Scientists have spent decades attempting to develop a vaccine for HIV, with limited success. Now, research published in Nature this week (September 21) suggests that the solution to achieving effective protection may partially lie in how the vaccine is delivered. By breaking the initial dose of a vaccine into multiple, escalating doses over the course of nearly two weeks, a team of researchers from the La Jolla Institute for Immunology was able to generate a longer-lasting immune response in rhesus macaques (Macaca mulatta) and higher levels of neutralizing antibodies following a booster dose when compared with a traditional, one-shot approach. 

There are still many steps to be taken before this can be translated into HIV treatment for humans, says Elizabeth Connick, who studies HIV pathogenesis and cure strategies at the University of Arizona College of Medicine and was not involved in this study. For example, she says it’s crucial to determine whether the neutralizing antibodies observed in vitro “actually protect people.” But so far, she adds, the potential clinical implications of this study are “very exciting,” and not only for HIV; the approach holds promise for the development of vaccines against other targets, too.

Previous research published in 2019 by the same team, led by La Jolla Institute for Immunology researcher Shane Crotty, found that rhesus monkeys that received a dose of HIV vaccine via a slow-release osmotic pump or by an initial escalating series of shots had a better immune response—for example, increased quantity and diversity of antibodies—than those that received the same dose in a single injection. “In the end, the best vaccines are the ones that actually can mimic what an actual infection looks like without making you sick, [and] slow delivery is probably better at that,” says Henry Sutton, also at La Jolla Institute for Immunology and a coauthor of the new study. Those earlier data, in which Sutton was not involved, showed that the immune response was still quite robust at the time the study ended, eight weeks after the first injection and six weeks after the last one. “The obvious question,” says Sutton, was what might happen if the experiment kept “going for another few months: How long would it take for that response to actually disappear?” 

In the end, the best vaccines are the ones that actually can mimic what an actual infection looks like without making you sick, [and] slow delivery is probably better at that.

—Henry Sutton, La Jolla Institute for Immunology

One of the key factors in triggering an effective immune response is training B cells to generate antibodies that bind and neutralize the pathogen in question. Once the vaccine antigen enters the body, B cells start to evolve through a process of random mutation and selection for cells that produce antibodies with better affinity for the antigen. This training takes place in structures called germinal centers that are transiently created in the lymph nodes nearest to the site of vaccination. Among the hypotheses derived from their earlier work, Crotty’s team wondered whether gradual antigen delivery favors the initial period of this evolution and whether it lengthens the lifespan of these training centers to give B cells more time to hone their antibodies and ultimately succeed against an elusive target such as HIV. 

See “Neutralizing HIV

To test this idea, the team now decided on a longer study period. They immunized 14 monkeys against the virus’s envelope protein in both their left and right thighs using three different strategies: Group 1 (six monkeys) received a conventional immunization in a single dose; its formulation included a classic adjuvant used in human vaccines called Alum. Groups 2 and 3 (four individuals each) received an escalating dose every other day over a period of 12 days; instead of Alum, the formulation included a new immune-stimulating adjuvant called saponin/MPLA nanoparticle. Finally, groups 1 and 2 received a booster as a single dose at week 10, while group 3 did so at week 30.

The immune response of those with the slow delivery was notably successful. In week 10, before any booster had been administered, the frequency of germinal center B cells binding to the HIV protein was 186 times higher in groups 2 and 3 than in group 1. Moreover, the B cells of group 3 monkeys (which waited more than six months before getting a booster) continued to have gene expression signatures that characterize active germinal centers, also showing improved affinity towards the target—likely as a result of the extended training period. This suggests that, even without any new antigen input, the bootcamp triggered by the initial vaccine in these monkeys was still working at least 191 days after the last shot.

University of Queensland immunologist Di Yu, who did not participate in this study, says that the earlier work by Crotty’s team in a way foreshadowed some of the new results, adding that “the really exciting part” is that they now used “the technology to step-by-step analyze what is happening in the immune system, rather than just [looking] into the outcome.” The team is able to demonstrate, using this strategy, “what we have hoped to see in a successful vaccine”—an ongoing and functional germinal center in which B cells continue to be trained and, thus, increase the antibodies’ affinity to the target, he says.

The longer spacing between the prime and the booster shots in group 3 also seemed to bear fruit. When Sutton and his colleagues tested the antibodies in the vaccinated and boosted monkeys’ sera against 12 different HIV variants in vitro, they found that the highest quality antibodies were those in group 3. While only one monkey in group 2 had the antibodies to neutralize more than half of the variants (ten), in group 3, the antibodies of three monkeys could neutralize eleven, ten, and eight variants, respectively. That a vaccine can generate antibodies to face different variants is generally desirable, but this is an even more pressing matter in the case of HIV, which mutates very rapidly. 

Connick says that it’s clear their strategy resulted in the production of broadly neutralizing antibodies, but she says it will be important to figure out how much of this successful outcome is related to the use of a different adjuvant and how much to the slow delivery. Sutton acknowledges that they can’t disentangle the role of both methodological aspects, but the team is currently aiming to do so in follow-up studies. Moreover, the experimental methodology and the results published in 2019 suggest that the role of the adjuvant may be important, but “we are fairly confident that the slow delivery is also playing a major role,” he writes in a follow-up email. 

Sutton further acknowledges that the strategy used in this new paper might not necessarily be protective against HIV—the monkeys were never infected with the virus, he notes. However, he and his colleagues are collaborating with other teams to combine their vaccination strategy with the design of specific proteins that elicit immune responses that are known to lead to broadly neutralizing antibodies. This new study on slow delivery is only a proof of concept that may be handy when designing vaccines against difficult targets, Sutton says. For instance, this model might also help in the development of a universal flu vaccine that provides protection against many different strains.