Antibodies mediate intracellular immunity through tripartite motif-containing 21 (TRIM21).

Abstract

Antibodies provide effective antiviral immunity despite the fact that viruses escape into cells when they infect. Here we show that antibodies remain attached to viruses after cell infection and mediate an intracellular immune response that disables virions in the cytosol. We have discovered that cells possess a cytosolic IgG receptor, tripartite motif-containing 21 (TRIM21), which binds to antibodies with a higher affinity than any other IgG receptor in the human body. TRIM21 rapidly recruits to incoming antibody-bound virus and targets it to the proteasome via its E3 ubiquitin ligase activity. Proteasomal targeting leads to rapid degradation of virions in the cytosol before translation of virally encoded genes. Infection experiments demonstrate that at physiological antibody concentrations TRIM21 neutralizes viral infection. These results reveal an intracellular arm of adaptive immunity in which the protection mediated by antibodies does not end at the cell membrane but continues inside the cell to provide a last line of defense against infection.

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Antibodies mediate intracellular immunity through
tripartite motif-containing 21 (TRIM21)
Donna L. Mallerya,1, William A. McEwana,1, Susanna R. Bidgooda,1, Greg J. Towersb, Chris M. Johnsona,
and Leo C. Jamesa,2
aProtein and Nucleic Acid Chemistry Division, Medical Research Council Laboratory of Molecular Biology, Cambridge CB2 0QH, United Kingdom; andbDivision
of Infection and Immunity, Medical Research Council Centre for Medical Molecular Virology, University College London, London W1T4JF, United Kingdom
Edited* by Douglas T. Fearon, University of Cambridge School of Clinical Medicine, Cambridge, United Kingdom, and approved October 12, 2010 (received for
review September 19, 2010)
Antibodies provide effective antiviral immunity despite the fact
that viruses escape into cells when they infect. Here we show that
antibodies remain attached to viruses after cell infection and
mediate an intracellular immune response that disables virions in
the cytosol. We have discovered that cells possess a cytosolic IgG
receptor, tripartite motif-containing 21 (TRIM21), which binds to
antibodies with a higher affinity than any other IgG receptor in
the human body. TRIM21 rapidly recruits to incoming antibody-
bound virus and targets it to the proteasome via its E3 ubiquitin
ligase activity. Proteasomal targeting leads to rapid degradation
of virions in the cytosol before translation of virally encoded
genes. Infection experiments demonstrate that at physiological
antibody concentrations TRIM21 neutralizes viral infection. These
results reveal an intracellular arm of adaptive immunity in which
the protection mediated by antibodies does not end at the cell
membrane but continues inside the cell to provide a last line of
defense against infection.
V
years, and this has given rise to a complex system of im-
munity traditionally divided into innate and adaptive responses.
Innate immunity comprises germ-line encoded receptors and
effector mechanisms that recognize pathogen-associated molec-
ular patterns (PAMPs) (1). The advantage of innate immunity is
that it is fast and generic; however, viruses are adept at avoiding
recognition by inhibiting innate immunity or by changing their
molecular patterns. In contrast, adaptive immunity can clear
a host of infection and provide protection against future in-
fection. Unlike the PAMP receptors of innate immunity, adap-
tive immunity uses proteins such as antibodies to target path-
ogens. Antibodies are unique in the human body in that they
evolve during the lifetime of an individual and can continue to
target evolving pathogens (2). The weakness of adaptive immu-
nity is that it can take 1 to 2 wk to reach full effectiveness.
Moreover, the dogma of antibody immunity for the last 100 y has
been that antibodies only provide extracellular protection (3). It
is thought that once a virus has entered the cytosol of a cell,
antibodies are helpless to prevent its infection.
Recently we described an intracellular cytosolic protein call-
ed tripartite motif-containing 21 (TRIM21) that is capable
of binding to an invariant region of antibody molecules via its
PRYSPRY domain (4). We found this activity to be structurally,
thermodynamically, and kinetically conserved across mammals
(5). However, antibodies are extracellular proteins, as are all
known mammalian IgG receptors (with the exception of FcRn,
which is intracellular but not cytosolic). It therefore seemed in-
congruous to us that TRIM21 should be a universally conserved
intracellular protein and yet be a high-affinity, highly specific IgG
receptor. We hypothesized that there might be an antibody ef-
fector mechanism inside cells, mediated by TRIM21. Here we
demonstrate the existence of this mechanism and its operation in
preventing infection by adenovirus. Our data reveal that humoral
immunity provides intracellular protection, whereby antibodies
iruses and their hosts have been coevolving for millions of
function inside cells by engaging with a potent effector mecha-
nism that targets virus for degradation.
