ST. LOUIS—One of the human body’s first responses to a viral infection is to make and release signaling proteins called interferons, which amplify the immune system’s response to viruses. Over time, many viruses have evolved to undermine interferon’s immune-boosting signal, and a paper published today in the journal Cell Host & Microbe describes a mechanism unique to the Ebola virus that defeats attempts by interferon to block viral reproduction in infected cells.
The newly published study explains for the first time how the production by the virus of a protein called Ebola viral protein 24 (eVP24) stops the interferon-based signals from ramping up immune defenses. With the body’s first response disabled, the virus is free to mass-produce itself and trigger the too-large immune response that damages organs and often becomes deadly as part of Ebola virus disease.
The study was led by scientists from Washington University School of Medicine in St. Louis in collaboration with researchers from the Icahn School of Medicine at Mount Sinai and the University of Texas Southwestern Medical Center.
“Our study is the first to show how Ebola viral protein 24 defeats the signal sent by interferons, the key signaling molecules in the body’s early response to Ebola virus infection,” said Dr. Christopher F. Basler, a professor of microbiology at the Icahn School of Medicine and an author of the newly published paper. “These newfound details of Ebola biology are already serving as the foundation of a new drug development effort, albeit in its earliest stages,” added Basler, who is also a researcher within the Mount Sinai Global Health and Emerging Pathogens Institute.
“We’ve known for a long time that infection with Ebola virus obstructs an important arm in our immune system that is activated by molecules called interferons,” said senior author Dr. Gaya Amarasinghe, an assistant professor of pathology and immunology at Washington University. “By determining the structure of an eVP24 in complex with a cellular transporter, we learned how Ebola does this.”
Amarasinghe’s lab solved the VP24-KPNA5 crystal structure. This showed that VP24 occupies the region of KPNA5 that also interacts with tyrosine-phosphorylated STAT1. This seemed to explain how VP24 blocks interferon responses. To prove this point, the Amarasinghe lab identified amino acid residues critical for the interaction and made mutant VP24 or KNPA5s. The Basler lab expressed wild-type and mutant VP24s in cell-based assays to confirm the hypothesis that the interaction is required for the block of STAT1 nuclear import and initiation of an interferon response. The Amarasinghe lab also performed in-vitro assays to show that the KPNA5 can, when bound to VP24, still bind classical nuclear localization signals. This latter result suggests that the virus very cleverly blocks STAT1 nuclear import but leaves other aspects of nuclear import functional.
The Basler and Amarasinghe labs have a longstanding and very productive collaboration, notes Basler, who has been studying immune evasion mechanisms of Ebola and Marburg viruses since 2000. Since 2009, the two groups have co-authored 19 papers. “Therefore, there was a very strong relationship between the groups going into the present study,” Basler tells DDNews. His focus is on the cell-based signaling pathway and virology aspects of the studies. Amarasinghe focuses on biochemical, biophysical and structural aspects of the study. The current study was driven by the Amarasinghe lab’s success in determining the crystal structure of VP24 in complex with KPNA5. The Basler lab had shown in 2006 that this interaction correlated with the ability of Ebola virus to block interferon responses. The Basler lab performed the cell-based assays associated with the functional characterization of the complex. The Amarasinghe lab did all the structural, in-vitro biochemical and biophysical studies
In 2006, Basler and colleagues found that the Ebola virus suppresses the human immune response through eVP24, but not how. Through a combination of molecular biology techniques, cell studies and tests that reveal protein structures, the current team led by Amarasinghe defined the molecular basis for how eVP24 achieves this suppression.
Understanding exactly how the Ebola virus targets the interferon pathway could help guide drug development moving forward. Basler describes how it may be possible to find an antibody or molecule that interferes with eVP24, or that works around its competition with STAT1, such that treatment of patients with extra interferon, long used against the hepatitis C virus for instance, might become useful against the Ebola virus.
“We feel the urgency of the present situation, but still must do the careful research to ensure that any early drug candidates against the Ebola virus are proven to be safe, effective and ready for use in future outbreaks,” said Basler, who is also principal investigator of an National Institutes of Health (NIH)-funded Center of Excellence for Translational Research focused on developing drugs to treat Ebola virus infections.
Given the deadly nature of the Ebola virus, Basler points out that the present study did not use live Ebola virus, so it did not require biocontainment. However, Basler is principal investigator of two NIH-funded Center Grants in which Amarasinghe also plays a key role. These centers also have as collaborators Dr. Alexander Bukreyev and Thomas Geisbert at University of Texas Medical Branch, who do BSL4 work with Ebola virus. These investigators will help with the next steps in these studies.