NK cells also contribute to the inhibition of preexisting leukemia following CD40L/IL-2 costimulation. Four groups of mice (seven per group) were challenged with 10⁵ A20 cells on day 1. On days 4 and 14, the animals were treated with irradiated A20 cells mixed with fibroblasts expressing either the CD40L or IL-2 gene or neo (control). Mice were depleted of NK cells by IV injection of the antiasialo GM-1 MoAbs 1 day before tumor challenge and again on 2 days per week for 2 weeks. The data are reported as mean mm3 ± SE.