Retrovirologist Harriet Robinson of the University ofMassachusetts Medical School has reported that pure DNA shotfrom guns worked "far better" than traditional subunit vaccinesto protect mice from influenza. Even by conventionalintramuscular and intravenous needle injection and nasal dripthe stripped-down DNA effectively immunized rodents andchickens against lethal flu-virus challenges that killed controls.Robinson reported these gene vaccine trials in the current issueof The Proceedings of the National Academy of Sciences (PNAS)under the title, "DNA vaccines: Protective immunizations byparenteral, mucosal and gene-gun inoculations."
Now Robinson's laboratory is extending these influenza vaccineexperiments to other animal models, such as ferrets, and alsobroadening them from flu to SIV, the primate version of theAIDS virus. Her team has just inoculated a group of monkeyswith a gene primed to encode gp120, the glycoprotein surfaceantigen that is also the target epitope for much human AIDSvaccine research.
"The simian monkey trial, which we have already initiated, isnow going through the vaccination-boosting phase," JosephSantoro, a senior scientist in Robinson's lab, told BioWorld. "Wewill challenge the animals with SIV after a period of time." Hesaid he expects these trials to last at least a year.
According to Robinson's rationale, stripped-down plasmid DNAoutstrips purified proteins or viral vectors as a means ofexpressing only the immunizing target protein in host cells. Inher murine flu trials, that antigen gene encoded hemagglutinin,the H1 influenza surface glycoprotein. The straight shots ofgenetic material relied on the host cells to take up andtransfect this gene and manufacture its antigen, which wouldthen trigger the immune defenses against influenza.
Choosing a body's portal of entry for such a vaccine is amultiple trade-off. Skin and the mucosal lining of airway andgut, which also offer access to pathogens, are lined withlymphoid tissues that provide high levels of local immunesurveillance. But these routes are 100 to 1,000 times lesscapable of transfecting DNA than muscle. And skin andperitoneum are favorite sites for testing immunizing antigens.
The Massachusetts team tried all of the above, plus biolisticpropulsion of DNA-coated gold beads into the epithelium. AsSantoro explained the process, the part
f mylar, which is then accelerated by electric spark dischargetoward a mesh screen. This stops the mylar, but the beadshurtle on through the mesh and penetrate an anesthetizedmouse's shaved skin surface.
"The DNA gun is by far the most efficient at delivering DNAvaccine into a tissue and achieving protection with less DNAthan by other routes," said Santoro. Specifically, twoimmunizations with beads carrying only 0.4 micrograms eachof DNA protected 95 percent of their target rodents against fluchallenge. At 0.04 mg, survival was still about 65 percent.Overall, gene-gun DNA delivery achieved protection with 250to 2,500 times less DNA than direct inoculations of purifiedDNA in saline, PNAS reported.
"We're very excited about this new technology, which has greatpotential application to developing subunit vaccines againstother viruses and pathogens," Santoro said.
Although the gene gun scored a bull's eye in efficienttransfection and vaccination, most of the customary inoculationpathways also confirmed the effectiveness of antigen-specificplasmid DNA. In these flu trials, mice 6 to 8 weeks old receivedtwo injections of the vaccine gene four weeks apart to the vein,peritoneum muscle, airway, skin or subcutaneously. Ten dayslater, they and control animals inhaled influenza virus solutions10 to 100 times the median lethal dose.
This challenge virus replicated in the airways and caused deathin the controls from pneumonia in a week or two -- but not in95 percent of mice (39 of 41) vaccinated intravenously,intraperitoneally or intramuscularly with high-dose DNA. Atlower doses survival declined. Of 24 challenged control mice,only three lived.
Also showing similar, though lower, dose-related survivalpatterns were 224 chickens vaccinated with genes for otherinfluenza epitopes.
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.