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*
Terry Fox Laboratory, British Columbia Cancer Agency,
Department of Medical Genetics, and
Department of Pathology and Laboratory Medicine, University of British Columbia, Vancouver, British Columbia, Canada
| Abstract |
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1 domain and
176 in the
2 domain, respectively. These mutant Dd cDNAs
were transfected into leukemic cell lines, and the binding of the
transfected cells to COS cells expressing Ly-49A or Ly-49C, as well as
their susceptibility to lysis by Ly-49A+ NK cells, was
examined. Only the mutation of the
2 domain glycosylation site
significantly reduced the binding of Dd to Ly-49A and
Ly-49C. Cells expressing Dd with the mutation at this site
were partially resistant to killing by Ly-49A+ NK cells.
These results suggest that, while carbohydrates linked to residue 176
seem to function as a part of the ligand structure for the Ly-49 family
of NK receptors, there are additional structural features involved in
this recognition. This glycosylation site is highly conserved among
murine class I MHC but is not found among those of other species,
suggesting that its role is unique to the murine immune system. It
further suggests that murine class I MHC and Ly-49 gene families may
have evolved in concert. | Introduction |
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1/
2 domains
of class I MHC (13).
Ly-49 contains a putative carbohydrate recognition domain (14, 15, 16), and
previous studies have shown that Ly-49A and -C bind sulfated
polysaccharides (17, 18). Furthermore, treatment of cells expressing
class I MHC with tunicamycin to inhibit glycosylation (17) or with
fucosidase (18) inhibited binding of class I MHC to Ly-49A or -C. These
results suggested an involvement of carbohydrates in the recognition of
class I MHC. However, the precise role of carbohydrates on class I MHC
in its interaction with the Ly-49 family of NK inhibitory receptors has
not yet been determined. The murine class I MHC molecules have two
conserved asparagine N-linked glycosylation sites, amino
acid residues at positions 86 in the
1 domain and 176 in the
2
domain. In this study, we report that the glycosylation at residue 176,
but not residue 86, is required for the binding of class I MHC to
Ly-49A and Ly-49C. We also report that other structural features are
likely to be involved in this interaction. Interestingly, unlike
residue 86, which is invariably conserved among all class I MHC
molecules thus far sequenced (19, 20, 21, 22), the glycosylation site at
residue 176 is conserved only among murine class I MHC and not those of
other species. These findings suggest that the Ly-49 family of murine
NK inhibitory receptors appear to have evolved simultaneously with
murine class I MHC molecules.
| Materials and Methods |
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The 5E6 (anti-Ly-49C and -I) and YE1/48 (anti-Ly-49A) mAbs have been described (23, 24). The 34-5-8S, 34-2-12S, and 34-4-20S hybridomas (anti-Dd) were obtained from the American Type Culture Collection (ATCC, Manassas, VA). Flow cytometric analysis was done as described (11). Briefly, 5 x 105 cells were incubated for 30 min on ice with the appropriate hybridoma supernatants, followed by two rinses with HBSS (containing 2% FCS) and an incubation with an FITC-conjugated secondary Ab. After a final rinse with HBSS/FBS, cells were stained with propidium iodide for detection of nonviable cells. Flow cytometric analysis was subsequently performed on the FACScan (Becton Dickinson, Mountain View, CA).
Site-directed mutagenesis
The cDNA encoding Dd was isolated by RT-PCR from the murine B lymphoma line A20. The Dd mut1 in which serine 88 is replaced by glycine residue was generated by a two-step PCR using Pfu I polymerase (Strategene, La Jolla, CA). The upstream PCR to generate a DNA fragment encoding the N-terminal portion of Dd that contained the mutation at residue 88 was conducted using oligonucleotide primers 5'-CTGGATCCCAGATGGGGGCGAT-3' and 5'-CCCTGGTTGTAGTAGCGCAGCG-3' (phosphorylated). The downstream PCR to generate a DNA fragment encoding the C-terminal portion of Dd was performed using primers 5'-CGCGGGCGGCTCTCACACACTC-3' (phosphorylated) and 5'-TTGAATTCATGTGTTAGTCTTGGTGGATGAAGG-3'. The two PCR fragments were blunt-end ligated and used as a template for the second round of PCR that used the primers 5'-CTGGATCCCAGATGGGGGCGAT-3' and 5'- TTGAATTCATGTGTTAGTCTTGGTGGATGAAGG3'. The Dd mut2 in which threonine in residue 178 is replaced by glycine was also generated in the same manner. The upstream PCR to generate the mutated DNA fragment was done using primers 5'-CTGGATCCCAGATGGGGGCGAT-3' and 5'-CCCAGCATTCCCGTTCTTCAGGTATC-3' (phosphorylated), and the downstream PCR was done using primers 5'-CTGCTGCGCACAGATCCCCC-3' (phosphorylated) and 5'-TTGAATTCATGTGTTAGTCTTGGTGGATGAAGG-3'. The second round of PCR was done using the same primers as those for the first mutation. The DNA fragments from the second round PCR were digested with BamHI and EcoRI, subcloned into pBluescript, and sequenced.
