Conformation-specific antibody Fab YEG Sc-G1 isolates an infectious ß-solenoid conformer of prions

Introduction: YEG mAb Sc-G1 (G1) is a monoclonal antibody generated using the structure-based prion vaccine 14R1. Recognizing a native, discontinuous epitope on a β-solenoid fold, it fails to bind PrP^Sc in its PIRIBS structure, as epitope residues would be spatially separated. Although, all recent ex-vivo high-resolution structures of PrP^Sc show a PIRIBS conformation, antibody G1 raises the possibility of alternative infectious prion conformers.

Objectives: To isolate the putative β-solenoid variant of PrP^Sc from infected RML brain homogenates using Fab G1 and determine their infectivity using bioassays.

Methods: Brain homogenates from RML-infected mice were immunoprecipitated using Protein-L beads coupled with Fab G1. Following incubation and washes, the complex of PrP^Sc-Fab G1 was eluted and collected pulldown fractions were inoculated intracerebrally into FVB mice. Experiments were performed in triplicates, including a set of controls with a non-PrP binding Fab (anti-HER2). Mice were monitored for clinical signs of prion disease.

Results and Discussion: Fab G1 immunoprecipitated a β-solenoid variant of PrP^Sc showing partial PK-resistance. Inoculation with G1-eluates produced clinical symptoms and incubation periods comparable to those of 1% brain homogenate controls. In contrast, mice inoculated with HER2-eluates have not developed clinical symptoms to date. These results suggest that G1 captures a subset of infectious PrP^Sc conformers consistent with a β-solenoid fold that are distinct from the widely accepted PIRIBS conformations of PrP^Sc.

Conclusion:  The findings support the concept of structural heterogeneity, with at least two distinct conformers in the same brain tissue sample, highlighting the utility of conformation-specific antibodies in prion disease research.