Abstract 3576: Molecular imaging of CD8 infiltration following combination immunotherapy in preclinical glioblastoma

Abstract Background: Novel immune-promoting therapeutics for glioblastoma multiforme (GBM), such as oncolytic herpes simplex viruses (oHSV) and immune checkpoint inhibitors (ICI), have the potential to improve overall survival and lead to long-term remission, however their clinical benefit remains inconsistent. Under standard of care imaging, assessment of immunotherapeutic response can be limited by apparent radiological tumor progression associated with treatment-induced inflammation and immune infiltration. This has led to a need for better understanding of immune cell dynamics and immunotherapy response in GBM. The objective of this study is to evaluate changes in CD8+ infiltration and its relation to therapy response, through positron emission tomography (PET) imaging, in preclinical GBM. Methods: GSC005-luc orthotopic GBM models (n= 40) were treated with saline, M002 oHSV, anti-PD1 or combination immunotherapy following three weeks of tumor growth. One-week post-treatment, [89Zr]-CD8 PET imaging was performed and biologically validated through ex vivo PET, autoradiography and staining for H&E and CD8 immunohistochemistry (IHC). Further, longitudinal changes in CD8 infiltration were evaluated via [89Zr]-CD8 PET imaging one- and three-weeks post-immunotherapy with responses monitored every three days via bioluminescence imaging (BLI). Statistical analysis involved one-way ANOVA and unpaired T-test, with p<0.05 considered significant. Results: Linear correlations were seen between in vivo PET signal and ex vivo uptake (r=0.61, p<0.01), autoradiography (r=0.46, p<0.01), and IHC tumor CD8+ cell density (r=0.55, p<0.01) one-week post-treatment. Immunotherapy efficacy, defined by decreased BLI signal, resembled clinical findings as only a subset of mice exhibited long-term positive response measured by BLI signal decrease (n=9/24). Response classification revealed increased CD8+ cell tumor localization, as measured by peak standardized uptake value (SUVpeak) tumor to background ratio (TBR), in non-responders (p<0.01) and decreased heterogeneity in signal distribution in responders (p<0.05) relative to controls early in the course of therapy. Conclusions: Early CD8 infiltration and uptake distribution from CD8-PET imaging provides potential imaging metrics of therapeutic response to oHSV immunotherapy in GBM. Development of these immune-focused imaging approaches for the assessment of positive immunotherapeutic effects in GBM is beneficial for the better understanding of immune cell dynamics and their relation to clinical outcomes. Citation Format: Carlos A. Gallegos, Yun Lu, Alessandro Mascioni, Fang Jia, Jennifer C. Clements, Patrick N. Song, Shannon E. Lynch, Jason M. Warram, James M. Markert, Anna Sorace. Molecular imaging of CD8 infiltration following combination immunotherapy in preclinical glioblastoma. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3576.