By
From * Institut National de la Santé et de la Recherche Médicale CJF 95-01, Institut Curie,
Section Recherche, 75005 Paris, France; the Second University of Milan, Department of
Biotechnology and Biological Sciences, 20126 Milan, Italy; the § Department of Immunology,
University Hospital, G04.614, 3584 CX, Utrecht, The Netherlands; and the
Division of Molecular
Genetics, Graduate School of Medicine, Chiba University, Chuo-ku, Chiba 260-8670, Japan
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Abstract |
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Dendritic cells (DCs) express several receptors for the Fc portion of immunoglobulin (Ig)G
(FcR), which mediate internalization of antigen-IgG complexes (immune complexes, ICs)
and promote efficient major histocompatibility complex (MHC) class II-restricted antigen presentation. We now show that Fc
Rs have two additional specific attributes in murine DCs: the
induction of DC maturation and the promotion of efficient MHC class I-restricted presentation of peptides from exogenous, IgG-complexed antigens. Both Fc
R functions require the
Fc
R-associated
chain. Fc
R-mediated MHC class I-restricted antigen presentation is extremely sensitive and specific to immature DCs. It requires proteasomal degradation and is dependent on functional peptide transporter associated with antigen processing, TAP1-TAP2. By
promoting DC maturation and presentation on both MHC class I and II molecules, ICs should
efficiently sensitize DCs for priming of both CD4+ helper and CD8+ cytotoxic T lymphocytes
in vivo.
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Introduction |
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ajor histocompatibility complex (MHC) class I molecules are generally complexed exclusively with peptides derived from cytosolic antigens (1). However, this picture is too restrictive to explain the priming of naive CD8+ T cells by bone marrow (BM)1-derived APCs (2): APCs also internalize exogenous antigens for processing and presentation on MHC class I molecules. The induction of CTL response due to exogenous antigen transfer was first examined in response to minor histocompatibility antigens, and was referred to as cross-priming (3). Recent results suggest that DCs may play a critical role in this process (4).
Indeed, dendritic cells (DCs) are the most potent APCs
for inducing differentiation of naive CD4+ and CD8+ T
cells into helper and cytotoxic T cells, respectively, and for
initiating primary and secondary immune responses (5, 6). To prime T cell responses, DCs require several separate
signals. The first is provided by antigens themselves, which
are processed into peptides and loaded intracellularly onto
MHC molecules. Efficient T cell priming also requires a
cell activation signal, delivered by either inflammatory cytokines (TNF- or IL-1) or bacterial components (such as
LPS). This signal induces expression of MHC and T cell
costimulatory molecules at the DC surface and causes migration from peripheral tissues to secondary lymphoid organs, where T cell priming occurs. Cognate CD4+ T cell
help is also required for efficient CD8+ T cell priming,
with antigen recognition by both CD4+ and CD8+ T cells
on the same DC (7). Therefore, this DC requires the simultaneous presentation of peptides from exogenous antigens on both MHC class I and II molecules.
Presentation of peptides derived from exogenous antigens on MHC class I molecules may occur through two different pathways (11). First, internalized antigens may exit endocytic compartments and, once in the cytosol, be processed by the proteasome into peptides which then reach the conventional transporter associated with antigen processing (TAP)1/2- dependent MHC class I antigen presentation pathway. Alternatively, internalized antigens may be processed in endocytic compartments, generating peptides which associate to preexisting MHC class I molecules, either in endosomes or at the cell surface after peptide regurgitation.
Regardless of the pathway, cross-priming in vitro after
fluid phase antigen internalization is generally very inefficient, since it requires very high antigen concentrationsin the
mg/ml range (11). Antigen coupling to or mixing with latex
beads forces internalization by phagocytosis and strongly enhances the efficiency of MHC class I-restricted antigen presentation in macrophages or DCs (12, 13). Phagocytosis of
bacteria (14, 15) or of apoptotic cells (4) also results in efficient MHC class I-restricted antigen presentation in macrophages and/or DCs. Thus, the pathway by which antigens are
internalized appears to influence the efficiency of presentation
on both MHC class I and II molecules.
