Regulation of T cell receptor expression in immature CD4+CD8+ thymocytes by p56lck tyrosine kinase: basis for differential signaling by CD4 and CD8 in immature thymocytes expressing both coreceptor molecules.

Abstract

SummarySignals transduced through the T cell antigen receptor (TCR) are modulated by the src family tyrosine kinase p56 kk (lck), which associates in mature T cells with the coreceptor molecules CD4 and CD8. Here we describe a novel function oflck in immature CD4+CD8 + thymocytes, that of regulating TCR expression. Activation of lck in immature CD4 + CD8 + thymocytes by intrathymic engagement of CD4 maintains low TCR expression by causing most TCR components to be retained and degraded within the endoplasmic reticulum. Importantly, activation of lck in immature CD4 § CD8 § thymocytes results from engagement of surface CD4 molecules, but not surface CD8 molecules, despite the nearly fourfold greater surface expression of CD8 than CD4. The competence of CD4 to activate Ick in CD4 + CD8 § thymocytes relates to the fact that a relatively large fraction of surface CD4 molecules (25-50%) are associated with intracellular lck molecules, whereas only 2% of surface CD8 molecules are associated with lck. The amount of lck associated with CD4 in CD4+CD8 + thymocytes is diminished by chronic CD4 engagement in the thymus, as activated lck molecules subsequently dissociate from CD4. Indeed, the amount of lck associated with CD4 in CD4 § CD8 § thymocytes is markedly increased in major histocompatibility complex (MHC) class II-mice that lack the intrathymic ligand for CD4 and in which surface CD4 molecules are consequently not engaged. Thus, the present study demonstrates that (a) activation of lck in CD4 + CD8 + thymocytes regulates distribution and expression of TCR components; (b) unlike CD4 molecules, CD8 molecules on CD4 + CD8 + thymocytes cannot efficiently activate lck despite their significantly greater surface expression; and (c) the amount of lck associated with CD4 in the CD4 +CD8 + thymocytes is inversely related to the extent of CD4 engagement by MHC class II molecules in the thymus.T he ability of immature precursor cells in the thymus to differentiate into mature T cells is dependent on the specificity of the TCR molecules that individual precursor cells express. Indeed, the TCR repertoire expressed by mature CD4 + and CD8 + T cells is selected from a broader TCR repertoire expressed by immature CD4+CD8 + thymocytes (1-3). Because the selection of immature CD4 + CD8 + thymocytes for differentiation into functional maturity is based on the specifidty of their surface TCR complexes, it is paradoxical that most CD4 + CD8 + thymocytes express few surface TCR complexes that are only marginally competent to mobilize intracellular calcium (4)(5)(6). We have previously demonstrated that the number of surface TCR complexes expressed by CD4 +CD8 + thymocytes is regulated, at least in part, by CD4 inhibitory signals generated in vivo (6-8). The molecular mechanism by which CD4 signals regulate TCR expression in immature CD4 + CD8 + thymocytes is not known, but it is known that such signals result in retention and degradation of most newly synthesized TCR chains within a pre-Golgi compartment (9).

T Cell Hyporesponsiveness Induced by Oral Administration of Ovalbumin Is Associated with Impaired NFAT Nuclear Translocation and p27kip1Degradation

“…2A, the 21-kDa phosphorylated form of TCR- was detected even in freshly prepared CD4 T cells. Endogenously phosphorylated TCR-has been previously reported in splenic T cells (34). In contrast, the hyperphosphorylated 23-kDa TCRwas detectable within 2 min after stimulation and the band intensities were significantly lower in the egg white diet group.…”

Developmental Alterations in Thymocyte Sensitivity Are Actively Regulated by MHC Class II Expression in the Thymic Medulla

“…It directs the double negative to DP transition, is central to the CD4/ CD8 lineage commitment decision (44), and mediates the normal homeostasis of peripheral T cells (45). Wiest et al (44) previously suggested that low TCR levels in DP thymocytes are tied to limiting expression of Lck; the total level of Lck increases in the SP thymocyte in parallel with increases in TCR levels. We postulated that medullary maturation was regulated by the relative accessibility of this newly expressed Lck to different surface receptors.…”

Inefficient Cell Spreading and Cytoskeletal Polarization by CD4+CD8+ Thymocytes: Regulation by the Thymic Environment

“…The -chain of the TCR-CD3 complex, which is phosphorylated on ITAM motifs in mature T cells upon TCR cross-linking (41) is constitutively phosphorylated at high levels in DP thymocytes (4). phosphorylation is Lck-dependent and results in constitutive association of inactive ZAP70 with phospho- (42,43). These modifications may play a causative role in the low level of surface TCR on DP thymocytes because phosphorylation is linked with TCR internalization in mature cells, and newly synthesized TCR-CD3 complexes in DP thymocytes are targeted for degradation (44).…”