ObjectivesTo identify the arthritogenic B cell epitopes of glucose-6-phosphate isomerase (GPI) and their association with rheumatoid arthritis (RA).MethodsIgG response towards a library of GPI peptides in patients with early RA, pre-symptomatic individuals and population controls, as well as in mice, were tested by bead-based multiplex immunoassays and ELISA. Monoclonal IgG were generated, and the binding specificity and affinity were determined by ELISA, gel size exclusion chromatography, surface plasma resonance and X-ray crystallography. Arthritogenicity was investigated by passive transfer experiments. Antigen-specific B cells were identified by peptide tetramer staining.ResultsPeptide GPI_293-307was the dominant B cell epitope in K/BxN and GPI-immunised mice. We could detect B cells and low levels of IgM antibodies binding the GPI_293-307epitopes, and high affinity anti-GPI_293-307IgG antibodies already 7 days after GPI immunisation, immediately before arthritis onset. Transfer of anti-GPI_293-307IgG antibodies induced arthritis in mice. Moreover, anti-GPI_293-307IgG antibodies were more frequent in individuals prior to RA onset (19%) than in controls (7.5%). GPI_293-307-specific antibodies were associated with radiographic joint damage. Crystal structures of the Fab–peptide complex revealed that this epitope is not exposed in native GPI but requires conformational change of the protein in inflamed joint for effective recognition by anti-GPI_293-307antibodies.ConclusionsWe have identified the major pathogenic B cell epitope of the RA-associated autoantigen GPI, at position 293–307, exposed only on structurally modified GPI on the cartilage surface. B cells to this neo-epitope escape tolerance and could potentially play a role in the pathogenesis of RA.