Single chain antibodies against the 37 kDa/67 kDa laminin receptor as tools for prion diseases therapy

Prions are unconventional pathogens that cause transmissible spongiform encephalopathies (TSEs). According to the "protein only" hypothesis, prions consist of an infectious protein that is capable of converting a normal host protein termed PrPc into a protease resistant form termed PrPSc. PrPSc is poorly degraded by the host and accumulates in the CNS. Normal biological functions of PrPc and mechanisms involved in neurodegeneration remain obscure. During the past two decades, considerable efforts have been made to elucidate prion diseases and in particular to identify PrP interactors for a better understanding in prion biology. A major break-through was the identification of the 37 kDa laminin receptor (LRP), which represents the precursor of the human 67 kDa high-affinity laminin receptor (LR), as the cell surface receptor for the cellular prion protein. We investigated the role of LRP/LR in the propagation of PrPSc in chronically infected cells by different approaches. Three strategies resulted in downregulation or blocking of LRP and prevented PrPSc accumulation in different scrapie infected neuronal cell lines (i) transfection with an antisense LRP RNA expression plasmid (ii) transfection with small interfering (siRNAs) specific for the LRP mRNA and (iii) incubation with the polyclonal anti-LRP antibody, W3. We observed that the treatment with W3 abolished PrPSc deposition and reduced PrPc levels after one week of incubation. PrPSc did not reappear in cells being cultured for 14 additional days without therapeutic antibody treatment. Taken together, these results indicate that LRP is not only required for PrPc metabolism under non-pathological conditions but also has a pivotal role in prion propagation in a cell culture model. LRP/LR appears then to be a promising potential target for the development of therapeutics for the treatment of prion disease. Due to these encouraging cell culture data, we decided to select single chain antibodies (scFv) encompassing a suitable format for therapy. ScFvs are composed of variable parts of heavy and light chains of an immunoglobulin that are connected by a peptidic linker. The antibodies were screened on recombinant GST::LRP employing a phage display strategy. Two scFvs termed N3 and S18 were screened and selected by ELISA. Both antibodies were further characterized by western blotting and FACS analysis: both N3 and S18 specifically recognized mouseLRP and humanLRP overexpressed in mammalian cells under denaturating conditions (western blot) and under native conditions at the cell surface (FACS). Epitope mapping revealed that as expected both scFvs are directed against the extracellular part of LRP: S18 and N3 recognized amino acid residues 225-233 and 273-278, respectively. The ability of N3 and S18 to interfere with LRP/PrP interaction was tested by pull-down assays. In contrast to the control scFv C9 directed against the pre-S1 coat-protein of hepatitis B virus, both anti-LRP scFvs were able to block the specific LRP/PrP binding. In order to investigate a potential curing effect of scFv S18 in vivo, this scFv was tested in a scrapie mouse model by passive immunization. The application of S18 by intra-peritoneal injection was able to reduce PrPSc deposition in the spleen in comparison to mice injected with PBS or C9. However the survival times of S18 treated animals was not increased. Anti-LRP scFv S18 seems to contribute to block prion propagation in the periphery but it is likely that this effect was not enough strong to have an impact on the CNS invasion. Thus, we hypothesized that a strategy targeting directly the brain should be more effective. In this context, an approach based on the expression of single chain antibodies as secretory molecules in the brain via an adeno-associated virus (AAV) vector was initiated. To assure secretion of the scFv expressed in mammalian cells, a signal sequence was fused to the scFvs. Tranfection experiments demonstrated that neuronal cells were able to express and secrete high quantities of both scFvs. Furthermore, the generated scFvs were still functional as shown by western blotting. To find the appropriate AAV serotype for scFv expression, neuronal cells were transduced with varying serotypes carrying a GFP. AAV serotype 2 was chosen due to (i) its good transduction performance in two neuronal cell lines and (ii) the possibility of its purification by affinity chromatography. The sequences encoding for the scFvs N3, S18 and C9 have been cloned in an AAV-based vector. The AAV system was also able to drive high expression of scFvs into the supernatant by transfection or transduction. rAAV-scFv particles were produced and purifed for further stereotaxic injections into mice. Although the investigation of this therapeutic strategy is still in progress in a murine scrapie model, we already proved that a single injection of rAAV led to the expression of scFvs into the brain of mice 30 days post injection. This study represents the first gene therapeutic approach for the treatment of prion diseases.