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* Department of Genetics, Osaka University Medical School, Osaka, Japan;
Laboratory for Innate Cellular Immunity, RIKEN Research Center for Allergy and Immunology, Yokohama, Kanagawa, Japan;
Core Research for Evolutional Science and Technology, Japan Science and Technology Corporation, Osaka, Japan;
Central Laboratories for Key Technology, Kirin Brewery Co., Fukuura, Kanazawa, Yokohama, Kanagawa, Japan; and
¶ Laboratory of Genetics, Integrated Biology Laboratories, Graduate School of Frontier Biosciences, Osaka University, Osaka, Japan
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Introduction |
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Apoptotic cells are rapidly engulfed by cells in the surrounding tissues, or by professional phagocytes such as macrophages or immature dendritic cells (7, 8). This is a critical feature of the apoptotic process: it prevents the release of potentially noxious or immunogenic intracellular materials from dying cells, thus preserving the integrity and function of the surrounding tissue. Therefore, when the capacity of the engulfment system is overwhelmed by a large number of apoptotic cells, for example, by activation of the Fas death receptor in the liver, cells undergo secondary necrosis, which may kill the animals by releasing toxic substances (9).
Phagocytes engulf apoptotic, but not healthy cells, indicating that the apoptotic cells present an "eat me" signal (or signals) to the phagocytes (10), and the phagocytes recognize the signal using a specific receptor. In the early stage of the apoptotic process, the surface of the plasma membrane dramatically changes, and phosphatidylserine (PS)3 that is on the inner leaflet of the plasma membrane in living cells is exposed to the cell surface, suggesting that PS is an "eat me" signal presented by apoptotic cells (11, 12). Other possible ligands in apoptotic cells are exposed sugars such as galactose, mannose, and N-acetylglucosamine, oxidized low density lipoprotein (LDL), and ICAM-3 (8, 13). Various molecules have also been proposed to act as a receptor for apoptotic cells or a bridging molecule between apoptotic cells and phagocytes. These include 
v
3 integrin, v5 integrin, class A scavenger receptor, CD36, CD14, PS receptor (PSR), a receptor tyrosine kinase called MER, thrombospondin, and milk fat globule epidermal growth factor 8 (MFG-E8) (8, 14, 15). The identification of a large number of candidate molecules suggests significant redundancy in apoptotic cell recognition and engulfment. To explain this redundancy and understand the role of individual receptors, Hoffmann et al. (16) recently proposed a two-step model for phagocytosis. In this model, apoptotic cells are first tethered to phagocytes through the interaction between putative ligands and receptors, then engulfed by phagocytes through PS-stimulated macropinocytosis.
In this study, we screened mouse macrophage cell lines for their ability to engulf apoptotic cells, and found one SV40-transformed cell line (BAM3) that engulfed apoptotic thymocytes, but not a T cell lymphoma line (WR19L). mAbs against BAM3 that inhibited the phagocytosis of apoptotic thymocytes were established. Purification of the Ag recognized by the mAbs revealed that it was Src homology 2 domain-bearing protein tyrosine phosphatase substrate-1 (SHPS-1) (17). The thymocytes were found to express CD47, the ligand for SHPS-1, also known as integrin-associated protein (18, 19), while WR19L did not express CD47. When WR19L cells were transformed with CD47, the transformants bound to SHPS-1-expressing phagocytes, and if they were induced to undergo apoptosis, they were engulfed by phagocytes in a PS-dependent manner. These data indicate that the interaction between CD47 expressed on the apoptotic cells and SHPS-1 expressed in phagocytes is involved in the tethering step, which is followed by PS-mediated engulfment.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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There are two alternatively spliced forms for the inhibitor of caspase-activated DNase (ICAD), ICAD-L for a long form and ICAD-S for a short form. The transgenic mice (ICAD-Sdm) expressing a caspase-resistant double mutant (dm) of ICAD-S were described previously (20). C57BL/6 mice and Armenian hamsters were purchased from Japan SLC (Hamamatsu, Japan) and Japan Oriental Yeast (Tokyo, Japan), respectively. A derivative of mouse T cell lymphoma, WR19L, expressing mouse Fas (W3), was described previously (21), and cultured in RPMI 1640 containing 10% FCS (Invitrogen, Carlsbad, CA). W3 was transfected by electroporation with pEF-BOS carrying DNA fragment coding for the FLAG-tagged ICAD-Ldm (22) to establish a cell line expressing the caspase-resistant form of ICAD-L (W3/Ildm). W3/Ildm was further transformed to express CD47 by introducing an expression vector for mouse CD47. Mouse NIH3T3, J774A.1, BAM3, and human 293T cells were cultured in DMEM containing 10% FCS. Mouse DNase II and SHPS-1 cDNAs were introduced into NIH3T3 cells by retrovirus-mediated transfection, as described previously (15). The recombinant soluble form of human Fas ligand (FasL) (23) and the D89E mutant of mouse MFG-E8L (15) were described previously. Rat mAbs against mouse Mac-1 (clone M1/70), CD47 (clone miap301), Fc
RII/III (clone 2.4G2), and mouse anti-rat IgG2a were purchased from BD PharMingen (San Diego, CA). Mouse anti-hamster IgG was from Jackson ImmunoResearch Laboratories (West Grove, PA).
