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CUTTING EDGE |
Receptor Cell Biology Section, Laboratory of Allergic Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Rockville, MD 20852
| Abstract |
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CD8+ T cells express NKG2D, but in mouse, it is only expressed by activated and memory CD8+ T cells. We purified human naive CD8+ T cells to show that NKG2D serves as a costimulatory receptor for TCR induced Ca2+ mobilization and proliferation. The resulting effector cells are skewed toward a type 1 phenotype and produce high levels of IFN-
and TNF-
. NKG2D ligands, MHC class I chain-related (MIC)A, MICB, and UL16-binding proteins are expressed on the proliferating cells and NKG2D is down-regulated. The addition of the homeostatic cytokines IL-7 and IL-15 to the culture medium not only enhances proliferation but also counteracts the down-regulation of NKG2D, more so than the addition of IL-2. These results indicate that NKG2D can regulate the priming of human naive CD8+ T cells, which may provide an alternative mechanism for potentiating and channeling the immune response. | Introduction |
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, TNF-
, and IL-2, whereas type 2 cells (Th2/Tc2) produce IL-4, IL-5, IL-10, and IL-13 (5).
NKG2D is expressed on almost all NK cells, CD8+ T cells, and 
T cells (6). Whereas only activated and memory mouse CD8+ T cells express NKG2D, all human CD8+ T cells express this receptor (6). NKG2D has no intrinsic signaling capacity, but attains this by association with the transmembrane signaling adaptor protein DAP10 (7). Several studies have shown that NKG2D is a primary activation receptor on NK cells, meaning that engagement of NKG2D alone is enough for the transmission of a signal that results in NK cell-mediated cytotoxicity (8). In contrast, several studies made with T cell clones and activated T cell lines have shown that NKG2D serves as a costimulatory receptor for TCR-mediated signals in a manner similar to CD28 (8, 9).
Multiple ligands have been described for human NKG2D, including MHC class I chain-related (MIC)A,2 MICB, and UL16-binding proteins (ULBP) (6, 10, 11). MICA and MICB are normally expressed only by intestinal epithelial cells (12). Tumor cell lines and primary tumors of epithelial origin express MICA and MICB (13). The expression of NKG2D ligands is also up-regulated in cells infected with certain pathogens (9, 14). In addition, NKG2D and NKG2D ligands also appear to play a role in the pathogenesis of diseases that are associated with type 1 (Th1/Tc1) T cell-dependent responses against autoantigens, such as diabetes in the NOD mouse (15), and celiac disease (16, 17) and rheumatoid arthritis in humans (18). Finally, and perhaps of great significance for the regulation of the immune response, IFN-
activated dendritic cells (DC) have been shown to express MICA and MICB (19).
Because the priming of naive T cells dictates the nature of the effector and memory T cell populations, we examined whether NKG2D costimulation could channel the development of these populations. Our results show that NKG2D is a costimulatory molecule for TCR-mediated signals in naive CD8+ T cells and that the engagement of NKG2D along with the TCR favors the development of Tc1 over Tc2 cells. In addition, we have found that after stimulation, CD8+ T cells express ligands for NKG2D, which presumably accounts for the observed down-regulation of NKG2D by activated human CD8+ T cells. This down-regulation is suppressed by the presence of the homeostatic cytokines IL-7 and IL-15.
| Materials and Methods |
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Allophycocyanin-conjugated anti-CD62L (BD Biosciences) and FITC-conjugated anti-CD45RO (eBiosciences) were used in three-color FACS analysis and for cell sorting. PE-conjugated anti-CD11a, CD25, CD27, CD28, CD62L, CD94, NKG2A (Beckman Coulter), CCR7, NKG2D (R&D Systems), CD45RA, CD45RO, CCR4 and LAIR-1 (BD Biosciences) were used for phenotypic analyses. Purified anti-CD3 mAb (clone UCH1, mouse IgG1; Beckman Coulter) and anti-NKG2D (clone 149810, mouse IgG1; R&D Systems) were used for cell stimulation. Anti-MICA mAb (Immatics Biotechnologies) and PE-conjugated goat anti-mouse (GAM) IgG (Beckman Coulter) were used in the indirect immunofluorescence assay. GAM IgG (Jackson ImmunoResearch Laboratories) was used for cross-linking of the primary Abs in the Ca2+ flux assay.
