FIG. 3.
Permissibility of Rov9, Rov9-B4, and Rov9-2A3 cell lines to a range of ovine scrapie isolates (a 10−3 dilution). The number of infected cells (spots) was determined using the SSCA. The cutoff values for determining samples to be positive or negative for infection, calculated from the average number of spots detected in the New Zealand-derived negative controls plus 3 times the standard deviation, are 107, 223, and 105 spots per 20,000 cells plated in the Rov9, Rov9-B4, and Rov9-2A3 cells, respectively. In some cases, the number of infected cells detected for the most sensitive subclones is an underestimate that is due to the ELISPOT plate reader being unable to separate the large numbers of individual spots. Error bars show standard errors of the means.