Non-glial specificities of immunocytochemistry for the glial fibrillary acidic protein (GFAP)

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Abstract
In an extensive immunocytochemistry study for glial fibrillary acidic protein (GFAP) of human neuropathological biopsy or autopsy tissue specimens examined for diagnostic or research purposes, rare non-glial specificities of the GFAP immunostain were observed: Schwann cells of some small nerves in salivary gland, renal capsule, and in epidural fat adjacent to a metastatic carcinoma, Schwann and satellite cells in a spinal ganglion invaded by tumor, chondrocytes of epiglottic cartilage, few cells of a malignant pleomorphic adenoma of salivary gland, most cells of a recurrent papillary meningioma with areas similar to the hemangiopericytic variant, and many cells of a renal carcinoma metastatic to brain; the primary renal tumor had been operated 4 years earlier and focally contained some GFAP-positive cells. To ascertain the specificity of such unexpected immunoreactivities for GFAP and to exclude possible crossreactivities with other intermediate filament (IF) proteins, a panel of different antibodies was used for immunocytochemistry with the peroxidase-antiperoxidase (polyclonal antisera) or labeled biotin-avidin (monoclonal antibodies) techniques: two monoclonal and four polyclonal anti-GFAP, three monoclonal and one polyclonal anti-cytokeratins (CK), and two monoclonal anti-vimentin (VIM) antibodies. Triple expression of GFAP, VIM and CK was found in the papillary meningioma (in patterns suggesting frequent co-localization), in the metastatic carcinoma (in patterns suggesting little co-localization), and in the pleomorphic adenoma (only few GFAP-positive cells). Co-expression of GFAP and VIM was seen in epiglottic chondrocytes and reactive astroglia; another metastatic carcinoma was labeled only for CKs. In the light of previous reports on non-glial specificities of the GFAP immunostain, and of the consistency of our immunostaining results obtained by all monospecific anti-GFAP antibodies as well as the lack of immunocytochemically evident crossreactivity with other IF proteins, authentic GFAP production by some rare non-glial tissues and tumors is suggested.