T-CELL RECEPTOR-INITIATED CALCIUM-RELEASE IS UNCOUPLED FROM CAPACITATIVE CALCIUM-ENTRY IN ITK-DEFICIENT T-CELLS

Itk, a Tec family tyrosine kinase, plays an important but as yet undef ined role in T cell receptor (TCR) signaling. Here we show that T cell s from Itk-deficient mice have a TCR-proximal signaling defect, result ing in defective interleukin 2 secretion. Upon TCR stimulation, Itk(-/ -) T cells release normal amounts of calcium from intracellular stores , but fail to open plasma membrane calcium channels. Since thapsigargi n-induced store depletion triggers normal calcium entry in Itk(-/-) T cells, an impaired biochemical link between store depletion and channe l opening is unlikely to be responsible for this defect. Biochemical s tudies indicate that TCR-induced inositol 1,4,5 tris-phosphate (IP3) g eneration and phospholipase C gamma 1 tyrosine phosphorylation are sub stantially reduced in Itk(-/-) T cells. In contrast, TCR-zeta and ZAP- 70 are phosphorylated normally, suggesting that Itk functions downstre am of, or in parallel to, ZAP-70 to facilitate TCR-induced IP3 product ion. These findings support a model in which quantitative differences in cytosolic IP3 trigger distinct responses, and in which only high co ncentrations of IP3 trigger the influx of extracellular calcium.