RNAi as Antiviral Therapy: The HIV-1 Case - RNA Interference from Biology to Therapeutics - page 221

cassettes may raise specific problems such as Drosha-mediated cleavage of the
vector RNA genome, but these problems can be countered by appropriate vector
design [ 53, 55 ]. Alternative viral vectors are available for delivery of a therapeutic
RNAi transgene, and these systems have been reviewed previously [ 55- 57 ] .
Another major question relates to the cell types that should be targeted in a gene
therapy for HIV-infected individuals. Hematopoietic CD34 + stem cells seed the dif-
ferent lineages of immune cells in the blood and organs and are, therefore, interest-
ing targets for such a gene therapy. This should be an ex vivo procedure, followed
by autologous transplantation of the cells into the same patient. The transduced
hematopoietic stem cells should durably supply all derived immune cells with the
antiviral arsenal, even after a single transduction event. In the presence of HIV-1,
one expects the preferential survival of these shRNA-expressing immune cells over
untreated cells. The latter cells are likely to become infected and will subsequently
be recognized and removed by the immune system. This survival benefit should
result in a gradual increase in the percentage of protected cells, which should even-
tually result in partial reconstitution of the immune system such that AIDS disease
progression is delayed or stopped.
Hematopoietic stem cells (HSC) transduced with a retroviral vector that encodes
an anti-HIV-1 ribozyme have already been evaluated in clinical trials [ 58, 59 ] . These
studies demonstrate the feasibility and safety of the proposed stem cell approach,
although little therapeutic effect was scored for the ribozyme. A recent study dem-
onstrated the safety of the lentiviral vector in combination with ex vivo targeting of
CD34 + cells [ 60 ]. Another option is the treatment of the mature CD4 + T cell popula-
tion, which represents the major target cell population for HIV-1. However, the gene
therapy should be applied repetitively in this scenario because the T cells have a
restricted life span [ 61 ] .
Treatment of hematopoietic stem cells has a disadvantage as well, as the number
of cells that can be obtained for manipulation is limited. The use of induced pluri-
potent stem cells (iPSC) could provide an elegant solution. The iPSC technology
makes it possible to produce patient-specific pluripotent stem cells by treatment of
somatic cells (e.g., fibroblasts or T cells) with specific reprogramming transcription
factors. These iPS cells are similar to human embryonic stem cells and are able to
differentiate into a wide variety of cell types. By transducing the iPSCs with a len-
tiviral vector encoding the antiviral genes, a continuous supply of therapeutic cells
could become available for transplantation. For instance, macrophages that express
a shRNA against CCR5 in combination with a gene encoding the restriction factor
TRIM5a were produced with this technology, and HIV-1 replication was substan-
tially inhibited in these cells [ 62 ] .

11.1.2 Where to Target the HIV-1 RNA Genome?

Several criteria can be used to identify the optimal targets for an RNAi attack on the
9-kb HIV-1 RNA genome. First, one could reason to select targets in the multiply
spliced HIV-1 mRNAs that are synthesized early upon virus infection and that