In vitro phagocytosis
In vitro phagocytosis was performed essentially as described previously (15). In brief, peritoneal macrophages (3 x 105 cells) prepared from thioglycolate-treated mice (12- to 16-wk-old C57BL/6 mice) or BAM3 (3 x 105 cells) were grown overnight in 24-well cell culture plates. For apoptotic cells, thymocytes from 4- to 8-wk-old ICAD-Sdm mice (20) were treated at 37°C for 4 h with 10 µM dexamethasone in DMEM containing 10% FCS, or W3/Ildm were treated at 37°C for 2 h with 0.2 µg/ml of FasL. The apoptotic cells (1.2 x 106 cells/well) were added to macrophages, and phagocytosis was allowed to proceed for 2 h in the presence of 5 µg/ml of rat anti-mouse FcRII/III. Macrophages were detached from the plates by incubating them in PBS containing 1 mM EDTA, and stained with PE-conjugated rat anti-mouse Mac-1, followed by TUNEL staining with FITC-labeled dUTP (Roche Molecular Biochemicals, Indianapolis, IN). Flow cytometry analysis was conducted using a FACSCalibur (BD Biosciences, San Jose, CA). Percentage of phagocytosis was defined as the percentage of TUNEL-positive cells in the Mac-1-positive population. In some cases, phagocytosis was evaluated by observing the cells under a microscope. In brief, BAM3 (1 x 105 cells) or NIH3T3 (2 x 104 cells) cells were cultured in 8-well Lab-Tek II chamber slides (Nalge Nunc International, Rochester, NY) that had been coated with 0.1% gelatin, and phagocytosis of apoptotic cells was as described above. After fixation with 1% paraformaldehyde, adherent cells were subjected to the TUNEL reaction using an Apoptag kit (Intergen, Purchase, NY) and diaminobenzidine black (Zymed Laboratories, South San Francisco, CA) as the chromogen, and counterstained with 0.5% methylgreen.
In vivo phagocytosis
In vivo phagocytosis was conducted essentially as described (24, 25). In brief, W3 or W3/CD47 cells (2 x 106 cells/ml) were incubated with 1 µM 5'-chloromethylfluorescein diacetate (CMFDA; Molecular Probes, Eugene, OR) at 37°C for 30 min in the serum-free RPMI 1640 medium, and treated with FasL, as described above. The CMFDA-labeled apoptotic cells (5 x 107 cells in 0.5 ml PBS) were administered i.v. into 8-wk-old female C57/BL/6 mice. One hour after injection, the spleens were treated with collagenase D (Roche Molecular Biochemicals), and the splenic dendritic cells were enriched by magnetic cell sorting using anti-CD11c microbeads (Miltenyi Biotec, Auburn, CA).
