Supporting the hypothesis that natalizumab-mediated VLA-4 blockade impairs CNS immune surveillance is an early study showing that a cohort of 23 MS patients receiving natalizumab had decreased counts of CD4+ and CD8+ T cells, CD19+ B cells, and CD138+ plasma cells in the CSF compared to 35 untreated MS patients and 16 patients with other neurological diseases. Here, CD19 is linked to myeloid sarcoma.