Surface TCR expression and sustained antigenic signaling in naïve T cells

Surface TCR expression level greatly influences T cell antigenic sensitivity. This level is rapidly downregulated when T cells are stimulated with strong TCR agonists, as engaged TCRs are internalized and degraded. However, T cells often require many hours of antigenic stimulus in order to gain effector function; somehow, they can achieve this despite the fact that TCR downregulation, once induced, has been reported to endure for up to several days. It has been unclear how sustained antigenic signaling could continue for an extended period during which a virtual depletion of the plasma membrane of TCR persists. By withdrawing antigenic contact after various times of stimulus, we determined the exact kinetics and magnitude of surface TCR recovery during T cell activation under different conditions. We discovered that surface TCR replenishment begins much more quickly and extensively for naïve T cells than was previously appreciated. In fact, T cell activation induces a new level of surface TCR expression, which can exceed the original, and is itself determined by CD28 costimulation, and antigen dose and duration. This upregulation occurs without bias regarding antigenic specificity, since both TCR species of dual-specificity T cells are upregulated in response to engaging a single TCR type. Continual TCR engagement not only immediately downregulates surface-replenishing TCRs, but also substantially contributes to the magnitude of T cell signal transduction and effector responses. Particularly, even after naïve CD4+ T cells have received sufficient antigenic signal to commit to proliferate, further stimulation can amplify the number of mitoses each precursor T cell undergoes. These findings explain: (1) that TCR engagement can be sustained for many hours because it is fueled by continual surface TCR replenishment, engagement, and consumption; (2) that the prolonged low level of surface TCR expression that has been previously observed post-stimulation reflects a dynamic, not static, state of these processes; (3) that extended TCR engagement is required to maximize the clonal expansion of naïve T cells