Preparation of mAbs, immunoprecipitation, and Western blotting
mAbs against BAM3 were prepared by immunizing Armenian hamsters, as described (15). In brief, 1.5 x 107 BAM3 cells were injected s.c. into hamsters twice with a 4-wk interval. Three days after a final booster (1.5 x 107 cells) into the footpads, lymphocytes from the popliteal and inguinal lymph nodes were fused with P3U1 mouse myeloma and selected in hypoxanthine/aminopterin/thymidine medium, and the supernatants were tested by an in vitro phagocytosis assay. After cloning, hybridomas were cultured in GIT medium (Nihon Seiyaku, Tokyo, Japan), and Abs were purified by protein A-Sepharose 4FF beads (Amersham-Pharmacia Biotech, Tokyo, Japan).
To identify molecules recognized by mAbs, proteins on the cell surface were labeled with EZ-Link sulfosuccinimidyl-6'-(biotinamido)-6-hexanamido hexanoate (Pierce, Rockford, IL), and the cells were lysed in buffer A (50 mM HEPES-NaOH buffer, pH 7.6, containing 1% Triton X-100, 0.1% SDS, 0.5% sodium deoxycholate, 150 mM NaCl, 1.5 mM MgCl2, 1 mM EGTA, 10% glycerol, 1 mM p-amidino PMSF, 1 µg/ml leupeptin, and 1 µg/ml pepstatin). The lysate was precleared with 40 µl of human IgG-Sepharose (Amersham-Pharmacia), followed by incubation for 2 h with 10 µg mAb and 15 µl of protein A-Sepharose 4FF beads. Proteins bound to the beads were eluted by boiling for 5 min in 0.017 M Tris-HCl buffer (pH 6.8) containing 0.56% SDS and 2% 2-ME, and were separated by PAGE. After the proteins were transferred to a polyvinylidene difluoride (PVDF) membrane (Millipore, Bedford, MA), the membrane was probed with HRP-conjugated streptavidin (Roche Molecular Biochemicals), and the biotin-streptavidin complex was visualized by an ECL system (Renaissance; DuPont-NEN Research Products, Boston, MA).
Identification of SHPS-1
Molecules recognized by the 15D9 mAb were purified from mouse J774A.1 cells by immunoprecipitation. In brief, the 15D9 mAb was covalently linked to protein A-Sepharose (2 mg protein/ml bed volume) using dimethyl pimelimidate (Pierce). J774A.1 cells (2.4 x 109) were lysed in 200 ml of buffer A, and the lysate was precleared human IgG-Sepharose (Amersham-Pharmacia), followed by incubation with protein A-Sepharose. The lysates were incubated for 2 h with 150 µl of 15D9 mAb protein A-Sepharose and washed with RIPA buffer containing 0.5 M NaCl, and proteins bound to the beads were eluted with 50 mM glycine-HCl buffer (pH 2.3) containing 150 mM NaCl and 0.1% Triton X-100. The sample was dialyzed against PBS, separated by PAGE, and blotted onto a PVDF membrane. After staining with Ponceau-S, the proteins were subjected to the protein sequence analysis with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry using a PerSeptive Biosystems (Framingham, MA) Voyager-DE/RP, as described (26).
Cloning of cDNAs for SHPS-1 and CD47
To isolate mouse SHPS-1 cDNA, two oligonucleotides (5'-TCTCCCTCCTTGCTCTGCAG and 5'-TCACTTCCTCTGGACCTGGA) were prepared based on the published sequence (27) (GenBank, D87968), and RT-PCR was conducted with RNA from BAM3. A cDNA for CD47 (19) was obtained from mouse thymocytes by RT-PCR using primers (5'-GGCGCGGAGATGTGGCCCTT and 5'-CACTTCCCTTCACCTATTCC) (GenBank, NM 010581). The amplified DNA fragments were inserted into pGEM-T-EASY (Promega, Madison, WI), and verified by DNA sequencing.
