Engagement of CD4 and CD8 accessory molecules is required for T cell maturation.

In order to address the role of CD4 and CD8 Ag in the process of positive selection in the thymus, antibodies against these molecules, which do not result in the elimination of mature lymph node T cells, were injected in vivo. The results indicate that even long-term injection of nondepleting anti-CD4 and anti-CD8 antibodies does not cause the loss of CD4 or CD8 positive lymph node cells, but it completely blocks the development of the corresponding subpopulation of mature thymocytes. Thus, it appears that the interaction of the CD4 and CD8 accessory molecules on developing thymocytes with a ligand in the thymic environment (probably MHC Ag) is necessary for the positive selection of thymocytes into the appropriate T cell lineage.