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Journal of Virology, September 2002, p. 8729-8736, Vol. 76, No. 17
0022-538X/02/$04.00+0 DOI: 10.1128/JVI.76.17.8729-8736.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Hemagglutinin Protein of Wild-Type Measles Virus Activates Toll-Like Receptor 2 Signaling
Karen Bieback,1 Egil Lien,2 Ingo M. Klagge,1 Elita Avota,1 Jürgen Schneider-Schaulies,1 W. Paul Duprex,3 Herrmann Wagner,4 Carsten J. Kirschning,4 Volker ter Meulen,1 and Sibylle Schneider-Schaulies1*
Institute for Virology and Immunobiology, University of Würzburg, D-97078 Würzburg,1
Institute for Medical Microbiology, Immunology, and Hygiene, Technical University of Munich, Munich, Germany,4
University of Massachusetts Medical School, Worcester, Massachusetts 01605,2
School of Biochemistry, The Queen's University of Belfast, Belfast BT7 9BL, Northern Ireland, United Kingdom3
Received 22 March 2002/
Accepted 3 June 2002

ABSTRACT
Pattern recognition via Toll-like receptors (TLR) by antigen-presenting
cells is an important element of innate immunity. We report
that wild-type measles virus but not vaccine strains activate
cells via both human and murine TLR2, and this is a property
of the hemagglutinin (H) protein. The ability to activate cells
via TLR2 by wild-type MV H protein is abolished by mutation
of a single amino acid, asparagine at position 481 to tyrosine,
as is found in attenuated strains, which is important for interaction
with CD46, the receptor for these strains. TLR2 activation by
MV wild-type H protein stimulates induction of proinflammatory
cytokines such as interleukin-6 (IL-6) in human monocytic cells
and surface expression of CD150, the receptor for all MV strains.
Confirming the specificity of this interaction, wild-type H
protein did not induce IL-6 release in macrophages from TLR2
-/- mice. Thus, the unique property of MV wild-type strains to activate
TLR2-dependent signals might essentially contribute not only
to immune activation but also to viral spread and pathogenicity
by upregulating the MV receptor on monocytes.

