4549 Background: The HCRN GU16-260 study showed that nivolumab (nivo) monotherapy is active in treatment-naïve patients (pts) with RCC. Although efficacy correlated with tumor cell (TC) PD-L1 expression, more clinically relevant biomarkers are needed. We previously showed that levels of CD8+ tumor-infiltrating lymphocytes (TIL) expressing PD-1 but not TIM-3 and LAG-3 (CD8+ PD1+ TIM3- LAG3-) were associated with response to nivo in previously treated clear cell RCC (ccRCC) pts (Pignon, 2019; Ficial, 2021). Here, we sought to validate these findings in the first-line setting. Methods: Primary tumor tissues from pts with ccRCC (n= 67) and non-ccRCC (n= 13) were analyzed by multiparametric immunofluorescence (IF), and the density of CD8+PD-1+TIM-3-LAG-3- TIL was quantified by image analysis. The correlation of the log-transformed biomarkers with objective response rate (ORR) and progression-free survival (PFS) was assessed in the ccRCC cohort and the combined (ccRCC + non-ccRCC) cohort (n=80) using Cox or logistic regression analysis, or Chi-square/Fisher’s exact test (as appropriate) for statistical inference – alpha level was set at 5% (1-sided). Results: In both the ccRCC and the combined cohort, density of CD8+PD-1+TIM-3-LAG-3- TIL (IF biomarker) measured as continuous variable was associated with ORR (ccRCC: OR = 1.54, p = 0.017; combined: OR = 1.35, p = 0.040) and PFS (ccRCC: HR = 0.79, p = 0.015; combined: HR = 0.85, p = 0.016). At a cutoff optimized for maximum sensitivity for ORR, ccRCC patients with a high (61/67) vs low (6/67) IF biomarker had higher ORR (44.3% vs. 0% p = 0.039) and longer median PFS (10.9 months vs. 4.2 months, p = 0.016). Of note, when combined with the IF biomarker, TC PD-L1 expression (≥1%) further separated the clinical outcomes of ccRCC pts. Conclusions: High levels of CD8+ PD1+ TIM-3-LAG-3- TILs were associated with response to first-line nivo in pts with advanced RCC and the combination with TC PD-L1 status further separated the outcomes of pts with ccRCC, validating our previous findings. Further investigations into the role of myeloid inflammation as determinant of clinical outcome to nivo therapy are ongoing. Clinical trial information: NCT03117309 . [Table: see text]