CX3CR1-Expressing Immune Cells Infiltrate the Tumor Microenvironment and Promote Radiation Resistance in a Mouse Model of Lung Cancer

SIMPLE SUMMARY: CX3CR1 is present in a subset of the immune cells in the tumor microenvironment; however, their function after radiation therapy remains unknown. We found that radiation induces a significant influx of CX3CR1-expressing immune cells, notably macrophages, into the tumor microenvironment. Co-culturing irradiated lung carcinoma cells with CX3CR1-deficient macrophages reduced proliferation and increased apoptosis of the cancer cells. Interestingly, deficiency of CX3CR1 in macrophages led to the redistribution of the irradiated cancer cells in the S-phase, parallel to increased expression of cyclin E1, required for cell cycle G1/S transition. In addition, deficiency in CX3CR1 in macrophages altered cytokine secretion with decreased interleukin 6, a mediator of cancer cell survival and proliferation. Lastly, we show that in vivo ablation of CX3CR1-expressing cells attenuates tumor progression following radiation and sensitizes the tumor to S-phase-specific chemotherapy. ABSTRACT: Introduction: Chemokine (C-X3-C Motif) Receptor 1 (CX3CR1) is present in 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: A mouse lung cancer model was performed by subcutaneously inoculating Lewis Lung Carcinoma (LLC) cells expressing luciferase (Luc-2) and mCherry cells in CX3CR1(GFP/GFP), CX3CR1(DTR/+), 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 and macrophages, 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, deficiency 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, the deficiency of CX3CR1 expression in macrophages altered the cytokine secretion with a decrease in interleukin 6, a crucial mediator of cancer cell survival and proliferation. Next, LLC cells were injected subcutaneously into CX3CR1(DTR/+) mice, sensitive to diphtheria toxin (DT), and WT mice. After injection, tumors were irradiated with 8 Gy, 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 chemotherapeutic gemcitabine with CX3CR1 cell ablation resulted in additional attenuation of tumor progression. Conclusions: 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.