Specific determination of the proteinase K-resistant form of the prion protein using two-site immunometric assays. Application to the post-mortem diagnosis of BSE.

The aim of this work was to establish an immunological test suitable for specifically detecting PrPres in tissues from animals or humans developing TSEs. We chose to use as detection method a conventional two-site immunometric assay (sandwich immunoassay) because over the last 20 years this technique has clearly been shown to be more sensitive and specific than other tests. We have established numerous two-site immunometric assays based on the use of monoclonal antibodies and suitable for measurement of PrPsen in various mammalian species (human, bovine, ovine, mouse and hamster). A detection limit below 100 pg/ml was estimated from standard curves established using ovine recombinant PrP. PrPres was selectively detected by processing samples (currently brain homogenates) to enable specific purification and concentration of PrPres, which was finally solubilized by a strong denaturing treatment. This sample-processing procedure can be achieved within 30 minutes. The capacity of this test to detect bovine PrPres was estimated in the framework of an evaluation study organized by the Directorate-General XXIV of the European Commission during May 1999. On this occasion, a blind test on 1400 brain stem samples taken from either healthy (1000) or BSE-infected (300) cows demonstrated 100% sensitivity and specificity. In addition, dilution experiments showed that the test can significantly detect PrPres in homogenates diluted 1/300 and was at least as sensitive as a conventional bioassay performed on mice.