Altered expression of beta-2-microglobulin in basaloid proliferations overlying dermatofibromas

The epidermis overlying 2-8% of dermatofibromas shows basaloid proliferations that are indistinguishable from superficial basal cell carcinoma. It is well established that basal cell carcinomas uniformly exhibit a strongly reduced expression of HLA class I molecules. Nineteen dermatofibromas with overlying basaloid proliferations were studied by immunohistochemistry using a monoclonal antibody against beta-2-microglobulin, the invariant chain of the HLA class I molecule. The basaloid proliferations exhibited the same strong reduction in expression of beta-2-microglobulin as demonstrated in basal cell carcinomas. We suggest that this phenomenon may represent a proliferative change induced by the mesenchymal cells of the underlying dermatofibroma.