Biomedical Engineering Reference
In-Depth Information
B virus antigen in lettuce (Ehsani
et al.
1997) and
cholera antigen in potato (Arakawa
et al.
1997). As
well as animal virus antigens, autoantigens associ-
ated with diabetes have also been produced (Ma
et al.
1997, Porceddu
et al.
1999). Plants have also
been infected with recombinant viruses expressing
various antigen epitopes on their surfaces. Cowpea
mosaic virus (CMV) has been extensively developed
as a heterologous antigen-presenting system (Porta
et al.
1994, Lomonossoff & Hamilton 1999). There
have been some recent successes in vaccination
trials using recombinant CMV vectors expressing
epitopes of HIV gp41 (McLain
et al.
1995, 1996a,b)
and canine parvovirus (Dalsgaard
et al.
1997). The
first clinical trials using a plant-derived vaccine
were conducted in 1997 and involved the ingestion
of transgenic potatoes expressing the B subunit of
the
E. coli
heat-labile toxin, which causes diarrhoea.
This resulted in a successful elicitation of mucosal
immunity in test subjects (Tacket
et al.
1994).
HIV (Wang
et al.
1993, Fuller
et al.
1997, Hinkula
et al.
1997); Ebola virus (Xu
et al.
1998) ), other
pathogens (e.g. tuberculosis (Huygen
et al.
1996) )
and even the human cellular prion protein in mice
(Krasemann
et al.
1996).
The DNA-vaccination approach has several addi-
tional advantages. These include the following:
• Certain bacterial DNA sequences have the innate
ability to stimulate the immune system (see Klinman
et al.
1997, Roman
et al.
1997).
• Other genes encoding proteins influencing the func-
tion of the immune response can be co-introduced
along with the vaccine (e.g. Kim
et al.
1997).
• DNA vaccination can be used to treat diseases that
are already established as a chronic infection (e.g.
Mancini
et al.
1996).
In principle, DNA vaccination has much in
common with gene therapy (discussed above), since
both processes involve DNA transfer to humans,
using a similar selection of methods. However, while
the aim of gene therapy is to alleviate disease, by
either replacing a lost gene or blocking the expres-
sion of a dominantly acting gene, the aim of DNA
vaccination is to prevent disease, by causing the
expression of an antigen that stimulates the immune
system.
DNA vaccines
The immune system generates antibodies in response
to the recognition of proteins and other large mole-
cules carried by pathogens. In each of the examples
above, the functional component of the vaccine
introduced into the host is the protein responsible
for the elicitation of the immune response. The intro-
duction of DNA into animals does not generate an
immune response
against the DNA molecule
, but, if
that DNA is expressed to yield a protein, that protein
can stimulate the immune system. This is the basis
of DNA vaccination, as first demonstrated by Ulmer
et al.
(1993). DNA vaccines generally comprise a
bacterial plasmid carrying a gene encoding the
appropriate antigen under the control of a strong
promoter that is recognized by the host cell. The
advantages of this method include its simplicity, its
wide applicability and the ease with which large
quantities of the vaccine can be produced. The DNA
may be administered by injection, using liposomes
or by particle bombardment. In the original demon-
stration, Ulmer and colleagues introduced DNA
corresponding to the influenzavirus nucleoprotein
and achieved protection against influenza infection.
Since then, many DNA vaccines have been used
to target viruses (e.g. measles (Cardoso
et al.
1996);
Selecting targets for new
antimicrobial agents
In attempting to develop new antimicrobial agents,
including ones that are active against intractable
pathogens such as the malarial parasite, it would be
useful to know which genes are both essential for
virulence and unique to the pathogen. Once these
genes have been identified, chemical libraries can be
screened for molecules that are active against the
gene product. Two features of this approach deserve
further comment. First, inhibition of the gene prod-
uct could attenuate the organism's virulence but
would not result in death of the organism
in vitro
;
that is, no effect would be seen in whole-organism
inhibition assays and so molecules only active
in
vivo
would be missed. Secondly, target genes can be
selected on the basis that there are no human coun-
terparts. Thus, active molecules will be less likely
to be toxic to humans. A number of different appro-
aches have been developed for identifying virulence
