Natalizumab differentially affects plasmablasts and B cells in multiple sclerosis

•Natalizumab decreases the frequency of plasmablasts among B cells in blood.•Natalizumab increases the absolute counts in blood of B cell subset except for plasmablasts.•Memory B cell absolute count in blood is lower in unstable natalizumab-treated patients.•CD49d expression is markedly higher on plasmablasts compared to other B cell subsets.•Cerebrospinal fluid plasmablasts frequency is reduced in the in natalizumab-treated patients.