Results
TRIM21 Mediates Intracellular Antibody Neutralization. It is assumed
that antibodies do not routinely enter the cytosol during viral
infection. To test this, we preincubated adenovirus (a model vi-
rus that causes respiratory disease) with different antibodies and
added the virions to cultured HeLa cells. Adenovirus was chosen
because it is a nonenveloped virus, and its capsid is naturally
exposed to serum antibody before cellular infection. This is in
contrast to enveloped viruses, in which the capsid is protected
from antibody. After 30 min of infection the cells were fixed, and
a fluorescent anti-IgG antibody was added to detect antibody-
coated virions. As can be seen in Fig. 1A, antibody-coated virions
successfully enter cells. Similar results were obtained using dif-
ferent polyclonal antihexon antibodies and human serum IgG.
Adenovirus enters the cell by binding to the coxsackie and ad-
enovirus receptor and becoming endocytosed. We found that
addition of antibody does not prevent this process and that an-
tibody remains attached to virus after entry. To address whether
antibody-coated virus is accessible to cytosolic TRIM21, we
costained for TRIM21. As shown in Fig. 1A, TRIM21 is effi-
ciently recruited to antibody-coated viral particles.
Next we tested the effect of TRIM21 recruitment to virions by
quantifying the levels of adenovirus infection. We used a virus
that carries a GFP gene so that infection efficiency could be
determined by flow cytometry analysis. A standard viral neu-
tralization assay was performed on HeLa cells pretreated with
control siRNA, TRIM21 siRNA, IFN-α , or IFN-α and TRIM21
siRNA (Fig. 1B). To take account of toxicity and variable cell
death between these different conditions, we measured the de-
crease in infection due to the addition of antibody. In the ab-
sence of antibody, adenovirus infected ≈50% of cells. The
percentage of infected cells decreased rapidly with increasing
antibody concentration, such that at 400 ng/mL antibody, in-
fection was reduced by ≈60-fold (Fig. 1B). However, in cells
depleted of TRIM21, addition of 400 ng/mL antibody had
a minimal effect on infection (≈3-fold).
During an immune response, IFN-α activates the transcription
of antiviral genes. TRIM21 is IFN-α regulated, and the modest
levels of endogenous TRIM21 protein are substantially increased
Author contributions: D.L.M., W.A.M., S.R.B., and L.C.J. designed research; D.L.M., W.A.M.,
S.R.B., C.M.J., and L.C.J. performed research; G.J.T. contributed new reagents/analytic
tools; D.L.M., W.A.M., S.R.B., and L.C.J. analyzed data; and D.L.M., W.A.M., S.R.B., and
L.C.J. wrote the paper.
The authors declare no conflict of interest.
*This Direct Submission article had a prearranged editor.
Freely available online through the PNAS open access option.
1D.L.M., W.A.M, and S.R.B. contributed equally to this work.
2To whom correspondence should be addressed. E-mail: lcj@mrc-lmb.cam.ac.uk.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
1073/pnas.1014074107/-/DCSupplemental.
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by IFN-α (Fig. 1C). Consistent with this regulation, pre-
incubation of cells with IFN-α markedly increased the effect of
antibody neutralization. IFN-α has pleiotropic effects, but with-
out addition of antibody we observed little impact on adenovirus
infection. To test whether IFN-α/antibody neutralization synergy
is TRIM21 dependent, we specifically depleted the TRIM21
levels that are up-regulated by IFN-α, leading to >20-fold re-
covery of infectivity (Fig. 1B). In all experiments, antibody
neutralization of viral infection directly correlated with TRIM21
levels (Fig. 1C). For example, cells expressing the most TRIM21
were almost 2 orders of magnitude more resistant to adenovirus
infection than those expressing the least.
