Recognition of citrullinated antigens such as vimentin, fibrinogen, and α‐enolase is associated with rheumatoid arthritis (RA). Emerging data suggest that the matrix protein aggrecan is also recognized as a citrullinated antigen. This study was undertaken to directly visualize Cit‐aggrecan–specific T cells and characterize them in patients with RA.

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 by tetramer staining and proliferation assays. Ex vivo tetramer staining was then performed 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.

We identified 6 immunogenic peptides, 2 of which were the predominant T cell targets in peripheral blood. These 2 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.

Our findings indicate that T cells that recognize citrullinated aggrecan are present in patients 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 used to monitor patients with RA or at‐risk subjects.