Download Intrinsic immunity: a front-line defense against viral attack

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Intrinsic immunity: a front-line defense
against viral attack
Paul D Bieniasz
In addition to the conventional innate and acquired immune responses, complex organisms have evolved an array of dominant,
constitutively expressed genes that suppress or prevent viral infections. Two major cellular defenses against infection by
retroviruses are the Fv1 and TRIM5 class of inhibitors that target incoming retroviral capsids and the APOBEC3 class of cytidine
deaminases that hypermutate and destabilize retroviral genomes. Additional, less well characterized activities also inhibit viral
replication. Here, the present understanding of these ‘intrinsic’ immune mechanisms is reviewed and their role in protection
from retroviral infection is discussed.
Evolution has equipped complex organisms with an array of tools to
suppress the replication of the viruses that parasitize them. The conventional innate and acquired immune systems are very effective at
reducing the burden of viral disease, but their most important
weakness is that that they work only in a responsive way. Thus, in a
naive host, a signal-dependent mobilization of antiviral effectors is
required. This response is triggered only by recognition of an
already replicating virus. Indeed, the effectiveness of adaptive
immunity is dependent on a considerable degree of immunological
‘education’. Obviously, this takes time. Meanwhile, the viral invader
can disseminate and sometimes irreversibly colonize the host.
Innate immunity provides antiviral defenses that can be deployed
more rapidly, sometimes within hours, but the mobilization of most
innate immune effectors generally requires not only recognition of
the presence of an unwelcome visitor but also some intracellular
and/or intercellular signaling event (such as, for example, interferon
release and induction of an antiviral state) that occurs in response to
viral replication that is already in progress.
A collection of activities that fall outside the conventional definitions of the innate and adaptive immune systems but nevertheless
provide potent protection from viral infection is called ‘intrinsic
immunity’ in this review. Some of the entities responsible for these
activities are called ‘restriction factors’ and for the most part are
constitutively expressed inhibitors of viral replication. Although
such activities could, in principle, be upregulated in response to
infection, their activity does not require any virus-triggered signaling or intercellular communication. These activities might be
considered to comprise the front line of host defense against viral
infection because they are active and, in some instances, most
effective in the context of the very first virus-cell interaction that
Aaron Diamond AIDS Research Center, New York, New York 10016, USA.
Correspondence should be addressed to P.D.B. ([email protected]).
Published online 20 October 2004; doi:10.1038/ni1125
NATURE IMMUNOLOGY VOLUME 5 NUMBER 11 NOVEMBER 2004
heralds the transmission of a virus to an immunologically naive
host. Therefore, a characteristic of intrinsic immunity is that it is
active in many cells cultured in the laboratory. Indeed, whereas the
in vitro cellular tropism of viruses is generally thought of as being
defined mainly by variation in certain host cell molecules required
for virus replication, similar variation in restriction factors is an
almost equally important tropism determinant. In fact, differential cell tropism has been the main tool that has been used to identify and characterize the components of intrinsic immunity
discussed herein.
Perhaps because they are the most widely studied viruses, or
perhaps because evolution has singled them out as uniquely
attractive targets, much of what is known about intrinsic immunity stems from studies on retroviruses. Although several decades
have passed since the first descriptions of a specific and constitutive antiviral activity resulting from the expression of a cellular
gene, the study of restriction factors can be reasonably described
as being in its infancy. At present, it barely receives mention in
most virology texts and it is unknown how many activities exist
and how many viruses are targeted. It may be that the present
knowledge of intrinsic immunity represents only a small fraction
of its true complexity.
Immunity by turning retroviruses against themselves
The simplest forms of what could be called intrinsic immunity are
special cases of viral interference, which arise because retroviral
genomes are inherited like cellular genes when they infect germline
cells. The ability to express ‘endogenous’ viral proteins in either
intact or defective forms can sometimes induce resistance to infection by related exogenous retroviruses. A classical example of interference among exogenous retroviruses occurs when cellular
receptors become blocked and/or downregulated as a consequence
of retroviral infection and expression of viral envelope proteins1,2.
Therefore, when an organism carries an endogenous retrovirus, its
cells can, in effect, synthesize their own exquisitely specific and
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effective viral entry inhibitor and become resistant to infection by
other retroviruses that use the blocked receptor. Although no examples of such a phenomenon are known in humans, inherited and
expressed retroviral envelope genes exist in mice3 and can indeed
provide robust protection against pathogenic exogenous retroviruses. Defective viral proteins that function as components of
multimeric viral protein complexes can also provide resistance by
forming inactive heteromultimers with replication-competent
exogenous virus-derived counterparts. For example, at least one of
the 20 or so endogenous Jaagsiekte sheep retrovirus loci encodes a
defective variant Gag protein that potently blocks the assembly of it
replication competent and pathogenic ‘cousin’4. Thus, viral proteins
encoded by the host genome can be highly efficacious and specific
inhibitors of viral replication by competing with or sequestering
exogenous viral proteins.
Fv1: The prototype restriction factor
The idea that cellular genes could encode constitutive inhibitors of
retroviral replication had its origin in genetic studies of laboratory
mice. These experiments, which began in the late 1960s, led to the
discovery of heritable traits that governed susceptibility to leukemia
induced by the Friend strain of murine leukemia virus (MLV)5,6.
Such traits were dubbed Friend virus susceptibility (Fv) genes, and
two (Fv1 and Fv4) were of special interest because they showed a
dominant, single locus pattern of inheritance, and cultured cells
from mice containing them were resistant to infection by FriendMLV7–11. Fv4 was shown to encode a retroviral envelope protein,
and to block infection by the aforementioned receptor interference
mechanism, but peculiar characteristics of Fv1-induced resistance
suggested that it would be unique and interesting. Fv1-mediated
resistance to MLV infection in vitro was partial, but resulted in a
considerable decrease in the frequency of leukemias in mice carrying restricted virus strains12,13. Two major allelic variants of Fv1,
called Fv1n and Fv1b, were shown to exist in laboratory mice and to
permit or restrict infection by specific classes of MLV strains10,11.
Thus, N-tropic MLV strains (N-MLV) efficiently infected cells from
mice that were Fv1n/n but not those from mice that were Fv1b/b,
whereas B-tropic strains (B-MLV) showed the opposite phenotype.
A third class of commonly used MLV strains, called NB-tropic,
resisted the effects of both Fv1n and Fv1b (ref. 10). Other Fv1 alleles
with distinguishable specificities, as well as null alleles, were also
shown to exist in laboratory and wild mice11,14.
