Polymorphism of antigen processing (TAP, LMP) and HLA class II genes in celiac disease

Variations in TAP1 and PSMB9 Genes Involved in Antigen Processing and Presentation Increase the Risk of Vitiligo in the Saudi Community

Background

The antigen processing 1 (TAP1) and proteasome 20S subunit beta 9 (PSMB9) genes are associated with strong susceptibility to many autoimmune diseases. Here, we explored whether TAP1/PSMB9 genetic variants, individually or combined, affected susceptibility to the complex, autoimmune-based skin disorder vitiligo.

Methods

Samples of genomic DNA from buccal cells of 172 patients with vitiligo and 129 healthy controls were analyzed using TaqMan™ genotyping assays for the TAP1 rs1135216 (A>G) and PSMB9 rs17587 (A>G) single nucleotide polymorphisms (SNPs). SNPStats software (https://www.snpstats.net) was utilized to choose the best interactive inheritance mode for selected SNPs.

Results

The genotype frequencies for the TAP1 rs1135216 and PSMB9 rs17587 SNPs were in Hardy-Weinberg equilibrium for cases (P= 0.11 and P= 0.10, respectively) but not for controls (P< 0.05). The TAP1 rs1135216 (D637G) and PSMB9 rs17587 (R60H) SNPs increased the risk of vitiligo four-fold and two-fold, respectively (odds ratio [OR]= 4.6; 95% confidence interval [CI], 3.2-6.5; P< 0.0001 and OR= 2.2; 95% CI, 1.5-3.1; P< 0.0001). The recessive model (G/G-D/G versus D/D) and the codominant model (R/R versus R/H) were the best models of inheritance for the rs113526 and rs17587 SNPs, respectively (OR= 16.4; 95% CI, 2.0-138; P= 0.0006 and OR= 1.7; 95% CI, 0.3-1.8; P= 0.013). Vulgaris, focal vulgaris, and acryl/acrofacial were the most common vitiligo subtypes in our sample (51%, 21%, and 19%, respectively). Heterozygous rs113526 (637D/G) and rs17587 (60R/H) were the most common genotypes in most vitiligo subtypes. The heterozygous 637D/G genotype and the 637G variant allele were significantly more common in patients with active disease than in patients with stable disease (P= 0.000052 and P= 0.0063, respectively).

Conclusion

Our findings suggest a crucial role for TAP1 rs1135216 and PSMB9 rs17587 in the risk and progression of vitiligo in the Saudi community. Genomic analyses are needed to identify more candidate genes and more genetic variants associated with vitiligo.

Variations in TAP1 and PSMB9 Genes Involved in Antigen Processing and Presentation Increase the Risk of Vitiligo in the Saudi Community

BACKGROUND

The antigen processing 1 (TAP1) and proteasome 20S subunit beta 9 (PSMB9) genes are associated with strong susceptibility to many autoimmune diseases. Here, we explored whether TAP1/PSMB9 genetic variants, individually or combined, affected susceptibility to the complex, autoimmune-based skin disorder vitiligo.

METHODS

Samples of genomic DNA from buccal cells of 172 patients with vitiligo and 129 healthy controls were analyzed using TaqMan™ genotyping assays for the TAP1 rs1135216 (A>G) and PSMB9 rs17587 (A>G) single nucleotide polymorphisms (SNPs). SNPStats software (https://www.snpstats.net) was utilized to choose the best interactive inheritance mode for selected SNPs.

RESULTS

The genotype frequencies for the TAP1 rs1135216 and PSMB9 rs17587 SNPs were in Hardy-Weinberg equilibrium for cases (P= 0.11 and P= 0.10, respectively) but not for controls (P< 0.05). The TAP1 rs1135216 (D637G) and PSMB9 rs17587 (R60H) SNPs increased the risk of vitiligo four-fold and two-fold, respectively (odds ratio [OR]= 4.6; 95% confidence interval [CI], 3.2-6.5; P< 0.0001 and OR= 2.2; 95% CI, 1.5-3.1; P< 0.0001). The recessive model (G/G-D/G versus D/D) and the codominant model (R/R versus R/H) were the best models of inheritance for the rs113526 and rs17587 SNPs, respectively (OR= 16.4; 95% CI, 2.0-138; P= 0.0006 and OR= 1.7; 95% CI, 0.3-1.8; P= 0.013). Vulgaris, focal vulgaris, and acryl/acrofacial were the most common vitiligo subtypes in our sample (51%, 21%, and 19%, respectively). Heterozygous rs113526 (637D/G) and rs17587 (60R/H) were the most common genotypes in most vitiligo subtypes. The heterozygous 637D/G genotype and the 637G variant allele were significantly more common in patients with active disease than in patients with stable disease (P= 0.000052 and P= 0.0063, respectively).

CONCLUSION

Our findings suggest a crucial role for TAP1 rs1135216 and PSMB9 rs17587 in the risk and progression of vitiligo in the Saudi community. Genomic analyses are needed to identify more candidate genes and more genetic variants associated with vitiligo.

Probiotics, Prebiotics and Other Dietary Supplements for Gut Microbiota Modulation in Celiac Disease Patients

To date, the only available treatment for celiac disease (CD) patients is a life-lasting gluten-free diet (GFD). Lack of adherence to the GFD leads to a significant risk of adverse health consequences. Food cross-contamination, nutritional imbalances, and persistent gastrointestinal symptoms are the main concerns related to GFD. Moreover, despite rigid compliance to GFD, patients struggle in achieving a full restoring of the gut microbiota, which plays a role in the nutritive compounds processing, and absorption. Pivotal studies on the supplementation of GFD with probiotics, such as Bifidobacterium and Lactobacilli, reported a potential to restore gut microbiota composition and to pre-digest gluten in the intestinal lumen, reducing the inflammation associated with gluten intake, the intestinal permeability, and the cytokine and antibody production. These findings could explain an improvement in symptoms and quality of life in patients treated with GFD and probiotics. On the other hand, the inclusion of prebiotics in GFD could also be easy to administer and cost-effective as an adjunctive treatment for CD, having the power to stimulate the growth of potentially health-promoting bacteria strains. However, evidence regarding the use of prebiotics and probiotics in patients with CD is still insufficient to justify their use in clinical practice.

Prevalence of Celiac Disease in a Long-term Study of a Spanish At-genetic-risk Cohort From the General Population

OBJECTIVES

To perform long-term celiac disease (CD) screening in an HLA-DQ2 (+) cohort from the general population and to assess the influence of risk genotypes on its development.

METHODS

In 2004, an HLA-DQ2 (+) cohort was selected. After the first CD screening at age 2 to 3 years, we performed a follow-up screening 8 to 10 years later. Antitransglutaminase 2 antibodies were determined using a rapid test kit. Results were confirmed by serum IgA antitransglutaminase 2 and IgA endomysial antibody determination. CD diagnosis was carried out by intestinal biopsies. Four HLA-DQ2 genotypic groups were used: G1: DQ2.5/DQ2.5 (G1A) or DQ2.5/ DQ2.2 (G1B); G2: DQ2.2/DQ7.5 (DQ2.5 trans); G3: DQ2.5/ X; G4: DQ2.2/X.

