A novel human autoantigen, endothelial cell growth factor, is a target of T and B cell responses in patients with Lyme disease

Presentation of autoantigens by HLA-DR molecules to CD4+ T cells is thought to be a central component of many autoimmune diseases (1), but in most instances, disease-relevant autoantigens have remained elusive. The problem is compounded by the fact that human autoimmune diseases are generally thought to be multifactorial, involving both genetic and environmental factors such as infection (2). Furthermore, in autoimmune diseases such as rheumatoid arthritis (RA) or lupus, multiple autoantigens are thought to be involved, and autoantibodies are often present months or years before the onset of clinical disease (3, 4), suggesting that additional critical factors are required to trigger tissue pathology (3). Even so, recognition of self-antigens is an essential component in the development of disease pathology. Lyme arthritis, a late manifestation of infection with the tick-borne spirochete, Borrelia burgdorferi (Bb) (5, 6), provides an important human model to study questions surrounding infection-induced autoimmunity. Lyme arthritis can usually be treated successfully with 1–2 months of oral or intravenous (IV) antibiotics, called antibiotic-responsive arthritis (7). However, in a small percentage of patients, proliferative synovitis persists for months or several years after apparent spirochetal killing with ≥3 months of oral and IV antibiotics, referred to as antibiotic-refractory arthritis (8). This disease course has been postulated to result from either persistent infection, retained spirochetal antigens, or infection-induced autoimmunity (9, 10). Against the persistent infection hypothesis, PCR and culture results of synovectomy specimens obtained in the post-antibiotic period have been uniformly negative (11), and relapse of infection has not been observed with the use of disease modifying anti-rheumatic drugs (DMARDs) after antibiotic therapy (8). Contrary to what might be expected with retained spirochetal antigens, T and B cell responses to Bb decline similarly in patients with refractory or responsive arthritis (12, 13), whereas inflammatory mediators in synovial fluid (SF), particularly IFN-γ, remain high or even increase in the refractory group during the post-antibiotic period (14). In support of the autoimmunity hypothesis, specific HLA-DR alleles, particularly the DRB1*0101 or 0401 alleles, are the greatest known genetic risk factor for antibiotic-refractory arthritis (15). As in other chronic inflammatory arthritides, HLA-DR molecules in antibiotic-refractory Lyme arthritis are intensely expressed in inflamed synovium (16). In a search for molecular mimicry between spirochetal and host proteins, partial sequence homology was found between the human peptides, LFA-1αL332-340 (17) and MAWD-BP280-288 (18), and an epitope of Bb outer-surface protein A (OspA163-175) (19), which binds refractory arthritis-associated HLA-DR molecules (15). However, only a minority of patients had low-level T cell reactivity with these self peptides, and none had autoantibody responses to these self proteins (18, 20). Later, Ghosh et al., identified human cytokeratin 10 as a cross-reactive target ligand recognized by anti-OspA antibodies in a small group of patients with refractory arthritis (3 of 15), but not in those with responsive arthritis (0 of 5) (21). Finally, several neural proteins have been reported to induce T or B cell responses in patients with neuroborreliosis (22–24) or post-Lyme syndrome (25). However, responses against neural proteins would be unlikely to explain antibiotic-refractory arthritis. In this study, we utilized discovery-based proteomics and translational research in an effort to identify autoantigens in synovial tissue, the target tissue of the immune attack in antibiotic-refractory Lyme arthritis. Based on this approach, we report here the identification of a novel autoantigen, endothelial cell growth factor (ECGF), which is a target of T and B cell responses in a subset of patients with Lyme disease, thereby providing the first direct evidence for autoimmune T and B cell responses in this illness.