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* Sir William Dunn School of Pathology, University of Oxford, Oxford, United Kingdom; and
DNAX Research Institute, Palo Alto, CA 94304
| Abstract |
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| Introduction |
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In addition to the inhibitor CD200R, several related genes have been identified in the mouse. These were termed CD200RL (for receptor like) (14), CD200R1, CD200R2, etc. (15, 16). These are predicted to be associated with DNAX activating protein (DAP)12, and this has been shown for two of the mouse genes, CD200RLa (CD200R4) and CD200RLb (CD200R3) (14), and also independently for CD200R3 (16). Thus, these proteins are expected to give activating signals and, hence, are similar to the many examples of paired receptors where closely related genes contain members that can give inhibitory signals and those that give activating signals through adaptors such as DAP12. In some cases, the ligands for the inhibitory forms do not bind the activating forms, such as with the signal regulatory proteins (SIRPs)3 (17, 18). The NKAT2 inhibitory p58 binds the HLA-C peptide complex with much higher affinity than the activating p50 (19); however, both activating Ly49H and inhibitory Ly49I can bind the same m157 mouse CMV protein, although affinities have not been measured (20). Both paired Ig-like type 2 receptor (PILR)
(inhibitory) and PILR
(activating) bind a CD99-related protein (21). With regard to CD200, it was reported that CD200-Fc fusion proteins did not bind to CD200RLa and CD200RLb (14), but a contradictory report suggested that similar reagents bound all the activating proteins as well as the inhibitory form (15). Clearly this has important functional implications and possibilities for therapies using CD200-Fc fusion proteins.
In this study we critically examine binding of CD200-Fc and other CD200 fusion proteins to CD200RL, proteins expressed at the cell surface and directly at the protein level, to determine the affinities of the interactions at 37°C. This establishes that CD200 is not a significant ligand for four CD200R-related proteins.
| Materials and Methods |
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A fusion protein consisting of the extracellular domain of mouse (m) CD200 fused to the Fc domain of mouse IgG1 mutated in the CH2 domain (D265A) to inhibit binding to FcRs (CD200-Fc) and control mutant IgG (cFc) were generated as previously described (22). The inability of the fusion proteins to bind FcRs was confirmed by staining the FcR-positive cell line J774 (data not shown). The clonings of CD200RLa and CD200RLb were described previously (14). Full-length CD200RLc was not obtained in that study. The complete extracellular region of CD200RLc was isolated by PCR from cDNA purified from mouse peritoneal exudate cells, and the extracellular region had the same sequence as that previously published (accession no. NM_206535). CD200RLe (accession no. BAC40774) was isolated by PCR from cDNA prepared from bone marrow mast cells derived from CD-1 mice (Charles River Laboratories) as previously described (14). The mCD200R 5' primer (5'-gtgcttaaccagattccactcc-3') and 3' primer (5'-tgttttcttgtattgtgtcatcagac-3') were used. Fusion proteins for mouse CD200R, CD200RLa, CD200RLb, and CD200RLc and CD200RLe with domains 3 and 4 of rat CD4 plus a biotinylation signal were prepared as previously described (14, 23). The boundary between the CD200RLa part and CD4 was MTTPRSTSIT, for CD200RLb TSILPSTSIT, for CD200RLc TTPSTSIT and for CD200RLe QGSTSIT (CD4 linker is underlined). Mouse CD200CD4d3+4 (4) was produced in CHO.K1 cells using the expression vector pEE14 and was subsequently purified from spent tissue culture medium by immunoaffinity chromatography using an OX68 mAb-Sepharose 4B column (3, 24). Before BIAcore analysis, the purified CD200 protein was fractionated by gel filtration on a Superdex S-200 column (Pharmacia Biotech) to exclude larger protein aggregates that are known to influence binding measurements (25).
Flow cytometry
Ba/F3 cells expressing mouse CD200R, CD200RLa, and CD200RLb and mAb recognizing mCD200R (DX109), mCD200RLa (DX87), and mCD200RLb (DX116) were generated as previously described (14). Surface expression of the CD200 receptors was analyzed by flow cytometry using the specific CD200 receptor mAb described above, followed by goat anti-rat PE-conjugated secondary Ab (Caltag Laboratories). CD200-Fc or cFc (2 µg/5 x 105 cells) was incubated on ice for 30 min at 4°C in PBS containing 1% BSA and 0.05% sodium azide. Cells were washed, and Fc was detected using a PE-conjugated rat anti-mouse IgG1 secondary Ab (clone A85-1; BD Biosciences). Analyses were performed on a FACScan flow cytometer (BD Biosciences).
