PD-L1 expression in oral premalignant and malignant lesions and its association with p53 expression

Introduction: Programmed death-ligand 1 (PD-L1) is a key immune checkpoint molecule that allows tumor cells to evade immune surveillance by promoting T-cell exhaustion within the tumor microenvironment (TME). Oral malignant lesions, of which 90% are oral squamous cell carcinoma (OSCC), are known to express abnormally high levels of PD-L1. Extensive research has explored PD-L1 in carcinogenesis and immunotherapy. The tumor suppressor protein p53 is often mutated in cancer, and its immunohistochemistry (IHC) assessment has been found to help predict malignant potential in oral premalignant lesions. Emerging evidence suggests p53 may influence immune activity in the TME by altering PD-L1 levels. However, how developing tumors modify the TME during the premalignant stage, and the role of p53 in this process, remain unclear. This study aims to evaluate the relationship between PD-L1 and p53 in oral premalignant and malignant lesions. Methods: A comparative cross-sectional study was conducted using patient data and oral tissue samples from the Oral Cancer Prediction Longitudinal Study and the BC Oral Biopsy Service. Inclusion criteria were biopsy confirmed oral epithelial dysplasia (OED) or OSCC. Exclusion criteria included HPV or EBV associated OED or OSCC, actinic cheilitis, and verrucous hyperplasia. IHC was performed on formalin-fixed, paraffin-embedded tissue to assess PD-L1 and p53 expression. We hypothesized that the malignant group (MA), defined as severe OED, carcinoma in situ (CIS), or OSCC, would show increased PD-L1 and mutant p53 expression compared with the premalignant group (PMA), which consists of mild or moderate OED. Samples were considered PD-L1 positive if ≥5% of dysplastic or tumor cells showed moderate to strong membranous staining. p53 was classified as mutant when showing either overexpression or complete loss. Fisher's exact test assessed group differences in categorical variables. Results: To date, 38 of 138 samples have undergone IHC analysis. PD-L1 positivity was higher in MA than PMA (40% and 17.4%), though not statistically significant with the current sample size. A significantly higher proportion of mutant p53 was observed in MA compared with PMA (86.7% and 34.8%, p=0.002). Thus far, no significant association was observed between PD-L1 and p53 expression, although an inverse trend in expression was noted. Additional samples are being processed to further evaluate expression patterns. Conclusion: Significant differences in p53 expression were observed between MA and PMA, while PD-L1 expression did not differ significantly between groups. Nonetheless, higher PD-L1 positivity in malignant lesions suggests its potential relevance and warrants further investigation. These findings contribute to a better understanding of immune checkpoint regulation and tumor suppressor dysfunction in the progression of oral premalignant lesions to malignancy.