Production of soluble SHPS-1 in human 293T cells
The soluble form of mouse SHPS-1 (sSHPS-1) was produced in human 293T cells, as described (15). In brief, a DNA fragment coding for the extracellular region of SHPS-1 (aa 1–365) was ligated with a DNA fragment coding for an isoleucine zipper motif and a FLAG epitope at the C-terminal end, and inserted into the pEF-BOS-EX vector (28). An expression plasmid for sSHPS-1
IgV mutant lacking the IgV-like domain (aa 29–147) was generated by means of rPCR. The expression plasmids were introduced into human 293T cells, and SHPS-1 secreted into the medium was purified using anti-FLAG M2 affinity gel (Sigma-Aldrich, St. Louis, MO).
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Results |
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Cells that express the caspase-resistant form of ICAD (ICAD-Ldm or ICAD-Sdm) do not undergo DNA degradation during apoptosis, but their DNA can be degraded by DNase II in macrophages after the apoptotic cells are phagocytosed (20, 29). Using this knowledge, we previously established a quantitative assay for the phagocytosis of apoptotic cells (15). This assay was used to examine the phagocytic activity of macrophage cell lines against two types of cells that were treated with apoptotic stimuli: thymocytes from ICAD-Sdm mice that were treated for 4 h with 10 µM dexamethasone, and mouse T cell lymphoma WR19L transformants expressing mouse Fas and ICAD-Ldm (W3/Ildm) that were treated for 2 h with FasL. Under these conditions, 
50% of the thymocytes and >90% of the W3/Ildm cells underwent apoptosis, as judged by annexin V staining. Of the several macrophage cell lines tested, BAM3 and J774A.1 were found to phagocytose thymocytes, but they phagocytosed W3/Ildm cells less efficiently.
BAM3 is a mouse macrophage cell line established by transformation of peritoneal macrophages with SV40 (30). As shown in Fig. 1, when BAM3 cells were incubated with freshly prepared thymocytes from ICAD-Sdm mice, few of the macrophages became TUNEL positive (b). In contrast, 41.5% of the BAM3 cells became TUNEL positive when they were incubated with the apoptotic thymocytes (c). This value was comparable to that observed with thioglycolate-elicited peritoneal macrophages (h), indicating that BAM3 cells could engulf apoptotic thymocytes as efficiently as peritoneal macrophages. In contrast, when the apoptotic W3/Ildm cells were used as prey, BAM3 did not phagocytose them at all (e), although peritoneal macrophages efficiently phagocytosed the W3/Ildm cells (j). These results suggested that WR19L cells lack molecules that were necessary to permit their phagocytosis by BAM3 cells.
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To identify the molecules involved in the phagocytosis of apoptotic cells by BAM3, mAbs against BAM3 cells were prepared in the Armenian hamster. By screening 2500 hybridomas with the assay described above, we identified two mAbs (15D9 and FG2) that inhibited phagocytosis of apoptotic thymocytes by BAM3 (Fig. 2A). They inhibited the phagocytosis in a dose-dependent manner, and the percentage of BAM3 engulfing dead cells was reduced to 9% in the presence of 10 µg/ml of 15D9 (Fig. 2B). Microscopic observation demonstrated that not only the number of cells that engulfed apoptotic cells, but also the number of engulfed apoptotic cells per macrophage decreased in the presence of 15D9 (Fig. 2C). The 15D9 and FG2 mAbs also inhibited the phagocytosis of apoptotic thymocytes by J774A.1 cells (Fig. 2D), suggesting that BAM3 and J774A.1 use the same molecule(s) for phagocytosis of apoptotic cells.
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4 kb in thioglycolate-elicited peritoneal macrophages, bone marrow-derived macrophages, or macrophage cell lines, but not in lymphoid, myeloid, and fibroblast cell lines (Fig. 3D), supporting the macrophage-restricted expression of SHPS-1 (32).
