CD4 and acute lymphoblastic leukemia: To confirm whether ALL can act as a costimulatory molecule, purified CD4+ T cells were simultaneously stimulated via immobilized anti-CD3 mAb plus soluble ALL. The results showed 39.5 ± 0.6% CD4+ T cell proliferation at 1 μg/mL anti-CD3 mAb, which increased 1.7-fold (69.1 ± 0.9%) in the presence of 5 μg/mL ALL (P = 0.03) only at 48 h of cell culture.