Fig. 6: M1 induces PD-L1 upregulation and overcomes PD-L1 blockade resistance.

A, B C57BL/6 mice were implanted subcutaneously in the right flank with B16F10 cells on day 0 and treated intravenously with ctrl (n = 6) or M1 (n = 6) (1 × 10⁷ pfu) once per day on days 6–10. The expression of PD-L1 on CD45⁻ and CD45⁺ CD3⁻ cells in tumor tissue samples were detected by flow cytometry on day 13. The P value was determined by a two-tailed Student’s t test. C C57BL/6 mice were implanted subcutaneously in the right flank with B16F10 cells on day 0 and treated intravenously with ctrl or M1 (1 × 10⁷ pfu) daily on days 7–11. A PD-L1 antibody or isotype control antibody was injected intraperitoneally on days 9, 12, 15, 18, and 21. Kaplan–Meier survival curves (right) are shown. ctrl, n = 8; M1, n = 7; PD-L1, n = 8; M1 + PD-L1, n = 9. The P value was determined by the log-rank test. D C57BL/6 mice were implanted subcutaneously in the right flank with RM-1 cells on day 0 and treated intravenously with ctrl or M1 (1 × 10⁷ pfu) once per day on days 6–10. The PD-L1 antibody or isotype control was injected intraperitoneally on days 8, 11, 14, 17, and 20. Kaplan–Meier survival curves (right) are shown. n = 7. The P value was determined by the log-rank test. E Tumor models were established, and M1 plus PD-L1 antibody treatment was conducted as shown in (C). Lymphocytes isolated from B16F10 tumor tissue samples (TILs), tumor-draining lymph nodes (TDLN), and the spleen on day 15 were cocultured with B16F10 cells for 48 h to evaluate cytotoxicity. n = 8. The P value was determined by one-way ANOVA with Dunnett’s test. F IFN-γ, TNF-a, and perforin were analyzed in supernatants of splenic lymphocytes cocultured with B16F10 cells. n = 8. The P value was determined by one-way ANOVA with Dunnett’s test. Data are shown as the mean ± SD. *P < 0.05; **P < 0.01; ***P < 0.001. ****P < 0.0001. See also Supplementary Fig. S7.