Activated kynurenine pathway metabolism by YKL-40 establishes an inhibitory immune microenvironment and drives glioblastoma development

Glioblastoma (GB) is the stage IV of glioma and mesenchymal GB represents the most common and malignant subtype characterized with elevated expression of a mesenchymal marker YKL-40 and resistance to immune drug therapy. Here, we determined if YKL-40 regulates kynurenine (Kyn) pathway (KP) metabolism that contributes to establishing an immune suppressive microenvironment in GB.

Tumor cells expressing YKL-40 from GB patients were isolated and activated cellular metabolisms were identified via gene microarray analysis. KP metabolism was determined by LC/MS/MS system. Indoleamine 2,3-dioxygenase 1 (IDO1), tryptophan 2,3-dioxygenase (TDO2), their regulatory transcription factors AhR and SRF were evaluated using WB. AhR and SRF transactivity was measured by luciferase reporter gene assays with binding motif mutation, while m6A-mediated AhR and SRF mRNA stability was determined in the presence of an METTL3inhibitor. YKL-40 and Kyn-induced tumor cell migration and CD8+ cytotoxic T cell (CTL) apoptosis were measured in cultured cells. Tumors cells expressing YKL-40 were injected to mouse brains to establish orthotpic tumor models. In GB, YKL-40, IDO1 and TDO2 expression was analyzed for correlation with patient survival.

KP metabolism was activated in YKL-40-expressing tumor cells. YKL-40 divergently regulated IDO1 and TDO2 via induction of AhR and SRF, respectively. mRNA levels of AhR and SRF were stabilized by decreased METTL3 and YTHDF2. YKL-40 and Kyn secreted from tumor cells and infiltrating M2 macrophages cooperated to enhance tumor cell migration and inhibit CTL immunity. In xenografts, tumors expressing YKL-40 displayed the elevated KP metabolism and macrophage infiltration, but decreased CTLs. Treatment with an anti-PD-1 antibody Tislelizumab significantly increased YKL-40+ mouse survival. In GB, YKL-40 was positively correlated with IDO1 expression and both were associated with decreased survival, whereas IDO1 was negatively correlated with TDO2.

YKL-40 upregulates IDO1 or TDO2 to activate KP metabolism, and coordinates with Kyn to establish an inhibitory tumor immune microenvironment, leading to tumor immune evasion.