Expression of PD-L1 in keratoacanthoma and different stages of progression in cutaneous squamous cell carcinoma

Background Programmed cell death 1 (PD-1) and its ligands (PD-L1) play a major role in the immune responses of a variety of cancers. Objectives To investigate the expression of PD-L1 in different progression forms of cutaneous squamous cell carcinoma (cSCC) and keratoacanthoma (KA). Methods We performed immunohistochemical staining of 21 KA, 26 actinic keratoses (AK), 20 Bowen's diseases (BD), and 26 high-risk cSCC. The staining patterns were assessed using the tumour proportion score and staining intensity evaluation. Immunohistology scores were statistically analysed. Results PD-L1 expression of tumour cells as well as tumour-infiltrating cells (TILs) was significantly higher in KA and cSCC when compared to AK and BD (P = 0.00028 and P = 0.00033, respectively). We observed a very strong positive correlation between the PD-L1 protein expression of tumour cells of KA and the PD-L1 protein expression of TILs (r = 0.97; P < 0.0001). A similar correlation was also found for cSCC (r = 0.86; P < 0.0001). The percentage of PD-L1 + tumours was 33.3% for KA and 26.9% for cSCC. Similarly, the percentage of PD-L1 + TILs in KA and cSCC was 33.3 and 34.6%, respectively. Conclusions PD-L1 is differently expressed in cSCC and closely related non-melanoma skin cancer. cSCC exhibit PD-L1 expression in a fourth of cases, indicating that PD1/PD-L1 inhibitors might be beneficial in a proportion of patients with an inoperable or metastatic cSCC. Unlike AK and BD, TILs and tumour cells of KA and cSCC present very similar PD-L1 expression profiles indicating a common immune escape mechanism.