Alternative Antigen Receptor (TCR) Signaling in T Cells Derived from ZAP-70-deficient Patients Expressing High Levels of Syk

Abstract ZAP-70-deficient patients present with nonfunctional CD4+ T cells in the periphery. We find that a subset of primary ZAP-70-deficient T cells, expressing high levels of the related protein-tyrosine kinase Syk, can proliferate in vitro. These cells (denoted herein as Sykhi/ZAP-70− T cells) provide a unique model in which the contribution of Syk to TCR-mediated responses can be explored in a nontransformed background. Importantly, CD3-induced responses, such as tyrosine phosphorylation of cellular substrates (LAT, SLP76, and PLC-γ1), as well as calcium mobilization, which are defective in T cells expressing neither ZAP-70 nor Syk, are observed in Sykhi/ZAP-70− T cells. However, Sykhi/ZAP-70− T cells differ from control T cells with respect to the T cell antigen receptor (TCR)-mediated activation of the MAPK cascades: extracellular signal-regulated kinase activity and recruitment of the JNK and p38 stress-related MAPK pathways are diminished. This distinct phenotype of Sykhi/ZAP-70− T cells is associated with a profound decrease in CD3-mediated interleukin 2 secretion and proliferation relative to control T cells. Thus, ZAP-70 and Syk appear to play distinct roles in transducing a TCR-mediated signal.