We confirmed the robustness of this phenotype by examining
the effect of different siRNA sequences, cell types, and types of
antibody. Different TRIM21 siRNAs with different target
sequences reversed antibody neutralization of adenovirus in-
fection by depleting TRIM21 levels (Fig. S1A). TRIM21 neu-
tralization occurred in a range of cell lines including HeLa,
HT1080, and TE671 (Fig. S1B). A stable TRIM21 knockdown
line was established in each case using an shRNA vector based
on the sequence of siRNA 2. In all cells, TRIM21 mediated
antibody neutralization of adenovirus (Fig. S1B). Finally, we
tested the effect on adenovirus neutralization of two different
anti-Ad5 polyclonal antibodies (Abd Serotec and Millipore) and
an anti-Ad5 hexon monoclonal. In every case, neutralization of
adenovirus was enhanced by TRIM21 up-regulation and re-
versed by TRIM21 depletion (Fig. S1C).
Unlike entry neutralization, antibody effector mechanisms are
reliant on interaction with the Fc domain. To test whether
TRIM21 neutralization was Fc dependent, we treated IgG with
pepsin to remove the Fc and generate Fab2 fragments. Fab2
fragments are bivalent, bind antigen with the same affinity as
IgG, and can cross-link viral epitopes. However, we found that
Fab2 fragments were not able to mediate efficient TRIM21
neutralization (Fig. 2A). Unlike with IgG, when using Fab2, IFN-
α treatment or TRIM21 depletion no longer dramatically af-
fected adenovirus infection. Thus in order for antibodies to
mediate intracellular viral neutralization they must contain an Fc
domain and TRIM21 must be present. These results reveal
a unique effector mechanism in humoral immunity that extends
protection into cells.
During the early stages of infection, in which innate immunity is
critical, IgM rather than IgG antibodies dominate the antibody
repertoire. We tested whether TRIM21 interacts with IgM and if
so the importance of TRIM21 in IgM viral neutralization. To
investigate TRIM21:IgM binding, we labeled the TRIM21
PRYSPRY domain with an Alexa 488 fluorophore and measured
its fluorescence anisotropy upon titration of IgM (Fig. 2B). The
resulting titration curve was fit (Materials and Methods) to give an
affinity (KD) of 16.8 ± 1.5 μM. This is weaker than the affinity with
which TRIM21 binds IgG Fc [0.2 μM under physiological con-
ditions (4)]. However, the affinity of TRIM21 to IgM is likely to be
significantly higher in vivo because full-length TRIM21 is a mul-
timer. Complement C1q, which binds IgM with nanomolar affin-
ity, has undetectable affinity when measured as a monomer (6).
NextwetestedtheeffectofserumIgMonadenovirus infection.
We found that pooled human serum IgM and TRIM21 operate
synergistically to neutralize adenovirus infection (Fig. 2C). Fur-
thermore, as with IgG, the neutralization of virus by IgM requires
TRIM21. This suggests that TRIM21-mediated immunity could
be effective in the early stages of an infection. Furthermore, it
demonstrates that TRIM21 utilizes a broader usage of isotypes
than classical Fc receptors, which are usually both isotype and
subtype specific.
Mechanism of TRIM21 Immunity. The previous experiments dem-
onstrate that there is an intracellular immune response mediated
by TRIM21 and antibodies that is capable of preventing viral
infection. Next we examined the mechanism by which this in-
tracellular neutralization occurs. We investigated the mechanism
in three ways. First, we determined how TRIM21 targets anti-
body and the thermodynamics of interaction. Second, we ex-
amined what events subsequent to targeting are required for
neutralization. Third, we asked how virus is neutralized.
TRIM21 is a multidomain protein consisting of RING, B Box,
coiled-coil, and PRYSPRY domains. We tested the role of these
domains in IgG binding using multiangle light scattering
(MALS) and fluorescence titration spectroscopy. Analysis of the
MALS data reveals that recombinant full-length TRIM21 forms
a stable dimer and not a trimer as previously reported (7) (Fig.
3A). There are conflicting data on the oligomeric state of other
TRIM proteins, which have been shown to be both trimers (8)
and dimers (9). Our results support the view that a dimer is the
preferred state for TRIM proteins. When mixed with IgG,
TRIM21 forms a stoichiometric complex consisting of one an-
tibody and one TRIM21 (Fig. 3A). Deletion of the RING do-
main alone resulted in a destabilized recombinant protein;
however, deletion of both RING and B Box did not affect
TRIM21 stability or its ability to dimerize. Fluorescence titration
spectroscopy revealed that full-length TRIM21 and ΔRING-Box
bound to IgG with a similar dissociation constant (KD) of <1 nM
(Fig. 3 B and C). Because the monomeric PRYSPRY domain
binds with ≈150 nM affinity (4, 5), this indicates that the coiled-
coil domain is required for TRIM21 dimerization and the si-
multaneous engagement of both IgG heavy chains. The sub-nM
affinity of TRIM21 for IgG makes TRIM21 the highest-affinity
antibody receptor in the human body. The evolution of such
a high-affinity interaction explains how TRIM21 efficiently tar-
gets virus.
