A severe defect in CRAC Ca²⁺ channel activation and altered K⁺ channel gating in T cells from immunodeficient patients

Engagement of the TCR triggers sustained Ca²⁺ entry through Ca²⁺ release-activated Ca²⁺ (CRAC) channels, which helps drive gene expression underlying the T cell response to pathogens. The identity and activation mechanism of CRAC channels at a molecular level are unknown. We have analyzed ion channel expression and function in T cells from SCID patients which display 1-2% of the normal level of Ca²⁺ influx and severely impaired T cell activation. The lack of Ca²⁺ influx is not due to deficient regulation of Ca²⁺ stores or expression of several genes implicated in controlling Ca²⁺ entry in lymphocytes (kcna3/Kv1.3, kcnn4/IKCa1, trpc1, trpc3, trpv6, stim1). Instead, electrophysiologic measurements show that the influx defect is due to a nearly complete absence of functional CRAC channels. The lack of CRAC channel activity is correlated with diminished voltage sensitivity and slowed activation kinetics of the voltage-dependent Kv1.3 channel. These results demonstrate that CRAC channels provide the major, if not sole, pathway for Ca²⁺ entry activated by the TCR in human T cells. They also offer evidence for a functional link between CRAC and Kv1.3 channels, and establish a model system for molecular genetic studies of the CRAC channel. JEM