pecific Aims This project examines the effects of immune checkpoint therapy (ICT) for the treatment of KS. The study includes samples from a clinical trial and a novel correlative assay using scRNA-seq to predict and evaluate responses. We will examine the response to nivolumab in a phase 2 expansion arm of 15 KS patients of our ongoing multicenter AIDS Malignancy Consortium (AMC) study of HIV-1 infected subjects (NCT02408861). The study includes individuals with KS refractory to systemic therapy, who will receive 480 ug of nivolumab every 28 days. Clinical study costs and other correlative studies, e.g. KSHV virus levels in plasma and blood mononuclear cells are supported by the AMC. For correlative studies, we utilize unbiased scRNA-seq with baseline and post2nd cycle biopsies from 5 individuals with greatest response, and 5 non-responders.