Recently, an immunocompetent in vivo mouse model was developed based on germ cell alkaline phosphatase (GCAP) transgenic (FVB/N × C3H) mice in which both placental alkaline phosphatase (PLAP)+ and GCAP+ solid MO4 tumors develop. A bispecific anti-PLAP/GCAP anti-mouse CD3 antibody (Ab) 7E8 × 7D6, previously shown to induce efficient dose-dependent T-cell proliferation and PLAP+ tumor cell lysis in the presence of recombinant IL-2 and the anti-mouse CD3 Ab 7D6, was used in this report in in vivo lysis experiments targeting GCAP+ tumors grown in GCAP+ transgenic mice. Mice received injections i.v. twice a week with PBS (group 1) or with 10 µg of the bispecific Ab 7E8 × 7D6, either alone (group 2) or combined with 1 µg of the anti-CD3 Ab 7D6 (group 3), starting 7 days after the tumor inoculation. A fourth group received a local treatment with mouse splenocytes precoated with 10 µg 7E8 × 7D6 and 1 µg 7D6. In between Ab injections, groups 2, 3, and 4 received 104 units recombinant IL-2 (i.v.) every day. Two weeks of treatment with the bispecific Ab either alone or combined with 7D6 resulted in a significant decrease of GCAP+ tumor cells in groups 2 and 3 (4 ± 3% and 10 ± 11% GCAP+ cells/tumor) as compared to the nontreated tumors (95 ± 5% GCAP+ cells), although tumor volumes were not significantly different (12 ± 15 cm3 and 14 ± 11 cm3 versus 16 ± 7 cm3). Apparently, the elimination of GCAP+ cells from the tumor seemed to favor conditions enabling the outgrowth of the few GCAP- cells originally present in the tumor inoculate. In contrast, tumor volumes in group 4 (local treatment) were significantly smaller (P < 0.03; 5 ± 10 cm3, 8 ± 11% GCAP+ cells) as compared to the nontreated group, probably due to the presence of higher amounts of Ab and infiltrated activated T cells (567 ± 322 CD5+ cells/mm2) capable of secreting cytostatic cytokines like tumor necrosis factor α and INF-γ as compared to groups 2 and 3 (266 ± 135 and 198 ± 86 CD5+ cells/mm2, respectively). In summary, this study clearly demonstrated that bispecific antibodies specifically concentrate cytotoxic T cells into a solid tumor in vivo, with subsequent elimination of the targeted tumor cell.
This work was supported in part by grants from the Kankerfonds of the ASLK, from the Vereniging voor Kankerbestrijding, and from the NIH (Grant CA 42595).