July 22, 1998 Researchers led by scientists at UC San Francisco say they have made a significant breakthrough in their attempt to reengineer the poliovirus so that it could some day be used as a transporting mechanism for inducing immunity against disease pathogens in humans.
The finding, reported in a recent issue of Proceedings of the National Academy of Science, could ultimately have implications for developing reengineered poliovirus vaccines for HIV, hepatitis B, other viral diseases and cancer, said the senior author of the study, Raul Andino, PhD, an assistant professor of microbiology at UCSF.
Recombinant viruses, engineered with foreign DNA fragments, are considered a promising approach to vaccine development because viruses replicate in a variety of host cell types and, in their natural states, induce responses from the immune system.
Two types of viruses, vaccinia and adenovirus, have already been used to develop recombinant vaccines for a variety of pathogens, but poliovirus could provide an alternative for situations in which these viruses cannot be used, Andino said. Moreover, poliovirus offers the advantages of safety and the ability to induce long-lasting protective immunity. It is also easy to administer orally, is sufficiently low in cost to enable distribution in the developing world, and can induce local immunity at mucus membranes sites. This targeted local immunity, said Andino, could potentially offer protection against respiratory or gastrointestinal pathogens or diseases that gain access through mucosal ports of entry, such as sexually transmitted diseases, including HIV.
In their study, conducted in mice, the UCSF investigators created a recombinant poliovirus vector that provided the first evidence that the reengineered vehicle could prompt the cell-mediated arm of the immune system into action, issuing a response from CD8 T-cells, which seek out and destroy pathogen-infested cells. Previously, recombinant poliovirus vectors had only been demonstrated to incite antibodies.
When the mice, genetically altered to express the poliovirus receptor, were inoculated, they developed a CD8 T-cell response. When they were subsequently injected with lethal doses of a malignant melanoma cell line naturally designed to prompt a CD8 T-cell response, all 40 of them were immune to the cancer. Evidence suggests, said Andino, that being able to incite both arms of the immune system--the antibody and cell-mediated arsenals--may be crucial for combating many infectious diseases and cancer. While antibodies latch onto infected cells and neutralize them, carrying out a crucial step in clearing viruses and preventing reinfection, cytotoxic (cell-killing) CD8+ T-cells actually kill virus-infected cells, thereby eliminating any reservoir of virus and preventing spread of infection.
"The generation of an effective CD8+ cytotoxic T-lymphocyte response is thought to be particularly important for control of virally-infected cells and tumors," said Andino.
The researchers' challenge in eliciting a CD8 T-cell response was to reengineer poliovirus so that when it infected cells it would produce an antigen, a foreign protein on the surface of infected cells, that would incite the specific cell-mediated response. All viruses express an antigen unique to their virus type on the surface of the cells they infect, and the immune system takes advantage of these inadvertent signals by issuing a response tailored to the specific antigen.
CD8+ cytotoxic T cells recognize antigens as short strands of protein bound to a class of molecules, known as major histocompatability class complex (MHC) I, that are located on the surface of cells. Therefore, the researchers needed to engineer the poliovirus in such a way that when it infected cells, the cells would process the antigen in a series of molecular steps that would ultimately lead it to be bound to the MHC I molecules on the cell surface. Their success in doing so was their major achievement.
In their test system, the researchers reengineered the poliovirus vector to express a chicken antigen called Ova. They then inoculated the transgenic mice with this viral vector, which prompted the desired cytotoxic T-lymphocyte response. Then, to test the immune response, they exposed the mice to lethal doses of a malignant melanoma cell line that also expresses the Ova antigen. The result: 100 percent of the animals were immune to the melanoma.
"In this study, we have shown that peptides derived from proteins expressed by poliovirus vectors reach the surface of the infected cell and are presented in a MHC class I-restricted manner," said Andino. "This is a crucial step in demonstrating the ability to manipulate poliovirus as an effective vaccine, and may have implications for understanding poliovirus pathogenesis."
While the melanoma mouse model was contrived, it may well be possible, he said, to create a recombinant poliovirus with an antigen that would prompt a CD8 T-cell response to cancers that actually affect humans.
Researchers at University of Massachusetts Medical Center also participated in the study.
The research was supported by a Public Health Service grant and several NIH grants.
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