ABSTRACT An estimated 44% of patients with cancer in the United States are eligible to receive immune checkpoint blockade (ICB). FDA-approved ICB agents include α-PD-1 and α-CTLA-4 antibodies, but the majority of patients do not benefit because their tumors are resistant to these agents. ICB treatment is expensive and may lead to serious toxicity. Prospective identification of patients with ICB-resistant cancers would reduce unnecessary risk and cost and give patients opportunities to seek more appropriate treatment options. To address the unmet need for a peripheral blood biomarker for ICB effectiveness, we performed immune profiling of ICB-treated patients with melanoma, using multiparametric flow cytometry to characterize immune cells in pretreatment peripheral blood. Our analyses revealed a new peripheral blood immune profile which we called Immunotype-1 (IT-1), defined in part by the presence LAG-3+CD8+ T cells as a promising biomarker of ICB resistance. This finding was validated in an independent dataset of metastatic urothelial cancer. Patients with IT-1 have inferior overall survival, progression free survival, and response rates to α-PD-1 blockade. Leveraging our leadership in the clinical development of ICBs, we have assembled one of the largest biobanks of peripheral blood samples from >600 ICB-treated patients across cancer types. In this proposal, we aim to test the hypothesis that IT-1 is a pan cancer biomarker for ICB, reflecting an exhausted, tumor-specific LAG-3+CD8+ T cell population whose function can be recovered for therapeutic benefit using α-LAG-3 blockade. The Specific Aims are to: 1) Phenotypically and functionally characterize the peripheral blood LAG-3+CD8+ T cell population and determine if this population is represented in the tumor microenvironment in patients with the IT-1 phenotype; 2) Determine the association between IT-1 and clinical outcome in ICB-treated patients across cancer types; and 3) Assess whether IT-1 identifies patients who will respond to relatlimab (α-LAG-3) + nivolumab (α-PD-1). Our project is rooted in strong clinical data and thus likely to identify a biomarker that is mechanism-based, clinically implementable, and most importantly, therapeutically actionable.