In the case of MHC class II-restricted presentation, a
major breakthrough came from the observation that antigens internalized through specific membrane receptors are
more efficiently presented to CD4+ T cells than they are
after fluid phase internalization (16). FcRs, which bind
antigen-IgG complexes (immune complexes, ICs [17]),
represent a privileged antigen internalization route for efficient MHC class II-restricted antigen presentation in DCs (18). Human DCs express several types of Fc
Rs, including type I (Fc
RI, CD64 [19]) and type II (Fc
RII, CD32
[18]). Fc
R expression by murine DCs has not been fully
examined. Importantly, in addition to IC internalization,
Fc
RI and Fc
RIII trigger cell activation (17) through the
associated
chain, which bears a motif called immunoreceptor tyrosine-based activation motif (ITAM), also required for IC internalization (20, 21).
Here, we examined the role of FcRs in DC activation
and in MHC class I-restricted presentation of peptides derived from internalized IgG-complexed antigens. We found
that Fc
R engagement in DCs triggers maturation and induces efficient MHC class I and II-restricted antigen presentation. These results suggest the existence of unknown connections between humoral and cytotoxic components of
immune responses.
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Materials and Methods |
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Mice.
DCs and Culture Medium.
Immature DCs were prepared as described (24). C57BL/6 andAntibodies, Cell Surface Staining, and Immunofluorescence.
The following antibodies were purchased from PharMingen: CD80/ B7.1 (1G10), CD40 (HM40-3), CD86/B7.2 (GL1), CD107a/ Lamp-1. Before labeling experiments, FcR blocking was performed by incubating cells with 2.4G2 supernatant. Staining was carried out according to standard techniques, and flow cytometry analysis was performed with a FACScan® (using CellQuest software; Becton Dickinson). For intracellular immunofluorescence, cells were fixed for 20 min in 3% paraformaldehyde and then permeabilized for 30 min in PBS containing 1% saponin, 5% BSA, and then stained in PBS containing 1% saponin, 5% BSA.Antigen Presentation Assay.
OVA batches from different companies were screened for the absence of presentation to B3Z cells with fixed cells. The selected batch (from Worthington) did not induce DC activation by immunofluorescence and flow cytometry. Presentation of OVA epitope 257-264 on Kb was detected using the T cell hybridoma B3Z, which carries aActivation Induced by ICs.
5 × 104 D1 cells/well were incubated with soluble OVA alone or in the presence of hen egg lysozyme (HEL)-ICs (at final concentrations: HEL 30 µg/ml, and mAbs anti-HEL, HyHEL5, and 5253C7, 15 µg/ml each [27]) and were overlaid with 5 × 104 B3Z cells/well and incubated for 24 h.H-2Kb Transfection of B Lymphomas.
IIA1.6 B lymphoma cells expressing Fc ![]() |
Results |
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To analyze the expression
and function of FcR in murine DCs, we first used a well-characterized, growth factor-dependent, spleen-derived DC
line called D1 (24). D1 cells display all of the phenotypic
characteristics of immature DCs: low levels of surface MHC
and costimulatory molecules, and abundant endocytic MHC
class II-containing compartments. Upon treatment with LPS or TNF-
, D1 cells show all of the phenotypical changes
characteristic of DC maturation (24).
We first analyzed the nature of the FcRs expressed by
D1 cells. Western blot analysis after immunoprecipitation
with the anti-Fc
RII and Fc
RIII 2.4G2 antibody showed
that D1 cells expressed Fc
RIIb1, Fc
RIIb2, and Fc
RIII
(Fig. 1 A). The two intermediate bands between Fc
RIIb1
and Fc
RIIb2 most likely represent Fc
RIIb1' and an unidentified spliced variant (29). Fc
RI are also expressed in
murine DCs, since mRNA encoding this receptor was
readily detected by reverse transcription PCR (not shown).
These results indicate that murine DCs express Fc
RI, II,
and III.
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Engagement of FcRI or III in macrophages triggers cell
activation, causing the production of various cytokines and
chemokines, as well as changes in expression of cell surface
proteins involved in antigen presentation (17, 30). To evaluate the ability of ICs to induce DC activation, D1 cells
were incubated for 24 h in the presence of OVA complexed to specific polyclonal anti-OVA IgG antibodies
(OVA-ICs) or LPS, which induces D1 maturation.
