XmAbCD40 enhances ADCC against primary tumor cells. ADCC was determined with a ⁵¹Cr release assay using primary tumor cells from patients with CLL or PCL as targets. Effector cells were fresh unstimulated PBMCs (CLL assays) or purified unstimulated NK cells (PCL assays). Tumor cells were labeled with ⁵¹Cr (100 μCi/10⁶ cells), opsonized with antibodies, and mixed with PBMCs or NK cells at an E/T ratio of 80:1 and 10:1, respectively; ⁵¹Cr release was measured 4 hours later. (A) Maximal lysis against samples from 4 individual CLL patients (assays used antibodies at 10 μg/mL). In each case, XmAbCD40 markedly increases maximal lysis relative to both the IgG1 analog and to rituximab. Data were obtained in triplicate and represent the mean ± SD. (B) Percentage ADCC against primary CLL. XmAbCD40 markedly improves potency and efficacy relative to the IgG1 analog (160- and 2-fold, respectively) and is approximately 50-fold more potent than rituximab. (C) Percentage ADCC against primary PCL. XmAbCD40 exhibits enhanced potency and efficacy relative to the IgG1 analog (4- and 3-fold, respectively), whereas rituximab has no effect; note that these target cells did not express CD20 as evidenced by flow cytometry (data not shown). (B-C) Data represent the mean ± SD of triplicate experiments for each of 2 (B) or 3 (C) PBMC donors.