Ganoderic acid Me induces the apoptosis of competent T cells and increases the proportion of Treg cells through enhancing the expression and activation of indoleamine 2,3-dioxygenase in mouse lewis lung cancer cells

•GA-Me increases IDO expression via IFN-γ.•GA-Me treatment increases apoptosis of T-cells via IDO.•GA-Me treatment increases kyn production in co-culture system of IDO expression.•GA-Me increases converted CD4+ CD25– T cells to CD4+ CD25+ T cells in vitro.•GA-Me treatment suppresses CD8+ T cells activation and cytolytic activity.

The indoleamine 2,3-dioxygenase-(IDO-) mediated microenvironment plays an important role in tumor immune escape. It is known that ganoderic acid Me can enhance IFN-γ expression and IDO is preferentially induced by IFN-γ. However, whether GA-Me can induce IDO expression has not been clarified yet. We established stable clones of IDO-overexpressing 2LL cells (2LL-EGFP–IDO). After co-culturing with IDO expressing or control vector-transfected 2LL-EGFP cells, T cell apoptosis was determined and the proportion of the regulatory T cells (Tregs) and CD8 + T cell subset was measured. The total cellular protein samples of 2LL-EGFP–IDO cells were isolated for detecting JAK–STAT1 signalling pathway. Co-culture supernatants were used to detect amino acids and cytokines. IDO transfected 2LL cells yielded high level of IDO enzymatic activity, resulting in complete depletion of tryptophan from the culture medium. We found that apoptosis occurred in T cells after cocultured with IDO + 2LL cells and the proportion of CD4 + CD25 + cells and FoxP3 + cells increased while CD8 + cells decreased. The specific inhibitor of IDO, 1-D-MT and GA-Me efficiently enhanced T cell apoptosis, increased Tregs, and reduced CD8 + T cells in vitro. Increased expression of IDO, p-JAK1 and p-STAT1 were confirmed by Western blot analysis. The levels of IFN-γ, IL-10, LDH and kynurenine in co-culture supernatant correspondingly increased, while tryptophan reduced. These results suggest that GA-Me contributing to IDO helps to create a tolerogenic milieu in lung tumors by directly inducing T cell apoptosis, restraining CD8 + T cell activation, and enhancing Treg-mediated immunosuppression.