Sustained TCR signaling is required for mitogen-activated protein kinase activation and degranulation by cytotoxic T lymphocytes.

Requirements for T cell activation are not fully established. One model is that receptor occupancy and down-regulation are essential for activation, and another, not necessarily mutually exclusive, model is that sustained signals are important. Here we examine the importance of signal duration in T cell activation. First, we demonstrate that immobilized, but not soluble cross-linked, Abs to CD3 stimulate degranulation by CTL. The cross-linked Abs are not deficient in their ability to signal since they stimulate the same tyrosine phosphorylation pattern as immobilized Ab, but it is very transient relative to that stimulated by immobilized Ab. Furthermore, novel decreased migratory forms of Lck occur to a significant extent only after stimulation with immobilized Abs. A dramatic difference in the duration of signals is very evident when mitogen-activated protein kinase (MAPK) activity is examined. Immobilized anti-CD3 stimulates very high levels of MAPK activation that is still detectable 1 h after stimulation. In contrast, cross-linked Ab stimulates only transient and incomplete activation of MAPK. Taken together, these results suggest that TCR engagement and induction of tyrosine phosphorylation alone are not sufficient for T cell activation and that the duration of TCR-stimulated signals is critical to attain a functional response.