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Common to all vectors is that a foreign gene is introduced into
the host cell so that the protein for which the foreign gene is
coding can be produced.
Viral Vectors
A viral vector can be based on
an attenuated virus which cannot replicate (produce new virus
particles) in the host but is able to introduce and express a
foreign gene in the infected cell. The recombinant virus is thereby
able to make a protein and display it to the immune system of the
host. Some key features of viral vectors are that they can elicit a
strong humoral and cell-mediated immune response, which result in
an immunological memory towards the gene in question. Viral vectors
are the most commonly used by researchers to develop vaccines for
the prevention and treatment of infectious diseases and cancer, and
of these, poxviruses (including canary pox, vaccinia, and fowl pox)
are the most common vector vaccine candidates
Bacterial Vectors
Similar to viral vectors,
in bacterial vectors the DNA encoding for an antigen is inserted
into its genome. The bacteria then express the antigen together
with its own proteins. Since some bacteria can survive in the
gastrointestinal tract, a bacterial vector is attractive for oral
delivery of antigens and induction of mucosal immune reaction.
However, bacterial vectors can also revert to a pathogenic form,
which can be controlled by antibiotics.
Plasmid Vectors
Plasmid vectors are
relatively small DNA molecules that can be used to carry a gene
into a target cell. These types of vectors, used as “naked DNA
vaccines”, can induce an immune response towards a gene product
when taken up by a host cell. DNA vaccines have been shown to
stimulate a cellular immune response and are easily produced at
relatively low cost. However, current DNA vaccines are often
inefficient in eliciting an adequate immune response in humans.
Furthermore, it is not exactly known how long the plasmid DNA
persists in the host cells and if the genetic material introduced
into the host cell nucleus could potentially integrate into the
host genome, potentially causing a significant safety risk.
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