Production of indoleamine 2,3-dioxygenase (IDO) by tumor cells, leading to tryptophan depletion and production of immunosuppressive metabolites, may facilitate immune tolerance of cancer. IDO gene is also expressed in dendritic cells (DC) upon maturation induced by lipopolysaccarides or IFN. We investigated IDO gene expression in melanoma cell lines and clinical specimens as compared to mature DC (mDC). Furthermore, we explored effects of L-kynurenine (L-kyn) and 3-hydroxyanthranilic acid (3-HAA) on survival and antigen-dependent and independent proliferation of CD8(+) cells. We observed that IDO gene expression in cultured tumor cells and freshly excised samples is orders of magnitude lower than in mDC, providing highly efficient antigen presentation to CD8(+) T cells. Non toxic concentrations of L-kyn or 3-HAA did not significantly inhibit antigen-specific CTL responses. However, 3-HAA, but not L-kyn markedly inhibited antigen-independent proliferation of CD8(+) T cells induced by common receptor gamma-chain cytokines IL-2, -7 and -15. Our data suggest that CD8(+) T cell activation induced by antigenic stimulation, a function exquisitely fulfilled by mDC, is unaffected by tryptophan metabolites. Instead, in the absence of effective T cell receptor triggering, 3-HAA profoundly affects homeostatic proliferation of CD8(+) T cells.