CD4 and melanoma: Inhibition of IDO1 by epacadostat in IFNγ-treated melanoma cells completely restored CD4+ and CD8+ T cell proliferation (Fig. 5C, D), demonstrating that T cell suppression by conditioned medium from IFNγ-treated melanoma cells is caused by increased IDO1 expression and high kynurenine levels (compare Fig. 4B-D).