A multi-center research team including scientists from the Vanderbilt Vaccine Center has come up with a clever “Trojan Horse” strategy for thwarting the highly lethal Ebola virus.
Using “bispecific” antibodies — two monoclonal antibodies combined into a single package — they first tricked the virus into revealing a normally hidden binding site required for infection. Then in a mouse model, they blocked the site, fully protecting the animals from Ebola infection.
Their findings, reported in this week’s Science magazine, suggest that this two-step, “deliver-and-block” strategy can provide broad protection against Ebola and other members of its hemorrhagic filovirus family, including the Marburg virus.
“We were intrigued to find this remarkable antibody that has the capacity to inhibit both Marburg and Ebola viruses,” said James Crowe Jr., M.D., Ann Scott Carell Professor in the Vanderbilt University School of Medicine and director of the Vanderbilt Vaccine Center at Vanderbilt University Medical Center.
“The team’s feat of delivering the antibody into cells using creative engineering tricks so that it can kill Ebola inside cells is very exciting,” Crowe said.
This advance is only the latest in a string of fundamental discoveries made during the past decade by a far-flung group of researchers including Crowe and four other corresponding authors of the paper.
The four are Kartik Chandran, Ph.D., and Jonathan Lai, Ph.D., of Albert Einstein College of Medicine in New York, John Dye, Ph.D., of the U.S. Army Medical Research Institute of Infectious Diseases in Fort Detrick, Maryland, and Javad Aman, Ph.D., of Integrated Biotherapeutics in Gaithersburg, Maryland.
Like other viruses, Ebola must “hijack” factors in the cells it infects to make copies of itself. As a first step, the virus enters a vesicle called an endosome inside the cell. There it commandeers two cellular enzymes called proteases to cut a sugar-bearing glycoprotein on its surface in two.
Cleavage of the glycoprotein reveals a previously hidden receptor-binding site that attaches to another cellular factor, a cholesterol transporter protein called Niemann–Pick C1 or NPC1. This step is essential for infection to occur.
Mutations in the NPC1 gene result in an abnormal protein that causes the rare lipid storage disorder Niemann-Pick type C disease. While patients with this disease are often quite ill, their abnormal NPC1 protein also renders them resistant to infection by Ebola and the related Marburg virus.
Last year, Crowe, Vanderbilt graduate student Andrew Flyak and colleagues at The Scripps Research Institute in La Jolla, California, reported that a human survivor of a severe Marburg infection had neutralizing antibodies that recognized and blocked the NPC1 binding site in Marburg virus
These antibodies also could bind to the Ebola virus, but only to the form of the virus inside cells.
Crowe and Flyak followed up that finding by generating a “monoclonal” antibody, called MR72, which specifically recognized and could block the NPC1 binding site. To actually prevent Ebola virus infection, however, they’d have to get the antibody into the endosome inside the cell where the action is taking place.
To do that, the researchers fused MR72 to another antibody, called FVM09, which recognizes and attaches to the Ebola glycoprotein before it is cut in two. The result was an immunological “Trojan horse.” Once the virus brought its antibody cargo into the endosome, MR72 went to work, and blocked infection.
“This Trojan horse bispecific antibody approach may also find utility against other viral pathogens known to use intracellular receptors,” they concluded.
Other contributors to the current study were Erica Ollmann Saphire, Ph.D., at Scripps and Zachary Bornholdt, Ph.D., now at Mapp Pharmaceutical in San Diego. The study was supported in part by National Institutes of Health grants AI109762, AI088027 and AI122403.
