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*
Terry Fox Laboratory, British Columbia Cancer Agency, Vancouver, Canada; and Departments of
Medical Genetics and
Pathology and Laboratory Medicine, University of British Columbia, Vancouver, Canada
| Abstract |
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| Introduction |
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The expression of multiple receptors with different specificities for MHC class I are thought to generate diverse specificities of NK cells to recognize target cells missing various self-MHC class I molecules and at the same time to maintain self-tolerance of NK cells. Recently, the receptor repertoires of human NK clones have been analyzed, and the results supported this concept (17). Unlike data on human NK cells, relatively little is currently known about murine NK cell receptor repertoires because of difficulties in establishing mouse NK cell clones. Murine NK cell receptor expression patterns have previously been examined by flow cytometric analysis of bulk NK cell populations using anti-Ly49 mAbs (18, 19). However, these studies are limited by the lack of mAbs to some receptors, cross-reactivities of some of the mAbs, and difficulty in simultaneously detecting more than three receptors on individual NK cells. In this study, we have developed a single-cell RT-PCR method to analyze the expression of receptor genes in individual NK cells. The results demonstrate a highly diverse pattern of coexpression of multiple receptors in individual murine NK cells from adult mice, suggesting a stochastic mechanism for receptor expression. In contrast, neonatal NK cells mostly express NKG2A but not Ly49.
| Materials and Methods |
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C57BL/6 mice were purchased from The Jackson Laboratory (Bar Harbor, ME).
Abs and flow cytometry
The hybridomas PK136 (anti-NK1.1) and 2.4G2 (anti-Fc receptor) were obtained from American Type Culture Collection (Manassas, VA). The mAbs YE1/48 (anti-Ly49A) and 5E6 (anti-Ly49C and I) have been described (11). The anti-Ly49G mAb 4D11 (12) was kindly provided by Dr. Stephen Anderson (National Cancer Institute-Frederick Cancer Research and Development Center, Frederick, MD). Anti-CD3-FITC was purchased from BIO/CAN Scientific (Mississauga, Canada). For sorting of NK cells, nylon wool-nonadherent spleen cells were first incubated with unlabeled 2.4G2 to block the Fc receptor and then stained for NK1.1 and CD3. NK1.1+ CD3- single cells were sorted into microtiter wells on a FACStarPlus cell sorter (Becton Dickinson, San Jose, CA).
Single-cell RT-PCR
A previously described method (20) for generating representative amplified total cDNA from small numbers of hemopoietic cells was used. In brief, single cells were deposited into 96-well plates containing 50 µl guanidinium isothiocyanate solution. RNA was isolated from each well and reverse transcribed using a special 60-mer oligo(dT) primer (1 µg/µl) (20) and Superscript II (Life Technologies, Gaithersburg, MD). The cDNA thus generated was subjected to poly(A) tailing by TDT and PCR amplified using the special oligo(dT) primer and Taq polymerase (Life Technologies) as follows: 94°C for 1 min, 55°C for 2 min (except for the first cycle which was performed at 37°C), and 72°C for 10 min for 40 cycles.
PCR amplification of NK cell receptor genes
The amplified total cDNA derived from each well was subjected to a second round of PCR amplification using consensus primer sets designed to match all of the different Ly49 gene sequences except Ly49B. Four percent of the total cDNA (2.0 µl) of each subpopulation was amplified by 35 cycles of PCR (94°C for 30 s, 55°C for 60 s, 72°C for 60 s). Annealing temperature for NKG2A amplification was 58°C. The following primers were used: consensus 5'-primer, 5'-TCCCA/GA/CA/GATGAA/G TGAAA/GCA/CA/GGA-3'; consensus 3'-primer, 5'-TC/TAAT C/GAGGA/GAATTTA/GTCCA-3'; Ly49B-specific 5'-primer, 5'-TCCGGGATGGACAAGAGAAA-3'; Ly49B-specific 3''-primer, 5'-TGGCACAGCTCCCTGGAAT-3'; NKG2A-specific 5'-primer, 5'-CGAAGCAAAGGCACAGA-3'; NKG2A-specific 3'-primer, ATGGCACAGTTACATTCATCAT-3'.
