Vaccine from fly spit
Fly saliva could protect us from a
dangerous disease.
7 August 2001
JOHN WHITFIELD
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| Worth spit: a new vaccine
works against the fly that carries Leishmania. |
| © SPL |
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An injection of fly spit sounds like medieval quackery, but
could be the vanguard of modern medicine. Researchers have used
sand fly saliva to develop a vaccine that protects mice against
leishmaniasis, a disease spread by the insect1.
This, the first potential vaccine against an insect-borne
disease derived from the carrier, rather than the parasite
itself, is based on chemicals that help the fly feed, preventing
blood clotting and dilating blood vessels.
Leishmania, a single-celled protozoon, infects about
12 million people worldwide. Different types of leishmaniasis
erode the mucus membranes of the mouth, nose and throat, or
internal organs. This latter condition is often fatal.
Jose Ribeiro, of the US National Institute of Allergy and
Infectious Diseases (NIAID) in Bethesda, Maryland, focused on
the skin-attacking Leishmania major, which is spread by
the Middle Eastern sand fly (Phlebotomus papatasi).
The bites of uninfected sand flies seem to protect mice from
Leishmania, the group had previously found. Apparently,
the same thing applies to people in South America, Africa and
around the Mediterranean, where the disease is common.
"Newcomers and children get sick, but they're the tip of the
iceberg of those who are infected," says Ribeiro.
Ribeiro's team isolated a protein from the fly's saliva that
provoked a particularly strong mouse immune response to
Leishmania. They identified the gene for this protein, and
injected it into mice. The mouse's cells made the protein from
the DNA, triggering their immune response.
When subsequently injected with a mixture of fly saliva and
Leishmania, the mice developed none of the disease
symptoms. They did not eliminate the parasite, but suppressed it
to one-hundredth of the level found in untreated mice. This is
similar to the response of humans who are 'immunized' by fly
bites, says Ribeiro.
"This is a very promising vaccine candidate," says
immunologist Heidrun Moll of the University of Wurzburg,
Germany. The importance of the carrier in the immune response to
insect-borne disease is widely acknowledged, says Moll, but
technical difficulties - such as breeding the insects and
keeping them contained - has slowed progress along this avenue.
About half a dozen potential leishmaniasis vaccines have been
derived from the parasite over the past decade, including one by
Moll herself. Eventually, people could be immunized using a
cocktail of vaccines against both fly and parasite, she says.
As well as different vaccines for different Leishmania
species, we may also need vaccines based on different species of
sand fly - salivary proteins vary a great deal between species,
says Ribeiro. "But this is not a big deal," he assures - it
takes less than two months to go from fly spit to vaccine
candidate.
Researchers are also targeting the insects that carry other
vaccines. One possible way to fight malaria is to develop
vaccines that "do harm" to mosquitos that suck them up in human
blood, says Filip Dubovsky of the Malaria Vaccine Initiative in
Washington DC. As yet, there are no candidate malaria vaccines
that exploit mosquito saliva. |