Curious features of Fv1-induced resistance included the finding
that the viral determinants for N and B tropism could be mapped
to residue 110 of the capsid protein (CA110)15,16. In addition, MLV
titration curves on resistant cells fit a ‘two-hit’ model whereby the
frequency of infection was proportional to the square of the inoculating virus dose17–20. Thus, the inhibition of a particular virus particle could be abrogated by prior or simultaneous infection by
other virus particles. Abrogating particles did not themselves need
to be infectious21,22, but they did need to be derived from a
restricted virus strain. These data indicated that Fv1 encoded a
unique inhibitor that targeted the incoming viral capsid but could
be saturated and overwhelmed by simultaneous challenge by multiple virion particles.
The later discovery that Fv1 was derived from a retrovirus Gag
gene23 and shared about 60% homology with hundreds of copies
of a particular endogenous retrovirus family found in many mammals suggested that it worked in a way that was akin to viral interference. Thus, the capsid-like Fv1 protein could potentially mimic,
and thereby bind to, individual CA molecules of the incoming
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MLV capsid in the same way that CA monomers multimerize.
However, although the precise mechanism by which Fv1 blocks
infection is still unknown, this model seems flawed because, other
than being of retroviral origin, Fv1 is unrelated to MLV Gag23.
Moreover, the amino acid (CA110) that governs Fv1n and Fv1b
sensitivity in MLV capsid probably resides on the outside of the
intact viral capsid and not in determinants predicted to make CACA contacts24,25. Conversely, the residues that differ between Fv1n
and Fv1b, and therefore determine restriction specificity, reside
toward the C terminus of the protein26,27. Thus, whereas Fv1 probably had its genesis as an endogenous retrovirus, it seems to have
been appropriated by the cell and to have acquired a completely
new and unexpected activity.
Fv1-like activities targeting multiple retroviruses
For many years, the existence of Fv1-type inhibitors of retrovirus
infection was thought to be unique to the mouse. However, related
activities were subsequently found to be very widespread in mammals. Indeed, cells from several mammalian species, including
humans, acted as if they were Fv1b/b in that they specifically resisted
N-MLV infection28. In humans, the supposed inhibitor was
referred to as Ref1 (for restriction factor 1) and the same capsid
residue (CA110) that controlled sensitivity to Fv1 also controlled
sensitivity to Ref1 (ref. 28). Paradoxically, the availability of the
cloned Fv1 enabled the demonstration that no genes very closely
related to Fv1 were present in any mammalian species except
mice23 and inspection of the human genome has shown no intact
Fv1-like endogenous retrovirus Gag sequences that seemed likely to
be responsible for an Fv1-like activity.
It also became clear that lentiviruses, including human immunodeficiency virus 1 (HIV-1), could be subject to similar restrictions29–32.
Inhibitors of lentivirus infection were shown to be saturable by
incoming virions and to target determinants in the lentiviral capsid29,33,34. Because of these similarities to Fv1 restriction, the
inhibitors presumed to be responsible for capsid-specific lentivirus
inhibition were called Lv1 (lentivirus susceptibility factor 1)29, and
their presence partly explained why HIV-1 could not replicate efficiently in nonhuman primates cells34. Moreover, the saturable property of Lv1 and Ref1 was exploited to indicate that each was capable
of inhibiting multiple retroviruses that shared little sequence homology35, although it was unclear whether Lv1 and Ref1 were different
entities or were species-specific variants of the same factor.
A rhesus monkey–derived cDNA that has several of the properties expected of Lv1 has been identified36. The cDNA confers strong
resistance to HIV-1 but not to simian immunodeficiency virus isolated from rhesus monkeys and encodes the ‘alpha’ spliced variant
of tripartite interaction motif 5 (TRIM5α). Depletion of TRIM5α
from rhesus cells using small interfering RNA substantially
increases their sensitivity to HIV-1 infection, and the CA protein is
the determinant that governs HIV-1 susceptibility to inhibition by
TRIM5α36. TRIM5 is one of a large family of proteins whose Nterminal domains contain RING, B-box and coiled-coil motifs (the
TRIM domain), linked to a variable C-terminal domain, which in
the case of TRIM5α is a B30.2 or SPRY domain37. Characterization
of TRIM5α from other species suggests that it has a general function in defense against retroviral infection (Fig. 1). Indeed, whereas
human TRIM5α lacks the ability to strongly inhibit HIV-1, both
human and nonhuman primate variants of TRIM5α seem capable
of blocking infection by other retroviruses. For example, the
human form of TRIM5α is the inhibitor previously referred to as
Ref1 (refs. 38–40). The African green monkey TRIM5α protein is
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particularly impressive in that it has the ability to inhibit at least
four highly divergent retroviruses, and this ‘broad spectrum’ of
inhibition explains the previously described cross-saturation phenomena35,38. Conversely, a rather unique form of TRIM5 exists in
owl monkeys, a New World primate41. In this species, a remarkable
retrotransposition event has resulted in the insertion of a
cyclophilin A (CypA) pseudogene into the TRIM5 locus and the
expression of a TRIM5-CypA fusion protein. Because HIV-1 CA is
thus far unique among retroviral CA proteins in its ability to bind
CypA, this finding explains why owl monkey cells specifically resist
HIV-1 infection and that restriction in that species requires the
CypA binding activity of HIV-1 CA42. Moreover, it also suggests
that the C-terminal domain of TRIM5α might be responsible for
restriction specificity, although other sequences in the TRIM
domain of TRIM5α are also required for inhibition of infection36.
At present, little is known of the molecular details by which Fv1
and TRIM5α actually prevent retrovirus infection. While Fv1 and
TRIM5α probably bind directly to incoming retroviral capsids, this
has not yet been formally proven, despite attempts in several laboratories to demonstrate Fv1-MLV capsid interaction. Difficulties arise
most likely because it is not easy to configure in vitro binding assays
that precisely recapitulate the capsid structure that forms during
virion maturation43,44, is delivered to the target cell cytoplasm during infection and is thought to be recognized by these factors.
