Keratoacanthoma and squamous cell carcinoma: study of PCNA and LeY expression

To determine whether keratoacanthoma (KA) is unique or a variant of squamous cell carcinoma (SCC), the expressions of proliferating cell nuclear antigen (PCNA) and LeY in 16 KA cases (11 fully developed and 5 involutional types; two of the involutional types transformed from the fully developed types) and 11 cases of well‐differentiated SCC were investigated by immunohistochemistry. The monoclonal antibodies used were PC‐10 for PCNA and BM‐1/JIMRO for LeY. LeY is one of the Lewis‐type antigens, and is thought to be related to apoptosis. In most of the cases of fully developed KA, the PCNA expression was linear or band‐like and was limited to the basal and suprabasal layers, while the LeY expression was seen in a diffuse pattern throughout the epidermis, except for the basal layer. However, in 5 of these cases, there were some areas in the basal and suprabasal layer where PCNA‐positive cells were few, while LeY expression was very strong. In the involutional Ka cases, the PCNA expression was very weak in the basal and suprabasal layers, and the LeY expression was strong throughout the epidermis. In the SCC cases, the PCNA and LeY expressions were very strong and diffuse throughout the tumor masses except for the lack of PCNA in the horny layer and the lack of LeY in the basal layer. These findings indicate that there are some differences between KA and SCC in the expression pattern of PCNA and LeY, and that apoptosis may occur in the LeY‐positive areas in the basal and suprabasal layers and thus cause the involution of KA.