Measurement of affinities of interactions of CD200 using surface plasmon resonance
Affinity and kinetic data were collected using a Biacore 2000 (Biacore) at 37°C as previously described (3). Briefly,
2500 response units (RU) of streptavidin was coupled to a CM5 research grade chip using amine coupling. Biotinylated mCD200RCD4d3+4-related proteins and control CD4d3+4 were each immobilized at
1000 RU. For kinetic analysis, serially diluted monomeric CD200CD4d3+4 purified proteins were injected at the indicated active concentrations over all four flow cells connected in series. The extinction coefficient, 53,590 M1cm1, was calculated. Kd values were obtained by both nonlinear curve fitting and Scatchard transformations to the binding data. The following were then passed over all four flow cells sequentially: cIg, CD200-Fc, specific CD200R mAb, and OX68 mAb recognizing CD4d3+4 common to all proteins (all at 20 µg/ml).
| Results |
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Fig. 1 shows the similarity in the amino acid sequences of the extracellular regions of mCD200R and related proteins. Ba/F3 cells were transfected with constructs for CD200R, CD200RLa, or CD200RLb, and the three receptors were expressed at high levels, as shown by binding of three specific mAb (Fig. 2, AC). A DAP12 construct was cotransfected with CD200RLa and CD200RLb, because previous studies had shown that this was necessary to obtain expression of CD200RLa and CD200RLb at the cell surface (14). Soluble fusion proteins consisting of the extracellular domains of mCD200 fused to the Fc binding domain of mouse IgG gave very strong binding to cells expressing CD200R and not to cells expressing the activating receptors CD200RLa and CD200RLb (Fig. 2, DF). No binding could be obtained for these proteins using a variety of CD200-Fc fusion protein concentrations, labeling times, and temperatures, indicating that CD200 is a ligand for CD200R, but not for the related activating gene products, CD200RLa and CD200RLb.
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The failure of CD200-Fc protein to bind to CD200RLa and CD200RLb is unlikely to be due to a lack of sensitivity in the assay, because very high levels of expression of all three receptors were obtained (Fig. 2, AC), and there was strong binding of CD200-Fc to the control CD200R (Fig. 2D). However, analysis using purified proteins allowed a quantitative comparison of interacting proteins to be determined. CD200R, CD200RLa, CD200RLb, and two gene products, CD200RLc and CD200RLe (see below), for which specific mAb were not available, were expressed as chimeric proteins consisting of the extracellular regions of these proteins together with two domains of rat CD4 as an antigenic tag and a sequence that allows biotinylation. The proteins were expressed transiently, biotinylated, concentrated, and bound to streptavidin-coated BIAcore chips along with a control protein, CD4d3+4, consisting of the two domains of CD4 present in each construct. Mouse CD200CD4d3+4 was expressed in Chinese hamster ovary cell lines and purified by affinity chromatography, followed by gel filtration to remove any aggregates that might produce anomalous binding. The resultant preparation was passed over the four proteins. Clear binding to CD200R was obtained, but not to CD200RLa, CD200RLb, or the control CD4d3+4 (Fig. 3A), confirming the data on binding to cells (Fig. 2). The interactions were weak as expected with rapid dissociation over a few seconds. Thus, high concentrations of mCD200 were used, which gave rise to a bulk effect as the protein passed over the chip. For the control protein, CD4d3+4, this was seen as signal that quickly reached equilibrium and was washed out rapidly. The specific binding of CD200 to CD200R showed an increased signal compared with the control, which also quickly reached equilibrium and then dissociated rapidly. CD200RLb gave the same signal as the control protein, indicating that there was no specific binding to CD200; trace binding was observed with CD200RLa, and this was quantitated further (see below and Fig. 4A).
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0.2 µM) than the monomeric CD200 protein in Fig. 3A and, as expected, gave more avid binding, with a slower on rate; equilibrium was not reached, and dissociation occurred over
5 min.
