CD28-B7 INTERACTIONS ARE NOT REQUIRED FOR INTRATHYMIC CLONAL DELETION

Activation of antigen specific T cells requires more than stimulation through the TCR - CD3 complex. A second or costimulatory signal is als o required, and this second signal can be delivered by interactions be tween CD28 and B7, ligands expressed on T cells and antigen presenting cells respectively. We have examined the role of the CD28 - B7 intera ction in superantigen mediated T cell activation and intrathymic negat ive selection by blocking B7 molecules with a high affinity soluble li gand, CTLA4Ig. In vitro T cell activation mediated by both virally enc oded endogenous and exogenous bacterial superantigens was significantl y blocked by the addition of CTL4AIg to cultures. However, intrathymic clonal deletion in vivo and in fetal thymic organ cultures was not in hibited by blocking B7 molecules. Therefore, although the CD28 - B7 co stimulation pathway is necessary for T cell activation, it does not ap pear to play a role in intrathymic clonal deletion.