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CUTTING EDGE |



*
Department of Medicine and
Gwen Knapp Center for Lupus and Immunology Research, University of Chicago, Chicago, IL 60637; and
Laboratory of Cardiovascular Biology, Harvard School of Public Health and Harvard Medical School, Boston MA 02115
| Abstract |
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14-J
281 TCR
transcript seen in NK T cells. Unlike wild-type NK T cells,
Ets1-deficient thymocytes failed to produce detectable levels of IL-4
following anti-CD3 stimulation. The absence of NK T cells in the
Ets1-deficient mice was not associated with defective expression of
CD1, an MHC class I molecule required for NK T cell development. We
conclude that Ets1 defines a novel transcriptional regulatory pathway
that is required for the development of both the NK and NK T cell
lineages. | Introduction |
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-chain composed of
V
14 and J
281 gene
segments (4, 5, 6, 7). NK T cells display a
CD44high Mel-14low
HSAlow 3G11low cell surface
phenotype that is similar to that of an activated effector T cell
(1, 2, 3, 8). In addition, like NK cells, NK T cells express
membrane-associated receptors such as NK1.1 and IL-2Rß (1, 9). Just as NK T cells have both T and NK cell cell-surface
attributes, they also manifest both T and NK cell effector functions.
Following TCR-mediated activation, NK T cells can secrete large amounts
of IL-4 and IFN-
, a characteristic of differentiated Th cells
(2, 8, 10). However, they have also been shown to display
cytotoxic activity, the mechanism of which is more comparable to that
of NK cells (11). Previous studies have suggested that NK
T cells have unique and important immunomodulatory roles in vivo,
including 1) participating in the elimination of mycobacterial
pathogens (12), 2) contributing to tumor rejection
(11), 3) providing help to B cells in mediating Ab
responses to protozoan parasites (13), and 4) regulating
autoimmune responses (14, 15, 16). Little is currently known about the transcriptional pathways that regulate the development and function of the NK T cell lineage. In this report, we have examined the role of the Ets1 transcription factor in NK T cell development. Ets1 is a member of the Ets family of winged helix-turn-helix transcription factors. Ets proteins bind to conserved purine-rich sequences surrounding a GGA core sequence motif and are important regulators of both vertebrate and invertebrate development (17). Several Ets family members are expressed in lymphoid cells, and many lymphoid-restricted genes contain functionally important Ets binding sites (17).
The generation of Ets1-deficient animals has provided an important reagent for understanding the role of Ets1 in the regulation of lymphoid cell development and function (18, 19, 20). Ets1 is not required for the development of mature T and B cells. However, Ets1-deficient mice display a severe defect in NK cell development and function. Moreover, these mice contain decreased numbers of peripheral CD8+ T cells and increased numbers of IgM-secreting plasma cells. Both CD4+ and CD8+ T cells from the Ets1-deficient animals display a severe defect in TCR-mediated activation that can be rescued by stimulation with phorbol ester plus ionomycin. In this paper we show that Ets1 is also required for the differentiation of the NK T cell lineage and, as such, defines a novel transcriptional pathway that regulates the development of both NK and NK T cells in mice.
| Materials and Methods |
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Ets1-deficient mice were generated as described previously (20). CD1-deficient mice were generated as previously described and had been backcrossed onto the BALB/c background for seven generations (21).
Antibodies
PE-conjugated anti-NK1.1 (PK136), FITC-conjugated
anti-CD8
(53-6.7), and CyChrome-conjugated anti-CD4 (RM4-5),
and anti-TCRß (H57-597) were purchased from PharMingen (San
Diego, CA). The hamster anti-mouse CD1 mAb has been described
previously (27). The 2.4G2 anti-FcR mAb (22) producing
hybridoma and bioreactor supernatant containing hamster anti-mouse
CD3
were kindly provided by Dr. Jeffrey A. Bluestone (University of
Chicago, Chicago, IL).
Isolation of lymphocyte populations
Thymus, lymph node, hepatocyte, and splenocyte suspensions were prepared as described previously (19, 20, 21).
Flow cytometric analysis
A total of 106 cells per staining reaction were washed in FACS buffer (0.1% BSA in PBS containing 0.01% sodium azide). Fc receptors were blocked with the addition of 2.4G2 culture supernatant. Cells were stained for 30 min at 4°C and washed in FACS buffer before analyzing on a FACScan flow cytometer (Becton Dickinson, Mountain View, CA).