Southern blot analysis of PCR product
PCR products were separated by agarose gel electrophoresis and transferred to nylon membranes. Oligonucleotide probes specific for each known Ly49 cDNA were synthesized so that each had a melting temperature (Tm) of 68°C. Probes for Ly49AH were previously described (21). The following probes were used: Ly49I, 5'-GGAACAGTGAAACCAAGACGGTT-3'; Ly49J, 5'-GGAATAGTGAAACCAATACTATT-3', NKG2A, 5'-TCCTTCGGAAGGGCAGAGGTCA-3'. The probes were 32P-end labeled using TdT and hybridized at 510°C below Tm to Southern blots, and the blots were washed at room temperature in 3x SSC, 1% SDS. The filters were exposed to x-ray films at -70°C.
| Results |
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NK1.1+ CD3- spleen
cells from 8-wk-old C57BL/6 mice were individually sorted into
microtiter wells by FACS, and expression of NK cell receptor genes in
each cell was analyzed by single-cell RT-PCR. Total cDNA was generated
from each well and amplified by a first-round PCR as described in
Materials and Methods. The amplified cDNA was then subjected
to a second round of PCR to amplify NK cell receptor cDNAs. For all
Ly49 except Ly49B, consensus primers were used, whereas specific
primers were used to amplify Ly49B and NKG2A. The individual receptor
cDNAs thus amplified were identified by specific oligonucleotide probes
by Southern hybridization. To verify the efficiency and specificity of
this method, the second-round PCR was first applied to individual cDNA
clones. As shown in Fig. 1
A,
all known Ly49 as well as NKG2A cDNAs were efficiently amplified by the
PCR, and the oligonucleotide probes specifically detected the
corresponding cDNAs. A mixture of all Ly49 cDNAs (Ly49ALy49J) was
also subjected to the second-round PCR, and the amplified Ly49
cDNAs were detected by the oligonucleotide probes. All the Ly49
cDNAs were amplified at similar efficiencies, indicating that the
second-round PCR is not biased toward preferential amplification of
some of the receptor genes because of competition among different Ly49
genes for PCR amplification (Fig. 1
B). Also, all the
receptor genes were efficiently amplified when 1000 NK cells were
subjected to this RT-PCR method.
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40% of adult NK cells
are stained with tetrameric Qa-1b that binds to
the CD94/NKG2A heterodimer (15). Two cells expressed only
Ly49D, an activating receptor, whereas all other NK cells expressed at
least one inhibitory NK cell receptor.
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Fig. 4
shows the frequencies of
cells expressing different numbers of receptors. The mean number of
receptors expressed per cell was 2.6. Limiting the analysis to the
receptors that have the ITIM in their cytoplasmic tails, the mean
number of receptors per cell was 2.0. When the frequencies of
coexpression of any two receptors were examined, the observed
frequencies were similar to those expected from the products of their
individual frequencies in most cases (Table II
). The frequencies of
coexpression of more than three receptors were also similar to the
expected values (data not shown). The only significant exception was
the frequency of Ly49D and G coexpression, which was slightly higher
than that expected from random association. The significance of this is
unknown.
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We also analyzed NK cell receptor expression in neonatal NK cells.
NK1.1+CD3- spleen cells
from 1-wk-old C57BL/6 mice were sorted as single cells, and 64 NK cells
were analyzed for the expression of NK cell receptors by single-cell
RT-PCR. The frequency of neonatal NK cells expressing Ly49 was
significantly lower than that of adult mice. Only low percentages of
cells expressed Ly49A, -C, -H, or -I (Fig. 5
). In contrast, almost 60% of neonatal
NK cells expressed NKG2A. The difference in the frequency of
NKG2A+ cells between adult and neonatal NK cells
was statistically significantly (
2 test,
p < 0.01). Therefore, it is likely that NKG2A
expression precedes Ly49 expression in NK cell ontogeny and that it is
the major inhibitory receptor on neonatal NK cells.