INTRODUCTION
In the course of acute measles, an efficient virus-specific
immune response is generated which mediates viral clearance
from the host and confers protection against reinfection. Paradoxically,
a general immunosuppression is also induced favoring secondary
infections, which are the major reason for the annual high morbidity
and mortality rates associated with measles. The magnitude and
duration of immune activation and immune suppression differ
between natural or experimental infection and vaccination (
20,
60). Studies addressing measles virus (MV)-induced immune suppression
mainly have focused on alterations of T-cell functions and viability
as a consequence of direct MV infection or contact-mediated
signaling (
53). In vitro observations suggest that MV infection
also enhances apoptosis of monocytes and dendritic cells (DC)
and affects their antigen-presenting capacity and cytokine release
(
31,
53). MV interaction with DC and monocytes is, however,
also associated with their maturation or activation, respectively,
and thus is important for induction of virus-specific immune
responses (
32,
39,
45,
54,
56). Strains expressing an MV wild-type-derived
hemagglutinin (H) protein reveal a particular tropism for DC
and are more efficient in inducing both DC maturation and immunosuppression
(
32,
48,
54). The mechanisms by which MV leads to these functional
alterations are largely unknown. Downregulation of interleukin-12
(IL-12) production in monocytes was linked to MV- or antibody-mediated
cross-linking of CD46, the receptor for certain MV strains (
29).
Lymphotropic MV wild-type strains and clinical isolates, with
few known exceptions (
43), fail to interact with CD46 but require
CD150 for cell entry (
15,
26,
49,
59). This molecule is absent
from unstimulated monocytes and immature DC (
33,
45,
48), and
it is thus unknown how infection of these cells by CD150-dependent
MV strains occurs.
Mammalian Toll-like receptors (TLRs) were implicated in the innate immune recognition of a variety of bacterial pathogens and bacterial products (2). Ten TLR family members were discovered, and several of these appear to play important roles in the activation of cells by various bacterial products. TLR2 is responsible for recognition of gram-positive bacteria (57, 65), bacterial lipoproteins (12, 42), and lipoteichoic acid (38, 55). TLR4 appears to be the main receptor for lipopolysaccharide (LPS) lipid A from gram-negative bacteria (41), TLR6 might be a coreceptor for TLR2 in recognizing certain bacterial structures (50, 58), and TLR9 and TLR3 mediate responses to CpG bacterial DNA and double-stranded RNA (dsRNA), respectively (3, 24). Hence, these receptors are able to discriminate between different bacteria and bacterial structures and thereby direct a proper immune response to a specific pathogen. Intracellular domains of the TLRs share motifs with the protein families of the IL-1 receptors, and a common intracellular pathway leading to activation of NF-
B and mitogen-activated protein kinases involves MyD88, IRAK, and TRAF6 (2). However, other signaling pathways upstream of NF-
B have been described which also include utilization of the phosphatidylinositol-3/Akt-kinase pathway by TLR2 (4). It has recently been demonstrated that mammalian TLR signaling can also be regulated by viral gene products. Vaccinia virus encodes gene products that interfere with proximal TLR signaling in the cytoplasm (11), and the fusion protein of respiratory syncytial virus (RSV) was found to activate cells via TLR4 and CD14 (35).
Using CHO cells stably overexpressing TLR2 or TLR4, we found that MV wild-type strains specifically activated cells via TLR2, and this was dependent on the expression of the H protein of the MV wild-type strain, WTF. The failure of attenuated MV strains to induce TLR2 activation correlated with a single amino acid exchange at position 481 which is, in turn, essential for the usage of CD46 as receptor by these strains. Importantly, MV expressing the wild-type H protein induced activation of TLR-responsive genes such as IL-1
/ß, IL-6, and IL-12 p40 in monocytes and also the expression of CD150, the receptor for all MV strains. Activation of TLR signaling by wild-type MV H protein may thus essentially contribute to the immune activation during measles, and loss of the capability to activate TLR2 may be considered as an attenuation marker.

MATERIALS AND METHODS
Cells and viruses.
MV vaccine strains AIK-C and Moraten were grown on Vero cells
(minimal essential medium with 5% fetal calf serum [FCS]); MV
wild-type strains WTF (isolated on lymphoblastoid, Epstein-Barr
virus-negative BJAB cells), Bilthoven (isolated on Epstein-Barr
virus-transformed primary B cells), Wü5404, Wü5679,
and Wü4797 (isolated on BJAB cells), the MV vaccine strain
Edmonston (ED) and the recombinant viruses [MV(WTF-F)ED, MV(WTF-H)ED,
MV(WTF-F/H)ED [
28], and MV(WTF-H481N

Y)ED] [referred to as ED(WTF-F),
ED(WTF-H), ED(WTF-F/H), and ED(WTF-H;N

Y) in this paper] on BJAB
cells in RPMI 1640 with 10% FCS. For the generation of ED(WTF-H;N

Y),
a point mutation was introduced in ED(WTF-H) at position 1441
(A to T), leading to amino acid Tyr (Y) instead of Asn (N) at
position 481. Viruses and uninfected BJAB cells, as mock control,
were harvested in phosphate-buffered saline and purified by
discontinuous sucrose gradient ultracentrifugation. Viruses
were titrated on marmoset lymphoblastoid B95a cells (maintained
in RPMI 1640-5% FCS). Except for the experiments shown in Fig.
1a to c, virus preparations were UV inactivated (1.5 J/cm
2),
analyzed for their glycoprotein content by Western blotting
using a rabbit serum raised against the cytoplasmic domains
of MV F or H proteins, respectively, and adjusted to equal concentrations
of glycoproteins. CHO cell clones (expressing functional hamster
TLR4 but nonfunctional hamster TLR2) EL1 (expressing the NF-

