CD8⁺ T-Cell Density Imaging with ⁶⁴Cu-Labeled Cys-Diabody Informs Immunotherapy Protocols

Purpose: Noninvasive and quantitative tracking of CD8⁺ T cells by PET has emerged as a potential technique to gauge response to immunotherapy. We apply an anti-CD8 cys-diabody, labeled with ⁶⁴Cu, to assess the sensitivity of PET imaging of normal and diseased tissue.Experimental Design: Radiolabeling of an anti-CD8 cys-diabody (169cDb) with ⁶⁴Cu was developed. The accumulation of ⁶⁴Cu-169cDb was evaluated with PET/CT imaging (0, 5, and 24 hours) and biodistribution (24 hours) in wild-type mouse strains (n = 8/group studied with imaging and IHC or flow cytometry) after intravenous administration. Tumor-infiltrating CD8⁺ T cells in tumor-bearing mice treated with CpG and αPD-1 were quantified and mapped (n = 6-8/group studied with imaging and IHC or flow cytometry).Results: We demonstrate the ability of immunoPET to detect small differences in CD8⁺ T-cell distribution between mouse strains and across lymphoid tissues, including the intestinal tract of normal mice. In FVB mice bearing a syngeneic HER2-driven model of mammary adenocarcinoma (NDL), ⁶⁴Cu-169cDb PET imaging accurately visualized and quantified changes in tumor-infiltrating CD8⁺ T cells in response to immunotherapy. A reduction in the circulation time of the imaging probe followed the development of treatment-related liver and splenic hypertrophy and provided an indication of off-target effects associated with immunotherapy protocols.Conclusions: ⁶⁴Cu-169cDb imaging can spatially map the distribution of CD8⁺ T cells in normal organs and tumors. ImmunoPET imaging of tumor-infiltrating cytotoxic CD8⁺ T cells detected changes in T-cell density resulting from adjuvant and checkpoint immunotherapy protocols in our preclinical evaluation. Clin Cancer Res; 24(20); 4976-87. ©2018 AACR.