Enhanced microglia activation and glioma tumor progression by inflammagen priming in mice with tumor necrosis factor receptor type 2 deficiency

Despite the fact that accumulation of microglia, the resident macrophages of the central nervous system (CNS) are the main feature of glioblastoma, the role of microglia in the progression of glioma is still arguable. Based on the correlation of inflammation with tumor progression, in this study, we attempt to determine if peripheral inflammation aggravates glioma expansion and the activation of microglia associated with the tumor. Experimental animals were administered intraperitoneally by inflammagen lipopolysaccharide (LPS) for 7 days (LPS priming) before intracerebral implantation of glioma cells. Moreover, a reduced level of tumor necrosis factor receptor type 2 (TNFR2) that is restricted to immune cells, neurons, and microglia has been found in patients with glioblastoma through the clinic analysis of monocyte receptor expression. Thus, in addition to wildtype (WT) mice, heterogeneous TNFR2 gene deficiency (TNFR2^+/−) mice and homogeneous TNFR2 gene knockout (TNFR2^−/−) mice were used in this study. The results show that peripheral challenge by LPS, Iba1⁺-or CD11b⁺-microglia increase in numbers in the cortex and hippocampus of TNFR2^−/− mice, when compared to WT or TNFR2^+/− mice. We further conducted the intracerebral implantation of rodent glioma cells into the animals and found that the volumes of tumors formed by rat C6 glioma cells or mouse GL261 glioma cells were significantly larger in the cortex of TNFR2^−/− mice when compared to that measured in LPS-primed WT or LPS-primed TNFR2^+/− mice. Ki67⁺-cells were exclusively clustered in the tumor of LPS-primed TNFR2^−/− mice. Microglia were also extensively accumulated in the tumor formed in LPS-primed TNFR2^−/− mice. Accordingly, our findings demonstrate that aggravation of microglia activation by peripheral inflammatory challenge and a loss of TNFR2 function might lead to the promotion of glioma growth.