section 23.8
DNA Vaccines
537
triphosphate, a toxic substance that kills dividing cells. As
a result, individuals with an ADA deficiency are defective
in both humoral and cell-mediated immunity.
In the mid-1980s, the first clinical gene therapy trials
were attempted to correct ADA deficiency in two chil-
dren with SCID. The ADA gene was cloned into a viral
vector and inserted into peripheral T cells removed from
the affected children. After a period of growth of the ge-
netically modified cells
in vitro,
they were reintroduced
into the patients. Synthesis of ADA could be detected
for a time but the activity disappeared as the introduced
cells died. Several other trials of gene therapy for ADA
deficiency showed promise, but it was still necessary to
maintain the children with SCID on enzyme replacement
therapy.
Finally, in 2000, successful treatment of an X-linked
form of SCID (SCID-X1) by gene therapy was reported
by a French medical team. Almost a year after a normal
gene was introduced into their cells, two children were
still synthesizing the enzyme that they lacked. This was
the first success for gene therapy in curing a disease after
years of effort.
Since the 1980s, hundreds of clinical trials of gene ther-
apy for cystic fibrosis, osteogenesis imperfecta, Gaucher’s
disease, Fanconi’s anemia, and several forms of cancer
have been attempted. The problems that need to be over-
come in developing a successful strategy for gene therapy
are:
1. Development of safe and effective vectors for cloning
the gene and inserting it into palients cells,
2. Regulation of expression of the desired gene product
so that it is produced at the right time and in the
correct amounts in the appropriate tissues, and
3. Stability of the inserted genetic construct in cells so
that the product will continue to be produced.
Many different viral vectors have been developed to
deliver genes to various organs in the body; these include
retroviruses that can insert themselves and the genes they
carry into the chromosomes of cells; adenovirus, a DNA
virus that causes respiratory infections, which has been in-
activated by removal of many viral genes; and lentiviruses,
slow-growing retroviruses. All of the viral vectors carry
with them some risks. In 1999, a gene therapy patient died
of complications from the use of adenovirus that was be-
ing tested as a therapy for an inherited liver disease caused
by a deficiency in the enzyme ornithine transcarbamylase
(OTC). Despite the problems that have beset gene therapy
trials, it is expected that development of safer vectors and
new techniques for delivering genes to cells will eventually
make gene therapy a vital part of medical treatment.
23.8 DNA Vaccines
An extension of gene therapy, and one that may turn out to
be of worldwide importance, is the use of naked DNA as
a vaccine to prevent viral diseases. For example, plasmid
DNA can be injected into tissues; upon entry, the DNA
expresses any cloned gene, such as a viral antigen, that is
carried by the plasmid. DNA vaccines have the advantage
that the viral protein that is expressed in the cells stimulates
both humoral and cell-mediated immunity. Fragments of
the synthesized viral protein are carried to the cells’ surface
where they stimulate CD
8
+ cytotoxic T cells and, thereby,
cell mediated-immunity (Figure 23-16).
In experiments with mice,
DNA vaccines
have proven
to be very effective. The gene coding for the core protein
of the influenza virus was cloned into a plasmid vector
and injected into mice. The mice developed immunity not
only to the strain of influenza from which the gene was
derived but from other strains of influenza virus as well.
Inducing an immune response with a viral core antigen is
thought to be more effective than capsid antigens because
viral core proteins from related viral strains do not differ
much in structure or antigenicity. Viral capsid proteins, on
the other hand, evolve rapidly; such changes, for example,
account for the different strains of influenza virus that arise
each year.
Before DNA vaccines can replace conventional vac-
cines that use inactivated or attenuated viruses, the safety
of the plasmid vectors must be rigorously proved. The plas-
mids might occasionally integrate into the host genome
or they might stimulate an immune response to tissues
containing the plasmid DNA. Either event might dictate
against the widespread use of DNA vaccines.
Antibodies to DNA
Both single- and double-stranded DNA are antigenic; an-
tibodies to DNA are normally found in the circulation,
but in some individuals overproduction of DNA antibod-
ies causes disease. In particular, patients with
systemic
lupus erythematosus (SLE)
show abnormal levels of an-
tibodies to double-stranded DNA. Antibodies to single-
stranded DNA also bind to bases, nucleosides, nucleotides,
oligonucleotides, and the ribose-phosphate backbone of
RNA. Antibodies to double-stranded DNA also bind to
base pairs, chromatin, nucléosomes, type IV collagen, and
the deoxyribose-phosphate backbone of DNA.
Antibodies to DNA consist of both IgM and IgG
classes. Healthy individuals usually have low-affinity IgM
antibodies to DNA; however, if these undergo an iso-
type switch to IgG, they may become pathogenic. Tests
for DNA antibodies help establish a diagnosis of SLE,
