CD4 and myeloid sarcoma: In summary, our phenotypic and functional characterization of circulating immune-cell subsets in independent discovery and validation cohorts in the context of aCD20 treatment initiation in MS patients and our examination of the association between immune-cell subsets and measures of disease activity observed both prior to and early following aCD20 treatment initiation provides insights into distinct in vivo interactions between B cells and CD4+ versus CD8+ T cell subsets, and implicates CD20-expressing T cells (and particularly CD8+ T cells) in developing MS disease activity.