SUMMARY/ABSTRACT Kaposi sarcoma (KS) is a virally-driven malignancy caused by Kaposi sarcoma-associated herpesvirus (KSHV) that remains a leading cause of global cancer-related morbidity and mortality among people living with HIV (PLWH). In our ongoing prospective KS cohort in Uganda (the “HIPPOS” study), treatment outcomes remain poor despite optimized treatment with antiretroviral therapy and chemotherapy, with an overall 1-year survival of only 64% among the first 200 study participants. Importantly, however, we observed differential outcomes among participants, with ~10% achieving a complete remission (CR), ~48% a partial response (PR), and ~25% progressive disease (PD). Understanding the biologic mechanisms underlying these response differences could inform the design of more effective therapies for KS. Findings from several of our studies based on the HIPPOS KS cohort suggest that ongoing KSHV replication is an important driver of KS tumorigenesis, and that KSHV expression profiles in KS tumors are likely modified by HIV co-infection and the host immune infiltrate. We hypothesize that control of KSHV replication and suppression of viral oncogenic gene products in KS tumors is necessary for KS tumor regression. We now propose to build on these observations by evaluating how signatures of KSHV, HIV, and host immune cell composition in KS tumors over time are associated with tumor response among participants in the HIPPOS cohort. To date, the HIPPOS cohort has enrolled over 280 participants (including both HIV-associated and HIV-negative KS), and we have established an extensive biorepository of clinically annotated specimens, including more than 1,500 plasma samples and 1,350 KS tumor biopsies. Importantly, these samples are collected serially before, during, and following treatment and linked to participant response assessments. We now propose to leverage this unique cohort to assess changes that occur in KSHV, HIV, and host immune signatures over the course of KS treatment and to determine how these signatures are related to KS tumor regression or progression. We will use existing data from the HIPPOS cohort as well as perform RNA- sequencing and multispectral immunohistochemistry on a subset of KS samples to characterize viral gene expression and host immune responses in the KS tumor microenvironment (TME). Our specific aims are: Aim 1. To determine if suppression of plasma KSHV replication over the course of treatment is associated with improved KS response or survival. Aim 2. To identify signatures of KSHV and HIV gene expression and host immune cell infiltration in KS tumors that are associated with response to treatment. Aim 3. To determine if the number and/or localization of tumor infiltrating lymphocytes (TIL) in the KS TME associated with KS treatment response.