Genetic Complexity in Spondyloarthritis: Contributions of HLA-B Alleles Beyond HLA-B*27 in Romanian Patients.

This study examined the distribution and disease associations of non-HLA-B*27 HLA-B alleles in Romanian spondyloarthritis (SpA) patients, aiming to address the underrepresentation of Eastern European populations in immunogenetic research. Methods: We analyzed 263 HLA-B*27-negative patients from Northeastern Romania fulfilling ASAS criteria. HLA-B genotyping was performed at two-digit resolution, and allele distributions were compared with two Romanian HLA-B*27-negative control groups (n = 335 and n = 1705 cases), using chi-square testing and logistic regression. Compared to controls, HLA-B*47 (p = 0.0007) and HLA-B*54 (p = 0.0013) were significantly enriched, while HLA-B*40 was underrepresented (p = 0.0287). Notably, HLA-B*54 was observed exclusively in axial SpA. Within the cohort, both HLA-B*13 and HLA-B*57 alleles were associated with psoriasis, while HLA-B*37 and HLA-B*41 alleles were clustered within the reactive arthritis group. The HLA-B*35 and HLA-B*18 alleles were the most frequently observed alleles across most clinical phenotypes. When comparing the frequency of HLA-B associations, the most common genotypes among SpA patients were B*08-B*18, B*13-B*35, and B*35-B*51. Notably, B*08-B*18 was more frequent in patients with radiographic sacroiliitis grade ≥ 2, while B*35-B*51 was more frequent in those with confirmed systemic inflammation, as indicated by elevated CRP or ESR levels. Analysis of peptide-binding patterns revealed a cluster of risk alleles, HLA-B*08, B*18, B*35, B*40, and B*54, sharing similar features, distinct from the canonical profile of B*27. These findings highlight the contribution of non-B*27 HLA-B alleles to SpA susceptibility in an Eastern European population and support the notion that HLA-B*27-negative SpA may represent a distinct clinical and immunological entity, driven by alternative pathogenic mechanisms. They also emphasize the importance of population-specific immunogenetic profiling and support expanding genetic characterization in HLA-B*27-negative patients.