Antibody to scrapie-associated fibril protein identifies a cellular antigen

Antibody to scrapie-associated fibril (SAF) protein was produced in a rabbit by a primary injection of 50 brain equivalents of native SAF protein emulsified in complete Freund's adjuvant and by a secondary inoculation of polyacrylamide gel which contained 50 brain equivalents of SAF protein emulsified with incomplete Freund's adjuvant. The antibody produced was found to decorate the SAF as demonstrated by the use of goat anti-rabbit colloidal gold second antibody. Immunoblots (dot blots and Western blots) revealed that the antibody reacted with scrapie hamster brain extracts but not with age-matched normal hamster brain extracts. However, the antibody also identified an immunoreactive host cellular protein with an apparent mol. wt. of 28 000 from unextracted brain homogenates of both uninfected and scrapie-infected hamster and mouse brains up to a dilution of 1/10(5) brain equivalent. The SAF purification procedures caused selective enrichment of SAF proteins with selective elimination of this immunoreactive cellular antigen. It may be possible that SAFs are composed of a host cellular protein which has been changed in such a way that it aggregates non-covalently to form fibrils and becomes partially resistant to protease digestion.