BY TAMING one of nature鈥檚 deadliest poisons, scientists could change the way
we vaccinate children against fatal diseases. So claim scientists in the US who
have come up with a vaccine that prevents lab animals contracting botulism.
Botulism is a potentially fatal disease caused by the neurotoxin from
Clostridium botulinum bacteria. Lance Simpson and his colleagues at
Jefferson Medical College in Philadelphia wanted to create a harmless version of
the protein that is able to produce an immune response. To do this they tinkered
with the gene for one strain of C. botulinum neurotoxin and then grew
the altered protein in bacteria.
The researchers then injected the altered toxin into mice. The toxin caused
none of its normal ill-effects, such as muscle twitching and paralysis, and
raised levels of antibodies to the toxin. When the mice later received a dose of
the unaltered toxin that would normally have been lethal, they survived. The
team鈥檚 results are published in this month鈥檚 Infection and Immunity (vol
65, p 4568).
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The vaccine could prove useful in protecting farm animals and horses against
botulism, say the researchers. But more importantly, the results of their
experiments indicate that botulism neurotoxin could serve as an oral vector for
other vaccines. This is because the heavy protein has the rare ability to pass
through the gut and into the bloodstream intact. Few if any other large proteins
touted as oral vaccine carriers are as impervious to digestion by the acid and
enzymes in the stomach.
鈥淭his is an extremely exciting idea, but until now we haven鈥檛 had a protein
that could be a practical carrier,鈥� says Simpson. 鈥淭his could be it. Though if
it does prove practical, it will be several years away.鈥�
Oral vaccines are potentially much cheaper to give than injections. The
savings would also allow many more children in the developing world to get vital
protection from infectious diseases. Some scientists suggest it may be possible
to 鈥済row鈥� vaccines in a fruit such as bananas. They could place the necessary
genes in them to code for the immunising proteins.
Simpson鈥檚 team are now tyring to find out how the 鈥渟afe鈥� botulinum
toxin will fare in carrying antigens from other important disease-carrying
groups, including tetanus and diphtheria.
鈥淚 think this is a fascinating piece of work,鈥� comments Julian Ma, an
immunologist at Guy鈥檚 Hospital in London. However, he cautions that oral
vaccines may not live up to everyone鈥檚 expectations. 鈥淭hey might protect you at
the level of the mucous membranes,鈥� says Ma. 鈥淏ut if the infection did make it
through into the general circulation, they could have the effect of actually
weakening the immune response.鈥� White blood cells can learn not to attack
proteins absorbed through the gut, a mechanism that prevents the immune system
attacking proteins in the diet.