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* Bone Marrow Transplantation Laboratory, Queensland Institute of Medical Research, Queensland, Australia;
Centre for Immunology and Cancer Research, University of Queensland, Princess Alexandra Hospital, Woolloongabba Queensland, Australia; and
Cooperative Research Centre for Chronic Inflammatory Diseases and Australian Research Council Special Research Centre for Functional Applied Genomics, Institute for Molecular Bioscience, University of Queensland, Queensland, Australia
| Abstract |
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| Introduction |
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| Materials and Methods |
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MacGreen mice were produced on a B6 x CBAF1 background as described previously (5). Homozygous op/op and heterozygous op/wt mice were supplied by the Herston Medical Research Centre (Brisbane, Australia). C57BL/6 mice were supplied by the Animal Resource Centre (Western Australia, Australia). All mice used were 512 wk of age.
Cytokine treatment
Recombinant progenipoietin-1 (ProGP-1) (Pfizer) was diluted in 1 µg/ml murine serum albumin in PBS before injection. Mice were injected s.c. with diluent or 20 µg/animal/day ProGP-1 once daily from days 7 to 1. Complete blood counts were performed on EDTA peripheral blood (PB) samples using a Sysmex SE-9000 (Sysmex) automated analyser.
Monoclonal Abs
The following mAbs were purchased from BD Pharmingen: FITC-conjugated CD11c (HL3) and IgG2a isotype control; PE-conjugated CD4 (GK1.5), CD8a (53-6.7), CD11b (M1/70), CD11c (HL3), CD40 (3/23), CD45R/B220 (RA3-6B2), CD80 (16-10A1), CD86 (GL1), I-A/I-E (2G9), and IgG2b isotype control; and PE-Cy5-conjugated CD8 CD4 and IgG2b isotype control. PE-conjugated CD115 was purchased from Serotec. Purified mAb against CD3 (KT3), CD19 (HB305), Gr1 (RB6-8C5), Thy1.2 (HO-13-4), Ter119, Fc
R II/III (2.4G2), and biotinylated F4/80 were produced in house.
Cell preparation
DC purification was undertaken as described previously (6). Briefly, low-density cells were enriched from digested lymphoid tissues, bone marrow, or lysis buffer-treated heparinized blood by Nycodenz density gradient centrifugation. In some experiments, non-DC lineage cells were depleted by coating with rat IgG Abs to B cells (CD19), T cells (CD3, Thy1), granulocytes (Gr-1), and erythroid cells (Ter-119). The coated cells were then removed by magnetic beads coupled to anti-rat IgG (Dynal ASA, Oslo, Norway). For mixed lymphocyte cultures (MLC), splenic T cells were purified by depleting B cells (B220 and CD19), monocytes (CD11b), granulocytes (Gr-1), and erythroid cells (Ter-119) using magnetic bead depletion. For culture experiments, highly purified DC populations (>98%) were obtained by FACS (MoFlo; DakoCytomation). Peritoneal cells were lavaged from the peritoneal cavity with HBSS containing EDTA (Sigma-Aldrich).
Real-time RT-PCR for CSF-1R
For real-time RT-PCR analysis, equivalent numbers of sort-purified cells were resuspended in TRIzol (Invitrogen Life Technologies), snap frozen on dry ice, and RNA extracted, according to the manufacturers protocol. cDNA was immediately reverse transcribed using avion myeloblastosis reverse transcriptase (Promega), according the manufacturers protocol, and cDNA stored at 20°C. Real-time PCR was undertaken using Platinum SYBR Green qPCR SuperMix UDG (Invitrogen Life Technologies) conducted on a Rotor-Gene3000 (Corbett Research) and data analyzed using Rotor-Gene v5.0 (Corbett Research). Primers used for CSF-1R reactions were 5'-CCACCATCCACTTGTATGTCAAAGAT-3' (forward) and 5'-CTCAACCACTGTCACCTCCTGT-3' (reverse). Primers used for
2-microglobulin reactions were 5'-TTTCTGGTGCTTGTCTCACTGACCG-3' (forward) and 5'-GCAGTTCAGTATGTTCGGCTTCCCA-3' (reverse). The thermal cycler conditions were as follows: 1 x 2 min 50°C, 1 x 10 min 95°C, 4050 cycles denaturation (15 s, 95°C), and combined annealing/extension (1 min, 60°C). CSF-1R cDNA copy numbers were then normalized for variations in the efficiency of RNA extraction and cDNA transcription against the
2-microglobulin housekeeping gene.
