AIDSWEEKLY Plus; Monday, February 15, 1999
Daniel J. DeNoon, Senior Editor

Recombinant adeno-associated virus (rAAV) is already known by gene-therapy researchers, who are exploring the use of its unique properties as a DNA delivery vehicle. Now vaccinologists hope to take advantage of AAV's unique properties in the quest for today's Holy Grail: an HIV vaccine.

To date, no experimental AIDS vaccine has succeeded in the induction of long-lasting HIV specific humoral responses without repeated boosting. But a preliminary experiment with macaque monkeys has shown that a single inoculation with rAAV carrying the HIV gp120 envelope glycoprotein gene induced a gp120-specific antibody response that was maintained for seven months.

Unlike naked DNA, which is only transiently expressed, DNA carried by rAAV is integrated into host cells and can be expressed for extended periods of time.

"Clearly there is a dramatic difference between the way AAV carries DNA to a cell and the way a plasmid does it," said Philip Johnson of Ohio State University, Columbus.

Johnson spoke in a symposium presentation to the 6th Conference on Retroviruses and Opportunistic Infections, held January 31-February 4, 1999, in Chicago, Illinois.

AAV is a nonpathogenic parvovirus originally discovered in the mid-1980s as a contaminant of laboratory cultures. The virus can integrate itself into host cells, but requires superinfection with another virus - usually adenovirus - to replicate. AAV is endemic in human populations, and up to 70 percent of people over the age of 10 years are seropositive for at least one of the three circulating serotypes.

The virus normally contains two genes. But Johnson and co-workers found that both genes could be deleted, leaving a virtually empty vector with a maximum 4.8 kb DNA-carrying capacity.

"The only thing that's left of the vector is its terminal repeats," Johnson said. "It's hard to call it a virus any more because it carries no viral genes."

The terminal repeat sequences are potent transducers of human cells with a wide cell-type tropism. They are also powerful promoters of gene expression.

To create an rAAV vaccine, Johnson and colleagues invented a technique whereby derived cell lines contain all the necessary elements.

"Just infect them with adenovirus and rAAV pops out," Johnson said. "Today we work with essentially pharmacologically pure particles."

Several features make the recombinant particles attractive vaccine vectors:

• They have broad cell tropism. In animal experiments, DNA carried by rAAV has been expressed in muscle, brain, and liver tissues.

• They are extraordinarily heat stabile, requiring no cold chain for storage. "You could put this in your backpack and take it to developing countries," Johnson said.

• Because rAAV is integrated into host cells, antigen is persistently expressed. This may be an important feature for HIV vaccines, which may require continual stimulation of cellular immune responses.

• Endogenous synthesis of antigen within cells may permit stimulation of antiviral cytotoxic lymphocyte (CTL) responses.

• rAAV vectors can be tested in primate models of AIDS.

Although rAAV induced HIV specific antibody responses in macaques, it also appeared able to stimulate CTL. One animal inoculated with an rAAV/SIV construct had clear SIV specific CTL responses nine months after a single inoculation.

Currently Johnson and colleagues are working with rAAV admixtures encoding HIV Env, Gag, Pol, and soluble Env proteins. Their early experiments suggest that intramuscular injections give the best results in nonhuman primates.

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