The conditions of protein production and immobilization permit the minimum of handling and are unlikely to cause denaturation. Three mAb specific for the three proteins were passed over the flow cells, showing specific binding of DX109 to CD200R and of DX116 to CD200RLb (Fig. 3, C and E). The DX87 mAb did not label CD200R and CD200RLb as expected, but only gave marginal labeling of CD200RLa (20 RU; Fig. 3D). However, this mAb did give very good labeling of CD200RLa expressed at the cell surface (Fig. 2). It is possible that this mAb reacts better with cell surface-expressed protein that might have different glycosylation compared with the soluble extracellular region of the fusion protein. (This phenomenon has been observed for another highly glycosylated cell surface protein, SIRP
(18).) This protein and indeed all the CD200Rs were reactive with the OX68 mAb that binds the CD4 tag present in the chimeric proteins (Fig. 3F), suggesting that all the proteins are correctly folded. In the unlikely event that the CD200RLa was completely inactive, the data from Fig. 2, B and E, clearly showed no CD200 binding to antigenically active CD200RLa expressed at the cell surface.
The experiment was repeated with a range of concentrations of CD200-CD4d3+4 protein, and the equilibrium binding levels are plotted in Fig. 4A. The affinity for CD200 binding to CD200R was calculated (Kd =
4 µM at 37°C) from curve fitting (Fig. 4A) and Scatchard analysis (Fig. 4B) and was comparable to that determined for the equivalent interaction in rats (Kd = 1 µM at 37°C) (3) and humans (Kd = 0.5 µM at 37°C) (14). The binding to CD200RLa was too weak to get accurate values, but extrapolation of the data would indicate a Kd >500 µM at 37°C, which is unlikely to be significant physiologically. There was no detectable binding of CD200-CD4d3+4 to CD200RLb.
Comparable experiments were conducted for CD200RLc, although there are no specific mAb recognizing this protein, and it is not known whether this protein is expressed on normal cells (14). This gene product also gave no significant binding to CD200 (Fig. 4C), whereas CD200R gave values similar to those shown in Fig. 4A. One CD200R mAb, OX110, cross-reacted weakly on this protein (data not shown).
A new CD200R-like gene (CD200RLe or CD200R5) present in some strains of mice
This gene (accession no. BAC40774) was characterized as an expressed sequence tag from the NOD mouse, noted in database searches (16), and termed CD200R5 (16). No mRNA expression was found in C57BL/6 mice (16), and we found no evidence of this gene in searches of the mouse genome (C57BL/6J;
www.ensembl.org/Mus_musculus/
), through established sequence tag searches, or through functional analysis. However, it was expressed in the CD1 mouse strain. The extracellular region of this protein was expressed as described above, and binding to CD200 was analyzed. This protein also did not bind CD200 (Fig. 4C). We term this gene product CD200RLe (RL stands for receptor like), rather than CD200R5, because it is not a receptor of CD200. It should be noted that it is probably not present in all mouse strains, and likewise, the expression of all the other gene products has not been tested in this mouse.
| Discussion |
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RI
-chain and the TCR
-chain (26, 27) that contains a negatively charged residue in its transmembrane that allows pairing with a variety of cell surface receptors with positively charged amino acid residues in their transmembrane regions. The cytoplasmic domain of DAP12 contains a consensus ITAM motif, which, when phosphorylated, recruits protein tyrosine kinases (26, 27, 28, 29). There is a charged residue in all the CD200RL gene products, and they probably all interact with DAP12 and have the potential to produce activating signals. The current analysis uses well-characterized mAb that are highly specific for the gene products and recognize the proteins on normal cells, whereas in the study by Gorczynski et al. (15) the Abs are less well characterized antipeptide reagents.
Analysis of the sequence similarities of the extracellular regions of the receptors shows that CD200R is most similar to the strain-specific CD200RLe (91% amino acid sequence identity), but is also similar to CD200RLa (84%) and CD200RLc (83%, although this lacks part of the N-terminal region), and is less similar to CD200RLb (39%). Mutagenesis analysis of the human CD200R has identified residues in predicted
strands C and F important for binding human CD200 (30). Analysis of this region in the mouse shows that it is highly conserved between the binding CD200R and the nonbinding CD200RLe. However, a single residue change can eliminate human CD200R-CD200 binding (30), and this sensitivity of the binding site to small changes in sequence may be important in the evolution of activating receptors from inhibitory receptors.
