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Basel Institute for Immunology, Basel, Switzerland
| Abstract |
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| Introduction |
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DC maturation can be conventionally followed by in vitro cultures upon addition of inflammatory cytokines (6, 7). Not only do DCs up-regulate the costimulatory molecules B7-1 and B7-2 and CD40, but they also lose their capacity to capture and process protein Ags as well as to synthesize MHC class II molecules (8, 9). Furthermore, the efficiency of the formation of immunogenic MHC class II/peptide ligands is reduced in the lysosomal compartments (10) and, possibly due to down-regulation of endocytosis, the half-life of MHC class II complexes on the cell surface is increased up to 10-fold (11, 12). The end product of this maturation is a highly immunostimulatory DC that retains the "memory" of the original antigenic exposure.
It is generally assumed that these events take place also in vivo when, during a cutaneous inflammation, LC leave the skin and migrate to lymphoid organs for presentation of the MHC-peptide complexes to T lymphocytes. However, herein, we show that in vivo-matured LC that reached the LNs still are able to capture and process exogenous Ags even with increased efficiency as compared to that of normal resident DCs in the LNs, suggesting that the paradigm of maturation based on in vitro studies is not strictly followed in vivo.
| Material and Methods |
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BALB/c and C57BL/6 mice were provided by IFFA-Credo (Saint Germain-sur-lAbresle, France). Transgenic mice expressing a MHC class II-restricted TCR specific for the OVA peptide323339 (DO11.10) have been described previously (13). Mice were bred under specific pathogen-free conditions according to Swiss federal law.
Activation of LC in vivo
Green fluorescent Cell Tracker (50 µl; Molecular Probes, Leiden, The Netherlands), dissolved 1:20 in a 50:50 (v/v) acetone:dibutyl phtalate mixture, was painted on the dorsal site of the ears. To ensure full DC maturation in vivo, one mouse group was injected intradermally (i.d.) with LPS (LPS from Escherichia coli, 100 ng/mouse; Difco, Detroit, MI). One day later, draining LNs were collected and DCs were isolated as previously described (14). Briefly, LNs were digested twice for 30 min at 37°C in IMDM supplemented with 5% FCS with 100 µg/ml collagenase D (Boehringer Mannheim, Mannheim, Germany) in a shaking water bath. Cells were recovered, resuspended in a Optiprep-gradient (Nycomed, Norway), and centrifuged at 600 x g for 15 min. Low-density cells in the interface were harvested and incubated for 30 min on ice with PE-labeled anti-CD11c and Cy5-labeled anti-CD40. CD11chighCD40high cells were sorted with a FACStarPlus (Becton Dickinson, Mountain View, CA) or analyzed using a FACSCalibur (Becton Dickinson) excluding propidium iodide-positive dead cells.
Phenotype of in vitro- and in vivo-matured LC
To determine the phenotype of in vivo-matured LC, ears were painted with green fluorescent Cell Tracker with or without additional LPS injection as described above. DCs were isolated 1 day later from the draining LNs and stained for 30 min on ice with biotinylated anti-CD11c, Cy5-labeled anti-CD40, and PE-labeled anti-B7-1, anti-B7-2, and anti-MHC class II, respectively. Cells were washed and incubated for another 30 min with streptavidin-PerCP (Beckton Dickison). Cell Tracker+ and Cell Tracker- cells in the fraction CD11chighCD40high were analyzed for expression of both costimulatory molecules B7-1 and B7-2 and MHC class II (I-A, I-E) by FACS analysis. In vitro-matured DCs were obtained by incubating overnight the sorted CD11chighCD40high cells in the presence of 100 ng/ml LPS.
Isolation of LC after skin irritation
Acetone/dibutyl phtalate mixture was painted on the dorsal site of the ears and LPS was injected i.d. After 30 min, ear skin from BALB/c mice was split in dorsal and ventral halves, cut into small pieces, and digested twice for 30 min at 37°C in IMDM supplemented with 5% FCS with 100 µg/ml collagenase D. Released cells were collected, resuspended in an Optiprep gradient (Nycomed, Oslo, Norway), and centrifuged at 600 x g for 15 min. Low-density cells in the interface were harvested and labeled for 30 min on ice with PE-labeled anti-MHC class II.