B
reporter plasmid ELAM.TAC), 3E10 (expressing ELAM.TAC and human
CD14), 3E10 hTLR2 (expressing ELAM.TAC, CD14, and human TLR2),
3E10 hTLR4 (expressing ELAM.TAC, CD14, and human TLR4), CHO
hTLR2 (only expressing human TLR2), EL1 mTLR2 (expressing ELAM.TAC
and murine TLR2), and 3E10 mTLR2 (expressing ELAM.TAC, human
CD14, and murine TLR2) (
41,
42,
44,
65) were maintained in Ham's
F-12-10% FCS containing 0.5 mg of G418 (Gibco BRL)/ml and/or
400 U of hygromycin B (Calbiochem)/ml, THP-1 cells in RPMI 1640-10%
FCS, and matured with 100 mM vitamin D
3 (Calbiochem) for 72
h. Human monocytes enriched from whole blood by density gradient
centrifugation using Ficoll-Paque Plus and Percoll (Amersham
Pharmacia) were maintained in RPMI 1640-10% FCS. Peritoneal
macrophages were isolated from C3H/HeN, C3H/HeJ, and C3H/TL2
-/- mice (
64) after thioglycolate elicitation and were maintained
in RPMI 1640-10% FCS-5 mM ß-mercaptoethanol. For cell
stimulation, LPS (
Escherichia coli serotype 0111:B4; Sigma)
(10 ng/ml), Pam
3CysSerLys
4 (PamCSK) (2.5 µg/ml) (EMC Microcollections,
Tübingen, Germany) was used. All reagents, cell lines,
and virus stocks were mycoplasma-free as determined by reverse
transcription-PCR and contained less than 10 pg of endotoxin/ml
as determined by a
Limulus lysate assay (Cape Cod Associates).
Reporter gene assay and IL-6 bioassay.
CHO cells (3
x 10
4/well) were stimulated in Ham's F12-10% FCS
for 12 h and analyzed by flow cytometry for surface expression
of CD25 using a phycoerythrin-conjugated antibody or an isotype
control (both from Beckmann Coulter). When indicated, a fusion-inhibiting
peptide (FIP) (Z-D-Phe-L-Phe-Gly-OH; Bachem) was added (final
concentration, 0.2 mM). Supernatants were assayed for IL-6 content
by the B9 cell proliferation assay (
1). Results are shown in
picograms per milliliter as means ± standard deviations
of triplicate measurements.
RNase protection assay.
Total cellular RNA was extracted from vitamin D3-treated THP-1 cells 12 h following stimulation with TriFast (PeqLab, Heidelberg, Germany), DNase I digested, and purified on RNeasy columns (Qiagen, Hilden, Germany). RNase protection assays were performed using a Riboquant multiprime kit (hCK-2; Pharmingen).
Enzyme-linked immunosorbent assay and immunocytochemistry.
THP-1 cells, human primary monocytes, or peritoneal mouse macrophages (105/well) were stimulated for 12 h. For blocking experiments, cells were preincubated with monoclonal antibodies specific for either CD14 (clone 3C10), TLR2 (clone TL2.1; kindly provided by T. Espevik, Trondheim, Norway [18]), or TLR4 (clone HTA125; kindly provided by K. Miyake, Saga, Japan) and viruses with a pool of three monoclonal MV-H-specific antibodies (L77, K83, and K29) or an MV-N-specific antibody (F227) (all three antibodies were generated in our laboratory) at a final concentration of 10 µg/ml in RPMI 1640 for 30 min. IL-6 or IL-12 p70 levels were determined in supernatants obtained 12 h after stimulation by using an enzyme-linked immunosorbent assay (R & D). For detection of IL-12 p70, cells were prestimulated with 1,000 U of human gamma interferon (h-IFN-
; Strathmann Biotech) for 30 min. Cells were analyzed for surface expression of HLA-DR (Pharmingen) and CD150 (clone 5C6; generated in our laboratory) (15) by using monoclonal antibodies or isotype-matched controls, respectively, followed by a secondary fluorescein-conjugated goat anti-mouse antibody.
Statistical analysis.
Statistical analysis was performed using Student's t test, and significance levels were determined based on the respective controls.

