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(293-307) was 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(293-307) epitopes, and high affinity anti-GPI(293-307) IgG antibodies already 7 days after GPI immunisation, immediately before arthritis onset. Transfer of anti-GPI(293-307) IgG antibodies induced arthritis in mice. Moreover, anti-GPI(293-307) IgG 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(293-307) antibodies. 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.