Uveal melanoma expression of indoleamine 2,3-deoxygenase: Establishment of an immune privileged environment by tryptophan depletion

The enzyme indoleamine 2,3-dioxygenase (IDO) catalyzes degradation of tryptophan, an essential amino acid required for lymphocyte activation and proliferation. Many tumors express IDO which implies that it acts as a mechanism to evade T cell-mediated immune attack, and also to establish an immunosuppressive tumor microenvironment. The purpose of this study was to determine whether primary and metastatic uveal melanoma expressed the IDO gene and whether uveal melanoma cells could deplete tryptophan. In situ expression of IDO in primary uveal melanoma from tumor bearing eyes and metastatic uveal melanoma liver tissues was determined by immunohistostaining with IDO-specific antibody. Reverse transcription PCR was used to assess IDO gene transcription by primary and metastatic uveal melanoma cell lines. IDO protein expression was determined by Western blot of uveal melanoma cell protein lysate. IDO catalytic activity was assessed by measuring the presence of kynurenine, a product generated by tryptophan degradation, in uveal melanoma culture supernatants. Primary uveal melanoma from tumor-bearing eyes and metastatic uveal melanoma from the liver did not express IDO in situ. IDO was not constitutively expressed in either primary or metastatic uveal melanoma cell lines. However, stimulation of primary and metastatic uveal melanoma cell cultures with interferon-gamma (IFN-γ) universally upregulated both IDO gene and protein expression. Culture supernatants from IFN-γ treated primary and metastatic uveal melanoma cell cultures contained elevated levels of kynurenine. Addition of the IDO inhibitor 1-methyl dl-tryptophan significantly diminished kynurenine levels in IFN-γ treated uveal melanoma cell cultures. The results from this study suggest that IFN-γ inducible IDO upregulation by primary and metastatic uveal melanoma may generate a local immune privileged microenvironment to promote escape from T cell-mediated immune surveillance.