RESULTS
Wild-type MV but not vaccine strains activate TLR2 signaling.
To analyze whether MV acts as a TLR agonist, CHO cells stably
transfected with a CD25 reporter gene driven by an NF-

B-dependent
endothelial cell leukocyte adhesion molecule (ELAM) promoter
together with human CD14 (3E10), human CD14/TLR2 (3E10 hTLR2),
or CD14/TLR4 (3E10 hTLR4) were used in fluorescence-activated
cell sorter reporter cell assays. As expected, LPS activated
CHO cells expressing CD14, whereas a synthetic lipopeptide,
PamCSK, required TLR2 coexpression (Fig.
1a). Corroborating
the findings of Kurt-Jones et al. (
35), RSV activated CD25 expression
in 3E10 hTLR4 cells to about 20% (results not shown). The MV
Edmonston (ED) vaccine strain did not induce CD25 expression
in all CHO clones across a wide range of infectivities, whereas
a lymphotropic MV strain, WTF, activated CD25 expression in
3E10 hTLR2 even at virus doses as low as a multiplicity of infection
(MOI) of 0.1 (Fig.
1a). In this and all further experiments,
activation of the reporter gene by all MV strains and mock preparations
tested did not exceed 5% in 3E10 and 3E10 hTLR4 cells, also
indicating that, in accordance with the
Limulus lysate assay
(data not shown), all reagents were essentially endotoxin free.
Induction of reporter gene expression in the following experiments
was thus normalized to the mock control on TLR2-expressing cells.
While additional MV vaccine strains tested also failed to induce
reporter gene activation (Fig.
1b, lanes 1 to 3), all MV wild-type
strains efficiently stimulated CD25 expression in 3E10 hTLR2
cells (Fig.
1b, lanes 4 to 8). MV WTF also activated CD25 expression
in CHO cells coexpressing murine TLR2 and human CD14, and less
efficiently in the absence of CD14 (Fig.
1c). Similarly, MV
WTF induced release of bioactive IL-6 from CHO cells expressing
human TLR2, and this was enhanced by coexpression of CD14 (Table
1).
TLR2-dependent cell activation does not require virus entry and replication.
Heat or UV inactivation did not affect the ability of MV WTF
to induce TLR2-dependent signaling, indicating that replication
was not required (Fig.
1d). Stable expression of either entry
receptor, CD46 or CD150, allows productive MV replication in
CHO cells (
14,
15,
26,
59). In contrast, neither TLR2 nor TLR4
supported viral entry into these cells, since only about 2%
of MV nucleocapsid protein-positive cells were detected in all
cultures and formation of syncytia did not occur (data not shown).
Inclusion of a peptide inhibitor (FIP) previously found to inhibit
mixing of the outer membrane leaflets and thereby preventing
an early step of viral membrane fusion (
63) did not affect CD25
induction, confirming that reporter gene activation resulted
from a surface interaction of MV WTF (Fig.
1d).
The MV WTF H protein is the viral component triggering both human and mouse TLR2 activation, and this property is abolished upon a single amino acid exchange.
To identify the agonist responsible for MV-dependent TLR2 activation, we used purified UV-inactivated ED, WTF, and recombinant MV based on the ED strain backbone in which the F and H glycoproteins were singly or doubly swapped for those of the WTF strain (28). Since equal MOIs, as used in the experiments shown in Fig. 1, do not necessarily reflect equal amounts of viral glycoproteins applied, for this and all subsequent experiments the glycoprotein concentration of the individual viruses was adjusted by Western blotting (data not shown) prior to incubation with the cells. While the ED and the recombinant MV strain expressing the WTF-F protein [ED(WTF-F)] did not induce TLR2 activation (Fig. 2, lanes 1 and 2), recombinants containing the H protein of the WTF strain [ED(WTF-H) and ED(WTF-F/H)] stimulated reporter gene activation in 3E10 hTLR2 cells at equivalent levels to those seen with WTF (Fig. 2, lanes 3, 5, and 6). Activation with WTF-H-containing recombinants was not restricted to human TLR2, since CHO cells stably expressing the murine orthologue were also susceptible (Fig. 2). To define the potential basis for the failure of MV vaccine strains to activate TLR2 signaling, a recombinant MV expressing WTF-H [ED(WTF-H;N
Y)] was used in which the Asn (N) found at position 481 in lymphotropic wild-type viruses was replaced by Tyr (Y), which is important for the high-affinity interaction of MV with CD46 (47). In contrast to recombinants expressing the WTF-H protein, this mutant, as the ED-H-expressing MV strains, did not induce TLR2 activation (Fig. 2, lane 4). Thus, the abilities of MV H protein to use CD46 as a receptor and to activate TLR2 are apparently inversely correlated, and amino acid 481 plays an important role in this switch in activity.
MV WTF triggers TLR signaling in monocytes and stimulates expression of CD150.
To establish whether MV also activates TLR2 signaling in monocytes,
we analyzed the induction of monokines in RNase protection assays.
Induction of transcripts specific for IL-1

