FGL2 as a Multimodality Regulator of Tumor-Mediated Immune Suppression and Therapeutic Target in Gliomas

Background: Fibrinogen-like protein 2 (FGL2) may promote glioblastoma multiforme (GBM) cancer development by inducing multiple immune-suppression mechanisms. Methods: The biological significance of FGL2 expression was assessed using the The Cancer Genome Atlast (TCGA) glioma database and tumor lysates analysis. The therapeutic effects of an anti-Fgl2 antibody and the role of immune suppression regulation by Fgl2 were determined in immune-competent, NOD-scid IL2Rgammanull (NSG), and Fc gamma RIIB-/- mice (n = 3-18 per group). Data were analyzed with two-way analysis of variance, log-rank survival analysis, and Pearson correlation. All statistical tests were two-sided. Results: In low-grade gliomas, 72.5% of patients maintained two copies of the FGL2 gene, whereas 83.8% of GBM patients had gene amplification or copy gain. Patients with high levels of FGL2 mRNA in glioma tissues had a lower overall survival (P =.009). Protein levels of FGL2 in GBM lysates were higher relative to low-grade glioma lysates (11.48 +/- 5.75 ng/mg vs 3.96 +/- 1.01 ng/mg, P =.003). In GL261 mice treated with an anti-FGL2 antibody, median survival was 27 days compared with only 17 days for mice treated with an isotype control antibody (P =.01). The anti-FGL2 antibody treatment reduced CD39(+) Tregs, M2 macrophages, programmed cell death protein 1 (PD-1), and myeloid-derived suppressor cells (MDSCs). FGL2-induced increases in M2, CD39, and PD-1 were ablated in Fc gamma RIIB-/- mice. Conclusions: FGL2 augments glioma immunosuppression by increasing the expression levels of PD-1 and CD39, expanding the frequency of tumor-supportive M2 macrophages via the Fc gamma RIIB pathway, and enhancing the number of MDSCs and CD39(+) regulatory T cells. Collectively, these results show that FGL2 functions as a key immune-suppressive modulator and has potential as an immunotherapeutic target for treating GBM.