(A) Schematic diagram depicting the experimental paradigm used to test the role of CD4⁺ T cells in TSLP-mediated immunity against breast cancer. Six- to eight-week-old K14-TSLP^tg Tslpr^–/– animals received WT or Tslpr^–/– CD4⁺ T cells and were implanted 2 days later with PyMt^tg Tslpr^–/– primary breast tumors. WT CD4⁺ T cells were congenically marked (CD45.1⁺). (B) WT CD4⁺ T cells were sufficient to receive TSLP signal and mount an antitumor immune response in the breast. Representative images are shown of K14-TSLP^tg Tslpr^–/– mice with their implanted tumors. Red arrow points to a small tumor in the K14-TSLP^tg Tslpr^–/– animal injected with WT CD4⁺ T cells. Shown are H&E-stained tumor sections isolated from K14-TSLP^tg Tslpr^–/– animals that received WT or Tslpr^–/– CD4⁺ T cells. Inset highlights an area of the tumor that shows reversion to a dilated gland structure in a small tumor that formed in the K14-TSLP^tg Tslpr^–/– mouse injected with WT CD4⁺ T cells. Scatter plot shows the average volume of the breast tumors forming in each K14-TSLP^tg Tslpr^–/– mouse that received WT or Tslpr^–/– CD4⁺ T cells at the time point when the tumors of the Tslpr^–/– T cell–injected animals had reached their terminal volume (n = 8 in each group; *P < 0.05, Student’s t test). Scale bars: 1 cm (mouse), 500 μm (histology). (C) WT CD4⁺ T cells isolated from growth-arrested breast tumors of K14-TSLP^tg Tslpr^–/– mice were polarized to Th2 cells and showed increased proliferation. Representative flow cytometric plots show the FOXP3 and GATA3 expression status of tumor-infiltrating CD4⁺ T cells in K14-TSLP^tg Tslpr^–/– mice that received WT or Tslpr^–/– CD4⁺ T cells. Congenically marked (CD45.1⁺) WT CD4⁺ T cells are highlighted in red. Numbers in the flow plots represent the percentage of cells within each box. Histograms show the percentage of Ki67⁺CD4⁺ T cells.