CD19 and rheumatoid arthritis: Analysis of PU.1 expression in paired PB and SF samples of RA patients revealed that SF-derived CD19+ cells have lower PU.1 levels than PB-derived CD19+ cells (P=0.002, Wilcoxon test), consistent with a higher expression of its negative regulator miR-155 in the synovial compartment (Fig. 5b).