As shown in Fig. 1 B, OVA-ICs, like LPS, induced a
marked increase in the surface expression of MHC class II,
CD86, and CD40 molecules (Fig. 1 B), phenotypic
changes characteristic of DC maturation (6, 24). Immunofluorescence and confocal microscopy analysis showed that
both LPS (Fig. 1 C, middle) and OVA-ICs (bottom) also
induced MHC class II redistribution to the plasma membrane (compared with Fig. 1 C, top), as lysosomes became
devoid of MHC class II molecules. Incubation with OVA
alone, or with the antibodies in the absence of OVA, induced no changes in the surface expression of any of the
markers analyzed or in DC morphology and MHC class II
localization (not shown). Similar results were obtained with fresh BM-derived DCs (BM-DCs, see below). Like other
maturation stimuli (31, 32), OVA-ICs induced an increase
in MHC class II synthesis and a strong decrease in their rate
of degradation (MHC class II half-life raised from 3-5 to
>40 h; not shown). Therefore, like LPS or TNF-, Fc
R
engagement by ICs induces murine DC maturation in vitro.
The other main consequence of
FcR engagement is IC internalization, which induces potent MHC class II-restricted presentation in various cell
types, including DCs (18, 33). However, during cross-priming, DCs also need to present exogenous antigens on MHC class I molecules to initiate CTL responses. To determine if ICs may participate in the acquisition of antigens
by DCs for MHC class I-restricted presentation, we next
examined presentation of an OVA-derived peptide to a
CD8+ T cell hybrid after Fc
R-mediated internalization of
OVA-ICs by murine D1 cells.
As shown previously (34), presentation of OVA
257-264/H-2Kb epitope to B3Z T cells (25) after OVA
uptake by fluid phase was only observed at very high, nonphysiological antigen concentrations of 1-10 mg/ml (Fig.
2). In contrast, presentation of the same epitope after internalization of OVA-ICs was observed at OVA concentrations ranging between 0.1 and 1 µg/ml (Fig. 2), i.e., three
to four orders of magnitude lower antigen concentrations
than uncomplexed OVA. As expected for ICs, the optimal
antigen to antibody ratio was achieved at lower antigen
concentrations as the antibody concentrations decreased
(Fig. 2). The highly efficient OVA presentation observed after OVA-IC internalization was not due to FcR engagement per se, since presentation of soluble OVA to B3Z T
cells was not modified by the presence of irrelevant HEL-ICs
(Fig. 2). HEL-ICs induced D1 maturation, as detected by
surface immunostaining of MHC and costimulation molecules (not shown). Therefore, formation of ICs allows efficient acquisition of antigens for peptide presentation to
CTLs on MHC class I molecules.
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The capacity of DCs to ingest and process exogenous antigens for presentation by MHC class II molecules is downmodulated upon DC maturation (37). Therefore, it was important to determine whether MHC class I-restricted presentation after IC internalization was also modulated during DC maturation. Like D1 cells, immature (untreated) BM-DCs presented OVA epitope 257-264 to B3Z cells more efficiently after OVA-IC internalization than after uncomplexed OVA uptake (Fig. 3, top left and middle left panels). In cells treated with LPS for 18 h, presentation after both OVA-IC and soluble OVA internalization was transiently and reproducibly enhanced (Fig. 3, top center and middle center panels). In contrast, MHC class I-restricted presentation after OVA-IC and soluble OVA internalization was almost completely abrogated in DCs treated with LPS for 48 h (Fig. 3, top right and middle right panels). Direct presentation of the 257-264 OVA peptide was not significantly modified by LPS treatment (Fig. 3, bottom panels). Therefore, as shown previously for MHC class II (31, 32), MHC class I-restricted presentation of exogenous antigens is first transiently upregulated and then shut down during DC maturation. Downregulation of MHC class I-restricted presentation by DCs upon maturation may prevent presentation of IgG-complexed antigens encountered after DC migration to secondary lymphoid organs.