RT-PCR analysis of V
14-J
281
expression
Five micrograms of total RNA was used to synthesize cDNA by
reverse transcription with random hexamer primers. The resulting cDNAs
were subjected to 38 cycles of amplification by the PCR (94°C for 1
min, 60°C for 1 min, and 72°C for 2 min) using synthetic
oligonucleotide primers specific for either
V
14-J
281,
V
8-J
2, or
C
1-C
2
(21). The amount of template cDNA used in each reaction
was normalized to the amount of HPRT mRNA (21). The levels
of TCR
cDNA in the samples were normalized to the
C
mRNA amplified with
C
1 and C
2 primers
(21).
In vitro thymocyte stimulation
Triplicate aliquots of 106 thymocytes were stimulated with plate-bound anti-CD3 mAb (10 µg/ml) for 48 h in flat-bottom 96-well plates. Levels of IL-4 in culture supernatants were determined by ELISA using a commercially available kit according to the manufacturers instructions (Endogen, Woburn, MA).
| Results |
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The Ets1-deficient mice were generated on a C57BL/6 x 129/J mixed background. The 129/J strain is of the NK1.2 allele. To identify Ets1 mutant and wild-type mice that carried the NK1.1 allele, peripheral blood from both wild-type and Ets1-deficient animals was analyzed by flow cytometry for NK1.1 expression. Using this protocol, we could detect NK1.1+ cells in the peripheral blood of both the wild-type (3.8% of PBL) and Ets1-deficient (0.5% of PBL) mice (data not shown). All experiments were performed using animals that were determined to express the NK1.1 allele by this flow cytometric assay.
To determine whether NK T cells develop in the Ets1-deficient animals,
we analyzed lymphocyte populations from the thymus, spleen, and
mesenteric lymph nodes. In addition, lymphocytes were isolated from the
liver, a compartment known to be rich in NK T cells in the mouse
(23). As shown in Fig. 1
A, there were significantly
decreased numbers of NK1.1+
CD4+ T cells in both the thymi and spleens of the
Ets1-deficient mice. In the liver, as in other lymphoid organs, the
majority of the NK T cells express only intermediate levels of the TCR
complex. Therefore, we gated on this population of TCR-intermediate
lymphocytes (Fig. 1
B, top left and
right) before analyzing the cells for
NK1.1+ expression (Fig. 1
B,
bottom left and right).
NK1.1+ T cells were almost undetectable in the
livers of the Ets1-deficient mice. Consistent with this marked decrease
in NK1.1+ T cells in the livers of these animals,
there was also a significant decrease in the numbers of cells
expressing an intermediate level of the TCR ß-chain that is
characteristic of hepatic NK T cells (Fig. 1
B, top left and
right). In addition to the decrease in
NK1.1+ hepatic T cells, we also observed a
significant reduction in the number of
TCRintCD4+NK1.1-
hepatic lymphocytes in the Ets1-deficient mice. The function of these
cells and their relationship to classical NK T cells remains unclear
(24). Taken together, these data suggested a severe defect
in NK T cell development in the Ets1-deficient mice.
|
14-J
281
transcripts in Ets1-deficient animals
As described above, NK T cells express a limited repertoire of TCR
- and ß-chains. The majority of NK T cells express a TCR
-chain
encoded by the
V
14-J
281 gene
segments (4, 5). In mice deficient for the
J
281 gene, no NK T cells were observed either
in central or peripheral lymphoid organs (11). Similarly,
mice deficient for the nonclassical MHC class I molecule, CD1, lack NK
T cells and display a marked reduction in
V
14-J
281 transcripts
in the thymus and liver (21, 25). Accordingly, we examined
the expression of
V
14-J
281 transcripts
in the livers of Ets1-deficient mice (Fig. 2
). There was a significant decrease in
the levels of
V
14-J
281 transcripts
in Ets1-deficient livers as compared with wild-type control livers.
This decrease was comparable to that observed in livers from the
CD1-deficient mice (Fig. 2
). Moreover, this decrease was specific for
the NK T-associated transcripts, because the wild-type, Ets1-deficient,
and CD1-deficient livers contained equivalent levels of
V
8-containing TCR
transcripts that are not
associated with NK T cell populations (Fig. 2
). Decreased levels of
V
14-J
281 transcripts
were also observed in the thymi and lymph nodes of the Ets1-deficient
animals (data not shown). These results were consistent with the NK1.1
flow cytometric data demonstrating a significant decrease in NK T cell
numbers in the Ets1-deficient mice.
