More
than a living syringe: Mosquito saliva alone triggers unexpected immune
response
Baylor College of
Medicine
Mosquito season is
around the corner, bringing with it a higher risk of catching potentially
serious diseases transmitted by their bite.
Mosquitoes also may
increase the severity of the diseases they transmit, and researchers think that
mosquito saliva plays an active role in this process.
A team of researchers
at Baylor College of Medicine has taken a closer look at the effect of mosquito
saliva alone and found that it can trigger an unexpected variety of immune
responses in an animal model of the human immune system.
"Billions of
people worldwide are exposed to diseases transmitted by mosquitoes, and many of
these conditions do not have effective treatments," said corresponding
author Dr. Rebecca Rico-Hesse, professor of molecular virology and microbiology
at Baylor College of Medicine.
"One of the
interests of my lab is to study the development of dengue fever, which is
caused by the dengue virus transmitted by mosquito Aedes aegypti."
The World Health
Organization has estimated that 100 million dengue virus infections and 22,000
deaths occur yearly worldwide, mostly among children.
According to the
Centers for Disease Control and Prevention, more than one-third of the world's
population lives in areas at risk of infection, making the dengue virus a
leading cause of illness and death in the tropics and subtropics.
"One of the main
limitations for studying dengue fever is that the dengue virus only causes the
disease in humans; no other animals can be used as models of the condition to
develop preventive and therapeutic measures," Rico-Hesse said.
"To overcome this
challenge, we have been working with a mouse model of the human immune
system."
These 'humanized mice'
were developed by other research groups from mice naturally born without their
own immune system.
These severely
immunodeficient mice received human stem cells that gave rise to many of the
components of the human immune system, creating a living humanized animal model
in which Rico-Hesse and her colleagues can study factors that may affect the
development of dengue fever.
"In 2012, we
demonstrated in these humanized mice that mosquito-bite delivery and
needle-injection delivery of dengue virus led to significantly different
disease developments," Rico-Hesse said.
"Importantly,
mosquito-bite delivery of the virus resulted in a more human-like disease than
the one we observed after needle-injection delivery of the virus. When the
mosquitoes delivered the virus, the mice had more of a rash, more fever and
other characteristics that mimic the disease presentation in humans."
These observations
support the idea that mosquitoes are not just acting like 'syringes,' merely
injecting viruses into the animals they feed on.
Their saliva seems to
contribute significantly to the development of the disease, which has prompted
Rico-Hesse and her colleagues to investigate what this role might be. They
began by determining the effect of bites from virus-free mosquitoes on the
human immune response of humanized mice.
An unexpected complex
response
To test the effect of
virus-free mosquito saliva on humanized mice, the researchers held a vial
containing mosquitoes against a footpad of anesthetized humanized mice,
allowing a total of four mosquitoes to feed on both footpads.
The researchers then
took blood and a number of other tissue samples six hours, 24 hours and seven
days after the mosquitoes bit the mice, and determined the levels of cytokines,
molecules that modulate the immune response, as well as the number and activity
of different types of immune cells.
They compared these
results with those obtained from humanized mice that had not been bitten by
mosquitoes.
To make the above
determinations, the researchers used highly-sensitive techniques -- flow
cytometry for immune cell analysis and multiplex cytokine bead array analysis
for cytokines -- that allowed them to dissect the immune responses in great
detail. This approach produced surprising results.
"We found that
mosquito-delivered saliva induced a varied and complex immune response we were
not anticipating," said co-author Dr. Silke Paust, assistant professor of
pediatrics at Baylor and Texas Children's Hospital.
"For instance,
both the immune cell responses and the cytokine levels were affected. We saw
activation of T helper cells 1, which generally contribute to antiviral
immunity, as well as activation of T helper cells 2, which have been linked to
allergic responses."
At various time
points, the levels and activities of other types of immune cells also increased
as others decreased. Overall, the researchers found evidence that mosquito
saliva alone can trigger long-lasting immune responses -- up to seven days
post-bite -- in multiple tissue types, including blood, skin and bone marrow.
"The diversity of
the immune response was most striking to me. This is surprising given that no
actual infection with any type of infectious agent occurred," said Paust,
who also is a member of the Dan L Duncan Comprehensive Cancer Center at Baylor
College of Medicine.
"These results
are evidence that components in the mosquito saliva can modulate the immune
response in humanized mice."
The researchers will
continue this study by investigating which of the more than 100 proteins in
mosquito saliva are mediating the effects on the immune system, or may help the
virus become more infectious.
Identifying these
proteins could help design strategies to fight transmission of dengue fever, as
well as other diseases caused by viruses also transmitted by Aedes
aegypti, such as Zika virus, chikungunya virus and yellow fever virus.
"We hope that our
work will inspire more research in this area with the long-term goal of using
our understanding of how saliva manipulates the immune system for therapeutic
purposes," said Paust.
