Induction of Interleukin‐6 Production by Rituximab in Human B Cells

Rituximab (RTX) is a chimeric monoclonal antibody specific for human CD20. The utility of CD20 targeting by RTX has revolutionized the treatment of B cell malignancies (1-3). In addition, RTX has found wide application in many autoimmune disorders, such as anti-neutrophil cytoplasmic antibody (ANCA) associated vasculitis, autoimmune bullous diseases, and rheumatoid arthritis (RA) (4). RTX results in B cell depletion through several effector mechanisms such as antibody-dependent cellular cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), and induction of apoptosis (5). Interestingly, none of these mechanisms has been able to account for the incomplete relationship between B cell depletion and clinical response in RA (6-8). Thus, it seems reasonable to consider alternative effector functions of RTX that are not associated with B cell depletion (9-12). In this regard, there is increasing appreciation of an alternative effector function of RTX, that of trogocytosis (13, 14). Trogocytosis refers to the process when one cell ‘gnaws’ or ‘shaves’ a portion of another cell's membrane following the creation of an immunologic synapse. This phenomenon has been described to occur in many cellular interactions, including antibody/FcγR engagement, T cell receptor/antigen presenting cell engagement, and B cell receptor/cell surface peptide engagement (13, 15-17). RTX mediated trogocytosis occurs when an FcγR bearing cell such as a monocyte engages the Fc portion of B cell-bound RTX and removes the RTX-CD20 complex, as well as a portion of the B cell membrane, without altering B cell viability (13, 14, 18, 19). The clinical significance of RTX mediated trogocytosis still is not fully understood, but holds considerable relevance as there is increased recognition of trogocytosis by several other antibody-based therapeutics, such as epratuzumab, daclizumab, trastuzumab, and cetuximab (20-22). We previously identified that RTX exhibits little or no complement dependent cytotoxicity on normal human B cells (14). Instead, in the presence of RTX, phagocytes mediate rapid trogocytosis of CD19 and CD20 in the absence of cell death. Since the absence of CD19 and CD20 from B cells is associated with immunodeficiency (23, 24), this raised the possibility that RTX-dependent trogocytosis might alter the phenotype and function of B cells. In this model, following trogocytosis, functionally impaired B cells might result from their CD19- or CD20-deficiency states. We tested this hypothesis by characterizing phenotypic and functional responses of B cells following RTX-dependent trogocytosis. Instead of seeing functional impairment of B cells, RTX mediated trogocytosis of human B cells results in a selective and marked increase in production and release of IL-6. As we describe, the effect of RTX on IL-6 production was surprisingly specific both in terms of the cytokines being affected but also in its requirement for trogocytosis. In addition to identifying a novel pathway that triggers IL-6 production by B cells, this observation may be relevant to acute and chronic effects of RTX in shaping the immune response including infusion reactions (6, 7).