SUMMARY ATR kinase is a central regulator of the DNA damage response (DDR) and cell cycle checkpoints. ATR kinase inhibitors (ATRi’s) combine with radiation to generate CD8 + T cell-dependent responses in mouse models of cancer. We show that ATRi’s induce CDK1-dependent origin firing across active replicons in CD8 + T cells activated ex vivo while simultaneously decreasing the activity of rate-limiting enzymes for nucleotide biosynthesis. These pleiotropic effects of ATRi induce deoxyuridine (dU) contamination in genomic DNA, R loops, RNA-DNA polymerase collisions, and type-1 interferons (IFN-1). Remarkably, thymidine rescues ATRi-induced dU contamination, cell death, and IFN-1 expression in proliferating CD8 + T cells. Thymidine also rescues ATRi-induced cancer cell death. We propose that ATRi-induced dU contamination contributes to dose-limiting leukocytopenia and inflammation in the clinic and CD8 + T cell dependent anti-tumor responses in mouse models. We conclude that ATR is essential to limit dU contamination in genomic DNA and IFN-1 expression.

Thymidine rescues ATR kinase inhibition induced deoxyuridine contamination in genomic DNA, cell death, and Type 1 interferon expression