As shown in Figure 2B for three representative donors, binding of the anti-NeuGcGM3 positive responders was not affected after trypsin treatment of L1210 cell surfaces. In contrast, binding was diminished in contrast to L1210 cell binding when the sera were incubated with L1210 cmah-kd cells. However, there is some degree of recognition of the L1210 cmah-kd cell line, presumably due to binding of the serum polyclonal antibodies to non-NeuGc-related antigens. No binding was detected against normal human PBMCs. Moreover, pretreatment of the positive sera with NeuGcGM3

but not with NeuAcGM3 strongly affected the percentage of L1210 stained cells (Fig. 2C). JQ1 purchase In concordance with the results obtained by ELISA, the percentage of tumor cells recognized by the healthy donors’ sera significantly decreased with increasing donor age (Fig. 2D). Also, the number of the healthy donors with serum containing antibodies able to recognize L1210 cell line decreased with age (Fig. 2E). Next, we tested whether the anti-NeuGcGM3 antibodies present in healthy human sera CT99021 research buy were able not only to recognize but also to induce the death of L1210 cells. Forty healthy donors’ samples, with positive binding to NeuGcGM3 by ELISA and to L1210 by flow cytometry, were incubated for 4 h at 37°C with L1210 cells,

and cell death was detected by PI incorporation. Thirty-five of the sera tested induced complement-mediated cell death of L1210 cells (Supporting Information Fig. 4). The anti-NeuGcGM3 mAb 14F7 and antibodies against Celastrol this antigen induced in NSCLC patients treated with the 1E10 anti-idiotypic vaccine are able to kill tumor cells by a complement-independent

mechanism [18, 20]. In order to test whether the anti-NeuGcGM3 antibodies present in healthy human sera share this property, the samples were heated at 56°C for 30 min to inactivate complement before evaluating their cytotoxic capacity. Interestingly, 11 out of 35 donors’ sera that induced complement-mediated cell death still showed cytotoxic capacity after complement inactivation (Fig. 3A). There was a positive correlation between the complement-independent cytotoxicity capacity and both the levels of anti-NeuGcGM3 antibodies measured by ELISA and tumor cell binding by flow cytometry (Supporting Information Fig. 5). Furthermore, ten of these 11 donors were less than 30 years of age. In order to define whether the anti-NeuGcGM3 anti-bodies present in normal human sera mediate this complement-independent cytotoxic effect, we evaluated cell death in tumor cell lines that express or do not express the NeuGcGM3 ganglioside. As shown in Figure 3B for three healthy donors, sera that induced the death of L1210 cells lacked this activity against malignant cells that do not express NeuGcGM3 ganglioside.