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Two pathways for MHC class I-restricted presentation of exogenous antigens have been described (11). One reaches the so-called "conventional" pathway after exogenous antigen transfer from endocytic compartments to the cytosol. This pathway requires proteasomal degradation, TAP-dependent transport of the peptides into the lumen of the endoplasmic reticulum, and association to newly synthesized MHC class I molecules (11). The second pathway is TAP and proteasome independent; it is inhibited by NH4Cl (which neutralizes lysosomal pH) but not by cycloheximide (CHX; an inhibitor of protein synthesis), suggesting that the generation of peptides and their association to MHC class I may occur in endosomes (11). We next determined which of these two pathways was used for MHC class I-restricted presentation after IC internalization.
Antigen presentation by D1 cells after OVA-IC internalization was strongly inhibited by CHX (Fig. 4 A), lactacystin (a specific proteasome inhibitor [38]; Fig. 4 B), and the peptide aldehyde N-acetyl-Leu-L-Leu-L-norleucinal (LLnL; Fig. 4 C), which inhibits both lysosomal proteases and the proteasome (38). Direct presentation of the 257-264 OVA peptide was not affected by any of these drugs (Fig. 4, A-C). To assess the specificity of the two protease inhibitors in DCs, lactacystin and LLnL were tested in parallel on the MHC class II-restricted presentation after OVA-IC internalization. Only LLnL, and not lactacystin, blocked MHC class II-restricted presentation of OVA peptide 323-339 on I-Ab to BO97.10-specific T cells (Fig. 4 D), indicating that lactacystin specifically inhibits proteasomal degradation at the concentrations used. MHC class I-restricted presentation after OVA-IC internalization was also sensitive to brefeldin A (which blocks protein transport from the endoplasmic reticulum, not shown).
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MHC class I-restricted presentation after OVA-IC internalization also required the TAP1-TAP2 peptide transporter. BM-DCs derived from TAP1-deficient mice did not present the OVA epitope after internalization of OVA-ICs, whereas direct presentation of the 257-264 OVA peptide to B3Z T cells was as efficient as with DCs derived from wild-type (wt) C57BL/6 mice (Fig. 4 E). Therefore, MHC class I-restricted presentation after IC internalization reaches the conventional MHC class I presentation pathway.
MHC Class I-restricted Presentation after IC Internalization Is DC Specific.However, efficient IC internalization is
not restricted to DCs. In B lymphoma cells, we showed
that expression of endocytic FcRs induces efficient MHC
class II-restricted presentation after IC internalization (20,
39). To determine whether Fc
R-mediated MHC class
I-restricted presentation of exogenous antigens is DC specific, we next examined antigen presentation after OVA-IC
internalization in B lymphoma cells. IIA1.6 B lymphoma
cells are an Fc
R clone derived from A20 B lymphoma
cells, one of the cell lines most widely used to analyze
MHC class II-restricted antigen presentation (20, 39). We
have shown previously that IIA1.6 cells expressing recombinant Fc
RIIb2 or Fc
RIII efficiently internalize IC and strongly promote MHC class II-restricted antigen presentation (20, 39). In addition, a chimeric receptor composed of
the lumenal and transmembrane domains of Fc
RII and
the cytoplasmic tail of the
chain (Fc
R-ct
) presents all
of the functional characteristics of Fc
RIII in terms of
internalization and antigen presentation (20, 33). I-Ad-
expressing B lymphoma IIA1.6 cells expressing recombinant Fc
RIIb2 or Fc
R-ct
chimeras were supertransfected with H-2Kb, and compared with D1 cells for MHC
class I and II presentation after OVA-IC internalization.
In FcR-ct
/H-2Kb-expressing B lymphoma cells,
MHC class II-restricted presentation of the 323-339 OVA
peptide on I-Ad to 54.8 T cell hybridomas was strongly enhanced (three to four orders of magnitude) after OVA-IC
internalization (Fig. 5 A). In D1 cells, presentation of the
same peptide on I-Ab to BO97.10 T cell hybridomas was
also enhanced by three to four orders of magnitude after
OVA-IC internalization (Fig. 5 B). Similar results were
obtained with cells expressing Fc
RIIb2 (not shown). Therefore, internalization of ICs results in efficient MHC class II-restricted presentation in both B lymphoma cells and DCs.
