Reducing the knowledge gap on filovirus antagonism of human innate immune responses

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Standardized in vitro system shines a spotlight onto interferon antagonist proteins from both well-known and lesser-appreciated ebolavirus and marburgvirus species

Text by Jonathan Guito

Filoviruses, the virus family that includes ebolaviruses and marburgviruses, are among the deadliest known pathogens. These viruses block host innate immune responses via three viral interferon antagonist proteins linked to disease severity. Understanding antagonist functions and designing therapies against them has been studied in many labs. However, most antagonist research has focused on only Ebola and Marburg viruses, leaving the other five known pathogenic filoviruses neglected. One ebolavirus, Reston virus, kills non-human primates but hasn’t yet caused disease in humans, and no one quite knows why.

In our study, we directly compare the three antagonists VP35, VP40 and VP24 from all known ebolaviruses and marburgviruses side-by-side using standardized vectors and reporters. This analysis shows differential antagonist potencies that may explain why some viruses are more pathogenic in primates than others, findings that may guide future antiviral research.

Our story was borne from an unusual reservoir of inspiration: a bat reservoir, in fact.

Bats are believed to be filovirus hosts. Egyptian rousette bats were recently identified by our lab and others as a natural reservoir of marburgviruses. Unlike primates, Marburg virus-infected bats remain clinically healthy. To understand why, we initiated comparative immune response studies in vitro between bats and humans. Unfortunately, effective tools to study bats were severely limited, and rousette cell lines proved resistant to transfection. We soon became awash with antagonist vectors, but had nothing to transfect except our “control” human cells.

With filovirus antagonist research in its second decade, a screen of additional antagonists in human cells was not expected to be a bastion of novelty. Even Reston virus, the presumed outlier, previously displayed antagonism. However, antagonists had always been investigated piecemeal, often in different systems, and relative activities across genera were unknown. Therefore, we forged ahead, in human cells, thinking a standardized filovirus antagonism study would be valuable.

I’ll never forget that first full-scale reporter assay. The novel elements within our story immediately took shape. VP35 proteins blocked interferon induction, as expected, but so did some VP40 and VP24 proteins. A few ebolavirus VP40 proteins showed inhibitory activity. And surprisingly, Reston virus VP24 behaved as the putative outlier many had long speculated, perhaps a clue to the virus’s “asymptomatic” phenotype in humans.

“Cautiously dumbstruck” is a good descriptor for that moment.

This project reminded me of an important lesson: sometimes in science, as in life, a door slammed shut can open a window full of hidden intrigue.

Figure legend

Filovirus antagonists – VP35 and VP24 for Ebola virus (EBOV), VP35 and VP40 for Marburg virus (MARV) – block human interferon (IFN) responses: VP35 blocks induction, while VP24 and VP40 are known to block signaling (blue arrows). Antagonists of many filoviruses – Sudan (SUDV), Taï Forest (TAFV), Bundibugyo (BDBV), Reston, (RESTV) and Ravn (RAVV) – remain incompletely or never studied. Using a standardized vector library for reporter assays measuring different IFN response pathways, relative antagonist potencies across all known ebolavirus and marburgvirus species were determined (red/green gradients). Red arrows highlight putative novel activities by ebolavirus VP24 and some VP40 proteins.

Introducing the author

virology-highlights-author-photo-updated

Jonathan Guito is an ORISE fellow within the Viral Special Pathogens Branch at the Centers for Disease Control and Prevention in Atlanta, GA, USA. He works with Dr. Jon Towner and Dr. César Albariño and currently focuses on innate immune responses to filovirus infections. Jonathan just completed his two-year tenure as an ASM/CDC fellow, during which time this reported work was conducted. He received his PhD from Rutgers University’s Biomedical and Health Sciences in Newark, NJ, USA, where he engaged in research on mechanisms of Kaposi’s sarcoma associated-herpesvirus (KSHV) lytic reactivation under the mentorship of Dr. David Lukac.

About the research

Novel activities by ebolavirus and marburgvirus interferon antagonists revealed using a standardized in vitro reporter system

Jonathan C. Guito, César G. Albariño, Ayan K. Chakrabarti, Jonathan S. Towner

Virology, Volume 501, 15 January 2017, Pages 147–165, open access