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CUTTING EDGE |


*
Department of Immunology, Mayo Clinic and Foundation, Rochester, MN 55905; and
Section on Lymphocyte Signaling, Cell Biology and Metabolism Branch, National Institutes of Health, Bethesda, MD 20892
| Abstract |
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RIII receptors and following direct contact with
NK-sensitive target cells. This NK stimulation induces the association
of LAT with several phosphotyrosine-containing proteins. In addition to
the biochemical evidence showing LAT involvement in NK cell activation,
a genetic model shows that LAT is required for FcR-dependent
phosphorylation of phospholipase C-
. Furthermore,
overexpression of LAT in NK cells leads to increased Ab-dependent
cell-mediated cytotoxicity and "natural cytotoxicity," thus
demonstrating a functional role for LAT in NK cells. These data suggest
that LAT is an important adaptor protein for the regulation of human NK
cell-mediated cytotoxicity. | Introduction |
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RIII receptor (Ab-dependent cell-mediated cytotoxicity
(ADCC)3) or, in a less
defined manner, through a direct interaction with certain malignant or
virus-infected target cells ("natural cytotoxicity") 2 .
The biologic functions of NK cells differ from the functions of other
lymphocyte subpopulations, e.g., B and T cells. However, the mechanism
of NK activation bears homology to that employed by other lymphocytes.
Although B cell receptor, TCR, and FcR differ in their extracellular
portions, these multichain immune recognition receptors (MIRR) all have
immunoreceptor tyrosine-based activation motifs in their
signal-transducing subunits and all share certain common features in
their intracellular signaling pathways 3 . Fc
RIII cross-linking on
NK cells leads to the activation of Src and Syk family kinases 4, 5, 6 .
Downstream of Syk family kinases are, among other molecules,
phospholipase C-
(PLC-
) 7, 8, 9 , the guanine nucleotide exchange
factor Vav 10 , and the adaptor protein SLP76 11, 12, 13 , all of which
are important regulators of NK cell-mediated cytotoxicity 7, 8, 14, 15 .
Early studies of lymphocyte signaling showed that an
36-kDa protein
undergoes tyrosine phosphorylation upon stimulation of
T and NK cells 16, 17, 18, 19 . Recently, this protein was cloned from T cells
and named linker for activation of T cells (LAT) 20 . This protein
lacks intrinsic catalytic activity, but it appears to function as an
adaptor integrating signals initiated by MIRR stimulation. LAT is a
transmembrane protein and has multiple tyrosine residues that, upon
phosphorylation, promote specific protein-protein
interactions with Src homology (SH)-2-containing proteins.
Palmitoylation of LAT seems to be important for its targeting to
glycolipid-enriched membrane subdomains in which specific interactions
with other proteins may occur 21 . Upon TCR cross-linking,
phosphorylated LAT associates with several signaling
proteins such as PLC-
1, Vav, SLP76, Grb2, and the p85 subunit of the
phosphatidylinositol 3-kinase (PI3K) 20 . A LAT mutant bearing
substitutions at the two C-terminal tyrosine residues fails to
associate with specific interacting proteins, suggesting that the full
adaptor function of LAT is dependent on the inducible tyrosine
phosphorylation of this protein. Moreover, the same LAT
mutant inhibits TCR-induced transcriptional activity of AP-1 and NFAT
when transiently expressed in Jurkat T cells. Therefore,
phosphorylated LAT seems to be playing a role in the
nucleation of several signaling complexes important for T cell
activation.
The function of LAT in NK cells is unknown. Phosphorylated p36 has been implicated as a direct target of the killer cell inhibitory receptor-associated phosphatase SHP-1 during inhibitory signaling in NK cells 19 . Phosphorylated p36 has also been found to bind to the adaptor protein Grb2 during the FcR stimulation of NK cells 18, 22 . This association may be involved in the activation of downstream effectors such as Ras and phosphatidylinositol 3-kinase. However, the precise role for LAT during NK cell activation and particularly its potential role in NK cell-mediated cytotoxicity have not been described. In this paper, we provide genetic, biochemical, and functional evidence that supports a key role for LAT in the regulation of human NK cell-mediated cytotoxicity.
| Materials and Methods |
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Unless otherwise indicated, all chemicals were obtained from
Sigma (St. Louis, MO). Human NK clones were cloned and passaged as
previously described 23 . Briefly, CD16+ PBL from healthy
donors were cloned by limiting dilution in RPMI 1640 supplemented with
10% human serum and 20 U/ml of IL-2. Freshly isolated PBMC were
obtained from healthy human donors and were monocyte-depleted by
adhesion to tissue-culture flasks. The human Jurkat T cell line, murine
mastocytoma line P815, and anti-CD3-producing hybridoma OKT3
(murine IgG2a) were obtained from American Type Culture Collection
(Manassas, VA). The HLA class I-deficient C1R and 721.221 cell lines
were kindly provided by Peter Cresswell (Yale University, New Haven,
CT) and Peter Parham (Stanford University, Palo Alto, CA),
respectively. The LAT-deficient ANJ3 clone was derived by selection of
randomly mutagenized Jurkat E6 cells for stable somatic mutants bearing
defects in TCR-induced Ca2+ mobilization 24 .
Anti-phosphotyrosine mAb 4G10 was purchased from Upstate Biotechnology
(Lake Placid, NY). Rabbit polyclonal antiserum to LAT was obtained by
immunization of rabbits with keyhole limpet hemocyanin-conjugated LAT
peptide (amino acids 106131). Polyclonal antisera to Vav, SLP-76, and
PLC-
1 have been previously described (Refs. 14, 15, and 7,
respectively).
