IL-2 bioactivity of anti-Id–IL-2 fusion proteins. (A) HT-2 cells were incubated with various concentrations of S5A8, chS5A8, chS5A8–IL-2, scFv5A8–IL-2 or recombinant mouse IL-2. (B) Irradiated 38C13 cells were incubated with 10 μg/mL of various anti-Id Abs and fixed with paraformaldehyde. HT-2 cells were then incubated with 1 × 10⁴ or 2 × 10⁴ Ab-coated cells. Proliferation was measured by ³H-thymidine uptake 16 to 24 hours later. All results are expressed as the mean cpm incorporated ± SD of triplicate wells.