RESULTS

CD prevalence after 10 years of follow-up was 5.8% (95% confidence interval 3.8-8.7). One of every 3 HLA-DQ2(+) children carried at least 1 haplotype DQ2.2 or DQ7. The homozygous genotype DQ2.5/DQ2.5 and the HLA-DQ2.5 trans genotype increased CD risk 4- and 3-fold, respectively. The homozygous genotype DQ2.5/ DQ2.2 did not increase the CD risk. Children carrying G1 or G2 genotypes were diagnosed with CD earlier and more frequently during the follow-up compare with those carrying G3 or G4 genotypes. Approximately 81% of children with spontaneous antibody negativization after the first screening maintained negative antibodies.

CONCLUSIONS

A repeated screening of at-risk children during their follow-up allowed us to diagnose new CD cases. In our cohort, HLA- DQ2.5 trans genotype conferred a higher risk in the development of CD than HLA- DQ2.5/DQ2.2. The majority of children with potential CD and CD autoimmunity at 10 years of age remained healthy.

Different DRB1*03:01-DQB1*02:01 haplotypes confer different risk for celiac disease

Celiac disease is associated with the HLA-DR3-DQA1*05:01-DQB1*02:01 and DR4-DQA1*03:01-DQB1*03:02 haplotypes. In addition, there are currently over 40 non-HLA loci associated with celiac disease. This study extends previous analyses on different HLA haplotypes in celiac disease using next generation targeted sequencing. Included were 143 patients with celiac disease and 135 non-celiac disease controls investigated at median 9.8 years (1.4-18.3 years). PCR-based amplification of HLA and sequencing with Illumina MiSeq technology were used for extended sequencing of the HLA class II haplotypes HLA-DRB1, DRB3, DRB4, DRB5, DQA1 and DQB1, respectively. Odds ratios were computed marginally for every allele and haplotype as the ratio of allelic frequency in patients and controls as ratio of exposure rates (RR), when comparing a null reference with equal exposure rates in cases and controls. Among the extended HLA haplotypes, the strongest risk haplotype for celiac disease was shown for DRB3*01:01:02 in linkage with DQA1*05:01-DQB1*02:01 (RR = 6.34; P-value < .0001). In a subpopulation analysis, DRB3*01:01:02-DQA1*05:01-DQB1*02:01 remained the most significant in patients with Scandinavian ethnicity (RR = 4.63; P < .0001) whereas DRB1*07:01:01-DRB4*01:03:01-DQA1*02:01-DQB1*02:02:01 presented the highest risk of celiac disease among non-Scandinavians (RR = 7.94; P = .011). The data also revealed 2 distinct celiac disease risk DR3-DQA1*05:01-DQB*02:01 haplotypes distinguished by either the DRB3*01:01:02 or DRB3*02:02:01 alleles, indicating that different DRB1*03:01-DQB1*02:01 haplotypes confer different risk for celiac disease. The associated risk of celiac disease for DR3-DRB3*01:01:02-DQA1*05:01-DQB1*02:01 is predominant among patients of Scandinavian ethnicity.

A case-control study on association of proteasome subunit beta 8 (PSMB8) and transporter associated with antigen processing 1 (TAP1) polymorphisms and their transcript levels in vitiligo from Gujarat

BACKGROUND

Autoimmunity has been implicated in the destruction of melanocytes from vitiligo skin. Major histocompatibility complex (MHC) class-II linked genes proteasome subunit beta 8 (PSMB8) and transporter associated with antigen processing 1 (TAP1), involved in antigen processing and presentation have been reported to be associated with several autoimmune diseases including vitiligo.

OBJECTIVES

To explore PSMB8 rs2071464 and TAP1 rs1135216 single nucleotide polymorphisms and to estimate the expression of PSMB8 and TAP1 in patients with vitiligo and unaffected controls from Gujarat.

METHODS

PSMB8 rs2071464 polymorphism was genotyped using polymerase chain reaction- restriction fragment length polymorphism (PCR-RFLP) and TAP1 rs1135216 polymorphism was genotyped by amplification refractory mutation system-polymerase chain reaction (ARMS-PCR) in 378 patients with vitiligo and 509 controls. Transcript levels of PSMB8 and TAP1 were measured in the PBMCs of 91 patients and 96 controls by using qPCR. Protein levels of PSMB8 were also determined by Western blot analysis.

RESULTS

The frequency of 'TT' genotype of PSMB8 polymorphism was significantly lowered in patients with generalized and active vitiligo (p = 0.019 and p = 0.005) as compared to controls suggesting its association with the activity of the disease. However, TAP1 polymorphism was not associated with vitiligo susceptibility. A significant decrease in expression of PSMB8 at both transcript level (p = 0.002) as well as protein level (p = 0.0460) was observed in vitiligo patients as compared to controls. No significant difference was observed between patients and controls for TAP1 transcripts (p = 0.553). Interestingly, individuals with the susceptible CC genotype of PSMB8 polymorphism showed significantly reduced PSMB8 transcript level as compared to that of CT and TT genotypes (p = 0.009 and p = 0.003 respectively).

CONCLUSIONS

PSMB8 rs2071464 was associated with generalized and active vitiligo from Gujarat whereas TAP1 rs1135216 showed no association. The down-regulation of PSMB8 in patients with risk genotype 'CC' advocates the vital role of PSMB8 in the autoimmune basis of vitiligo.

Transporter TAP1-637G and immunoproteasome PSMB9-60H variants influence the risk of developing vitiligo in the Saudi population

We evaluated whether TAP1-rs1135216 (p.637D>G) and PSMB9-rs17587 (p.60R>H) were significantly associated with the risk and severity of vitiligo among Saudi patients. One hundred seventy-two subjects were genotyped for the TAP1-rs1135216 and PSMB9-rs17587 variants using endonuclease digestions of amplified genomic DNA. The TAP1-rs1135216 and PSMB9-rs17587 mutant alleles were strongly associated with vitiligo, with odds ratios showing five fold and two fold risks (P < 0.0001 and P = 0.007, resp.). In TAP1-rs1135216, the 637G mutant allele was more frequent in cases (74%) than in healthy controls. In cases, the 60H mutant allele PSMB9-rs17587 was less frequent (42%) than the wild-type 60R allele (58%). Vitiligo vulgaris was the most common type of disease, associated with the DG (55%) and GG (46%) genotypes for rs1135216 and with the RH genotype (59%) for rs17587. The heterozygous 637DG and 60RH genotypes were each linked with active phenotypes in 64% of cases. In conclusion, the TAP1-rs1135216 and PSMB9-rs17587 variants are significantly associated with vitiligo, and even one copy of these mutant alleles can influence the risk among Saudis. Vitiligo vulgaris is associated with genotypes containing the mutant G and H alleles.

