Insects don’t make the cells and antibodies characteristic of the vertebrate adaptive immune response. As a result, scientists assumed for years that insects rely on innate immune defenses that are neither heritable nor directed at any pathogen in particular. Over the last 20 years, though, evidence has emerged that invertebrates do inherit some types of immunity from their parents, but it’s still not clear how or how often it happens. In a study published December 15 in Cell Reports, researchers show that fruit flies (Drosophila melanogaster) and mosquitoes (Aedes aegypti) pass immunity to viruses to their progeny for multiple generations.
“The authors present a very thorough set of experiments that detail the existence of this transfer of some kind of immunological memory to the offspring from generation to generation,” says Barbara Milutinović, a postdoc at the Institute of Science and Technology Austria who did not participate in the work. Some scientists still discuss immunity in Drosophila as though it’s only innate and not tailored to certain pathogens, she adds, but “this study shows us . . . that there is something really specific going on.”
Maria Carla Saleh, an immunologist at Institut Pasteur in Paris, and her colleagues set out to understand immunity in insects because mosquitoes, while the vectors of so many viruses that make people sick, they themselves are unharmed by these infections. “The same virus will produce a fatal disease in humans, but the mosquito will be super happy,” she says. They focused on so-called persistent viral infections: those with high viral titers but no fitness costs for their insect hosts. Understanding the basis of persistent infections might make it possible to push the mosquito out of this persistent state—either toward succumbing to the pathogen or clearing it, she explains.
Definitely the information seems to be being transmitted as DNA.—Maria Carla Saleh, Institut Pasteur
In Saleh’s lab, researchers often work with fruit flies because they have similar immune systems to mosquitoes and fruit flies are much easier to work with. In 2018, the team showed that D. melanogaster that were exposed to a virus as larvae successfully fought it off as adults whereas naïve adults didn’t. The next question was whether or not this sort of immune priming could be passed from parent to offspring, too.
In the new study, Saleh and colleagues injected female fruit flies with either recombinant Sindbis virus or a mock injection of the same volume containing no virus. Sindbis virus is carried by mosquitoes in Europe, Asia, Africa, and Australia. Infections are usually mild in people and may include arthritis, fever, and rash. In contrast, Sindbis in fruit flies produces high viral titers but doesn’t make them sick, nor does it pass from mother to offspring. After they received the injection, the females mated and laid eggs. Once the next generation reached adulthood, the researchers injected the new generation with the same recombinant Sindbis virus. The flies whose mothers had been infected with virus ended up with lower viral titers and lower activity of the reporter gene engineered into the virus than did the offspring of mothers that were mock infected before mating.
The researchers determined that the protective effects of a virus infection in the founder generation persisted at least five generations. They saw similar levels of transgenerational antiviral protection when they repeated the experiments with three other positive-sense single-stranded RNA viruses, but not with a double-stranded RNA virus or a negative-sense single-stranded RNA virus.
To determine whether or not intact virus was necessary for antiviral immunity in the progeny, the researchers constructed double-stranded RNA corresponding to two different parts of the Sindbis virus, which mimics intermediate forms of the virus created during viral replication. When injected into female flies, this chunk of double-stranded RNA also protected their offspring from subsequent viral challenges. “That was mind blowing because it means that somehow the system is recognizing that this double-stranded RNA . . . is exogenous or that that is something that you should create immunity against,” says Saleh.
Next, the authors either injected female Aedes aegypti mosquitoes with chikungunya virus—a positive-sense single-stranded RNA virus that can cause headaches, muscle pain, and joint swelling in people—or fed them an infected blood meal. When the researchers subsequently exposed the mosquitoes’ offspring to the virus, the insects had lower viral titers than those whose mothers had been mock infected.
In their 2018 study, Saleh and colleagues showed that viral DNA corresponding to some portion of a viral RNA sequence was present in adult flies after they’d been exposed to a virus as larvae. On a hunch that viral DNA was playing a role in the transgenerational immunity they’d observed in the new study, the authors looked at levels of both viral DNA and RNA in flies that had been infected with one of two positive-sense single-stranded RNA viruses and in their progeny. They found both viral RNA, as expected for a current infection, and viral DNA in the mothers, but the offspring did not have any viral RNA, just viral DNA.
“Definitely the information seems to be being transmitted as DNA,” says Saleh. But “the paper opens a lot of questions. For example, this protection or this transmission of antiviral immunity, we could only observe it for positive single-stranded RNA viruses. Why?”
Previous work has shown “that these sorts of mechanisms that confer some kind of memory exist in invertebrates—or at least some invertebrates—and they maybe just look quite different to what we’re used to seeing in the vertebrate immune system,” says Matt Ballinger, a biologist at Mississippi State University who did not participate in the study. “Probably for the next five years or ten years, [the field] is going to be working on trying to figure out exactly how that happens. They presented quite a few new steps, just in this paper alone, that are worth following up on.”
In addition to the focused questions about viral DNA and how the positive-sense single-stranded RNA viruses do this, there are human health implications for this work, according to Bryony Bonning, an insect biologist at the University of Florida who did not participate in the study.
“There are several viruses with single-stranded RNA genomes that are transmitted to humans by mosquitoes,” including dengue, chikungunya, Zika, yellow fever, and West Nile viruses, says Bonning. “Understanding how the mosquito might have an immune mechanism to suppress the replication of these human viruses . . . you could imagine that downstream, if you get a handle on that, there might be something that could be used to try to suppress the transmission of the viruses to humans.”