Inhibition of cell growth induced by anti-TCII antibody is abrogated by free Cbl. Cells were seeded at 2,000 cells/well (in 0.1-mL volumes), cultured in RPMI lacking Cbl in which the folic acid was replaced with 1 μmol/L 5-methyltetrahydrofolic acid and 1 μmol/L homocysteine (medium II), and then supplemented with 10 % QUSO-treated FCS. Serial dilutions of CN-Cbl were added to cultures and cell growth measured, using the MTT reduction assay, after 5 days of culture. (□), No antibody, no recombinant human apo-TCII; (▪), no antibody, 25 ng/mL apo-TCII; (○), antibody (1 μg/mL), no apo-TCII; (•), antibody (1 μg/mL), 25 ng/mL apo-TCII. (A) mAb 2-2 was added where indicated (•,○); (B) mAb 5-18 was added where indicated (○,•). Results are expressed as the mean ± SEM of three replicate data points. Background represents cell viability in the absence of Cbl and apo-TCII with (○) and without (□) antibody added, or absence of Cbl and presence of 25 ng/mL apo-TCII with (•) and without (▪) antibody added.