Is germline targeting the future of HIV vaccine development?

Despite four decades of effort, traditional vaccine approaches have been unable to prevent HIV acquisition. Because the virus mutates rapidly and hides from the immune system, most experts think more sophisticated vaccine strategies will be necessary and that the research and development process will probably take years.

One novel strategy, known as germline targeting, uses a series of primer and booster vaccines to train B cells – the immune system’s antibody factories – to recognise HIV and produce broadly neutralising antibodies (bnAbs) that can deactivate the virus. To overcome HIV’s ability to hide, scientists create ‘immunogens’, or artificial constructs containing viral proteins designed to provoke a robust immune response. Seven research teams published results from early studies of this approach to coincide with HIV Vaccine Awareness Day on 18 May.

“Driving the process to fruition – the production of bnAbs that neutralise primary HIV-1 viruses – remains to be achieved,” Dr Rogier Sanders of Amsterdam University Medical Center and Dr John Moore of Weill Cornell Medicine wrote in a commentary accompanying the reports. “However, these studies provide valuable guidance as to what the next immunogens in the sequence might look like.”

A new type of vaccine

Traditional prevention vaccines teach the immune system to fight invaders it hasn’t encountered yet. B cells produce antibodies, which bind to foreign proteins called antigens and either neutralise the invader directly or call in additional immune defences. CD8 killer T cells destroy abnormal cells, such as cancerous or virus-infected cells. CD4 helper T cells coordinate the process. Once the threat has waned, a subset of memory B cells and T cells remain on guard, ready to fight the same invader in the future.

Many vaccines aim to replicate the natural process of developing immunity without the risk of becoming ill. They typically contain a weakened version or pieces of a virus or other pathogen that can be recognised by immune cells. To date, no traditional vaccine candidates have been able to provide adequate protection against HIV in large trials. One reason is that the vast majority of people do not develop strong natural immunity against HIV.

“A vaccine generally mimics the body’s natural immune reaction to a virus, which usually results in it being cleared,” according to Dr Anthony Fauci, former director of the US National Institute of Allergy and Infectious Diseases. “But with HIV, it doesn’t. So a vaccine has to elicit an immune response that’s better than nature, and that’s hard to do.”

(Read more about the search for HIV prevention vaccines and why they are so difficult to make.)

People with HIV usually do produce antibodies against the virus, but these mostly target specific parts that are highly variable, so they don’t recognise new viral mutations. What’s more, the conserved parts of the virus envelope that don’t change much as it evolves are well hidden and normally invisible to the immune system. Only around 15% of people with HIV naturally produce broadly neutralising antibodies that target these conserved parts and recognise multiple strains of HIV. Most people, however, do possess rare immature B cells that have the genetic capacity to make these powerful antibodies. 

Germline targeting uses a series of vaccines in a stepwise manner to encourage the development and multiplication of these specialised precursor B cells and then train them to produce bnAbs. First, a primer vaccine selectively recruits capable naive B cells, they migrate to germinal centres in lymph nodes to mature, and a sequence of boosters helps them learn to recognise the right targets and make effective bnAbs.

As Sanders and Moore describe it, “An initial germline-targeting immunogen primes desirable naive B cells, a suitably modified second immunogen ‘shapes’ the now memory B cells and then a final ‘polishing’ immunogen completes the bnAb maturation process.” (The shaping and polishing steps might actually require multiple vaccine iterations.)

In a natural immune response, bnAbs emerge slowly over time as the virus and host evolve together. Early antibodies drive viral mutation, and these escape viruses in turn trigger B cells to produce antibodies that match. The germline targeting approach aims to speed up this process. By utilising the messenger RNA (mRNA) technology used for COVID-19 vaccines, scientists can produce these tailored booster vaccines faster.

Germline targeting

In 2022, Professor William Schief of the Scripps Research Institute and colleagues described a novel immunogen dubbed eOD-GT8 60mer, which consists of 60 copies of an engineered version of HIV’s gp120 envelope protein fused to a virus-like lipid nanoparticle. The immunogen was designed to activate immature B cells that can learn to generate bnAbs similar to VRC01. Originally isolated from an individual who naturally controlled the virus, VRC01 targets HIV’s CD4 binding site.

“VRC01-class bnAbs offer several advantages for germline-targeting strategies, including that they have been isolated multiple times from people with HIV, which implies that the human immune system can reproducibly make such antibodies,” according to Sanders and Moore.

In the IAVI G001 trial, all but one of the 36 HIV-negative participants who received a vaccine containing eOD-GT8 60mer produced desired precursor B cell responses. After a booster, these cells made antibodies with greater affinity for the virus. These findings “demonstrate[d] for the first time that one can design a vaccine that elicits made-to-order antibodies in humans,” Schief said at the time. The following year, researchers reported that the vaccine also stimulated strong HIV-specific T-cell responses in most study participants.

Those findings showed that the GT8 60mer primer vaccine can kick-start the process of bnAb production. Now, two new papers describe the next steps. To shepherd maturing B cells toward producing the desired bnAbs, they are exposed to a series of booster immunogens that look more and more like natural HIV envelope proteins.

In the first study, published in Science Immunology, researchers at Scripps and the Ragon Institute tested a vaccine that contains mRNA encoding genetic instructions for eOD-GT8 60mer. They found that the initial vaccine primed B cells in mice, and three different booster immunogens encouraged these precursor B cells to mature and produce VRC01-like bnAbs.

