Utilizing PET Imaging To Visualize Immune Activation and Tumor Apoptosis

Immunotherapy-induced tumor apoptosis is one of the crucial pathways in tumor cell death. This study aimed to explore the potential of PET imaging for noninvasively visualizing pivotal processes in immunotherapy, specifically immune activation and tumor apoptosis, by targeting granzyme-B and caspase-3. Bioinformatic analyses validated granzyme-B and caspase-3 expression in cancer tissues and their associations with immune infiltration and patient prognosis using the GEPIA and TIMER databases. Two radiolabeled probes, [⁶⁸Ga]Ga-GZP and [⁶⁸Ga]Ga-AC3, were used to specifically target granzyme-B and caspase-3 for PET imaging, respectively. CT26 xenograft tumor models were assigned to PD-1 inhibitor or PBS control groups to receive treatment every 3 days, with imaging conducted at baseline and after each treatment. Imaging results showed significantly increased tumor uptake of both [⁶⁸Ga]Ga-GZP and [⁶⁸Ga]Ga-AC3 in the ICB-treated group compared to controls, indicating early molecular changes in immune activation and tumor apoptosis. Immunofluorescence analysis further supported these findings, revealing upregulated granzyme-B and caspase-3 expression in treated tumor tissues. Immunohistochemistry also confirmed increased T-cell infiltration and elevated levels of effector molecules, such as IFN-γ and TNF-α, in the ICB group. This study demonstrates that granzyme-B and caspase-3 PET/CT can noninvasively visualize early molecular changes in immunotherapy-induced CD8⁺ T cell activation and tumor apoptosis. These noninvasive diagnostic techniques hold significant promise for future clinical applications, particularly for a more accurate evaluation of immunotherapy efficacy.