Cell culture
Sort-purified CD11c+c-fms/EGFPneg PB cells were cultured at 106/ml for 79 days in 10% FCS/IMDM (JRH Biosciences) supplemented with GM-CSF, IL-3, and IL-4 (all at 100 ng/ml). Sort-purified CD11chigh and CD11cdimB220+ DC were cultured for 18 h at 106/ml in 10% FCS/IMDM supplemented with GM-CSF, IL-3 (each at 100 ng/ml), LPS (1 µg/ml), and phosphorothioated oligo CpG (1668; 0.5 µM) (7). For MLC, 106 magnetic bead-purified C57BL/6 T cells were cultured for 5 days with varying numbers of sort-purified CD11chigh DC. [3H]Thymidine (1 µCi/well) was added on day 4, and proliferation was determined 18 h later using a Betaplate Reader (Wallac).
| Results |
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We first compared c-fms expression by peritoneal macrophages and splenic DC subsets using three-color flow cytometry. For these experiments, DC were highly enriched from MacGreen and C57BL/6 spleens by density gradient centrifugation and magnetic bead depletion of lineage-positive cells. CD11chigh (myeloid) and CD11cdimB220+ (plasmacytoid) cells were gated (Fig. 1a) and examined for c-fms transgene reporter expression. Peritoneal F4/80+ macrophages and both DC subsets in MacGreen mice expressed c-fms/EGFP; however, the level of expression in the F4/80+ macrophages and the CD11chigh DC was higher than that of the CD11cdimB220+ DC subset (Fig. 1b). In contrast to F4/80+ macrophages, which expressed high levels of surface CSF-1R using a CD115 mAb, only low-level surface CSF1R was detected on either DC subset, and again, the CD11chigh DC staining was brighter than that of the CD11cdimB220+ DC subset (Fig. 1b). The differential expression of CSF-1R by peritoneal F4/80+ macrophages and the two DC subsets was confirmed by real-time PCR analysis. The expression of CSF-1R mRNA in F4/80+ macrophages was 5-fold that of CD11chigh DC, which was 2-fold higher than the CD11cdimB220+ DC. (Fig. 1c). As expected, CSF-1R mRNA was not detected in CD4+ T cells. Finally, the presence of the EGFP reporter gene in the MacGreen mice did not alter the stimulatory capacity of CD11c+ DC (Fig. 1d).
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6% of blood mononuclear cells.
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Because a CD11c+MHC-IIneg DC precursor population of similar frequency to that of the CD11c+c-fms/GFPneg PB population was recently identified in murine PB (8), we investigated whether the CD11c+c-fms/GFPneg PB population related to this DC precursor. To limit the number of animals required, we treated the MacGreen mice with ProGP-1, a chimeric protein with receptor agonist activity for both fetal liver tyrosine kinase-3 (Flt3) and G-CSF receptor that is known to expand both CD11chigh and CD11cdimB220+ DC in blood and lymphoid organs (9, 10). ProGP-1 pretreatment of the MacGreen mice resulted in a 100-fold increase in the blood white cell count and a 40-fold expansion of blood CD11c+ DC, and this expansion was associated with both the CD11c+c-fms/GFPpos and CD11c+c-fms/GFPneg populations (Fig. 4a). Phenotypic analysis of the CD11c+c-fms/GFPneg population from ProGP-1-treated animals revealed the cells to be MHC class IIneg, CD8neg, CD4neg, F4/80neg, CD11b+, CD62Ldim, and B220+ (Fig. 4b). Notably, this phenotype is identical to that of the DC precursor described by del Hoyo (8).