Nonetheless, it may be easier to demonstrate capsid-binding activity for TRIM5α and it seems almost inescapable that TRIM5-CypA
will bind HIV-1 capsid, as CypA itself has this activity45. Thus, Fv1,
TRIM5α and TRIM5-CypA are intracellular neutralizing factors
that recognize and probably bind to incoming retroviral capsids. At
present, however, it is unclear whether the capsid–restriction factor–binding event itself is sufficient for restriction. Potentially, this
could be the case with the restriction factor interfering with some
essential capsid–host factor interaction, accelerating or retarding
capsid disassembly or trapping the capsid such that it cannot traffic
appropriately within the cell cytoplasm. Both TRIM5α and Fv1
seem to be multimeric37,46, and polyvalent interactions between
restriction factor and capsid could plausibly have substantial effects
on disassembly and/or trafficking. Alternatively, restriction could
involve recruitment of additional factors that lead to the destruction of the retroviral core. In the case of TRIM5α, there are hints
that the ubiquitination system might be brought to bear on incoming viral capsids: TRIM5α contains a RING domain whose mutation reduces restriction activity36, and ubiquitin conjugation
activity has been demonstrated for an alternatively spliced TRIM5
variant47. It may be that TRIM5 ubiquitin ligase activity can
explain earlier observations that proteasome inhibitors can
increase HIV-1 infectivity and the stability of nascent reverse transcripts48,49. In addition, given the involvement of ubiquitin in
transmembrane protein trafficking50, TRIM5α-mediated ubiquitination could target retroviral capsids to the lysosomal lumen,
although incoming retroviral capsids are not thought to be membrane bound. At present, all of these models are very speculative,
and given the apparently different subcellular localization of
TRIM5α and Fv1 (refs. 37,46) and the fact that TRIM5α prevents
the accumulation of reverse transcripts whereas Fv1 does not36,51,
it would seem that there are at least subtle differences in restriction
mechanism or timing (Fig. 1). It is surprising, therefore, that at Fv1
activity seems, at least when it is expressed in human cells, to be
dependent on TRIM5α39. Clearly, much remains to be learned
about what actually occurs when Fv1 or TRIM5α blocks infection
by an incoming retrovirus.
Cytidine deamination as an antiretroviral defense
A second major cellular antiretroviral activity does not prevent the
cell from becoming infected but is intended to ensure that the viral
progeny of that infection are ‘poisoned’. The realization that cytidine deaminases are potent intrinsic inhibitors of retrovirus replication arose from studies of the HIV-1 virion infectivity factor
(Vif) protein. For some years, it was known that the requirement
for Vif during HIV-1 replication was governed by host factors:
some cell lines, called ‘permissive’, were able to support infection
by Vif-deleted HIV-1 strains, whereas others, called ‘nonpermissive’, could not52. The nonpermissive phenotype was shown to be
dominant in heterokaryons53,54 and defined by the cell in which
the virus was assembled, rather than its target52. This finding provoked the search for gene products that could imprint a noninfectious phenotype on HIV-1 virions assembled in otherwise
permissive cells. This search yielded APOBEC3G, a homolog of the
known cytidine deaminases55 Other enzymes in this class include
APOBEC1 (apolipoprotein editing complex 1), which edits
apolipoprotein B mRNA in a highly site-specific way and activation-induced cytidine deaminase (AID), which edits DNA at the
immunoglobulin locus to promote antibody diversification.
APOBEC3G is incorporated into HIV-1 particles and cytidine
deamination reactions ensue during reverse transcription in the
subsequent generation of target cells56–59 (Fig. 2). Because
APOBEC3G specifically deaminates single stranded DNA, the
minus strand of nascent viral DNA, which is synthesized first, is
targeted primarily60. Cytidine deamination yields uracil, and this
can have one of two consequences: First, a U residue on the minusstrand template is replicated to an A residue on the plus strand so
the net effect of cytidine deamination event is a G-to-A mutation
(Fig. 2). However, the mutation is only established if the viral
DNA survives; uracil-containing DNA is a target for uracil
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Figure 1 Capsid-specific restriction factors. After entry into the cytoplasm,
retroviral capsids can be recognized and infection blocked by one of many
factors. Fv1 is unique to the mouse and blocks infection by MLV only, in an
Fv1 allele– and MLV strain–specific way. TRIM5α, which is present in most
primates, can block infection by a range of retroviruses including N-tropic
MLV, equine infectious anemia virus (EIAV), simian immunodeficiency virus
(SIV) and HIV-1. The precise spectrum of TRIM5α antiretroviral activity
depends on its species of origin. A unique form of TRIM5 exists in owl
monkeys, due to transposition of a CypA pseudogene, and the resulting
fusion protein inhibits HIV-1 because of the latter’s CypA binding activity.
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Figure 2 Inhibition of retroviral replication
by cytidine deamination and its restoration
by lentivirus Vif proteins. APOBEC3G and/or
APOBEC3F cytidine deaminases can be
incorporated into retroviral particles by virtue
of interactions with viral nucleic acid and/or
protein components. Thereafter, cytidine
deamination on the nascent minus-strand
DNA (inset) ultimately results in G-to-A
hypermutation of the plus strand. APOBEC3G
and APOBEC3F preferentially mutate GG
and GA dinucleotides, respectively, although
this preference is not absolute. Uracilcontaining DNA is predictably less stable,
and hypermutated DNA may encode defective
viral proteins. To counteract this antiviral
activity, lentivirus Vif proteins recruit the
Cul5–elongin B –elongin C–Rbx1 ubiquitin
ligase complex to APOBEC3G or APOBEC3F
in the virus-producing cell, resulting in
its polyubiquitination and degradation
by proteasomes.
DNA-glycosylase (UDG). Its action leaves an abasic site, which is
predictably a target for specific endonucleases. In fact, several
reports have documented the incorporation of UDG into HIV-1
particles through interactions with the virion proteins Vpr and/or
integrase61–64. Although the Vpr mutations that abolish its interaction with UDG also apparently modulate the frequency with
which mutations arise in the HIV-1 genome65, whether this occurs
as a consequence of virion encapsidated UDG action on APOBECmodified viral DNA is yet to be established. Nonetheless, the overall consequence of APOBEC3G incorporation into HIV-1 virions
is the subsequent generation of viral DNA that is hypermutated
and has impaired stability.
In general, APOBEC3G-mediated hypermutation should be fatal,
depending on the number and position of deamination events within
a viral genome, and may be especially so because of APOBEC3G’s target site specificity; GG dinucleotides are preferred56–59. Thus, any
tryptophan codon (TGG) that is targeted becomes a stop codon, a
mutation that would usually be lethal to a retrovirus. There have been
several gene duplication events at the APOBEC locus in primates66,
such that primates have several APOBEC3 genes while mice have only
one, and at least one of the additional genes, APOBEC3F, that is
closely related to APOBEC3G is expressed in lymphocytes and
encodes a cytidine deaminase that has very potent antiretroviral
activity67–70. APOBEC3F has a subtle, but perhaps important, target
site preference for hypermutation (GA rather than GG). Because GAto-AA substitution would less frequently result in the formation of
new stop codons, it is possible that APOBEC3F might be less potently
lethal to the virus than APOBEC3G.