Skin organ culture
Ear skin from BALB/c mice was split in dorsal and ventral halves and cultured in a 24-well plate (one ear per well) in the presence of 100 ng/ml LPS. Skin-derived DCs were obtained 1 or 2 days later by collecting the cells migrated from the skin explants into the culture medium and identified as MHC class II-expressing cells.
In vitro stimulation assays
For the in vitro T cell assays, LC obtained from the peripheral
LNs (CD11chighCD40high fraction) were sorted on
the basis of green fluorescence (Cell Tracker+ and Cell
Tracker- fractions) using a FACStarPlus, as
described above, obtaining a purity of >97%. For the MHC class
II-restricted OVA presentation assay, 5 x 103 Cell
Tracker+ and Cell Tracker- LCs obtained from
treated mice and unstimulated LC were cocultured with 5 x
104 CD4+ OVA-specific T cells in the presence
of different concentrations of OVA (10 µM to 100 nM). For comparison,
104 sorted CD11chighCD40high were
in vitro cultured with cytosine-guanosine oligonucleotides (1 µM),
LPS (100 ng/ml), GM-CSF (50 ng/ml) + TNF-
(50 ng/ml), or
without any stimulus. After 24 h, stimuli were washed away and DCs
were cocultured with 5 x 104 CD4+
OVA-specific T cells.
For the MLR, different numbers of sorted DC (H-2d, 2 x 1031.25 x 102) were added to 1 x 105 purified allogeneic T cells obtained from spleen of C57BL/6 (H-2b) mice.
T cell proliferation was assessed by [3H]thymidine (1 µCi/well) uptake in a 16-h pulse after 4 days for the MLR and 2 days for MHC class II-restricted presentation assay.
Determination of fluid phase and receptor-mediated endocytosis
For Ag-pulsing experiments, an enriched LN DC population of Cell Tracker-treated mice (with or without i.d. LPS) and in vitro-activated CD11chighCD40high were incubated 1) for 30 min with 100 µg/ml Cy5-labeled OVA at 4°C and 37°C or 2) for various times (15, 30, and 60 min) with Cy5-labeled mannosylated-BSA and Cy5-labeled BSA at 4°C and 37°C. The cells were washed three times with cold PBS containing 1% BSA and stained with biotinylated anti-CD11c and PE-labeled anti-CD40 for 30 min on ice. After another 30 min with streptavidin-PerCP, four-color FACS analysis was performed.
| Results and Discussion |
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We have previously defined at least four distinct DC
subpopulations in the peripheral LNs by different CD11c and CD40
expression patterns (staining profile is shown in Fig. 1
A). One DC subset, the
CD11chighCD40high cells, consists of immigrated
LCs (iLC) from the epidermis, since only this fraction occasionally
displays Birbeck granules, expresses E-cadherin, and carries
epicutaneously applied cellular dye (15). This
identification of progeny of skin LC in the peripheral LNs allowed us
to study their functional behavior after migration and maturation
processes in vivo.
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24 h later that correspond to the massive, but transient peak of iLC
(15). In the absence of concomitant LPS injection, Cell
Tracker+ cells (
70%) were clearly detectable after
20 h in the LC-derived subset of
CD11chighCD40high cells (Fig. 1
To correlate the migration of LC with maturation in vivo, we analyzed
the expression of MHC class II and of two different costimulatory
molecules, B7-1 and B7-2, their up-regulation being the hallmark for DC
maturation generally (18, 19, 20). Indeed both costimulatory
molecules were up-regulated in iLC bearing the Cell Tracker dye,
whereas Cell Tracker- iLC from the same LNs display
similar expression levels as iLC obtained from LNs of untreated mice
(Fig. 1
C). The LPS injection induced only a small additional
boost of expression of both costimulatory molecules when compared to
Cell Tracker+ iLC obtained from mice treated only with the
solvent. Intradermal injection of LPS also stimulated slightly the Cell
Tracker- fraction, probably due to diffusion of some
traces of LPS through the lymphatics into the draining LNs. MHC class
II levels were comparably high on all iLC sets analyzed.
