4518Background: Mutation load and neoantigen burden predict benefit from anti-PD1 therapy across several cancers but are generally low in RCC. Pan-cancer comparisons highlight RCC for its enriched tumor immune infiltrate and a high proportion of indels, which may be immunogenic. We investigated these phenomena in anti-PD1 treated patients (pts) with comparison to pts receiving tyrosine kinase inhibitors (TKI). Methods: Whole exome sequencing (WES) was done for 77 tumors from 2 anti-PD1 treated pt cohorts; 25 pts provided RNAseq data. Overall mutation burden, indels, and predicted neoantigens were quantified. ESTIMATE and CIBERSORT algorithms were used to infer abundances of immune infiltrate and detailed immunophenotypes from RNAseq. We investigated correlation of molecular features with overall survival (OS) and durable clinical benefit (DCB = progression free survival textgreater 6mo). A cohort of TKI-treated pts with no prior anti-PD1, previously analyzed as part of the TCGA, was used for comparison (35 with WES…