Citrullinated Aggrecan Epitopes as Targets of Auto-reactive CD4+ T cells in Patients with Rheumatoid Arthritis.

OBJECTIVE: Recognition of citrullinated antigens such as vimentin, fibrinogen, and alpha-enolase is associated with rheumatoid arthritis (RA). Emerging data suggests that the matrix protein aggrecan is also recognized as a citrullinated antigen. The goal of this study was to directly visualize cit-aggrecan specific T cells and characterize them in subjects with RA.

METHODS: Citrullinated aggrecan peptides with likely DRB1*04:01 binding motifs were predicted using a previously published scanning algorithm. Peptides with detectable binding were assessed for immunogenicity by HLA tetramer staining, followed by single cell cloning. Selectivity for citrullinated peptide was assessed through tetramer staining and proliferation assays. Ex vivo tetramer staining was then employed to assess frequencies of aggrecan specific T cells in peripheral blood. Finally, disease association was assessed by comparing T cell frequencies in RA patients and controls and correlating aggrecan specific T cells with levels of aggrecan specific antibodies.

RESULTS: We identified six immunogenic peptides, two of which were the predominant T cell targets in peripheral blood. These two epitopes were citrullinated at HLA binding residues and shared homologous sequences. RA patients had significantly higher frequencies of cit-aggrecan-specific T cells than healthy subjects. Furthermore, T cell frequencies were significantly correlated with antibodies against citrullinated aggrecan.

CONCLUSION: Our findings indicate that T cells that recognize citrullinated aggrecan are present in subjects with RA and correlate with antibodies that target this same antigen. Consequently, aggrecan-specific T cells and antibodies are potentially relevant markers that could be utilized to monitor patients with RA or at risk subjects. This article is protected by copyright. All rights reserved.