OBJECTIVE:The aim of this study was to assess the potential contribution of HLA-class I MICA and HLA-B gene polymorphisms towards the pathogenesis of giant cell arteritis (GCA). METHODS:Ninety-eight biopsy-proven GCA patients and 225 ethnically matched controls from Lugo, Northwest Spain, were genotyped for the MICA-TM microsatellite polymorphism using a polymerase chain reaction (PCR)-based method. Genotyping of HLA-B was performed using PCR and detection with a reverse sequence-specific oligonucleotide (SSO) probes system. RESULTS:A significant difference in the distribution of the alleles of MICA between patient and control groups (P = 0.005) was found. This was due to an increased frequency of the MICA A5 allele in GCA patients compared with controls (26 vs 13.6%; P = 0.0001; P(C) = 0.0005; OR 2.2, 95% CI 1.4-3.4). In addition, the HLA-B*15 allele showed a higher frequency in GCA patients compared with controls (P = 0.004; P(C) = 0.04; OR 2.7, 95% CI 1.3-5.7). Interestingly, the association observed with the MICA A5 allele seems to be independent of linkage disequilibrium with HLA-B, as well as independent of that previously described with HLA-DRB1*04. Remarkably, simultaneous presence of MICA A5 and HLA-B*15 or HLA-DRB1*04 genetic markers leads to an increase in the OR obtained for each individual genetic marker (MICA A5 + B*15 OR 3.2; MICA A5 + DRB1*04 OR 5.8). CONCLUSIONS:Our results provide the first evidence that the MICA and HLA-B genes are independently associated with the genetic susceptibility to GCA, and suggest that several genes within the MHC might have independent effects in the susceptibility to this systemic vasculitis.