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CUTTING EDGE |
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* Department of Microbiology and Immunology,
Cancer Research Institute, and
Department of Medicine, University of California San Francisco, San Francisco, CA 94143; and
Department of Medicine, Veterans Administration Medical Center, San Francisco, CA 94143
| Abstract |
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| Introduction |
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Although in some circumstances cross-linking of DAP12-associated receptors on macrophages can initiate cytokine production, under different conditions DAP12 inhibits the secretion of these same cytokines when they are induced by TLRs or FcRs (3). This dual functionality of DAP12 was revealed by the finding that TLR- or FcR-mediated activation of DAP12-deficient bone marrow-derived macrophages resulted in more cytokine secretion than did activation of wild-type macrophages. The inhibitory function of DAP12 in macrophages requires the tyrosines within the ITAM and presumably involves the syk tyrosine kinase, as syk-deficient bone marrow-derived macrophages also exhibit enhanced cytokine production in response to TLR-induced activation. Therefore, the initial steps in the signal transduction cascade for activation and inhibition of cytokine production mediated by DAP12 are similar.
Critical to understanding DAP12s inhibitory function is whether an associated receptor is required to inhibit TLR-induced cytokine production. It is possible that DAP12, regardless of the receptor with which it is associated, can give a constitutive inhibitory signal. Alternatively, a specific DAP12-paired receptor may be required. DAP12-associated receptors in myeloid cells include: MDL-1, TREM-1, TREM-2, TREM-3, TREM-5 (CD300LB), PILR
, MAIR-II (CD300c), SIRP
, CD200R2, CD200R3, and CD200R4 (2). We set out to identify which, if any, DAP12-associated receptors can provide inhibitory signals in macrophages.
| Materials and Methods |
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DAP12-deficient mice (tyrobp/) (19) were backcrossed nine generations to C57BL/6. Protocols were approved by our Institutional Animal Care and Use Committee.
Retrovirus
The HA-DAP12 construct was generated by introducing DNA encoding amino acids 28114 of DAP12 into the pMX-pie vector (containing an IRES-GFP) encoding a CD8 leader segment and HA tag. The HA-DAP12 TM mutant changed an aspartic acid at position 52 to alanine. The TREM-1+DAP12 and TREM-2+DAP12 chimeras were generated by replacing the HA tag of the HA-DAP12 TM mutant with the extracellular domains of TREM-1 (aa 1200) or TREM-2 (aa 1169).
Macrophage culture, infection, and stimulation
Bone marrow-derived macrophages were infected with VSVg-pseudotyped retroviruses (3). Cells were stimulated with 0.01 µM CpG DNA ODN1826 (InvivoGen) or 10 µg zymosan (InVitroGen) for 4 h with 10 µg/ml Brefeldin A (Sigma-Aldrich). For FcR activation, 24-well dishes were coated with 2.4G2 (3), and cells were added to the wells with Brefeldin A for 4 h. Receptors were detected with biotinylated anti-TREM-1 (a gift from M. Colonna, Washington University, St. Louis, MO), anti-TREM-2 clone no.150 (5) or anti-HA mAb (Roche) and PE-labeled streptavidin (BD).
shRNA-mediated knockdown
Lentiviruses were generated by using pLentiLox 3.7. A nonspecific shRNA virus (15) was provided by H. Neumann (European Neuroscience Institute, Göttingen, Germany). The knockdown lentivirus, containing the TREM-2 sequence gaagcggaatgggagcaca, was as described (RNAi no.2, (5)). At day 3 of culture, 1 x 106 cells per well were plated in 6-well dishes. 1015 µl of VSVg-pseudotyped, concentrated lentivirus and 8 µg/ml polybrene were added, and cells were centrifuged at 1024 x g for 1 h at RT. After 6 h, the medium was replaced. At day 6, cells were re-plated and on day 7 stimulated with TLR ligands.
TREM-2 Fc staining
0.5 µg of TREM-2 Fc or NKp30 Fc (R&D Systems) was precomplexed with 2.5 µg PE-labeled anti-human IgG Fc F(ab')2 (Jackson ImmunoResearch Laboratories) for 30 min. 2 x 105 cells were stained for 1 h on ice. For blocking, 10 µg F(ab')2 of control or anti-TREM-2 mAb (no. 78) was preincubated with TREM-2 Fc.
| Results and Discussion |
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To understand whether DAP12 must associate with a receptor to inhibit TLR and FcR-induced TNF production in macrophages, we made a variant of DAP12 that cannot pair with receptors. A mutant DAP12, in which the aspartic acid in the transmembrane was mutated to an alanine (DAP12 TM mutant), was generated. This aspartic acid is required for receptor pairing, as it interacts with an arginine or lysine in the transmembrane of DAP12-pairing receptors (4). Unlike wild-type DAP12, which is retained in the cytoplasm and degraded in the absence of a pairing receptor, the DAP12 TM mutant is expressed on the cell surface in the absence of any partner (4). HA-DAP12 and HA-DAP12 TM mutant were introduced into DAP12-deficient bone marrow-derived macrophages with retroviruses containing an IRES-GFP for identification of transduced cells. These cells were then stimulated with CpG DNA, which signals through TLR9, with zymosan, which signals through TLR2, or by plating on wells coated with 2.4G2, a mAb that binds Fc
RII (CD32) and Fc
RIII (CD16). After 4 h, TNF production was assessed by intracellular staining. DAP12 reduced TNF production of DAP12-deficient macrophages to a level similar to that seen in wild-type macrophages in response to CpG DNA, zymosan, or FcR stimulation, but the DAP12 TM mutant had no effect on TNF production (Fig. 1 and data not shown). DAP12 and the TM mutant were expressed similarly on the surface of the transduced macrophages and were functional in a mouse T cell reporter system (data not shown). Therefore, DAP12 must associate with a receptor present in the macrophage to inhibit TLR- and FcR-induced TNF production.