Transporter TAP1-637G and immunoproteasome PSMB9-60H variants influence the risk of developing vitiligo in the Saudi population

We evaluated whether TAP1-rs1135216 (p.637D>G) and PSMB9-rs17587 (p.60R>H) were significantly associated with the risk and severity of vitiligo among Saudi patients. One hundred seventy-two subjects were genotyped for the TAP1-rs1135216 and PSMB9-rs17587 variants using endonuclease digestions of amplified genomic DNA. The TAP1-rs1135216 and PSMB9-rs17587 mutant alleles were strongly associated with vitiligo, with odds ratios showing five fold and two fold risks (P < 0.0001 and P = 0.007, resp.). In TAP1-rs1135216, the 637G mutant allele was more frequent in cases (74%) than in healthy controls. In cases, the 60H mutant allele PSMB9-rs17587 was less frequent (42%) than the wild-type 60R allele (58%). Vitiligo vulgaris was the most common type of disease, associated with the DG (55%) and GG (46%) genotypes for rs1135216 and with the RH genotype (59%) for rs17587. The heterozygous 637DG and 60RH genotypes were each linked with active phenotypes in 64% of cases. In conclusion, the TAP1-rs1135216 and PSMB9-rs17587 variants are significantly associated with vitiligo, and even one copy of these mutant alleles can influence the risk among Saudis. Vitiligo vulgaris is associated with genotypes containing the mutant G and H alleles.

Human leukocyte antigen DQ2.2 and celiac disease

Patients with celiac disease (CD) lacking both human leukocyte antigen (HLA)-DQ2.5 in cis (DQA1*05:01, DQB1*02:01) or trans (DQA1*05:05, DQB1*02:02) configuration and HLA-DQ8 (DQA1*03:01, DQB1*03:02) are considered to be rare. Therefore, absence of these genotypes is commonly used to exclude the diagnosis of CD. To investigate whether this approach is justified, the HLA-distribution in 155 children with CD was studied. A total of 139 (89.7%) patients carried HLA-DQ2.5. Of the remaining patients, 7 (4.5%) carried HLA-DQ8. Interestingly, the 9 (5.8%) patients lacking HLA-DQ2.5 and HLA-DQ8 carried HLA-DQA1*02:01 and -DQB1*02:02 (HLA-DQ2.2). Therefore, HLA-DQ2.2 should be included as an important HLA-type related to CD.

European Society for Pediatric Gastroenterology, Hepatology, and Nutrition guidelines for the diagnosis of coeliac disease

OBJECTIVE

Diagnostic criteria for coeliac disease (CD) from the European Society for Paediatric Gastroenterology, Hepatology, and Nutrition (ESPGHAN) were published in 1990. Since then, the autoantigen in CD, tissue transglutaminase, has been identified; the perception of CD has changed from that of a rather uncommon enteropathy to a common multiorgan disease strongly dependent on the haplotypes human leukocyte antigen (HLA)-DQ2 and HLA-DQ8; and CD-specific antibody tests have improved.

METHODS

A panel of 17 experts defined CD and developed new diagnostic criteria based on the Delphi process. Two groups of patients were defined with different diagnostic approaches to diagnose CD: children with symptoms suggestive of CD (group 1) and asymptomatic children at increased risk for CD (group 2). The 2004 National Institutes of Health/Agency for Healthcare Research and Quality report and a systematic literature search on antibody tests for CD in paediatric patients covering the years 2004 to 2009 was the basis for the evidence-based recommendations on CD-specific antibody testing.

RESULTS

In group 1, the diagnosis of CD is based on symptoms, positive serology, and histology that is consistent with CD. If immunoglobulin A anti-tissue transglutaminase type 2 antibody titers are high (>10 times the upper limit of normal), then the option is to diagnose CD without duodenal biopsies by applying a strict protocol with further laboratory tests. In group 2, the diagnosis of CD is based on positive serology and histology. HLA-DQ2 and HLA-DQ8 testing is valuable because CD is unlikely if both haplotypes are negative.

CONCLUSIONS

The aim of the new guidelines was to achieve a high diagnostic accuracy and to reduce the burden for patients and their families. The performance of these guidelines in clinical practice should be evaluated prospectively.

Integration of genetic and immunological insights into a model of celiac disease pathogenesis

Celiac disease (CD) is a gluten-sensitive enteropathy that develops in genetically susceptible individuals by exposure to cereal gluten proteins. This review integrates insights from immunological studies with results of recent genetic genome-wide association studies into a disease model. Genetic data, among others, suggest that viral infections are implicated and that natural killer effector pathways are important in the pathogenesis of CD, but most prominently these data converge with existing immunological findings that CD is primarily a T cell-mediated immune disorder in which CD4(+) T cells that recognize gluten peptides in the context of major histocompatibility class II molecules play a central role. Comparison of genetic pathways as well as genetic susceptibility loci between CD and other autoimmune and inflammatory disorders reveals that CD bears stronger resemblance to T cell-mediated organ-specific autoimmune than to inflammatory diseases. Finally, we present evidence suggesting that the high prevalence of CD in modern societies may be the by-product of past selection for increased immune responses to combat infections in populations in which agriculture and cereals were introduced early on in the post-Neolithic period.

Long-term and acute effects of gliadin on small intestine of patients on potentially pathogenic networks in celiac disease

Celiac disease (CD) is a complex, immune-mediated intolerance to gliadin that develops in genetically susceptible individuals. Although the main driving force of the disease is an aberrant autoimmune response, several other pathogenic mechanisms, many still unidentified, are also involved. In order to describe at a network level the alterations provoked by a gliadin insult on the intestinal mucosa of patients, we compared the expression profiles of biopsies from 9 active and 9 treated patients (long-term effects of gliadin), and of 10 biopsies from gluten-free diet treated patients that were incubated in vitro with or without gliadin (acute effects) and integrated significantly altered transcripts into potentially pathogenic biological processes. Using information on Kyoto Encyclopedia of Genes and Genomes pathways and Gene Ontology terms represented among the differentially expressed genes, we observed important dysfunction in several complex networks, including those related to cell-cell communication, intracellular signaling, ubiquitin-proteasome system, cell cycle/apoptosis and extracellular matrix. The reconstruction of the role of these biological networks in the development of the intestinal lesion in CD provides a comprehensive picture of key events that contribute to the disease, and could point towards novel functional candidates that might be potential therapeutic targets or responsible for genetic susceptibility.

HLA class II influences humoral autoimmunity in patients with type 2 autoimmune hepatitis

BACKGROUND/AIMS

Type 2 autoimmune hepatitis (AIH) is characterized by the presence of anti-liver kidney microsome (anti-LKM-1) and/or anti-liver cytosol type 1 (anti-LC1) autoantibodies. However, the correlation between these autoantibodies and the genetic background has not been studied.

METHODS

Frequencies of HLA class II alleles were compared between the 60 Caucasian children with type 2 AIH and 313 control subjects. The anti-LKM1 antibody reactivity directed against antigenic sites of CYP2D6 was analysed by ELISA.

