CD200, a type I transmembrane glycoprotein, can interact with its receptor CD200R, which plays an inhibitory role in the activation of microglia—the resident macrophages of the central nervous system. In this study, the rat C6 glioma cell line (C6-1) that was previously characterized with high in vivo tumorigenicity was found to generate CD200 mRNA abundantly. However, CD200 expression was barely detected in another C6 glioma cell clone (C6-2) that was previously found to display low tumorigenic behavior. The results from CD200 immunohistochemistry on human glioma tissue array also showed that tumor cells in Grade I–II astrocytoma expressed a lower level of CD200 immunoreactivity than those detected in Grade III–IV glioblastoma multiforme. C6-1 transfectants with stable downregulation of CD200 gene expression using lentivirus knockdown approach were generated (C6-KD). Microglia and iNOS+ cells were increased when microglia were co-cultured with C6-KD cells. The colony formation of C6-KD was also augmented when those cells were co-cultured with microglia. Yet, increased colony formation of C6-KD transfectants in the co-culture with microglia was effectively suppressed by interleukin (IL)-4 and IL-10. The in vivo results indicated that the tumor formation of C6-1 cells in rat brain was promoted after CD200 gene knockdown. Moreover, CD11b+ activated microglia and iNOS+ microglia were highly accumulated in the tumor site formed by C6-KD. In conclusion, our findings demonstrate that the downregulation of CD200 expression in CD200-rich glioma cells could foster the formation of an activated microglia–associated tumor microenvironment, leading to glioma progression.
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