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| Discussion |
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70% of NK cells from 1-wk-old mice and 40% of adult NK cells (data
not shown). However, considering that CD94/NKG2C also binds Qa-1b
tetramer (S. Lohwaser, A. Kubota, M. Salcedo, and F. Takei, unpublished
results), the frequencies of NKG2A+ NK cells
determined by the single-cell RT-PCR are in good agreement with those
estimated by Qa-1b binding. Although CD94 expression was not determined
in this study because of unknown technical difficulties, these results
also suggest that NKG2A+ NK cells likely coexpress CD94. Among the
adult and neonatal NK cells analyzed in this study, none expressed
Ly49B, -E, or -F. These genes were readily detected by the current
method when the number of NK cells to be analyzed was increased to
1000. Therefore, the lack of detection of NK cells expressing these
receptors in this study was most probably due to the low frequency of
NK cells expressing these genes. This study revealed, for the first time, the full complexity of the receptor repertoires of murine NK cells. Previous flow cytometric analyses have shown that more than two Ly49 molecules can be coexpressed on some NK cells. However, these studies were limited to the receptors for which specific mAbs are available. The results showed that the frequency of Ly49 coexpression is close to that expected from the individual frequency (product rule) (18, 19), suggesting that different Ly49 genes are randomly activated at a low frequency. Our study extended this product rule to all possible combinations of known murine NK cell receptors, including NKG2A. Although NKG2 and Ly49 belong to separate gene families, they follow the similar product rule. Up to six different receptors have been detected in individual NK cells. There seems to be no limitation to the number of receptors a single NK cell can express. However, the frequency of NK cells expressing several receptors is low simply because of low probability of simultaneously expressing multiple genes, each of which has <50% probability of expression in NK cells. The stochastic coexpression of multiple NK receptors generates very diverse receptor repertoires as demonstrated by the single-cell RT-PCR analysis. Of 62 adult NK cells examined in this study, 42 different combinations of receptor coexpression were observed, implying a high degree of heterogeneity among NK cells.
When compared with the receptor repertoires of human NK cells (17), our study revealed strong similarities between murine and human NK cell receptor repertoires despite their structural difference. Both human and murine NK cells express the nonclassical MHC class I-specific receptors CD94/NKG2 and classical MHC class I-specific receptors, the Ly49 family of C-type lectins in mouse and the KIR, Ig superfamily members, in humans. Each receptor is expressed on a subset of NK cells, and coexpression of receptors is stochastic. From an evolutionary perspective, the divergence of MHC class I-specific NK cell receptors is thought to occur to adapt to the rapid evolution of MHC class I polymorphism (22). Humans and mice have distinctive class I genes with unique patterns of polymorphism (23). Thus, MHC class I receptors of mice and humans would have evolved independently since the separation of these two species, which may explain why structurally different NK cell receptors are used. The similarity between human and murine NK cell receptor repertoires indicates an evolutionary advantage of the system. It allows NK cells to express not only self-specific receptors but also multiple receptors in various combinations. The stochastic coexpression of multiple receptors in individual NK cells would generate a high degree of diversity in overall NK cell specificities.
It has been proposed that NK cells are potentially self-reactive but
maintained to be self-tolerant because of inhibitory receptors specific
for self-MHC class I (17, 24, 25). According to this
hypothesis, every NK cell must express at least one inhibitory receptor
recognizing self-MHC class I molecules (26). In C57BL/6
mice, Ly49C and possibly Ly49I (R. Lian, D. L. Mager, and F.
Takei, unpublished results) recognize self-MHC. The 5E6 mAb that
recognizes both Ly49C and -I stains only
60% of NK cells.
Therefore, the recently characterized murine NKG2A may be an important
inhibitory receptor for the recognition of self-MHC. The NKG2A/CD94
heterodimer has been shown to bind to the nonclassical MHC class I
Qa-1b (15). Our studies have shown
that NKG2A mRNA is detected in 32% of adult NK cells. However, 23% of
adult NK cells tested in this study did not express Ly49C, Ly49I, or
NKG2A mRNA. These results suggest that murine NK cells may express
additional self-MHC-specific inhibitory receptors yet to be
identified.
The proportion of NK cells expressing each Ly49 receptor is low at birth and increases during ontogeny (27, 28). Consistent with this, the frequency of Ly49-expressing NK cells in neonatal mice was low in our assay. It has been reported that fetal and neonatal NK cells cultured in the presence of cytokines in a stroma-free system are deficient in Ly49 expression but preferentially kill tumor cells and blast cells deficient in the expression of MHC class I molecules (28, 29, 30). This suggested that fetal and neonatal NK cells may express inhibitory receptors other than Ly49. Indeed, our current results showed that many neonatal NK cells express NKG2A mRNA. It is likely that CD94/NKG2A plays a major role in self recognition of NK cells in early ontogeny, as suggested by the highly conserved features of Qa-1 and HLA-E (31, 32).
| Acknowledgments |
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| Footnotes |
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2 A.K. and S.K. contributed equally to this work. ![]()
3 Address correspondence and reprint requests to Dr. Fumio Takei, Terry Fox Laboratory, British Columbia Cancer Research Center, 601 West 10th Avenue, Vancouver, British Columbia, V5Z 1L3 Canada. E-mail address: ![]()
4 Abbreviations used in this paper: KIR, killer cell-inhibitory receptor; ITIM, immune receptor tyrosine-based inhibitory motif. ![]()
Received for publication March 22, 1999. Accepted for publication April 22, 1999.
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