Monoclonal
Antibodies (MAb):
Monoclonal antibodies are
clones (exact copies) of antibodies. These antibodies are then
used in research and in the production of vaccines and laboratory tests,
such as a heartworm ELISA test. There are two primary methods of
monoclonal antibody production: in
vitro
manufacture in cell cultures and in vivo production
in laboratory animals, such as mice, rabbits and goats.
The classic method of monoclonal
antibody production is made by fusing a tumor cell (which is rapidly
dividing) with a lymphocyte (antibody-producing cell) in the lab.
This yield a hybridoma--a lymphocyte that manufactures large
amounts of one specific antibody. This hybridoma is then combined
with an adjuvant, an irritating substances that causes inflammation,
stimulating the immune response. One of the common and most
controversial adjuvants is Freund's complete adjuvant
(FCA), which not only enhances immune production, but also may cause pain. This hybridoma-adjuvant mixture is then injected into a
laboratory animal, such as a mouse or a rabbit. Ascites (fluid
accumulation) containing a high concentration of monoclonal antibodies
results.
In vivo production
of monoclonal antibodies has several advantages. A high yield of
concentrated monoclonal antibodies is produced rapidly in an
animal. Labor, materials and skill requirements are minimal and
process is relatively inexpensive. However, ascitic production of
antibodies is painful for the mouse (the most commonly used species),
and must be justified by the investigator. A variety of
alternatives have been developed and validated and many IACUCs severely
limit approval of the procedure in animals.
Return
to the top of the page
Xenotransplantation:
Xenotransplantation is
the implanting of animal cells, tissues and organs into humans. At
least 50,000 people in the United States are waiting for transplants and
four thousand die each year while waiting for an organ: there just
aren't enough human-origin tissues and organs available.
Xenotransplants may be used as a bridge to keep patients alive until
human organs are available or may serve as a suitable alternative to
human organs.
Actually,
xenotransplantation isn't new: pig
heart valves have been used since 1974 to replace defective or
worn-out valves in people. Isolated experimental
organ transplants from animals to humans have occurred over the past
few decades, including the transplantation of a baboon
heart into Baby Fae in 1984.
Swine are the most common
source of organs and tissues for transplantation into humans.
Their organs are about the same size as human organs, their gestation is
fairly short and litters are large, making more organs available.
In addition, swine are currently raised for meat and leather, so raising
pigs as organ and tissue donors is less controversial than using
primates for this purpose. Swine can be raised as axenic animals
in isolators, decreasing the chance of disease being transferred from
the pigs to human recipients. In addition, pig cells can be
genetically engineered with human DNA to increase similarities and
decrease the chance of rejection.
There
is considerable controversy associated with xenotransplantation,
including concerns about the ethics of raising and killing animals to harvest organs and
tissues. An important issue is the possibility of the
development of xenogenic infections--diseases that are transmitted
through xenotransplants or that emerge from viruses that are not
dangerous to the donor animal, but are transformed into serious
pathogens in man. The U.S. Public Health Service has established guidelines
on infectious disease issues in xenotransplantation.
The
Food and Drug Administration published a
fact
sheet on xenotransplantation in 1996 and the Public Broadcasting
Service aired "Organ
Farm" on Frontline in 2001.
Return
to the top of the page
Transgenic
Technology:
Transgenics
is the integration of a functional sequence of DNA (a transgene)
into a chromosome and the subsequent inheritance of that new gene by the
offspring.
Selective
breeding has been used for thousands of years to create different
species and breeds of animals, such as the domestic cat, the chihuahua
and quarter horse.
Inbreeding is the selective breeding of animals to stabilize dominant
genes and eliminate recessive genes by mating brothers and sisters or
parents and offspring. The first true mouse inbred strain was
developed about 1910. Many new inbred strains, such as nude mice,
were created by selective breeding of mutant animals.
Recombinant
DNA technology was developed in the 1970s, and by the 1980s, routine
insertion of DNA into rodent embryos began.
Viral
vector-mediated transgene introduction was the first method used to
create transgenes and is still in use, particularly in human gene
therapy trials. Viruses are modified so that they do not infect
the host's cells, but instead are used as the vehicle to introduce a
foreign gene into the early embryo.
Microinjection
(pronuclear-mediated transgene
introduction) was first used in the 1980s
and is a common method of transgene production. The prepared DNA
segment is injected into a mouse embryo which is in the pronuclear
phase. This transgene will be integrated into every cell of the
resulting animal.
Targeted
transgene insertion via homologous recombination exchanges DNA fragments
at an identified site on the chromosome. This allows targeting of
where the transgene fits into the chromosome. It can also be used
to create knockouts--animals that have had a specific genes removed,
thereby making an animal more susceptible to disease or creating a
deficiency disease.
Most transgenic
animals are mice, with a few rats, and may be patented. There are three
primary applications for transgenic animals: models for human
disease processes, targeted production of pharmaceuticals and selective
improvement of a species.
Transgenic
mice are used for hepatitis B studies, research in HIV-related
pathology, especially Karposi's sarcoma and Alzheimer's disease.
Transgenic mice are more sensitive to mutagens and carcinogens,
decreasing the number of animals needed for a study.
Animals,
especially farm
animals, are being used as bioreactors for the production of
pharmaceuticals such as insulin and human growth factors.
Modification
of an animal's anatomy and physiology is more controversial than the
other uses of transgenic animals. Improvements such as decreased
body fat in swine, more milk production in cattle or faster racehorses
could be achieved by transgenic technology. Currently, most
genetic improvement is limited to plants not intended for human
consumption.
Transgenic cow production:
Return
to the top of the page
Reading
Recommendations: In-text links.
