ATR inhibition induces NF-κB-dependent immunogenicity in Merkel cell carcinoma independent of cGAS-STING

There is a major unmet need to develop therapies for patients whose tumors do not respond to PD-1 blockade. Inhibition of the ATR-mediated DNA replication checkpoint has been reported to augment antitumor immunity, although the underlying mechanisms remain poorly understood, particularly in cGAS-STING-deficient tumors. Here, we investigated the effects of ATR inhibition (ATRi) on tumor immunogenicity in Merkel cell carcinoma (MCC), an aggressive skin cancer commonly driven by Merkel cell polyomavirus (MCPyV). Among 92 primary MCC tumors, Ki-67 positivity was observed in 50% of MCPyV-positive and 83% of MCPyV-negative tumor cells, suggesting reliance on ATR-mediated replication stress responses. Notably, MCC frequently exhibits cGAS-STING deficiency, especially in MCPyV-positive tumors. In representative MCPyV-positive and -negative MCC cell lines, ATRi enhanced cytotoxicity and, alone or combined with low-dose radiation, induced proinflammatory NF-κB transcriptional programs. These included increased expression of MHC class I, antigen-processing machinery, interleukins, chemokines, and interferon-related genes. ATRi also increased surface calreticulin exposure and subsequent phagocytosis by human monocyte-derived macrophages. These findings support ATRi-mediated immune reprogramming in MCC cells through an NF-κB signaling cascade, regardless of cGAS–STING functional status. These observations support clinical evaluation of ATR inhibitors in immunotherapy-refractory disease (NCT05947500) and suggest candidate biomarkers of therapeutic response.