Indoleamine 2,3-dioxygenase regulates T cell activity through Vav1/Rac pathway

The immunoregulatory enzyme indoleamine 2,3-dioxygenase (IDO) suppresses T-cell responses and promotes immune tolerance in tumor resistance. A previous study determined that IDO inhibits Vav1 mRNA expression and the activation of Vav1 and its downstream targets in T cells in the guanine exchange factor (GEF)-independent pathway. The current study aims to determine whether IDO induces T-cell immunosuppression through Vav1/Rac signaling pathway, which is a GEF-dependent pathway. The correlation between Vav1 mRNA expressions in T cells of tumor infiltrating lymphocytes and the levels of IDO expression in lung cancer tissues from lung cancer patients was detected. HEK293 cells were stably transfected with human IDO (HEK293-IDO). T cells were isolated from human blood. HEK293-IDO cells were co-incubated with T cells in the presence or absence of an anti-CD3 antibody to activate T cell receptor (TCR) and/or 1-methyl-l-tryptophan (1-MT) to inhibit IDO activity. The early signaling proteins in T-cytoskeleton regulation through Vav1/Rac pathway of T cell were determined. A significant and negative correlation was observed between IDO and Vav1 expression in the tumor microenvironment. IDO, which was produced by HEK293-IDO cells, significantly inhibited the expression of Vav1, which resulted in defective F-actin reorganization. Thus, TCR signaling initiation was damaged. The effects on T-cells induced by the co-culture of HEK293-IDO cells with T cells were attenuated by 1-MT. Results indicate that the inhibitory effects of IDO on T cell immune responses may occur through the down-regulation of Vav1 protein expression and the suppression of Vav1/Rac cascade. These studies provide insight into the mechanisms of immune escape induced by IDO.