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Department of Immunobiology, DNAX Research Institute of Molecular and Cellular Biology, Palo Alto, CA 94304; and
Department of Immunology, University Hospital, Utrecht, The Netherlands
| Abstract |
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| Introduction |
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The role of inhibitory receptors has been extensively studied on NK cells. Ligation of human killer cell inhibitory receptors (KIRs) on NK cells by MHC class I molecules on the target cells, leads to inhibition of target cell lysis (reviewed in 3). KIRs are also expressed on a small subset of human T cells, predominantly CD28- memory cells, and are functional in inhibiting cytotoxicity and cytokine production of these cells (4, 5, 6, 7). Recently, additional families of inhibitory receptors have been identified both in humans and mice (reviewed in 8). Many of these inhibitory receptors are related to the KIRs (3). In mice, the gp49B molecule (9) functions as an inhibitory receptor on mouse mast cells and NK cells (10, 11). Similarly, cross-linking of gp91 or paired Ig-like receptors leads to inhibition of B cell receptor-mediated activation of B cells in vitro (12, 13, 14, 15). The ligands of gp49 and paired Ig-like receptors are not known yet.
In humans, a novel group of inhibitory molecules has been identified that is mainly expressed on monocytes and B cells, although some family members are found on NK and T cells. These molecules have been called Ig-like transcripts, leukocyte Ig-like receptors, monocyte/macrophage Ig-like receptors, or HM18, by the laboratories that reported the first cDNA sequences (16, 17, 18, 19, 20, 21). Some of the family members bind MHC class I molecules or viral MHC class I homologues (18, 19, 22). Cross-linking of the receptors by mAb inhibits the Ag presentation by APC (17), as well as activation of B cells, T cells, NK cells, and macrophages (22, 23).
We previously reported on the identification of leukocyte-associated Ig-like receptor-1 (LAIR-1) (24). LAIR-1 is a member of the Ig superfamily that is expressed on the majority of human PBMCs, including NK, T, B, monocytes, and dendritic cells, as well as the majority of thymocytes. It is a type I transmembrane glycoprotein with a single Ig-like domain in the extracellular region and a cytoplasmic tail containing two ITIMs. Cross-linking of LAIR-1 on human NK cells by mAb delivers a potent inhibitory signal that is capable of decreasing target cell lysis by both resting and activated NK cells in vitro (24).
Compared with other inhibitory receptors reported so far, LAIR-1 is unique with its broad expression pattern, which implies that it will play a role in a variety of immune interactions. Here we describe another member of the LAIR family and demonstrate that LAIR-1 can function as an inhibitory receptor on effector T cells.
| Materials and Methods |
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Peripheral blood from healthy donors was purchased from Stanford Blood Center (Stanford, CA). PBMC were isolated by Ficoll-Hypaque centrifugation. T cell clones (TCCs) were established as described (4) and maintained in culture using the method of Yssel et al. (25).
The HLA class I-deficient EBV-transformed lymphoblastoid cell line
721.221 has been described (26). DT287 is a stable transfectant of
721.221, expressing human Fc
RII (CD32), generated in our laboratory.
293 cells expressing SV40 large T Ag (293T) were generously provided by
Dr. T. Kitamura (DNAX, Palo Alto, CA). Jurkat and P815 were obtained
from the American Type Culture Collection (Manassas, VA). YT-2C2 is a
NK-like tumor cell line that was kindly provided by Dr. K. Smith
(27).
Monoclonal Abs
The mAb against LAIR-1 (DX26, IgG1) (24) has been described. The anti-CD3 mAb (CLB-T3/4.1, IgG1) was a kind gift of Dr. René van Lier (CLB, Amsterdam, The Netherlands). All other mAbs were generously provided by Becton Dickinson Immunocytometry Systems (San Jose, CA).
Cytotoxicity assay
Cell lines were labeled with 51Cr and used as targets in a 4- or 7-h radioisotope release assay, as described (28). Data are expressed as the mean of triplicate cultures. Spontaneous radioisotope release did not exceed 10% of maximum release as determined by lysis of target cells with 10% Triton-X 100. The percentage of specific lysis was calculated as: [(cpm specific 51Cr release - cpm spontaneous 51Cr release)/(cpm maximum 51Cr release - cpm spontaneous 51Cr release)] x 100.
Expression cloning
A cDNA library from Jurkat T cells was generously provided by Dr. Terrill McClanahan (DNAX). cDNA cloning by transient expression in 293T cells was performed as described (29) with modifications (30).
cDNA was sequenced using the dideoxy termination technique with Sequenase 2.0 kit (United States Biochemical, Cleveland, OH) and by using an automated nucleic acid sequencer (Applied Biosystems, Foster City, CA).
