GAINESVILLE, Fla. – 1975 Nobel Prize winner and HIV pioneer David Baltimore was the keynote speaker last Wednesday at the Florida Genetics 2012 Symposium. The program this year was dedicated to University of Florida’s Kenneth Berns, the father of adeno-associated virus therapy. Baltimore’s presentation, “AAV to the Rescue,” reflected the great strides research has achieved since Berns’ initial studies on the virus at the UF Genetics Institute.
Baltimore began by describing the unique problems HIV presents to human immune systems and vaccination attempts.
He said that despite its reputation, HIV is “not a particularly powerful virus.” He noted that it can take a hundred instances of unprotected sexual contact for infection to occur.
What HIV lacks in potency it makes up for by being extremely elusive to the human immune system, primarily because the virus is unaffected by most human antibodies. This is due to a thick outer carbohydrate layer, a binding site inaccessible to most antibodies and an additional binding site that is only accessible after the virus has already infected immune system cells by way of CD4 receptors. HIV also mutates quickly, requiring any vaccine attempts to be especially potent.
Years into the HIV pandemic, researchers were finally able to identify antibodies capable of fighting the virus on a broad scale, and while they were discovered too late to protect their hosts, Baltimore said they are the source of much of the current HIV vaccine research.
“…By that time, the people who make them have so much virus in their system that they can mutate against any antibody that come along, even the best,” Baltimore said.
However, the logistics of producing and dispersing the capable antibodies in viable quantities, especially in the developing countries where HIV is pandemic, are nearly insurmountable. Baltimore’s research focuses on leveraging specific genes from these antibodies so that the host body will produce vaccine-level amounts itself, a process called vectored immunoprophylaxis (VIP).
The key to the process is the family of adeno-associated viruses. Berns was the first to utilize its unique qualities that now make it a gene therapy leader. The virus is harmless to humans, has long-lived expression from a single administration, expresses inserted genes strongly and is already present in about 80 percent of humans. Its only drawback is a small genome, only about 5000 bases to humans’ 3 billion, which limits the size of genes it can carry.
Baltimore’s lab chose a strand of AAV in which to insert the genes necessary to produce b12, one of the HIV antibodies. The altered AAV was injected into the muscles of mice, which began to produce b12 in copious amounts for an extended period of time. Mice, genetically modified to have immune systems similar to humans, were injected with the AAV and then unnaturally large doses of HIV before weekly monitoring of their CD4 levels.
The presence of HIV is indicated by an almost immediate drop of CD4 counts in the blood. Mice injected with the AAV maintained or increased their CD4, demonstrating the potency of the process. The b12 antibodies are only effective against a limited number of HIV strands, however, so their genes were soon replaced in the AAV by another, VRC01, which is effective against almost all known strands.
Following the conclusion of their study in January, Baltimore and his team have continued to explore the possibilities of VIP. They tested it against HIV strands with slightly different mechanics and delivery through a mucosal membrane, its normal transmission method. All experiments found the VRC-type antibodies produced by modified AAV extremely effective in protecting mice against HIV.
With extensive experimental data to support it, the VIP process is transitioning to clinical development in humans. Although it is cheap and stable, Baltimore had to find a way to reverse the production of antibodies in patients in the case that they responded to them detrimentally. Using a Cre-Lox process, he was able to overcome limits set by AAV’s small genome to successfully stop the antibody’s production.
“The cost for us to make a batch of virus for clinical tests is enormous,” Baltimore said. “But I can only imagine that if we had a mechanized process to make virus that the yield from cells is so enormous, that we could make it at quite a reasonable cost.”
Baltimore said VIP can work for other viruses as well. Due to the rapid mutation of HIV once it is established in the immune system however, he believes it is unlikely that VIP can be used as a cure.