SPI1 and rheumatoid arthritis: The aims of our study were therefore: (i) to assess the expression of miR-155 in B cells of RA patients in multiple biological compartments (PB, SF and synovial tissue, respectively), (ii) to evaluate the possible association between miR-155 expression and B-cells activation features (defined as ACPA positivity and ectopic synovial GC frequency), (iii) to assess the relationship between miR-155 and its target PU.1 in synovial tissue and circulating B cells of RA patients and (iv) to investigate the impact of miR-155 on RA B-cell function.