Fc dependent mechanisms are necessary for ADCC and effective depletion of murine B cells by humanized anti-CD19 mAb. (131.27)

B cell depletion is an effective treatment for autoimmune diseases and B cell malignancies. Successful B cell depletion may be limited by the surface expression of the target antigen. CD19 represents an attractive molecular target that is expressed on the surface of early B cell progenitors through the latter stages of B cell differentiation. In this study we developed a fully humanized CD19 monoclonal antibody mAb 19-1 and two variants with either reduced (19-2) or enhanced (19-3) Fc-FcR binding and evaluated their potential to deplete B cells. In vitro antibody-dependent cellular-mediated cytotoxicity (ADCC) activity of 19-3>19-1>19-2 on human peripheral blood B cells as well as B cell lines (Raji, Daudi, and Ramos) by LDH release or Granzyme B assays. We evaluated in vivo depletion in human CD19 transgenic mice using 19-1, 19-2, and 19-3. 19-1 at 50 ug/mouse i.v. depleted most B cells in blood, spleen, and bone marrow after 7 days following treatment. 19-2 showed little depletion of B cells at a high dose, whereas 19-3 efficiently eliminated B cells in the spleen and blood at low dose. These data demonstrate that transgenic mice expressing human antigens provide an excellent model for testing humanized antibodies in depletion studies, and that Fc-FcR interactions represent the most important mechanism. Targeting B cells with anti-CD19 mAb may be a more effective strategy than existing B cell depletion protocols.

B-cell antigen receptor activates transcription factors NFAT (nuclear factor of activated T-cells) and NF-κB (nuclear factor κB) via a mechanism that involves diacylglycerol

Engagement of the B-cell antigen receptor (BCR) induces the activation of various transcription factors, including NFAT (nuclear factor of activated T-cells) and NF-κB (nuclear factor κB), which participate in long-term biological responses such as proliferation, survival and differentiation of B-lymphocytes. We addressed the biochemical basis of this process using the DT40 chicken B-cell lymphoma. We discovered that Bruton's tyrosine kinase (BTK) and phospholipase C-γ2 (PLC-γ2) are required to activate NFAT and NF-κB, and to produce the lipid second messenger diacylglycerol in response to BCR cross-linking. Therefore the functional integrity of the BTK/PLC-γ2/diacylglycerol signalling axis is crucial for BCR-directed activation of both transcription factors NFAT and NF-κB.