The present study found a correlation between PD-L1 expression with p53 and a-SMA expression. Furthermore, both p53-positive and a-SMA-positive rates were identified to increase with higher CPS.

Higher PD-L1 positivity of lymphocytes in oral squamous cell carcinoma has been associated with poorer prognosis [11]. While the PD-L1 positivity rate was higher in T2 cases compared to T1 cases in our study, T3 and higher cases were not examined. The checkmate-048 study also reported an improvement in 1-year survival in patients with CPS ≥ 20 compared to cetuximab therapy [8]. However, in the present study, two patients with CPS ≥ 20 revealed recurrence, and subsequent treatment at other hospitals makes it certain whether immune checkpoint inhibitors were utilized.

p53 has been shown to play roles in apoptosis, DNA repair, and cell cycle regulation in cancer cells [12]. p53 mutations are found in most cancer types, including oral squamous cell carcinoma, and have been reported to be a poor prognostic factor [13]. Tojyo et al. previously reported a correlation between PD-L1 expression and mutant p53 expression in oral squamous cell carcinoma [14]. Additionally, PD-L1 is overexpressed in patients with malignant transformation of oral leukoplakia [15], which is possibly related to p53 mutations. However, it remains unclear whether wild-type p53 regulates PD-L1 expression, and it has been reported that miR-34a suppresses PD-L1 expression in breast cancer [16]. With these, Tojyo et al. predicted that wild-type p53 suppresses PD-L1 expression; however, mutation abolishes this function and upregulates PD-L1 expression, leading to a correlation between p53 and PD-L1 expression [14].

CAFs, a main component of tumor stroma, are shown to be poor prognostic factors in oral cancer [17,18,19]. These are considered to have various subtypes; however, one of the representative markers is α-SMA, a myofibroblast marker, and α-SMA positive CAFs have been reported to have various tumor-promoting properties [20]. CAFs have also been reported to be observed in esophageal squamous epithelial neoplasia and CIS [21]. Furthermore, growth factors derived from CAFs promote proliferative potential during the carcinogenic process [22]. In this study, only one case of eleven CIS cases showed the presence of α-SMA-positive fibroblasts. CAFs Although PD-L1 expression is induced by CAFs in lung adenocarcinoma [23], no reports on PD-L1 expression in oral cancer have been conducted to the extent of our research efforts. C-X-C motif chemokine ligand (CXCL) [2, 5] has been reported to be involved in the mechanism by which CAFs regulate PD-L1 expression [23, 24]. Meanwhile, PD-L1 has been reported to transform lung fibroblasts into myofibroblasts [25]. These aforementioned findings suggest that CAFs and PD-L1 may regulate each other's expression via cell–cell interactions. The ratio of CD8-positive T cells to CAFs may be useful in predicting prognosis and immunotherapy response [26], and further exploration of the relationship between CAFs and PD-L1 expression in oral cancer is necessitated. In contrast, among α-SMA-positive CAFs, the existence of CAFs involved in immunotherapy resistance is becoming clear, but with still many unknowns [20].

As mentioned previously, a limitation of this study included not examining patients with advanced disease beyond T3, and stage III and IV patients exhibited higher PD-L1 expression than stage I and II patients [11]. Only one case of cervical lymph node metastasis out of 37 cases was considered in this study, which poses challenges in capturing the characteristics of the cases. Additionally, since no cases were identified to have actually undergone immune checkpoint therapy, reflecting the actual treatment effect was not feasible. Hence, further investigation is recommended for staining patterns in patients with recurrent disease, cervical lymph node metastases, and postoperative immunotherapy, and increase the number of variations of cases to provide more robust evidence for our findings.