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Because CSF-1R was found to be associated with DC maturation, we asked whether CSF-1 signaling was required for the generation and/or differentiation of DC. For these studies, we used op/op mice, which have a null mutation in the CSF-1 gene that results in congenital osteopetrosis (11) due to a deficiency of osteoclasts and macrophages (12). As expected, F4/80-positive peritoneal macrophages were largely absent in op/op mice (Table I). Examination of DC within the op/op spleen revealed a 43 and 70% reduction in the CD11chigh DC and CD11cdimB220+ compartments, respectively, compared with the normal heterozygous op/wt littermates. In contrast, the relative frequency of the CD4+, CD8+, or CD4CD8 subsets of the CD11chigh population was unchanged.
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Functional testing of sort-purified CD11chigh DC in MLC revealed that op/op DC were as potent at stimulating allogeneic T cell (C57/BL6) proliferation as normal heterozygote op/wt DC (Fig. 6a). Furthermore, following 18 h of culture in the presence of GM-CSF, IL-3, LPS, and CpG, op/op and op/wt CD11chigh DC class II and costimulatory molecule expression was equivalent (Fig. 6b). Interestingly, following culture, the op/op CD11cdimB220+ DC exhibited enhanced expression of CD40 and CD86 as compared with the op/wt CD11cdimB220+ DC.
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| Discussion |
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CSF-1 drives the expansion and differentiation of macrophages, which are well established to be of myeloid lineage. MacGreen mice express EGFP driven by the c-fms promoter, thus permitting identification of myeloid-derived cells. In this study, we demonstrate that the MacGreen mouse represents a highly sensitive means of amplifying CD115. This allowed assessment of the potential for expression of the CSF-1R and confirmed that the majority of all tissue DC subsets expressed c-fms/EGFP. Conversely, up to 30% of CD11c+ DC within the PB lacked c-fms/EGFP expression. The c-fms/EGFPnegCD11c+ PB population exhibited a phenotype (class IIneg, CD11b+, B220+, and CD62L+) identical to that of a previously described blood DC precursor (8) and could be induced to express c-fms/EGFP upon differentiation in vitro. Although the data suggest that these c-fms/EGFPnegCD11c+ cells are DC precursors, we cannot exclude the possibility that these are CD11c+ cells of a separate lineage (e.g., lymphoid) or stage of trafficking. However, these putative DC precursors resemble common myeloid progenitors purified from bone marrow in which control elements of the c-fms locus are assembled into active chromatin, and c-fms mRNA expression is very low (13). The expression of c-fms is induced rapidly during macrophage lineage commitment. In contrast to the intense c-fms/EGFP transgene expression seen in mature DC populations, surface c-fms (CD115) was expressed on DC at low levels only (3-log less than on F4/80-positive macrophages), suggesting that the majority of c-fms expression in DC is not surface bound. This finding does not imply that the CSF-1R is inactive. For example, Langerhans cells have been shown to express functional CSF-1R and respond to CSF-1 (14). c-fms mRNA is barely detectable in bone marrow myeloid progenitors, yet the cells are clearly CSF-1 responsive (13). Tissue macrophages express surface CSF-1R at much higher levels than proliferating progenitors, and in these cells, the receptor mediates endocytic clearance of the growth factor (15). The level of c-fms mRNA is also down-modulated posttranscriptionally by GM-CSF, which promotes DC differentiation (16, 17), and surface CSF-1R is down-modulated acutely by TLR agonists (18). In contrast, EGFP remains intracellular and therefore is not subject to variables that influence both initial expression at the surface and subsequent degradation. Therefore, the expression of the CSF-1R reporter gene is not expected to be perfectly correlated with the presence of the receptor on the cell surface. Taken together, these data suggest that expression of c-fms promoter activity is a marker of maturation within all DC subsets, and its absence identifies immature DC precursors within the PB.
