Few pathogens inspire fear to the degree that prions do. As the culprit behind such mysterious diseases as mad cow disease and Familial Fatal Insomnia, prions are avoided at all costs. In December of 2003 more than 30 countries closed their borders to all beef imports from the United States because just one cow from the state of Washington tested positive for mad cow disease (1). But what are prions? And why do they evoke such aggressive responses?
Prions are not viruses but rather a misfolded form of a particular protein, called prion protein (PrP). This protein, whose function remains unclear, is found throughout the body of healthy individuals in its properly folded state, known as PrPC. When an individual is exposed to a prion, the misfolded form of PrP known as PrPSc, the prion can cause the normal protein to adopt its misfolded state. The normal PrPC is converted into the abnormal PrPSc, which can then go on to convert more healthy proteins into their pathogenic form. Eventually amyloid plaques of these abnormal proteins build up, mostly in neuronal tissue, causing transmissible spongiform encephalopathies, holes in the brain that continue to grow until the individual has passed away.
Prion diseases are universally fatal and there is no currently approved treatment to slow their advance (P). Because these diseases occur due to a single misfolded protein, they are not reliant on a nucleic acid based entity for transmission, as is every other known transmissible disease. It is for this reason that prions are not susceptible to normal sterilization procedures, including high temperatures and UV radiation. Transmission can occur solely due to ingestion of infected neural tissue or, as more recently suggested, via inhalation of air droplets (2). Because there is no known treatment for prion diseases countries tend to go to relatively extreme measures to prevent prions from crossing their borders.
However, new research suggests that the use of PrP antibodies, both prophylactically and after infection has taken root, might help slow prion disease progression (3, 4, 5, 6). These antibodies bind to specific regions of the normal PrP protein, helping them resist conversion to their prion form (4). However, these have mostly involved in vitro studies, which do not necessarily mean that there are practical clinical uses for anti-PrP antibodies in treating prion diseases. One of the main hurdles to overcome is getting these antibodies to target tissues in the central nervous system at concentrations high enough to have an effect. In order to do this the blood brain barrier (BBB), which is normally impermeable to antibodies, must be overcome. In the past this has meant inserting the drugs directly into the brain of mouse models. However, it would be more suitable in human patients for a less invasive delivery method to be used, especially considering the possible transmission of prions that could occur during brain surgery.
To address this problem a recent study by Ohsawa et al. (2013) treated prion-infected mice with anti-PrP antibodies using peripheral injections, which were administered via the tail vein on a weekly basis. Treatment began 120 days after infection in order to determine whether these antibodies would affect late stage disease progression. While these antibodies have been shown to slow the progression of prion diseases when administered directly to the central nervous system, it was unclear whether the antibodies would be able to reach the brain in sufficient doses to have their desired effect. These antibodies were fluorescently labeled to allow observation of spatial distribution by immunohistochemistry. Levels of PrPC and PrPSc were determined for various brain sections via immunoblotting (P).
Immunohistochemistry revealed that peripheral administration of the antibodies did in fact lead to successful delivery to the brain. Areas where the concentrations of antibodies were high enough to be detected include the cerebellar medullas, thalami, and hippocampus. The mean survival time for mice treated with antibodies was longer than those that received the control, although these results failed to reach statistical significance. Mice that were given the antibodies but did not survive significantly longer than control mice were found to have less well-distributed concentrations of the antibodies in their brains. Immunoblotting, likewise, revealed that administration of anti-PrP antibodies during late stage prion disease correlated with a higher percentage of normal PrP proteins as compared with their pathogenic form (P).
It has been suggested that late stage prion disease might be associated with an increase in BBB permeability (6). This would certainly explain the ability of the antibodies used in this study to reach critical regions of the brain. However, the low sample number of this study calls some of its results into question. More research is needed to determine the factors that contribute to the differences observed in antibody distribution for some of the mice used. For this the relationship between prion disease progression and BBB permeability should be focused on. In addition, while this treatment has promising results suggesting the ability of peripherally injected antibodies to slow prion diseases a cure still remains elusive. Antibodies might be able to help normal PrP proteins resist conversion but without a mechanism to clear the abnormal prions the patient would still remain infected and capable of transmitting the disease to others. Future research should focus on ways to minimize the effects of already converted PrPSc proteins.
P. Ohsawa, Natsuo, Chang-Hyun Song, Akio Suzuki, Hidefumi Furuoka, Rie Hasebe,
and Motohiro Horiuchi. "Therapeutic Effect of Peripheral Administration of
an Anti-Prion Protein Antibody on Mice Infected with Prions." Microbiology
and Immunology 57 (2013): 288-97. (Ohsawa et al. 2013)
1. New York Times. "Japan Deems Beef Standards Lax in Canada and U.S." New York
Times 20 Jan. 2004. (New York Times (2004))
2. Haybaeck, Johannes, Mathias Heikenwalder, Britta Klevenz, Petra Schwarz, Ilan
Margalith, Claire Bridel, Kirsten Mertz, Elizabeta Zirdum, Benjamin Petsch,
Thomas Fuchs, Lothar Stitz, and Adriano Aguzzi. "Aerosols Transmit Prions
to Immunocompetent and Immunodeficient Mice." Plos Pathogens 7.1
(2011): E1001257. (Haybaeck et al. 2011)
3. Enari, Masato, Eckhard Flechsig, and Charles Weissmann. "Scrapie Prion Protein
Accumulation by Scrapie-Infected Neuroblastoma Cells Abrogated by
Exposure to a Prion Protein Antibody." Proceedings of the National Academy
of Sciences 98.16 (2001): 9295-299. (Enari et al. 2001)
4. Kim, Chan-Lin, Ayako Karino, Naotaka Ishiguro, Morikazu Shinagawa, Motoyoshi
Sato, and Motohiro Horiuchi. "Cell-surface Retention of PrPC by Anti-PrP Antibody Prevents
Protease-resistant PrP Formation." Journal of General Virology 85.11 (2004): 3473-482.
5. Peretz, David, Anthony Williamson, Kiotoshi Kaneko, Julie Vergara, Estelle Leclerc,
Gerold Schmitt-Ulms, Ingrid Mehlhorn, Giuseppe Legname, Mark Wormald,
Pauline Rudd, Raymond Dwek, Dennis Burton, and Stanley Prusiner.
"Antibodies Inhibit Prion Propagation and Clear Cell Cultures of Prion Infectivity." Nature 412
(2001): 739-43. (Peretz et al. 2001)
6. Trevitt, Clare, and John Collinge. "A Systematic Review of Prion Therapeutics in
Experimental Models." Brain 129 (2006): 2241-265.