RESULTS

HLA-DQB1 *0201 allele was found to be the primary genetic determinant of susceptibility to type 2 AIH by conferring the highest odd-ratio (OR = 6.4). HLA-DRB1 *03 allele was significantly increased (P < 0.0001) among patients with both anti-LKM1 and anti-LC1 autoantibodies as well as in those with only anti-LC1(+) compared to those with anti-LKM1(+) alone. In contrast, HLA-DRB1 *07 allele was significantly associated (P < 0.0001) with anti-LKM1(+) alone compared to groups with both anti-LKM and anti-LC1 or with LC1+ alone. Children with the DRB1 *07 allele develop anti-LKM1 autoantibodies having a more restricted specificity (2 epitopes) than to those having HLA-DRB1 *03 allele (5 epitopes).

CONCLUSIONS

The HLA-DR locus is involved in autoantibody expression, while the DQ locus appears to be a critical determinant for the development of type 2 AIH.

The pathogenesis of coeliac disease

Coeliac disease is a common condition and its prevalence in UK is now thought to be approximately 1:100. It is being diagnosed and treated more frequently as awareness at the primary care level has increased. Coeliac disease is a complex disorder and is frequently associated with other disease processes. The management of these patients needs to take on a holistic approach, whilst the physician needs to be aware of the rare complications. This article gives an up-to-date review of the literature written on the pathogenesis of coeliac disease. We have attempted to paint a picture from beginning to end, whilst clarifying the grey areas in between. General epidemiological factors are reviewed before looking at genetic risk factors. We assess the sensitivity and specificity of the investigative modalities available for clinical use and comment on optimum management of these patients thereafter. The future of coeliac disease looks promising for patients with several novel therapies on the horizon. Whilst further work is still needed to breed out the toxic epitopes from wheat, novel therapies may come from other areas such as the work aimed at restoring normal tolerance to gluten.

HLA-DQ relative risks for coeliac disease in European populations: a study of the European Genetics Cluster on Coeliac Disease

Coeliac disease is an enteropathy due to an intolerance to gluten. The association between HLA-DQ genes and CD is well established. The majority of patients carry the HLA-DQ heterodimer encoded by DQA1*05/DQB1*02, either in cis or in trans. The remaining patients carry either part of the DQ heterodimer or DQA1*03-DQB1*0302. The aim of the study was to estimate the risks associated with different DQ genotypes in European populations. HLA information was available for 470 trio families from four countries: France (117), Italy (128), and Norway and Sweden (225). Five DQA1-DQB1 haplotypes were considered and control haplotype frequencies were estimated from the set of parental haplotypes not transmitted to the affected child. The possible genotypes were grouped into five genotype groups, based on the hierarchy of risk reported in the literature. A north-south gradient in the genotype group frequencies is observed in probands: homogeneity is strongly rejected between all country pairs. For each country, the relative risks associated with each genotype group were computed taking into account the control haplotype frequencies. Homogeneity of relative risks between countries was tested pairwise by maximum likelihood ratio statistics. The hypothesis of homogeneity of relative risks is rejected (P is approximately 10(-6)) for all country pairs. In conclusion, the gradient in the genotype group frequencies in probands is not only due to differences in haplotype frequencies but also due to differences in genotype relative risks in the studied populations; the relative risks associated with each DQ genotype group are different between northern and southern European countries; neither are they ordered in the same way.

[Genetics and immunopathogenics aspects of the celiac disease: a recent vision]

BACKGROUND

Celiac disease, or gluten-sensitive enteropathy, is a strongly inherited condition. Although the genetic association of CD with the DQ2 and DQ8 HLA haplotypes has been known for long, others HLA and non-HLA genes are also important in the development of the disease. Celiac disease results of the combined effect of different normally functioning genes' products. The tissue damage in celiac disease is immunologically mediated and several effector mechanisms are responsible for the disease expression. The interplay between genetic, immunological and environmental factors explains the large spectrum of clinical, histological and serological alterations observed in the different stages of the disease development, pointing out to the polygenic nature of celiac disease.

CONCLUSION

The recent advances in the understanding of the immunopathogenesis, genetics and diagnoses of celiac disease have allowed the revision of strict concepts and previous criteria and their adequation to the new evidences, aiming a better diagnostic and orientation to celiac patients and relatives.

Is TAP2*0102 allele involved in insulin-dependent diabetes mellitus (type 1) protection?

In this study, we have investigated the frequencies of TAP1 and TAP2 alleles in a group of 226 persons, living in La Reunion Island, consisting of 70 patients with insulin-dependent diabetes mellitus (IDDM) and most of their first degree relatives (i.e., 156 parents and full sibling subjects) and previously HLA DQB1, DQA1, and DRB1 genotyped. The population of this island is constituted by a particular structure of highly crossbreeding people. Interestingly, the new TAP2*0104 allele, previously discovered by our team in Reunion Island, was found to be increased in the IDDM population and the calculated HRR was relatively high (HRR = 3.3). This result seems to be due to a positive linkage disequilibrium between TAP2*0104 allele and the highly diabetogenous DQB1* 0201-DQA1* 0501-DRB1 0301 haplotype (HRR = 9), which suggests that TAP2*0104 cannot be considered as an additional predispositional factor, but more as a genetic susceptibility marker of IDDM. In addition, we show that minor alleles (TAP2D, *0102, *0103, *0104) are associated with a restricted number of HLA DQ-DR haplotypes and each of them exhibits a preferential linkage with one particular haplotype. In contrast with other alleles, and despite a HRR value close to 1, we show that TAP2*0102 allele contributes significantly to a drastic reduction of the diabetogenic effect of DQB1*0201-DQA1*0301.1-DRB*0701 haplotype. Indeed, this haplotype, which is usually preferentially transmitted to affected children, is dominantly transmitted to healthy children when it is associated with TAP2*0102. Therefore, this allele seems to contribute to genetic protection to IDDM.

Genes of the LMP/TAP cluster are associated with the human autoimmune disease vitiligo

Genes within the class II region of the major histocompatibility complex (MHC), including genes involved in antigen processing and presentation, have been reported to be associated with several autoimmune diseases. We report here that the LMP/TAP gene region is significantly associated with vitiligo, a disorder in which biochemical defects and/or autoimmune destruction cause melanocyte loss and resulting skin depigmentation. Case/control analyses revealed genetic association of vitiligo in Caucasian patients with an early age of onset with the transporter associated with antigen processing-1 (TAP1) gene. A family-based association method revealed biased transmission of specific alleles from heterozygous parents to affected offspring for the TAP1 gene, as well as for the closely linked LMP2 and LMP7 genes encoding subunits of the immunoproteasome. No association with vitiligo was found for the MECL1 gene, which encodes a third immunoproteasome subunit and is unlinked to the MHC class II region. These results suggest a possible role for the MHC class I antigen processing and/or presentation pathway in the antimelanocyte autoimmune response involved in vitiligo pathogenesis.