In the second study, published in Science Translational Medicine, the Scripps team, working with scientists at the US National Institutes of Health’s Vaccine Research Center and vaccine-maker Moderna, tested the first booster immunogen, dubbed core-g28v2 60mer. Mice boosted with core-g28v2 60mer after an initial eOD-GT8 60mer primer produced bnAbs closer to VRC01 than mice that got a placebo booster. What’s more, the mRNA version of core-g28v2 60mer neutralised ‘pseudoviruses’ that are similar to HIV but missing a sugar molecule that hides the CD4 binding site.

A phase I clinical trial (IAVI G002; NCT05001373) of the eOD-GT8 60mer primer (which Moderna calls mRNA-1644) and the core-g28v2 60mer booster (mRNA-1644v2-core) is currently underway.

Two other studies, published in Science, described the development of a different immunogen, dubbed N332-GT5, designed to elicit the production of B cells capable of making another bnAb called BG18. Prior HIV prevention and treatment studies have shown than combining different bnAbs will likely be necessary to prevent viral escape.

One research team showed that an N332-GT5 primer vaccine activated precursor B cells in all eight vaccinated monkeys. The second team showed that adding one of two new booster immunogens (B11 or B16), especially if delivered via mRNA, drove further maturation of these B cells in mice, leading to bnAbs with increased affinity for HIV.

MPER vaccine

Meanwhile, researchers at the Duke Human Vaccine Institute reported on another vaccine candidate that targets a usually hidden part of HIV’s envelope that remains stable as the virus evolves. As HIV’s envelope proteins break apart in preparation for cell entry, the gp41 membrane proximal external region (MPER) is briefly exposed.

The HIV Vaccine Trials Network’s HVTN 133 trial evaluated an engineered immunogen consisting of peptides in a lipid nanoparticle designed to train B cells to generate bnAbs that recognise and block MPER.

At last year’s International AIDS Society Conference on HIV Science, Dr Wilton Williams reported that while most study participants had good immune responses, one person developed an anaphylactic reaction to the polyethylene glycol (PEG) in the vaccine. The study was stopped, and the researchers plan to substitute a PEG-free formulation.

Williams and colleagues further described the findings in Cell. When the trial was halted, 15 out of 20 healthy HIV-negative volunteers had received two of the four planned vaccine doses and five had received three doses. A majority had binding antibody responses to the MPER peptides targeted by the vaccine after just two jabs. After the second dose, the most potent bnAbs neutralised 15% of tier 2 HIV strains (virus that is harder to neutralise) and 35% of clade B strains (the most common HIV subtype in Europe and North America). Furthermore, the precursor B cells appeared to remain in a state of development, allowing them to evolve along with the virus.

“To get a broadly neutralizing antibody, a series of events needs to happen, and it typically takes several years post-infection,” Williams said in a Duke press release. “The challenge has always been to recreate the necessary events in a shorter space of time using a vaccine. It was very exciting to see that, with this vaccine molecule, we could actually get neutralising antibodies to emerge within weeks.”

In another pair of studies, published in Nature Immunology, the Scripps team and collaborators developed a priming immunogen designed to induce immature B cells to produce a specific MPER-targeted bnAb known as 10E8, which provides “exceptionally broad neutralization.” Unfortunately, the 10E8 binding site on HIV’s envelope is hidden in a crevice, making it hard for antibodies to reach.

To train bnAbs to reach the hidden region, the researchers created nanoparticle ‘scaffolds’ that mimic HIV’s natural structure. Vaccines that delivered protein nanoparticles triggered precursor B-cell responses in mice and monkeys, and mRNA nanoparticles also did so in mice. The same immunogens also induced precursor B cells that could produce another gp41-directed bNAb called LN01. 10E8 precursor B cells migrated to germinal centres in mice, but they were soon displaced by higher affinity B cell competitors. Only one MPER precursor clone was able to “close the affinity gap” and establish long-term germinal centre residency and maturation, according to the study authors.

Looking ahead

While these findings are promising, they are early steps in a long process. As earlier vaccine trials show, interventions that work in mice and monkeys do not necessarily translate to humans. Researchers have not yet tested whether the novel vaccine approaches protect animals exposed to HIV or its simian cousin SIV, much less whether they can prevent HIV acquisition at the population level. And if the vaccines do work in people, it is unknown how long protection might last.

What’s more, vaccine trials have become more challenging now that highly effective pre-exposure prophylaxis (PrEP) is widely available and more people are on effective antiretroviral treatment with an undetectable viral load, meaning they do not transmit the virus. Add to this the fact that complex vaccine regimens requiring multiple doses are unlikely to be feasible in the real world.

But while a widely accessible HIV prevention vaccine is a high bar, this research could pay off in other ways. Therapeutic vaccines that stimulate the production of bnAbs could potentially contribute to a functional cure, or long-term remission off antiretrovirals, in people living with HIV. And the knowledge gained will yield dividends beyond HIV.

“Although HIV-1 vaccine researchers have not yet succeeded in their goals, technical developments in this field have consistently had wider influences,” Sanders and Moore wrote. These studies “exemplify progress in the rational design of germline-targeting HIV-1 vaccines, and what is being learned will guide germline-targeting programs for inducing bnAbs against other human pathogens such as coronaviruses and influenza and hepatitis C viruses.”