Immunomodulatory Effects of High-Dose Irradiation Regimens in Renal Cell Carcinoma: Insights from an In Vitro Model with Human Peripheral Blood Mononuclear Cell

Background: Stereotactic ablative radiotherapy (SABR) is increasingly used in the treatment of localized and metastatic renal cell carcinoma (RCC), a malignancy traditionally considered radioresistant. Beyond direct cytotoxicity, SABR may promote immunogenic cell death and modulate the tumor immune microenvironment, though the underlying mechanisms remain incompletely understood. Objectives and Methods: This study examined the immunomodulatory effects of two high-dose irradiation regimens (8 Gy and 3 × 8 Gy) in an in vitro model using two RCC cell lines (ACHN, Caki-2) and peripheral blood mononuclear cells (PBMCs) from healthy donors. Results: The 3 × 8 Gy regimen more effectively reduced tumor cell viability and proliferation, particularly in ACHN cells, suggesting differential radiosensitivity. Both regimens induced secretion of IL-6, IL-8, TGF-β, and VEGF, with levels varying by cell line and dose. Caki-2 cells exhibited a cytokine profile consistent with a pro-inflammatory and potentially immunosuppressive phenotype. Conditioned media from irradiated cells were used to stimulate PBMCs, revealing divergent responses. Media from 3 × 8 Gy-irradiated ACHN cells enhanced PBMC proliferation and increased CD8+ T cells and CD11c+ monocytes, along with IFN-γ, IL-2, and TNF-α secretion, suggesting immunostimulatory effects. Conversely, media from Caki-2 cells had minimal impact on PBMC proliferation and increased TGF-β levels. Conclusions: These results indicate that high-dose irradiation can differentially modulate immune responses in RCC cell lines, depending on tumor intrinsic properties and irradiation regimen. Further in vivo studies are warranted to validate these findings and support development of SABR immunotherapy combinations guided by predictive immune biomarkers.