PROJECT SUMMARYOral squamous cell carcinoma (OSCC) is the sixth most common human cancer worldwide. Approximately 30%of the oral premalignant lesions (OPLs) progress to OSCC a process that may have a multifocal origin and canbe promoted by carcinogens such as those found in tobacco. Our long-term goal is to identify the geneticalterations that promote high risk of progression to OPLs and to determine how those alterations modulate theresponse of OPLs to preventive strategies. The TP53 gene (also known as p53) and CDKN2A are the mostfrequently mutated genes in oral cancer also found altered in OPLs. p53 GOF mutations and genomic alterationsthat result in loss of the CDKN2A gene associate with cold immune microenvironments in OPLs and OSCCswith high risk of progression to carcinoma and with extremely poor outcomes in OSCC patients. We hypothesizethat the early appearance of mutations in p53 and CDKN2A inactivation modulate the oral tissuemicroenvironment and predispose OPLs to progress to OSCC. To test this hypothesis we will study mouse modelsthat develop OPLs upon exposure to the tobacco-surrogate 4NQO in the presence of p53 and/or CDKN2Amutations. Patients with high-risk OPLs could benefit from preventive strategies designed to block the malignantprogression of OPLs. However previous attempts with different chemopreventive agents have not beensuccessful. Recently immune checkpoint blockade with antibodies directed at programmed cell death protein 1(PD-1) has been shown to improve the survival of patients with advanced OSCC in clinical trials confirming theimportance of the immune system in containing progression of invasive tumors. Moreover our previous studiesconfirmed by multiple independent groups demonstrated that anti-PD-1 antibodies can also prevent theprogression of OPLs to OSCC in a 4NQO mouse model for oral carcinogenesis. Our preliminary studies indicatethat the p53 and CDKN2A status of the OPLs may determine the response to anti-PD-1-mediatedimmunoprevention. In this proposal we will assess the long-term benefits of anti-PD-1-mediated oral cancerprevention to determine whether PD-1 blockade administered in a preventive setting can confer survivalbenefits and to assess how p53 and CDKN2A mutations affect the sustained response to PD-1 blockade. Toovercome resistance to anti-PD-1 we hypothesize that reactivation of p53 in OPLs carrying p53 mutationssensitizes the oral lesions to anti-PD-1. Our mouse models will allow us to test this hypothesis in vivo.