Forty years into the HIV pandemic, the virus has defeated every vaccine candidate thrown at it. A study published today in Nature may have finally found a crack in the armor.

Researchers from Karolinska Institutet, The Scripps Research Institute, and Emory University report that a new vaccination strategy generated broadly cross-neutralizing antibodies in non-human primates — the kind of immune response that, until now, had only been observed in a small fraction of people after years of living with HIV. Every animal that received the liposome-based vaccine produced antibodies targeting the apex of HIV’s envelope spike, a conserved region shared across diverse viral strains.

The results do not mean a human vaccine is imminent. But they represent something the field has never achieved: a vaccination protocol that reliably produces the specific class of antibodies long considered the holy grail of HIV research.

Why the apex matters

HIV is a nightmare for vaccine design. It mutates furiously, cloaks its critical structures in host-derived sugar molecules, and presents a moving target to the immune system. The sole target for neutralizing antibodies on the virus’s surface is the trimeric envelope glycoprotein, or Env spike.

Broadly neutralizing antibodies — bNAbs — can cut through those defenses. They arise infrequently in people living with HIV, typically after years of chronic infection and co-evolution between the virus and the immune system. Among these, antibodies targeting the apex of the Env spike are particularly attractive for vaccine design because they often require less somatic hypermutation — the process by which antibodies mature and sharpen their targeting — than other bNAbs to gain broad neutralizing activity.

The problem has been getting a vaccine to produce them. Replicating virus stimulates the immune system in ways that recombinant protein vaccines simply do not. Decades of attempts using soluble Env proteins have failed to elicit these apex-directed responses.

Fat bubbles with a viral disguise

The breakthrough in this study came from how the vaccine was packaged.

Rather than injecting free-floating Env proteins, the researchers covalently attached multiple copies of stabilized HIV Env trimers to synthetic liposomes — essentially, tiny fat-based spheres studded with viral proteins. This “multivalent display” means the immune system encounters a dense, repeating array of the target, mimicking how a real virus presents itself to B cells.

As Mónika Ádori, a researcher at Karolinska Institutet and co-author of the study, explained: the approach “enabled multiple copies of the virus’s surface protein to be presented to the immune system simultaneously, thereby strengthening the immune response.”

The team primed rhesus macaques with Q23 strain-derived trimers arrayed on liposomes, then sequentially boosted them with heterologous trimers from different HIV strains — ZM233, WITO, 001428, RW020, and CH119. The idea was to train the immune system to recognize features shared across variants, not just the specifics of one strain.

The numbers

Twelve macaques were immunized: six with soluble trimers and six with the trimer-liposome formulation. Both groups received the same adjuvant and the same sequential boosting regimen.

The liposome group outperformed the soluble group at every stage. After the second immunization, electron microscopy-based polyclonal epitope mapping detected apex-directed antibody densities in all six liposome-immunized animals, compared with four of six in the soluble group.

After six immunizations, serum antibodies from all six liposome-vaccinated animals neutralized more than 49% of viruses in a 67-virus panel spanning multiple HIV clades. Two animals — designated Q8 and Q12 — neutralized 70% and 64% of the panel, respectively.

High-resolution cryo-electron microscopy structures of monoclonal antibodies isolated from four different macaques showed that they bind the Env apex in a manner strikingly similar to PG9, a well-known broadly neutralizing antibody isolated from a human infected with HIV.

The long road from macaques to humans

These are the strongest vaccination-elicited broadly neutralizing antibody responses reported in any animal model. But significant hurdles remain.

The animals received six immunizations over roughly 18 months with a complex sequence of different Env trimers — a regimen that would be difficult to implement at scale. The immune system of a rhesus macaque is not identical to a human’s. And neutralization in a laboratory panel of pseudoviruses, while an essential benchmark, is not the same as protection against real-world HIV transmission.

Gunilla Karlsson Hedestam, professor at Karolinska Institutet and a shared senior author of the study, noted that “discussions are now underway about how the strategy could be taken forward into clinical studies.”

That is research-speak for: this is promising, but there is hard work ahead. Still, after 40 years of failure, a vaccine strategy that produces broadly neutralizing antibodies in every single vaccinated animal is not a incremental step. It is a genuine proof of concept the field has never had before.

Sources