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IgV) were prepared. As shown in Fig. 3E, sSHPS-1 inhibited the phagocytosis of apoptotic thymocytes by BAM3 in a dose-dependent manner. In contrast, sSHPS-1IgV had no effect on phagocytosis by BAM3. These results indicated that SHPS-1 is required for phagocytosis of apoptotic cells by BAM3, and that the 15D9 and FG2 mAbs worked as antagonists for this process by binding to the IgV domain of SHPS-1. Requirement of CD47 for the engulfment of apoptotic cells
CD47, also called integrin-associated protein, can bind SHPS-1 (18, 19, 33). A FACS analysis with anti-CD47 revealed that freshly prepared thymocytes expressed CD47 (Fig. 4Aa). The expression level of CD47 on apoptotic thymocytes was comparable to that on the fresh thymocytes (b). The sSHPS-1 bound to the living as well as apoptotic thymocytes (c and d), and preincubation of thymocytes with sSHPS-1 inhibited the binding of anti-CD47 (e). These results indicated that primary mouse thymocytes express CD47 that has the ability to bind to SHPS-1, and that the expression of CD47 did not change during apoptosis. In contrast, WR19L cells expressed little CD47 on their surface (f), and did not bind sSHPS-1 (g).
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The apoptotic cells are engulfed by marginal zone dendritic cells in the spleen (24, 25). To examine the effect of CD47 on the clearance of apoptotic cells in vivo, W3/Ildm and W3/Ildm/CD47 cells were labeled with CMFDA, treated with FasL, and injected i.v. into mice. As shown in Fig. 4D, 15% of the CD11c-positive dendritic cells contained the apoptotic W3/Ildm cells after 1 h. This percentage of the dendritic cells carrying the apoptotic cells increased to 22% when W3/Ildm/CD47 cells were injected into the mice. These results indicated that the surface expression of CD47 contributed to the efficient clearance of apoptotic cells in vivo.
Corequirement of PS and CD47 for engulfment
PS is known to be exposed on apoptotic cells and to work as a signal for their engulfment (8, 11). We previously identified MFG-E8-L as a bridging molecule between apoptotic cells and macrophages (15). A mutant MFG-E8-L, D89E, which has a point mutation in an RGD motif, can tightly bind PS on apoptotic cells, but fails to bind to macrophages. Thus, D89E works as a dominant-negative form of MFG-E8-L, or it masks PS exposed on the apoptotic cells (15). To examine whether PS is required for the phagocytosis of apoptotic cells by BAM3, D89E was added to the phagocytosis assay with BAM3 cells. As shown in Fig. 5A, 1 µg/ml D89E strongly inhibited the phagocytosis of apoptotic thymocytes by BAM3. A similar complete inhibition was observed when apoptotic W3/Ildm/CD47 cells were used as prey (data not shown). Because the 15D9 anti-SHPS-1 mAb also efficiently inhibited the phagocytosis of apoptotic cells by BAM3 (Fig. 5A), these results suggested that BAM3 recognized PS as well as CD47 for the engulfment of apoptotic thymocytes.
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Discussion |
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CD47 is a peculiar transmembrane protein of the Ig superfamily, with a single IgV-like domain at its N terminus, and five transmembrane segments (41). It is broadly expressed in various cells, including lymphocytes and hemopoietic cells (42, 43). A variety of functions has been suggested for CD47, which include migration of neutrophils, platelet aggregation, costimulation for T cell activation, and caspase-independent killing of B cell lymphoma (44, 45). One of the well-supported functions of CD47 is to mediate cell-cell adhesion (18, 33), and the tethering effect of CD47 in the phagocytosis of apoptotic cells is likely to use similar mechanisms. In contrast, Oldenborg et al. (46) reported that the expression of CD47 prevents the engulfment of RBC by splenic macrophages, which apparently contradicts our results. Because CD47-expressing lymphocytes and thymocytes are efficiently engulfed by various macrophages in vitro and in vivo (this study, and see Ref. 47), it is unlikely that CD47 works as a "don’t eat me" signal in lymphocytes. CD47 in lymphocytes is known to be localized to the membrane rafts, specialized domains in the plasma membrane enriched in cholesterol and sphingolipids (48). Whether the apparent discrepancy between lymphocytes and erythrocytes is due to the cell specificity or a different conformation of CD47 on the plasma membrane remains to be studied. In any case, the finding of a T cell lymphoma (WR19L) deficient in CD47 will provide an excellent experimental system to examine the function of this protein. In particular, it may be interesting to examine the role of CD47 in lymphomatogenesis in relation to the engulfment by macrophages.
