PD-L1 Immuno-PET Reveals Systemic Effects of Localized Oncolytic Virotherapy in a Mouse Model of Head and Neck Cancer

Immune checkpoint inhibitors can trigger antitumor immunity, yet most patients, including those with head and neck cancer, show limited benefit because of insufficient immune activation at the tumor site. Oncolytic virotherapy (OV) may overcome this by priming tumors at the injection site for a stronger response to immune checkpoint inhibitors. However, there are limited data on how OV influences systemic immune responses in uninjected tumors and secondary lymphoid organs (spleen, lymph nodes), largely because of the difficulty of sampling these tissues. We investigated whether immuno-PET can noninvasively track systemic and intratumoral changes in programmed death ligand 1 (PD-L1) expression after intratumoral OV administration in a syngeneic mouse model of oral carcinoma. Methods: ⁸⁹Zr-DFO-PD-L1_mAb was injected into murine oral cancer (MOC) models, including MOC1, MOC2, and MOC2(PD-L1) tumor-bearing mice. Whole-body PET/CT static scans were performed 48 h after injection. On the basis of in vitro OV sensitivity, the MOC1 model was selected to study the effects of OV on PD-L1 expression after a single intratumoral dose of RP1, a proprietary strain of oncolytic herpes simplex virus. Immuno-PET scans with concomitant biodistribution studies were performed on days 3 and 7 after OV administration. Radiomics features were extracted from immuno-PET scans, and immunohistochemistry was performed to confirm PD-L1 expression levels. Intratumoral cytokines and CD8 T-cell infiltration were also assessed. Results: RP1 injection significantly increased ⁸⁹Zr-DFO-PD-L1_mAb uptake in spleens and tumor-draining lymph nodes, as observed in the immuno-PET images acquired 3 d after treatment. By day 7, uptake levels in these organs returned to pretreatment levels. In contrast, no transient increase in radioconjugate uptake was observed in tumors treated with RP1 compared with controls. The levels of intratumoral and intrasplenic ⁸⁹Zr-DFO-PD-L1_mAb uptake were consistent with PD-L1 immunohistochemistry performed on representative sections. Radiomics analysis of tumors and spleens revealed several features occurring on day 3 after administration of OV. Elevated levels of intratumoral type I interferon, followed by CD8 T-cell infiltration, indicated local immune stimulation. Conclusion: The present work highlights the potential of anti-PD-L1 immuno-PET to noninvasively assess spatiotemporal changes in PD-L1 expression on a whole-body scale. This method can help guide and optimize OV, a task that would be challenging to achieve with a single biopsy alone.