CD28-B7 interactions function to co-stimulate clonal deletion of double-positive thymocytes

Negative selection of thymocytes only occurs if next to signals through the TCR, additional antigen-presenting cell (APC)-derived signals are also provided. It has been unclear which molecular interactions lead to the generation of these signals. In particular, the involvement of CD28 and its ligands B7-1 and B7-2 has been controversial. In the present study, we re-address this issue and first confirm that cross-linking CD28 molecules on thymocytes can indeed complement TCR-derived signals for induction of deletion upon TCR engagement with antibodies. Furthermore, we extend these findings by documenting that also peptide agonist-induced deletion can be co-stimulated by antibody-mediated engagement of CD28. Additionally, blocking B7-1 or B7-2 reduces negative selection induced by both anti-CD3 and peptide agonist in suspension cultures and in fetal thymic organ culture. At the same time, prominent co-stimulation of TCR-induced deletion could be provided by a B7-negative cell line. Together these results definitively demonstrate that CD28-B7 interactions can function to co-stimulate induction of clonal deletion, while yet to be identified B7-independent co-stimulatory signals can fulfil this function as well.