Although type III IFNs are currently in clinical trials as potential therapeutic agents to eradicate HCV infection, their mechanisms of action are not fully understood. Here we report that IFN- λ expression is increased in cHCV patients, both in blood and liver. Our results provide novel evidence for the immunomodulatory action of IFN- λ in anti-HCV immunity and add new complexity to the understanding of the anti-HCV potential of IFN- λ.
We found elevated levels of serum IFN- λ upon cHCV; these data are in agreement with some 
but not other 
recent studies. We further provide novel data that, in contrast to patients with active HCV infection, serum levels of IFN- λ in individuals with HCV SVR were comparable to HCV-naïve control. These data indicate that HCV presence is needed for increased IFN- λ production; similar findings were reported in other hepatotropic viral infections including HBV and and HIV 
infections. This was further supported by our observation, also confirmed by Diegelmann et al 
, that the presence of liver inflammation due to metabolic disturbances (such as in non-alcoholic steatohepatitis) did not lead to increased serum levels of IFN- λ.
We also found robust expression of the IFN- λ receptor (IFN- λR) in dendritic cells and to a lesser extent in T cell populations. We observed expression of the other component of the IFN- λR, IL-10R2, which is also shared between IL-10 and IL-22 
. While increased levels of IL-10 have been reported in HCV infection, to date there is no evidence for IL-10 modulating IFN- λR function. Our finding of the inhibitory effect of IFN-λ on dendritic cells indicates that distribution of IFN- λR may influence tissue-specific effects of IFN- λ during HCV infection.
HCV is difficult to eradicate and this could be related to its prolific capacity to manipulate the immune system. Here we showed that IFN- λ acts as a modulator of immune responses during HCV infection in three inter-related areas described below.
First, we demonstrated that IFN- λ directs DCs to a regulatory phenotype with increased expression of negative regulatory molecules. The IFN- λ-exposed DCs exhibited diminished capacity to stimulate T cell proliferation in a PD-1/PD-L1 dependent manner with contribution from the imbalanced cytokine milieu, such as low IL-12 and IL-2 and/or high IL-10 production. In fact, the functional potential of the IFN- λ-generated DCs, shown here, closely resembles the functional deficits seen in myeloid dendritic cells of HCV patients, as previously reported by our group, and others, although these changes were not identified in other patient cohorts (reviewed in 
). A limitation of our study is that monocyte-derived DCs may not fully represent the in vivo
DC life cycle. However, our data suggest a PD-1/PD-L1-dependent effect of IFN- λ on the stimulatory capacity of DCs. We observed comparable effects of a PD-1 antibody on control DCs that express less PD-L1 levels thus lending support to a combinatory defect, including, but most likely not limited to, negative co-stimulatory receptors and cytokines. Nevertheless, the newly-discovered inhibitory effects of IFN-λ on DCs in our study are not surprising, taking into account the functional redundancy between IFN- λ and IFN-α and impaired DC response to IFN-α in patients with HCV infection 
Second, we show that IFN- λ-exposed DCs are potent triggers involved in the proliferation of Tregs. We and others reported increased frequency of Tregs upon HCV infection 
. Here we observed that IFN- λ-treated DCs trigger the proliferation of pre-existing Tregs and failed to induce de novo
Third, we found that the Tregs expanded in the presence of IFN- λ-DC have preserved their inhibitory capacity in a new MLR. These characteristics of the IFN- λ-DC-expanded Tregs closely resemble findings in primary CD4+CD25+FoxP3+ Tregs of HCV patients 
Finally, in light of the fact that IFN-
λs induce a subset of ISGs stimulated by Type I IFNs and not a unique subset of genes 
, our novel finding of the immunomodulatory effects of IFN-λ on DCs require further investigations. Further studies will be needed to identify whether the immunomodulatory effect of IFN-λ on the MDCs/Tregs system may constitute a desirable anti-inflammatory potential, in parallel or in addition to its previously-reported direct anti-viral effects of IFN-λ against HCV 
. Alternatively, IFN-λ may play have a detrimental role due, at least in part, to its role in Tregs expansion.
In conclusion, we found increased levels of IFN-λ, IL-28, and IL-29 in serum and increased expression of these cytokines and their receptor in the liver during chronic HCV infection. We also identified the functional effect of IFN- λ on DCs, which results in dendritic cell-dependent expansion of regulatory T cells. Our study suggests that the immunomodulatory effects of IFN- λ on DC and T cell populations will need to be considered in future clinical studies involving IFN- λ therapy in HCV infection.