Interestingly, the highest concentration of IFN-α induces up to 5 pg/mL of IL-6, a concentration similar to that found in both plasma and CSF of HCV patients who developed IFN-α-induced depression (Wichers et al., 2006; Raison et al., 2009), and this is the concentration of IL-6 that we use in our in vitro experiments. Here, IL6 is linked to major depressive disorder.