Cell isolation and sorting
Human CD8+ T cells were isolated from buffy coat by negative selection using a CD8+ T cell isolation kit (Miltenyi Biotec). Naive CD8+ T cell populations, based on CD62L+CD45RO phenotype, were isolated at the NIAID sorting facility. After sorting, cells were resuspended in T cell medium: IMDM medium (Invitrogen Life Sciences) containing 10% human AB serum (Valley Biomedical) and supplemented with GlutaMAX (Invitrogen Life Sciences) and penicillin/streptomycin (BioSource International).
Ca2+ flux assay
Naive CD8+ T cells were washed and resuspended in HBSS (Invitrogen Life Sciences) with 1% FCS at 5 x 106 cells/ml. Then, cells were labeled with Fluo-4 (2 µg/ml) and Fura Red (5 µg/ml) (Molecular Probes) for 30 min at 30°C. Cells were washed two times and resuspended at 2 x 106 cells/ml. Then, they were incubated in a water bath at 37°C for 5 min, followed by FACS. To establish a baseline, cells were acquired for 30 s, at which point primary mAb were added, and acquisition of cells was continued for 30 s. The primary mAb were cross-linked with GAM IgG. Data were analyzed using the FlowJo 4.2 software package (Treestar).
Cell activation/analysis
Unlabeled or CFSE-labeled naive CD8+ T cells (0.5 x 106 cells in 0.5 ml) were added to 48-well plates coated with anti-CD3 mAb alone at 0.1 µg/ml, or in conjunction with 1 µg/ml of anti-NKG2D mAb. Cells were cultured for different time points and then transferred into noncoated wells. At the time of transfer, 50 U/ml human recombinant IL-2 (NCI-FRDC, Frederick, MD) or 10 ng/ml IL-7 plus 10 ng/ml IL-15 (R&D Systems) were added to each well. To favor Tc2 development, whole or naive CD8+ T cells were cultured with 50 U/ml IL-2, 10 ng/ml IL-4, and 1 µg/ml anti-human IFN-
mAb (BD Biosciences). Cells were analyzed by flow cytometry at days 4 and 7.
Cytokine analysis
Naive CD8+ T cells stimulated and cultured as above were harvested at day 7, washed and adjusted to 1 x 106 cells/ml in T cell medium. Cells were cultured in wells coated with 0.1 µg/ml anti-CD3 mAb in a recall assay for 24 h at which time supernatants were collected for cytokine analysis. The human Th1/Th2 cytokine cytometric bead array kit (BD Biosciences) was used to quantitatively measure cytokine levels.
RT-PCR analysis
Total RNA was isolated from unstimulated cells at day 0 and anti-CD3 or anti-CD3 + anti-NKG2D-stimulated cells at day 7, using RNeasy Mini kits (Qiagen), and synthesis of cDNA was performed using the SuperScript III first-strand synthesis system for RT-PCR (Invitrogen Life Sciences). The PCR reaction was done using specific pairs of primers to detect MICA, MICB, ULBP-1, -2, and -3. Primer sequences for the NKG2D ligands were ULBP-1 forward, 5'-AGGCCAAAGCCTTTGCTTCT-3', and reverse, 5'-AAATTCTTCAAGCCACATC-3'; ULBP-2 forward, 5'-CAGTCACACCTGTCAGTCC-3', and reverse, 5'-TCCCATTGAGAAGTAATG-3'; ULBP-3 forward, 5'-AGGTCTTATCTATGGGTCAC-3', and reverse, 5'-GGGTTCCAGCCTCTTCTTCCTG-3'. Specific primers for MICB were previously described (19). The housekeeping gene GAPDH was detected with the following primers: forward, 5'-CCGTCTAGAAAAACCTGCC-3', and reverse, 5'-GCCAAATTCGTTGTCATACC-3', primers.
