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Double-positive thymocytes developing in the thymus eventually must become single-positive thymocytes and commit to a specific lineage of T cells, either CD4$^+$ or CD8$^+$. There are three proposed models of lineage commitment: (1) instructive model, (2) stochastic model, and (3) kinetic signaling model.

The kinetic model of signaling has been favored over the others because it is thought that the CD8 molecule is downregulated on developing thymocytes leaving CD4 still on the surface. When this happens, if the thymocyte receives a continuous signal through the TCR, it becomes a CD4$^+$ T cell; if not, then it becomes a CD8$^+$ T cell. These continuous or disruptive signals are thought to induce transcription factors related to commitment choice. For thymocytes to commit to a CD4$^+$ T cell lineage, it must upregulate
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. For thymocytes to commit to a CD8$^+$ T cell lineage, it must upregulate
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.
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