Different Role of Apaf‐1 in Positive Selection, Negative Selection and Death by Neglect in Foetal Thymic Organ Culture

Apoptotic protease‐activating factor 1 (Apaf‐1) is a component of the apoptosome which is required for the activation of procaspase‐9. As Apaf‐1 knockout (KO) (Apaf‐1‐/‐) mice die before birth, the role of Apaf‐1 during thymic selection was investigated using 5 day foetal thymic organ culture (FTOC) of thymi obtained at gestational day 15. There was a lower ratio of CD4 single‐positive (SP) to CD8 SP cells and decreased apoptosis of CD4+CD8+ (DP) thymocytes from Apaf‐1‐/‐ mice compared with wild‐type. To determine if these defects resulted in increased production of neglected thymocytes, the Apaf‐1‐/‐ mice were crossed with the T‐cell receptor (TCR)‐α‐chain KO mice. There was no difference in thymocyte development in the thymi of TCR‐α‐/‐Apaf‐1‐/‐ and TCR‐α‐/‐Apaf‐1+/+ mice 5 days after FTOC. To determine if Apaf‐1 is involved in apoptosis during death by negative or positive selection, FTOC of the thymus of Apaf‐1‐/‐ Db/HY TCR‐αβ transgenic (Tg) mice was carried out. There was decreased apoptosis of the HY clonal‐specific M33+ thymocytes and an increased percentage of the autoreactive CD8+M33+ thymocytes in male, but not female Apaf‐1‐/‐ Db/HY TCR Tg mice. Our data suggest that Apaf‐1 is not involved in positive selection or death by neglect, but may have a partial role in negative selection during early thymic T‐cell development.