Diagnostic tests based on fragments of antibodies offer a route to detecting both known and yet-undiscovered species of Ebola virus, according to a new study. Researchers developed these single-domain antibodies, known as nanobodies, against five species of Ebola and used them to detect a sixth species that was unknown when they began their experiments 6 years ago (ACS Infect. Dis. 2022, DOI: 10.1021/acsinfecdis.1c00478). Universal tests such as these could one day help control disease outbreaks and offer quicker diagnosis of infections.
Five different species of Ebola emerged at different times between 1976 and 2007 and caused outbreaks in different regions. Vaccines and treatments have been developed for the Zaire ebolavirus, and they aren’t useful against the others, says Daniel Bausch, director of emerging threats and global health security at the nonprofit Foundation for Innovative New Diagnostics, who was not involved in the study. “This approach of trying to find conserved antibodies that cut across various different species has a lot of utility.”
To develop the diagnostic test, Andrew Hayhurst and Laura Jo Sherwood of the Texas Biomedical Research Institute homed in on a nucleoprotein that is produced by all known species of the virus. Research on therapies has focused on blocking a glycoprotein on the virus’s surface. The glycoprotein target is “good for therapeutics but highly variable,” Hayhurst says. “We weren’t interested in neutralizing the virus but detecting it.”
The team purified nucleoprotein molecules from the five species, injected a pair of llamas with the proteins, and isolated and sequenced the antibodies the animals produced, which tend to be smaller than human versions. The researchers then cloned the antibody fragments that bound to the viral proteins to produce libraries of nanobodies. By screening these libraries, they identified two nanobodies that strongly bound to all five nucleoproteins. Compared with full-size antibodies, nanobodies are easier and less expensive to produce in large quantities and can be stored and used at a wide range of temperatures, making them ideal for handheld diagnostics in remote settings.
The researchers fused the nanobodies to an enzyme that triggers a fluorescence reaction when bound to the nucleoprotein. When treated with this construct, cells expressing one of the five Ebola nucleoproteins glowed green. The assay also detected a sixth species named Bombali ebolavirus, identified in bats in 2016 in Sierra Leone, as well as it did Zaire, the best characterized species thus far. The data suggest the test could potentially identify still-unknown Ebola viruses when they emerge. Although Bombali has not caused human disease yet, people in the region encounter bats frequently, creating the potential for spillover into humans.
Virus nucleoproteins are not usually a target for developing treatments, but future experiments could test whether these nanobodies could be further engineered to produce antiviral therapies in addition to diagnostics, says virologist Nicholas Wu of the University of Illinois at Urbana-Champaign, who was not involved in the work. The study is “robust and comprehensive,” he says. “A major part of developing such a test is developing the right reagents. . . . Now, it’s just a question of deploying it.”
This story was updated on Jan. 28, 2022, to indicate that the cells treated with the nanobody-enzyme construct are not infected with Ebola species but are engineered to express Ebola nucleoproteins.
This story was updated on Jan. 25, 2022, to add scale information to the image caption and to add that the researchers began their nanobody experiments 6 years ago.