MHC-I⁻MHC-II⁺tumors respond to PD-1 blockade. (A) In vivo antitumor efficacy of anti-PD-1 mAb against A20/OVA or A20/OVA/B2MKO tumors. Tumor cells (5.0 × 10⁶) were inoculated subcutaneously. Mice were grouped when the tumors reached ∼100 mm³ (day 0), and anti-PD-1 mAb was administered on days 0, 3, and 6 (n = 6 per group). Tumor growth was monitored every 3 days. (B) In vivo antitumor efficacy of anti-PD-1 mAb against E.G7/B2MKO and MC-38/B2MKO tumors. The in vivo experiments were performed as described in panel A (E.G7, 5.0 × 10⁶, and MC-38, 1.0 × 10⁶). (C) Tumor growth of E.G7/B2MKO and E.G7/B2MKO/CIITA tumors in immunocompetent (C57BL/6) and immunocompromised (B6 SCID) mice. Tumor cells (5.0 × 10⁶) were inoculated subcutaneously into C57BL/6 and B6 SCID mice on day 0 (n = 6 per group). Tumor growth was monitored every 3 days. (D) In vivo antitumor efficacy of anti-PD-1 mAb against E.G7/B2MKO/CIITA and MC-38/B2MKO/CIITA tumors. The in vivo experiments were performed as described in panel A (E.G7, 5.0 × 10⁶, and MC-38, 1.0 × 10⁶). All in vivo experiments were performed twice with similar results. *P < .05; **P < .01.