Lack of association between transporter associated with antigen processing (TAP) and HLA-DM gene polymorphisms and antibody levels following measles vaccination

The transporter associated with antigen processing (TAP) and human leukocyte antigen-DM (HLA-DM) genes are involved in the antigen-processing pathway of both HLA class I and class II-restricted antigen presentation. We hypothesized that polymorphisms within the TAP and DM genes may influence antibody levels following measles vaccination. We examined TAP and DM polymorphisms in 242 school children from Olmsted County, Minnesota, USA who received one dose of measles-mumps-rubella-II (MMR-II) vaccine at the age of 15 months. Based on the level of serum measles-specific immunoglobulin G (IgG) antibodies, subjects were classified as seronegatives (n = 72) or seropositives (n = 170). We determined TAP1 and TAP2 allele types by polymerase chain reaction (PCR) amplification of specific alleles (PASA) and determined DM allele type by PCR amplification followed by direct sequencing of the polymorphic sites. We analysed the data for any TAP or DM allelic association with antibody levels post measles vaccination using the chi-square test and univariate linear regression analysis. We found no trend in the overall distribution of TAP and DM genotype frequencies between seronegative and seropositive subjects, suggesting that TAP and DM polymorphism and antibody levels following measles vaccination are not directly associated. In addition, we did not find an association between TAP (TAP1, P = 0.71; TAP2, P = 0.87) or DM (DMA, P = 0.42; DMB, P = 0.71) homozygosity and seronegativity to measles vaccine in this study group. Our study suggests that TAP and DM gene polymorphisms do not influence antibody levels post measles vaccination.

HLA types in celiac disease patients not carrying the DQA1*05-DQB1*02 (DQ2) heterodimer: results from the European Genetics Cluster on Celiac Disease

Genetic susceptibility to celiac disease is strongly associated with HLA-DQA1*05-DQB1*02 (DQ2) and HLA-DQA1*03-DQB1*0302 (DQ8). Study of the HLA associations in patients not carrying these heterodimers has been limited by the rarity of such patients. This European collaboration has provided a unique opportunity to study a large series of such patients. From 1008 European coeliacs, 61 were identified who neither carry the DQ2 nor DQ8 heterodimers. Fifty seven of these encoded half of the DQ2 heterodimer. The remaining 4 patients had a variety of clinical presentations. Three of them carried the DQA1*01-DQB*05 haplotype as did 20/61 of those carrying neither DQ2 nor DQ8. This may implicate a role of the DQA1*01-DQB*05 haplotype. None of these four patients carried the DQB1*06 allele that has previously been reported in this sub-group of patients. Of the 16 DQ2 heterodimer negative patients without DRB1*04 or DRB1*07 haplotypes, it was inferred that none encoded the previously implicated DRB4 gene as none had a DRB1*09 haplotype. These results underline the primary importance of HLA-DQ alleles in susceptibility to celiac disease, and the extreme rarity of celiac patients carrying neither the DQ2 or DQ8 heterodimers nor one half of the DQ2 heterodimer alone.

Polymorphisms of human TAP2 detected by denaturing gradient gel electrophoresis

The human transporter associated with antigen processing (TAP1 and TAP2) genes are located in the human leukocyte antigen (HLA) class II region of the genome and encode proteins that form a heterodimer essential for the transport of endogenous peptides into the endoplasmic reticulum for assembly with HLA class I molecules. Type 1 diabetes is an autoimmune disease that is associated with the HLA region of the genome, with HLA class II genes conferring the greatest statistical risk. The presentation of self-peptides by HLA class I molecules is defective in individuals with this disease, and both TAP1 and TAP2 are potential contributors to this defect. Denaturing gradient gel electrophoresis (DGGE) was applied to screen all 11 exons and the 3' flanking region of TAP2 for polymorphisms in individuals with type 1 diabetes patients and controls. Seventy polymorphisms, including 51 in introns, 4 in the 3' flanking region, and 15 in exons, were identified. Sequencing of polymorphic DNA fragments revealed several new polymorphisms, including a Gln --> Arg substitution at codon 611 and a GT --> GC polymorphism affecting the donor splice site of intron 4, that might be of functional significance. None of the polymorphisms examined differed in frequency between individuals with type 1 diabetes and controls.

HLA-DQA1 and HLA-DQB1 genetic markers and clinical presentation in celiac disease

BACKGROUND

Patients with celiac disease are diagnosed at any age and can exhibit a wide range of clinical manifestations. The reasons for this are unclear. The aim of this study was to investigate a possible correlation between the HLA-DQA1 and HLA-DQB1 genetic markers and clinical features of celiac disease.

METHODS

A total of 133 patients with celiac disease were tested for the HLA-DQA1 and HLA-DQB1 genes. Their corresponding allele and haplotype frequency distributions were estimated from the phenotypes found. The results were correlated with data from the clinical records.

RESULTS

The DQ2 molecule was found in 93% of the patients, and DQ2 or DQ8 was found in 98%. The DQA1*0201-DQB1*0202 haplotype showed strong linkage disequilibrium. DQ2 homozygosis was significantly associated with female sex, earlier age at diagnosis, and shorter delay between onset of symptoms and diagnosis. Double-dose DQB1*02 (01-02) allele was more frequent in patients with the classic presentation of the disease.

CONCLUSIONS

The genetic markers investigated may prove useful for diagnosing and managing celiac disease. With some clinical variables, correlations not previously described were found. These correlations have a moderate strength and, therefore, must be confirmed by other studies.

Celiac disease-associated autoimmune endocrinopathies

Celiac disease (CD) is an autoimmune disorder induced by gluten intake in genetically susceptible individuals. It is characterized by the presence of serum antibodies to endomysium, reticulin, gliadin, and tissue transglutaminase. The incidence of CD in various autoimmune disorders is increased 10- to 30-fold in comparison to the general population, although in many cases CD is clinically asymptomatic or silent. The identification of such cases with CD is important since it may help in the control of type I diabetes or endocrine functions in general, as well as in the prevention of long-term complications of CD, such as lymphoma. It is believed that CD may predispose an individual to other autoimmune disorders such as type I diabetes, autoimmune thyroid, and other endocrine diseases and that gluten may be a possible trigger. The onset of type I diabetes at an early age in patients with CD, compared to non-CD, and the prevention or delay in onset of diabetes by gluten-free diet in genetically predisposed individuals substantiates this antigen trigger hypothesis. Early identification of CD patients in highly susceptible population may result in the treatment of subclinical CD and improved control of associated disorders.

TAP1 and TAP2 polymorphism in Spanish patients with ankylosing spondylitis

OBJECTIVE

HLA-B27 is strongly associated with ankylosing spondylitis (AS); however, the association is not absolute and additional susceptibility factors in the MHC region could play a role. We studied the influence of polymorphism in the transporter associated with antigen processing (TAP) genes, including point mutations not previously analyzed.

METHODS

HLA-B*27 typing and subtyping as well as TAP1 and TAP2 typing were performed by PCR-RFLP. Forty-four AS individuals were compared to 61 ethnically matched random individuals and 35 B*27-positive healthy unrelated individuals as controls.

