Editing T cell (section)

====Negative selection====
Negative selection removes thymocytes that are capable of strongly binding with "self" MHC molecules. Thymocytes that survive positive selection migrate towards the boundary of the cortex and medulla in the thymus. While in the medulla, they are again presented with a self-antigen presented on the MHC complex of [[medullary thymic epithelial cells]] (mTECs).<ref name="pmid20431619">{{cite journal|vauthors=Hinterberger M, Aichinger M, Prazeres da Costa O, Voehringer D, Hoffmann R, Klein L|title = Autonomous role of medullary thymic epithelial cells in central CD4(+) T cell tolerance|journal=Nature Immunology|volume=11|issue=6|pages=512–519|date=June 2010|pmid=20431619|doi=10.1038/ni.1874|s2cid = 33154019|url = https://hal.archives-ouvertes.fr/hal-00531148/file/PEER_stage2_10.1038%252Fni.1874.pdf}}</ref> mTECs must be [[Autoimmune regulator]] positive (AIRE<sup>+</sup>) to properly express tissue-specific antigens on their MHC ''class I'' peptides. Some mTECs are [[Phagocytosis|phagocytosed]] by [[Dendritic cell|thymic dendritic cells]]; this makes them AIRE<sup>−</sup> [[Antigen-presenting cell|antigen presenting cells]] (APCs), allowing for presentation of self-antigens on MHC ''class II'' molecules (positively selected CD4<sup>+</sup> cells must interact with these MHC class II molecules, thus APCs, which possess MHC class II, must be present for CD4<sup>+</sup> T-cell negative selection). Thymocytes that interact too strongly with the self-antigen receive an [[apoptosis|apoptotic]] signal that leads to cell death. However, some of these cells are selected to become [[Treg]] cells. The remaining cells exit the thymus as mature [[naive T cell]]s, also known as recent thymic emigrants.<ref>{{cite journal|vauthors=Pekalski ML, García AR, Ferreira RC, Rainbow DB, Smyth DJ, Mashar M, Brady J, Savinykh N, Dopico XC, Mahmood S, Duley S, Stevens HE, Walker NM, Cutler AJ, Waldron-Lynch F, Dunger DB, Shannon-Lowe C, Coles AJ, Jones JL, Wallace C, Todd JA, Wicker LS|title=Neonatal and adult recent thymic emigrants produce IL-8 and express complement receptors CR1 and CR2|journal=JCI Insight|volume=2|issue=16|date=August 2017|pmid=28814669|pmc=5621870|doi=10.1172/jci.insight.93739}}</ref> This process is an important component of [[central tolerance]] and serves to prevent the formation of self-reactive T cells that are capable of inducing autoimmune diseases in the host.