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CUTTING EDGE |
Institute of Medical Microbiology, Immunology and Hygiene, Technische Universität München, Munich, Germany
| Abstract |
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| Introduction |
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Immature DCs lack costimulatory signals required for productive T cell activation but are well equipped to sample Ag, for example via receptor-mediated endocytosis or fluid phase pinocytosis (6). Because of its selectivity for the Ag in question, the efficacy of Ag internalization via receptor-mediated endocytosis is high, examples being C-type lectins that bear mannose specificity (6), Ag/Ab immune complexes that bind FcRs on DCs (7), or Ag delivered via apoptotic cells (8).
CpG oligonucleotides covalently linked to soluble Ags result in enhanced receptor-like endocytosis of Ag, Ag cross-presentation by DCs, and activation of DCs into professional APC (9). Immunostimulatory CpG DNA motifs activate immature DCs via Toll-like receptor (TLR)9 (10, 11, 12) at late (lysosome-associated membrane protein 1+) endosomal organelles (13). This raises the fundamental question of whether TLR9 effects receptor-mediated endocytosis of covalently linked Ag in addition to its known ability to activate the Toll-IL-1 signaling pathway. To address this issue, we analyzed Ag uptake, cross-presentation, and cross-priming of CpG DNA-linked OVA in TLR9-deficient DCs in vitro and in vivo. We now report that TLR9 does not impact receptor-mediated endocytosis but is essential for cross-priming.
| Materials and Methods |
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Cell lines and in vitro culture medium
EL-4 (H-2b) thymoma cells were purchased from the American Type Culture Collection (Manassas, VA). B3Z, a somatic T cell hybrid generated by fusing the OVA/Kb-specific cytotoxic clone B3 with a lacZ-inducible derivative of BW5147 fusion partner (14), was kindly provided by Dr. B. Kelsall (National institutes of Health, Bethesda, MD). Cells were cultured in RPMI 1640 supplemented with 10% (v/v) heat-inactivated FBS, 100 IU/ml penicillin G, 100 IU/ml streptomycin sulfate (all from Biochrom, Berlin, Germany) and 50 µmol/L 2-ME (Life Technologies, Karlsruhe, Germany) at 37°C and 5% CO2.
Generation of GM-CSF- and Flt3 ligand-cultured DC from bone marrow
Flt3 ligand-supplemented bone marrow cell cultures were generated as described (9), and cells were used at 10 days of culture. Generation of GM-CSF-induced, bone marrow-derived DC cultures was performed as previously described (15). Cells were used at days 57.
Reagents
Chicken egg albumin (OVA) was from Sigma-Aldrich (Taufkirchen, Germany). The peptide SIINFEKL (OVA peptide 257264) was custom synthesized by Research Genetics (Huntsville, AL). FITC-labeled OVA was purchased from Molecular Probes (Leiden, The Netherlands).
Phosphothioate-modified immunostimulatory CpG DNA was custom synthesized by MWG (Ebersberg, Germany). The phosphothioated sulfhydryl-modified oligodeoxynucleotides (ODN; TriLink Biotechnologies, La Jolla, CA) used throughout this study consisted of 20 bases and contained a CpG motif (1668: 5'-S-TCCATGACGTTCCTGATGCT-3'). OVA was incubated with the cross-linker sulfo-maleimidobenzoyl-N-hydroxysuccinimide ester (S-MBS; Pierce, Bonn, Germany) in a 50 mM EDTA-PBS buffer (pH 7.0) at a molar ratio of 1:10 for 1 h at room temperature. The sulfhydryl-modified ODN were reduced in a 50 mM 1,4-DTT-PBS solution. Subsequently unbound S-MBS and 1,4-DTT were removed by chromatography on a Bio-Rad P-6 gel column (Bio-Rad, Munich, Germany). The activated ODN were incubated with the linker-modified OVA at a molar ratio of 5:1 for 2.5 h at room temperature and thereafter L-cysteine was added to quench reactive S-MBS. Free ODN were removed by chromatography on a Superdex 75HR column (Amersham Biosciences, Freiburg, Germany). Purified conjugates were analyzed on a 620% gradient SDS-PAGE and consecutively silver-stained. To determine the ratio of bound ODN on OVA, a 415% gradient nondenaturing, nonreducing PAGE was run and silver-stained or visualized using ethidium bromide staining. Protein concentration was determined by the Lowry method (Pierce). The batches of CpG-OVA conjugates used in this study had a ratio of 2.5 CpG DNA molecules linked to one OVA molecule.
