Distinct phases in the positive selection of CD8+ T cells distinguished by intrathymic migration and T-cell receptor signaling patterns

Developing T cells are positively selected in the thymus to ensure that their antigen receptors can interact with self-MHC. For CD8 T cells, this process takes days to complete, yet the steps involved are poorly understood. We followed a synchronized wave of cells undergoing positive selection within three-dimensional thymic tissue. Surprisingly, migration from the cortex to the medulla occurred before CD4 down-regulation and while thymocytes still required TCR signaling for efficient positive selection. There was a gradual change in the pattern of calcium signaling over time, with an upward shift in basal intracellular calcium correlating with increased speed and brief signaling events. Our data have interesting implications for how positive and negative selection shape the mature CD8 T-cell repertoire.