The enhancement of surface B7-1 and B7-2 expression on in vivo-migrated
iLC led to substantially higher immunostimulatory ability. In fact,
when cocultured with allogeneic T cells, the Cell Tracker+
fraction was clearly more immunogenic than the unstimulated Cell
Tracker- iLC fraction obtained from LNs of treated or
untreated mice (Fig. 2
). Again, matured
iLC obtained from LPS-treated mice were slightly more immunostimulatory
than matured iLC obtained from mice painted with the solvent containing
Cell Tracker only.
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In vivo-, but not in vitro-matured LC efficiently present soluble protein to Ag-specific T cells
Based on the observation that LC undergo a maturation process
during the transition from the skin into the LNs, we monitored the
capacity of matured iLC to take up and present soluble proteins, such
as OVA, to Ag-specific T cells. Experiments in vitro have clearly
demonstrated that DC maturation abolishes the ability to endocytose
native proteins (21, 22) and concomitantly increases the
stability of MHC class II complexes formed on their cell surface prior
to maturation (11). Consistently, also in our hands, in
vitro-matured LC were unable to present OVA to Ag-specific T cells
(Fig. 3
A). However, to our
surprise, this was not the case when the maturation occurred in vivo.
When Cell Tracker+ and Cell Tracker- cells
(i.e., matured and naive, respectively) were purified from draining LNs
24 h after skin irritation, pulsed with OVA, and then cocultured
with Ag-specific CD4+ T cells, both fractions were capable
of stimulating Ag-specific proliferation. Furthermore, as shown in Fig. 3
B, immigrated Cell Tracker+ iLC were even more
efficient in presenting OVA to Ag-specific CD4+ T cells
when compared to the Cell Tracker- iLC or iLC obtained
from LNs of untreated mice. As a note, contaminating peptides in our
OVA preparation can not explain the above result, because OVA peptide
is presented equally well by in vitro- and in vivo-matured DC (data not
shown).
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To further elucidate the potential mechanistic differences between
in vivo- and in vitro-matured LC, these cells were pulsed with
Cy5-labeled OVA. As reported previously and also confirmed herein, in
vitro matured LC were unable to actively take up OVA at 37°C (Fig. 4
A). Remarkably, in
vivo-matured LC behaved differently. They had not shut down their
capacity to endocytose, but showed an even increased capacity compared
to that of normal resident DCs (Fig. 4
A). Interestingly, as
shown in Fig. 4
B, in vivo-matured iLC were also more potent
in receptor-mediated Ag uptake when compared to unstimulated normal DC,
as shown by the uptake of mannosylated BSA. This was most likely due to
a higher expression of a biological active mannose receptor, since DC
pulsed with Cy-5-labeled mannosylated BSA at 4°C revealed about three
times more cell surface binding to matured vs immature iLC (median of
fluorescence of 640 vs 196). In contrast, in vitro-stimulated DCs were
incapable of taking up not only BSA, but also mannosylated BSA (Fig. 4
B).
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3 days) they survive there. Concluding remarks
Maturing LC are not the only travelers in transit from the skin to the draining LNs, but also antigenic material thereof use the same lymphatics. Hence, it would make teleological sense that DCs, in transit and in final destination, while actually swimming in antigenic lymph, would not stop the uptake and processing of Ags, but would continue to do so and thereby to become "super" APCs ready to encounter T cells.
| Acknowledgments |
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| Footnotes |
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2 Abbreviations used in this paper: LC, Langerhans cell; DC, dendritic cell; iLC, immigrated Langerhans cell; LN, lymph node; i.d., intradermal. ![]()
Received for publication September 12, 2000. Accepted for publication April 9, 2001.
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