BACKGROUNDAutoimmune diseases often have strong genetic associations with specific HLA-DR alleles. The synovial lesion in chronic inflammatory forms of arthritis shows marked upregulation of HLA-DR molecules, including in postinfectious Lyme arthritis (LA). However, the identity of HLA-DR-presented peptides, and therefore the reasons for these associations, has frequently remained elusive.METHODSUsing immunopeptidomics to detect HLA-DR-presented peptides from synovial tissue, we identified T cell epitopes from 3 extracellular matrix (ECM) proteins in patients with postinfectious LA, identified potential Borreliella burgdorferi-mimic (Bb-mimic) epitopes, and characterized T and B cell responses to these peptides or proteins.RESULTSOf 24 postinfectious LA patients, 58% had CD4+ T cell responses to at least 1 epitope of 3 ECM proteins, fibronectin-1, laminin B2, and/or collagen Vα1, and 17% of 52 such patients had antibody responses to at least 1 of these proteins. Patients with autoreactive T cell responses had significantly increased frequencies of HLA-DRB1*04 or -DRB1*1501 alleles and more prolonged arthritis. When tetramer reagents were loaded with ECM or corresponding Bb-mimic peptides, binding was only with the autoreactive T cells. A high percentage of ECM-autoreactive CD4+ T cells in synovial fluid were T-bet-expressing Th1 cells, a small percentage were RoRγt-expressing Th17 cells, and a minimal percentage were FoxP3-expressing Tregs.CONCLUSIONAutoreactive, proinflammatory CD4+ T cells and autoantibodies develop to ECM proteins in a subgroup of postinfectious LA patients who have specific HLA-DR alleles. Rather than the traditional molecular mimicry model, we propose that epitope spreading provides the best explanation for this example of infection-induced autoimmunity.FUNDINGSupported by National Institute of Allergy and Infectious Diseases R01-AI101175, R01-AI144365, and F32-AI125764; National Institute of Arthritis and Musculoskeletal and Skin Diseases K01-AR062098 and T32-AR007258; NIH grants P41-GM104603, R24-GM134210, S10-RR020946, S10-OD010724, S10-OD021651, and S10-OD021728; and the G. Harold and Leila Y. Mathers Foundation, the Eshe Fund, and the Lyme Disease and Arthritis Research Fund at Massachusetts General Hospital.