(A) Schematic diagram outlining the experimental design used to test the efficacy of topical calcipotriol treatment in blocking breast cancer development. Six- to eight-week-old WT and Tslpr^–/– animals were used as tumor recipients. The animals were treated with 20 nmol calcipotriol or EtOH on their ears every 3 days, starting before adoptive T cell transfer (in a subgroup of Tslpr^–/– animals) and PyMt^tg Tslpr^–/– primary breast tumor implantation. (B) Topical calcipotriol treatment on ears led to elevated TSLP levels in the circulation. Bar graph shows the average levels of TSLP in the serum of WT mice treated with calcipotriol or EtOH (*P < 0.05, Student’s t test). (C and D) TSLP induction by calcipotriol blocked breast tumor growth in a TSLP- and CD4⁺ T cell–dependent manner. Endogenous TSLP expressed by breast tumor cells also acted to suppress breast tumor growth. (C) Graph demonstrates the average tumor volumes in 5 cohorts of mice tested (n = 6 in each group; *P < 0.05 compared with the WT plus EtOH group, Student’s t test). (D) Representative images of the implanted primary breast tumors are shown (red arrow points to small tumors). Scale bar: 1 cm. (E) CD4⁺ T cells in calcipotriol-treated WT mice and WT CD4⁺ T cells transferred into calcipotriol-treated Tslpr^–/– mice were polarized to Th2 cells. Representative flow cytometric plots of tumor-infiltrating CD4⁺ T cells in recipient mice are shown (numbers in each box represent the percentage of CD4⁺ T cells). Congenically marked (CD45.1⁺) WT CD4⁺ T cells are highlighted in red. Bar graph shows the percentage of Th1 and Th2 cells and Tregs within the breast tumors in each group (n = 6 per group; *P < 0.05 compared with the WT plus EtOH group, Student’s t test). The data were reproduced in 3 independent experiments.