Impact of immune checkpoint protein expression in tumor cells and tumor infiltrating CD8+ T cells on clinical benefit from PD-1 blockade in metastatic clear cell renal cell carcinoma (mccRCC)

477 Background: PD-1 blockade with nivolumab has demonstrated efficacy in mccRCC patients who have failed prior therapy; however, durable benefit is observed only in a subset of patients. Correlative studies have demonstrated that tumor PD-L1 expression alone fails to reliably identify patients likely to benefit. Therefore, the development of more robust predictive markers is warranted. Methods: We studied 20 mccRCC patients treated with nivolumab (n=17) or atezolizumab (n=3) with distinct clinical outcomes: 8 pts who experienced durable clinical benefit (DCB) for ≥ 12 months and 12 pts with limited clinical benefit (LCB) for ≤ 6 months from start of therapy. Expression of PD-L1 and PD-L2 on tumor cells and density of tumor infiltrating CD8+ T cells and Foxp3+ cells were evaluated by immunohistochemistry. Percentages of tumor infiltrating CD8+T cells expressing the immune-inhibitory molecules PD-1, TIM-3, or LAG-3 either alone or in combination were determined by multiplex immunofluorescence, using the Inform algorithm (Perkin Elmer). The association between clinical benefit (CB) and biomarker expression was assessed using Fisher exact test. Results: Baseline patient characteristics were similar in the DCB and LCB groups. When analyzed separately, tumor cell expression of PD-L1 or PD-L2 showed no association with CB. However, when positivity was defined as expression of either or both PD-L1(≥ 1% cutoff) and PD-L2 (≥ 5% cutoff), a significant association with CB was found (7/8 DCB vs 4/11 LCB, p = 0.04). A low percentage of tumor infiltrating CD8+PD-1+ T cells co-expressing either or both TIM-3 and LAG-3 (75thpercentile cutoff) was also significantly associated with CB (8/8 DCB vs 7/12 LCB, p = 0.05). Conclusions: Our results suggest that expression of a PD-1 ligand on tumor cells (PD-L1 and/or PD-L2), and a low percentage of severely exhausted T cells, i.e. CD8+PD-1+ T-cells co-expressing the immune-inhibitory receptors TIM-3 and/or LAG-3, might represent valuable predictive biomarkers for response to PD-1 blockade in mccRCC. Independent validation in a larger patient cohort is ongoing.