AbstractObjectiveInterleukin‐6 (IL‐6) receptor inhibition by tocilizumab was recently licensed for the treatment of rheumatoid arthritis (RA). IL‐6 induces in vitro differentiation of B cells into antibody‐forming cells; however, the in vivo effects of IL‐6 inhibition on the B cell compartment are currently not known. The purpose of this study was to examine this feature.MethodsSixteen patients with active RA were treated in an open‐label study with tocilizumab (8 mg/kg every 4 weeks). Immunophenotyping was performed at baseline, week 12, and week 24.ResultsMemory B cell subsets declined significantly during tocilizumab therapy. Preswitch memory B cells decreased from a median of 19.6% to 12.3% at week 24 and postswitch memory B cells declined from a median of 18.6% to 15.0% at week 24 (P = 0.04). In parallel, CD19+IgA+ and CD19+IgG+ B cells decreased significantly. The proportion of IgA‐expressing B cells fell from a median of 9.2% at baseline to 4.3% at week 12 and to 3.6% at week 24 (P = 0.01). IgG+ B cells declined from a median of 6.7% at baseline to 4.9% at week 12 (P = 0.007) and 2.8% at week 24 (P = 0.01). In parallel, serum levels of IgA and IgG were significantly diminished at week 24 (P < 0.05). There was a good correlation between relative and absolute numbers of IgA+ B cells with serum IgA at week 24.ConclusionTocilizumab induced a significant reduction in the frequency of peripheral preswitch and postswitch memory B cells. In addition, the number of IgG+ and IgA+ B cells declined and correlated well with reduced serum immunoglobulin levels. The data indicate that IL‐6 blockade affects the B cell hyperreactivity in RA patients.