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In D1 cells, as shown previously, soluble MHC class
I-restricted presentation was observed after soluble OVA at
high concentrations and after OVA-IC internalization at
low antigen concentrations (Fig. 5 D). In contrast, B lymphoma cells expressing FcR-ct
(Fig. 5 C) or Fc
RIIb2
(not shown) were completely incompetent for MHC class
I-restricted presentation of OVA-derived peptide 257-
264, after internalization of OVA-ICs or uncomplexed
OVA at high concentrations. In contrast, when incubated
with the OVA 257-264 synthetic peptide, both D1 cells
and B lymphoma cells activated B3Z cells (Fig. 5, C and D).
Like B lymphoma cells, IFN-
-treated peritoneal macrophages were capable of directly presenting OVA peptide 257-
264 to B3Z cells, but not after OVA-IC or uncomplexed
OVA internalization (not shown). Therefore, in contrast to
MHC class II-restricted presentation, the Fc
R-mediated pathway for presentation of exogenous antigens by MHC
class I molecules is restricted to DCs.
The
results presented thus far suggest that FcRs are involved in
both the induction of DC maturation and antigen uptake for efficient MHC class I-restricted presentation of exogenous antigens. Importantly, Fc
RI and Fc
RIII trigger cell
activation in a variety of cell types through an ITAM found
in the associated
chain (17). To determine the nature of
the Fc
Rs involved in the triggering of DC maturation by
ICs, we prepared BM-DCs from wt mice and from mice
deficient for the Fc
RI- and Fc
RIII-associated
chain
(22). Surface expression of Fc
RII and III (as detected by
the mAb 2.4G2) was decreased but not abolished in BM-DCs from the
chain
/
mice (Fig. 6 A), confirming that
BM-DCs from the
chain
/
mice still express Fc
RII
and suggesting that BM-DCs from wt C57BL/6 mice expressed both Fc
RII and III.
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To determine the requirement for chain in DC activation, the ability of LPS and OVA-ICs to induce maturation
was analyzed. LPS induced a marked increase of the surface
expression of MHC class II, CD86, and CD40 molecules
on DCs from wt and
chain
/
mice (Fig. 6 B), as well as
all of the morphological modifications characteristic of DC
maturation (not shown). In contrast, OVA-ICs did not induce any detectable maturation in BM-DCs from
/
mice, whereas they induced maturation of BM-DCs from
wt mice (Fig. 6 B). Therefore, DCs from
chain
/
mice
presented a selective defect in IC-induced maturation.
The involvement of the chain in MHC class I-restricted
presentation was tested next. As expected, BM-DCs from
both wt and
chain
/
mice presented soluble OVA and
the 257-264 OVA peptide to the B3Z T cell hybridomas
with similar efficiencies (Fig. 7). BM-DCs from wt mice
also presented the OVA epitope after OVA-IC internalization at low antigen concentrations (Fig. 7). In contrast,
DCs from
/
mice did not show any significant MHC
class I-restricted presentation after incubation with OVA-ICs
(Fig. 7). These DCs had also lost the ability to present
OVA peptide 323-339 in association to I-Ab MHC class II
molecules (not shown). In addition, Fc
R-mediated IC
internalization in BM-DCs from
chain
/
mice was decreased, as measured both biochemically and by immunofluorescence and confocal microscopy (not shown),
suggesting that the absence of antigen presentation by
BM-DCs from
chain
/
mice results from inefficient IC
internalization.
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Therefore, the FcR-associated
chain is required for
both induction of DC maturation by ICs and promotion of
MHC class I-restricted presentation, indicating that Fc
RI
and/or Fc
RIII (the two
chain-associated Fc
Rs) are required for the functions of ICs in DCs.
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Discussion |
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Our results evidence a novel receptor-mediated pathway
for antigen acquisition for MHC class I-restricted presentation. This FcR-mediated cross-priming pathway is DC
specific and is inactive in mature DCs. It requires the
TAP1-TAP2 transporter and is sensitive to lactacystin, a
proteasome inhibitor, indicating that after internalization,
IgG-complexed antigens are transferred into the cytosol
and reach the conventional MHC class I antigen presentation pathway. Fc
R engagement also induces full DC activation, reflected by increased levels of MHC and costimulatory molecules (CD40, CD80, and CD86) at the cell
surface. Simultaneous induction of maturation and MHC
class I and class II-restricted presentation by a single receptor-ligand interaction should result in efficient T cell priming in vivo.
