Alloreactive donor T cells destroy Aire⁺mTECs during aGVHD, allowing escape of autoreactive T cells that contribute to cGVHD. (A) In healthy individuals, mTECs (purple) mediate negative selection of thymocytes (blue) to eliminate autoreactive T cells (green) recognizing self-peptides presented on MHC molecules. (B) During aGVHD, alloreactive donor T cells (yellow) damage the thymus and eliminate mTECs, allowing escape of autoreactive T cells (green) that contribute to the pathogenesis of cGVHD in the periphery. Professional illustration by Patrick Lane, ScEYEnce Studios.