Review Article | Published:

Cell death and immunity

Negative selection — clearing out the bad apples from the T-cell repertoire

Nature Reviews Immunology volume 3, pages 383391 (2003) | Download Citation

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Abstract

Dead cells are a prominent feature of the thymic landscape as only 5% of developing thymocytes are exported as mature T cells. The remaining thymocytes die by one of two mechanisms; most thymocytes die because they are not positively selected and do not receive a survival signal, whereas a minority of thymocytes undergo T-cell receptor (TCR)-mediated apoptosis, a process known as negative selection. Negative selection is extremely important for establishing a functional immune system, as it provides an efficient mechanism for ridding the T-cell repertoire of self-reactive and potentially autoimmune lymphocytes. This review discusses several cellular and molecular aspects of negative selection.

Key points

  • CD4+CD8+ double-positive (DP) thymocytes undergo one of three fates in the thymus: positive selection, negative selection or death by neglect. Negative selection of thymocytes that express T-cell receptors (TCRs) with high affinity for self-peptide–MHC (5% of the total) deletes potentially self-reactive thymocytes, generating a largely self-tolerant peripheral T-cell repertoire.

  • Most negative selection is thought to occur in the thymic medulla as this contains two types of specialized antigen-presenting cell — dendritic cells and thymic epithelial cells (TECs). Medullary TECs transcribe genes that are normally expressed in peripheral tissues.

  • Negative selection can occur before or after positive selection and in thymocytes at all stages of development. So, positive and negative selection are probably independent, and not sequential, events.

  • Negative selection in response to high-affinity ligands might be due to increased TCR occupancy or a slower 'off-rate' (kinetic proofreading).

  • A second co-stimulatory signal, in addition to the TCR signal, might be required for negative selection. However, there are discrepancies between blocking experiments and genetically deficient mice in this regard.

  • The kinetics of mitogen-activated protein kinase (MAPK) signalling are thought to determine positive- versus negative-selection signals. Extracellular signal-regulated kinase (ERK) is induced more rapidly during negative selection, which might determine the particular transcription factors (such as NUR77 and NF-κB) that are triggered.

  • Linker for activation of T cells (LAT) and phosphatase and tensin homologue (PTEN) are also thought to be involved in negative selection.

  • Death-domain-containing proteins, such as CD95 (FAS) and tumour-necrosis factor receptors (TNFRs), that are involved in the apoptosis of peripheral T cells are not thought to be important for negative selection. Rather, negative selection might occur independently of death domain signalling.

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Acknowledgements

I am indebted to the efforts of many laboratories that have contributed to this field and apologize to those whose work was not cited. Thanks to J. Sprent and G. Werlen for interesting discussions, and D. Gil-Pagès, B. Hausmann, D. Naeher, A. Schrum, G. Werlen and E. Teixeiro-Pernas for reading the manuscript. My laboratory is supported by grants from the Swiss National Science Foundation and Hoffmann La Roche, Ltd., and by a generous gift from Novartis, AG.

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  1. Laboratory of Transplantation Immunology and Nephrology, University Hospital Basel, Hebelstrasse 20, CH-4031 Basel, Switzerland.  ed.palmer@unibas.ch

    • Ed Palmer

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Glossary

RADIATION BONE-MARROW CHIMAERAS

Animals are lethally irradiated to destroy most of their haematopoietic-lineage cells then given bone-marrow cells from a donor animal to reconstitute their haematopoietic cells. In the context of thymic development, positive selection is mediated by radiation-resistant cells in the cortex of the host's thymus. Negative selection is mediated by residual epithelial cells from the host's thymus and, by antigen-presenting cells that are derived from the transferred bone marrow.

ENDOGENOUS SUPERANTIGENS

These are type II cell-surface proteins, which are encoded by mouse mammary tumour viruses (MMTVs) that have integrated into the mouse germline. These viral proteins are presented by MHC class II molecules and bind to the T-cell receptor through its Vβ domain. Because these endogenous proteins are self-antigens and are expressed by antigen-presenting cells in the thymus, they delete large numbers of thymocytes that express a particular Vβ domain.

PEPTIDE–MHC TETRAMERS

Recombinant MHC molecules can be engineered to contain an antigenic peptide in their peptide-binding groove. These peptide–MHC monomers are biotinylated and then used to form tetrameric complexes by binding to fluorescently labelled streptavidin (streptavidin has four biotin-binding sites). Tetramers can be used to stain T cells expressing T-cell receptors that are specific for that peptide–MHC complex.

SURFACE PLASMON RESONANCE

The detection of alterations in plasmon waves generated at a metal–liquid interface. Changes in surface plasmon resonance are a function of the mass of molecules bound to the interface, so this technique allows sensitive detection of ligand binding in real time without requiring the chemical modification of ligands to enable their detection.

FETAL THYMIC ORGAN CULTURE

(FTOC). Removal of day-16 fetal thymi allows the analysis of antigen-driven positive- and negative-selection events during in vitro culture.

SUBTRACTIVE HYBRIDIZATION

A technique that is used to enrich messenger RNA (complementary DNA) sequences that are specifically expressed by a particular cell line or under particular physiological conditions.

CASPASES

Family of cytosolic proteases that contain a cysteine residue in the active site and that cleave their substrate after an aspartic-acid residue. They can be divided into inflammatory caspases (1, 4, 5 and 11), which cleave and activate pro-inflammatory cytokines, and pro-apoptotic caspases, which cleave and activate pro-apoptotic substrates. Pro-apoptotic caspases consist of initiator caspases (2, 8 and 9), which, in turn, cleave and activate effector caspases (3, 6 and 7).

HY-TCR TRANSGENIC MICE

Mice transgenic for an αβ T-cell receptor specific for a peptide derived from the male antigen HY.

APOPTOSOME

An apoptotic protein complex formed from the union of apoptotic protease activating factor 1 (APAF1), cytochrome c and dATP with pro-caspase-9. Complex formation leads to the cleavage and activation of caspase-9, which activates caspase-3 and other effector caspases, leading to cell death.

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