TCR activation of ZAP70 is impaired in CD4⁺CD8⁺ thymocytes as a consequence of intrathymic interactions that diminish available P56(lck)

The fate of developing CD4⁺CD8⁺ thymocytes is determined by signals transduced through surface TCR complexes. Here, we report that cross- linking of TCR on CD4⁺CD8⁺ thymocytes fails to activate ZAP70 protein tyrosine kinase and fails to initiate downstream signaling events, unless the TCR are coaggregated with surface coreceptor molecules. TCR signaling in CD4⁺CD8⁺ thymocytes is impaired because the number of available p56(lck) molecules is diminished by intrathymic CD4-Ia interactions that initially activate p56(lck) molecules, which are subsequently degraded. As a consequence of intrathymic CD4-Ia interactions, TCR chains are initially phosphorylated to recruit ZAP70 molecules, but the recruited ZAP70 molecules are not subsequently phosphorylated, resulting in TCR complexes that are stably associated with inactive ZAP70 molecules. Thus, intrathymic interactions that diminish p56(lck) regulate TCR signaling thresholds and affect TCR structure in developing CD4⁺CD8⁺ thymocytes.