T cell antigen receptor engagement abrogates CD4-mediated T cell deletion in vivo

We have previously shown that the engagement of CD4 by specificantibody in the mouse initiates a T cell apoptosis responsewith the following features: spleen and lymph node CD4⁺ T cellsmigrate into the bloodstream within minutes of anti-CD4 administrationwhere they exhibit the phenotype of null cells. If they arecapable of expressing functional Fas protein on their surfacethey degrade their DNA and disintegrate rapidly. We show herethat the engagement of the T cell antigen receptor blocks theCD4-medlated deletion process in mouse. Anti-CD4-reactive Tcells avoid the exodus into the bloodstream when their TCR Isengaged by anti-CD3 or by a superantigen, do not modulate surfacereceptors and are not deleted. In contrast to the apoptoslsinducingCD4-specific antibody which causes migration of lymphocytesfrom lymphoid organs into the blood stream, the T cell-activatingCD3-specific antibody causes lymphoid cell redistribution inthe opposite direction, from the bloodstream to lymphoid organs.The TCR-mediated protection of T cells against CD4-mediateddeletion lasts for several hours but ceases before the T cellsbecome blasts.