To investigate the effect of filgotinib on phenotype, B cell receptor (BCR) usage and functional parameters of circulating B cells expressing ACPA in patients with ACPA-positive RA that show incomplete response to standard, medium-dose methotrexate (MTX) monotherapy.

B cells expressing anti citrullinated protein antibodies (ACPA) in patients with rheumatoid arthritis (RA) display an activated, proliferative phenotype. Experimental data indicate that ACPA and ACPA-expressing B cells are actively involved in driving the disease process in RA. The present study is based on the hypothesis that targeted intervention with filgotinib as a means to interfere with the activation of B cells in early, active, ACPA-positive RA can reverse the activated, proliferative phenotype of citrullinated antigen-specific B cells.