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To determine which receptor is required for inhibition, we generated receptor-DAP12 chimeras for two candidate receptors, TREM-1 and TREM-2. These chimeras contained the extracellular domains of TREM-1 or TREM-2 fused to the DAP12 TM mutation to ensure that they would not associate with endogenous receptors. DAP12-deficient macrophages transduced with these chimeras were activated with CpG DNA, zymosan and anti-Fc
RII/III mAb, and TNF production was assessed. For TLR and FcR activation, the TREM-2+DAP12 chimera inhibited the TNF production in DAP12-deficient macrophages (Fig. 2A-C). By contrast, the TREM-1+DAP12 chimera did not reduce cytokine production. Two other receptor-DAP12 chimeras, MAIR-II+DAP12 and PILR
+DAP12, failed to inhibit TNF production of the DAP12-deficient macrophages (data not shown). Both MAIR-II and PILR
are expressed on the surface of bone marrow-derived macrophages as determined by flow cytometry (data not shown).
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To confirm that inhibition through the TREM-2 chimera is ITAM-dependent, we generated an ITAM-mutant TREM-2+DAP12 chimera. The ITAM mutant did not inhibit TNF production by DAP12-deficient macrophages in response to CpG DNA or zymosan (Fig. 3, A and B). In comparison with the TREM-2+DAP12 chimera, the ITAM mutant was expressed more highly on the transduced macrophages (Fig. 3C). A functional ITAM reduces the expression of the TREM-2+DAP12 chimeric receptor, supporting a model of ligand-induced internalization or intracellular retention of TREM-2.
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We hypothesized that the inhibitory effect of TREM-2 is caused by ligands on macrophages. Macrophages were stained with a TREM-2 Fc fusion protein complex. Macrophages stained with the TREM-2 Fc, but not with an irrelevant fusion protein (Fig. 5A). F(ab')2 fragments of a TREM-2 mAb blocked the binding of the TREM-2 Fc (Fig. 5B). Freshly isolated resident peritoneal macrophages also stained with mAb to TREM-2 and with the TREM-2 Fc reagent (data not shown). Thus, macrophages express a TREM-2 ligand. These data explain the ability of TREM-2 to cause inhibition in macrophage cultures, as well as the apparent ligand-induced, ITAM-dependent internalization or intracellular retention of TREM-2 seen upon expression of the TREM-2+DAP12 chimera in macrophages.
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In mouse microglia, mAb-induced TREM-2 cross-linking resulted in cytoskeletal rearrangements (15). This same study correlated TREM-2 expression with the ability of microglia to phagocytose apoptotic neurons-TREM-2 overexpression increased phagocytosis, whereas TREM-2 knockdown decreased phagocytosis. Of note, TREM-2 overexpression in microglia led to reduced amounts of TNF and iNOS mRNA after coculture with apoptotic neurons, whereas TREM-2 knockdown resulted in an almost two-fold increase in TNF and iNOS mRNA (15). These data suggest that TREM-2 signaling positively regulates phagocytosis, but negatively regulates inflammatory responses in microglia, as we have shown in macrophages. Apoptotic neurons may express TREM-2 ligands, which can give either activating or inhibitory signals. Alternatively, the microglia may themselves express TREM-2 ligands, similar to macrophages. Other DAP12-associated receptors have been implicated in inhibition of TLR responses in IFN-producing cells (IPC). Signaling through Siglec-H in mouse IPC and NKp44 in human IPC inhibits CpG DNA-induced type I IFN production, and DAP12-deficient IPC secrete increased type I IFNs in response to CpG DNA (16, 17).
In humans, mutation of either DAP12 or TREM-2 results in Nasu-Hakola syndrome, a rare disease characterized by bone cysts and presenile dementia (10, 18). This phenotype implies a role for the TREM-2-DAP12 receptor complex in osteoclast and microglia differentiation and function. Because TREM-2 and DAP12 have been implicated in the activation of myeloid cells (8, 9), one might predict immunodeficiencies in humans lacking these molecules. In contrast, although these patients live 4050 years, they do not have increased susceptibility to infections. Thus, the predominant role of DAP12 and its associated receptors, such as TREM-2, might be in the inhibition, rather the activation, of the innate immune system.
| Acknowledgments |
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| Disclosures |
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| Footnotes |
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1 This work was supported by an Arthritis Investigator Award (to J.A.H.), National Institutes of Health Grants AI068129 (to L.L.L.), AR050038 (to M.C.N.), and CA87922 (to W.E.S.), Department of Veterans Affairs (to M.B.H. and M.C.N.), American Cancer Society (to M.C.N.), Rosalind Russell Arthritis Center (to M.B.H., M.C.N., and W.E.S.), and Abbott Scholar in Rheumatology (to M.B.H.). L.L.L is an American Cancer Society Research Professor. ![]()
2 Address correspondence and reprint requests to Dr. Lewis L. Lanier, 513 Parnassus Avenue, HSE 1001G, Box 0414, San Francisco, CA 94143-0414. E-mail address: lewis.lanier{at}ucsf.edu ![]()
3 Abbreviation used in this paper: TREM, triggering receptor expressed on myeloid cells. ![]()
Received for publication April 7, 2006. Accepted for publication June 12, 2006.
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