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CD8-expressing T cells are the main effector cells in cancer immunotherapy. Treatment-induced changes in intratumoral CD8 + T cells may represent a biomarker to identify patients responding to cancer immunotherapy. Here, we have used a 89 Zr-radiolabeled human CD8-specific minibody ( 89 Zr-Df-IAB22M2C) to monitor CD8 + T-cell tumor infiltrates by PET. The ability of this tracer to quantify CD8 + T-cell tumor infiltrates was evaluated in preclinical studies following single-agent treatment with FOLR1-T-cell bispecific (TCB) antibody and combination therapy of CEA-TCB (RG7802) and CEA-targeted 4-1BB agonist CEA-4-1BBL. In vitro cytotoxicity assays with peripheral blood mononuclear cells and CEA-expressing MKN-45 gastric or FOLR1-expressing HeLa cervical cancer cells confirmed noninterference of the anti-CD8-PET-tracer with the mode of action of CEA-TCB/CEA-4-1BBL and FOLR1-TCB at relevant doses. In vivo , the extent of tumor regression induced by combination treatment with CEA-TCB/CEA-4-1BBL in MKN-45 tumor-bearing humanized mice correlated with intratumoral CD8 + T-cell infiltration. This was detectable by 89 Zr-IAB22M2C-PET and γ-counting. Similarly, single-agent treatment with FOLR1-TCB induced strong CD8 + T-cell infiltration in HeLa tumors, where 89 Zr-Df-IAB22M2C again was able to detect CD8 tumor infiltrates. CD8-IHC confirmed the PET imaging results. Taken together, the anti-CD8-minibody 89 Zr-Df-IAB22M2C revealed a high sensitivity for the detection of intratumoral CD8 + T-cell infiltrates upon either single or combination treatment with TCB antibody-based fusion proteins. These results provide further evidence that the anti-CD8 tracer, which is currently in clinical phase II, is a promising monitoring tool for intratumoral CD8 + T cells in patients treated with cancer immunotherapy. SIGNIFICANCE: Monitoring the pharmacodynamic activity of cancer immunotherapy with novel molecular imaging tools such as 89 Zr-Df-IAB22M2C for PET imaging is of prime importance to identify patients responding early to cancer immunotherapy., (©2020 American Association for Cancer Research.)