Immunotherapy for Infectious Diseases

238 Dornburg and Pomerantz
cells (5). Thus, AIDS patients usually die from secondary infections (e.g., tuberculosis,
pneumonia) or cancer (e.g., Kaposi’s sarcoma). To prevent the destruction of the
cells of the immune system, a diverse array of efforts is now under way to make such
cells resistant to HIV-1 infection. This approach has been termed intracellular immunization
(6).
HIV-1 replicates via a classic retroviral life cycle (Fig. 1). Virus entry is mediated
by the binding of the viral envelope protein to a specific receptor, termed CD4, which
is expressed on the cell surface of T-lymphocytes and certain monocyte/macrophage
populations. However, in contrast to other retroviruses, other receptors are also required
for cell entry. Such coreceptors (e.g., CXCR-4 and CCR5) have been found to be
chemokine receptors. Efforts are being made to develop genetic antivirals, which interfere
with the first step of viral infection (Fig. 2).
After entry into the cell, the viral RNA is reverse-transcribed into viral DNA by the
viral reverse transcriptase (RT). The resulting preintegration complex is then actively
transported across the nuclear membrane. Thus, in contrast to C-type retroviruses, HIV-
1 is capable of infecting quiescent cells. Many attempts are now also under way to
endow immune cells with genes that would prevent reverse transcription and/or integration
(Fig. 2).
Lentiviruses also express a number of critical regulatory genes from multiplyspliced
mRNAs. Thus, a series of studies are currently under way to test the potential
of genetic antivirals directed not only against the structural core and envelope proteins
(e.g., matrix proteins, RT, integrase, protease) but also against some regulatory proteins,
which are specific and essential for the life cycle of lentiviruses. HIV-1 contains
six regulatory genes, which are involved in the complex pathogenesis. For example, the
Tat gene is the major transcriptional transactivator of HIV-1 and is essential for activity
of the long terminal repeat (LTR) promoter. The Tat protein stimulates HIV-1 transcription
via an RNA intermediate called the transactivation response (TAR) region,
which is found just downstream of the 5� LTR. The product of the Rev gene ensures
the transport of unspliced viral RNA from the nucleus to the cytoplasm. Tat and Rev
are absolutely essential for HIV-1 replication, and therefore became major targets for
the development of genetic antivirals. Such antivirals attack the virus after integration
into the chromosomes of the host and are aimed at preventing or reducing particle formation
and/or release from infected cells (Fig. 2).
Other critical accessory proteins include Vpr (which leads to G2 arrest in the cell
cycle of infected cells), Nef (which stimulates viral production and activation of
infected cells), Vpu (which stimulates viral release), and Vif (which seems to augment
viral production in either early or late steps in the viral life cycle. These regulatory proteins
may be somewhat less crucial to viral load and replication in comparison with Tat
and Rev. Consequently, antiviral agents, which attack these proteins, are less likely to
significantly prevent infection and/or the spread of the virus.
Potential genetic inhibitors of virus replication should have four features, which
overcome the shortcomings of conventional treatments: First, they should be directed
against a highly conserved moiety in HIV-1, which is absolutely essential for virus
replication, eliminating the chance that new mutant variants may arise that can escape
this attack. Second, they must be highly effective and must greatly reduce or, ideally,
completely block the production of progeny virus. Third, they must be nontoxic. A