Influenza infection causes an increase in indoleamine 2, 3-dioxygenase (IDO) activity in the lung parenchyma. IDO catabolizes tryptophan into kynurenine, leading to immune dampening. Multiple cell types express IDO, and while IFN-γ upregulates IDO in dendritic cells and macrophages, it is unclear how IDO is affected in respiratory epithelial cells during influenza infection. In this study, the role of IFN-λ in IDO regulation was investigated after influenza infection of respiratory epithelial cells. IDO1 expression increased concurrently with IFN-λ expression. In differentiated NHBE cells, the IDO metabolite was released basolaterally. Recombinant IFN-λ upregulated IDO1 activity, and silencing of IFN-λ decreased IDO1 expression during influenza infection. During IFN-λ stimulation, most differentiated cell types are able to express IDO but during influenza infection, IDO is primarily expressed in uninfected cells. These studies show a role for IDO in the host response to influenza infection, and they provide insights into novel approaches for enhancing vaccine responses and therapeutic approaches.

Influenza infection induces an increase in the level of indoleamine 2,3-dioxygenase (IDO) activity in the lung parenchyma. IDO is the first and rate-limiting step in the kynurenine pathway where tryptophan is reduced to kynurenine and other metabolites. The depletion of tryptophan, and production of associated metabolites, attenuates the immune response to infection. The impact of IDO on the primary immune response to influenza virus infection was determined using the IDO inhibitor 1-methyl-d,l-tryptophan (1MT). C57BL/6 mice treated with 1MT and infected with A/HKx31 influenza virus had increased numbers of activated and functional CD4+ T-cells, influenza-specific CD8+ T-cells and effector memory cells in the lung. Inhibition of IDO increased the Th1 response in CD4+ T-cells as well as enhanced the Th17 response. These studies show that inhibition of IDO engenders a more robust T-cell response to influenza virus, and suggests an approach for enhancing the immune response to influenza vaccination by facilitating increased influenza-specific T-cell response.

The generation of a heterosubtypic memory T cell response is important for cross-protective immunity against unrelated strains of influenza virus. One way to facilitate the generation of the memory T cell population is to control the activity of immune modulatory agents. The enzyme, indoleamine 2,3-dioxygenase (IDO), is upregulated during influenza infection by the interferon response where IDO activity depletes tryptophan required in T cell response. In this study, IDO activity was pharmacologically inhibited with 1-methyl-tryptophan (1MT) during the primary response to influenza virus infection and the effect on the memory T cell response was evaluated. 1MT treatment improved the memory T cell response to influenza virus challenge by increasing interferon gamma expression by CD4 and CD8 T cells, and numbers of lung virus-specific CD8 + T cells, and increased the Th1 response as well as modifying the immunodominance hierarchy to increase the number of subdominant epitope specific CD8 + T cells, a feature which may be linked to decreased regulatory T cell function. These changes also accompanied evidence of accelerated lung tissue repair upon virus challenge. These findings suggest that modulation of IDO activity could be exploited in influenza vaccine development to enhance memory T cell responses and reduce disease burden.

Influenza virus is recognized by PRRs, which are critical in the early response to virus infection and induction of proinflammatory cytokines. IDO is increased in the lung of mice immediately following influenza infection, and the presence of IDO has been shown to mediate immune suppression through depletion of trp and reduction in IL-6 production. To determine the role of IDO activity in the early immune response to influenza infection, IDO activity was inhibited using the synthetic analog, 1MT. The results show that IDO inhibition enhanced proinflammatory cytokine gene and protein expression at 24 and 48 h postinfection, respectively, compared with control-treated mice and affected PRR expression. The enhanced proinflammatory response in the presence of 1MT was attributed to macrophages in the airways, as Raw264.7 and primary AMs showed enhanced production of IFN-β, IL-1β, IL-6, and TNF-α in the presence of 1MT. These findings provide important knowledge for the role of IDO during initial host response to influenza infection.

The 2009 swine-origin pandemic H1N1 (pH1N1) influenza virus transmitted and caused disease in many individuals immune to pre-2009 H1N1 influenza virus. Whilst extensive studies on antibody-mediated pH1N1 cross-reactivity have been described, few studies have focused on influenza-specific memory T-cells. To address this, the immune response in pre-2009 H1N1 influenza-immune mice was evaluated after pH1N1 challenge and disease pathogenesis was determined. The results show that despite homology shared between pre-2009 H1N1 and pH1N1 strains, the effector memory T-cell response to pre-2009 H1N1 was generally ineffective, a finding that correlated with lung virus persistence. Additionally, pH1N1 challenge generated T-cells reactive to new pH1N1 epitopes. These studies highlight the importance of vaccinating against immunodominant T-cell epitopes to provide for a more effective strategy to control influenza virus through heterosubtypic immunity.

The generation of heterosubtypic CD8 T cell responses is important for cross-protective immunity against unrelated strains of influenza. The need to maintain tolerance in the lung, however, limits the overall breadth and duration of CD8 T cell responses, warranting a better understanding of mechanisms of immunoregulation at this site. Influenza infection induces pulmonary expression of the tryptophan catabolizing enzyme IDO. Depletion of tryptophan results in suppression of T cell expansion or effector function, T cell anergy, or deletion. We hypothesized that ablation of IDO activity during CD8 T cell priming would enhance the subsequent CD8 T cell memory pool. To test this in vivo, we conditionally ablated IDO activity during the priming phase using the inhibitor 1-methyl tryptophan (1MT). 1MT-treated animals challenged with a lethal dose of a heterosubtypic influenza virus generated a more robust Th1 response, with enhanced migration to the lung airways. Interestingly, the immunodominance hierarchy was also shifted to favor subdominant CD8 T cell epitopes in both 1MT-treated and IDO-deficient animals, likely a result of observed decreased regulatory T cell function. While 1MT treatment did not affect cytotoxicity or viral clearance, pulmonary pathology was reduced in the absence of IDO. Together these data provide evidence that modulation of IDO activity in the context of influenza infection could be exploited in vaccine development to enhance memory CD8 T cell responses.