CD200R resembles other gene families, such as SIRP, killer cell Ig-like receptor, NKG2A/C, LY49, dendritic cell immunoreceptor/dendritic cell immunoactivating receptor, and PILR, that have both activating and inhibitory forms (31, 32, 33, 34, 35). The numbers of genes can vary considerably between species and strains, and this is also observed with CD200R, for which there are several related genes in the mouse, but only one in the human, and the number in mice may vary. Like some other paired receptors, the inhibitory CD200R binds a host protein. In addition, there is an example where a CD200 homologue (K14) in the HHV8 virus binds the inhibitory receptor (9). The ligands of CD200RLa, CD200RLb, CD200RLc, and CD200RLe and the functional and biological significance of these receptors remain unknown; however, it is possible that these receptors have evolved to interact with bacterial or viral components in a manner similar to that recently described for a DAP12 pairing member of the Ly49 family and mouse CMV m157 protein (20, 36).
| Acknowledgments |
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| Disclosures |
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| Footnotes |
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1 This work was supported by the Medical Research Council and the Cellular Immunology Unit hybridoma fund. DNAX is supported by Schering Plough Corp. ![]()
2 Address correspondence and reprint requests to Dr. A. Neil Barclay, Sir William Dunn School of Pathology, University of Oxford, South Parks Road, Oxford, U.K. OX1 3RE. E-mail address: neil.barclay{at}path.ox.ac.uk ![]()
3 Abbreviations used in this paper: SIRP, signal-regulatory protein; cFc, control mutant IgG; RU, response unit; DAP, DNAX activating protein; PILR, paired Ig-like type 2 receptor. ![]()
Received for publication March 28, 2005. Accepted for publication June 6, 2005.
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(SIRP
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K. Sato, K. Eizumi, T. Fukaya, S. Fujita, Y. Sato, H. Takagi, M. Yamamoto, N. Yamashita, A. Hijikata, H. Kitamura, et al. Naturally occurring regulatory dendritic cells regulate murine cutaneous chronic graft-versus-host disease Blood, May 7, 2009; 113(19): 4780 - 4789. [Abstract] [Full Text] [PDF] |
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R. Gorczynski, I. Khatri, L. Lee, and I. Boudakov An Interaction between CD200 and Monoclonal Antibody Agonists to CD200R2 in Development of Dendritic Cells That Preferentially Induce Populations of CD4+CD25+ T Regulatory Cells J. Immunol., May 1, 2008; 180(9): 5946 - 5955. [Abstract] [Full Text] [PDF] |
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T. Kojima, K. Obata, K. Mukai, S. Sato, T. Takai, Y. Minegishi, and H. Karasuyama Mast Cells and Basophils Are Selectively Activated In Vitro and In Vivo through CD200R3 in an IgE-Independent Manner J. Immunol., November 15, 2007; 179(10): 7093 - 7100. [Abstract] [Full Text] [PDF] |
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W. Cui, E. Cuartas, J. Ke, Q. Zhang, H. B. Einarsson, J. D. Sedgwick, J. Li, and A. Vignery CD200 and its receptor, CD200R, modulate bone mass via the differentiation of osteoclasts PNAS, September 4, 2007; 104(36): 14436 - 14441. [Abstract] [Full Text] [PDF] |
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D. Hatherley, K. Harlos, D. C. Dunlop, D. I. Stuart, and A. N. Barclay The Structure of the Macrophage Signal Regulatory Protein {alpha} (SIRP{alpha}) Inhibitory Receptor Reveals a Binding Face Reminiscent of That Used by T Cell Receptors J. Biol. Chem., May 11, 2007; 282(19): 14567 - 14575. [Abstract] [Full Text] [PDF] |
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R. Gorczynski No tumor-CD200 expression, please! Blood, December 15, 2006; 108(13): 3958 - 3958. [Full Text] [PDF] |
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J. Zhang, A. Raper, N. Sugita, R. Hingorani, M. Salio, M. J. Palmowski, V. Cerundolo, and P. R. Crocker Characterization of Siglec-H as a novel endocytic receptor expressed on murine plasmacytoid dendritic cell precursors Blood, May 1, 2006; 107(9): 3600 - 3608. [Abstract] [Full Text] [PDF] |
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M. C. Jenmalm, H. Cherwinski, E. P. Bowman, J. H. Phillips, and J. D. Sedgwick Regulation of Myeloid Cell Function through the CD200 Receptor J. Immunol., January 1, 2006; 176(1): 191 - 199. [Abstract] [Full Text] [PDF] |
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