In the true manner of an ‘arms race’ between virus and host,
lentiviruses have acquired a unique way of counteracting APOBEC
restriction. The HIV-1 Vif protein simultaneously binds to APOBEC3G
and to the Cul5–elongin B–elongin C–Rbx1 ubiquitin ligase complex
and, as a result, APOBEC3G becomes polyubiquitinated and degraded
by proteasomes71–75 (Fig. 2). Additional mechanisms have been proposed for Vif ’s activity76,77, but the fact that dominant negative
mutants of Cul5 abolish the effect of Vif indicates that these contribute
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in relatively minor ways to Vif function71. APOBEC3F is also degraded
as a consequence of Vif expression, although it seems slightly less Vifsensitive than does APOBEC3G67,70. Even when Vif is present, G-to-A
substitutions are unusually frequent, suggesting that APOBEC3G or
APBOBEC3F degradation is incomplete. Thus, some level of APOBECmediated mutation is probably tolerated and might even be even
encouraged by lentiviruses to generate diversity in the face of an evolving array of antiviral acquired immunity. The fact that circulating
lentivirus genomes are relatively adenosine rich is probably because
they bear the ‘scars’ of both APOBEC3G and APOBEC3F action67,69,70.
Other factors, other viruses and intrinsic immunity
In principle, DNA-specific, cytoplasmic cytidine deaminases such as
APOBEC3G and APOBEC3F might target any virus whose life cycle
includes cytoplasmic DNA replication (such as, for example,
poxviruses and hepadnaviruses). However, although APOBEC3G
can inhibit the amplification of hepatitis B virus replicons in culture, this is not due to its DNA-editing activity78. At present, the relevance of this finding to natural hepatitis B virus infection is
somewhat unclear because APOBEC3G is not expressed in the
healthy liver. Nonetheless, it is possible that hepatitis could induce
APOBEC3G and/or APOBEC3F expression. Some strains of hepatitis B virus apparently carry specific G-to-A changes that affect
pathogenesis79, and it is possible that cytidine deamination could be
responsible for such changes.
Intuitively, it would seem somewhat more troublesome for cells to
make use of cytidine deamination as a constitutive defense against
DNA viruses whose genome replicates in the nucleus; this would
likely lead to hypermutation of cellular DNA and cell death or transformation. However, the death of a few virus-infected cells is not necessarily catastrophic for the organism as a whole, and findings suggest
that the expression of the nuclear cytidine deaminase AID is induced
in B cells after infection by the RNA virus hepatitis C virus80. Thus,
AID or other nuclear cytidine deaminases could in principle be components of responsive innate immunity and induce both cell and viral
death in response to infection.
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Viruses that replicate exclusively using RNA genomes and intermediates are likely to be troublesome targets for cytidine deaminases,
because their genomes replicate in an environment that contains a
complex mixture of cellular RNAs. Thus, an RNA-specific antiviral
cytidine deaminase would probably require a substantial degree of
sequence specificity, which immediately provides opportunities for
viral escape. Nonetheless, if specificity could be established simply by
confining editing activity to viral particles or replication sites, rather
than by targeting sequence-specific sites in viral genomes, then an
effective antiviral strategy could result. Indeed, certain cytidine
deaminases do seem capable of causing C-to-T changes in retroviral
genomes through editing of RNA, perhaps in virion particles81.
The genomes of certain viruses seem to be the target of another
previously unknown host defense factor that likely recognizes RNA:
zinc-finger antiviral protein inhibits the accumulation of cytoplasmic
RNA for some retroviruses and alphaviruses and, in the case of
alphaviruses, is capable of very large decreases in viral yield82,83. Its
mechanism of action may be related to that of the tristetraprolin-like
proteins that it resembles, which induce the degradation of cytokine
mRNAs by binding to AU-rich sequences in their 3′ untranslated
regions and recruiting them to the exosome. How zinc-finger antiviral protein distinguishes viral from cellular RNAs is not yet known.
The existence of yet another intrinsic immune mechanism has been
suggested; namely inhibition of virus assembly or release84. The viral
Vpu protein is required for the efficient release of HIV-1 from certain
cell lines but not others85–87, and fusion of cells that do or do not
require the presence of Vpu for efficient virus release results in the formation of heterokaryons that show the Vpu-dependent virus release
phenotype84. This observation indicates the existence of a dominant
inhibitor of HIV-1 release that is counteracted by Vpu. Moreover,
because Vpu can facilitate the release of many divergent retroviruses85,
then the presumed release inhibitor must have a unique mechanism of
action that targets virus release in a rather general way, but what the
inhibitor is and how it works are completely unknown at present.
Intrinsic immunity and prevention of viral disease
As can be gleaned from the discussion above, there is a very rapidly
expanding literature describing a broad array of previously unknown
cellular activities that are capable of inhibiting viral replication in
vitro. Are these factor important in protection from viral diseases?
Using retroviruses as an example, it might be thought somewhat
unlikely. Retroviruses replicate and mutate at a far higher frequency
than their hosts, and just as it seems rather trivial for HIV-1 to evolve
resistance to antiretroviral drugs (at least when applied one at a time),
then cellular inhibitors of retrovirus replication that target a specific
step in the retroviral life cycle should be readily escaped. Indeed, the
propensity of retroviruses such as HIV-1 to generate huge numbers of
mutants means that even adaptive immunity seems to have only limited effectiveness in the context of a chronic infection.
One way for intrinsic immunity to be more effective than might be
otherwise predicted is by evolving multiple ways to attack. As has
been learned using chemotherapeutic approaches to treat infectious
agents or tumors, multiple attacks are almost always better than one.
Thus, cells may have evolved, in effect, TRIM- and APOBEC-based
‘combination therapy’. Both of the main TRIM- and APOBEC-based
inhibitors of retroviruses are members of multigene families that have
elaborated during mammalian evolution37,66. There are dozens of
TRIM genes in mammalian genomes, many of which express alternatively spliced mRNAs, and both TRIM1 and TRIM19 (also known as
promyelocytic leukemia protein) have some antiretroviral activity
against MLV and HIV-1, respectively88,89. Arsenic treatment of cells,
which is known to disrupt promyelocytic leukemia protein, often
enhances retrovirus infection88,90, and although this activity was
originally thought to be mediated through TRIM19 (ref. 88), the
apparent sensitivity of TRIM5 to arsenic39 means that perhaps additional TRIM proteins might be the target through which arsenic
enhances retroviral infection. How extensive and frequent antiviral
activity is among TRIM proteins remains to be determined, but it is
possible that the potent antiretroviral activity showed by TRIM5α
reflects a general antiviral function of the TRIM gene family, and
clearly it would be more difficult for a retrovirus to escape a concerted
attack on its capsid by multiple different TRIM proteins.
The sensitivity of APOBEC3G to Vif proteins is governed by one (or
a few) APOBEC amino acid(s) that differ(s) between APOBEC3G and
APOBEC3F91–94. Thus, to be truly effective, a Vif protein must counter
two different targets. Although HIV-1 Vif has apparently been able to
adapt to suppress the antiviral activity of both APOBEC3G and
APOBEC3F, perhaps it would not have been able to do so if at the same
time viral replication were suppressed by an inhibitory TRIM protein.
