Scientists at the University of North Carolina, Chapel Hill say they have found a new target for human antibodies that could hold the key to a vaccine for a globally widespread mosquito-borne disease.
Using an experimental technique new to the dengue field, the labs of Ralph Baric, Ph.D., and Aravinda de Silva, Ph.D., showed that a molecular hinge where two regions of a protein connect is where natural human antibodies attach to dengue 3 to disable it. Their study (“Dengue virus envelope protein domain I/II hinge determines long-lived serotype-specific dengue immunity finding”), published in the Proceedings of the National Academy of Sciences, illustrates that after primary infection most human antibodies that neutralize the virus bind to the hinge region.
It's the first study to demonstrate how these binding sites–composed of just 25 amino acids–can be genetically swapped out for amino acids from another dengue type without disrupting the integrity of the virus.
“This gives us a lot of insight into how human antibodies work,” said Dr. de Silva, a professor of microbiology and immunology in the UNC School of Medicine. “And there could be a lot of translational aspects to this; it could lead to a new way to create vaccines for other diseases.”
“The four dengue virus (DENV) serotypes, DENV-1, -2, -3, and -4, are endemic throughout tropical and subtropical regions of the world, with an estimated 390 million acute infections annually,” write the investigators. “Infection confers long-term protective immunity against the infecting serotype, but secondary infection with a different serotype carries a greater risk of potentially fatal severe dengue disease, including dengue hemorrhagic fever and dengue shock syndrome. The single most effective measure to control this threat to global health is a tetravalent DENV vaccine.”
Making a truly effective dengue vaccine has proven difficult because of a phenomenon called antibody-dependent enhancement. People infected with one type of dengue usually develop a natural immune response that rids the body of the virus and prevents a repeat infection of that same virus type. But if those people are infected with a second type of dengue, the virus is enhanced because of that first immune response.
To prove the importance of the hinge, Dr. Baric was able to pinpoint the structurally complex, nonlinear 25-amino-acid hinge domain and remove it from dengue 3 particles. His group, led by William Messer, Ph.D., then developed strategies to recover dengue viruses from DNA clones and replace the dengue 3 hinge with a replicated 25-amino-acid chain from dengue 4. Essentially, Dr. Baric turned dengue 3 into dengue 4.
The genetically mutated virus survived and grew in cell cultures and in primates. Then the researchers exposed the mutant virus to dengue 3 antibodies, which typically bind to dengue 3. But they had no effect on the genetically modified dengue. They then showed in cell lines that the virus could be neutralized by antibodies directed against dengue 4. In collaboration with researchers at the University of Puerto Rico, the de Silva-Baric team was able to show that the new virus infected primates, which developed antibodies against dengue 4.
“These results amount to a paradigm shift,” Baric said. De Silva added, “This told us that the epitope we thought was important was indeed the main site for antibody binding. If antibodies had been able to bind to other sites on the virus, then we would have seen a small drop in protection against dengue 3.”
Some antibodies would have bound to those other sites and offered some level of protection. “Instead, we saw a complete loss of protection.”
If Drs. De Silva and Baric, who are conducting similar experiments with dengue 1 and 3, can isolate the major epitopes for each dengue type, then they could potentially genetically modify a virus with all four epitopes; this could lead to a vaccine against all four types of dengue virus.