Activation of syk but NOT Src-family protein tyrosine kinases is sufficient to mediate apoptosis in T cells

The TCR complex is expressed in association with src-related protein tyrosine kinases and syk/zap70 family kinases. The activation of those PTKs is well documented upon ligation of the TCR complex, and is believed to be critical in directing T-cell responses. In order to explore the functional role of PTKs in activation-driven cell death (apoptosis), we constructed chimeric receptors composed of extracellular/transmembrane domains of murine CDS and full-length sequences of src protein kinases of blk, lyn, ick and fyn, or syk and zap70 kinases as a cytoplasmic domain, respectively. The chimeras were introduced into Al.l mouse T-cell hybridoma cells. Upon stimulation of the chimeras with anti-CD8 mAb, substantial increases in tyrosine-phosphorylation of multiple intracellular substrates were observed in CD8:syk, CD8:blk, CD8:lyn, CD8:lck and CD8:fyn cell lines, but not in CD8:zap70. A reproducible difference in the substrate pattern between src family kinases and syk kinase was also seen. These results indicates the activation of the chimeric receptors. Interestingly, when incubated in the anti-CDS-coated wells, CD8:syk could mediate IL-2 production and apoptosis as evaluated by cell viability and DNA fragmentation assays, whereas the other chimeric receptors failed to do so. These results suggest that while activation of src family kinases is important in early signal transduction, the activation of syk kinase is required and sufficient for the functional response of T cells.