The idea that intrinsic immunity is most effective in the context of
multiple attacks on a single virus is no less important than in considerations of conventional immunity and chemotherapy.
To further increase its effectiveness, intrinsic immunity may target
viral components or steps in the viral life cycle that are not amenable
to the generation of escape mutants. If one considers how a virus
would escape an inhibitor that recognizes and binds to one of its components, the simplest escape would be to acquire mutations in the
inhibitor-binding site. For example, escape from APOBEC3G or
APOBEC3F could involve the acquisition of mutations that cause a
failure to incorporate them into viral particles. However, in this
example it seems that APOBEC3G incorporation into retroviral particles is not a very specific process and may require only nonspecific
RNA95–99. Because it would be impossible for retroviruses to replicate
in the absence of packaged RNA, then escape from the antiviral affects
of APOBEC3G should be very difficult indeed. Nonspecific virion
incorporation might explain why lentiviruses have taken the very
demanding evolutionary step of acquiring a new gene whose sole purpose seems to be to remove APOBEC3G and APOBEC3F from the
virus-producing cell. It is unclear how retroviruses that lack Vif genes
escape the inhibitory cytidine deaminases, but it may be less simple
than avoiding cells in which they are expressed69,100.
The idea of somewhat ‘relaxed’ specificity may also be important in
the context of TRIM5-based restriction. Each of the TRIM5α variants
tested thus far is capable of inhibiting retroviruses with very little
sequence homology. However, the monomeric CA protein structures
and the multimeric hexagonal lattice configurations that they adopt
in the context of a viral capsid are probably conserved among retroviruses25. Thus, it is reasonable to suppose that these ‘shapes’ might
be important determinants of recognition, with large interaction surfaces between inhibitor and capsid, rather than short peptide motifs.
In such a scenario, escape from the inhibitor might be very difficult. A
parallel is the rather broad specificity with which Toll-like receptors
recognize their ligands and trigger innate immune responses101; in
this case, a rather small number of invariant receptors can provide
defense against a rather large number of pathogens by recognizing
‘patterns’ rather than highly specific antigens. Nonetheless, in the case
of TRIM5α escape is clearly possible, because retroviruses from a
given species are not strongly restricted by TRIM5α variants
expressed by the same species. For HIV-1 in humans, there is evidence
that HIV-1 capsid evolved CypA-binding activity as a viral defense
against capsid-specific restriction factors42, although whether human
TRIM5α imposed this requirement is not clear at present.
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Based on the species-specific effects of some restriction factors, it
is reasonable to suppose that they might evolve specifically to protect from zoonotic infections, in some cases. Clearly, TRIM5α and
APOBE3G tend to be more effective suppressors of retroviruses
from heterologous species36,38–40,77,89. One can only speculate, for
example, how many times retroviruses attempted to colonize
human populations only to be thwarted by one or both of these factors. Alternatively, restriction factor ‘mutants’ in a host population
could be selected because they acquire resistance to retroviruses that
are endemic to their contemporaries that express inactive ‘wildtype’ restriction factors. In principle, a selective advantage conferred to the host by restriction factor mutation could ultimately
lead to extinction of a given retrovirus within a species (our
genomes carry ample evidence of extinct retroviruses). Thus, our
view of the role of intrinsic immunity in protection from viral
attack should not be colored by the fact that in most cases, we probably study instances in which it has failed. HIV-1 in humans, for
example, seems largely resistant to the effects of both human
TRIM5α and APOBEC3G. Thus, the modern forms of these factors
may have been evolved to tackle viruses that they now rarely or
never encounter, rather than those that now circulate. In fact, it is
unclear whether human TRIM5α, APOBEC3G and/or APOBEC3F
provide any disease-ameliorating effects at all against retroviruses
that now threaten human health.
Concluding remarks
The study of restriction factors and intrinsic immunity to viruses has
substantially increased in prominence recently. The replication of all
viruses, but perhaps that of retroviruses in particular, is intimately
tied to cellular processes. Probably because of this there has been a
tendency to view any negative regulator of a cellular process that is
used by viruses as a ‘restriction factor’. Whereas some degree of negative regulation of cellular processes may indeed have evolved to suppress viral infections, cells almost certainly did not evolve negative
regulation simply to combat viruses, and caution should be exercised
before a negative regulator is dubbed a restriction factor. For the identified factors described here, there is reasonable indication, based on
viral specificity or on the existence of viral ‘countermeasures’, that
certain cellular proteins have evolved primarily or exclusively to combat viral infection. Nonetheless, the evolution of viruses and their
hosts are inextricably intertwined, and studies of intrinsic immunity
have uncovered new and interesting dimensions in the complex and
ongoing battle between viruses and their hosts. Perhaps we are just
beginning to understand how cells defend themselves against viruses.
ACKNOWLEDGMENTS
I thank T. Hatziioannou for comments on the manuscript; and B. Cullen, M.
Malim and M. Palmarini for sharing manuscripts before publication. Supported
by the National Institutes of Health, the American Foundation for AIDS Research,
GlaxoSmithKline.
5.
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10.
11.
12.
13.
14.
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COMPETING INTERESTS STATEMENT
The author declares that he has no competing financial interests.
31.
Published online at http://www.nature.com/natureimmunology/
32.
33.
1.
2.
3.
4.
Sommerfelt, M.A. & Weiss, R.A. Receptor interference groups of 20 retroviruses
plating on human cells. Virology 176, 58–69 (1990).
Miller, A.D. & Wolgamot, G. Murine retroviruses use at least six different receptors
for entry into Mus dunni cells. J. Virol. 71, 4531–4535 (1997).
Ikeda, H. & Sugimura, H. Fv-4 resistance gene: a truncated endogenous murine
leukemia virus with ecotropic interference properties. J. Virol. 63, 5405–5412
(1989).
Mura, M. et al. Late viral interference induced by transdominant Gag of an endogenous retrovirus. Proc. Natl. Acad. Sci. USA 101, 11117–11122 (2004).
1114
34.
35.
36.
Odaka, T. & Yamamoto, T. Inheritance of susceptibility to Friend mouse leukemia
virus. 11. Spleen foci method applied to test the susceptibility of crossbred progeny between a sensitive and a resistant strain. Jpn. J. Exp. Med. 35, 311–314
(1965).
Lilly, F. Susceptibility to two strains of Friend leukemia virus in mice. Science 155,
461–462 (1967).
Suzuki, S. FV-4: a new gene affecting the splenomegaly induction by Friend
leukemia virus. Jpn. J. Exp. Med. 45, 473–478 (1975).
