Filaggrin distribution in keratoacanthomas and squamous cell carcinoma.

The cellular distribution of filaggrin, a histidine-rich protein present in the granular and horny layers of normal epidermis, was investigated in 27 human keratoacanthomas and squamous cell carcinomas of the skin using an indirect immunohistochemical procedure with rabbit antimouse filaggrin immunoglobulin. Using the same working dilution of the primary antibody, normal epidermis, as well as all keratoacanthomas, exhibited a positive immunoperoxidase or immunofluorescence stain in the granular and horny layers; whereas, all squamous cell carcinomas, except one, were negative. With higher concentrations of primary antibody, two additional carcinomas showed a weak, positive staining. We suggest that the absence of filaggrin in squamous carcinomas could be of use in the differential diagnosis of cutaneous tumors.