Target Discovery and Validation Reviews and Protocols

190 Houdebine
of the intestinal epithelium, which is accessible only when enterocytes divide.
The mouse model correctly reproduced this phenomenon. They are also good
candidates for internalizing foreign molecules as soon as they are associated
with internalin (148). L. monocytogenes is known to impact cells other than
enterocytes in the organism. This type of tool could study these mechanisms.
Mice sensitive to measles have been obtained by transferring to them the
viral receptor CD46 gene (149). Similarly, mice harboring the gene for the
polymyelitis receptor are used as models to study viral injection (150).
Hepatitis C infection is more complex. This virus does not infect cells in
vitro, and in vivo models are currently strictly needed. Severe combined
immunodeficient mice harboring human hepatocytes can be infected by serum
from patients suffering from hepatitis C. This model has been greatly improved
by using homozygous transgenic mice expressing the U-plasminogen activator
gene in their liver under the control of an albumin promoter. The grafted human
hepatocytes survived a longer time in mice (151).
The HIV-1 virus is known to have two major receptors in human cells, CD4
and CCR5. Transgenic rabbits expressing the human CD4 gene were shown to
become seropositive after HIV-1 injection (152). Although the virus replicated
in rabbit cells, it was unable to generate disease. The CCR5 receptor could have
been added to the CD4 rabbits. Because of the difficulty in maintaining rabbits
in appropriate confinement, and for other reasons, many potential mouse models
were generated (153). It finally seemed that transgenic rats expressing all
the HIV-1 genes except gag and pol showed pathology having many similarities
to human AIDS (154). Transgenic mice overexpressing the soluble domain
of the pseudorabies virus receptor are highly resistant to infection by the virus.
This model strongly suggests that pigs expressing the same transgene could be
protected against the Aujeszky disease (101).
5. Conclusions and Perspectives
Transgenesis is increasingly used not only to study gene regulation and function
but also to create models for the study of human diseases. The trend is to
use a lower number of more sophisticated models. It is indeed difficult to generate
relevant models because the function of many genes remains unknown or
the genes have not been clearly identified. Genetic background is an important
consideration (155). Transgenesis is more efficient in some strains of mice such
as FVB/N (156). Yet, the genetic background of this strain is not the best for all
the models. Backcrossing is then needed to transfer the transgene to more
appropriate genetic background. That ES cells can be used only in two strains
of mice is a clear limitation to the rapid generation of relevant models. Recent
studies have identified some of the genes that are required for the maintenance
of the pluripotency of ES cells. The transfer of these genes in embryonic cells