailed study of a second cohort with 18 potential autoantibody targets revealed marked heterogeneity in autoantibody responses against islet cell autoantigens including two polymorphic variants of ZnT8. A subset of T1D subjects exhibited autoantibodies against several organ-specific targets including gastric ATPase (11%), thyroid peroxidase (14%), and anti-IgA autoantibodies against tissue transglutaminase (12%). Although a few T1D subjects showed autoantibodies against a lung-associated protein KCNRG (6%) and S100-β (8%), no statistically significant autoantibodies were detected against several cytokines. Analysis of the overall autoantibody profiles using a heatmap revealed two major subgroups of approximately similar numbers, consisting of T1D subjects with and without organ-specific autoantibodies. Within the organ-specific subgroup, there was minimal overlap among anti-gastric ATPase, anti-thyroid peroxidase, and anti-transglutaminase seropositivity, and these autoantibodies did not correlate with islet cell autoantibodies. Examination of a third cohort, comprising prospectively collected longitudinal samples from high-risk individuals, revealed that anti-gastric ATPase autoantibodies were present in several individuals prior to detection of islet autoantibodies and before clinical onset of T1D. Taken together, these results suggest that autoantibody portraits derived from islet and organ-specific targets will likely be useful for enhancing the clinical management of T1D.