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Naive peripheral T cells do not produce IL-4 upon stimulation
through the TCR. In contrast, thymic NK T cells have been shown to
produce significant quantities of IL-4 following TCR cross-linking
(2, 8, 10, 21). To determine whether such IL-4-producing
NK T cells were present in the thymi of Ets1-deficient animals,
thymocytes were isolated from wild-type and Ets1-deficient mice and
cultured on anti-CD3 coated plates for 48 h. Thymocytes from
control wild-type animals produced significant quantities of IL-4
following TCR cross-linking in vitro (Fig. 3
). In contrast, no IL-4 was detectable
in the supernatants from anti-CD3-stimulated Ets1-deficient
thymocytes (Fig. 3
), suggesting that the NK T cells were absent or
nonfunctional in the Ets1-deficient mice.
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The absence of NK T cells in the Ets1-deficient animals suggested that these mice either lack the extrinsic signals required for the development of this lymphocyte population, or are intrinsically incapable of responding to these developmental signals due to the absence of an NK T cell developmental pathway that is transcriptionally regulated by Ets1. It has been demonstrated previously that the nonclassical MHC class I molecule, CD1, can be recognized by hybridomas generated from murine NK1.1+ T cells (26) and that CD1 is required for the development of this lineage in mice (21, 25).
To determine whether the defect in NK T cell development in the
Ets1-deficient mice was the result of decreased CD1 expression,
thymocytes and splenic B cells from wild-type and Ets1-deficient
animals were analyzed for CD1 expression by flow cytometry. There was
no significant difference in the levels of cell surface CD1 expression
on wild-type and Ets1-deficient lymphocytes (Fig. 4
). Ets1-deficient lymphocytes also
expressed normal levels of class I MHC as assessed by flow cytometry
(data not shown). Thus, the defect in NK T cell development seen in the
Ets1-deficient mice is not due to decreased levels of expression of CD1
in these animals.
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| Discussion |
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It is not possible from our data to conclude if the defect in NK T cell development observed in the Ets1-deficient mice is cell autonomous or instead reflects a defective Ets1-regulated developmental signal that is required for NK T cell differentiation or survival. We have shown previously that Ets1 is expressed at high levels in wild-type NK cells and T cells (20). However, because it is difficult to purify large numbers of NK T cells away from NK cells, we are currently unable to determine whether Ets1 is also expressed in the NK T cell lineage.
The markedly reduced numbers of NK T cells seen in the thymus, spleen, and liver of the Ets1-deficient mice strongly suggest that the NK T cell defect observed in these animals reflects the deficient production or survival of these cells. We and others have shown previously that Ets1-deficient T cells display a defect in their ability to respond to stimulation through the CD3 components of the TCR complex (19, 20). Although this defect is not sufficient to block the development of mature single positive T cells, more recent studies have demonstrated partial defects in both positive and negative selection of Ets1-deficient TCR transgenic T cells (T. Walunas and J. Leiden, manuscript in preparation). These findings are consistent with a model in which decreased TCR-mediated signaling in the absence of Ets1 partially, but incompletely, blocks T cell maturation and selection. The affinity of the NK T cell TCR for selecting elements such as CD1 remains unknown. However, if this is a low affinity interaction, this same Ets1-related defect in TCR signaling might be predicted to have a more profound effect on NK T cell selection and maturation resulting in the marked reductions in NK T cell numbers seen in the Ets1-deficient animals.
The decreased numbers of NK T cells in the livers and lymph nodes of the Ets1-deficient animals might also reflect defects in the survival or population of peripheral lymphoid organs by NK T cells. Such defects could result from deficient expression of a number of different growth factors, growth factor receptors, or adhesion molecules. Our previous studies have not demonstrated defects in the production of IL-2, IL-15, IL-12, or their receptors in the Ets1-deficient mice (20). Ongoing studies designed to elucidate the normal targets of Ets1 in both lymphocytes and other cell lineages that are important for NK and NK T cell differentiation and survival should help to elucidate the components of the Ets1-dependent pathways(s) that regulate the development and survival of these two important lymphoid cell lineages.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Jeffrey M. Leiden, Harvard School of Public Health, Laboratory of Cardiovascular Biology, Building II, Room 117, 677 Huntington Avenue, Boston, MA 02115. E-mail address: ![]()
Received for publication December 6, 1999. Accepted for publication January 11, 2000.
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