Gardner, M.B., Rasheed, S., Pal, B.K., Estes, J.D. & O’Brien, S.J. Akvr-1, a dominant murine leukemia virus restriction gene, is polymorphic in leukemia-prone wild
mice. Proc. Natl. Acad. Sci. USA 77, 531–535 (1980).
Rasheed, S. & Gardner, M.B. Resistance to fibroblasts and hematopoietic cells to
ecotropic murine leukemia virus infection; an Akvr-1R gene effect. Int. J. Cancer
31, 491–496 (1983).
Hartley, J.W., Rowe, W.P. & Huebner, R.J. Host-range restrictions of murine
leukemia viruses in mouse embryo cell cultures. J. Virol. 5, 221–225 (1970).
Pincus, T., Hartley, J.W. & Rowe, W.P. A major genetic locus affecting resistance to
infection with murine leukemia viruses. I. Tissue culture studies of naturally occurring viruses. J. Exp. Med. 133, 1219–1233 (1971).
Rowe, W.P. Studies of genetic transmission of murine leukemia virus by AKR mice.
I. Crosses with Fv-1 n strains of mice. J. Exp. Med. 136, 1272–1285 (1972).
Rowe, W.P. & Hartley, J.W. Studies of genetic transmission of murine leukemia
virus by AKR mice. II. Crosses with Fv-1 b strains of mice. J. Exp. Med. 136,
1286–1301 (1972).
Kozak, C.A. Analysis of wild-derived mice for Fv-1 and Fv-2 murine leukemia virus
restriction loci: a novel wild mouse Fv-1 allele responsible for lack of host range
restriction. J. Virol. 55, 281–285 (1985).
DesGroseillers, L. & Jolicoeur, P. Physical mapping of the Fv-1 tropism host range
determinant of BALB/c murine leukemia viruses. J. Virol. 48, 685–696 (1983).
Kozak, C.A. & Chakraborti, A. Single amino acid changes in the murine leukemia
virus capsid protein gene define the target of Fv1 resistance. Virology 225,
300–305 (1996).
Decleve, A., Niwa, O., Gelmann, E. & Kaplan, H.S. Replication kinetics of N- and
B-tropic murine leukemia viruses on permissive and nonpermissive cells in vitro.
Virology 65, 320–332 (1975).
Duran-Troise, G., Bassin, R.H., Rein, A. & Gerwin, B.I. Loss of Fv-1 restriction in
Balb/3T3 cells following infection with a single N tropic murine leukemia virus particle. Cell 10, 479–488 (1977).
Pincus, T., Hartley, J.W. & Rowe, W.P. A major genetic locus affecting resistance to
infection with murine leukemia viruses. IV. Dose-response relationships in Fv-1sensitive and resistant cell cultures. Virology 65, 333–342 (1975).
Tennant, R.W., Otten, J.A., Brown, A., Yang, W.K. & Kennel, S.J. Characterization
of Fv-1 host range strains of murine retroviruses by titration and p30 protein characteristics. Virology 99, 349–357 (1979).
Boone, L.R., Innes, C.L. & Heitman, C.K. Abrogation of Fv-1 restriction by genomedeficient virions produced by a retrovirus packaging cell line. J. Virol. 64,
3376–3381 (1990).
Bassin, R.H., Duran-Troise, G., Gerwin, B.I. & Rein, A. Abrogation of Fv-1b restriction with murine leukemia viruses inactivated by heat or by gamma irradiation.
J. Virol. 26, 306–315 (1978).
Best, S., Le Tissier, P., Towers, G. & Stoye, J.P. Positional cloning of the mouse
retrovirus restriction gene Fv1. Nature 382, 826–829 (1996).
Taylor, W.R. & Stoye, J.P. Consensus structural models for the amino terminal
domain of the retrovirus restriction gene Fv1 and the murine leukaemia virus capsid proteins. BMC Struct. Biol. 4, 1 (2004).
Ganser, B.K., Cheng, A., Sundquist, W.I. & Yeager, M. Three-dimensional structure
of the M-MuLV CA protein on a lipid monolayer: a general model for retroviral capsid assembly. EMBO J. 22, 2886–2892 (2003).
Bock, M., Bishop, K.N., Towers, G. & Stoye, J.P. Use of a transient assay for studying the genetic determinants of Fv1 restriction. J. Virol. 74, 7422–7430 (2000).
Bishop, K.N., Bock, M., Towers, G. & Stoye, J.P. Identification of the regions of Fv1
necessary for murine leukemia virus restriction. J. Virol. 75, 5182–5188 (2001).
Towers, G. et al. A conserved mechanism of retrovirus restriction in mammals.
Proc. Natl. Acad. Sci. USA 97, 12295–12299 (2000).
Cowan, S. et al. Cellular inhibitors with Fv1-like activity restrict human and simian
immunodeficiency virus tropism. Proc. Natl. Acad. Sci. USA 99, 11914–11919
(2002).
Besnier, C., Takeuchi, Y. & Towers, G. Restriction of lentivirus in monkeys. Proc.
Natl. Acad. Sci. USA 99, 11920–11925 (2002).
Munk, C., Brandt, S.M., Lucero, G. & Landau, N.R. A dominant block to HIV-1
replication at reverse transcription in simian cells. Proc. Natl. Acad. Sci. USA 99,
13843–13848 (2002).
Hofmann, W. et al. Species-specific, postentry barriers to primate immunodeficiency virus infection. J. Virol. 73, 10020–10028 (1999).
Owens, C.M., Yang, P.C., Gottlinger, H. & Sodroski, J. Human and simian immunodeficiency virus capsid proteins are major viral determinants of early, postentry
replication blocks in simian cells. J. Virol. 77, 726–731 (2003).
Dorfman, T. & Gottlinger, H.G. The human immunodeficiency virus type 1 capsid
p2 domain confers sensitivity to the cyclophilin-binding drug SDZ NIM 811.
J. Virol. 70, 5751–5757 (1996).
Hatziioannou, T., Cowan, S., Goff, S.P., Bieniasz, P.D. & Towers, G. Restriction of
multiple divergent retroviruses by Lv1 and Ref1. EMBO J. 22, 385-94 (2003).
Stremlau, M. et al. The cytoplasmic body component TRIM5α restricts HIV-1 infection in Old World monkeys. Nature 427, 848–853 (2004).
VOLUME 5 NUMBER 11 NOVEMBER 2004 NATURE IMMUNOLOGY
© 2004 Nature Publishing Group http://www.nature.com/natureimmunology
REVIEW
37. Reymond, A. et al. The tripartite motif family identifies cell compartments. EMBO
J. 20, 2140–2151 (2001).
38. Hatziioannou, T., Perez-Caballero, D., Yang, A., Cowan, S. & Bieniasz, P.D.
Retrovirus resistance factors Ref1 and Lv1 are species-specific variants of
TRIM5α. Proc. Natl. Acad. Sci. USA 101, 10774–10779 (2004).
