Ablation of CX3CR1-expressing Immune Cells after Radiation Attenuates Tumor Progression and Sensitizes Cancer Cells to S-phase Specific Chemotherapy

Introduction: Chemokine (C-X3-C Motif) Receptor 1 (CX3CR1) is present on a subset of the immune cells in the tumor microenvironment (TME) and plays an essential and diverse role in cancer progression. However, its potential function in the irradiated TME remains unknown.Materials and Methods: Mouse lung cancer model was performed by subcutaneously inoculating Lewis Lung Carcinoma (LLC) cells expressing luciferase (Luc-2) and mCherry cells in CX3CR1GFP/GFP , CX3CR1DTR/+ , and wild–type (WT) mice. Bioluminescence imaging, clonogenic assay, and flow cytometry were used to assess tumor progression, proliferation, and cell composition after radiation.Results: Radiation provoked a significant influx of CX3CR1-expressing immune cells, notably monocytes, into the TME. Co-culturing irradiated LLC cells with CX3CR1-deficient monocytes and macrophages resulted in reduced clonogenic survival and increased apoptosis of the cancer cells. Interestingly, depletion of CX3CR1 in macrophages led to a redistribution of the irradiated LLC cells in the S-phase, parallel to increased expression of cyclin E1, required for cell cycle G1/S transition. In addition, deletion of CX3CR1 expression in macrophages altered the cytokine secretion with a decrease of interleukin-6, a crucial mediator of cancer cell survival and proliferation. Next, LLC cells were injected subcutaneously into CX3CR1DTR/+ mice, sensitive to diphtheria toxin (DT), and WT mice. After injection, tumors were irradiated with 8Gy, and mice were treated with DT, leading to conditional ablation of CX3CR1-expressing cells. After three weeks, CX3CR1 depleted mice displayed reduced tumor progression. Furthermore, combining the S-phase specific chemotherapy gemcitabine with CX3CR1 cell ablation resulted in additional attenuation of tumor progression.Conclusion: CX3CR1-expressing mononuclear cells invade the TME after radiation therapy in a mouse lung cancer model. CX3CR1 cell depletion attenuates tumor progression following radiation and sensitizes the tumor to S–phase-specific chemotherapy. Thus, we propose a novel strategy to improve radiation sensitivity by targeting the CX3CR1-expressing immune cells.