1139 - Tumor Immune Microenvironment Modulation by Single-Fraction vs. Multifraction SBRT in an Orthotopic NSCLC Mouse Model
Presenter(s)
L. Bouchard1, R. Haroun1, C. Rutihinda1, M. Bouchard2, and A. J. Oweida1; 1Université de Sherbrooke, Sherbrooke, QC, Canada, 2Centre Hospitalier Universitaire de Sherbrooke, Sherbrooke, QC, Canada
Purpose/Objective(s): Stereotactic body radiotherapy (SBRT) is used for curative intent of non-operable early-stage non-small cell lung cancer (NSCLC). However, it remains unclear how different fractionation protocols can effectively modulate the tumor immune microenvironment, especially in orthotopic models transposable to the clinic. The aim of this study is to compare the immunogenicity profile of highly radioresistant NSCLC tumors treated with two commonly used clinical fractionation schedules at our institution: 30Gy/1 fraction (SF-SBRT) versus 48Gy/4 fractions (MF-SBRT). The hypothesis is that MF-SBRT induces a stronger anti-tumor immune response and greater immunogenic cell death than SF-SBRT.
Materials/Methods: Lung Lewis carcinoma (LLC) cells were implanted via intrapleural injection in the right lung of C57BL/6 mice. All mice developed tumors by day 7 as determined by micro-CT imaging and were allocated to receive either SF or MF-SBRT. Mice were monitored for tumor growth using micro-CT scans and treated using a small-animal image-guided radiation platform. Seven days post irradiation, lungs and spleens were harvested, and immune cells were characterized using multicolor flow cytometry and immunohistochemistry (IHC). Treatment groups were compared using the Student’s t-test with significance set at a P value < 0.05.
Results: Data from 12 mice were analyzed. When compared to MF-SBRT, SF-SBRT showed increased infiltration of CD8+ T cells, but did not have a statistically significant difference on CD4+ helper T cells. Activated CD8+CD44+ tumor infiltrating lymphocytes (TILs) were significantly 1.5 times higher in the SF-SBRT group (30.75%) compared to MF-SBRT (20.15%). Interestingly, PD-1 expression on CD3+CD8+ cytotoxic T cells was directly correlated to fractionation with exhausted CD8+PD-1+ TILs being significantly 2.7 times lower in the SF-SBRT group (0.63%) compared to the MF-SBRT group (1.72%).
Conclusion: SF-SBRT induced superior immunogenicity compared to MF-SBRT in NSCLC tumors in an orthotopic model by promoting infiltration and activation of CD8+ TILs and reducing the proportion of exhausted PD-1+ TILs. These results may help identify new radioimmunotherapy strategies to increase local and distant control after SBRT for NSCLC patients.