Immunotherapy for Infectious Diseases

Gene Therapy for HIV-1 Infection 245
Fig. 4. Retroviral helper cells derived from C-type retroviruses. A C-type retroviral provirus.
(Top) The DNA intermediate of a retrovirus. The protein-coding genes (gag-pol and env) are
flanked by cis-acting or controlling sequences, which play essential roles during replication.
(Bottom) In a retroviral helper cell, the retroviral protein coding genes, which code for all virion
proteins, are expressed (ideally) from heterologous promoters (pro) and polyadenylated via a heterologous
polyadenylation signal sequence (poly A). To minimize reconstitution of a full-length
provirus by recombination, the gag-pol and env genes are split to different gene expression vectors.
In the retroviral vector, the viral protein coding sequences are completely replaced by the
gene(s) of interest. Since the vector contains specific encapsidation sequences (E), the vector
genome is encapsidated into retroviral vector particles, which bud from the helper cell. The virion
contains all proteins necessary to reverse transcribe and integrate the vector genome into that of
a newly infected target cell. However, since there are no retroviral protein coding sequences in
the target cell, vector replication is limited to one round of infection. LTR, long terminal repeat.
Unanswered Questions
Even if we can transduce enough cells in vivo with anti-HIV-1 genes to inhibit virus
replication significantly, many other questions remain to be answered. For example, it
is not clear whether a cell that has been endowed with a certain HIV-1 resistance gene
will be able to fulfill its normal biologic function in vivo. Will the body be able to eliminate
all HIV-1-infected cells or will the infected person become a life-long carrier of
the virus, which is still replicating in the body although at levels that cause no clinical
symptoms owing to the presence of HIV-1 resistance genes? Will the patient be capable
of infecting new individuals? Finally, because genetic therapies can all be overcome
with in vitro challenges using very high amounts of HIV-1, will there be a difference
in antiviral effects in peripheral blood versus lymphoid tissues?