In this Perspectives article, we describe how Behçet disease and several clinically distinct spondyloarthropathies-all associated with MHC class I (MHC-I) alleles such as HLA-B(*)51, HLA-C(*)0602 and HLA-B(*)27 and epistatic ERAP-1 interactions-have a shared immunopathogenetic basis.
We suggest that lower affinity of peptide binding may be the basis for inefficient tolerance to HLA-B*5101-binding self-peptides, a predisposing factor for the development of Behçet disease.
Our data suggest that the robust HLA-B*51 association in Behçet's disease is explained by a variant located between the HLA-B and MICA genes (rs116799036: odds ratio (OR) = 3.88, P = 9.42 × 10(-50)).
Three risk loci shared with ankylosing spondylitis and psoriasis (the MHC class I region, ERAP1 and IL23R and the MHC class I-ERAP1 interaction), as well as two loci shared with inflammatory bowel disease (IL23R and IL10) implicate shared pathogenic pathways in the spondyloarthritides and Behçet's disease.
Three risk loci shared with ankylosing spondylitis and psoriasis (the MHC class I region, ERAP1 and IL23R and the MHC class I-ERAP1 interaction), as well as two loci shared with inflammatory bowel disease (IL23R and IL10) implicate shared pathogenic pathways in the spondyloarthritides and Behçet's disease.
These facts suggest that the pathogenic gene of Behçet's disease is not the HLA-C gene (HLA-Cw*14 and/or HLA-Cw*15) but the HLA-B gene (HLA-B51) itself or a non-HLA gene residing in the centromeric side of the HLA-B gene rather than in the telomeric side around the HLA-C gene.
Combined with its requirement for HLA-B*51, these data suggest that a hypoactive ERAP1 allotype contributes to Behçet's disease risk by altering the peptides available for binding to HLA-B*51.
Rare IL10 coding variants were not detected in BD patients, but we identified 28 known single nucleotide polymorphisms with minor allele frequencies ranging from 0.010 to 0.390, and five novel non-coding variants in five heterozygous cases. ss836185595, located in the IL10 3' untranslated region, was also detected in one Iranian control individual and therefore is not specific to BD.
HLA-B51 molecules themselves may be responsible, at least in part, for the neutrophil hyperfunction in Behçet's disease; a significant correlation was observed between the neutrophil hyperfunction and the possession of HLA-B51 phenotype, regardless of the presence of the disease, in both humans and HLA-B transgenic mice.
The distinct effects of both enzymes on the HLA-B*51 peptidome provide a basis for their differential association with Behçet's disease and suggest a pathogenetic role of the B*51:01 peptidome.
The Behçet's disease (BD)-associated human leukocyte antigen (HLA) allele, HLA-B*51 (B*51), encodes a ligand for a pair of allelic killer immunoglobulin-like receptors (KIR) present on cytotoxic cells-KIR3DL1, which inhibits their cytotoxicity, and KIR3DS1, which activates their cytotoxic activity.
GWAS have also shown the potential associations between ERAP single nucleotide polymorphisms (SNP) loci and susceptibility to several autoimmune diseases, and ERAP1 and ERAP2 polymorphisms are related to HLA class I-associated diseases, including ankylosing spondylitis and Behçet's disease.
In IL10, rs1518111 was nominally associated with BD before and after adjustment for population stratification (odds ratio [OR] for T allele 1.20, 95% confidence interval [95% CI] 1.02-1.40, unadjusted P [P(unadj) ] = 2.53 × 10(-2) ; adjusted P [P(adj) ] = 1.43 × 10(-2) ), and rs1554286 demonstrated a trend toward association (P(unadj) = 6.14 × 10(-2) ; P(adj) = 3.21 × 10(-2) ).