New and unexpected findings in mice challenge a widespread theory that connects a perceived rise in allergies and asthma to the emergence of modern hygiene. This idea, called the hygiene hypothesis, suggests that allergies come from overreactive and bored immune systems without enough exposures to bacteria, fungi and parasites.
"Our study does not directly contradict the hygiene hypothesis. However, I believe that it is the first proof-of-concept study confirming that diverse microbial exposures as well as infections are not the sole nor primary factors driving the dramatic rise of allergic diseases," said Stephan Rosshart, a corresponding author of the Science Immunology study and microbiome researcher at Universitatsklinikum Freiburg.
After raising laboratory mice in semi-natural settings with ample microbes, Rosshart and his colleagues found that the "wilding" mice still responded strongly to allergen challenge tests. These surprising results show that the relationship between microbiota and the immune system is more complicated than understood.
"[This work] may help to recalibrate scientists' view on the hygiene hypothesis, prompting the field to have a closer look at other factors such as indoor living, physical activity, pollutants and chemical compounds present in the modern world," added Rosshart.
A Sanitation Story
The hygiene hypothesis dictates that as the world became more sanitation-focused, people encountered fewer germs and parasites. This lack of stimulation for the immune system led white blood cells — especially eosinophils, which defend against parasitic infections that are no longer as commonplace — to behave in dysfunctional ways. In favor of this idea, allergies to hay fever and other plants and disorders such as eczema and asthma do seem to be on the rise.
"But it's really not as simple as it sounds," said Jonathan Coquet, another corresponding author and T cell (another white blood cell type) development specialist at Karolinska Institutet. "We are still some ways from knowing precisely how different microbes can have a beneficial impact on our health and using this knowledge to our advantage as a population."
For example, rising rates of children with asthma in America and Europe started to appear in the 1960s, but practices like handwashing and showering were already prevalent by the 1920s. So hygiene and microbial interactions cannot be the only drivers for these symptoms and conditions.
"The reason for this increase in allergic diseases in the modern world is still not very clear," reiterated Junjie Ma, a first author on the paper at Columbia University who specializes in exploring how microbiota can impact immune cells.
Teasing out the many contributors to this increase in allergies has been hard to do. One thing that might help would be getting more details about the relationship between microbial encounters and allergy development.
"Although there is increasing evidence how microbiota regulates different immune responses, … there is no known mechanism for how diversity of microbial environment influences the development and management of allergies," confirmed Egon Urgard, a co-first author on the paper who studies T cell-related inflammatory tissue pathways at Karolinska Institutet.
Of Mice and Microbes
To try to home in on this mechanism, the group harnessed a previously designed approach for raising laboratory mice with a more natural bacterial microbiome. Essentially, they took genetically standardized, pathogen-free baby mice and placed them in semi-natural environments with hay, compost and contaminated particles called fomites from truly wild mice. As young mice grew and intermingled, they gained microbiome diversity, and their immune systems received plenty of stimulation.
"It is not just gut microbes that the mice are exposed to. The skin, vaginal tract through which the mice are born, and the entire environment provides the mice with more exposure. Importantly, it does so from the moment the mice are born, which was critical, since microbial exposures as newborns and infants are thought to play the greatest role in protecting us from allergy," Coquet explained.
He and his co-authors tested the wilding mice with several allergens, including house dust mite extract, IL33 allergy-associated signaling chemical or cytokine, and an airway-irritating fungus (Alternaria alternata). What they found was startling. The wildings had similar type 2 immune responses to their genetically identically, but sterile-environment laboratory counterparts during these tests.
"When we talk about a type 2 immune response, we are talking about cells that produce specific cytokines that directly trigger the symptoms of allergic diseases and asthma," said Coquet.
In particular, the wildings were marked by expansion of newly differentiated type 2 helper T (TH2) cell and existing memory T H2 cells in the lungs. Those cells can produce chemicals that signal to eosinophils and can help trigger asthma and airway inflammation.
"Our study shows that manipulation of the microbiota to improve health is not straightforward and that we need to consider not only when in life infections are acquired, but also the sequence and duration of microbial exposures," said Susanne Nylén, an author on the paper and specialist in host-parasite immune interactions at Karolinska Institutet.
Noteworthy Next Steps
Moving forward, the researchers have plans to continue investigating the roles that microbes have on allergy development. In particular, Nylén plans to explore whether exposing baby wildings to a subset of parasites called helminths could subdue their allergic responses. Her motivation comes from existing preliminary experiments, which indicated that some helminth infections can lead to reductions in allergic symptoms.
Meanwhile, other avenues for exploration include seeing how the route of allergen exposure impacts mice's responses.
"We have not rigorously tested how these mice deal with allergens on the skin — think eczema — or in the intestines — think food allergy," said Coquet. He also hopes to delve into whether there might be a protective effect on wildings' allergy responses across generations.
Ultimately, according to Rosshart, the study's main takeaway revolves around the concept of health as a multi-faceted state with compounding factors or "little things," which include genetics, microbial exposures and environmental conditions.
"Science has to unravel these 'little things' and make the information available to the public domain, so that they may come together in the right way," he concluded.