Next we looked at what happens to virus after TRIM21 is
recruited and the role of the RING and B Box domains. Because
RING domains often display E3 ubiquitin ligase activity, we
A
BC
Fig. 1.
microscopy images of adenovirus-infected HeLa cells. Adenovirus precoated
in antibody and detected after infection with an Alexa Fluor 546 secondary
(red) can be seen inside cells. The images are a Z-projection. (Scale bars, 10
μm; 2 μm in Insets.) (B) Cells treated with IFN-α, TRIM21 siRNA, siRNA control,
or IFN-α and TRIM21 siRNA were infected with GFP adenovirus at different
polyclonal antibody concentrations. Adenovirus infection is reduced by 2
logs in cells expressing the highest levels of TRIM21. (C) Western blot of
TRIM21 protein levels in each condition.
TRIM21 mediates intracellular antibody neutralization. (A) Confocal
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hypothesized that TRIM21 may target bound virus for degra-
dation via ubiquitination. Cells possess two pathways for degra-
dation of ubiquitinated material: the proteasome and autophagy.
To explore the role of these pathways in TRIM21 neutralization
of virus we performed viral infection experiments in the presence
of MG132 (a proteasome inhibitor) and 3-methyladenine (3-
MA; an autophagy inhibitor). The autophagy inhibitor had no
effect on infectivity; however, MG132 significantly reversed
TRIM21 neutralization of infectivity (Fig. 3D). Titration experi-
ments showed a direct correlation between increasing levels of
MG132 and reduced neutralization (Fig. 3E). The ability of
MG132 to reverse neutralization was dependent upon the pres-
ence of antibody and TRIM21. Moreover, addition of MG132
could not recover infection in cells depleted of TRIM21, showing
that TRIM21 and proteasome function are essential components
in the same neutralization pathway (Fig. 3F).
To determine whether ubiquitination is essential to target virus
to the proteasome, we tested the ability of full-length TRIM21
and ΔRING-Box recombinant proteins to neutralize infection.
We incubated protein with antibody-coated virions and allowed
the virus to infect cells depleted of TRIM21. As can be seen in
Fig. 4A, deletion of the RING and B Box domains prevents
TRIM21 neutralization of virus. We attempted to repeat these
experiments in cells overexpressing TRIM21; however, this led to
loss of function. Loss of function could be partially reversed with
IFN, suggesting that overexpression titrates essential cofactors
rather than generates inactive protein. To confirm that the neu-
tralization we observe with recombinant proteins correlates with
ubiquitin ligaseactivity, wecompared theabilityoffull-length and
ΔRING-Box proteins to autoubiquitinate. Although deletion of
the RING and B Box domains has no effect on IgG binding, it
abolishes autoubiquitination (Fig. 4B). Thus both TRIM21
ubiquitination activity and proteasomal function are required for
A
B
C
Fig. 2.
the Fc domain. (B) TRIM21 binds IgM with an affinity of 16.8 ± 1.5 μM. (C) Serum IgM antibodies can be used by TRIM21 to neutralize virus. Depletion of
TRIM21 reverses this effect, and IFN-α increases it. Error bars in all panels were calculated from triplicate experiments.
TRIM21 mediates neutralization using serum IgG and IgM but not Fab2 fragments. (A) Antibody-dependent TRIM21 neutralization of virus requires
A
BC
D
E
F
Fig. 3.
