Consistent with the MEDI-565 binding data, CHO SV CEA did not trigger the activation of T cells from healthy donors in the presence of MEDI-565, as measured by the up-regulation of the IL-2Ra chain/CD25 protein on either CD8 or CD4 T cells ( Figure 6C and D) . This T cell activation marker has been shown previously to correlate temporally with the release of cytokines from T cells activated by MEDI-565 [18] and other BiTE antibodies [23, 25] . Likewise MEDI-565 also did not mediate the lysis of target cells expressing the splice variant ( Figure 6B ). In contrast, MEDI-565 activated T cells and induced killing of CHO FL CEA or CHO FL+SV CEA cells with similar levels of potency (EC 50 values: CHO FL CEA, EC 50 = 75635 ng/mL; CHO FL+SV CEA, EC 50 = 59643 ng/mL; p = 0.79). Somewhat higher maximal T cell activation and maximum achievable cytotoxicity levels were observed with CHO FL+SV CEA cells relative to CHO FL CEA cells, and may be explained by the higher full length CEA expression in the former, as described above.