Enhanced Fc and complement activity of Fc‐modified avelumab boosts anti‐tumor activity but promotes NK cell fratricide

Abstract Objectives The anti‐PD‐L1 antibody avelumab has demonstrated efficacy across multiple cancer types. Avelumab primarily blocks the PD‐1/L1 immune checkpoint, while inducing antibody‐dependent cellular cytotoxicity (ADCC) from CD16a + NK cells. However, subsets of patients possess lower‐affinity CD16a allotypes that limit ADCC capacity of monoclonal antibodies. In this study, we modified the Fc domain of avelumab with characterised mutations to enhance ADCC across all CD16a allotypes. Methods Comparisons between the wild‐type avelumab and modified ‘AveFc5M’ were carried out to assess the impacts of introduced mutations on PD‐L1 blockade, ADCC induction and complement‐dependent cytotoxicity (CDC) induction. To assess ADCC, a range of PD‐L1 + target cells were coated with mab prior to incubation with effector cells, with both high‐ and low‐affinity CD16a allotypes represented. Human serum was employed for assessing CDC. Results Both antibodies exhibited equivalent PD‐L1 blocking activity. Notably, the modified AveFc5M displayed significantly enhanced ADCC across diverse tumor cell–effector cell combinations. Fc mutations also conferred the ability to mediate complement‐dependent cytotoxicity (CDC) against a PD‐L1 + lymphoma cell line. However, short term activation of NK cells or long‐term expansion on artificial antigen presenting cells, led to upregulation of PD‐L1 on NK cells. Addition of avelumab or AveFc5M to these NK cell populations induced a marked reduction in viable NK cells via ADCC‐mediated fratricide. Conclusion These findings demonstrate that Fc engineering of avelumab can substantially augment ADCC and CDC activity against PD‐L1 + tumors. However, the enhanced effector function of AveFc5M led to NK cell fratricide, potentially limiting NK cell‐dominated anti‐cancer responses.