The Affinity/Avidity and Length of Exposure to the Deleting Ligand Determine Dependence on CD28 for the Efficient Deletion of Self-Specific CD4+CD8+ Thymocytes

Whether the CD28/B7 signaling pathway is essential for the negative selection of immature CD4+CD8+ (DP) thymocytes expressing self-specific αβ TCRs is a controversial issue. In this study we examined the role of CD28 in the deletion of thymocytes that express either the H-Y or the 2C transgenic TCR. In H-2b male mice that expressed the H-Y TCR, negative selection of DP H-Y TCR+ thymocytes occurred very efficiently and this deletion was unaffected by the CD28−/− mutation. In H-2b 2C mice, where the deletion of DP 2C TCR+ thymocytes occurred less efficiently, the CD28−/− mutation led to a higher recovery of DP thymocytes. Using an in vitro deletion assay, a requirement for the CD28 signaling pathway in the deletion of DP H-Y TCR+ thymocytes was evident at low, but not high, densities of the antigenic ligand. Similar results were also observed in an in vivo assay for the deletion of these thymocytes. Intraperitoneal administration of an anti-CD3ϵ mAb led to the intrathymic deletion of DP H-Y TCR+ thymocytes in a CD28-dependent manner at the 24-h time point. However, the CD28 dependence was less evident at the 40-h time point. These results indicate that the dependence on CD28 for the efficient deletion of self-specific thymocytes is determined by the concentration, affinity/avidity, and length of exposure to the deleting ligand.