Immunotherapy for Infectious Diseases

242 Dornburg and Pomerantz
domains of both the heavy and light chain of an antibody and are expressed from a single
gene. The resulting single-chain antibody can bind to its antigen with similar affinity
as an Fab fragment of the authentic antibody molecule.
sFvs against viral proteins (e.g., the envelope, integrase, RT, matrix, Rev, and Tat
proteins) that lack a hydrophobic signal peptide are expressed intracellularly and are
retained in the cytoplasm. They are capable of binding to specific domains of HIV-1
proteins and have been shown to prevent integration of the HIV-1 into the host chromosome
or to block virus maturation. Some intracellular sFvs have been found to
greatly reduce the ability of HIV-1 to replicate; other sFvs have had moderate success
(4,10,11).
RNA-Based Inhibitors
Antisense RNAs and Ribozymes
Artificial antisense oligonucleotides (RNAs or single-stranded DNAs) have been
successfully used to selectively suppress the expression of various genes. Furthermore,
the presence of double-stranded RNA inside the cell can induce the production of interferon
and/or other cytokines, stimulating an immune response. Indeed, it has also been
reported that the expression of RNAs capable of forming a double-stranded RNA molecule
with the HIV-1 RNA (antisense RNAs) can significantly reduce the expression
of HIV-1 proteins and consequently the efficiency of progeny virus production (12–18).
Ribozymes are very similar to antisense RNAs. They bind to specific RNA sequences,
but they are also capable of cleaving their target at the binding site catalytically (Fig.
3). Thus, they may not need to be overexpressed to fulfill their biologic function. Many
sites in the HIV-1 genome have been successfully targeted. However, several questions
are being asked, e.g., can an efficient subcellular colocalization be obtained, in particular
in vivo? Will the target RNA be efficiently recognized owing to secondary and tertiary
folding of the target RNA? Will RNA binding proteins prevent efficient binding?
Experimentation in several laboratories has addressed these problems, and clinical trials
have been initiated to test the therapeutic effect of ribozymes in AIDS patients.
RNA Decoys
RNA decoys are mutant RNAs that resemble authentic viral RNAs that have crucial
functions in the viral life cycle. They mimic such RNA structures and decoy viral
and/or cellular factors required for the propagation of the virus (8). For example, HIV-
1 replication largely depends on the two regulatory proteins Tat and Rev. These proteins
bind to specific regions in the viral RNA, the TAR loop and the Rev response
element (RRE), respectively. Tat binding to TAR is crucial in the initiation of RNA
transcription, and Rev binding to RRE is essential in controlling splicing, RNA stability,
and the transport of the viral RNA from the nucleus to the cytoplasm. These two
complex secondary RNA structures within the HIV-1 genome appear to be unique for
the HIV-1 virus, and no cellular homologous structures have been identified. Thus,
such structures appear to be valuable targets for the attack with genetic antivirals.
HIV-1 target cells have been endowed with genes that overexpress short RNAs containing
TAR or RRE sequences. These RNA molecules capture Tat or Rev proteins,
preventing the binding of such proteins to their actual targets. Consequently, HIV-1
replication is markedly impaired. This strategy has the advantage over antisense RNAs