Second-generation lymphoma sensitivity to CD20 mAb. (A) Method for isolating second-generation BL3750 lymphomas from mice given primary cells. (B) CD20 expression by second-generation lymphomas with control mAb values subtracted. The gray bar indicates primary BL3750 cells; black bars indicate second-generation lymphomas from CD20 mAb-treated mice; white bars indicate second-generation lymphomas from control mAb-treated mice. (C) Second-generation lymphoma resistance to CD20 mAb is not driven by CD20 mAb selection in vivo. Mice were given second-generation lymphoma cells isolated from mice previously treated with either control (top panels) or CD20 (bottom panels) mAb’s as indicated. One day later, littermates were given either CD20 (closed circles) or control (open circles) mAb. Survival plots represent pooled results for 2 to 3 independent experiments (n = 5-15 mice per group) with significant differences between groups indicated. (D) Secondary lymphoma sensitivity to CD20 mAb as in Figure 2, with prior control or CD20 mAb exposure indicated. Significant differences between sample means are indicated. **P ≤ .01. (E) Second-generation lymphoma survival does not correlate with CD20 expression. Scatter plot comparing the percent sensitivity of secondary lymphoma cells with immunotherapy vs CD20 MFIs.