RESULTS

The frequency of the TAP1B allele was significantly greater in the patient group compared with the random controls (corrected p value (p(c)) = 0.035; odds ratio = 15.8, p = 0.01). A greater frequency was also evident when B*27-positive patients and B*27- positive healthy controls were compared, although it did not reach statistical significance. No differences were observed in TAP2 alleles between the groups studied.

DISCUSSION

We did not find a primary association between TAP2 polymorphism and AS susceptibility. Formal confirmation of a linkage between the TAP and HLA-B loci would probably require family studies.

Celiac disease: strongly heritable, oligogenic, but genetically complex

Celiac disease, or gluten-sensitive enteropathy, is a small intestinal disorder which affects up to 1:250 people in the United States. Disease development has a strong genetic component, with a sibling relative risk (lambda(s)) of 30. One susceptibility locus is the MHC region, with a particular association with the HLA-DQ alleles DQA1*0501 and DQB1*0201. However, haplotype sharing studies suggest that genes within the MHC complex contribute no more than 40% to the sibling familial risk of disease. This means that the stronger genetic risk is likely to be conferred by a small number of non-HLA-linked genes. Genome-wide linkage studies, plus linkage and association studies of candidate loci have been used to try to identify these genes. However, these studies have either failed to detect loci, or produced inconsistent results. Such difficulties in identifying susceptibility genes are encountered when investigating any complex genetic disorder. Information from the Human Genome Project, coupled with new technology for high throughput single nucleotide polymorphism typing may help to identify the non-HLA determinants of celiac disease in the future.

Genetics of celiac disease

Celiac disease (CD) is a chronic inflammatory disease of the gut resulting from ingestion of gluten, occurring in genetically susceptible individuals. The strong genetic association of CD with the DQ2 and DQ8 HLA heterodimers has been known for long, but others non-HLA genes are involved. In order to identify susceptibility genes to CD, several studies have been performed, based on either linkage analyses or candidate gene approaches. This review describes these different studies and their results. The hypothesis of the implication of the DR53 heterodimer in the HLA region has been proposed. The existence of a susceptibility locus on chromosome 5q has been evidenced through linkage analysis and candidate gene strategies have revealed the role of CTLA-4 and of the immunoglobulin gamma genes in the disease.

Synergistic effect of two HLA heterodimers in the susceptibility to celiac disease in Tunisia

The DR and DQ HLA genotypes of 94 Tunisian children affected with celiac disease are analyzed so that we can gain a better understanding of the HLA component of this disease. All of them carry at least one of two specific heterodimers: a DQ heterodimer, encoded by DQA1*0501, DQB1*0201 and/or a DR heterodimer, encoded by the nonpolymorphic gene DRA and the DRB4 gene. Quantifying the relative penetrances of all susceptible genotypes gives evidence for a synergistic effect of these two heterodimers and for a dose effect of the alleles encoding the beta chains of these two heterodimers. The DR3DR7 individuals have the greatest risk. They present the two kinds of heterodimers and carry two DQB1*0201 alleles. Celiac disease is the first HLA-associated disease for which the at-risk genotypes are so well delineated.

Characterization of LMP polymorphism in homozygous typing cells and a random population

Within the class II region of the MHC are several genes whose products are involved in processing antigen for HLA class I presentation. Two such genes, LMP2 and LMP7, encode products that are incorporated into a multicatalytic proteinase complex which serves as the major pathway for protein degradation for class I peptide presentation. Polymorphic residues have been identified in both LMP2 and LMP7. In this report, we describe an ARMS-PCR method to distinguish LMP7 alleles. We applied this method to characterize these alleles in addition to LMP2 alleles in 50 homozygous typing cells (HTC) as well as in a panel of 110 random individuals. Of the four possible combinations of LMP2 and LMP7, we observed three in the HTC population, while all four were observed in the random population. The frequencies at which allele combinations were observed were similar to that predicted by individual allele frequencies. We also analyzed the possibility of linkage disequilibrium of LMP2 and LMP7 alleles with TAP1, TAP2, and specific HLA class I alleles in both populations. From this data, there seems to be no apparent linkage disequilibrium and no indication that particular combinations of LMP2 and LMP7 have been maintained.

A novel PCR-based methodology to determine TAP allele frequencies in population studies

The TAP genes have been extensively studied as candidate susceptibility genes for autoimmune and infectious diseases. TAP1 and TAP2 have two and three polymorphic sites respectively which are used for the allele assignment following WHO nomenclature. The usual techniques employed to determine the TAP alleles cannot unequivocally ascertain the alleles present in subjects which are heterozygous at more than one polymorphic position within the genes. This results in an inability to determine absolute TAP allele frequencies in population studies. The aim of this study was to devise a PCR-based method to unambiguously assign TAP alleles to all subjects. The novel method was tested in Oriental Type I diabetic and control subjects. The technique is a valuable tool for allele assignment in heterozygous subjects solely using PCR.

HLA-DQ2-negative celiac disease in Finland and Spain

Genetic susceptibility to celiac disease (CD) is strongly associated with DQA1*0501 and DQB1*02 (= DQ2). To study whether CD patients without DQ2 share other MHC class II or TNF alleles, we screened DQ2-negative patients in Finland and Spain. Twelve of 84 (14%) Finnish patients and 13 of 189 (6%) Spanish patients were negative for DQ2. We observed that all but two of altogether 25 DQ2-negative patients had the DR4 DQ8 haplotype, or either DQA1*0501 or DQB1*02 alone. Also, all but three were positive for DRB4*01. The only patients without any of these alleles were both positive for DR 13. There was a clear difference between Finland and Spain: Ten (83%) of the 12 Finnish DQ2-negative patients but only five (38%) of the 13 Spanish patients had DRB1*03, DQA1*03, DQB1*0302 (= DQ8) alleles. Of the Spanish patients, eight (62%) had DQB1*02 without DQA1*0501 and three (23%) had DQA1*0501 without DQB1*02. None of the TNF, TAP, or DPB1 alleles was found to be significantly associated with CD. Our results indicate that in addition to the DQ2 heterodimer, the other major risk alleles for CD are DR4 DQ8, and either DQA1*0501 or DQB1*02 alone. Patients without these alleles appear to be very rare, only two (0.7%) were identified in altogether 253 patients tested.

Human TAP1 polymorphisms detected by denaturing gradient gel electrophoresis

Presentation of endogenous peptides by major histocompatibility complex class I (MHC) molecules is controlled, in part, by the Tap1 and Tap2 genes in the MHC class II region that encode a heterodimeric peptide transporter. Polymorphisms of human Tap1 in normal individuals have now been investigated systematically by denaturing gradient gel electrophoresis (DGGE) analysis of fragments of genomic DNA generated by the polymerase chain reaction. Polymorphisms identified by distinctive DGGE band patterns were confirmed by DNA sequencing. In addition to four previously described polymorphisms in the open reading frame, DGGE detected three new polymorphisms: a G-->T substitution in the promoter region, a 10-bp insert in intron 9, and a G-->T substitution 80-bp downstream of the translation termination codon.

