Science Editor

By using three separate immune stimulants and packaging them in a nanoparticle that mimics a live virus, researchers at Emory University were able to induce immune responses to flu virus, which were much stronger than those induced by the current vaccines – and that lasted long enough to rival the immune responses induced by live viruses.

The new findings came out of studies of the yellow fever vaccine, which is extremely long-lasting: One shot can confer lifelong immunity. Many other vaccines, in contrast, wane over time, needing periodic booster shots to stay effective.

"That's always been a mystery: How do you get lifelong immunity?" senior author Bali Pulendran told BioWorld Today. Pulendran is professor of pathology and laboratory medicine at Emory University School of Medicine and a researcher at Yerkes National Primate Research Center, and the senior author of the paper, which was published in the Feb. 24, 2011, issue of Nature.

The longest-lasting vaccines have two characteristics: They are live viral vaccines, and they stimulate Toll-like receptors (TLR) of the innate immune system.

So, Pulendran said, his team wondered: "Is it possible to design a particle that would look like a live virus?"

Several years ago, they showed that the answer to that question was yes. They managed to come up with a nanoparticle that was the size of a virus and immunological characteristics.

In the study now published in Nature, the team used this nanoparticle vaccine to deliver a combination of influenza antigen and ligands that stimulate two specific TLRs, TLR4 and TLR7.

They found that with their approach, they could induce immune responses that were both more intense and longer lasting than those induced by classical vaccines.

The team tested flu vaccines with the new system in both mice and monkeys. In mice, the vaccine protected the animals from both avian and swine flu, and induced both antibodies and T-cell responses that lasted for more than 18 months – which is pretty much the life span of a mouse. The team also compared the magnitude of the immune response to antigen delivered with and without the TLR stimulation and nanoparticle delivery, and found that their system increased the monkey's immune responses by at least fivefold.

Surprisingly, though the TLRs are part of the innate immune system, which is part of the body's early response to infection, the vaccine did not stimulate the early immune response any more than a comparison vaccine.

"I would have predicted the opposite," Pulendran said, adding that stimulating the innate immune system should create a greater early response but not necessary affect how long that response stays strong. But in his team's experiments, "the innate system programs the adaptive responses to become long-lived."

Pulendran said that for the long-lasting immune responses there is "a strict requirement for both TLR ligands to work together. But putting them into a nanoparticle certainly helps.

"We spent quite some time trying to develop TLR ligands without the nanoparticles." In that case, "the magnitude of the response is considerably lower." He contended that the nanoparticles work by focusing the delivery of the TLR ligands to the receptors.

Pulendran and his team are currently doing additional primate studies in malaria, HIV and dengue fever, and "hopeful" that positive results in those experiments could lead to Phase I studies. Both TLR ligands, as well as the material the nanoparticles are made of, are already individually FDA-approved, which will make translation to the clinic easier.

But Pulendran said that the work also points to "fascinating basic biology that we would like to pursue."

For example, another surprising finding was that to elicit a maximum response, the antigen had to be delivered separately from the TLR ligands. "This is quite an enigma. . . . We don't really understand why."

That said, Pulendran noted, the separate particle approach could make it easier to make clinical use of his team's discoveries, by enabling the production of what he called a "generic adjuvant" consisting only of the TLR activators. "If tomorrow there is a new pathogen, and you have to expedite vaccine design, you could couple it to the antigen of your choice."