Transfections
The cell lines COS-1, RBL-1, GM979, and C1498 were obtained from ATCC and cultured in DMEM supplemented with 5% FCS. Ly-49A and -C cDNAs that had been cloned into the pAX142 expression vector were transfected into COS cells using Lipofectamine (Canadian Life Technologies, Burlington, ON) according to the manufacturers protocols. Transfection of RBL-1, GM979, and C1498 cells were performed by electroporation at 0.45 kV and 125 µFD, and stable transfectants resistant to G418 (Canadian Life Technologies) were subsequently selected for high expression by limiting dilution, panning, or cell sorting.
Immunoprecipitation
GM979 transfectants were surface biotinylated and lysed with lysis buffer (1% Triton X-100 lysis buffer containing 1% BSA, 150 mM NaCl, and 0.1% NaN3 in 10 mM Tris-HCl, pH 7.5) as described (25). The clarified lysates were incubated overnight at 4°C with anti-Dd (34-5-8S)-coupled beads (50 µl of a 1:1 slurry). The beads were then washed four times with lysis buffer (without BSA and NaN3), and bound proteins were eluted with 100 µl 2x SDS sample buffer containing 50 mM DTT and boiling for 5 min. The eluted proteins were subsequently analyzed by SDS-PAGE and transferred onto nitrocellulose for Western blotting. Following incubation with peroxidase-conjugated streptavidin, proteins were detected using the ECL (Amersham, Arlington Heights, IL) chemiluminescence method.
N-glycosidase F treatment
Following immunoprecipitation as above, Dd bound to anti-Dd-mAb-coupled beads were washed and resuspended with PBS, treated with two units of N-glycosidase F (Boehringer Mannheim Canada, Laval, QC) for 6 h or 18 h at 37°C, and finally eluted with 100 µl 2x SDS sample buffer.
Binding assays
Twenty-four hours after transfection, COS cells were trypsinized and replated on 6-cm dishes (Falcon, Oxnard, CA). Forty-eighty hours later, Dd transfectants were added to COS cells. For the adhesion of RBL-1 transfectants, dishes containing COS cells were pretreated with heat-inactivated BSA (0.1% in PBS) for 1 h at 37°C to reduce nonspecific binding. Adhesion assays were performed for 2 h at 37°C or 15 min at room temperature for GM979 and RBL-1 cells, respectively. The plates were subsequently rinsed six times with prewarmed media and photographed on a Nikon Diaphot microscope. For quantitative assays, GM979 cells were radiolabeled with 51Cr (Mandel Scientific, Guelph, ON), and equivalent amounts of radioactivity were subsequently added to COS cells. After the unbound cells were removed by rinsing with prewarmed media, the remaining bound cells were lysed with 10% Triton X-100 and counted for radioactivity.
NK cell isolation
Spleens from C57BL/6 mice were homogenized and passed through a nylon wool column (Polysciences, Warrington, PA). Flow-through cells were then further enriched for NK cells using the immunomagnetic separation method, StemSep (StemCell Technologies, Vancouver, BC). The NK cells were subsequently cultured for 3 days in 1000 U/ml of murine IL-2 before purification of the Ly-49A+ subpopulation using the MACS separation method (Miltenyi Biotec, Auburn, CA). After 6 additional days of culture in the presence of IL-2, the Ly-49A+ NK cells were used in standard 51Cr release cytotoxicity assays.
| Results |
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To determine the role of the conserved N-linked
carbohydrates of class I MHC in the recognition of class I MHC by
Ly-49, two glycosylation-deficient mutants of Dd were
generated. The Dd mut1 lacks the glycosylation site at
residue 86 due to a replacement of serine by glycine at residue 88. In
the Dd mut2, residue 176 is deficient in glycosylation due
to a substitution of threonine at residue 178 by glycine. The wild-type
and mutant Dd cDNA clones were subsequently transfected
into the murine erythroleukemic cell line GM979, the rat basophilic
leukemia line RBL-1, and the murine lymphoma cell line C1498, and
transfectants expressing Dd were established. Flow
cytometric analysis of the transfected cells using the
conformation-dependent anti-Dd mAb 34-5-8 showed that
wild-type Dd and both mutant Dd were expressed
at comparable levels (Fig. 1
A). No differential binding
of other mAb that recognize Dd, including 34-4-21, 34-2-12,
and 34-4-20, to wild-type and mutant Dd was observed (data
not shown), indicating that the mutations did not significantly disrupt
the conformation of Dd. To verify that the expressed mutant
Dd proteins were indeed deficient at one glycosylation
site, Dd was immunoprecipitated from the transfected cells
using the 34-5-8 mAb and analyzed by SDS-PAGE. The mutant
Dd had lower apparent molecular masses than that of the
wild-type (Fig. 1
B), presumably because lack of
glycosylation at the mutated sites would reduce the overall mass.
