During the early stages of a pandemic, viruses tend to evolve in ways that enhance their ability to reproduce and spread, rather than to evade the host's immune system. The genetics and sex of the host influence how a novel virus adapts to a new environment, but scientists don't fully understand how these factors work individually or together.
New research on mice led by University of Utah biologists sheds fresh light on this mystery by demonstrating that some hosts appear to serve as "evolutionary accelerators" that could enable viruses to gain virulence more quickly.
"Some hosts seem to select for virulence-associated mutations, virulence traits that affect other hosts as well. This supports the idea that if a virus infects them, then it'll become worse for the entire population. We don't know this yet, but it's what our work indicates that could be happening," said Rodrigo Costa, a postdoctoral researcher in the U's School of Biological Sciences. That gain in virulence appears to occur more quickly in female mice of a specific strain.
Costa is the lead author of the National Institutes of Health-funded study that appears in Nature Communications.
Do more resistant hosts help accelerate virus virulence?
This study shows that both host genetics and sex deeply influence how influenza viruses evolve. Host-driven effects could help explain why some viruses become more dangerous in certain populations as they evolve. Understanding these patterns could help public health officials model outbreaks and design control strategies.
The research also underscores the power of experimental evolution of pathogens to reveal how they attack and circumvent host defenses, according to principal investigator Wayne Potts, a U professor of biology.
"These revelations are often unpredictable. For example, when we designed these experiments, our favored hypothesis was that increased viral genetic diversity–via inoculations of viruses collected and mixed from multiple infected hosts-would be a major factor influencing virulence evolution, something we also tested," Potts said. "There was some support for this hypothesis, but it was minor compared to the influence of host sex and genotype and the observation that more resistant hosts select for greater viral virulence."
The team's experiments discovered that when the virus infects a naïve host, it retains its virulence from the previous host encounters, a relatively novel finding which implies that there could be specific hosts that act as 'evolutionary accelerators.'
"If we found that this is also true in humans, then in the future, with enough sequencing data and more knowledge of these interactions and what these proteins do, maybe we can predict which hosts will be more likely to make the virus more virulent and immunize those people first," Costa said.
How the experiments were run: A tale of two mice
The research team evolved influenza A virus separately in male and female mice from two different strains used in laboratory research-BALB/c (white) and C57BL/6 (black) mice-creating 28 different viral lineages.
"At the beginning of a pandemic, the virus comes from some animal and infects a new host species, where it acquires mutations that allow it to replicate faster. And that's what we're seeing with our experimental work. It's emulating that first wave of infection and that first time a virus sees a population of new hosts," Costa said.
They simulated early waves of a pandemic by infecting lab mice with the A/Hong Kong/1/1968, the original H3N2 strain responsible for the Hong Kong flu pandemic that killed up to 4 million people globally between 1968 and 1970. Then they observed the pathogen's evolution over 10 passages through the mice.
"We see at the end what the virus is doing in terms of replication, how fast or how much it's replicating," Costa said. "We measure its virulence, the damage to the host, the weight loss, and then also sequence the virus."
The team used a new technique that maps mutations onto the 3D structure of viral proteins to determine which regions were under selection.
"We can look at the individual mutations that the virus acquired in each of the hosts," Costa said. And this is where the results were somewhat unexpected.
In short, the researchers discovered that some combinations, such as female BALB/c mice, can cause the virus to become more virulent, whereas in the C57BL/6 mice, the virus did not become as deadly or genetically different from the original virus.
Notably, initial evidence that a virus can evolve differently in males and females came at the level of protein-protein interactions. For example, in the NS1 protein, which helps the virus evade the immune system, mutations in female-adapted viruses were all found at a specific site, while those in male-adapted viruses were spread out across the same region of the protein. (See embedded video illustration.)
In contrast, viruses evolving in the black mice had fewer mutations overall, instead producing more defective copies of their genome, which muted the self-destruct mechanisms that cells usually activate when infected. This offers the first direct evidence from live animals showing that host genetics alone can influence how often a virus' replication leads to the accumulation of defective viral genomes, possibly as a different way to deal with viral infection
"It's a good start in the direction of mitigating a virus, but we don't have enough information yet. These observations depend on the specific pathogen we're studying and also the host," Costa said. "These are mice, not humans, but if the idea that different sexes and genotypes affect the evolution of the virus differently is true, this can be used, for example, to inform disease control strategies where the individuals or groups more likely to select for virulence get vaccinated first."
This research is to appear in an upcoming edition of Nature Communications under the title, "Host genotype and sex shape influenza evolution and defective viral genomes." The journal posted an unedited early version on April 15. Co-authors include scientists from the University of Michigan, Stanford University, as well as professors Jon Seger and Frederick Adler of the U School of Biological Sciences, along with former undergraduates and lab assistants Kaili Curtis, Kort Zarbock, Justin Kelleher and Lehi Acosta-Alvarez. Funding came from the National Institutes of Health.
