A Functional FCGR2A Haplotype Associated with Rheumatoid Arthritis Determines Circulating Soluble FcγRIIa Levels and Immune Complex Signalling

Fc{gamma}RIIa, encoded by FCGR2A, is a widely expressed Fc receptor implicated in autoimmunity and infectious disease susceptibility. To fine-map the rheumatoid arthritis (RA) association at the complex FCGR locus, we combined gene-specific resequencing, genetic association studies in UK and Spanish European cohorts, functional genomics, structural biology, biophysical analyses, and cellular assays. We identified a common European FCGR2A haplotype (2A.3), defined by Q27W, H131H, and the RA-associated SNP rs12746613, which showed the strongest association with RA. Multi-omics analyses demonstrated that 2A.3 is associated with reduced expression of the soluble FCGR2A splice variant and lower circulating soluble Fc{gamma}RIIa levels. Functional studies revealed altered IgG interactions and delayed Fc{gamma}RIIa signal transduction associated with Q27W, while structural analyses found no evidence for stable ectodomain dimerisation. Together, these findings identify 2A.3 as an important functional contributor to RA susceptibility and provide mechanistic insight into how FCGR2A variation may influence immune regulation and disease risk in Europeans.