Viral Research Gets Batty to Study Spillovers

Hemorrhagic fevers caused by members of the filovirus family have high fatality rates but remain poorly understood.¹ Scientists suspect that these viruses enter the human population through zoonotic spillover events, but only one, Marburg virus, has been identified in the Egyptian fruit bat, Rousettus aegyptiacus.² Although studying bat-derived viruses offers glimpses into how these entities survive in their original host and how these may first emerge in humans, most research still uses human isolates.

Joseph Prescott, an immunologist and virologist at the Robert Koch Institute, studies Marburg virus infection in Egyptian fruit bats and the animal’s immune response to it. “The end goal is trying to compare what's happening in the natural reservoir… to what's happening in humans,” he explained. In a recent paper published in npj Viruses, he and his team explored human macrophage activity against a bat-isolated Marburg virus and found distinct responses between individuals.³

“These are critical questions to better understand what happens,” said Gaya Amarasinghe, a virologist at Washington University who studies host pathogen interactions in RNA viruses. “What drives the species crossing? And more importantly, what are the adaptations and what happens in different species?”

Prescott and his team isolated monocytes from blood donors and cultured them into macrophages, which were then infected with bat-derived Marburg virus that expressed a fluorescent reporter. “[Bat-derived Marburg virus] would, presumably be what's initially infecting a human. And then we know that macrophages and dendritic cells are some of the initial target cells, so we're trying to model the spillover,” he said.

While the virus infected macrophages from all donors, its efficiency in doing so ranged from 10-80 percent of the cell population. To explore this further, the team investigated the changes in gene expression by RNA sequencing cells of five donors after Marburg virus infection or stimulation with lipopolysaccharide (LPS), a potent macrophage activator.

Photograph of green fluorescence, representing Marburg virus, in macrophages overlaid on a brightfield image of the cells in gray.

A bat isolate of Marburg virus expressing a fluorescent protein (green) infected human macrophages.

Ivet Yordanova

Macrophages from four of the five donors showed comparable gene expression profiles with LPS stimulation, upregulating proinflammatory cytokine and chemokine genes. However, while cells in three of these donors activated expression of antiviral chemokines and cytokines in response to Marburg virus, macrophages of two donors did not activate gene expression in immune-related genes. Additionally, these two donors had the lowest viral loads in their supernatant.

“The comparison [to] LPS was great, but what about other viruses?” Amarasinghe inquired, saying it would be interesting to know if these outcomes were specific to Marburg virus or more general to viruses in these individuals.

The team then identified distinct baseline gene expression profiles of the donor macrophages, with the Marburg virus-responders exhibiting greater expression of receptors suspected to be important in viral infection, such as dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN) and multiple other C-type lectins. Looking at secreted inflammatory cytokines and chemokines as a more functional readout of macrophage responses, the researchers showed that macrophages of nonresponders produced less C-X-C motif chemokine ligand 10 (CXCL10) compared to responders.

Amarasinghe said that while the differences in macrophage responses to Marburg virus and LPS were interesting, he pointed out that the donors’ ages were not given, which could help answer why these differences exist. Prescott also acknowledged that while it was exciting to him and his team to observe the differences in donor responses, the current sample size is small. “The other thing is,” he added, “We don't know exactly what sequence of virus spills over into humans, so it could be that this isolate is a little bit different than what would make a human sick.”

However, the present study better replicates not only the initial conditions in a spillover event, but also what will be circulating in the bats to allow the researchers to study and compare those responses to that of humans in the future. “It definitely addresses a key gap,” Amarasinghe said.