Treatment of immunoprecipitated wild-type Dd with
N-glycosidase F for 6 h to cleave off
N-linked carbohydrates yielded three bands in SDS-PAGE,
corresponding to undigested, partially digested, and fully digested
Dd (Fig. 1
B, upper panel). The
apparent size of the two mutant Dd corresponded to the
partially digested wild-type molecule, and treatment with
N-glycosidase F reduced the sizes of mutant Dd
to that of the fully digested wild-type Dd. Furthermore,
prolonged treatment with the enzyme subsequently resulted in complete
deglycosylation of all the Dd molecules to its lowest
molecular mass (Fig. 1
B, lower panel). These
results confirmed that the glycosylation of Dd was indeed
disrupted by the mutations as expected.
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Binding of mutant Dd to Ly-49 was determined by
adhesion of Dd-transfected cells to Ly-49-transfected COS
cells. Flow cytometric analysis showed high levels of Ly-49A and Ly-49C
expression on the transfected COS cells (Fig. 2
A). GM979 transfected with
wild-type Dd readily bound to Ly-49A-transfected COS cells
(Fig. 2
B). Similarly, GM979 transfected with Dd
mut1 also bound to Ly-49A-transfected COS cells, and no significant
effect of this mutation was observed in the binding assay. In contrast,
Dd mut2-transfected GM979 failed to bind to
Ly-49A-transfected COS cells. This was not due to the expression level
of this mutant Dd, because flow cytometric analysis (Fig. 1
A) showed the level of this mutant Dd to be
slightly higher than that of Dd mut1. None of the GM979
lines bound to control COS cells transfected with vector alone, whereas
untransfected GM979 cells did not bind to Ly-49A-transfected COS cells,
indicating that the binding in this assay is mediated by Dd
and Ly-49A. Quantitative analysis using 51Cr-labeled
transfected GM979 cells in the presence and absence of YE1/32
(anti-Ly-49A mAb) also confirmed the binding of wild-type
Dd and Dd mut1 to Ly-49A (Fig. 2
C).
The binding of Dd mut2 was significantly lower even in the
absence of the blocking Ab.
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The binding of Dd on target cells to Ly-49A on NK
cells has been shown to inhibit cytotoxicity of Ly-49A+ NK
cells (7). Therefore, we tested whether the very low level of binding
of Dd mut2 to Ly-49A results in defective inhibition in NK
cytotoxicity. For this study, we were unable to use RBL-1 or GM979
cells, because the former labeled poorly with chromium and the latter
cells were NK-resistant. Therefore, we transfected wild-type and mutant
Dd into the murine leukemic cell line C1498, which is
highly sensitive to NK cytotoxicity. The expression levels of
Dd on the transfectants were comparable (Fig. 4
A). In cytotoxicity assays,
untransfected C1498 cells were readily lysed by Ly-49A+ NK
cells. C1498-expressing wild-type Dd or Dd mut1
were resistant to Ly-49A+ NK cells, whereas Dd
mut2-transfected C1498 cells were only partially resistant to
Ly-49A+ NK cells (Fig. 4
B). In four independent
experiments, the cytotoxic killing of Dd mut2-transfected
C1498 was consistently higher than that of wild-type Dd or
Dd mut1 transfectants, while no statistically significant
difference was seen between wild-type Dd and Dd
mut1. The cytotoxicities were restored to the level of control
untransfected C1498 cells in the presence of mAb to Ly-49A (YE1/48) or
F(ab')2 fragment of anti-Dd (34-5-8),
demonstrating that the resistance of these cells to NK killing is due
to the specific recognition of Dd by Ly-49A. These results
indicate that the recognition of Dd mut2 by Ly-49A is less
efficient than that of wild-type Dd or Dd mut1.
