Researchers have failed to reproduce the results of a landmark study that claimed to show mice that recover from infection can pass on stronger immunity to their future pups. New data from a separate group that conducted nearly identical experiments in parallel show no benefit to future generations of the animals, according to a report published last week (January 20) in Nature Immunology.
“As much as we tried to look for any evidence of transmission, I mean, there was literally nothing. It was like the most insignificant set of results that we’ve ever had,” says Luis Barreiro, a geneticist at the University of Chicago, who worked on the replication attempt. “It was a farfetched idea to start with, I thought.”
The original study was published in Nature Immunology in October. It reported the results of experiments that exposed adult mice to infectious fungi or zymosan, particles made from yeast used to stimulate the immune system. When either parent was subjected to a real or simulated infection, the offspring showed a stronger immune response to potential pathogens, including E. coli bacteria, than controls whose parents hadn’t been subjected to an immune system challenge. They had lower numbers of the bacteria in their lungs and liver, as well as higher concentrations of immune cells and pro-inflammatory cytokines. The effect persisted further: offspring of these second-generation mice also showed a lower bacterial burden after infection.
Transgenerational transmission of traits related to the immune system had been seen before in plants, birds, and invertebrates, including flies, beetles, and worms, but the October paper was the first to claim such an effect in mammals, and it raised the question of whether the same mechanism could be at work in people. In fact, the findings were so striking that the journal only accepted the paper after key experiments, originally conducted at the University of Athens, were replicated in a separate lab at University Hospital in Lausanne, Switzerland.
As much as we tried to look for any evidence of transmission, I mean, there was literally nothing. It was like the most insignificant set of results that we’ve ever had.—Luis Barreiro, University of Chicago
“The frustrating aspect of this is that honestly, I don’t really have a good explanation as to why the results are so different,” Barreiro says. “We did pretty much everything that they did.”
The original study authors, led by Radboud University immunologist Mihai Netea, published an accompanying response to the new report by Barreiro and his colleagues. The response proposes that the different results come down to “elusive” effects of the environment such as differences in mouse substrains, housing, diets, or microbiomes. A careful read of the two papers’ methods, for instance, reveals that Netea and colleagues used six-week-old male mice in their initial generation, while Barreiro’s first generation of mice were up to two weeks older and included both sexes.
Netea says differences between the experimental protocols “certainly exist,” adding that he welcomes the new paper, as it’s important that the details of both studies are published so that other groups can dig deeper into precisely what underlies the contrasting results. “And then in the future, hopefully, we will understand where . . . the difference [is] coming from,” he says.
Other researchers in the field agree that environmental factors could be responsible for the differing findings. “The lack of replication is significant, but there are several reasons for this,” says Deepshika Ramanan, a microbiologist who studies the nongenetic transfer of immunological traits at Harvard Medical School and was not involved in either study. “Changes in the environment of the mice, especially the microbiome, having an effect on reproducibility is pretty common,” she notes.
Both Netea and Barreiro stress the relationship between their groups is friendly and that they cooperated with one other, including sharing data and experimental materials.
“The only way to resolve this issue is to do more studies, by different groups, while trying to carefully control for environmental changes and effects,” says Ramanan.
The answer won’t come any time soon. “The problem with these types of studies is that they are long,” says Netea. Both studies took about six years to complete because the experiments required multiple generations of animals and each took more than a year to run. “It will take some time, but I think it’s worthwhile,” Netea says.