Escherichia coli-derived LPS was purchased from Sigma-Aldrich.
Immunization of chromium release assay
For induction of CTL, OVA and adjuvant were injected into both hind footpads of mice. Four days later, draining lymph nodes (LN) were removed and a single-cell suspension was prepared. LN cells (3 x 106 cells/ml) were cultured for an additional 4 days in medium conditioned with 10 IU/ml rIL-2. The chromium-51 release assay was performed as described (13). SIINFEKL peptide-untreated cells served for specificity control. Specific lysis was calculated according to the formula: percent specific lysis = (cpmsample - cpmspontaneous release/maximum release - cpmspontaneous release) x 100.
Uptake and activation analysis, mAbs
To examine uptake of FITC-labeled CpG-OVA in vitro by bone marrow derived Flt3 DC, cells were exposed to FITC-labeled OVA, mixed with CpG DNA 1668 or 1668-OVA-FITC conjugates (0.5 h at 37°C), washed with ice-cold 2% FCS-PBS containing 2 mM EDTA, and stained with the allophycocyanin-labeled anti-CD11c (clone HL3).
To analyze activation of DC, GM-CSF-cultured bone marrow-derived DC (15) were incubated with 10 µg/ml OVA conjugated with 1.14 µM CpG DNA. Cells were cultured for 24 h, and thereafter washed and stained with allophycocyanin-labeled anti-CD11c (clone HL3), FITC-labeled anti-CD4 (clone 3/23), anti-CD80 (clone 16-10A1), and anti-CD86 (clone GL1). FACS analysis was performed using a FACSCalibur flow cytometer (BD Biosciences, Heidelberg, Germany) acquiring at least 30,000 events per sample. FACS data were analyzed using CellQuest software (BD Biosciences).
mAbs and corresponding isotype controls were purchased from BD PharMingen (Heidelberg, Germany).
Presentation assay
Presentation of SIINFEKL after CpG DNA-aided uptake of OVA by DCs was assayed as previously described (13) by measuring induction of lacZ activity in SIINFEKL/Kb-specific T cell hybridoma B3Z (14). To this, 1 x 105 GM-CSF-cultured DC were incubated with the indicated reagents for 18 h at 37°C. Plates were washed, and 1 x 105 B3Z cells were added to each well. After additional incubation overnight at 37°C, cells were fixed with 0.5% glutaraldehyde for 10 min and incubated with X-Gal solution (Taufkirchen, Germany) (16) at 37°C. After 68 h, blue B3Z cells were counted under the microscope.
Detection of cytokines
For determination of cytokines, the supernatant of stimulated cells was collected. Cytokine release (IL-12p40 and TNF-
; R&D Systems, Wiesbaden, Germany) was assayed by ELISA as described by the manufacturer.
| Results and Discussion |
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(Fig. 3b) upon in vitro uptake of immunostimulatory CpG-OVA conjugates. Of note, the DC activation power of CpG DNA linked to protein (Fig. 3) was somehow reduced compared with free CpG DNA (9). Whether processing of OVA when linked to CpG DNA is still TAP-dependent has not yet been analyzed. However, in general, DCs of TLR9-/- were fully able to up-regulate CD40 and CD80 and secrete cytokine as shown by stimulation via TLR4 with LPS (Fig. 3).
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| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Hermann Wagner, Institute of Medical Microbiology, Immunology and Hygiene, Technische Universität München, Trogerstrasse 9, D-81675 Munich, Germany. E-mail address: h.wagner{at}lrz.tu-muenchen.de ![]()
3 Abbreviations used in this paper: DC, dendritic cell; TLR, Toll-like receptor; ODN, oligodeoxynucleotide; S-MBS, sulfo-maleimidobenzoyl-N-hydroxysuccinimide ester; LN, lymph node; wt, wild type. ![]()
Received for publication December 20, 2002. Accepted for publication January 28, 2003.
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receptor-mediated induction of dendritic cell maturation and major histocompatibility complex class I-restricted antigen presentation after immune complex internalization. J. Exp. Med. 189:371.Related articles in The JI:
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