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| Discussion |
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The view that asparagine 176-linked carbohydrates on Dd plays a role in the interaction with Ly-49 is consistent with previous findings. Ly-49A and -C have been shown to bind certain polysaccharides (16, 17, 18). Furthermore, binding of class I MHC to Ly-49C is also inhibited by treatment of class I MHC-positive cells with tunicamycin (17) or fucosidase (18). These results indicate that carbohydrates on class I MHC are involved in the recognition by Ly-49. On the other hand, it is unlikely that the binding of class I MHC to Ly-49 is mediated solely by carbohydrates. It has been shown that recognition of class I MHC by Ly-49 depends on the conformation of the former since Ly-49A is unable to recognize class I MHC lacking bound peptides (13). Therefore, Ly-49 most probably recognizes a combination of carbohydrates and the peptide backbone structure of class I MHC. This is consistent with our previous finding that the binding of Ly-49 to class I MHC involves not only the carbohydrate recognition domain of Ly-49 but also a portion of the stalk region immediately adjacent to the carbohydrate recognition domain (14, 16). The involvement of carbohydrates of class I MHC in the binding to Ly-49 implies that NK cytotoxicity may be regulated not only by the expression level of class I MHC on target cells but also by the pattern of glycosylation of class I MHC.
While this manuscript was under review, Matsumoto et al. reported similar studies with different results (26). In their studies, mutations at the conserved glycosylation sites of Dd had no detectable effects on inhibition of NK cytotoxicity or binding of Dd-transfected C1498 cells to Ly-49A-transfected Chinese hamster ovary cells. The reasons for the discrepancies are unknown at this time, but they may be due to differences in cells used, mutations (substitutions of asparagines with glutamines in their studies vs substitution of serine or threonine with glycine in this study), or expression levels of Dd on target cells. Our studies have shown that mutation of the glycosylation site at residue 176 reduces the binding of Dd to Ly-49, but the binding is sufficient to induce partial inhibition of cytotoxicity of Ly-49A+ NK cells. It is conceivable that, if the expression level of this mutant Dd on target cells is high enough, it can protect the targets from cytotoxicity.
It is of interest that glycosylation at residue 176, but not residue
86, of class I MHC is involved in the binding to Ly-49. Residues 86 and
176 are located at the end of the
1 and
2 domains, respectively,
and are outside of the peptide binding groove. The residue 86
glycosylation site is invariably conserved among all class I MHC thus
far sequenced (19, 21). In contrast, the residue 176 glycosylation site
is conserved only among murine class I MHC (Fig. 5
). A survey of various class I MHC
sequences in the data base showed that this glycosylation site in the
2 domain is conserved among all classical murine class I MHC
including Kd, Kb, Kk,
Kf, Dd, Db, Dk, and
Df but is not found in any others examined. In addition to
the class I MHC sequences listed in Figure 5
, those of human (10 HLA-A,
30 HLA-B, 7 HLA-C), six rat, 13 chimpanzee, 5 orangutan, 14 gorilla, 3
baboon, 3 gibbon, 11 rhesus monkey, 6 cat, 3 rabbit, 3 cow, 5 horse, 2
cheetah, 2 squirrel, 2 frog, and 4 fish were examined. The
glycosylation site at residue 176 was not found among any of the
nonmurine class I MHC. This indicates that this glycosylation site in
the
2 domain likely arose after the divergence of mice from other
species and has became fixed in all murine class I MHC genes.
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-chain with ß2 microglobulin and peptides (19, 27, 28). On the other hand, the functional role of carbohydrates on residue
176 of murine class I MHC has not yet been described. Since it is
conserved among murine class I MHC and not found among others, its
function should be uniquely attributed to mice. The results in this
study show that carbohydrates on residue 176 function as a part of the
ligand structure for the murine NK receptor Ly-49. Therefore, it is
possible that murine class I MHC conserved this glycosylation site
during evolution to maintain the function to serve as a ligand for the
Ly-49 family. The complex and highly polymorphic Ly-49 family likely
evolved in response to the rapid evolution of murine class I MHC. The
lack of this N-linked glycosylation in class I MHC of other
species suggests that the murine Ly-49 family diverged from other NK
receptors with respect to how they recognize class I MHC on target
cells. Although Ly-49 has also been found in rats (29, 30), direct
interaction between rat Ly-49 and rat class I MHC has not been
demonstrated. The absence of glycosylation sites in the
2 domain of
rat class I MHC suggests that binding specificities of rat Ly-49 may be
quite different from murine Ly-49. Moreover, efforts to isolate
homologous human Ly-49 genes using mouse probes have not been
successful. While a human Ly-49 gene family may exist, our results
support the hypothesis that it may be significantly diverged from mouse
Ly-49, reflecting the rapid evolution of genes involved in immune
functions.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Fumio Takei, Terry Fox Laboratory, British Columbia Cancer Research Centre, 601 West 10th Avenue, Vancouver, BC V5Z 1L3, Canada. E-mail address: ![]()
Received for publication March 9, 1998. Accepted for publication May 4, 1998.
| References |
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