The continuous traces represent the refractive index signal (left-hand axis), and the short horizontal lines represent the calculated mass at each sampling
interval (1 s) within each peak (right-hand axis). Analysis indicates that TRIM21 is dimeric with a mass of 107 kDa, that IgG has a mass of 154 kDa, and that
TRIM21:IgG complex yields a peak corresponding to free IgG and a 1:1 complex with mass ≈280 kDa. (B and C) Steady-state fluorescence titration of IgG with
full-length TRIM21 (B) and ΔRING-Box TRIM21 (C) reveals an affinity of 0.6 ± 0.1 nM and 0.9 ± 0.2 nM, respectively. (D) TRIM21 neutralization is reversed by
the proteasome inhibitor MG132 but not the autophagy inhibitor 3-MA. Error bars were calculated from triplicate experiments. (E) Direct correlation be-
tween MG132 concentration and reversal of TRIM21 neutralization. MG132 only reverses neutralization in the presence of antibody. (F) Proteasomal deg-
radation, TRIM21, and antibody are necessary factors in the same pathway of viral neutralization.
Mechanism of TRIM21 neutralization. (A) SEC MALS chromatograms of TRIM21 (black), IgG (light gray), and TRIM21 in complex with IgG (dark gray).
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viralneutralization.However,althoughtheinvitroassayindicates
that TRIM21 is an active ligase, it cannot be assumed that it is
autoubiquitination that drives proteasomal targeting in vivo.
Although E3 ubiquitin ligases are known to autoubiquitinate,
it is the transfer of ubiquitin to substrate that is thought to be
important for proteasomal targeting. However, we hypothesized
that proteasomal targeting via autoubiquitination would allow
TRIM21 to neutralize any virus and prevent evolution of viral
mutants that escape ubiquitin conjugation. Consistent with this
mechanism, although we found that TRIM21 efficiently forms
ubiquitin chains on itself, we found no detectable ubiquitination
of either IgG or virus in our in vitro ubiquitination assay
(Fig. 4C). This suggests that recruitment to the proteasome is not
dependent upon direct ubiquitination of either the antibody or
virus but rather autoubiquitination of TRIM21 or a recruited
cellular cofactor. Proteasomal recruitment through TRIM21
autoubiquitination is consistent with the extremely high affinity
with which TRIM21 has evolved to bind antibody. If TRIM21
were transferring ubiquitin to antibody or virus through normal
enzymatic turnover, then a high affinity would translate as
a highly inefficient KM. However, we cannot rule out that
a capsid protein other than hexon is ubiquitinated or that al-
though undetected in vitro, ubiquitination of hexon or antibody
occurs in vivo. Finally, to demonstrate that the intracellular
TRIM21-associated viral particle is ubiquitinated, we examined
infected cells by confocal microscopy and stained for ubiquitin.
As can be seen in Fig. 4D, virions colocalized with TRIM21 are
also positive for ubiquitin.
To determine what happens to virus after TRIM21-mediated
targeting to the proteasome, we performed a fate-of-capsid time-
course experiment. In this experiment we infected cells with virus
preincubated with antibody and compared the levels of hexon
protein (viral capsid) in infected HeLa cells with those in cells
depleted of TRIM21. By 2 h after infection there was markedly
less hexon in HeLa compared with TRIM21-depleted cells
(Fig. 5). This indicates both that TRIM21 mediates degradation
of virus and that it is a rapid process. Addition of MG132 slowed
the decline in hexon levels, confirming that virus is being physi-
cally degraded in a proteasome-dependent manner. Because
proteasomal targeting by TRIM21 requires virus to be antibody-
bound, we also looked at the antibody levels in infected cells. We
found that destruction of virus is paralleled by disposal of anti-
body (Fig. 5). In contrast to hexon and antibody, we saw little
change in TRIM21 levels after 2 h. Treatment with MG132 did
not increase TRIM21 levels, suggesting either that TRIM21 is
not degraded by the proteasome or that ubiquitination and
degradation are coupled to IgG binding. In the latter case, only
a fraction of the total pool of cellular TRIM21 would be de-
graded. We also cannot rule out the possibility of a feedback
loop that maintains TRIM21 levels.
The combination of antibody targeting and TRIM21 autoubi-
quitination implies that no direct viral interactions are required
for neutralization. To test this, we transfected cells with strepta-
vidin latex beads coated in antistreptavidin antibody. We found
that TRIM21 is efficiently recruited to the antibody-bound beads
(Fig. 6). Furthermore, TRIM21-associated beads are positive for
ubiquitin. This result suggests that direct interaction with an in-
vading viral particle may not be required to recruit TRIM21 and
that virus may not be the target of ubiquitination. We believe that
TRIM21-mediated neutralization may be effective against other
nonenveloped viruses.