This pathway is not the first described for cross-priming in vitro. Fluid phase internalization can result in MHC class I-restricted presentation, but only at very high antigen concentrations (3-10 mg/ml in the OVA system), in both macrophages and DCs (34, 36). Interestingly, induction of macropinocytosis was shown to result in cross-priming in vitro (35). In addition, phagocytosis somehow favors MHC class I-restricted presentation of exogenous antigens in macrophages and DCs. Indeed, internalization of bacteria by macrophages or DCs (12, 14, 15) and phagocytosis of apoptotic bodies by DCs (4) also result in cross-priming. Likewise, antigen coupling to (12), or in some case mixing with (13), synthetic beads forces antigen phagocytosis and remarkably increases the efficiency of MHC class I antigen presentation (peptides derived from OVA may then be presented at 1-3 µg OVA/ml). Thus, the mode of antigen internalization influences cross-priming in vitro.
The mode of IC internalization (endocytosis or phagocytosis) in DCs is still unclear. However, our results exclude
the possibility that the efficient cross-priming observed with
ICs is due to FcR-independent phagocytosis. Indeed, in
DCs from
chain
/
mice, MHC class I-restricted presentation was not observed, demonstrating that the DCs (and the
Fc
Rs they express) and not the eventual particulate form of
the antigen are determinant for cross-priming with ICs.
FcR-mediated cross-priming is TAP dependent and
sensitive to the proteasome inhibitor lactacystin, suggesting
that IgG-complexed antigens are transferred from endocytic compartments into the cytosol. Although the
mechanism of this transfer is still unclear, it was shown previously that macropinocytosis results in increased antigen
delivery to the cytosol (40). After IC internalization at low
antigen concentrations, we observed antigen transfer to the
cytosol by immunofluorescence and confocal microscopy
(Rodriguez, A., unpublished results). However, the efficient cross-priming observed in DCs after IC internalization was not due to an overall effect of Fc
R engagement,
since simultaneous engagement of Fc
Rs by irrelevant ICs
did not increase the efficiency of cross-priming with soluble OVA internalized by fluid phase (Fig. 2) or its transfer to the cytosol (not shown). This observation also suggests
that, to be transferred into the cytosol after internalization,
antigens need to be targeted by Fc
Rs to a particular population of endosomes or lysosomes.
In contrast, IC internalization in other cell types expressing endocytosis-competent FcRs, like macrophages or
transfected B lymphoma cells, did not result in MHC class
I-restricted presentation. The molecular bases of this DC
specificity are still unclear. They are certainly not related to
the ability of Fc
Rs to mediate IC internalization or cell
activation, which are both efficient in macrophages or
transfected B lymphoma cells. In contrast, we found that
antigen transfer to the cytosol is inefficient in these two cell
types compared with DCs (Rodriguez, A., unpublished results). Therefore, the specificity of DCs for cross-priming
might be related to a selective ability of DCs to deliver antigen from endosomes or lysosomes into the cytosol.
The other major effect of FcR engagement in DCs is
induction of maturation. Indeed, all of the phenotypical,
morphological, and functional modifications caused by inflammatory cytokines or LPS were also induced by ICs. In
addition, none of these modifications were observed with
DCs from
chain
/
mice, demonstrating the implication
of the
chain in the induction of DC maturation by ICs.
We found here that mouse D1 cells (as well as BM-DCs)
express the two
chain-dependent Fc
Rs, Fc
RI (CD64)
and Fc
RIII (CD16). It is not clear to date which of these two receptors is required for DC maturation, but it will be
directly addressed using Fc
RI and/or Fc
RIII
/
mice.
Whether FcRI or Fc
RIII is used, the involvement of
the Fc
R-associated
chain in DC maturation indicates
that an ITAM-bearing receptor triggers DC activation.
