A. Serum ACTH level was measured using ELISA. Serum was collected 14 days (brain) or 20 days (flank) after SB28 tumor implantation or from mice without a tumor. n=4–5/group. Two-way ANOVA with Tukey’s multiple comparison test (*p<0.05, **p<0.01, ***p<0.001). B. Serum ACTH level from mice after intracranial tumor implantation with MB49 (5,000 cells/mouse) or B16-F10 (40,000 cells/mouse). Unpaired student t-test (*p<0.05, **p<0.01). C. Serum ACTH level from gonadally intact SB28-bearing (brain) mice treated with vehicle (Veh, corn oil) or enzalutamide (Enza, 10 mg/kg, i.p.). Serum samples were collected at endpoint. n=5/veh, n=3/enza. D. Serum ACTH level from castrated SB28-bearing (brain) mice treated with vehicle (Veh, corn oil) or testosterone cypionate (TC, 250 μg/injection, s.c., weekly). Serum samples were collected at endpoint. n=10/veh, n=9/TC. Unpaired t-test (*p<0.05, ***p<0.001). E. Proposed model depicting how the loss of testosterone and the presence of a brain tumor synergistically activate the HPA axis and regulate anti-tumor immunity.