Correlation of CD8 PET images with immunostaining and histology of tumor sections. Wild-type C57BL/6 mice were inoculated with one million breast cancer cells (epithelial PB2 cells in A or mesenchymal PB3 cells in B). Mice were imaged 2 wk after inoculation by PET/CT using anti-CD8 ⁸⁹Zr-PEG20-VHH X118. (A and B) A transverse PET-CT image taken through a cross section of the tumor in mice bearing an epithelial (A) or mesenchymal (B) tumor (n = 5 for each cohort; one representative animal shown). (C) Hematoxylin and eosin (H&E) staining of tumor samples. (D) Immunohistochemistry (CD8⁺ cells) of paraffin-embedded, formalin-fixed tumor sections shows homogeneous infiltration of CD8⁺ T cells into PB2 (epithelial tumor). For PB3 (mesenchymal tumor), CD8⁺ T cells remained mostly peripheral. Top and bottom panels are from the same sections at different magnifications. See supporting information in Fig. S5 for full-size immunohistology and H&E images.