, IL-1ß,
IL-12 p40, and IL-6 was detected with PamCSK and UV-inactivated
WTF in vitamin D
3-treated THP-1 cells, while ED had no effect
(Fig.
3, lanes 3, 5, and 10, and Table
2). Induction of these
transcripts was also seen with recombinants expressing the WTF-H
protein (Fig.
3, lanes 7 and 9, and Table
2) but not with those
containing the ED-H protein, and with ED(WTF-H;N

Y) (Fig.
3,
lanes 5, 6, and 8, and Table
2). To investigate whether induction
of TLR2-responsive genes by MV is also observed in primary human
monocytes, these were treated with ED, WTF, and the recombinant
viruses and the release of IL-6 was analyzed as a marker. Synthesis
of IL-6 was significantly enhanced with viruses containing the
WTF-H protein, but not with those expressing ED-H or WTF-H;N

Y
protein (Fig.
4a). Although the overall levels of IL-6 were
lower, similar observations were made in THP-1 cells (data not
shown). Production of IL-12 p70 from monocytes was not detected
with either virus strain, irrespective of IFN-

priming (data
not shown). To confirm that IL-6 induction in monocytes was
dependent on both wild-type MV H protein and TLR2, antibody
blocking experiments were performed. Pretreatment of WTF with
anti-H antibodies or monocytes with TLR2-specific antibodies
led to reduced efficiency of WTF-induced IL-6 production from
these cells (Fig.
4c). MV-N-specific or TLR4-specific antibodies
were used as irrelevant isotype controls and had no effect (Fig.
4c). Interestingly, pretreatment of monocytes with a CD14-specific
antibody also interfered with the WTF-mediated IL-6 induction
(Fig.
4c). Inhibition was, however, only partial and this might
be explained for the TLR-specific antibodies by their known
variable blocking efficiencies. Clearly supporting the importance
of TLR2 activation by MV WTF-H protein, however, was the finding
that peritoneal macrophages isolated from TLR2
-/- mice did not
show virus strain-specific differences in IL-6 production, while
those isolated from C3H/HeN and C3H/HeJ mice released enhanced
levels of this cytokine when treated with WTF (Fig.
5).
Since these viruses induced TLR2 signaling and monocyte functions
consistent with activation, levels of surface molecules on these
cells might also be stimulated. Expression levels of HLA-DR
were augmented upon treatment of monocytes with LPS, PamCSK,
and viruses containing the WTF-H protein, but not with those
expressing the ED-H or WTF-H;N

