Previous studies have reported that the 132-140 portion of PrP C [8] or the 132-156 region of PrP [50] [51] [52] [53] are important for the generation of PrP Sc . Rigter et al. [54] found two high affinity binding regions for protein-protein interactions using ovine peptide-arrays: (i) sheep-PrP peptides 43-102, including the amino-terminal octarepeats, and (ii) sheep-PrP peptides 134-177 Figure 2B for representative western blots). PrP Sc western blots were quantitated and the amount of inhibition was determined relative to N2a/22L control cultures. The controls consisted of cells both in the absence of Mab and in the presence of normal mouse IgG. The % PrP Sc inhibition plotted represents the mean 6 SD from three independent experiments as described in Methods. doi:10.1371/journal.pone.0041626.g002 which encompasses most of the scrapie susceptibility-associated polymorphisms in sheep. Moroncini et al. [55] found that residues within the 89-112 and 136-158 segments of PrP C are key components of the PrP C -PrP Sc complex. Beringue et al. [11] reported that antibodies exclusively binding PrP C were relatively inefficient inhibitors of PrP Sc accumulation compared with antibodies that additionally recognize disease-associated PrP isoforms. Féraudet et al. [56] screened 145 anti-PrP Mabs for their capacity to inhibit PrP Sc replication in infected N2a or Rov9 cells. They identified four different linear epitopes that hindered the PrP C to PrP Sc conversion: the amino terminal region 26-35, the octarepeat region 59-89, the intermediate region 97-102, and the central region 130-160. The observation that antibodies that bind to the amino terminus of the prion protein are capable of inhibiting conversion suggests that the Figure 3 . A. Western blot of 263K brain homogenate that was used as the seed for PMCA. Dilutions of the brain homogenate was prepared and either untreated (lanes 1 and 2) or PK-treated (lanes 3 and 4) prior to SDS-PAGE and western blotting. A 1023 dilution prior to and after PK demonstrates the three protein banding pattern typical for 263K brain homogenate whereas no bands are visible at a 1024 dilution of the same homogenate. B. Western blotting of the PMCA products following sPMCA40 in the absence and presence of PrP-Specific Mabs. Fourty cycles of serial PMCA was carried out in the absence or presence of Mabs as described in the text. Each Mab was added at a final concentration of 12, 25, and 50 mg/ ml. Following PK treatment, the PMCA products were subjected to SDS-PAGE, western blotted and immunostained for PrPSc. The protein bands were quantitated and the level of PrPSc inhibition, relative to the no Mab and normal mouse IgG controls, were determined. doi:10.1371/journal.pone.0041626.g003 Figure 4 . Influence of Mabs 3F4 and 5D6 on PrP Sc formation following sPMCA 40 . Mabs 3F4 and 5D6 were added to sPMCA 40 at final concentrations of 0-50 mg/ml. The PMCA products were PK treated and western blotted. The PrP Sc was quantitated and the level of PrP Sc inhibition was determined relative to control reactions. doi:10.1371/journal.pone.0041626.g004 endogenous proteolytic cleavage occurs after the site of conversion.