Transporter associated with antigen-processing (TAP) genes and susceptibility to tuberculoid leprosy and pulmonary tuberculosis

We have studied TAP polymorphism in a panel of 40 healthy individuals, 57 patients with pulmonary tuberculosis (PTB) and 50 with tuberculoid (TT) leprosy from North India. Only TAP2-A/F occurred with a significantly increased frequency in PTB patients as compared to controls (82.5% vs. 52.5%, P < 0.002, Pc < 0.01) giving a high relative risk of 4.3. On the other hand, TAP2-B was significantly increased in TT leprosy as compared to controls (76% vs. 47.5%, Pc < 0.003, RR 3.5) particularly in patients positive for HLA-DR15 than controls carrying DR15 (77.5% vs. 50%, P < 0.03, RR = 3.4). Further, TAP2-B allele was positively associated with DR15 negative PTB patients as compared to the DR15 positive group (43.8% vs. 17.1%, P < 0.04, RR = 0.3). This study along with our earlier studies on HLA association in mycobacterial diseases suggests that in addition to HLA-DR15 alleles in the TAP2 region influence susceptibility to PTB and TT leprosy.

Molecular mechanisms of class I major histocompatibility complex antigen processing and presentation

The presentation of antigenic peptides by class I major histocompatibility complex molecules plays a central role in the cellular immune response, since immune surveillance for detection of viral infections or malignant transformations is achieved by CD8+ T lymphocytes which inspect peptides, derived from intracellular proteins, bind to class I molecules on the surface of most cells. The transporter associated with antigen processing selectively translocates cytoplasmically derived peptides of appropriate sequence and length into the lumen of the endoplasmic reticulum where they associate with newly synthesized class I molecules. The translocated peptides are generated by multicatalytic and multisubunit proteasomes which degrade cytoplasmic proteins in a ATP-ubiquitin-dependent manner. This review discusses our current molecular understanding of class I antigen processing and presentation.

Absence of primary association between DM gene polymorphism and insulin-dependent diabetes mellitus or celiac disease

The DMA and DMB genes encode class II-like heterodimetric molecules located in a specialized endocytic compartment, where they facilitate efficient loading of antigenic peptides on HLA class II molecules. Both genes are located within the MHC class II region and present a limited allelic polymorphism. Here we report the distribution of DM alleles in a group of 75 IDDM patients, 72 CD patients, and 162 random controls. We found a pronounced decreased frequency of DMA*0102 in both patient groups relative to controls. This difference was, however, mainly secondary to a strong negative linkage disequilibrium (LD) between this allele and the IDDM and CD-associated DRB1*03 allele. The DMB phenotype frequencies were similar in CD patients and controls. By contrast, we observed a decreased frequency of DMB*0101 and an increased frequency of DMB*0102 and DMB*0104 in IDDM patients. These differences disappeared when matching individuals for DRB1*03 or DRB1*04 alleles, which was in accordance with strong negative LD between DMB*0101 and DRB1*04 or DQB1*0302 alleles, and positive LD between DMB*0104 and DQB1*0201. Our data suggest that the apparent associations of IDDM or CD with given DM alleles are mostly secondary to primary associations with alleles at the DRB and DQB loci.

Linkage disequilibrium between HLA class II (DR, DQ, DP) and antigen processing (LMP, TAP, DM) genes of the major histocompatibility complex

TAP, LMP and DM genes map within the major histocompatibility complex (MHC) class II region between the DQB1 and DPB1 loci, and are involved in the processing of peptides bound to HLA class I or class II molecules. In order to determine the various linkage disequilibria existing between these genes and HLA class II genes, we have analyzed TAP1, TAP2, LMP2, DMA, DMB, DRB1, DQA1, DQB1 and DPB1 polymorphisms in 162 unrelated healthy Caucasian individuals. Many positive or negative associations were observed between alleles at these loci, such as between DR/DQ and TAP2, DM or LMP, between DP and DMB, and between TAP2 and DM, TAP2 and LMP. Conversely, no linkage disequilibrium was detected between some closely related genes (DR/DQ and TAP1, TAP1 and TAP2, LMP2 and DM), in agreement with the existence of recombination hot spots in this region. Other weak linkage disequilibria are likely to exist in this region. These data allow to define some conserved MHC class II haplotypes including HLA class II and TAP, LMP and DM alleles. Furthermore, the knowledge of such linkage disequilibria is of outstanding importance in order to avoid misinterpretation of the data when studying MHC class II associations with autoimmune diseases.

Clinical aspects of coeliac disease in children with insulin-dependent diabetes mellitus

Coeliac disease (CD) is heterogeneous in its clinical presentation and pathological expression. Silent, latent and potential forms represent the submerged part of the so-called "coeliac iceberg". The association of insulin-dependent diabetes mellitus (IDDM) and CD has been widely reported. For the screening of CD in diabetic patients, anti-reticulin R1 (ARA-R1) and anti-endomysium (AEA) antibodies are more reliable markers than anti-gliadin (AGA) antibodies. Recent studies have reported an increased prevalence of CD in children with IDDM. In our experience intestinal biopsy confirmed a diagnosis of CD in 6 out of 172 diabetic patients, with a prevalence of 3.5%. Only occasionally does CD precede the onset of IDDM; more often CD is diagnosed shortly or sometimes years after the onset of diabetes. Typical gastrointestinal complaints of CD (such as diarrhoea, abdominal distension) are rare in IDDM patients, while atypical isolated signs or symptoms of CD are more common, in particular sideropenic anemia, short stature, delayed puberty, epilepsy, hypertransaminasemia, dyspeptic symptoms, herpetiform dermatitis, and recurrent aphthous stomatitis. It is recommended that all diabetic children, even those asymptomatic, should be screened yearly for CD, using a combination of AGA plus ARA-R1 and AEA.

Comparative study of the role of professional versus semiprofessional or nonprofessional antigen presenting cells in the rejection of vascularized organ allografts