Cell stimulation
Cells were stimulated as previously described 25 and lysed in
buffer containing 20 mM Tris-HCl, 40 mM NaCl, 5 mM EDTA, 50 mM NaF, 30
mM Na4P2O7, 0.1% BSA, 1 mM
Na3VO4, 1 mM PMSF, 5 µg/ml aprotinin, 5
µg/ml leupeptin, and 1% Triton X-100 (pH 7.4). Insoluble material
was removed by centrifugation for 5 min at 14,000 rpm. Cell lysates
were subjected to immunoprecipitation for 12 h with the indicated
rabbit antiserum bound to protein A-Sepharose beads. Eluted proteins
were resolved by SDS-PAGE and transferred electrophoretically to
Immobilon-P membranes (Millipore, Bedford, MA). Tyrosine
phosphorylated proteins were detected with 4G10 mAb,
followed by sheep anti-mouse IgG coupled to horseradish peroxidase
(Amersham, Buckinghamshire, England) and with the enhanced
chemiluminescence detection system (Amersham). LAT, PLC-
1, SLP76,
and Vav were analyzed with specific rabbit antisera and detected with
protein A-horseradish peroxidase.
Vaccinia viruses
Recombinant vaccinia viruses encoding myc-tagged, wild-type LAT
(myc.LAT wt), a double tyrosine mutant (myc.LAT Y171/191F), CD16, and
constructs were made as previously described 14 . Cells (2 x
106/ml) were infected at a multiplicity of infection of
20:1 for 1 h in serum-free RPMI 1640 at 37°C and then for the
remainder of the indicated infection time at 106 cells/ml
in RPMI 1640 supplemented with 10% bovine calf serum.
Cytotoxicity assays
The Cr51-release assays measuring direct NK cell-mediated cytotoxicity or reverse ADCC were performed as previously described 23 .
| Results and Discussion |
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RIII
Ab (3G8) for the indicated periods of time, and the phosphotyrosine
content of LAT-immunoprecipitates was detected. As shown in Fig. 1
RIII stimulation 5, 6 .
|
RIII
cross-linking, stimulation of NK cells with HLA class I-deficient,
NK-sensitive C1R cells led to an increase in the tyrosine
phosphorylation of LAT (Fig. 1
Following TCR stimulation of T cells, LAT associates with a number of
signaling molecules, including PLC-
and Grb2 20 . We also detected
coimmunoprecipitation of LAT with PLC-
following NK cell stimulation
with anti-FcR Ab (data not shown). Association of LAT with PLC-
implied its potential role in coupling receptor stimulation to PLC-
tyrosine phosphorylation. To test if LAT is necessary
for the activation of PLC-
, we used a LAT-deficient Jurkat subline,
ANJ3, as a genetic model (Fig. 2
A). Wild-type Jurkat, ANJ3,
or ANJ3 cells reconstituted with the myc-tagged wild-type LAT
(ANJ3-wt65, ANJ3-wt67) were infected with CD16- and
-encoding
recombinant vaccinia viruses. Fc
RIII was equally expressed on the
surface of each of the T cell lines (data not shown). Stimulation of
ANJ3 cells through Fc
RIII or TCR leads to the tyrosine
phosphorylation of multiple cellular proteins,
similarly to the stimulation of parental Jurkat cells (Fig. 2
B). However, although LAT does not seem to be required for
the phosphorylation of many of the signaling proteins
(SLP-76 and ZAP-70, for example), a selective loss of
phosphorylation is detected for a band that migrates in
the same position as PLC-
1. To confirm the identity of this band as
PLC-
1, we repeated the experiment using PLC-
1 immunoprecipitates.
As shown in Fig. 2
C, PLC-
1 is tyrosine
phosphorylated upon stimulation of Jurkat cells
(lanes 2 and 3). This inducible
phosphorylation is completely blocked in the
LAT-deficient ANJ3 cells (lanes 5 and 6).
Reconstitution of the wild-type LAT into ANJ3 cells rescues the
phosphorylation of PLC-
1 (lanes 8
and 9). These results directly demonstrate the regulatory
role of LAT in the activation of PLC-
induced by the TCR and
Fc
RIII receptors.
|
R mAb (reverse ADCC; Fig. 3
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Another potential function of LAT is the regulation of inhibitory
signaling in NK cells. Interaction with MHC class I-expressing target
cells led to dephosphorylation of p36 and prevented
its association with PLC-
and adaptor protein Grb2 19 . The loss of
association between PLC-
and p36 corresponded with diminished IP3
production but did not influence phosphorylation of
PLC-
upon stimulation, thus implicating spatial restraints as a
mechanism of regulating PLC-
activity. In other studies, PLC-
was
found to be dephosphorylated upon killer cell inhibitory
receptor engagement 26, 27 . Thus, the regulation of signals initiated
by the inhibitory receptors on NK cells and the potential role of LAT
in inhibitory signaling are not resolved.
In this paper, we have shown through biochemical and functional assays the importance of LAT for the activation of human NK cells. The role of the association of LAT with other signaling proteins, as well as the exact place of LAT in the signaling cascade from MIRRs, awaits future investigations.
| Footnotes |
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2 Address correspondence and reprint requests to Dr. Paul J. Leibson, Department of Immunology, Mayo Clinic, 200 First Street SW, Rochester, MN 55905. E-mail address: ![]()
3 Abbreviations used in this paper: ADCC, Ab-dependent cell-mediated cytotoxicity; PLC, phospholipase C; MIRR, multichain immune recognition receptors. ![]()
Received for publication October 30, 1998. Accepted for publication December 30, 1998.
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