511 Background: Prior studies have correlated FGFR3 mutations (mut) and increased PPARg expression with decreased T-cell infiltration raising the possibility that these might represent mediators of PD-1/PD-L1 blockade resistance in luminal UC and targets for combination strategies. The causality of these associations remains unclear and the clinical relevance in PD-1/PD-L1 blockade treated patients has been underexplored. Methods: Archival tumor specimens from patients enrolled on CheckMate 275, a phase 2 trial of nivo in patients with platinum-resistant metastatic UC, underwent whole exome sequencing (WES) and gene expression profiling (HTG EdgeSeq). CD8 expression was determined by immunohistochemistry. Results: WES and gene expression data was available for 139 and 217 patients, respectively; the baseline characteristics and outcomes of both cohorts were similar to the full CheckMate 275 cohort. FGFR3mut were detected in 15 patients (11%) and correlated with higher FGFR3 gene expression. Neither FGFR3mut nor FGFR gene expression correlated with CD8 expression (Pearson's r -0.13; p=0.16) or with response rate (RR), progression-free survival (PFS), or overall survival (OS); RR in FGFRmut vs. FGFRwt = 20% vs 20%. PPARg gene expression was inversely correlated with CD8 (Pearson’s r -0.44; P <0.001) and IFNg gene expression (Pearson’s r -0.52; P <0.001). Higher PPARg gene expression was associated with inferior outcomes (Table); RR in PPARg highest vs lowest tertile = 15% vs 24%. Conclusions: FGFR3 alterations (mut/increased expression) do not preclude response to nivo and were not associated with decreased CD8. Higher PPARg expression was associated with decreased CD8, lower RR to nivo, and significantly shorter PFS and OS. These data support exploring therapeutic modulation of PPARg or downstream mediators in an effort to overcome resistance to PD-1/PD-L1 blockade. Cox proportional hazards model. [Table: see text]