The immune systems of transplant recipients are progressively challenged with exposure to the multiple lineages of donor cells that comprise the vascularized organ allograft. Each lineage of such donor tissue constitutively expresses or can be induced to express varying densities of MHC antigens ranging from no expression of MHC to MHC class I only to both MHC class I and class II. In addition, the cell surface expression of a diverse assortment of costimulatory and cell adhesion molecules also varies in density in a tissue specific fashion within the allograft. The MHC class I/II molecules displayed on the donor cells contain within their clefts a constellation of processed protein antigens in the form of peptides derived from intracellular and to some extent extracellular sources. Therefore, the potential for each cell lineage to induce alloactivation and serve as a target for allospecific immune responses is dependent on the diversity and density of peptide-bearing MHC molecules, costimulatory molecules, and cell adhesion molecules. In addition, the T cell receptor repertoire of the recipient also contributes to the magnitude of the allogeneic response. Consequently, the variety of clinical outcomes following organ transplantation even with the institution of potent immunosuppressive (drug) therapies is not surprising, as it appears reasonable for such therapies to influence the allogeneic response against distinct lineages differentially. Our failure to prevent chronic human allograft rejection may therefore be due to our limited appreciation of the full spectrum of alloactivating experiences encountered by host T cells as they interact with donor cells of diverse tissue lineages. Investigations by our laboratory of the immunopathogenesis of chronic cardiac allograft rejection have revealed an intrinsic inability of human cardiac myocytes to process and present antigens, not only for primary but also for secondary alloimmune responses. One obvious explanation for this phenomenon is the fact that cardiac myocytes do not constitutively express MHC class II molecules and express only low levels of class I molecules. However, this immunological unresponsiveness is maintained even after the induction of MHC class II and upregulation of MHC class I on these cells by interferon-gamma (IFN-gamma). Similar results have also been reported for cells of different tissue lineages (e.g. chondrocytes, keratinocytes, neural cells). Until now, cells have been defined as professional or nonprofessional for the purposes of defining their potential for antigen presentation to T cells. Professional antigen presenting cells have been identified as cells that are of haematopoietic origin, that constitutively express MHC class I and class II molecules as well as potent costimulatory molecules, and that are able to induce both primary and secondary immune responses, whereas nonprofessional antigen presenting cells are not bone marrow derived, do not constitutively express MHC class II, but may in some cases initiate primary and secondary immune responses after induction of MHC class II antigen by proinflammatory cytokines (e.g. IFN-gamma). The findings of our laboratory and others suggest that cells of certain lineages be considered in the separate class of 'nonantigen presenting cells'. Indeed, nonprofessional antigen presenting cells can be reclassified into three categories: semiprofessional-, nonprofessional-, or nonantigen presenting cells that are able to present antigen to and activate naive T cells, activated T cells, or no T Cells, respectively. The aim of this review is to identify and (re)examine the antigen presentation characteristics of cells of different tissue lineages in terms of their ability to activate different subsets of T cells. This approach is taken in an attempt to synthesize these concepts into a unified picture of T cell activation in the context of antigen processing and presentation by different cell types.

TAP1 and TAP2 polymorphism in HLA-B27-positive subpopulations: no allelic differences in ankylosing spondylitis and reactive arthritis

The polymorphic TAP1 and TAP2 genes encode subunits of the transporter that delivers peptides to the HLA class I molecules. Because the polymorphism of the TAP genes has been shown to affect peptide transport, it has been suggested that TAP genes are potential regulators of the immune response. We studied TAP1 and TAP2 polymorphism in two multifactorial HLA-B27-associated diseases, ankylosing spondylitis (N = 30) and reactive arthritis (N = 30), in order to establish whether TAP genes are involved in the different pathogenesis of these diseases. Healthy HLA-B27-positive individuals (N = 55) were chosen as the primary controls and 93 individuals represented the random Finnish population as secondary controls. We found differences between the random and HLA-B27-positive populations, thus suggesting that certain TAP alleles are prevalent in HLA-B27 haplotypes. No differences were found between the AS and ReA groups nor between either of them and the healthy HLA-B27-positive controls. Thus it seems unlikely that TAP polymorphism, ar the level studied, has a dominant role in the pathogenesis of these diseases. However, a family study is needed in order to determine whether the same TAP complexes are carried by the same haplotypes in these diseases.

TAP polymorphism does not influence transport of peptide variants in mice and humans

The major histocompatibility complex (MHC)-encoded transporter associated with antigen processing (TAP) delivers cytosolic peptides to the lumen of the endoplasmic reticulum (ER) for presentation by MHC class I molecules. For the rat, it has been demonstrated that TAP polymorphism results in the selection of different sets of peptides, the nature of the C terminus being of particular importance. Here, we investigated whether TAP polymorphism in mice and humans has functional consequences for transport of peptide sets variable at the C-terminal residues. Using cell lines of H-2d, H-2k, and H-2dxk haplotype and a panel of human lymphoblastoid cell lines expressing eight different TAP alleles, we detected species-specific transport patterns, but no significant influence of TAP polymorphism on peptide selection. In addition, peptides with different core sequences were translocated to the same extent by different TAP. These results suggest that a major contribution of human TAP polymorphism to disease progression and autoimmunity is not very likely.

Reevaluation of the relative risk for susceptibility to celiac disease of HLA-DRB1, -DQA1, -DQB1, -DPB1, and -TAP2 alleles in a French population

In a population of 46 children with CD recruited in the Paris area of France, an excess of DRB1*03 and DRB1*07 alleles and of DR3/DR7, DR3/DR3 and DR11(or 12)/DR7 phenotypes was found (RRs of 6.3, 9.3, 24.6, 15, and 15.1, respectively), which is reminiscent of the markers of susceptibility observed in southern rather than in northern European celiac patients. More importantly, the highest association with CD was not found in individuals expressing the DQA1*0501-DQB1*0201 heterodimer in single dosage (RR = 24.9) or in homozygous state, but in people co-expressing one copy of DQA1*0501-DQB1*0201 on one haplotype and a second copy of DQB1*0201 on the second haplotype (RR = 35.7). This suggests that in our population either DQB1*0201 or a gene closely linked to DQB1*0201 influences the susceptibility to CD conferred by the DQA1*0501-DQB1*0201 heterodimer. Significant positive or negative RRs conferred by some TAP2 or DPB1 alleles were found. However, they were moderate compared to the RR conferred by the expression of a second copy of DQB1*0201. Moreover, they were no longer significant when patients were compared with HLA-DR matched controls. This suggests that associations of CD with TAP2 and DPB1 alleles are secondary to linkage disequilibria and argues against the contribution of these alleles in resistance and/or susceptibility to CD. Thus the "raison d'être" of a "DQB1*0201 second haplotype effect" in susceptibility to CD remains to be elucidated.

TAP2 association with insulin-dependent diabetes mellitus is secondary to HLA-DQB1

IDDM is known to be associated with genes of HLA complex, particularly alleles of HLA-DQ. The 40-kb TAP gene complex is located approximately 150 kb centrometric to the DQB1 locus. The TAP1-TAP2 protein heterodimer is required for normal expression levels of class I, molecules on the surface of cells. While present evidence implicates HLA-DQ as the major susceptibility locus in IDDM, as class I expression apparently plays a role in the progression of disease, the possibility exists that the association attributed to HLA-DQ is in fact due to an association with the TAP genes. Several studies have concluded that the alleles of TAP1 are not significantly associated with IDDM; this report concentrates on the more telomeric TAP2 locus. During this investigation, six previously described TAP2 alleles were identified in 208 normal Caucasians and 241 Caucasian diabetics. Sequence analysis of cDNA clones identified a seventh allele of TAP2, TAP2*F, which contains an arginine-to-cystine interchange at amino acid position 651. Overall, our results indicate only a modest association of IDDM with TAP2; however, the newly described TAP2*F allele was found to be significantly increased in a modest subset of our large diabetic population. These data, generated from the same population of controls and diabetics we previously studied at all other relevant MHC loci, provide additional evidence that the HLA susceptibility to IDDM maps to HLA-DQ.