Role of CX3CR1 Signaling in Malignant Transformation of Gliomas

Abstract

Abstract INTRODUCTION High-grade gliomas (HGGs), including the most common primary brain tumor, glioblastoma (GBM), may arise from malignant transformation of low-grade gliomas (LGGs). While LGGs are often clinically indolent, GBMs have dismal outcomes despite maximal therapy. Accumulating evidence suggests that chemokine signaling directly contributes to malignant progression of LGGs by altering tumor behavior or impacting the immune microenvironment. Here, we examined the role of CX3CR1 signaling in malignant transformation of LGGs. METHODS First, patients with malignantly transformed LGGs were genotyped for the presence of the common loss-of-function CX3CR1 V249I polymorphism, and median overall survival was compared between the genotypes. Second, RNA sequencing data was analyzed for differential gene expression based on genotype. Third, surgical samples were examined for altered expression of M2 macrophage markers and microvessel density between the genotypes. Finally, genetically engineered murine model was leveraged to generate endogenous intracranial gliomas with targeted expression of CX3CL1 and CX3CR1, individually or in combination. RESULTS Heterozygosity (V/I) or homozygosity (I/I) for the loss-of-function CX3CR1 polymorphism is associated with significantly better median overall survival in patients with malignantly transformed LGGs, compared to the wild type genotype (V/V). In addition, HGGs from V/I and I/I genotypes exhibit significantly decreased levels of CCL2, important for the recruitment of M2 macrophages, as well as decreased levels of ANGPT1 and MMP9, which mediate angiogenesis. This correlates with reduced intratumoral accumulation of CD204-positive macrophages and microvessel density in tumors from V/I and I/I patients. Finally, in the RCAS-PDGFB driven model of LGG, co-expression of CX3CL1 and CX3CR1 promotes more malignant tumor phenotype and shorter tumor-free survival in association with increased intratumoral microglia and macrophage infiltration and microvessel density. CONCLUSION Taken together, our results show that CX3CL1-CX3CR1 signaling promotes malignant transformation of LGGs via accumulation of glioma associated macrophages and microglia and increased angiogenesis.