| Results and Discussion |
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and TNF-
, and/or by cytolytic mechanisms. In this regard, we investigated the functional capacity of TCR-stimulated CD8+ T cells with or without co-stimulation by NKG2D. Cells cultured for 7 days were restimulated with anti-CD3 mAb for 24 h in a recall assay, followed by analyses of the cytokine production profile. Our results show that cells that were primed with anti-CD3 mAb in the presence of NKG2D costimulation tend to produce more type 1 cytokines (IFN-
and TNF-
). This, along with a lack of increase in type 2 cytokines (IL-4 and IL-5) (Fig. 3A), suggests that NKG2D is a proinflammatory coreceptor. To further confirm that NKG2D costimulation favors the development of Tc1 vs Tc2 cells, we checked the expression levels of the type 2 cell marker CCR4 (22) by the proliferating cells. TCR or TCR + NKG2D-stimulated whole or naive CD8+ T cells were cultured in the presence of IL-2 or in IL-2 + IL-4 and anti-IFN-
mAb, in the case of the latter, with the objective of promoting Tc2 development. In cells cultured in type 2 polarizing conditions, we observed that NKG2D costimulation inhibited the up-regulation of CCR4 compared with cells that received only TCR stimulation (Fig. 3B).
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The duration and strength of the Ag-induced signal, as well as the presence of costimulation, has a direct impact on the ability of the cells to attain fitness, which is defined by the capacity to respond to homeostatic cytokines and to resist death by neglect (23). Our results show that NKG2D costimulated human naive CD8+ T cells are functionally more fit, because they respond better to homeostatic cytokines, than cells that have been only TCR stimulated. Moreover, we also show that homeostatic cytokines have the ability to prevent the down-regulation of NKG2D expression. This down-modulation is presumably due to the interaction of NKG2D with its ligands expressed by activated T cells (Fig. 4) and by mature DC (19, 26) (data not shown). An attractive hypothesis is that the IL-15 produced by DC (26) and presented in trans by the IL-15R
(27) to naive CD8+ T cells prevents the down-regulation of NKG2D expression by the CD8+ T cells helping to prolong fitness of activated CD8+ T cells possibly by overriding down-modulation signals expected from the expression of NKG2D ligands by mature DC. Our long-term goal is to address issues raised by this hypothesis.
Mice are the experimental tool of choice for the study of the immune system in vivo. However, as 65 million years of evolution might suggest, there are significant differences between humans and mice, in both innate and adaptive immunity, and such differences should be taken into account (28). Here, distinctive from the mouse, we provide evidence that NKG2D acts as a costimulatory receptor for priming human naive CD8+ T cells, diminishing the TCR-triggering threshold and significantly affecting the functional features of the generated effector cells. Given that NKG2D ligand expression can be readily induced on DC, it seems likely that NKG2D plays an important role in the defense against infections and tumor development in humans (6, 9, 13, 14) but, not surprisingly, can also play a role in the development of autoimmunity (15, 16, 17, 18). Consequently, NKG2D could be a very good target for the development of therapies for several diseases.
| Disclosures |
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| Acknowledgments |
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| Footnotes |
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1 Address correspondence and reprint requests to Dr. Francisco Borrego, Twinbrook II Room 205, Receptor Cell Biology Section, Laboratory of Allergic Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, 12441 Parklawn Drive, Rockville, MD 20852-1742. E-mail address: Fborrego{at}niaid.nih.gov ![]()
2 Abbreviations used in this paper: MIC, MHC class I chain-related; ULBP, UL16-binding protein; DC, dendritic cell(s); GAM, goat anti-mouse. ![]()
Received for publication December 15, 2004. Accepted for publication February 14, 2005.
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