Role of enzyme Indoleamine 2, 3-dioxygenase (IDO) in the development of sepsis-induced immunosuppression

Immunosuppression has been shown to be one long-term sequels of severe sepsis, which is mainly characterized by the expansion of regulatory T cells (Tregs). However, the mechanisms underlying Tregs expansion after sepsis remain poor understood. Indoleamine 2,3-dioxygenase (IDO), an enzyme that initiates the kynurenine pathway of tryptophan degradation, has been implicated in promoting Tregs generation. Therefore, we propose to investigate the role of IDO in the development of sepsis-induced immunosuppression. The results presented here demonstrated that there is an increase in both IDO protein expression and IDO enzymatic activity in spleen of sepsis-surviving mice. Employing a melanoma mouse model as a second challenge in sepsis-surviving mice, we found that pharmacological inhibition of IDO suppressed the enhanced tumor growth observed in sepsis-surviving mice. Importantly, inhibition of IDO decreased the expansion of Tregs in sepsis-surviving mice, leading to reduced CD11b+ Ly6G+ cells frequency in spleen and activation of INF-? production by CD8+ in draining lymph, after tumor challenge. Moreover, the results suggest that aryl hydrocarbon receptor (AhR) is associated with increased IDO expression found in sepsis-surviving mice. Furthermore, we identified that a CD11C+ population of cells are expressing IDO in spleen of sepsis surviving mice. In addition, CD11C+ cells isolated from spleen of sepsis-surviving mice, presented a higher capacity to induce a regulatory phenotype in naïve CD4+ CD25- T than CD11C+ isolated from naïve mice. Taken together, our results suggest that AhR activation by kynurenine during acute phase of sepsis is important to IDO expression in a specific CD11C+. This new sepsis-induced CD11C+ IDO+ population is important to Treg cells expansion and immunosuppression development in sepsis-surviving mice. (AU)