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T cell receptor (TCR)-induced death of immature CD4+CD8+ thymocytes by two distinct mechanisms differing in their requirement for CD28 costimulation: implications for negative selection in the thymus.

Punt JA, Havran W, Abe R, Sarin A, Singer A - J. Exp. Med. (1997)

Bottom Line: Negative selection is the process by which the developing lymphocyte receptor repertoire rids itself of autoreactive specificities.One mechanism requires simultaneous TCR and costimulatory signals initiated by CD28.We propose that these mechanisms represent two distinct clonal deletion strategies that are differentially implemented during development depending on whether immature thymocytes encounter antigen in the thymic cortex or thymic medulla.

View Article: PubMed Central - PubMed

Affiliation: Experimental Immunology Branch, National Cancer Institute, Bethesda, Maryland 20892, USA.

ABSTRACT
Negative selection is the process by which the developing lymphocyte receptor repertoire rids itself of autoreactive specificities. One mechanism of negative selection in developing T cells is the induction of apoptosis in immature CD4+CD8+ (DP) thymocytes, referred to as clonal deletion. Clonal deletion is necessarily T cell receptor (TCR) specific, but TCR signals alone are not lethal to purified DP thymocytes. Here, we identify two distinct mechanisms by which TCR-specific death of DP thymocytes can be induced. One mechanism requires simultaneous TCR and costimulatory signals initiated by CD28. The other mechanism is initiated by TCR signals in the absence of simultaneous costimulatory signals and is mediated by subsequent interaction with antigen-presenting cells. We propose that these mechanisms represent two distinct clonal deletion strategies that are differentially implemented during development depending on whether immature thymocytes encounter antigen in the thymic cortex or thymic medulla.

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A CD28-independent mechanism of TCR-mediated death  of DP thymocytes. DP thymocytes from either B6 (Ly 5.1+) or CD28  KO (Ly5.1+) mice were co-cultured with Ly 5.2+ APC in the presence  or absence of platebound anti-TCR. Harvested cells were stained with  both anti-Ly5.1 antibody and EtBr. DP thymocytes were distinguished  from APCs by expression of Ly5.1.
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Figure 6: A CD28-independent mechanism of TCR-mediated death of DP thymocytes. DP thymocytes from either B6 (Ly 5.1+) or CD28 KO (Ly5.1+) mice were co-cultured with Ly 5.2+ APC in the presence or absence of platebound anti-TCR. Harvested cells were stained with both anti-Ly5.1 antibody and EtBr. DP thymocytes were distinguished from APCs by expression of Ly5.1.

Mentions: Our studies using an array of antibodies to known molecules on the surface of DP thymocytes did not identify any proteins other than CD28 that could cooperate with TCR to induce DP thymocyte apoptosis. To determine if any such molecule existed, we asked whether APC expressed ligands, known or unknown, for surface molecules that would cooperate with TCR to induce DP thymocyte apoptosis. In this coculture experiment, we stimulated DP thymocytes from wild-type and CD28 KO mice with platebound anti-TCR in the presence of APC. We confined our assessment of EtBr staining to DP thymocytes by excluding APCs from the analysis using an allelic marker, Ly5. DP thymocytes from wild-type mice died in response to TCR stimulation in the presence of APC. More importantly, DP thymocytes from CD28 KO mice also died in response to TCR stimulation in the presence of APC, indicating that APC possess ligands that engage molecules other than CD28 that can induce death of TCR-stimulated thymocytes. It is important to note that this CD28-independent mechanism of DP thymocyte death strictly requires engagement of TCR on DP thymocytes, as APCs did not kill unstimulated CD28 KO DP thymocytes (Fig. 6). From these data we can conclude that APC express or secrete ligands for surface molecules on DP thymocytes that are capable of cooperating with the TCR to induce TCR-dependent but CD28-independent apoptosis.


T cell receptor (TCR)-induced death of immature CD4+CD8+ thymocytes by two distinct mechanisms differing in their requirement for CD28 costimulation: implications for negative selection in the thymus.

Punt JA, Havran W, Abe R, Sarin A, Singer A - J. Exp. Med. (1997)

A CD28-independent mechanism of TCR-mediated death  of DP thymocytes. DP thymocytes from either B6 (Ly 5.1+) or CD28  KO (Ly5.1+) mice were co-cultured with Ly 5.2+ APC in the presence  or absence of platebound anti-TCR. Harvested cells were stained with  both anti-Ly5.1 antibody and EtBr. DP thymocytes were distinguished  from APCs by expression of Ly5.1.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2199155&req=5

Figure 6: A CD28-independent mechanism of TCR-mediated death of DP thymocytes. DP thymocytes from either B6 (Ly 5.1+) or CD28 KO (Ly5.1+) mice were co-cultured with Ly 5.2+ APC in the presence or absence of platebound anti-TCR. Harvested cells were stained with both anti-Ly5.1 antibody and EtBr. DP thymocytes were distinguished from APCs by expression of Ly5.1.
Mentions: Our studies using an array of antibodies to known molecules on the surface of DP thymocytes did not identify any proteins other than CD28 that could cooperate with TCR to induce DP thymocyte apoptosis. To determine if any such molecule existed, we asked whether APC expressed ligands, known or unknown, for surface molecules that would cooperate with TCR to induce DP thymocyte apoptosis. In this coculture experiment, we stimulated DP thymocytes from wild-type and CD28 KO mice with platebound anti-TCR in the presence of APC. We confined our assessment of EtBr staining to DP thymocytes by excluding APCs from the analysis using an allelic marker, Ly5. DP thymocytes from wild-type mice died in response to TCR stimulation in the presence of APC. More importantly, DP thymocytes from CD28 KO mice also died in response to TCR stimulation in the presence of APC, indicating that APC possess ligands that engage molecules other than CD28 that can induce death of TCR-stimulated thymocytes. It is important to note that this CD28-independent mechanism of DP thymocyte death strictly requires engagement of TCR on DP thymocytes, as APCs did not kill unstimulated CD28 KO DP thymocytes (Fig. 6). From these data we can conclude that APC express or secrete ligands for surface molecules on DP thymocytes that are capable of cooperating with the TCR to induce TCR-dependent but CD28-independent apoptosis.

Bottom Line: Negative selection is the process by which the developing lymphocyte receptor repertoire rids itself of autoreactive specificities.One mechanism requires simultaneous TCR and costimulatory signals initiated by CD28.We propose that these mechanisms represent two distinct clonal deletion strategies that are differentially implemented during development depending on whether immature thymocytes encounter antigen in the thymic cortex or thymic medulla.

View Article: PubMed Central - PubMed

Affiliation: Experimental Immunology Branch, National Cancer Institute, Bethesda, Maryland 20892, USA.

ABSTRACT
Negative selection is the process by which the developing lymphocyte receptor repertoire rids itself of autoreactive specificities. One mechanism of negative selection in developing T cells is the induction of apoptosis in immature CD4+CD8+ (DP) thymocytes, referred to as clonal deletion. Clonal deletion is necessarily T cell receptor (TCR) specific, but TCR signals alone are not lethal to purified DP thymocytes. Here, we identify two distinct mechanisms by which TCR-specific death of DP thymocytes can be induced. One mechanism requires simultaneous TCR and costimulatory signals initiated by CD28. The other mechanism is initiated by TCR signals in the absence of simultaneous costimulatory signals and is mediated by subsequent interaction with antigen-presenting cells. We propose that these mechanisms represent two distinct clonal deletion strategies that are differentially implemented during development depending on whether immature thymocytes encounter antigen in the thymic cortex or thymic medulla.

Show MeSH
Related in: MedlinePlus