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

There is an urgent unmet need to develop novel therapeutic strategies for tumors that do not respond to immune checkpoint inhibition (ICI) via PD-1 pathway blockade. The ATR-mediated DNA replication checkpoint has been reported to have immune-augmenting properties; however, the mechanisms underlying these properties are not well characterized. Here we explore the potential immunogenic effects of ATR inhibition in Merkel cell carcinoma (MCC), a cancer that is particularly relevant due to its high proliferative index and frequent response to anti-PD-(L)1 therapy. ATR inhibition induced tumor cell cytotoxicity in both Merkel cell polyomavirus-positive and UV-induced MCC cell lines in the absence of exogenous DNA damage. ATR inhibition alone or in combination with low-dose radiation induced numerous proinflammatory TNF-NF-κB signals as assessed via bulk transcriptomic profiling. These included increased expression of MHC class-I alleles, antigen processing machinery, interleukins, chemokines and interferon genes associated with anti-tumor immune responses in diverse tumor types. In parallel, we observed enhanced surface exposure of the “eat-me” signal calreticulin on MCC cells and subsequent phagocytosis by human monocyte-derived macrophages. Given that MCC tumors are often cGAS-STING-deficient (including two cell lines examined here), these ATRi-induced mechanisms are significant as they were activated regardless of cGAS-STING functional status. These data provide a mechanistic basis for the clinical evaluation of ATRi in advanced ICI-refractory MCC (NCT05947500), and suggest biomarkers that may be associated with response in human MCC tumors treated with ATRi.