Glial fibrillary acidic protein expression in pleomorphic adenoma, chordoma, and astrocytoma. A comparison of three antibodies.

Glial fibrillary acidic protein (GFAP) is a major constituent of glial cytoplasmic intermediate filaments. Glial fibrillary acidic protein expression has been accepted as a marker of astroglial differentiation or origin. However, GFAP expression has been demonstrated in a variety of normal and neoplastic tissues outside the central nervous system, including pleomorphic adenomas, chordomas, bone, and cartilage. It has been postulated that coexpression of GFAP and vimentin in neoplastic myoepithelial cells in pleomorphic adenomas reflects early chondroid differentiation. Glial fibrillary acidic protein expression in chondromyxoid and chordoid tumors was studied in formaldehyde solution-fixed, paraffin-embedded sections of 20 pleomorphic adenomas and 10 chordomas by the immunoperoxidase method with the use of commercially available monoclonal (n = 2) and polyclonal (n = 1) antibodies. All pleomorphic adenomas and chordomas demonstrated expression of GFAP with the use of the polyclonal antibody (Biomeda Corp [Foster City, Calif]). Variable expression of GFAP was present in 90% (18/20) and 70% (14/20) of pleomorphic adenomas, and in 20% (2/10) and 0% of chordomas, with the use of the two monoclonal preparations (Dakopatts [Glostrup, Denmark] and BioGenex Laboratories [San Ramon, Calif]), respectively. Normal brain tissue and eight astrocytomas were used as "controls" to compare staining intensity and quality between the polyclonal and monoclonal anti-GFAP antibodies. Glial fibrillary acidic protein positivity with the polyclonal antibody was more intense than that with either monoclonal antibody despite similar (congruent) distributions of tumor cell types that were stained in control brain and astrocytoma tissues. The GFAP polyclonal antibody was more frequently immunoreactive than the monoclonal antibodies, particularly in cells that exhibited chondroid differentiation. These findings may have practical application in surgical pathology.