Abstract 148: Release of the TNF-family member BAFF by NK cells contributes to the resistance of chronic lymphoid leukemia cells to direct and Rituximab-induced NK reactivity

Proceedings: AACR Annual Meeting 2014; April 5-9, 2014; San Diego, CA NK cells are cytotoxic lymphocytes that are particularly important for the immunosurveillance of leukemia. Due to their ability to mediate antibody-dependent cellular cytotoxicity (ADCC), NK cells substantially contribute to the therapeutic benefit of antitumor antibodies like Rituximab, which is a crucial component in the treatment of B cell malignancies. Available data indicate that the ability of NK cells to target malignant cells and to mediate ADCC is compromised in Chronic Lymphoid Leukemia (CLL), but the underlying mechanisms are still unclear (Jaglowski et al., Blood 2010). With the TNF family member B cell activating factor (BAFF) that regulates, among others, the proliferation and survival of B cells, we believe to have identified a factor which contributes to these clinical observations. We report that NK cells express BAFF on the mRNA level and release the protein in a soluble form with the levels depending on activation state. Notably, relevant BAFF surface expression was neither detected on resting nor activated NK cells. Using a tetrazolium salt based colorimetric assay we found that NK cell-derived BAFF enhances the metabolic activity of primary CLL cells. In addition, BAFF protects CLL cells from chemotherapy-induced cell death e.g. upon treatment with Fludarabine and Cyclophosphamide. Notably, the BAFF-specific antibody Belimumab, which is approved for the treatment of systemic lupus erythematosus, was found to block the leukemia cell-protective effects of BAFF and to restore the sensitivity of the leukemia cells to chemotherapy. Furthermore, BAFF was found to substantially reduce direct NK cell lysis and also Rituximab-mediated ADCC in cultures with primary CLL cells as measured by Europium cytotoxicity assays. Our data on the capacity of BAFF to enhance the metabolic activity of B-CLL cells and their resistance to NK cell cytotoxicity suggest its involvement in the compromised ability of NK cells to combat lymphoid as compared to myeloid leukemias (Pende et al., Blood 2005) and the reportedly impaired ability of NK cells to mediate ADCC upon Rituximab treatment in CLL. Furthermore, our findings point to a possible benefit of combinatory approaches employing Rituximab and Belimumab for immunotherapy of B cell malignancies. Citation Format: Julia Wild, Benjamin J. Schmiedel, Andreas Maurer, Stefanie Raab, Pascal Schneider, Helmut R. Salih. Release of the TNF-family member BAFF by NK cells contributes to the resistance of chronic lymphoid leukemia cells to direct and Rituximab-induced NK reactivity. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 148. doi:10.1158/1538-7445.AM2014-148