Monday 5 sept 2022

Introduction and Aims. Fibroblast-like synoviocytes (FLS) are critically important cells in post-infectious Lyme arthritis (LA) and in other chronic inflammatory arthritides. In inflamed tissue, FLS upregulate cell-surface expression of antigen-presenting molecules, especially HLA-DR molecules. However, little is known about the antigens presented by these cells. To better understand disease pathogenesis, we identified T cell epitopes of FLS-derived extracellular matrix (ECM) proteins, determined Borreliella (Borrelia) burgdorferi (Bb)-mimic epitopes, and characterized T and B cell responses to these peptides or proteins.

Methods. HLA-DR-presented peptides (T cell epitopes) of FLS-derived ECM proteins were identified directly from synovia of post-infectious LA patients using mass spectrometry. T and B cell responses to these proteins were determined by ELISpot and ELISA assays. Bb-mimic epitopes were identified by BLAST searches and tested for immunogenicity. Tetramer reagents were used to test directly the binding of these epitopes to patients’ CD4+ T cells and to determine the subtypes of these cells.

Results. Using immunopeptidomics to examine LA patients’ synovial tissue, we identified 3 HLA-DR-presented peptides (T cell epitopes) of fibroblast-derived ECM proteins – fibronectin-1, laminin B2, and collagen Va1. When tested for immunogenicity, 14 of 24 patients (58%) with post-infectious LA had CD4+ T cell responses to >1 of these T cell epitopes, and 9 of 52 (17%) had antibody responses to >1 of these proteins. These responses were found almost exclusively in patients with post-infectious LA, the period when massive FLS proliferation develops. Post-infectious LA patients with these T cell responses had significantly increased frequencies of HLA-DRB1*04 or DRB1*1501 alleles compared with antibiotic-responsive LA patients (P=0.0003), and among post-infectious LA patients, the duration of arthritis in the post-infectious period was significantly longer among those who had responses to FLS-derived protein epitopes compared with those who lacked these responses (P=0.05)  Of the 8 patients who had T cell reactivity with the collagen Va1^1730-1750 epitope, 5 also had responses to a similar Bb-mimic epitope in a previously unrecognized BBQ62-like protein, which is encoded on lp28-2. Tetramer reagents bound the autoreactive or Bb-mimic epitope, and a small percentage of cells bound both the borrelial and collagen epitopes. A high percentage of autoreactive CD4+ T cells to this one collagen epitope were T-bet-positive Th1 cells, intermediate percentages were GATA-2-positive Th2 cells or RoRgt-positive Th17 cells, and only a small percentage were FoxP3-expressing Treg cells.

Conclusion. These findings offer new insights into the pathogenesis of post-infectious LA. We propose a hypothesis linking 1) infection of collagen in joints with markedly inflammatory Bb strains, 2) induction of hyperinflammatory responses and marked proliferation of FLS, and 3) molecular mimicry between borrelial and collagen epitopes leading to collagen-reactive CD4+ T cells, which likely play a role, in part, in prolonging joint inflammation in post-infectious LA.