Additional therapeutic and/or prophylactic strategies have been and continue to be pursued. Vaccination with recombinant mouse PrP delays the onset of prion disease in mice [46] . Passive immunization with anti-PrP antibodies was shown not only to inhibit formation of PrP Sc in a cell-free system [47] , but was also shown to prevent infection of susceptible N2a cells [7] and to inhibit prion replication in infected cells [8, 47, 48] . The effectiveness of these treatments were also dependent on when they were administered relative to the time of infection.