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Positive and negative selection of developing thymocytes in the thymus Double-positive (DP) thymocytes that have passed the checkpoints that occur during somatic recombination are tested by antigen-presenting cells (APCs) or cortical thymic epithelial cells (cTECs) for the affinity of their T-cell receptor for MHC class I or II molecules presenting antigen.
Positive and negative selection of developing thymocytes in the thymus Thymocytes that cannot bind to MHC molecules or that bind with too high an affinity die by apoptosis.
Positive and negative selection of developing thymocytes in the thymus Those with a low or moderate affinity for MHC class I or II molecules presenting antigen first select for their coreceptor to become single-positive (SP) thymocytes and then undergo a second round of negative selection in the medulla.
Positive and negative selection of developing thymocytes in the thymus Self antigens from endocrine tissues are expressed in mTECs due to the action of autoimmune regulator (AIRE). SP thymocytes in the medulla that bind to MHC molecules presenting antigen on the surface of either medullary thymic epithelial cells (mTECs) or dendritic cells die by apoptosis.
Positive and negative selection of developing thymocytes in the thymus SP thymocytes that pass negative selection from mTECs or dendritic cells presenting self antigens within the medulla exit the thymus as mature T lymphocytes.