Programmed death-ligand 1 expression correlates with diminished CD8+ T cell infiltration and predicts poor prognosis in anal squamous cell carcinoma patients

OBJECTIVE: Increased expression of programmed death-ligand 1 (PD-L1) on tumor cells can be found in various malignancies; however, very limited information is known about its role in anal squamous cell carcinoma (ASCC). This study explored PD-L1 expression in ASCC patients and its association with patients’ clinicopathological features, CD8+ T cell infiltration, and prognosis. METHODS: Formalin-fixed paraffin-embedded tumor samples from 26 patients with ASCC were retrieved. The levels of PD-L1 expression on the membrane of both tumor cells and tumor-infiltrating mononuclear cells (TIMCs) were evaluated by immunohistochemistry. CD8+ T cell densities, both within tumors and at the tumor–stromal interface, were also analyzed. Baseline clinicopathological characteristics, human papilloma virus (HPV) status, and outcome data correlated with PD-L1-positive staining. RESULTS: PD-L1 expression on tumor cells and TIMCs was observed in 46% and 50% of patients, respectively. Nineteen patients (73%) were HPV positive, with 7 showing PD-L1-positive staining on tumor cells and 9 showing PD-L1-positive staining on TIMCs. Increasing CD8+ density within tumors, but not immune stroma, was significantly associated with decreased PD-L1 expression by both tumor cells and TIMCs (P=0.0043 and P=0.0007). Patients with negative PD-L1 expression had significantly better progression-free survival (P=0.038 and P=0.0443) and a non-statistically significant trend toward longer overall survival (P=0.0882 and P=0.1222) compared with patients with positive PD-L1 expression. CONCLUSION: PD-L1 is widely expressed on the membrane of tumor cells and TIMCs in ASCCs. Its negative impact on prognosis may be due to the diminished CD8+ T cell infiltration within tumors.