DC numbers are reported to be normal in op/op mice, and previous studies have noted GM-CSF to be the primary cytokine determining DC development rather than CSF-1 (19, 20). In elegant studies of macrophage and DC differentiation in vitro, IL-6 from stromal cells has been shown to enhance CSF-1R expression on monocytes and promote CSF-1-CSF-1R internalization. This results in the subsequent differentiation of monocytes to macrophages (21). In contrast, TNF-
blocks CSF-1R expression and internalization, leading to DC differentiation (22). These data suggest that the absence of the CSF-1R expression would preferentially block macrophage differentiation, consistent with the phenotype of the CSF-1 (op/op) and CSF-1R-deficient mice (23). Because previous studies used poorly quantitative immunohistological techniques and predate the description of DC subsets, we revisited the influence of CSF-1 on DC development in these mice using a quantitative approach encompassing the wider understanding of DC subset heterogeneity as published recently (24). These data confirm that the absence of CSF-1 results in a 5070% reduction in DC numbers, supporting the view that the low levels of CSF-1R on DC are, indeed, functional. CSF-1 is also required for optimal production of osteoclasts, hence, the osteopetrotic phenotype of the op/op mice. However, the op/op mice recover relatively normal osteoclastogenesis with age, which is attributed to the ability of vascular endothelial growth factor-A and/or flt3 ligand, to provide partial compensation (25, 26). Both these ligands act through type III tyrosine kinase receptors closely related to c-fms. Given the biology of flt3 ligand as a DC promoter, it is very likely that it also provides partial compensation for the absence of CSF-1, and of course, GM-CSF (which actually cross-modulates CSF-1 signaling (27)) may also contribute to DC homeostasis in the absence of CSF-1.
The majority of splenic CD11chigh DC is myeloid in origin based on repopulation experiments and the relative frequency of myeloid vs lymphoid precursors in vivo (2). In normal mice these myeloid DC can be further divided into CD4+, CD8+, and CD4CD8 subsets (6), and while the relative generation of these subsets is not dependent of CSF-1 signaling, the optimal generation of CD11chigh myeloid DC in total does require CSF-1 (Table I). Although we were unable to document a functional defect in the ability of myeloid DC to stimulate allogeneic T cell responses, this does not exclude more subtle defects, and therefore, we are currently studying the functional characteristics of DC from op/op mice in detail. DC have been proposed as a separate lineage to monocytes and macrophages based on the absence of CSF-1R on PB and bone marrow-derived DC (28, 29). However, more recently the CSF-1R was reported to be expressed on monocytes before their differentiation to DC and the down-regulation of CSF-1R by TNF-
appears to be a critical component in driving DC (as opposed to macrophage) development (22). Our data confirm and highlight the differential expression of CSF-1R on macrophages and DC, and we propose that the differentiation of each lineage is at least in part determined at the cellular level by c-fms expression. Furthermore, the expression of c-fms by both myeloid and plasmacytoid DC and their dependence on CSF-1 as seen by the current studies in op/op mice also favors a common lineage for these two DC subsets. Recent studies confirming the ability of plasmacytoid DC to differentiate into myeloid DC following stimulation by dsRNA and type I IFNs further supports this concept of plasticity within DC subset lineages and their differentiation (30). Thus, although both common myeloid and lymphoid progenitors can give rise to both plasmacytoid and myeloid DC in vivo, these subsets can no longer be unambiguously defined on arbitrary assumptions of lineage (reviewed in Ref. 31). Our data provides further support of this concept, suggesting that the majority of tissue DC are of myeloid origin during steady state and that a close relationship between DC and macrophage biology exists in vivo.
| Disclosures |
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| Footnotes |
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1 This work was supported in part by grants from the National Health and Medical Research Council and Queensland Cancer Fund. G.R.H. is a Wellcome Trust Senior Overseas Research Fellow. ![]()
2 Address correspondence and reprint requests to Dr. Kelli MacDonald, Bone Marrow Transplantation Laboratory, Queensland Institute of Medical Research, 300 Herston Road, Herston, Queensland 4006, Australia. E-mail address: kelliM{at}qimr.edu.au ![]()
3 Abbreviations used in this paper: DC, dendritic cell; EGFP, enhanced green fluorescent protein; op/op, osteopetrotic; ProGP-1, progenipoietin-1; PB, peripheral blood; MLC, mixed lymphocyte culture; Flt3, fetal liver tyrosine kinase-3. ![]()
Received for publication November 8, 2004. Accepted for publication May 13, 2005.
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