CD27 and rheumatoid arthritis: FACS analysis of CD19+ cells subpopulations in paired PB-SF samples of RA patients shows that SF compartment is characterized by significantly lower proportion of naive B cells (IgD+CD27-) (P<0.001) and higher proportion of post-switched memory (IgD-CD27+) (P=0.002, Mann-Whitney U-test) and double negative memory B cells (IgD-CD27-) (P=0.01, Mann-Whitney U-test) compared to the PB compartment (Supplementary Fig. 2).