An immunohistochemical study of pleomorphic adenomas of the salivary gland: glial fibrillary acidic protein-like immunoreactivity identifies a major myoepithelial component.

An immunohistochemical study of 34 pleomorphic adenomas of the major salivary glands demonstrated phenotypic differences among the various morphologic regions in these tumors. The phenotypes expressed were comparable to those of normal salivary gland cells. In the normal glands, myoepithelial cells were immunoreactive for glial fibrillary acidic protein (GFAP), S-100 protein, and keratin; acinic cells exhibited strong, predominantly nuclear S-100 staining and weaker keratin staining; intercalated ducts had both cytoplasmic and nuclear S-100 positivity; and several epithelial antigens were observed throughout the ductal system. In the tumors, the presence of classic epithelial markers (including carcinoembryonic antigen, epithelial membrane antigen, secretory component, and keratin) in the luminal cells of ducts and the intense immunoreactivity with GFAP (with weaker keratin and S-100 staining) in periductal and stromal cells indicated distinct epithelial and myoepithelial differentiation. Solid epithelioid areas consisted phenotypically of intercalated duct/acinic cells and/or myoepithelial cells, the former exhibiting predominant nuclear S-100 positivity. The presence of GFAP-like immunoreactivity in normal myoepithelial cells strongly supports the extensive involvement of this cell in pleomorphic adenomas. The spectrum of phenotypes expressed adds weight to existing evidence for pleomorphism rather than a mixed origin of this tumor. The combination of keratin, S-100, and GFAP immunostaining is particularly useful in identifying the component cells in pleomorphic adenomas of the salivary glands.