Fig. 1

TC11 ER+ tumors are immunosuppressed, anti-estrogen resistant and poorly responsive to immune checkpoint inhibitors. A TC11 cells were orthotopically transplanted to the caudal mammary fat pads of syngeneic FVB/N young adult females. i. immunostaining for CD8 (arrowheads); ii. immunostaining for PD-L1 (arrowheads). Original magnifications ×200; scale bars = 50 µm. B Schematic of treatment timeline. Three weeks after transplantation, when tumors were palpable, weekly treatments with fulvestrant (Fulv) or oil vehicle control were initiated. After ten additional days, three doses of ICIs (α-PDL1, α-CTLA-4) were administered (days 31, 34 and 37 after transplantation). Tumor sizes were measured biweekly. C–F Neither fulvestrant nor α-PDL1, alone or in combination, impacts tumor growth (C, D, F) or survival (E). C Tumor volumes (mean ± SEM, n = 6 for single treatments, n = 9 for Fulv+α-PDL1). D Comparison of mammary tumor volumes on day 46 after transplantation, when all animals in all treatment groups were alive. One way ANOVA identified no significant differences. E Kaplan Meier analysis of survival (log-rank test, n.s., no significant differences). F Tumor growth curves in individual mice shown in (C). G-K α-CTLA4 (α-C4) modestly reduces tumor growth and increases survival, which is significantly improved in combination with fulvestrant. G Tumor volumes (mean ± SEM, n = 4–5). H Comparison of mammary tumor volumes on day 49 after transplantation, when no animals had reached end stage. Differences among treatments were determined by one way ANOVA, followed by Tukey post tests (*, p < 0.05). I Comparison of mammary tumor volumes on day 56 after transplantation, when only animals treated with α-CTLA4 and those treated with Fulv/α-CTLA4 combination were alive. Unpaired t test (*, p < 0.05). J Kaplan Meier analysis of survival (log rank tests, *, p < 0.05; **, p < 0.01). K Tumor growth curves in individual mice shown in (G)