Finding prions (PrPSc) in blood is like looking for a needle in a hay stack. Recent developments in concentration and amplification of prions in blood might help solve this problem.
Individuals with prion infected blood can contain TSE infectivity. Research has already proven that BSE and scrapie can be transmitted between sheep via blood transfusion, and scientists are almost certain that red cell donations from two pre-clinical vCJD patients were responsible for infecting two blood transfusion recipients. The presence of infectivity in blood of vCJD patients demands special measures to prevent the spread of the disease via transfusions. That makes the development of sensitive and specific ante mortem tests for donor screening one of the major focal points in research today.
Blood tests
At the current time, the most promising approach for development of sensitive blood tests involve the concentration of PrPSc from blood. This is followed by sensitive detection of the concentrated prion protein. A new approach currently being pursued by Prionics to concentrate and detect the TSE agent in blood uses the antibody 15B3 which only binds PrPSc, and not the normal form of the prion protein (see Figure 1). Diagnosis can be made through the analysis of fluorescence in the sample.
Prion amplification
Regardless of the method used to concentrate and detect PrPSc, further sensitivity might be achieved by amplifying the amount of PrPSc in blood. The protein misfolding cyclic amplification (PMCA) uses PrPSc present in a positive sample as a seed. Normal PrP in the sample is converted to PrPSc (see Figure 2). Using this method, Castilla and coworkers succeeded in detecting PrPSc in blood of hamsters that were experimentally infected with TSE. The reproducibility of the PMCA method is hampered by the fact that it uses brain homogenate as a source of normal PrP (PrPC), a material that contains numerous other biological substances and therefore not be standardized. Furthermore, until now the only successful results has been with hamster blood. Even though the PMCA amplification method seems a promising tool for increasing the sensitivity of a TSE blood test, it seems unlikely that the first ante mortem tests will make use of the amplification step.
