CD20 immunotherapy does not select for treatment-resistant lymphomas. (A) Method for isolating and naming representative BL3750 lymphoma family members. Primary lymphoma cells were transferred into mice that were then given either CD20 (closed circles) or control (open circles) mAb as in Figures 2 and 3. Second-generation lymphomas were subsequently collected and adoptively transferred as in Figure 3. This process was repeated for subsequent generations. (B) Representative changes in BL3750, BL6454, and BL5239 lymphoma family member sensitivities to CD20 immunotherapy during progressive in vivo passages. Whether the transferred lymphomas were primary lymphomas or were isolated from mice treated with either control or CD20 mAb is indicated. Survival plots represent pooled results for 2 to 3 independent experiments (n = 4-11 total mice per group). Significant cumulative survival differences between groups are indicated. (C) CD20 immunotherapy during progressive in vivo passages does not select for CD20-deficient lymphomas. Line graphs compare the sensitivity of representative lymphoma cells with CD20 immunotherapy (closed triangles) vs their CD20 expression levels (closed squares) over 3 to 7 generations for the lymphoma families shown in panel B.