Unconventional presentation of an immunodominant HLA-DR15-restricted nephritogenic proteinase 3 epitope implicated in vasculitis

Proteinase 3 (PR3) anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis is associated with HLA-DR15, implicating HLA-DR15-restricted CD4⁺ T cell autoimmunity in the disease. However, the PR3 epitopes presented by HLA-DR15 and the autoreactive CD4⁺ T cell responses they elicit are undefined. Using humanized hPR3.DR15 ⁺ mice, we identified PR3 _216-231 as an immunodominant human PR3-derived CD4⁺ T cell epitope presented by HLA-DR15. Immunopeptidomic profiling confirmed the natural processing and HLA-DR15-restricted presentation of PR3 _216-231 . Structural analyses of HLA-DR15-PR3 _216-231 complexes revealed an unconventional mode of HLA class II antigen presentation, in which a 10 amino acid peptide core occupies the 9 amino acid binding groove, causing a central kink in the bound peptide. PR3 _216-231 -specific CD4⁺ T cells from both hPR3.DR15 ⁺ mice and patients with PR3-ANCA-associated vasculitis exhibited convergent T cell receptor features, and patients with active vasculitis displayed a clonally expanded, PR3 _216-231 -specific CD4⁺ TCR repertoire. Immunization of hPR3.DR15 mice with PR3 _216-231 induced cell-mediated glomerulonephritis characterized by increased renal infiltration of CD4⁺ T cells and macrophages together with segmental glomerular necrosis. These findings identify a nephritogenic PR3-derived CD4⁺ T cell epitope presented by HLA-DR15 in ANCA-associated vasculitis and define a new extended core binding pattern, broadening our understanding of HLA class II peptide presentation.