In vitro infectivity assay for prion titration for application to the evaluation of the prion removal capacity of biological products manufacturing processes

Prions can be detected and quantified currently by using either immunoassays such as Western-blot, ELISA or conformation dependent immunoassay, or an infectivity assay in laboratory animals (bioassay). While immunoassays are inexpensive and rapid, they are based on the detection of PrPSc, the abnormal isoform of the prion protein, a surrogate marker for prion infectivity. The bioassay is considered the gold-standard analytical method for measuring prion infectivity, but it is very costly and time-consuming, involving the destruction of large numbers of animals. The use of the transgenic MovS6 cell line is described for the development of an in vitro tissue culture infectivity assay (TCIA) for prion detection and quantitation. Compared to a bioassay, the TCIA is rapid (∼8 weeks), easy to implement, much less expensive, and requires far fewer animals. After titrating concomitantly a prion-infected brain homogenate sample by Western-blot, TCIA and bioassay, data show that the sensitivity of the TCIA is close to that of the bioassay, since 1 TCID50 corresponds to 4 ID50, and 80-fold more sensitive than the Western-blot. The application of the TCIA to the evaluation of prion removal in biological products manufacturing processes is described using a 15 nm-nanofiltration step of human albumin as a model.