Y protein (data not shown). Freshly
isolated monocytes from healthy individuals do not express CD150.
However, this molecule is upregulated after monocyte activation
(
45), possibly induced by TLR signals. Indeed, we observed upregulation
of CD150 on monocytes by WTF-H-containing viruses and not with
viruses containing the ED-H and WTF-H;N

Y protein (Fig.
4b).
Supporting the importance of TLR2 and WTF-H in this process,
induction of CD150 by WTF-H protein-expressing viruses was sensitive
to antibodies specific to TLR2 and MV-H but not MV-N (Fig.
4d).
Thus, lymphotropic wild-type MV activates human monocytes through
TLR2 and triggers surface expression of the MV receptor CD150.

DISCUSSION
Although crucial for overcoming acute measles and conferring
long-lasting immunity, the mechanisms of immune induction, and
particularly those concerning innate immunity, are largely unknown.
The major findings provided by our study are (i) MV can activate
cellular signaling via TLR2, (ii) this biological property is
confined to wild-type strains, (iii) the H protein of MV wild-type
strains is a TLR2 agonist, (iv) a single amino acid exchange
is sufficient to abolish the agonistic activity of this protein,
and (v) MV wild-type strains induce the expression of their
cellular receptor, CD150, through TLR2 in monocytes.
After the first descriptions of viral proteins acting as TLR agonists, the F protein of RSV (35) and the env proteins of murine retroviruses (51), both of which activate TLR4, we have described that the H protein of MV, one of the most important human viral pathogens, recruits TLR2 for cell activation. Lymphotropic wild-type MV specifically activates cells through TLR2, but not TLR4. Thus, MV WTF-mediated TLR activation only occurred on CHO cells transfected to express either human or murine TLR2 and not on those expressing endogenous TLR4 without or together with human TLR4 (Fig. 1a). Confirming that TLR4 activation is not involved in our system, all our reagents were essentially free of endotoxin, as determined by Limulus lysate assays, and inclusion of polymyxin B did not inhibit MV WTF-induced activation of 3E10 hTLR2 cells (data not shown). TLR4 activation by the RSV F protein required coexpression of CD14, which is an essential coreceptor for most TLR2 and TLR4 agonists (2, 12), and this molecule also enhanced TLR2 activation by MV WTF. Thus, reporter gene activation and induction of IL-6 in CHO cells is more efficient in the presence of CD14 (Fig. 1c and 2; Table 1), and antibodies directed against CD14 reduce MV WTF-H protein-induced IL-6 production by monocytes (Fig. 4c). It is evident that the ability of MV wild-type strains to induce TLR2 signaling is independent of the MV receptors CD46 and CD150, which are not expressed by CHO cells. Additionally, both THP-1 cells and unstimulated primary human monocytes do not express CD150 (Fig. 4b) (45). Apparently, CD46 is also not involved, since viruses binding CD46 such as ED barely activate monokine and CD150 expression, whereas viruses not binding CD46 do (Fig. 4).
Most interestingly, the ability to activate TLR2 signaling in both CHO cells and monocytes was confined to lymphotropic MV wild-type strains. The failure of vaccine strains to induce this signaling is not likely to reflect the requirement of coexpression of a heterologous TLR such as TLR6, which is required for TLR2-dependent activation by certain agonists (40, 50, 58). This can be deduced, since ED and ED(WTF-F) fail to activate human monocytes expressing both TLR2 and TLR6 (61) (Fig. 4). The differential ability of MV strains to activate TLR-dependent signaling also argues against a role of TLR3, recently identified as a receptor for dsRNA (3). This is particularly so because induction of IFN-
