TRYPTOPHAN METABOLISM AND CD8+T-CELL FUNCTION

L-tryptophan is the least common essential amino acid and is necessary for cellular proliferation. In the context of immune regulation, lack of tryptophan in tissue microenvironments and the production of its bioactive metabolites has been implicated as an immunosuppressive phenomenon. The rate limiting enzyme in tryptophan degradation is indoleamine 2,3 dioxygenase (EDO), which metabolizes tryptophan along the L- kynurenine pathway. When induced by interferon signaling, IDO promotes anergy and apoptosis in naive T-cells, and enables the differentiation and activation of T-regulatory cells. However, the role that IDO and tryptophan metabolism play in epitope specific CD8+T-lymphocyte (CTL) function has not yet been investigated. By using a pharmacological inhibitor (1-methyl-D-tryptophan) and knockout (KO) mice independently, IDO was depleted to examine its role in shaping the CTL responses to SV40 large T antigen (TAg) and influenza A virus (IAV). EDO KO and inhibition increased the frequency of CD8+T-cells specific for defined epitopes in both models, as measured by intracellular cytokine staining (ICS). This increase in cell frequency was mediated by a proliferative advantage exhibited by immunodominant EDO KO T-cells when compared with WT counterparts. Bioactive tryptophan metabolites such as L- kynurenine did not play a role in this system, and failed to decrease T-cell responses in either the TAg or IAY model. In addition, IDO KO potentiated the cytotoxic effector function of CTLs in vivo, but did not affect animal morbidity in a disease model of IAV infection