Effect of rituximab on B cells in rheumatoid arthritis peripheral blood and bone marrow. Rituximab preferentially depletes B cells in peripheral blood (PB) and reduces activated B cells in the PB and the bone marrow (BM) of rheumatoid arthritis (RA) patients. (a) The percentage of PB (n = 11) CD19+ B cells was significantly reduced following 3 months of treatment with rituximab. A nonsignificant reduction in B cells was also observed in the BM (n = 8) of RA patients. (b) Representative flow cytometry analysis of PB CD19+ B cells in a RA patient at baseline (left) and after 12 weeks of rituximab treatment (right). (c) Rituximab depletes activated CD19+HLA-DR+ B cells both in the PB and in the BM of RA patients. (d) Representative flow cytometry histograms of HLA-DR expression in PB and BM CD19+-gated cells of a RA patient at baseline and after treatment with rituximab. *P < 0.05 for paired analysis, **P < 0.05.