Although general health and luck play some role, HIV infection is, to some extent, a numbers game - repeated exposure to the virus most likely will lead to infection. However, it is true that there are people who apparently are resistant to HIV infection.
Those people do not become infected despite repeated high-risk behavior, but a notable indication of HIV exposure for such individuals is the existence of antibodies to a part of an HIV envelope protein that is known - after the one-letter-each code for its amino acid sequence - as the ELDKWA epitope.
For HIV to infect the immune system cells that are its main target, it must first enter the body by crossing the mucosal membranes via a process known as transcytosis. Antibodies to the ELDKWA epitope are known to block both transcytosis and actual infection of immune system cells, and thus, the antibodies are an obvious candidate for vaccine development. However, efforts to reliably raise antibodies against the epitope for use in an AIDS vaccine have proved elusive to date.
But there's hope. In the Sept. 3, 2004, online edition of the Proceedings of the National Academy of Sciences (print version due Sept. 14) researchers from the School of Life Sciences and Biodesign Institute at Arizona State University and from the Institut Cochin de Genetique Moleculaire in Paris reported they have constructed a chimeric protein that can induce in mice the production of antibodies against the ELDKWA epitope. Further experiments showed that those antibodies were able to block transcytosis in cell culture.
The key to the researchers' success was prodding into action the mucosal, rather than the systemic, immune system.
"When the immune system is engaged via the mucosal immune system as an entry point, both mucosal and systemic immune reactions are activated," said Tsafrir Mor, assistant research professor at the Biodesign Institute and senior author of the study. "By contrast, when the systemic immune system is activated directly via the injection of immunogens, there is no corresponding mucosal antibody production. The other advantage of the mucosal immune system is that it is interconnected; so you can immunize orally and get a response in the respiratory tract or the vagina."
The paper, which is titled "A mucosally targeted subunit vaccine candidate eliciting HIV-1 transcytosis blocking Abs," describes how the researchers first constructed a chimeric protein out of a subunit of cholera toxin (the "CTB" subunit), which is able to efficiently present antigens to the mucosal immune system, and a piece of HIV envelope that included the ELDKWA epitope (the "P1" part of the chimera). Mice were immunized with either CTB alone, the CTB-P1 chimeric protein or the chimeric protein plus an adjuvant. When immune responses were measured, it was shown that mice immunized with the chimeric protein mounted mucosal and systemic immune responses to P1.
Serum, vaginal secretions and feces then were collected from the mice to see if the immunoglobulins they produced could block HIV from crossing mucosal membranes. The researchers grew epithelial cell lines to construct an artificial mucosal membrane, and HIV-infected cells were added to the apical part of the barrier. Incubation with immunoglobulins isolated from excretions of the immunized mice reduced the amount of virus that was able to cross the mucosal barrier by up to 50 percent.
Several control experiments support the notion that the result was due to antibodies against the P1 part of the chimera. A reduction in virus crossing was observed using Ig that came from mice immunized with the CTB-P1 construct with or without adjuvant, but not from mice immunized with CTB alone. Blocking the antibodies with free P1 peptide also re-established transcytosis. Finally, depleting the samples of systemic IgG antibodies did not affect transcytosis blockage, but depleting them of mucosal IgA antibodies did.
Eat Your Vegetables - They're Good For You!
The researchers also are looking at a novel production method for delivering the vaccine, should it prove successful in clinical trials. They showed that plants could produce the vaccine, and they ultimately would like to use tomatoes as the production system.
Vaccine production in plants has several advantages. It is heat-stable, for one, and can be delivered orally, most likely in the form of freeze-dried and dehydrated plant tissue packed into gel capsules. Also, processing would be fairly minimal. And, as Charles Arntzen, founding director of the Biodesign Institute and a co-author of the study, told BioWorld Today: "Once the technology has gone into making the plant, it is infinitely scaleable. This is an advantage particularly for new vaccines with no proven market value, because production can be grown according to market demands."
When asked about potential risks of plant-based vaccine production, Arntzen said that "the biggest risk is not using a new technology that could save lives." But there are risks associated with allergies, equivalent to vaccines that are produced in eggs or yeast. With any protein vaccine, there also is the risk of tolerance (i.e., the lack of an immune response).
Mor and his colleagues next will attempt to demonstrate that their fusion protein can not only block transcytosis, but also neutralize HIV infection. Research to express the fusion protein in plants and demonstrate its oral effectiveness also are ongoing.
The group, and its academic collaborators, also has National Institutes of Health funding to evaluate in clinical studies CTB and other fusion proteins for their effectiveness in inducing immune responses against other sexually transmitted diseases, as well as for clinical studies of the fusion protein described in the current paper. Assuming further preclinical research pans out, they hope to enter the clinic by early 2006.
Ultimately, they expect their work to be aligned with other research. Arntzen said: "I would not want to say that we have discovered a new HIV vaccine. What we have hopefully discovered is a necessary component of a multicomponent vaccine."