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CUTTING EDGE |
Molecular Immunology Group, Institute of Molecular Medicine, John Radcliffe Hospital, Oxford, United Kingdom
| Abstract |
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| Introduction |
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Until recently, two cell surface receptors, TNF-R1 and Fas, had been identified that could promote apoptosis when contacted by their ligands, TNF and Fas-L (5). Recently discovered death receptors include LARD/DR3/Wsl-1/Apo-3/TRAMP (TNF receptor-related apoptosis-mediating protein), which is expressed on lymphocytes and regulated by alternative splicing (6, 7, 8, 9, 10), and the receptors for TRAIL/Apo-2L (11, 12), DR4, and TRICK2 (13, 14).
DR4 and TRICK2 are expressed on PBL, but the expression of TRAIL on a wide variety of tissues provides a potential problem for circulating lymphocytes. We have cloned a new TRAIL receptor, LIT (lymphocyte inhibitor of TRAIL), which we believe can competitively inhibit TRAIL to protect resting lymphocytes from inappropriate apoptosis.
| Materials and Methods |
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The EST subset of the NCBI (National Center for Biological Information) database was screened for homology to TRICK2 and DR4; four clones were identified and sequenced. Multiple tissue human Northern blots (Clontech, Palo Alto, CA) were hybridized with a 32P random-primed probe encompassing the nerve growth factor receptor (NGFR) domains of LIT.
CD4+, CD8+, and B cells were purified by absorption onto magnetic beads coated with Abs to CD4, CD8, and CD19. RT-PCR (30 cycles) was conducted on cDNA produced from these samples with primer pairs spanning around 400 bp of the extracellular domains of LIT, TRICK2, and DR4 or 600 bp of TRAIL.
TRAIL apoptosis assay
293 T cells were cotransfected by calcium phosphate
precipitation with pCDNA3 TRAIL (40 µg/15-cm plate) and CD8
cDNA
(10 µg/plate). Cotransfected cells were isolated using
anti-CD8-coated magnetic beads (Dynal, Chantilly, VA) at 48 h.
Cells (105/well) were seeded into 96-well plates with
5 x 103 51Cr-labeled Jurkat T cells and
soluble Fc fusion proteins, used at 20 µg/ml. Supernatants were
harvested at 12 h and counted by scintillation on a beta plate
counter. The percentage of specific release was calculated as
((experimental - background)/(maximum - background)) x
100. Fusions between the IgG1 constant domain and extracellular domains
of DR4 amino acids 104 to 217, Fas amino acids 17 to 173, TNF-R1 amino
acids 24 to 210, TRICK2 amino acids 57 to 183, and LIT amino acids 35
to 152 were produced in 293T cells.
| Results and Discussion |
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Searching the expressed sequence tag subset of the NCBI database
for the sequences of TRICK2 and DR4 revealed a novel sequence with a
high degree of homology in the extracellular domain. Four clones were
sequenced, and the predicted amino acid sequence of LIT is shown in
Figure 1
aligned with the extracellular
domains of TRICK2 and DR4. LIT is a type I membrane protein and has a
23-amino acid signal sequence, a 213-amino acid extracellular domain,
and a 23-amino acid transmembrane domain. The mature protein has a
predicted molecular mass of 24.9 kDa and has five consensus sites for
N-linked glycosylation. The extracellular domain has two and
a half NGFR repeats that have 54 and 69% amino acid identity to the
repeats of TRICK2 and DR4, respectively. During the preparation of this
manuscript, reports describing the cloning of an identical molecule
TRID/DcR1 have appeared (15, 16).
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Of the death receptors described to date, TNF-R1 and LARD have four NGFR repeats; Fas has three; DR4, TRICK2, LIT, and the chicken death receptor CAR1 (17) have two. The NGFR repeats in LIT are most similar in structure to domains 2 and 3 of TNF-R1. These two repeats make contacts with the ligand molecule in the TNF/TNF-R1 crystal structure (18). Thus, the two domain receptors contain the core binding surface, and the function of the extra repeats found in TNF-R1 and LARD is unclear, although the N-terminal repeat may help to prevent self aggregation (19).
LIT is a receptor for TRAIL
Biotinylated LIT-Fc fusion protein was used to stain 293T cells
cotransfected with CD8/TRAIL or CD8/empty vector (Fig. 2
A). This allowed us to
gate for TRAIL-transfected cells using an anti-CD8 mAb. Cells
transfected with TRAIL, but not with an irrelevant second vector, stain
positively with LIT-Fc, demonstrating that LIT is a receptor for
TRAIL.
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Expression of LIT
Northern blot analysis of LIT expression is shown in Figure 3
. Several mRNA species can be seen on
this blot, running at 1.4, 2, 2.4, 3, 3.8, 4, and 6 kb. Expression is
particularly marked in PBL, where the 1.4-kb band predominates; this
corresponds to the LIT clones we sequenced. The 1.8-kb message
predominates in spleen and is also found in thymus. We were not able to
isolate any of the larger LIT transcripts from cDNA libraries, and they
probably represent alternative 3' untranslated sequences, although the
possibility does exist that some may encode isoforms of LIT possessing
a cytoplasmic domain.
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Although TRAIL and its death receptors are quite widely expressed, these molecules may have important immunoregulatory roles. The up-regulation of TRAIL upon lymphocyte activation mirrors the rise in Fas-L expression and may provide another weapon to the armory of cytotoxic T cells. The down-regulation of LIT upon lymphocyte activation exposes activated cells to apoptosis and may thus play a role in activation-induced death of lymphocytes. Finally, the majority of cell in the thymus are immature lymphocytes, most of which will die by apoptosis; it may be that the low expression of LIT on thymocytes exposes them to TRAIL apoptosis during negative selection.
Fas-L expression by HIV-infected cells and malignant tumors (22, 23) may deliver a death signal to cytotoxic T cells expressing Fas, protecting the diseased cells from immune attack. It is therefore rather puzzling that tumors that would normally be supposed to develop mechanisms to evade apoptosis seem to lose expression of the protective LIT molecule and become exposed to TRAIL killing.
A complex regulatory mechanism needs to be in place to protect lymphocytes that express a set of apoptosis receptors from an untimely death. Several mechanisms have evolved to mediate this protection, such as tight regulation of ligand expression in the case of Fas, alternative pre-mRNA splicing to exclude the death domain in LARD, expression of antiapoptotic proteins, such as Bcl-2, and interfering with the signaling cascade by molecules such as Fas-associated protein-1 and the recently described cellular FLICE inhibitory proteins (FLIPs) (24, 25, 26). Our finding of LIT, a molecule without a cytoplasmic domain, represents an additional mechanism to control apoptosis signaled by cell surface receptors. A more detailed analysis of the expression and function of the TRAIL/DR4/TRICK2/LIT molecules in the immune system awaits the generation of an mAb panel and knockout animals.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Gavin R. Screaton, Molecular Immunology Group, Institute of Molecular Medicine, John Radcliffe Hospital, Oxford, United Kingdom OX3 9DS. ![]()
3 Abbreviations used in this paper: Fas-L, Fas ligand; LARD, lymphocyte-associated receptor of death; TRICK2, tumor necrosis factor-related apoptosis-inducing ligand receptor inducer of cell killing-2; TRAIL, tumor necrosis factor-related apoptosis-inducing ligand; LIT, lymphocyte inhibitor of tumor necrosis factor-related apoptosis-inducing ligand; NGFR, nerve growth factor receptor; RT-PCR, reverse transcription-polymerase chain reaction. ![]()
Received for publication August 18, 1997. Accepted for publication October 7, 1997.
| References |
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