Co-engagement of CD8 with the T cell receptor is required for negative selection.

Although it is established that the CD8 and CD4 co-receptors are involved in T-lymphocyte recognition and activation in the periphery, it is less clear whether these molecules participate in thymic selection events. Analysis of thymic selection in mice transgenic for T cell-receptor genes or for major histocompatibility complex (MHC) genes, or mice injected with antibodies against CD8, CD4 or MHC molecules, is consistent with the participation of CD8 and CD4 in thymic selection. But antibody-mediated crosslinking of surface receptors in thymic organ cultures has indicated that CD8 is not involved in thymic deletion. We show here that mice transgenic for a mutant MHC class I molecule that cannot interact with CD8 do not delete CD8-dependent T cells reactive with the wild-type molecule. This finding unequivocally establishes that for negative selection in the thymus, CD8 must interact with the same MHC class I molecule as the T cell receptor.