/ß as reported after TLR3 triggering with dsRNA might, if at all with MV, preferentially occur with vaccine and not with wild-type strains (46). The unique requirement for TLR2 in MV WTF monocyte activation was further confirmed with macrophages from TLR2-/- animals (Fig. 5). Since vaccine strains did not activate TLR2 even when applied at high doses and exchange of the H protein converted them into TLR2 agonists, amino acid differences within this protein were more likely to be important. We showed that a single amino acid exchange at position 481 within the WTF-H protein is sufficient to abolish activation of TLR2 signaling (Fig. 2). The structure of the MV H protein has not been resolved as yet; however, a model has been predicted (36). According to this model, amino acids 481 is located within one of the predicted six sheets of the most-membrane-distal domain, which has a propeller-like structure. Although amino acid changes are likely to alter the surface of the molecule, this can only be firmly established once the H protein structure is known. Remarkably, however, the amino acid at position 481 was also found to essentially determine binding to and usage of CD46 as an entry receptor for attenuated MV strains (9, 27, 37, 47). Thus, upon attenuation of wild-type strains in tissue culture and adaptation to utilization of CD46, the property to activate TLR2 signaling is concomitantly lost. This biological property may play an important role for the attenuation of MV vaccine viruses.
Monocyte activation in measles, as assessed by monokine induction, was addressed directly in one study where spontaneous release of IL-6 from peripheral blood mononuclear cells (PBMC) was found to be enhanced (21). Other studies applying clinical material or experimentally MV-infected macaques have relied on secondary cytokine responses in PBMC or monocyte cultures after restimulation in vitro (6, 7, 21). These cannot be compared to our experiments where primary consequences of MV interaction with directly challenged cells were investigated. In vaccinees, spontaneous production of IL-1
from PBMC was suppressed while levels of IL-1ß were not affected, and serum levels of tumor necrosis factor alpha (TNF-
) were lower than those seen in controls (62). While MV wild-type strains can induce monocyte activation by triggering TLR2 signaling, attenuated, CD46-adapted strains are obviously, albeit to a more limited extent, also able to activate antigen-presenting cells (APC) in vivo and in vitro in a TLR2-independent manner. After infection of primary human monocytes and THP-1 cells with a Vero cell-adapted and thus CD46-utilizing wild-type strain, low-level secretion of IL-1ß and TNF-
occurred (39), and in monocyte-derived DC cultures the replication-competent MV Halle strain, which is closely related to MV ED, induced low levels of IL-12 p40 and high levels of IL-1ß transcripts (56). Mechanisms of TLR2-independent monocyte activation are unknown as yet but may include CD46 ligation. In T cells, activation of p120CBL, Vav, Rac, and ERK and stimulation of proliferation were seen upon CD3-CD46 coligation (5, 66), and in murine RAW264.7 monocytic cells ligation of transgenic CD46 enhanced IFN-
-induced NO production after MV infection (30). In human macrophages, a CD46-specific antibody able to also block MV binding stimulated IL-12 p40 production, suggesting that positive signals can be induced via binding to this particular domain (34). Independent of CD46 ligation, MV replication can also stimulate NF-
B activation directly in certain cell types (13, 23).
Although activation of TLR signaling may play an important role for immune induction by MV wild-type strains, regulation of this pathway by MV wild-type viruses may also contribute to viral pathogenicity and induction of immunosuppression. Thus, CD150 was upregulated on monocytes by LPS, phytohemagglutinin (PHA), and two lymphotropic MV wild-type strains, both live and UV inactivated (45). Together with our observations that CD150 is induced on monocytes treated with WTF-H-containing viruses (Fig. 4b), this is, to the best of our knowledge, the first example of a virus triggering the expression of its own receptor. Partial suppression of this induction by TLR2-specific antibodies strongly supports the notion that TLR2 signaling is important for CD150 induction in monocytes (Fig. 4d). Our analyses do not allow us to distinguish whether CD150 upregulation results directly from TLR signaling or is induced indirectly via released cytokines such as IL-1
