Anti-CD8 ⁸⁹Zr-labeled PEG20-X118-VHH detects lymphoid organs and tumor-infiltrating CD8⁺ lymphocytes. (A and B) PET-CT images of tumor-bearing mice (A, B16 tumor; B, Panc02 tumor) injected with ⁸⁹Zr-PEGylated VHH (n = 3 for each experiment). (C) Enlarged view of the tumor and draining lymph nodes. (D) A cross-section of the tumor shows the intratumoral distribution of infiltrated CD8⁺ T cells. (E) Enlarged view 2D and 3D representation of the cross section in D shows CD8⁺ T cells deep inside the tumor. (F) Biodistribution of PET signals in different organs and in the tumors. Error bars represent standard deviation. (G) Flow cytometry analysis on the Panc02-infiltrating immune cells confirmed infiltration by CD8⁺ T cells (n = 3).