Intrathymic signalling in immature CD4+CD8+ thymocytes results in tyrosine phosphorylation of the T-cell receptor zeta chain.
Thymic selection of the developing T-cell repertoire occurs in immature CD4+CD8+ double-positive thymocytes and is thought to be mediated by signals transduced by T-cell antigen receptor (TCR) molecules and possibly by CD4 and CD8 accessory molecules as well. It is not known, however, which signal-transduction mechanisms function in immature CD4+CD8+ thymocytes on engagement of TCR, CD4 or CD8 molecules. In mature T cells, CD4 and CD8 molecules are each associated with the src-like protein tyrosine kinase p56 lck and signals transduced by TCR and CD4 activate tyrosine kinases that phosphorylate TCR-zeta chains and other intracellular substrates. Consequently, we examined whether tyrosine kinases could be similarly activated in immature CD4+CD8+ thymocytes. Unexpectedly, we found that TCR-zeta chains from CD4+CD8+ thymocytes were already phosphorylated in vivo, and that dephosphorylation of this TCR subunit occurred on removal of CD4+CD8+ cells from their intrathymic environment. Rephosphorylation of TCR-zeta in cultured CD4+CD8+ thymocytes occurred rapidly in vitro, either in response to cross-linking of TCR, CD4 or CD8 by specific monoclonal antibodies, or on cell-cell contact. These observations indicate that tyrosine kinases are activated in vivo in immature CD4+CD8+ thymocytes undergoing thymic differentiation and selection. They also indicate that TCR, CD4 and CD8 molecules can function in CD4+CD8+ thymocytes as signalling molecules to activate tyrosine kinases and that phosphorylated TCR-zeta serves as a marker of these signalling events.
Nakayama
T
T
Experimental Immunology Branch, National Cancer Institute, Bethesda, Maryland 20892.
Singer
A
A
Hsi
E D
ED
Samelson
L E
LE
D000818:Animals; D000945:Antigens, Differentiation, T-Lymphocyte; D015496:CD4-Positive T-Lymphocytes / Q000502:physiology*; D016827:CD8 Antigens; D002454:Cell Differentiation; D051379:Mice; D008815:Mice, Inbred Strains; D010766:Phosphorylation; D019000:Phosphotyrosine; D011505:Protein-Tyrosine Kinases / Q000502:physiology; D011948:Receptors, Antigen, T-Cell / Q000502:physiology* / Q000648:ultrastructure; D013950:Thymus Gland / Q000502:physiology*; D013997:Time Factors; D014443:Tyrosine / Q000378:metabolism
D000945:Antigens, Differentiation, T-Lymphocyte; D016827:CD8 Antigens; D011948:Receptors, Antigen, T-Cell; D019000:Phosphotyrosine; D014443:Tyrosine; D011505:Protein-Tyrosine Kinases