A Role for Accessibility to Self-Peptide-Self-MHC Complexes in Intrathymic Negative Selection

Abstract

Whether intrathymic-positive and -negative selection of conventional αβ T cells occur in anatomically distinct sites is a matter of debate. By using a system composed of two distinct immune receptors, the Y-Ae mAb and the 1H3.1 (Vα1/Vβ6) TCR, both directed against the 52–68 fragment of the I-Eα-chain (Eα52–68) bound to I-Ab, we examined the occurrence of negative selection imposed in vivo by a self-peptide-self-MHC class II complex with differential tissue expression. 1H3.1 TCR-transgenic (Tg) mice were bred to mice having an I-Eα transgene with expression directed to all MHC class II-positive cells, restricted to thymic epithelial cells, or restricted to B cells, dendritic cells, and medullary thymic epithelial cells. All 1H3.1 TCR/I-Eα double-Tg mice revealed a severely diminished thymic cellularity. Their lymph node cells were depleted of Vβ6+CD4+ cells and were unresponsive to Eα52–68 in vitro. The absolute number of CD4+CD8+ thymocytes was drastically reduced in all combinations, indicating that negative selection caused by an endogenously expressed self-determinant can effectively occur in the thymic cortex in vivo. Moreover, both cortical epithelial cells and, interestingly, the few cortical dendritic cells were able to support negative selection of CD4+CD8+ thymocytes, albeit with a distinct efficiency. Collectively, these observations support a model where, in addition to the avidity of the thymocyte/stromal cell interaction, in vivo negative selection of autoreactive TCR-Tg T cells is determined by accessibility to self-peptide-self-MHC complexes regardless of the anatomical site.