39. Keckesova, Z., Ylinen, L.M. & Towers, G.J. The human and African green monkey
TRIM5alpha genes encode Ref1 and Lv1 retroviral restriction factor activities.
Proc. Natl. Acad. Sci. USA 101, 10780–10785 (2004).
40. Perron, M.J. et al. TRIM5α mediates the postentry block to N-tropic murine
leukemia viruses in human cells. Proc. Natl. Acad. Sci. USA 101, 11827–11832
(2004).
41. Sayah, D.M., Sokolskaja, E., Berthoux, L. & Luban, J. Cyclophilin A retrotransposition into TRIM5 explains owl monkey resistance to HIV-1. Nature 430, 569–573
(2004).
42. Towers, G.J. et al. Cyclophilin A modulates the sensitivity of HIV-1 to host restriction factors. Nat. Med. 9, 1138–1143 (2003).
43. von Schwedler, U.K. et al. Proteolytic refolding of the HIV-1 capsid protein aminoterminus facilitates viral core assembly. EMBO J. 17, 1555–1568 (1998).
44. Ganser, B.K., Li, S., Klishko, V.Y., Finch, J.T. & Sundquist, W.I. Assembly and
analysis of conical models for the HIV-1 core. Science 283, 80–83 (1999).
45. Luban, J., Bossolt, K.L., Franke, E.K., Kalpana, G.V. & Goff, S.P. Human immunodeficiency virus type 1 Gag protein binds to cyclophilins A and B. Cell 73,
1067–1078 (1993).
46. Yap, M.W. & Stoye, J.P. Intracellular localisation of Fv1. Virology 307, 76–89
(2003).
47. Xu, L. et al. BTBD1 and BTBD2 colocalize to cytoplasmic bodies with the RBCC/tripartite motif protein, TRIM5delta. Exp. Cell Res. 288, 84–93 (2003).
48. Schwartz, O., Marechal, V., Friguet, B., Arenzana-Seisdedos, F. & Heard, J.M.
Antiviral activity of the proteasome on incoming human immunodeficiency virus
type 1. J. Virol. 72, 3845–3850 (1998).
49. Butler, S.L., Johnson, E.P. & Bushman, F.D. Human immunodeficiency virus cDNA
metabolism: notable stability of two-long terminal repeat circles. J. Virol. 76,
3739–3747 (2002).
50. Hicke, L. A new ticket for entry into budding vesicles-ubiquitin. Cell 106, 527–530
(2001).
51. Pryciak, P.M. & Varmus, H.E. Fv-1 restriction and its effects on murine leukemia
virus integration in vivo and in vitro. J. Virol. 66, 5959–5966 (1992).
52. Gabuzda, D.H. et al. Role of vif in replication of human immunodeficiency virus
type 1 in CD4+ T lymphocytes. J. Virol. 66, 6489–6495 (1992).
53. Madani, N. & Kabat, D. An endogenous inhibitor of human immunodeficiency virus
in human lymphocytes is overcome by the viral Vif protein. J. Virol. 72,
10251–10255 (1998).
54. Simon, J.H., Gaddis, N.C., Fouchier, R.A. & Malim, M.H. Evidence for a newly discovered cellular anti-HIV-1 phenotype. Nat. Med. 4, 1397–1400 (1998).
55. Sheehy, A.M., Gaddis, N.C., Choi, J.D. & Malim, M.H. Isolation of a human gene
that inhibits HIV-1 infection and is suppressed by the viral Vif protein. Nature 418,
646–650 (2002).
56. Mangeat, B. et al. Broad antiretroviral defence by human APOBEC3G through
lethal editing of nascent reverse transcripts. Nature 424, 99–103 (2003).
57. Zhang, H. et al. The cytidine deaminase CEM15 induces hypermutation in newly
synthesized HIV-1 DNA. Nature 424, 94–98 (2003).
58. Harris, R.S. et al. DNA deamination mediates innate immunity to retroviral infection. Cell 113, 803–809 (2003).
59. Lecossier, D., Bouchonnet, F., Clavel, F. & Hance, A.J. Hypermutation of HIV-1
DNA in the absence of the Vif protein. Science 300, 1112 (2003).
60. Yu, Q. et al. Single-strand specificity of APOBEC3G accounts for minus-strand
deamination of the HIV genome. Nat. Struct. Mol. Biol. 11, 435–442 (2004).
61. Sleigh, R., Sharkey, M., Newman, M.A., Hahn, B. & Stevenson, M. Differential
association of uracil DNA glycosylase with SIVSM Vpr and Vpx proteins. Virology
245, 338–343 (1998).
62. Bouhamdan, M. et al. Human immunodeficiency virus type 1 Vpr protein binds to
the uracil DNA glycosylase DNA repair enzyme. J. Virol. 70, 697–704 (1996).
63. Selig, L. et al. Uracil DNA glycosylase specifically interacts with Vpr of both human
immunodeficiency virus type 1 and simian immunodeficiency virus of sooty
mangabeys, but binding does not correlate with cell cycle arrest. J. Virol. 71,
4842–4846 (1997).
64. Willetts, K.E. et al. DNA repair enzyme uracil DNA glycosylase is specifically incorporated into human immunodeficiency virus type 1 viral particles through a Vprindependent mechanism. J. Virol. 73, 1682–1688 (1999).
65. Mansky, L.M., Preveral, S., Selig, L., Benarous, R. & Benichou, S. The interaction
of vpr with uracil DNA glycosylase modulates the human immunodeficiency virus
type 1 In vivo mutation rate. J. Virol. 74, 7039–7047 (2000).
66. Jarmuz, A. et al. An anthropoid-specific locus of orphan C to U RNA-editing
enzymes on chromosome 22. Genomics 79, 285–296 (2002).
67. Wiegand, H.L., Doehle, B.P., Bogerd, H.P. & Cullen, B.R. A second human antiretroviral factor, APOBEC3F, is suppressed by the HIV-1 and HIV-2 Vif proteins.
EMBO J. 23, 2451–2458 (2004).
68. Zheng, Y.H. et al. Human APOBEC3F is another host factor that blocks human
immunodeficiency virus type 1 replication. J. Virol. 78, 6073–6076 (2004).
69. Bishop, K.N. et al. Cytidine deamination of retroviral DNA by diverse APOBEC proteins. Curr. Biol. 14, 1392–1396 (2004).
70. Liddament, M.T., Brown, W.L., Schumacher, A.J. & Harris, R.S. APOBEC3F properties and hypermutation preferences indicate activity against HIV-1 in vivo. Curr Biol
14, 1385–1391 (2004).