D
A
B
C
Fig. 4.
RING and B Box domains does not. (B) TRIM21 is an active E3 ligase, but deletion of RING and B Box domains prevents autoubiquitination. (C) TRIM21 does not
ubiquitinate viral capsid (hexon) or bound antibody. (D) Confocal microscopy Z-projection showing HeLa cells infected with antibody-coated adenovirus.
TRIM21-colocalized virions are positive for ubiquitin.
TRIM21 E3 ubiquitin ligase function is essential for viral neutralization. (A) Recombinant full-length TRIM21 neutralizes virus, but TRIM21 lacking the
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Discussion
We have discovered that there is a system of intracellular im-
munity through which antibodies mediate the neutralization of
virus inside the cytosol of infected cells. This intracellular system
combines features traditionally associated exclusively with either
adaptive or innate immunity. Pathogen targeting is provided by
adaptive immunity in the form of antibodies, whereas neutrali-
zation is provided by an intracellular receptor (TRIM21) and an
innate degradation pathway. This combination of features may
make it difficult for viruses to evolve and escape.
Most importantly, our results show that humoral immunity is
not limited to extracellular protection but can neutralize a virus
even after it has entered a cell. Indeed, the antibody effector
function provided by TRIM21 offers a different kind of pro-
tection from known extracellular mechanisms. Extracellular ef-
fector functions are mainly provided by professional cells and are
based on immune surveillance. In contrast, TRIM21 is expressed
in most tissues and not just professional cells. Crucially, encap-
sulating immunity within host cells means that every infection
event is an opportunity for neutralization. This may provide
more effective protection against viral spread than surveillance.
Moreover, TRIM21 provides a noncytotoxic mechanism of im-
munity, in contrast to T cell killing or antibody-dependent cel-
lular cytotoxicity that target an infected cell rather than a virus.
Because TRIM21 neutralization occurs rapidly, before tran-
scription of viral genes, this offers the possibility of “curing”
rather than killing an infected cell. Finally, because TRIM21
utilizes both IgM and IgG, it is capable of operating alongside
both innate immunity during the early stages of infection and
adaptive immunity to provide long-term protection.
It is conceivable that in addition to making use of adaptive and
innate immunity to neutralize viruses, TRIM21 also helps to
activate these pathways. Because TRIM21 detects virus early in
cellular infection it is well placed to stimulate immune signaling,
for example by stimulating NF-κB. Similarly, because TRIM21
uses the proteasome to dispose of virus, this may provide a
way for signaling directly to adaptive immunity through MHC
presentation.
It is possible that TRIM21 has been contributing to many
antibody neutralization experiments over the last 100 y. Indeed,
because we see that TRIM21 mediates potent antibody neu-
tralization of adenovirus, it will be important to reassess whether
the antibody neutralization of other viruses is caused by a block
to entry or is TRIM21 dependent. This may be an important
consideration in vaccine design, because effective vaccines may
need to stimulate TRIM21 immunity. We suggest that a good
predictor of TRIM21 involvement in the antibody neutralization
of other viruses is a synergistic relationship between IFN and
antibody. Unexplained synergy between IFN and antibody has
been reported for enterovirus 70 (10) and coxsackie virus (10).
TRIM21 may also contribute to viral neutralization in experi-
ments in which no antibody is added because the calf serum used
in routine tissue culture contains a repertoire of antibodies of
potentially cross-reactive specificity.
The existence of a TRIM21/antibody intracellular immune
response may help to resolve crucial unexplained observations in
viral infection. For instance, early antibody neutralization ex-
periments on the newly identified adenovirus revealed that there
is a linear-log relationship between antibody dilution and neu-
tralization and that prolonged antibody incubation does not in-
crease neutralization (11). It has since been shown that antibody
neutralization of poliovirus occurs even when viruses are allowed
to preadhere to target cells (12). It has also been observed that
a single IgG is sufficient to mediate neutralization of poliovirus
(13, 14) and adenovirus (15) and that only five to six IgG mol-
ecules are required for rhinovirus (16). Finally, intact antibodies
have long been known to be far more effective than their pro-
teolysed fragments. In 1943 Kalmanson and Bronfenbrenner
showed that digestion with papain reactivated poliovirus in-
fectivity but not through dissociation of binding (17). In-
triguingly, several groups later showed that virus could be
reneutralized by addition of anti-Ig (18, 19). These findings could
be explained through a cross-linking or conformational model
of neutralization, but they are also consistent with an Fc effec-
tor mechanism.
