CD4⁺ and CD8⁺ cells are required for the CAT-13–induced antitumor protection in vivo. (A) Naive mice were depleted of CD4⁺ cells by intraperitoneal injections of the anti-CD4 mAb GK1.5 starting on day −1. Both depleted (, n = 12) and nondepleted control mice (■, n = 12) were then injected with 5 × 10⁵ EL4-huCD20 cells on day 0 and then received CAT-13 mAb therapy. Untreated mice (□, n = 6) were also used as controls (log-rank analysis, *P = .01). (B) Surviving CAT-13–treated mice received intraperitoneal injections of GK1.5 mAb starting on day 69 (, n = 9) or not (■, n = 9) and were intravenously challenged with 5 × 10⁵ EL4-huCD20 cells on day 70. Untreated mice (□, n = 5) were used as controls (log-rank analysis, **P = .005). (C) Wild-type (n = 7) and CD8 KO C57Bl/6 mice (n = 18) were intravenously injected on day 0 with 5 × 10⁵ EL4-huCD20 cells. Wild-type animals received the CAT-13 mAb therapy (□, n = 7). CD8 KO mice were divided into 2 groups. The first group was left untreated and used as a control (▷, n = 6). The second group of CD8 KO mice received the CAT-13 mAb therapy (◀, n = 18; log-rank analysis, †P = .01; ††P < .001). (D) Both wt (■, n = 4) and CD8 KO (◀, n = 10) surviving CAT-13–treated animals were intravenously challenged with 5 × 10⁵ EL4-huCD20 cells on day 70. Untreated mice (□, n = 6) were used as controls (log-rank analysis, †††P = .02). Black arrows indicate CAT-13 mAb injections.