Bispecific Antibody-mediated Lysis of Placental and Germ Cell Alkaline Phosphatase Targeted Solid Tumors in Immunocompetent Mice¹

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.