| The developmental pathway of TCRalphabeta+CD4 -CD8beta- intestinal intraepithelial lymphocytes (unconventional iIEL) has remained unclear despite major advances in understanding about the development of most other TCRalphabeta+ cell lineages. By cloning and expressing TCRs isolated from unconventional iIEL, we demonstrated that DPhigh thymocytes that expressed these TCRs strongly recognized MHC class I or class II in a coreceptor dependent manner, became DPdullCD69highPD-1 high thymocytes, and largely underwent Bim- and caspase 3-mediated apoptosis. Yet, a few cells consistently escaped thymic negative selection and were able to selectively populate the unconventional iIEL lineage. Conversely, cloning of TCRs isolated from the pool of negatively selected thymocytes selectively gave unconventional iIEL upon forced expression in wild-type thymocytes. Our results clearly demonstrate that TCR specificity drives commitment to the unconventional iIEL lineage, that DPdullCD69highPD-1 high thymocytes are the first post-signaling precursor to unconventional iIEL, and that most unconventional iIEL are escapees of thymic negative selection. Altogether, these results suggest that by downregulating CD4 and CD8beta coreceptors, unconventional iIEL largely lose their ability to recognize self ligands, and instead, are subsequently recycled into long-lived innate-like cytolytic effectors. |
