Overheated
mice do not defend against flu in lab study
Heat waves can reduce
the body's immune response to flu, according to new research in mice at the
University of Tokyo. The results have implications for how climate change may affect the future of vaccinations and nutrition.
Climate change is
predicted to reduce crop yields and nutritional
value, as well as widen the ranges of disease-spreading insects.
However, the effects of heat waves on immunity to influenza had not been
studied before.
University of Tokyo Associate Professor Takeshi Ichinohe and third-year doctoral student Miyu Moriyama investigated how high temperatures affect mice infected with influenza virus.
Flu in a heat wave
"Flu is a
winter-season disease. I think this is why no one else has studied how high
temperatures affect flu," said Ichinohe.
The influenza virus
survives better in dry, cold air, so it usually infects more people in winter.
However, Ichinohe is interested in how the body responds after infection.
The researchers housed
healthy, young adult female mice at either refrigerator-cold temperature (4
degrees Celsius or 39.2 degrees Fahrenheit), room temperature (22 C or 71.6 F),
or heat wave temperature (36 C or 96.8 F).
When infected with flu,
the immune systems of mice in hot rooms did not respond effectively. Most
affected by the high heat condition was a critical step between the immune
system recognizing influenza virus and mounting a specific, adaptive response.
Otherwise, heat-exposed
mice had no other significant changes to their immune system: They had normal
reactions to flu vaccines injected under the skin. Moreover, bacteria living in
the gut, which are increasingly becoming regarded as important for health,
remained normal in the mice living in hot rooms.
Temperature and
nutrition
Notably, mice exposed to
high temperature ate less and lost 10 percent of their body weight within 24
hours of moving to the hot rooms. Their weight stabilized by day two and then
mice were infected by breathing in live flu virus on their eighth day of
exposure to heat.
Mice living in heat wave
temperatures could mount a normal immune response if researchers provided
supplemental nutrition before and after infection. Researchers gave mice either
glucose (sugar) or short-chain fatty acids, chemicals naturally produced by
intestinal bacteria.
In experiments at room
temperature, researchers surgically connected mice so that body fluids moved
freely between underfed and normally fed mice, both infected with influenza.
The fluids from normally fed mice prompted the immune systems of underfed mice
to respond normally to the flu virus.
"Does the immune
system not respond to influenza virus maybe because the heat changes gene
expression? Or maybe because the mice don't have enough nutrients? We need to
do more experiments to understand these details," said Moriyama.
The results may shed
light on the unfortunate experience of getting sick again while recovering from
another illness.
"People often lose
their appetite when they feel sick. If someone stops eating long enough to
develop a nutritional deficit, that may weaken the immune system and increase
the likelihood of getting sick again," said Ichinohe.
Future of infection
An important area of
future study will be the effect of high temperature on different types of
vaccinations. Flu vaccines injected into the upper arm use inactivated virus,
but vaccines sprayed into the nose use live attenuated (weakened) virus.
"The route of
delivery and the type of virus both may change how the immune system responds
in high temperatures," said Moriyama.
Until more research can
clarify what these findings may mean for humans, Ichinohe and Moriyama
cautiously recommend a proactive approach to public health.
"Perhaps vaccines
and nutritional supplements could be given simultaneously to communities in
food-insecure areas. Clinical management of emerging infectious diseases,
including influenza, Zika, and Ebola, may require nutritional supplements in
addition to standard antiviral therapies," said Ichinohe.
The researchers are
planning future projects to better understand the effects of temperature and
nutrition on the immune system, including experiments with obese mice, chemical
inhibitors of cell death, and different humidity levels.
About the research
The strain of flu used
in this research was A/PR8/34 influenza virus, a type of H1N1 flu originally
isolated in Puerto Rico in 1934 and now common in laboratory mouse studies.
This research is a
peer-reviewed experimental study with mice published in the Proceedings
of the National Academy of Sciences of the United States of America Latest
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