We have demonstrated TRIM21-mediated restriction of ade-
novirus; however, we believe that TRIM21 should be effective
against other nonenveloped viruses. The streptavidin bead
experiments suggest that any intracellular antibody-coated par-
ticle is capable of being targeted by TRIM21.
1 2 4 6 1 2 4 6 1 2 4 6 1 2 4 6
Hours post-infection
WB:Hexon WB:Antibody WB: Trim21 WB: Actin
HeLa
HeLa + MG132
siTRIM21
Fig. 5.
protein and antibody are rapidly degraded in a TRIM21-dependent manner. Addition of MG132 partially rescues degradation.
TRIM21 mediates rapid degradation of virus. Western blots of hexon, antibody, and TRIM21 protein levels 1–6 h after infection. Adenovirus hexon
Dapi T21!
Ub!
Bead!
Merge!
Fig. 6.
and transfected into cells recruit TRIM21 and colocalize with ubiquitin. (Scale bars, 10 μm; 5 μm in Insets.)
Intracellular antibody-coated beads recruit TRIM21 and are ubiquitinated. Streptavidin-conjugated latex beads coated with antistreptavidin antibody
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Materials and Methods
For full details, see SI Materials and Methods.
Cells Lines and Viral Production. HEK293T, HeLa, TE671, and HT1080 were
maintained in standard conditions. 293F cells (Invitrogen) were grown in
serum-free Freestyle medium (Invitrogen). Adenovirus Ad5-GFP (20) was
grown in transcomplementation cell line 293F for 72 h and purified to 108to
109IU/mL. Transient siRNA knockdowns were performed in six-well plates
with 150 pmol of oligo per 1 × 105cells.
Virus Neutralization Assays. HeLa cells in six-well plates were infected with 5 ×
104infectious units (IU) AdV5-GFP for 30 min and then incubated for 48 h
before fixing. Goat antiadenovirus polyclonal antibody was used in all
virus neutralization assay experiments unless otherwise stated (AB1056;
Millipore).
Immunofluorescence. HeLa cells were seeded onto coverslips in 24-well plates
and infected essentially as described above, then fixed and stained with
appropriate primary and secondary antibodies. Confocal images were
taken using a Zeiss 63× lens on a Jena LSM 710 microscope (Carl Zeiss Mi-
croImaging).
Fluorescence Titration. Full-length and ΔRING-Box recombinant TRIM21 was
expressed as maltose binding protein-fusion proteins in Escherichia coli and
purified using amylose resin and size-exclusion chromatography. Fluores-
cence experiments were performed by titrating IgG using a Cary Eclipse
fluorescence spectrophotometer.
Fluorescence Anisotropy. The PRYSPRY domain of TRIM21 was expressed and
purified as previously described (4, 5). The protein was labeled with Alexa
Fluor 488 5-SDP ester (Invitrogen) and its polarized fluorescence measured at
530 nm using a Cary Eclipse fluorescence spectrophotometer. IgM (Athens
Research and Technology) was titrated into 50 nM PRYSPRY and the change
in anisotropy averaged over 5 s.
SEC MALS. Size-exclusion chromatography (SEC) MALS was performed using
a Wyatt Heleos II 18-angle light-scattering instrument coupled to a Wyatt
Optilab rEX online refractive index detector. Samples were prepared as
described above and passed through the light-scattering and refractive in-
dex detectors in a standard SEC MALS format.
In Vitro Ubiquitination Assays. In vitro assays were carried out largely as
previously described (21). Reaction mixtures were incubated at 37 °C for 1 h
then visualized by Western blot for TRIM21 (1:500, sc-25351; Santa Cruz
Biotechnology), Ad5 hexon (donkey anti-goat IgG HRP 1:5,000 sc-2056; Santa
Cruz Biotechnology), or ubiquitin (1:1,000, FK-2; Enzo Life Sciences)
as indicated.
ACKNOWLEDGMENTS. An initial stock of adenovirus Ad5-GFP was kindly
provided by Dr. Olivier Danos (Cancer Biology, University College London
Cancer Institute, London, United Kingdom).
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| www.pnas.org/cgi/doi/10.1073/pnas.1014074107 Mallery et al.