Lung cancer is diagnosed late, has a 5 year survival of only 15%, and kills more people than colorectal, breast and prostate cancer combined. Less than 30% of lung cancers are resected and the majority of patients are treated with cisplatin and ionizing radiation (IR). We show that inhibition of DNA damage signaling by ATR kinase during treatment with cisplatin and IR is well-tolerated and leads to durable responses in mouse xenograft and genetic models of lung cancer. Quite unexpectedly, we show that in addition to potentiating DNA damage, two clinical ATR kinase inhibitors (ATRi’s), with unrelated structures, block expression of the immune checkpoint protein PD-L1 and increase presentation of MHC class I antigens in lung cancer cells after IR. Our finding that crosstalk exists between DNA damage signaling and immune checkpoints has not been described previously and is the focus of this proposal. Immune-inhibitory pathways, termed immune checkpoints, are coopted by tumor cells to evade cytotoxic immune cells. PD-1 is expressed on cytotoxic T cells and its ligand PD-L1 is upregulated in lung cancers. PD-L1 binding by PD-1 prevents the activation of cytotoxic T cells. Immune checkpoint blockade using anti-PD-L1 and anti-PD-1 antibodies restores anti-tumor immune responses and is emerging as an exciting lung cancer therapy. We propose that ATRi’s inhibit DNA repair and cell cycle checkpoints potentiating the DNA damage induced by cisplatin and IR while concurrently inhibiting PD-L1 expression and restoring anti-tumor immune responses. Our objective in this proposal is to define the mechanisms that connect DNA damage signaling and immune checkpoints. This objective will be accomplished by the following Specific Aims. Aim 1: To determine how ATRi’s inhibit PD-L1 expression in lung cancer cells after IR. This aim will define the contribution of ATR, ATM, IRF-1, NF-κB and p53 to PD-L1 expression after IR. Aim 2: To determine how ATRi’s increase MHC class I expression on lung cancer cells after IR. This aim will identify ATR, ATM, and p53 signaling that inhibits protein synthesis and MHC class I presentation after IR. Aim 3: To identify ATRi-induced PDL-1/PD-1 immune checkpoint blockade in lung cancer after IR. This aim will identify ATRi-induced immune checkpoint blockade in lung cancer after IR. The outcomes of these Aims will identify mechanisms that connect DNA damage signaling to immune self- tolerance. This will define a novel therapeutic opportunity to use ATRi’s to potentiate the DNA damage induced by cisplatin and IR while concurrently inducing immune checkpoint blockade.