Positive selection of CD4+ and CD8+ T cells.

Significant progress has been made in characterizing intermediates and defining individual steps of positive selection, providing important insights into mechanisms of CD4/CD8 lineage commitment. New evidence suggests that specific recognition of peptides may be important for positive selection of CD4+ T cells. Several studies have defined signal-transduction pathways important for positive selection and have provided evidence that distinct signaling pathways may regulate positive versus negative selection.