SHPS-1, the receptor for CD47, is expressed in only limited cell types such as monocytes, macrophages, and immature dendritic cells that have a strong ability to engulf apoptotic cells (18, 32, 49). It is a receptor-type glycoprotein of the Ig superfamily, and carries two immunoreceptor tyrosine-based inhibitory motifs that can be tyrosine phosphorylated (17). The phosphorylated SHPS-1 recruits and activates tyrosine phosphatases such as SHP-1 and SHP-2 to regulate the signal transduction evoked by a variety of stimuli, including signals through receptor tyrosine kinase, Toll receptor, and integrins (31). Apoptotic cell clearance is known to be noninflammatory (8), and apoptotic cells even actively suppress inflammation (50). The engagement of SHPS-1 with CD47 inhibits the endotoxin-induced production of proinflammatory cytokines in human immature dendritic cells (51). It is therefore likely that the interaction between CD47 and SHPS-1 in the phagocytosis process causes an anti-inflammatory effect, in at least some subsets of macrophages. In this study, we reconstituted the engulfment process with NIH3T3 cells expressing SHPS-1. Mutational analysis of SHPS-1 may allow us to examine in more detail the mechanism for the anti-inflammatory function of apoptotic cells.
We previously identified a protein (MFG-E8-L) that strongly binds to PS on apoptotic cells and has the ability to pass the dying cells to phagocytes (15). The thioglycolate-elicited peritoneal macrophages express MFG-E8-L abundantly, while BAM3 and J774A.1 cells do not express it (15). The anti-SHPS-1 mAb had no significant effect on the ability of thioglycolate-elicited peritoneal macrophages to engulf apoptotic thymocytes. Furthermore, the addition of MFG-E8-L rendered BAM3 capable to engulf the apoptotic WR19L cells to some extents (data not shown). These results indicate that binding of MFG-E8 to PS on apoptotic cells, and to v3 integrin on macrophages tethers the apoptotic cells to macrophages. Although we cannot rule out the possibility that peritoneal macrophages use a tethering mechanism other than CD47-SHPS-1, these results suggest that if the direct recognition of PS by its receptor (or bridging molecule) is strong enough to recruit the apoptotic cells to phagocytes, an extra tethering system may not be necessary at least in vitro. In contrast, the tethering process alone was not sufficient to promote phagocytes to engulf the cells. That is, BAM3 cells required PS to be exposed on the surface of apoptotic cells to engulf them. Whether any of the proposed receptors for PS such as PSR, CD36, CD14, LDL receptor, and scavenger receptors (12, 37, 38, 39, 52) is used in BAM3 cells for the PS-mediated engulfment of apoptotic cells remains to be clarified. In any event, establishing and screening macrophage cell lines will help to resolve whether each macrophage has a single unique system or redundant systems for the engulfment of apoptotic cells.
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Acknowledgments |
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Footnotes |
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2 Address correspondence and reprint requests to Dr. Shigekazu Nagata, Osaka University Medical School, B-3, Department of Genetics, 2-2 Yamada-oka, Suita, Osaka 565-0871, Japan. E-mail address: nagata{at}genetic.med.osaka-u.ac.jp
3 Abbreviations used in this paper: PS, phosphatidylserine; CMFDA, 5' chloromethylfluorescein diacetate; FasL, Fas ligand; ICAD, inhibitor of caspase-activated DNase; LDL, low density lipoprotein; MFG-E8, milk fat globule epidermal growth factor 8; PSR, PS receptor; PVDF, polyvinylidene difluoride; SHPS-1, Src homology 2 domain-bearing protein tyrosine phosphatase substrate-1; sSHPS-1, soluble SHPS-1; dm, double mutant.
Received for publication July 23, 2003. Accepted for publication September 23, 2003.
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References |
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v3-dependent ligand binding. J. Cell Biol. 123:485.
translocation in its synergistic activation of T cells. J. Biol. Chem. 276:7672.: down-regulation of IL-12 responsiveness and inhibition of dendritic cell activation. J. Immunol. 167:2547.Related articles in The JI:
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