Frequency of B cells and B cell subgroups before and after natalizumab treatment. (A) Schematic overview of the clinical course of MS and PML in our splenectomized patients. PE, plasma exchange. (B) MRI over the course of disease. On admission, an axial flair-attenuated inversion recovery (FLAIR)-weighted scan of the brain displayed signal hyperintense lesion in the frontal- parietal lobe (arrow), 3 weeks after diagnosis lesion volume increased in the course of the IRIS, 6 weeks after diagnosis nearly the whole frontal- parietal lobe is affected (arrow). T1-weighted images with Gd displayed a very faint Gd enhancement (arrow), 3 weeks after diagnosis and 6 weeks after diagnosis lesion volume with Gd enhancement increased (arrow). (C) The percentage of CD19+ cells within the lymphocyte gate was assessed by flow cytometry in relapsing-remitting MS (RRMS) patients with active disease before and 3 to 6 months after initiation of Natalizumab therapy (n = 14) and in the splenectomized patient. Different time points marked by a cross; T0, before natalizumab therapy; T1, at the time of PML; T2, 3 weeks after PE. (D) The percentages of specialized B cell subpopulations (CD19 + CD27+ memory cells, IgM + memory cells, and IgM + IgD + marginal cell-like B cells) were assessed by flow cytometry in natalizumab-treated RRMS control patients (n = 5; mean + 95% CI) and the splenectomized PML patient at different time points marked by a cross. T0, before natalizumab therapy; T1, at the time of PML; T2, 3 weeks after PE. (E) Dot blot of CD19 + CD27+ memory B cells within the lymphocyte gate in the splenectomized patient and a representative, natalizumab-treated RRMS control.