The Prognostic Impact of PD-L1 and CD8 Expression in Anal Cancer Patients Treated with Chemoradiotherapy

Abstract

The programmed death ligand-1 (PD-L1) and programmed death-1 (PD1) signaling axis is exploited by cancer cells as a mechanism to evade the host’s immune surveillance. The expression of PD-L1 has been shown to be prognostic in many cancer types and expression status has been used in consideration of checkpoint inhibitor immunotherapy treatment. However, there are very limited and conflicting data on the prognostic impact of PD-L1 in patients with anal cancer. The objectives of this retrospective study were to measure expression of PD-L1 and CD8 in patients with anal cancer treated chemoradiation (CRT), and to correlate tumor expression with overall survival (OS). A total of 102 patients with anal cancer treated with primary CRT at two tertiary care cancer centers between 2000 and 2013, with available pre-treatment tumor, were included. Tissue microarrays from tumor specimens were stained for PD-L1 and CD8. PD-L1 expression in tumor and in stroma was quantified using HALO image analysis software, and the results were interpreted using novel quantitative methods comparing average pixel intensity of the tissue sample normalized to reference cell lines. The density of CD8 cells within the tumor was interpreted by a specialist pathologist semi-quantitatively, using a 0-4 scoring system. Kaplan-Meier analysis with log rank was used to determine significance in association between tumor markers and OS. Cox multivariate analysis (forward stepwise Wald) was used to explore independent predictors of OS. Of the 102 patients, 65 (64%) had sufficient tumor sample available for full analysis. There were no differences in baseline characteristics between tested and not tested cases. Mean follow up was 14.1 years; the female:male ratio was 2.5:1. Most patients had T2 disease; the majority were squamous histology (88%). We analyzed commonly used PD-L1 expression levels from the literature, and determined that the 5% cut-point was most strongly prognostic for OS. Approximately half the patients had tumoral PD-L1 expression ≥5%. Patients with tumoral PD-L1 ≥5% had better OS vs those with lower expression, HR=0.29 (CI 0.10-0.78), p=0.009; 10 years OS: 84% for PD-L1 ≥5% vs. 46% for PD-L1 <5%. On univariate analysis, OS was associated with PD-L1 status, as well as T stage, N stage, ECOG status and gender. On multivariate analysis, PD-L1 ≥5% remained statistically significant for better OS, HR=0.35 (CI 0.12-0.99), p=0.047. 33 of 65 (51%) of tumors had high CD8 levels (3 or 4). There was no association between CD8 status and OS; further stratifying PD-L1 high patients by CD8 did not improve the prognostic impact vs PD-L1 status alone. This is the first study reporting significant association of PD-L1 expression with OS in patients with anal cancer treated with CRT. PD-L1 status warrants consideration in the prognostication of patients with anal cancer. Future studies are required to determine the benefit of alternative treatment strategies based on PD-L1 status.