Association between PD-1 expression on tumor-infiltrating regulatory T cells and resistance to first-line nivolumab in advanced clear cell renal cell carcinoma: Insights from the HCRN GU16-260 clinical trial.

590 Background: Inhibition of PD-1 signaling in regulatory T cells (Tregs) has been shown to enhance their immunosuppressive function. We previously demonstrated that high levels of PD-1 expression on tumor-infiltrating Tregs are associated with resistance to nivolumab (nivo) monotherapy in pretreated patients with advanced clear cell renal cell carcinoma (ccRCC) from the CheckMate-025 trial (Denize et al, 2024). Here, we aim to validate these findings in the first-line setting. Methods: Primary tumor tissues from patients with ccRCC (n=70) treated with first-line nivo in the HCRN GU16-260 clinical trial were analyzed using multiparametric immunofluorescence (IF). The percentage of tumor-infiltrating PD-1+ Tregs was quantified by image analysis. Associations with objective response rate (ORR) and progression-free survival (PFS) were evaluated using logistic and Cox regression models, along with Fisher’s exact test and log-rank test for statistical inference, as appropriate. The alpha level was set at 5% (one-sided). Results: In line with our previous findings, the percentage of PD-1+ Tregs, measured as a continuous variable, did not have a statistically significant association with PFS [HR = 2.62; p = 0.169] or ORR [OR = 0.72; p = 0.42]. Using an optimized cut-off based on the minimal p-value for ORR, patients with a high percentage (≥36%) of PD-1+ Tregs (8/70) experienced shorter median PFS (3.4 vs. 10.9 months; p = 0.001) and a trend toward lower ORR (12.5% vs. 43.6%; p = 0.093) compared to those with a low percentage (62/70). Conclusions: PD-1 expression on tumor-infiltrating Tregs is associated with worse outcomes to first-line nivo therapy in patients with advanced ccRCC, confirming our previous findings. These results highlight the therapeutic potential of targeting Tregs to overcome resistance and improve the efficacy of PD-1 blockade, which is currently being evaluated in a clinical trial (HCRN GU22-587).