The rabies virus kills 59,000 people each year, many of them children. Some victims, especially children, do not realize they have been exposed to danger until it is too late. For others, intensive rabies treatment is out of the question: treatment is not universally available, and the average cost of $3,800 places an unimaginable economic burden on most people around the world.
Rabies vaccines are much cheaper and easier to administer than treatments. But these vaccines also have a major disadvantage:
“Vaccinations against rabies do not offer lifelong protection. You should have your pets vaccinated every year for three years,” says LJI Professor Erica Ollmann Saphire, Ph.D. “Currently, human and pet rabies vaccines are made from killed viruses. But this process of inactivation can distort the molecules – hence these vaccines don’t present the immune system with the correct shape. If we made a better vaccine, better designed and better structured, would immunity last longer? »
Saphire and his team, in collaboration with a team led by Hervé Bourhy, Ph.D., at the Institut Pasteur may have discovered the path to better vaccine design. In a new study published in Scientists are progressingResearchers share one of the first high-resolution glimpses of the rabies virus glycoprotein in its vulnerable ‘trimeric’ form.
“Rabies glycoprotein is the only protein that rabies expresses on its surface, meaning it will be the primary target for neutralizing antibodies during infection,” says Heather Callaway, Ph.D., LJI postdoctoral fellow, who is a researcher of the study. first author.
“Rabies is the deadliest virus we know. It’s such a part of our history – we’ve lived with its spectrum for hundreds of years,” adds Saphire, who is also President and CEO of LJI. “Nevertheless, scientists have never observed the organization of its surface molecule. Understanding this structure is important for designing more effective vaccines and treatments — and understanding how rabies and other similar viruses enter cells. »
Rage the shapeshifter
Scientists aren’t exactly sure why rabies vaccines don’t offer long-term protection, but they do know that its shape-shifting proteins pose a problem.
Like a Swiss army knife, the rabies glycoprotein has sequences that unfold and fold up when needed. The glycoprotein can switch back and forth between prefusion forms (prior to fusion with a host cell) and postfusion forms. It can also collapse from a trimeric structure (where three copies come together in a bundle) to a monomer (one copy by itself).
This shapeshifting gives the rage a kind of camouflage cloak. Human antibodies are built to recognize a unique site on a protein. You can’t track when a protein changes to mask or displace these sites.
The new study gives scientists a critical picture of the correct shape of glycoproteins to target for antibody protection.
Finally capture the glycoprotein
Callaway worked for three years to stabilize and freeze the rabies glycoprotein in its trimeric form. This “prefusion” form is the form that the glycoprotein takes before it infects human cells.
Callaway combined the glycoprotein with a human antibody, which helped her identify a site where the viral structure is vulnerable to antibody attack. The researchers then took a 3D image of the glycoprotein using state-of-the-art cryo-electron microscopy equipment at LJI.
The new 3D structure highlights several key features that the researchers had not previously seen. Importantly, the structure reveals two key structural elements of the virus, called fusion peptides, as they occur in real life. These two sequences connect the lower part of the glycoprotein to the viral membrane, but protrude into the target cell upon infection. It is very difficult to get a stable picture of these sequences. In fact, other rabies researchers had to cut them up to image the glycoprotein.
Callaway solved this problem by trapping the rabies glycoprotein in detergent molecules. “This allowed us to see how the fusion sequences are attached before they detach during infection,” says Saphire.
Now that scientists have a clear view of this viral structure, they can better design vaccines that tell the body how to make antibodies against the virus.
“Rather than being exposed to more than four different forms of protein, your immune system should really only see one — the good one,” says Callaway. “It could lead to a better vaccine. »
Prevent a family of viruses
Saphire hopes stronger and broader immunity could help people who come into regular contact with animals, such as veterinarians and wildlife carers, as well as the billions of people who might accidentally come into contact with a rabid animal. Endemic to every continent except Antarctica, rabies infects many species including dogs, raccoons, bats and skunks.
This new work could also open the door to a vaccine to protect against the entire Lyssavirus genus, which includes rabies and similar viruses that can spread between humans and other mammals.
The next step in this work is to acquire more images of the rabies virus and its relatives with neutralizing antibodies. According to Callaway, scientists are working to resolve several of these structures that could uncover antibody targets that lyssaviruses share.
“Because we didn’t have these rabies virus structures in this conformational state before, it was difficult to design a broad-spectrum vaccine,” says Callaway.
Other authors of the Structure of the Rabies Virus Glycoprotein Trimer Bound to a Pre-Fusion Specific Neutralizing Antibody study are Dawid Zyla, Florence Larrous, Guilherme Dias de Melo, Kathryn M. Hastie, Ruben Diaz Avalos, Alyssa Agarwal, and David Corti.
This study was supported by the National Institutes of Health (Grants 5T32AI07244-36 and 5F32AI147531-03) and an Early Postdoctoral Mobility Fellowship from the Swiss National Science Foundation (P2EZP3_195680). A portion of this research was supported by NIH grant U24GM129547 and was conducted at OHSU’s PNCC 742 and is accessible through EMSL (grid.436923.9), a DOE Office of Science user facility sponsored by the Office of Biological Research and Environmental. Confocal microscopy on the Zeiss LSM 880 was supported by NIH Equipment Grant 745 S10OD021831.