The immunoregulatory
enzyme indoleamine 2,3-
dioxygenase (IDO) is expressed by a subset of murine plasmacytoid DCs (pDCs) in tumor-draining LNs, where it can potently activate Foxp3 regulatory T cells (Tregs). We now show that IDO functions as a
molecular switch in tumor-draining LNs, maintaining Tregs in their normal suppressive
phenotype when IDO was active, but allowing
inflammation-induced conversion of Tregs to a polyfunctional T-helper
phenotype similar to proinflammatory TH17 cells when IDO was blocked.
In vitro, conversion of Tregs to the TH17-like
phenotype was driven by
antigen-activated
effector T cells, and required IL-6 produced by activated pDCs. IDO regulated this conversion by dominantly suppressing production of IL-6 in pDCs, in a GCN2-
kinase dependent fashion.
In vivo, using a model of established B16
melanoma, the combination of an IDO-inhibitor
drug plus anti-tumor vaccine caused upregulation of IL-6 in pDCs and in situ conversion of a majority of Tregs to the TH17 phenotype, with marked enhancement of CD8+
T cell activation and anti-tumor
efficacy. Thus, Tregs in tumor-draining LNs can be actively re-programmed
in vitro and
in vivo into T-helper cells, without the need for physical depletion, and IDO serves as a key
regulator of this critical conversion.