DURING thymic development, T cells that can recognize foreign antigen in association with self major histocompatibility complex (MHC) are selected for survival (positive selection) and autoreac-tive T cells are eliminated (negative selection). Both of these selective events are mediated by interaction between the T-cell receptor (TCR) and the peptide-MHC complex1. But the signalling pathways that lead to cell survival or to cell death are still unclear. ZAP-70 is a protein tyrosine kinase (PTK) that is associated with the TCR signalling subunits (CD3 and ζ) and is expressed in T cells and natural killer cells2. It has been shown that ZAP-70 plays a crucial role in T-cell activation2-5 and development6-8. Here we show that mice lacking ZAP-70 had neither CD4 nor CDS single-positive T cells, but human ZAP-70 reconstituted both CD4 and CDS single-positive populations. Moreover, ZAP−/− thymocytes were not deleted by peptide antigens. Natural killer cell function was intact in the absence of ZAP-70. These data suggest that ZAP-70 is a central signalling molecule during thymic selection for CD4 and CD8 lineage.
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