Genotoxic chemotherapy impedes complement dependent cytotoxicity via Chk1-mediated CD59 regulation

The DNA damage response (DDR) is a central regulator of cancer cell fate, coordinating both pro-death and pro-survival pathways in response to genotoxic stress. Here, we reveal an unexpected role for the DDR at the cell surface, in mediating immune evasion from complement-dependent cytotoxicity (CDC), an innate immune mechanism exploited by therapeutic monoclonal antibodies (mAbs). In the context of diffuse large B-cell lymphoma (DLBCL), where the anti-CD20 mAb rituximab utilizes CDC, we show that genotoxic chemotherapy induces expression of membrane-bound complement regulatory proteins (mCRPs) CD46, CD55, and CD59, thereby reducing CDC sensitivity and compromising rituximab activity. In this setting, CD59 emerged as the dominant DDR-induced inhibitor of complement-mediated killing. A high-throughput kinase inhibitor screen identified checkpoint kinase 1 (Chk1) as a critical mediator of this response. Mechanistically, DNA damage activates Chk1, enhancing CD59 transcription via an Sp1-bound promoter. Co-immunoprecipitation mass spectrometry revealed a Chk1 dependent remodelling of Sp1-associated complexes to a transcriptionally active state with recruitment of the histone acetyltransferase KAT2A. These findings expand the role of the DDR in immune resistance at the tumor cell surface, and highlight a negative interaction between chemotherapy and monoclonal antibodies that may require sequential administration or targeting of the Chk1– Sp1–CD59 axis.