CD4 and acute lymphoblastic leukemia: Lastly, in a syngeneic model of Eμ−ret B-ALL, mice that failed to control non−immunogenic wild−type ALL blasts exhibited an upregulation of PD−1 and CTLA−4 on both CD4+ and CD8+ splenic T cells, whereas mice receiving B-ALL cells that express GFP/luciferase (which act as a model antigens) did not (80).