CD27 and rheumatoid arthritis: The most prominent differentially expressed marker was HLA-DR, which was increased in ACPA+ RA compared to controls across B-cell clusters, including clusters that represent both naïve (clusters a, c, d: IgD+CD27−) and memory (clusters f & g: IgD−CD27+) B-cell populations, irrespectively of the Ig isotype (Fig. 1B, fold change = 1.6–2.1).