By David Ivanov, Biochemistry ’15
Oral vaccines are known to be a convenient and effective method for treatment or prevention of diseases caused by pathogenic microorganisms. The difficulty of developing such vaccines is due to the often inhospitable environment of the stomach and intestinal tract because of low pH, or acidity, as well as enzymes that can digest or destroy biological molecules. Using a virus-like particle to deliver the vaccine is an advantageous method for getting around these and other barriers in the host organism.
A virus-like particle, or VLP, is a biological particle that resembles a virus, but contains no genetic information and thus cannot infect host cells. VLP’s can be formed by inserting and expressing just the genes for creating the viral capsid, which is a shell made up of protein subunits that protects the infectious genetic information in wild-type, or normal, viruses. The expressed capsid proteins can then self-assemble into the VLP. The capsid also has domains, or structural areas, that are responsible for recognizing suitable host cells to infect and inserting the viral genome.
The use of a VLP does, however, pose another challenge. The capsid has a domain which can contain an epitope, which is a small structural region of the capsid that can be bound to by antibodies. A host organism can then acquire immunity to the specific VLP that has been developed as carrier of the vaccine, also called a vector, which would trigger the immune system of the host to destroy the virus before successful delivery of the vaccine. Researchers at the UC Davis PIOMS Institutional Program led by Dr. Holland Cheng have made significant progress in developing a solution to this dilemma.
The researchers used the hepatitis E virus, or HEV, as a viral vector for an oral vaccine. In order to escape the anti-HEV immunity of the host, which could have been acquired through actual infection or repeated use of the same vector, the researchers had to mutate, or modify, the region on the capsid that is recognized by the immune system. The researchers took a short amino acid sequence derived from the structure of the HIV-1 virus that stimulated an immune response and inserted it into the epitope, which in the antibody-binding site of the HEV VLP. The researchers in effect swapped out the epitope of the HEV capsid and inserted an epitope of the HIV-1 virus using recombinant DNA technology, and were able to produce these fusion VLP’s that were derived from more than one organism, in this case HEV and HIV-1.
The new VLP was then tested to see how it would interact with antibodies for HEV and HIV-1, respectively. As expected, the new VLP reacted strongly with the antibody against the HIV-1 epitope, confirming that the insertion of the HIV-1 epitope into the HEV capsid had been a success. However, the new VLP barely reacted at all with the antibody against HEV, with a 1-2% reactivity compared to a wild-type control HEV VLP, whose reactivity was set at 100%. The researchers then tested the structural integrity of the new capsid. Previous attempts had been made to mutate the capsid of HEV, but the insertion made at different places had always interfered with the structural assembly of the capsid protein complex. Such interference can negatively impact its resilience against digestive enzymes, specifically proteolytic (protein digesting) enzymes commonly found in the digestive tract, such as trypsin. When in the presence of trypsin, the new VLP did experience some proteolytic cleavage at the individual protein subunits, the capsid still held, and the structure was preserved. Future research can be done to explore other inserts into the HEV VLP vector that might bind to other specific areas, which could be used to target therapies to specific regions while taking advantage of the protective benefits and primary infection site of the HEV VLP capsid.
Jariyapong P, Xing L, van Houten NE, Li TC, Weerachatyanukul W, Hsieh B, Moscoso CG, Chen CC, Niikura M, Cheng RH. Chimeric hepatitis E virus-like particle as a carrier for oral-delivery. Vaccine. 2013 Jan 2;31(2):417-24. doi: 10.1016/j.vaccine.2012.10.073. Epub 2012 Oct 26.