Plants have great potentials as bioreactors for the production of
pharmaceutical proteins. For safety, product quality, and costs plants
have important advantages compared to animals and microorganisms (fungi
and bacteria): plants are not infected by human pathogenic viruses,
perform protein synthesis and modifications more similarly to mammals
than
to microorganisms and have maintenance costs much lower than those of
transgenic animals, animal cell cultures or bacteria and yeast fermentors.
Immunoglobulins A (IgA) are antibodies present in the secretions of our
body (in digestive, respiratory and reproductive tracts, milk, saliva and
tears). Specific IgA produced by bioreactors could be used as
pharmaceuticals against the numerous diseases that originate by
infections
of our body's cavities. However, the levels of IgA obtained up to now in
transfected mammalian cells are too low to represent a commercially
viable
system of production. IgA are oligomers composed of four different
proteins and are therefore the product of four different genes. By
transforming tobacco plants with the four genes, scientists at the Guy's
Hospital in London have produced a recombinant IgA against the bacterium
Streptococcus mutans, which is the major causing agent of dental caries.
IgA purified from the leaves of these transgenic plants has been tested
on
human volunteers, giving very promising results in inhibiting
colonization
of the oral cavity by S. mutans.
In tobacco, this IgA is produced in much higher amounts than in animal
cell cultures. IgA are largely secreted in the intercellular spaces of
the
plant leaves (an expected behaviour, because IgA are also secreted by the
cells that produce them in our body), where they stably accumulate. In
our
Institute we have however found that a relevant percentage of the IgA
molecules produced by the plant cells ends up in the vacuoles, where it
is
largely degraded. Avoiding this vacuolar delivery would therefore further
increase the accumulation of IgA in tobacco plants. Why are IgA sent to
vacuoles when synthesized in plants? Vacuoles are the inner hydrolytic
compartments of plant cells, similarly to lysosomes of animal cells. In
plants and animals, proteins that perform their functions in hydrolytic
subcellular compartments have signals (short amino acid sequences) that
direct them to these compartments, whereas proteins destined for
secretion
do not have such signals. However the signals and their receptors for
plant vacuoles differ from those for animal lysosomes. In collaboration
with British scientists we have now discovered that one of the four
components of IgA has at its very end an amino acid sequence that is
recognized, probably fortuitously, as a signal for vacuolar delivery in
plants. When this sequence is deleted by genetic engineering, IgA is
totally secreted by plant cells. This finding opens the way to completely
efficient secretion of IgA in plants and therefore to increased
production
of this important pharmaceutical.
Reference:
Hadlington JL, Santoro A, Nuttall J, Denecke J, Ma JK-C, Vitale A and
Frigerio L (2003) The C-terminal extension of a hybrid immunoglobulin A/G
heavy chain is responsible for its Golgi-mediated sorting to the vacuole.
Molecular Biology of the Cell, in press.
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