Using clone 6_2G3, we wished to assess whether a patientderived antibody has potential cytotoxic activity against tumor cells. We tested the tumor cell killing potential of this antibody using a real-time live-dead cell cytotoxicity assay using as targets metastatic melanoma cells recognized by this clone (Figure 7 & Supporting Videos S1 and S2). In these experiments, U-937 human monocytic cells which express Fcc receptors served as effector cells [40] and A-375 melanoma cells were used as target cells to evaluate antibody-dependent cellular cytotoxicity (ADCC) of tumor cells mediated by patient-derived IgG antibodies. We tested two monoclonal antibodies, both derived from Patient 6 ( Table 1) : (1) the 6_2G3 antibody, which bound to A-375 cells and not melanocytes and (2) the 6_2D10 antibody, which did not bind to A-375 cells or melanocytes in the cell based ELISA prior to limiting dilution, which served as a non-tumor-reactive control (Figure 7) . After 2 hours in culture, 18% (95% CI = -5 to 41%) of tumor cells given the melanoma-specific antibody were viable, compared to 95% (95% CI = 86 to 104%) of the tumor cells given the nonmelanoma specific antibody (P,0.0001) (Figure 7, A, left) . Relative to tumor cell fluorescence at the start of the assay, mean green/live tumor cell fluorescent intensity was reduced to 64% for the non-tumor specific antibody (6_2D10) compared to 18% for the tumor-specific antibody (6_2G3) (Figure 7, A, right) . These results highlight the potential of a patient derived tumor-specific antibody to kill tumor cells by antibody-dependent cell cytotoxicity (Figure 7 , B and see Videos S1 and S2). For tumor cells treated with the tumor-specific 6_2G3 antibody, we also observed a significant (P = 0.0002) reduction in the movement of monocytic effector cells in contact with tumor (13 mm, 95% CI = 10 to 17 mm) compared to effector cells not in contact with tumor (26 mm, 95% CI = 21 to 31 mm) (Figure 7, C and D) . Using the 6_2D10 non-specific antibody, no significant (P = 0.3) difference was observed for the movement of effector cells not in contact with tumor cells (20 mm, 95% CI = 15 to 25 mm) compared to those in contact with tumor cells (25 mm, 95% CI = 18 to 31). With this example, we demonstrate that a patient-derived tumor-specific antibody is capable of engaging immune effector cells in antibodydependent cellular cytotoxicity against tumor cells. Taken together, these data suggest that systemic melanoma-specific mature B cell responses may be present in patients with melanoma and may harbor the potential to be activated against cancer cells. Table 1 