Generation of stable transfectants of LAIR-1
Stable transfectants of LAIR-1a and -1b were generated as described (31). In short, LAIR-1a and LAIR-1b-encoding cDNAs were subcloned into the pMX puro retroviral vector (provided by T. Kitamura, DNAX), and transfected into 293T cells together with a packaging vector (31, 32). The supernatant was used to infect YT.2C2 cells. Cells were drug selected and transfectants with high expression of LAIR were sorted, using the FACStar flow cytometer (Becton Dickinson).
Biochemistry
To biochemically characterize the Ags recognized by anti-LAIR-1 mAb, cells were labeled with 125I and Ags immunoprecipitated by the procedure described previously (33). Immunoprecipitates were separated by SDS-PAGE and visualized by exposure to a Phosphor Imager screen.
| Results |
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We previously reported that human NK cells express two
biochemically distinct forms of LAIR-1. The majority of the LAIR-1
protein in NK cells has an apparent molecular weight of
40 kDa, but
we also detected a form of LAIR-1 of
34 kDa (24). Molecular cloning
of LAIR-1 from an NK cell library, revealed a cDNA that encoded a
protein of
40 kDa (now designated LAIR-1a), in agreement with the
majority of the immunoprecipitated LAIR-1 protein in NK cells (24).
Immunoprecipitation of LAIR proteins from the human T cell tumor Jurkat
revealed exclusive expression of the
34 kDa species, while human
PBMC (containing a majority of T cells) expressed both, but seem to
have more of this short form (Fig. 1
).
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34 kDa LAIR-1
protein in Jurkat cells (Fig. 3
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To compare the capacity of LAIR-1a and LAIR-1b to function as
inhibitory receptors, the LAIR-1-negative NK cell line, YT.2C2, was
stably transfected with LAIR-1a or LAIR-1b. Anti-LAIR-1 mAb efficiently
inhibited the cytotoxic activity of both the LAIR-1a and
LAIR-1b-transfectants but not the wild-type YT.2C2 against the human
Fc
RII (CD32)-transfected EBV-transformed B cell line, 721.221/CD32
(Fig. 4
, lower panels).
Cytolysis of the FcR-negative parent line, 721.221, however, was not
affected by anti-LAIR-1 mAb, indicating that FcR cross-linking of
LAIR-1a or -1b was required to deliver the negative signal (Fig. 4
, upper panels). These data confirmed that the inhibitory
effect of anti-LAIR-1 mAb on cells naturally expressing LAIR-1 (24)
was due to LAIR-1 cross-linking. Furthermore, it showed that both
LAIR-1a and LAIR-1b were able to function as inhibitory receptors in an
NK cell line upon mAb cross-linking.
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Since LAIR-1 is expressed on the majority of human T cells, we set out to study whether it is also capable of inhibiting T cell function, in addition to its inhibitory function on NK cells. Initially, we did not see any effect of anti-LAIR-1 mAb on T cell function in vitro. Proliferation of peripheral blood T cells in response to recall Ag, alloantigen, or anti-CD3 mAb could not be inhibited by anti-LAIR-1 mAb (data not shown). Cytotoxic activity against FcR-bearing targets mediated by several TCCs stimulated by anti-CD3 or superantigen could also not be inhibited by anti-LAIR-1 mAb (data not shown).
However, we did see inhibitory effects of the anti-LAIR-1 mAb when
we used TCCs generated from the CD28-negative subset of
CD3+CD8+ T cells from peripheral blood. Some of
these TCCs are able to exhibit spontaneous cytotoxic activity against
targets without addition of anti-TCR/CD3 mAb. Cytotoxic activity of
these TCCs against the EBV B cell line 721.221, transfected with the
human Fc
RII, was effectively inhibited by anti-LAIR-1 mAb (Fig. 5
A). Three clones were able to
also spontaneously lyse P815 target cells, and this cytotoxicity was
also inhibited by the addition of anti-LAIR-1 mAb (Fig. 5
B). This demonstrates that LAIR-1 can function to
inhibit the cytotoxic activity of human CD28- TCCs in
vitro.
