Curative properties of antibodies against prion protein: a comparative in vitro study of monovalent fragments and divalent antibodies.

Prion diseases, which include Creutzfeldt-Jakob disease (CJD) in humans, are a group of devastating neurodegenerative disorders for which no therapy is yet available. However, passive immunotherapy appears to be a promising therapeutic approach, given that antibodies against the cellular prion protein (PrPc) have been shown in vitro to antagonize deposition of the disease-associated prion protein (PrPSc). Nevertheless, in vivo deleterious side effects of injected anti-PrP antibodies have been reported, mainly due to their Fc fragments and divalence. In this context, we examined here the ability of five Fabs (monovalent fragments devoid of the Fc part), prepared from antibodies already characterized in the laboratory, to inhibit prion replication in infected neuronal cells. We show that all Fabs (which all retain the same apparent affinity for PrPc as their whole antibody counterpart, as measured in EIA experiments) recognize quite well membrane bound-PrP in neuronal cells (as shown by flow cytometry analysis) and inhibit PrPSc formation in infected cells in a dose-dependent manner, most of them (four out of five) exhibiting a similar efficiency as whole antibodies. From a fundamental point of view, this report indicates that the in vitro curative effect of antibodies i) is epitope independent and only related to the efficiency of recognizing the native, membrane-inserted form of neuronal PrP and ii) probably occurs by directly or indirectly masking the PrPc epitopes involved in PrPSc interaction, rather than by cross-linking membrane bound PrPc. From a practical point of view, i.e. in the context of a possible immunotherapy of prion diseases, our data promote the use of monovalent antibodies (either Fabs or engineered recombinant fragments) for further in vivo studies.