Janeway Figure 7.11. Changes in cell-surface molecules allow thymocyte populations at different stages of maturation to be distinguished. The most important cell-surface molecules for identifying thymocyte subpopulations have been CD4, CD8, and T-cell receptor complex molecules (CD3, and the T-cell receptor α and β chains). The earliest cell population in the thymus does not express any of these. As these cells do not express CD4 or CD8, they are called â€˜double-negativeâ€™ thymocytes. These precursor cells give rise to two T-cell lineages, the minority population of γ:δ T cells (which lack CD4 or CD8 even when mature), and the majority α:β T-cell lineage. Development of prospective α:β T cells proceeds through stages where both CD4 and CD8 are expressed by the same cell; these are known as â€˜double-positiveâ€™ thymocytes. At first, double-positive cells express the pre-T-cell receptor (pTα:β). These cells enlarge and divide. Later, they become small resting double-positive cells in which low levels of the T-cell receptor (α:β) itself are expressed. Most thymocytes (~97%) die within the thymus after becoming small double-positive cells. Those cells whose receptors can interact with self MHC molecules lose expression of either CD4 or CD8 and increase the level of expression of the T-cell receptor. The outcome of this process are the â€˜single-positiveâ€™ thymocytes, which, after maturation, are exported from the thymus as mature single-positive CD4 or CD8 T cells.
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