PRTN3 and rheumatoid arthritis: These data highlight that ACPA+ RA patients possess expanded clonal lineages of CD8+ T cells that can be activated by citrullinated antigens presented by HLA class I. Given that cit-fibrinogen and certain other citrullinated antigens are extracellular, our findings suggest that citrullinated antigens may be cross-presented in HLA class I to activate cytotoxic CD8+ T cells in RA.