Human Immunodeficiency Virus (HIV), a retrovirus, is the etiological agent of acquired
immunodeficiency syndrome (AIDS). AIDS is characterized by the failure of the immune
system to protect against not only HIV but also common secondary viral and bacterial
infections. Viruses such as HIV-1 are intracellular pathogens that require host cell
machinery to maintain and generate new progeny. In that process, viruses disturb the
normal components of intrinsic immune responses within infected cells. After HIV infection,
the expression of various cellular restriction factors is notably up-regulated due
to interferon activation, partly because genes for most of these restriction factors
such as APOBEC3, TRIM, BST2/Tetherin, contain interferon-responsive promoters. Importantly,
these molecules have been focused on for both elite controllers as well as long-term
non-progressor AIDS patients.
The best characterized HIV restriction factors are encoded by the APOBEC3 and TRIM
gene families. The interplay between these cellular and viral proteins appears to
be an important factor in deciding the ultimate disease outcome. Interestingly, HIV
has also armed itself to counter various cellular intrinsic defense mechanisms, thereby
overcoming the intrinsic responses mounted by the host. Therefore, today there is
an intense effort in the HIV/AIDS field to define both important host-encoded antiviral
restriction factors and viral counter-defense mechanisms that play key roles in the
pathogenesis. Along these lines, the list of both restriction factors (proteins that
counter specific viral proteins) and repressive factors (inhibitors of HIV life cycle)
has been growing continuously. These factors not only include APOBECs, TRIM5α, p21/waf1,
and SAMHD1, which act on the incoming virus [1], but also TRIM22, TRIM28/KAP1, and
BST2/Tetherin, which exert their effect on post integration, assembly and budding.
In a recent manuscript by Liu et al., the authors utilized an siRNA screening method
to knockdown expression of 19,121 human genes [2]. They specifically looked for knockdown
of restriction factors that would rescue the early stages of HIV replication from
post entry to integration. The assay utilized viral pseudotypes HIV89.6R and HIV8.2N
which are capable of a single round of infection. To optimize the screening, they
used negative control siRNAs targeting cyclophilin B, PLK1 and GFP in target HeLa-CD4
cells. They identified 114 genes that affected a wide range of cellular activities
that could be classified as genes that defend against retroviral invasion. These factors
fall into various functional categories such as receptor signaling, vesicle trafficking,
transcription, mRNA processing, DNA/RNA surveillance, cross-nuclear membrane transport
and ubiquitination. Interestingly, they did not observe the presence of the classical
RNA-recognizing PRRs such as toll like receptors TLR3, 7 or 8 or RIG-1 like receptors
or MDA-5 in their screening. Genes such as AP2M1, DNM2, SETDB1, PAF1, CTR9 and RTF1
were identified and further characterized. Some of the gene products were previously
shown to be involved in the regulation of endocytosis (AP2M1 and DNM2), HIV pre-integration
complex transportation from cytoplasm to nucleus (NPIP, an interacting partner of
nuclear pore components such as NUP62), PAF 1 complex and protein methylase (SETDB1).
Their detailed investigation demonstrated the role of PAF1 complex as an important
repressive factor during intrinsic defense against HIV infection. Specifically, the
PAF1 complex appears to block HIV replication during the early events from post entry
to integration of proviral DNA into host genome. Intriguingly, they found that PAF1
complex is present ubiquitously in various cell lineages, which are common targets
of HIV that include monocytes, macrophages and T lymphocytes. Moreover, PAF1 complex
not only effectively represses HIV infection but also inhibits infection of evolutionarily
similar retroviruses such as HIV-2 and SIV.
SETDB1, another interesting protein identified, harbors enzymatic activity as lysine
methyltransferase. The SETDB1 possesses specificity for lysine 9 of histone H3 and
plays an important role in silencing transcription by depositing specific histone
marks, namely H3K9me2-3 during cell differentiation [3,4]. Additionally, SETDB1 has
been shown to induce Tat methylation, which results in reduced viral production [5],
and on the other hand methyl transferase inhibitors (similar to siRNA used here) lead
to the enhancement of virus production [6]. The finding of SETDB1 in the screen of
Li et al. further establishes the role of TRIM28 during proviral integration, which
it primarily executes by recruiting SETDB1 to the pre-integration complex [7]. It
will be interesting to determine how this enzyme regulates the activity of other viral
pre-integration proteins such as RT, Gag or IN on specific lysine residues, which
could acquire both acetylating and methylation marks.
In another article by Woods at al., the authors confirmed the role of HECT domain
and RCC1-like domain-containing protein 5 (HERC5), an interferon inducible gene that
restricts the early stages of HIV assembly [8]. Cells expressing HERC5 released 4.0-fold
less infectious virus than the control cells after a single round of replication.
Furthermore using published databases, the authors found that HERC5 expression is
significantly increased in patients in acute and chronic stages of infection but not
in non-progressors. Collectively, the data are consistent with previous reports [9-11],
where HERC5 restriction is different from the well established anti-HIV-1 activities
of ISG15-only expression.
Thus, these studies add a few more candidates to the ever growing list of HIV repressive
and restriction factors that inhibit HIV life cycle at various stages (Figure 1).
Taken together, these investigations are novel and open the door for a better understanding
of how host cellular factors control infection. Future confirmatory experiments using
primary infections of T-cells vs. macrophages and field isolate of the virus to elucidate
viral targets for each of these important factors and to elicit possible escape mutants
to their restriction will shed more light on this fascinating area of research.
Figure 1
Two new host repressive factors which inhibit HIV life cycle at different steps. PAF1
complex seems to inhibit early events of viral life cycle from reverse transcription
to integration step. On the other hand HERC5 appears to act at the later part of viral
life cycle, that is during the earlier stage of new viral particle assembly, most
probably by regulating the ISGylation of Gag protein of HIV.