/ß, which is also induced by this pathway (Fig. 3) and which is known to enhance CD150 expression on mature DC (33). Again in DC, upregulation of CD150 was seen after treatment with LPS but not with lipopeptides which activate TLR2 signaling (10). In contrast, we observed that TLR2-agonistic MV were able to enhance expression of CD150 in monocytes (Fig. 4b). This could reflect differences in the cell type used or a differential ability of TLR2 ligands to induce CD150 expression. Maturation of DC with TLR2 agonists such as PamCSK and the 19-kDa lipopeptide from Mycobacterium tuberculosis has also been described, although expression of CD150 was not addressed (25).
The requirement of CD150 as an entry receptor for MV wild-type strains has been clearly documented (15, 26, 49, 59), and there is compelling evidence for the crucial importance of this molecule in monocyte infection with MV wild-type but not vaccine strains (45). In this study, induction of CD150 on the surface of monocytes by compounds such as LPS, PHA and, interestingly, two MV wild-type strains was seen. While replication of MV vaccine strains, which can enter these cells via CD46 (which is also expressed on unstimulated monocytes), was unaffected by the presence of CD150-specific antibodies, replication of wild-type MV was efficiently blocked by these antibodies. Supporting the importance of CD150 as receptor for MV wild-type strains, the tropism of MV for DC was linked to the expression of the WTF-H protein (48). Thus, induction of CD150 on monocytes via TLR2 may play an essential role in pathogenicity by conferring susceptibility to lymphotropic MV wild-type strains and thereby enhancing not only viral spread but also depletion of monocytes by apoptosis (16, 17). In addition, deregulation of APC functions, such as loss of allostimulatory activity, has been documented after infection, particularly with MV wild-type strains (19, 22, 32, 54). MV strains strongly interacting with CD46 may even actively block TLR activation by bacterial cell wall components. In monocyte cultures, ligation of CD46 by antibodies, but also MV, efficiently prevented the induction of IL-12 by subsequent LPS or Staphylococcus aureus strain Cowan stimulation (29). Similarly, downregulation of stimulated IL-12 release was seen with replication competent but also, albeit less efficiently, with UV-inactivated MV (Halle strain) in DC T-cell cultures (19). Interestingly, surface contact with the MV glycoprotein complex abolishes the activation of the phosphoinositol-3/Akt kinase pathway by IL-2 in T cells (8). Since this particular pathway was found to be involved in TLR2 signaling in monocytes (4), MV could also possibly modulate TLR signaling via this pathway in these cells. Lastly, induction of cross-tolerance to restimulation by TLR agonists after initial activation of TLRs was observed with many bacterial components (38, 52). If triggering of TLR2 signaling by MV on APC were to induce tolerance to subsequent activation by bacterial cell wall components, this could play an important role in the high sensitivity to opportunistic infections associated with acute cases of measles.

ACKNOWLEDGMENTS
We thank Michael Rehli, Bert Rima, Sieghart Sopper, and Stefan
Niewiesk for helpful discussions, Ian Johnston for generating
the MV recombinants, Douglas T. Golenbock for CHO transfectants,
Terje Espevik and K. Miyake for providing antibodies, and Sieglinde
Löffler, Maren Klett, Sylvia Fichte, Unni Nonstad, and
Mari Sörensen for excellent technical assistance.
We also thank the Deutsche Forschungsgemeinschaft, the Bundesministerium für Bildung und Forschung, The Wellcome Trust, the Research Council of Norway, the Norwegian Cancer Society, and the European Communities for financial support.

FOOTNOTES
* Corresponding author. Mailing address: Institute for Virology and Immunobiology, University of Würzburg, Versbacher Str. 7, D-97078 Würzburg, Germany. Phone: 49-931-201-49895. Fax: 49-931-201-49553. E-mail:
s-s-s{at}vim.uni-wuerzburg.de.


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Journal of Virology, September 2002, p. 8729-8736, Vol. 76, No. 17
0022-538X/02/$04.00+0 DOI: 10.1128/JVI.76.17.8729-8736.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
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