Fc
R cross-linking causes activation of protein tyrosine kinases (PTKs) from the src family (17). These PTKs phosphorylate tyrosine residues in the ITAM, thus inducing association to syk PTK, leading to Ca2+ release from
intracellular stocks and to a wide variety of biological responses. Our observation represents the first evidence of
induction of DC activation and maturation through an
ITAM-containing receptor. Interestingly, the
chain ITAM
also bears an internalization signal that mediates endocytosis
of Fc
RIII (20), and more recently, we showed that the
ITAM also determines
chain-mediated transport from
endosomes to lysosomes (41). Thus, Fc
Rs may initiate
DC maturation and simultaneously target antigen to the
appropriate endocytic compartment, where peptides are
loaded onto MHC class II and from where antigens are
transferred into the cytosol.
Indeed, CD4+ T cells play an important role in antiinfectious CD8+ T cell-mediated responses, even if they are
dispensable in some of them (42). The antigen presentation on both MHC class I and class II molecules that we
observed after IC internalization by DCs would ensure an
optimal stimulation of both CD4+ and CD8+ T cells. At
the end of primary responses and in the course of secondary
immune responses, the production of specific antibodies induces formation of complexes between antigens derived
from infected cells or tumor cells and specific IgGs. These
complexes could be taken up by FcRs on DCs. After internalizing ICs, DCs would then present the antigen to
specific CD4 T cells, which activate them through interactions implicating costimulatory molecules like CD40-CD40L and convert them into DCs capable of priming
CD8+ T cells (7). However, since Fc
R engagement
also induces efficient maturation, IC-activated DCs could
potentially prime CD8+ T cells directly, bypassing cognate
CD4+ T cell help. This mechanism could operate in certain antiviral and/or antitumoral immune responses. However, it could also induce inappropriate CTL responses,
since the absence of CD4+ T cells would not allow a control of specificity, i.e., a "double check."
In what physiological situation might ICs trigger CTL responses? Specific antibodies, which may potentially form ICs, are produced during most immune responses, including those where the final effectors are CTLs. ICs may participate in DC-mediated CD8+ T cell priming in the case of secondary immune responses, when specific IgGs may be produced very rapidly. In the case of ongoing immune responses, which in many cases correspond to situations of immunosuppression, such as chronic infections or tumors, cross-priming through ICs may contribute to the establishment of specific tolerance. ICs have also been reported to play critical roles in several autoimmune diseases. Amplification of anti-self CTL responses by DCs that have acquired autoantigens from ICs may contribute to the triggering or development of autoimmune pathologies.
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Footnotes |
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Address correspondence to Sebastian Amigorena, Institut Curie, 12 rue Lhomond, 75005 Paris, France. Phone: 33-1-42-34-63-89; Fax: 33-1-42-34-63-82; E-mail: sebas{at}curie.fr
Received for publication 6 August 1998 and in revised form 21 October 1998.
We thank S. Viel for technical assistance; N. Shastri (Berkeley, CA) and C. Watts (Dundee, UK) for the anti-OVA T cell hybridoma B3Z, and C. Watts for providing us with OVA; J.-C. Guery (Toulouse, France) for the anti-OVA T cell hybridoma BO97.10; and Dr. A. Sarukhan (Necker Institute), Dr. P. Pereira (Institut Pasteur), and Dr. P. Benaroch (Institut Curie) for helpful discussions and critical comments on the manuscript.
This work was supported by grants from Institut National de la Santé et de la Recherche Médicale, Centre National de la Recherche Scientifique, and Ligue Nationale Contre le Cancer. A. Regnault is funded by the Ligue Nationale Contre le Cancer, C. Théry by the Société de Secours des Amis des Sciences, and A. Rodriguez by the TMR Fellowship from the EEC.
Abbreviations used in this paper
BM-DC, bone marrow-derived DC;
CHX, cycloheximide;
DC, dendritic cell;
FcR-ct
, chimeric receptor
composed of the lumenal and transmembrane domains of Fc
RII and the
cytoplasmic tail of the
chain;
HEL, hen egg lysozyme;
IC, immune
complex;
ITAM, immunoreceptor tyrosine-based activation motif;
LLnL, aldehyde N-acetyl-Leu-L-Leu-L-norleucinal;
OVA-IC, OVA-containing IC;
PTK, protein tyrosine kinase;
TAP, transporter associated with
antigen processing;
wt, wild-type.
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