Requirement for a complex array of costimulators in the negative selectionof autoreactive thymocytes in vivo

Autoreactive thymocytes can be deleted at an immature stage of their develo pment by Ag-induced apoptosis or negative selection. In addition to Ag, neg ative selection also requires costimulatory signals from APC. We recently u sed a fetal thymus organ culture system to show that CD5, CD28, and TNF coo peratively regulate deletion of autoreactive thymocytes. Although these exp eriments provided strong evidence for the action of several costimulators i n negative selection, we wished to demonstrate a role for these molecules i n a physiologically natural model where thymocytes are deleted in vivo by e ndogenously expressed Ags. Accordingly, we examined thymocyte deletion in c ostimulator-null mice in three models of autoantigen-induced negative selec tion. We compared CD5(-/-) CD28(-/-) mice to CD40L(-/-) mice, which exhibit ed a profound block in negative selection in all three systems. Surprisingl y, only one of the three models revealed a requirement for the CD5 and CD28 costimulators in autoantigen-induced deletion. These results suggest that an extraordinarily complex array of costimulators is involved in negative s election. We predict that different sets of costimulators will be required depending on the timing of negative selection, the Ag, the signal strength, the APC, and whether Ag presentation occurs on class I or class II MHC mol ecules.