Glial fibrillary acidic (GFA) protein in Schwann cells: fact or artifact?

Antisera to the glial fibrillary acidic (GFA) protein stained a subpopulation of Schwann cells in cryostat sections of rat sciatic nerve by indirect immunofluorescence and by the peroxidase-antiperoxidase (PAP) procedure. The staining pattern was entirely different from that obtained with vimentin antisera, which uniformly decorated endoneurial tubes. Electron microscopic examination of sciatic nerve provided a possible explanation for the relatively small number of Schwann cells decorated by GFA antisera: 10 nm filaments were mainly confined to Schwann cell processes surrounding nonmyelinated axons. A marked increase in GFA-positive Schwann cells and in Schwann cells containing filaments by electron microscopy was observed in sciatic nerves undergoing Wallerian degeneration. Conversely, immunochemical procedures failed to demonstrate the presence of antigen reacting with GFA antisera in extracts of sciatic nerve, both normal and degenerated. These include absorption experiments, double immunodiffusion, immunoaffinity chromatography, and immunoradiometric assay. Two explanations may be considered for these findings: i) Schwann cell intermediate filaments and GFA protein share common antigenic determinants, the immunohistological methods being more sensitive to detect cross-reactivity as compared to immunochemical procedures on tissue extracts; and ii) the binding of anti-GFA to Schwann cell 10 nm filaments is not due to immunological cross-reactivity.