IDO2-AhR axis as central regulator of the kynurenine pathway in glioblastoma

Abstract

Purpose: Upregulation of the Kynurenine Pathway (KP) in Glioblastoma (GBM) plays an important role in driving its treatment-resistant immunosuppressive microenvironment. Factors driving this exaggerated pathway remain poorly understood. Our aim was to explore the correlation between key KP markers; IDO1, IDO2, TDO2, its primary effector target aryl hydrocarbon receptor (AhR) and a comprehensive set of clinical- and tumour characteristics.

Methods: Tissue samples from 108 newly diagnosed GBM patients were analyzed for the expression of TDO2, IDO1, IDO2, and AhR using immunohistochemistry and QuPath software. Exploratory analyses were conducted to evaluate correlations between KP marker expression and clinical, radiological, and molecular data.

Results: IDO1 expression was primarily correlated with inflammatory blood markers, while TDO2 was correlated with patient age, gender, smoking habit and medication use. In contrast, AhR and IDO2 demonstrated hardly any correlations with clinical or tumour characteristics. Notably, IDO2 exhibited a strong association with AhR expression and tumour cell density, with no observed correlation between AhR and either IDO1 or TDO2.

Conclusions: We validated the inflammatory influences on IDO1 expression and found that TDO2 was mostly correlated with medication and patient characteristics. We could not confirm IDO1 and TDO2 as most prominent drivers of AhR activity in the KP. However, we found a strong correlation between IDO2-AhR which may be responsible for the sustained and enhanced immunosuppression within the tumour microenvironment. This could explain recent failures of IDO1 and TDO2 antagonists and might redirect future studies to intervene in the kynurenine-AhR-IDO2 axis.

Keywords: AhR; Glioblastoma; IDO; Kynurenine; TDO2.

Fig. 1

Variables analyzed in relation to IDO1, IDO2, TDO2 and AhR expression in glioblastoma

Fig. 2

Images of two representative glioblastoma TMA cores demonstrate hematoxylin and eosin (H&E) staining alongside immunohistochemical (IHC) detection of IDO1, IDO2, TDO2, and AhR, with immunoscoring performed using QuPath. Cells are color-coded based on the expression levels of Kynurenine pathway markers, determined by mean DAB staining intensity: cytoplasmic staining for IDO1, IDO2, and TDO2, and nuclear staining for AhR. Expression levels are represented as follows: blue (negative), yellow (weak positive), orange (moderate positive), and red (strong positive). Abbreviations: H&E (hematoxylin and eosin), TDO2 (tryptophan 2,3-dioxygenase), IDO1 (indoleamine 2,3-dioxygenase 1), IDO2 (indoleamine 2,3-dioxygenase 2), and AhR (aryl hydrocarbon receptor)

Fig. 3

Flowchart of the patient selection process

Fig. 4

The IDO2-AhR Axis as Central Driver of the Kynurenine Pathway in Glioblastoma