71. Yu, X. et al. Induction of APOBEC3G ubiquitination and degradation by an HIV-1
Vif-Cul5-SCF complex. Science 302, 1056–1060 (2003).
72. Sheehy, A.M., Gaddis, N.C. & Malim, M.H. The antiretroviral enzyme APOBEC3G is
degraded by the proteasome in response to HIV-1 Vif. Nat. Med. 9, 1404–1407
(2003).
73. Marin, M., Rose, K.M., Kozak, S.L. & Kabat, D. HIV-1 Vif protein binds the editing
enzyme APOBEC3G and induces its degradation. Nat. Med. 9, 1398–1403
(2003).
74. Mehle, A. et al. Vif overcomes the innate antiviral activity of APOBEC3G by promoting its degradation in the ubiquitin-proteasome pathway. J. Biol. Chem. 279,
7792–7798 (2003).
75. Conticello, S.G., Harris, R.S. & Neuberger, M.S. The Vif protein of HIV triggers
degradation of the human antiretroviral DNA deaminase APOBEC3G. Curr. Biol. 13,
2009–2013 (2003).
76. Stopak, K., de Noronha, C., Yonemoto, W. & Greene, W.C. HIV-1 Vif blocks the
antiviral activity of APOBEC3G by impairing both its translation and intracellular
stability. Mol. Cell 12, 591–601 (2003).
77. Mariani, R. et al. Species-specific exclusion of APOBEC3G from HIV-1 virions by
Vif. Cell 114, 21–31 (2003).
78. Turelli, P., Mangeat, B., Jost, S., Vianin, S. & Trono, D. Inhibition of hepatitis B
virus replication by APOBEC3G. Science 303, 1829 (2004).
79. Ngui, S.L., Hallet, R. & Teo, C.G. Natural and iatrogenic variation in hepatitis B
virus. Rev. Med. Virol. 9, 183–209 (1999).
80. Machida, K. et al. Hepatitis C virus induces a mutator phenotype: enhanced mutations of immunoglobulin and protooncogenes. Proc. Natl. Acad. Sci. USA 101,
4262–4267 (2004).
81. Bishop, K.N., Holmes, R.K., Sheehy, A.M. & Malim, M.H. APOBEC-mediated editing of viral RNA. Science 305, 645 (2004).
82. Gao, G., Guo, X. & Goff, S.P. Inhibition of retroviral RNA production by ZAP a
CCCH-type zinc finger protein. Science 297, 1703–1706 (2002).
83. Bick, M.J. et al. Expression of the zinc-finger antiviral protein inhibits alphavirus
replication. J. Virol. 77, 11555–11562 (2003).
84. Varthakavi, V., Smith, R.M., Bour, S.P., Strebel, K. & Spearman, P. Viral protein U
counteracts a human host cell restriction that inhibits HIV-1 particle production.
Proc. Natl. Acad. Sci. USA 100, 15154–15159 (2003).
85. Gottlinger, H.G., Dorfman, T., Cohen, E.A. & Haseltine, W.A. Vpu protein of human
immunodeficiency virus type 1 enhances the release of capsids produced by gag
gene constructs of widely divergent retroviruses. Proc. Natl. Acad. Sci. USA 90,
7381–7385 (1993).
86. Geraghty, R.J., Talbot, K.J., Callahan, M., Harper, W. & Panganiban, A.T. Cell typedependence for Vpu function. J. Med. Primatol. 23, 146–150 (1994).
87. Sakai, H., Tokunaga, K., Kawamura, M. & Adachi, A. Function of human immunodeficiency virus type 1 Vpu protein in various cell types. J. Gen. Virol. 76,
2717–2722 (1995).
88. Turelli, P. et al. Cytoplasmic recruitment of INI1 and PML on incoming HIV preintegration complexes: interference with early steps of viral replication. Mol. Cell 7,
1245–1254 (2001).
89. Yap, M.W., Nisole, S., Lynch, C. & Stoye, J.P. Trim5α protein restricts both HIV-1
and murine leukemia virus. Proc. Natl. Acad. Sci. USA 101, 10786–10791
(2004).
90. Berthoux, L. et al. As2O3 enhances retroviral reverse transcription and counteracts
Ref1 antiviral activity. J. Virol. 77, 3167–3180 (2003).
91. Bogerd, H.P., Doehle, B.P., Wiegand, H.L. & Cullen, B.R. A single amino acid difference in the host APOBEC3G protein controls the primate species specificity of
HIV type 1 virion infectivity factor. Proc. Natl. Acad. Sci. USA 101, 3770–3774
(2004).
92. Schrofelbauer, B., Chen, D. & Landau, N.R. A single amino acid of APOBEC3G
controls its species-specific interaction with virion infectivity factor (Vif). Proc.
Natl. Acad. Sci. USA 101, 3927–3932 (2004).
93. Mangeat, B., Turelli, P., Liao, S. & Trono, D. A single amino acid determinant governs the species-specific sensitivity of APOBEC3G to Vif action. J. Biol. Chem. 279,
14481–14483 (2004).
94. Xu, H. et al. A single amino acid substitution in human APOBEC3G antiretroviral
enzyme confers resistance to HIV-1 virion infectivity factor-induced depletion.
Proc. Natl. Acad. Sci. USA 101, 5652–5657 (2004).
95. Svarovskaia, E.S. et al. Human APOBEC3G is incorporated into HIV-1 virions
through interactions with viral and nonviral RNAs. J. Biol. Chem. 279,
35822–35828 (2004).
96. Cen, S. et al. The interaction between HIV-1 Gag and APOBEC3G. J. Biol. Chem.
279, 33177–33184 (2004).
97. Alce, T.M. & Popik, W. APOBEC3G is incorporated into virus-like particles by a
direct interaction with HIV-1 Gag nucleocapsid protein. 279, 34083–34086 J.
Biol. Chem. (2004).
98. Zennou, V., Perez-Caballero, D. & Bieniasz, P.D. APOBEC3G incorporation into HIV1 particles J. Virology (in the press).
99. Schafer, A., Bogerd, H.P. & Cullen, B.R. Specific packaging of APOBEC3G into
HIV-1 Virions is mediated by the nucleocapsid domain of the Gag polyprotein precursor. Virology (in the press).
100.Kobayashi, M. et al. APOBEC3G targets specific virus species. J. Virol. 78,
8238–8244 (2004).
101.Janeway, C.A.Jr. & Medzhitov, R. Innate immune recognition. Annu. Rev. Immunol.
20, 197–216 (2002).
NATURE IMMUNOLOGY VOLUME 5 NUMBER 11 NOVEMBER 2004
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