Hepatitis virus-like particles as potential cancer treatment
UC Davis researchers have developed a way to use the empty shell of a Hepatitis E virus to carry vaccines or drugs into the body. The technique has been tested in rodents as a way to target breast cancer, and is available for commercial licensing through UC Davis Office of Research.
Hepatitis E virus is feco-orally transmitted, so it can survive passing through the digestive system, said Marie Stark, a graduate student working with Professor Holland Cheng in the UC Davis Department of Molecular and Cell Biology.
Cheng, Stark and colleagues prepared virus-like particles based on Hepatitis E proteins. The particles do not contain any virus DNA, so they can’t multiply and spread and cause infections.
Hepatitis E virus-like particles can be modified so that molecules such as LXY30, which binds to cancer cells, can be attached to them.
Such particles could be used as vaccines that are delivered through food or drink. The idea is that you would drink the vaccine, and after passing through the stomach the virus-like particles would get absorbed in the intestine and deliver vaccines to the body.
But the particles could also be used to attack cancer. Stark and Cheng did some tinkering with the proteins, so that they carry sticky cysteine amino acids on the outside. They could then chemically link other molecules to these cysteine groups.
They worked with a molecule called LXY-30, developed by researchers at the UC Davis Comprehensive Cancer Center, which is known to stick to breast cancer cells. By using a fluorescent marker, they could show that virus-like particles carrying LXY-30 could home in on breast cancer cells both in a laboratory dish and in a mouse model of breast cancer.
Hepatitis E Virus Like Particle
HEV, a non-enveloped RNA virus, is feco-orally transmitted. The viral capsid protein, when expressed in insect cells, can self-assemble into icosahedral T=1 virus like particles (VLPs) with 60 subunits forming the icosahedral 2-, 3-, and 5-fold axes. The VLP has 30 protrusions at the 2-fold axes of the surface with large depressions at the 3- and 5-fold axes. Like the virus, HEV VLPs are resistant to harsh environmental conditions, such as acidic and intestinal pH, and extremes of temperature. This makes HEV VLP highly suitable for use as an orally-deliverable nanocarrier and vaccine/therapeutic agent. The monomeric capsid protein has three distinct domains- the Shell (S), Middle (M) and Protrusion (P) domains. The secondary structures of the S and M domains are analogous to similar domains in many other viral capsids. The S domain forms the inner scaffold of the particle, while the M domain is closely associated with the shell and is located around the icosahedral 3-fold. A long proline rich hinge connects the M and P domains, and this makes the HEV VLP structure uniquely modular (Figure 1). This implies that extensive modification of the surface-protrusion forming domain (P) does not affect the ability of the dimeric building blocks to assemble into VLPs.
We have carried out surface modification of the P domain, through genetic engineering or chemical conjugation to express foreign peptides or other moieties. HEV VLPs expressing p18, predominant antigenic region of the V3 loop of HIV-1 gp120, when administered orally in mice, generated a strong tag-specific humoral and cell-mediated immune response (Figure 2).
Additionally, chemical modification of key mutated residues of the P domain allowed the attachment of a breast cancer targeting ligand to the VLP surface, and this modified VLP successfully trafficked to tumor sites in vivo and in cells (Figure 3).
Modularity allows the VLP to be disassembled and re-assembled in vitro, and re-assembly seems to be Calcium dependent. Two critical residues- asp269 and asp270 form a Calcium dependent salt bridge at icosahedral 2 fold interface, and this interaction allows for encapsulation of negatively charged DNA in the particle (Figure 4). When DNA encoding HIV Gag protein was encapsulated in HEV VLPs and orally administered in mice, Gag protein was expressed in gut epithilial sections (Figure 5), and a strong Gag specific immune response was also generated, both humorally and cell mediated. Preliminary studies done in mice need to be further corroborated in non-human primates, whose immune systems resemble the human immune system more closely.
We plan to expand our current knowledge of HEV VLP as a vaccine/ DNA carrier to understand the mechanism by which the vaccine capsule interacts with immune cells. By fine-tuning various parameters of vaccine development, such as optimizing the efficiency of DNA encapsulation and modifying the VLP surface for better targeting to specific cell types, we wish to develop a simple but effective targeting tool that has enormous clinical implications. Additional modification of the particle can enable labeling for various imaging modalities, such as electron microscopy and in situ fluorescence imaging.
So perhaps one day, cancer patients might drink their medicine and UC Davis-designed virus-like particles carrying anticancer drugs will home in on their target.