The activation threshold of CD4+ T cells is defined by TCR/peptide-MHC class II interactions in the thymic medulla.
Immature thymocytes that are positively selected based upon their response to self-peptide-MHC complexes develop into mature T cells that are not overtly reactive to those same complexes. Developmental tuning is the active process through which TCR-associated signaling pathways of single-positive thymocytes are attenuated to respond appropriately to the peptide-MHC molecules that will be encountered in the periphery. In this study, we explore the mechanisms that regulate the tuning of CD4(+) single-positive T cells to MHC class II encountered in the thymic medulla. Experiments with murine BM chimeras demonstrate that tuning can be mediated by MHC class II expressed by either thymic medullary epithelial cells or thymic dendritic cells. Tuning does not require the engagement of CD4 by MHC class II on stromal cells. Rather, it is mediated by interactions between MHC class II and the TCR. To understand the molecular changes that distinguish immature hyperactive T cells from tuned mature CD4(+) T cells, we compared their responses to TCR stimulation. The altered response of mature CD4 single-positive thymocytes is characterized by the inhibition of ERK activation by low-affinity self-ligands and increased expression of the inhibitory tyrosine phosphatase SHP-1. Thus, persistent TCR engagement by peptide-MHC class II on thymic medullary stroma inhibits reactivity to self-Ags and prevents autoreactivity in the mature repertoire.
Stephen
Tom Li
TL
Department of Medicine, University of Pennsylvania School of Medicine, Philadelphia, PA 19104, USA.
Tikhonova
Anastasia
A
Riberdy
Janice M
JM
Laufer
Terri M
TM
D000818:Animals; D015496:CD4-Positive T-Lymphocytes / Q000201:enzymology / Q000276:immunology* / Q000378:metabolism; D002454:Cell Differentiation / Q000235:genetics / Q000276:immunology; D015536:Down-Regulation / Q000235:genetics / Q000276:immunology; D004847:Epithelial Cells / Q000276:immunology / Q000378:metabolism; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D000949:Histocompatibility Antigens Class II / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D008024:Ligands; D008213:Lymphocyte Activation / Q000235:genetics / Q000276:immunology*; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D008822:Mice, Transgenic; D010446:Peptide Fragments / Q000276:immunology* / Q000378:metabolism; D053824:Protein Tyrosine Phosphatase, Non-Receptor Type 6 / Q000096:biosynthesis / Q000235:genetics; D011948:Receptors, Antigen, T-Cell / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D013950:Thymus Gland / Q000166:cytology / Q000254:growth & development / Q000276:immunology* / Q000378:metabolism*
D000949:Histocompatibility Antigens Class II; D008024:Ligands; D010446:Peptide Fragments; D011948:Receptors, Antigen, T-Cell; D048049:Extracellular Signal-Regulated MAP Kinases; D053824:Protein Tyrosine Phosphatase, Non-Receptor Type 6; C509248:Ptpn6 protein, mouse