B cell CD19 is widely transferred between immune cells [CD11b+]

When immune cells interact, they frequently exchange membrane-bound antigens. Our evolving understanding of these processes challenges the cellular specificity of lineage markers and therapeutically applied monoclonal antibodies. We here report that CD19, an assumingly exclusive B cell marker, is transferred via trogocytosis when B cells activate T cells as antigen-presenting cells. In a B cell-driven model of experimental autoimmune encephalomyelitis, CD19+ T cells accordingly expanded and showed enhanced features of activation, differentiation, and encephalitogenicity. A similar profile of augmented pathogenic properties of CD19+ T cells was detected in patients with chronic CNS demyelination. Here, CD19+ T cells were found to be concomitantly depleted by inebilizumab, an approved anti-CD19 antibody, which may support its effectiveness. Furthermore, we report that CD19 is found on myeloid cells after phagocytosis of apoptotic B cells. These results highlight the commonness of membrane and antigen transfer between interacting cells and provide instrumental considerations for monoclonal antibody therapies.