Investigation of indoleamine 2,3-dioxygenase 1 expression in uveal melanoma

Abstract

The purpose of this study was to investigate indoleamine 2,3-dioxygenase 1 (IDO1) expression and its implications in uveal melanoma (UM). Bioinformatics analysis was performed on microarray data (GSE22138 and GSE27831) from the Gene Expression Omnibus (GEO) database to evaluate IDO1 expression in mRNA level. Ninety-two cases in the database were divided into the IDO1-high group (46 cases) and IDO1-low group (46 cases). Paraffin embedded tumor sections from 27 patients with UM were studied by immunofluorescence. The mRNA results showed that IDO1 expression was inversely correlated with tumor thickness (9.93 ± 3.33 mm in IDO1-high group vs. 11.56 ± 2.38 mm in IDO1-low group) (p = 0.016) and metastatic rate (30.4% in IDO1-high group vs. 69.6% in IDO1-low group, p < 0.001). The IDO1-high group showed higher immune cell gene expression: CD3D (6.56 ± 1.0 vs. 5.46 ± 0.53, p < 0.0001), CD4 (4.72 ± 0.4 vs. 4.2 ± 0.42, p < 0.0001), and CD68 (6.17 ± 1.23 vs. 5.53 ± 0.77, p = 0.015). Further analysis showed that immune-suppressive T regulatory cell genes (CD3D, CD4, IL2RA and FOXP3) were expressed in 67.4% (31/46) cases in the IDO1-high group and 23.91% (11/46) cases in the IDO1-low group. In addition, IDO1 and interferon gamma (IFNG) mRNA expression were strongly correlated (r = 0.70, p < 0.0001). The correlation analysis of different immune checkpoints showed that IDO1 was positively correlated with CD274(PDL1), but not CTLA4 or PDCD1.The disease-free survival (DFS) in the IDO1-high/IFNG-high group was better than that of the IDO1-low/IFNG-low group. The IDO1 immunostaining result showed that 2 cases in 18 UMs with Bruch's membrane (BM) rupture and 7 out of 9 cases without BM rupture were scored high (Grade 2-3) (p = 0.001). Comparing the immune cells staining results between IDO1-high group and IDO1-low group, higher percentage of patients in the former group had high levels of T cells and macrophages infiltration, but only the difference in macrophage was statistically significant (CD68, 77.78% vs. 27.78%, p = 0.04). The analysis based on GEO data and the result from immunostaining study are consistent with each other. In conclusion, the expression of IDO1 is probably induced by IFNγ from infiltrated immune cells in UM. BM rupture is an important indicator of IDO1 expression level and distribution pattern. The complex role of IDO1 may limit its therapeutic effect in UM.

Keywords: Bruch's membrane; Immune escape; Indoleamine 2,3-dioxygenase 1; Uveal melanoma.