Rituximab-Mediated ADCC Is Augmented by Concomitant Interference with Inhibitory Self-Recognition by Human NK Cells.

The anti-CD20 monoclonal antibody, rituximab is widely used in the treatment of non-Hodgkin lymphomas. However, clinical responses to rituximab are variable. It has been demonstrated that rituximab can lead to tumor cell death by engaging the cellular immune system through antibody dependent cellular cytotoxicity (ADCC). NK cells have been shown to play a critical rule in eliminating rituximab coated B-cells, and the efficiency of killing depends on the interaction between the Fc portion of rituximab and the FcγRIII (CD16) activating receptor on NK cells. NK cell function is regulated by a complex balance of inhibitory and activating signals that enable the cells to survey their surrounding and selectively target and kill targets that do not display a “self” ligand (the “missing self hypothesis”). We hypothesized that interference with inhibitory self-recognition would augment rituximab-induced NK cell-mediated ADCC. Initial studies with the 721.221 B51 (HLA Bw4+) CD20+ cell line and NK92.26.5 cells transduced with human CD16 suggested that interference with KIR3DL1 recognition of Bw4 augmented tumor lysis in the presence of rituximab. To further test this hypothesis we employed human NK cells and autologous EBV transformed B cells from normal volunteers, and blocked the KIR3DL1 inhibitory receptor on NK cells using (Fab′)2 fragments of the DX9 antibody, in conjunction with rituximab exposure. Inhibitory blockade promoted rituximab-mediated cytotoxicity by peripheral blood mononuclear cells in three separate HLABw4+, KIR3DL1+ volunteers. These results suggest that manipulating the balance between inhibitory and activating receptors on NK cells might be applied to improve ADCC and ultimately lead to an improvement in response rates to rituximab and related lymphoma-directed antibodies that mediate ADCC. Supported by R01CA50633.