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Peripheral blood T cells are able to lyse target cells in vitro
when stimulated via the TCR/CD3 complex. This cytotoxic activity is
thought to be due mainly to the "effector" population of
CD8+ T cells (34). PBMCs from 15 healthy donors were
assayed directly upon isolation in a 7-h 51Cr release assay
for lysis of P815 cells in the presence of anti-CD3 mAb. A total of
12 of 15 donors showed significant specific lysis. This lysis was
inhibited up to 90% by the addition of anti-LAIR-1 mAb in all but
one of these donors (Fig. 6
),
demonstrating that LAIR-1 is able to inhibit the cytotoxic activity of
circulating effector T cells. The cytolysis was not inhibited by a
control IgG1 mAb added in the same concentrations, excluding the
possibility that the inhibition was due to competition of anti-CD3
and anti-LAIR for FcRs (Fig. 6
C).
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| Discussion |
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NK cells express mainly LAIR-1a and to a lesser extent LAIR-1b (Fig. 1
)
(24). Jurkat T cells express LAIR-1b, and peripheral T cells seem to
have a preference for LAIR-1b (Fig. 1
). However, this is not absolute,
since RT-PCR analysis of mRNA of T cells from several different healthy
donors revealed the presence of both LAIR-1a and LAIR-1b transcripts
(data not shown). Being alternative splice variants, the expression of
LAIR-1a and LAIR-1b, could, in principle, be regulated in a cell-type
specific manner but could also represent simply a stochastic process.
Our data suggest that LAIR-1a and LAIR-1b do not differ functionally.
When transfected into the NK cell line YT.2C2, both forms are capable
of inhibiting the cytotoxic function of these cells. One cannot
exclude, however, that absence of the 17-aa stretch in the stalk region
might lead to a different conformation of the extracellular domain,
which could influence ligand-binding.
We previously reported on the molecular cloning of LAIR-2 (LAIR-2b in
Fig. 2
B) (24), a putative secreted family member of LAIR-1,
with 84% amino acid homology in the Ig domain. Searching a database of
expressed sequence tags (EST) revealed a LAIR-2-like clone (Genbank
accession number: AA133246; LAIR-2a in Fig. 2
B). The EST
clone was sequenced and was found to be identical to LAIR-2b, with the
exception of a 17-aa insertion (data not shown). The 17-aa stretch
present in LAIR-2a and missing in LAIR-2b is in exactly the same
position as the 17-aa stretch in LAIR-1 (Fig. 2
B). This
finding suggests that LAIR-1 and LAIR-2 are derived from a common
ancestral gene. The biological role of the secreted LAIR family members
is as yet unclear.
We also demonstrate here that LAIR-1 is not only an inhibitory receptor on NK cells, but that anti-LAIR-1 mAb is also able to inhibit cytotoxic responses in T cells, indicating that LAIR-1 provides a mechanism of regulation for T cells. However, there is a restriction as to which T cell functions can be inhibited via LAIR-1. Inhibition was only observed when T cells derived from the effector T cell population were used in cytotoxicity assays. First, inhibition of TCCs was only observed in the case of spontaneous cytotoxicity in vitro, which can only be mediated by a minority of the CD8+CD28- TCC. Similarly, Poggi et al. (36) reported on a restricted capacity of Abs recognizing the p40 molecule to inhibit T cell function. Anti-p40 mAb inhibited anti-Vß8-induced, but not anti-CD3-induced cytotoxicity of only some Vß8+ TCC (36). Recent studies have shown that anti-p40 mAbs stain LAIR-1 transfectants and therefore define the same Ag as anti-LAIR-1 mAb (L.M., unpublished observations).
We now also demonstrate inhibition of the cytotoxic activity of freshly isolated T cells from peripheral blood. This cytotoxicity is reported to be due mainly to the effector CTLs, which express the phenotype CD8+CD45RA+CD27- and are different from memory and naive cells (34). These T cells are all CD28- T cells and are known to have low proliferative potential and high perforin-mediated cytotoxic activity in vitro (37, 38). The number of CD28- cells increases with age (39) and these cells have shorter telomere lengths, reminiscent of an active replicative history (40). Our finding that LAIR-1 cross-linking can inhibit the cytotoxicity of these T cells suggests that LAIR-1 provides a mechanism for down-regulation of ongoing immune responses against tissues bearing ligands for this receptor.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence to Dr. Linde Meyaard, Department of Immunology, University Hospital Utrecht, Heidelberglaan 100 (Rm. F03.8.21), 3584 CX Utrecht, The Netherlands. E-mail address: ![]()
3 Abbreviations used in this paper: ITIM, immunoreceptor tyrosine-based inhibitory motif; KIR, killer cell inhibitory receptor; TCC, T cell clone; LAIR, leukocyte-associated Ig-like receptor. ![]()
Received for publication December 10, 1998. Accepted for publication February 25, 1999.
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