Induction of type I IFN is a key feature of the innate immune antiviral response, and we show here an age-related impairment in production of type I IFN by DCs from older donors after infection with WNV in vitro. This is consistent with our previous findings of an age-dependent reduction in TLR ligand-stimulated cytokine production by monocytes, macrophages, and blood DCs [20
] and impairments in PI3K signaling, critical for type I IFN production [35
], which has recently been shown by others to be impaired in aging in humans and mice [36
]. Induction of type I IFN-dependent genes occurs as a highly regulated process [28
]. Although we did not observe any age-related deficiencies in the initial IFN response (IRF3), we showed an impaired increase in STAT1, IRF7, and IRF1 in DCs from older donors infected with WNV, suggesting inhibition of the positive-feedback regulation of type I IFN. Our results with DCs are distinct from our previous studies in macrophages, which showed higher levels and increased nuclear translocation of STAT1 in the early responses (3 h) to WNV infection by older donors [22
]. It is tempting to speculate that the different age-related responses of the 2 cell types may reflect either impairment of different mechanisms or related or compensatory effects.
Murine studies of age-associated defects in IFN-α production in pDCs, consistent with our current findings, showed reduced upregulation of IRF7 in pDCs of aged mice that is related to oxidative stress [37
]. Oxidative stress inhibits IFN-α-induced antiviral gene expression by blocking the JAK–STAT pathway [38
], and macrophages deficient in the antioxidant molecule myeloid HO-1 show reduced expression of IFN-β and of primary IRF3 target genes [39
]. Our study suggests a further role for oxidative stress in inhibition of IFN-β production by DCs through age-dependent upregulation of Axl.
Impairments in innate immunity in aging, such as the reduced DC functions shown here, may contribute to age-related defects in T cell immunity, which have been reported to play an important role in the susceptibility of old mice to WNV [40
]. We have shown here that DCs from older donors have reduced costimulatory markers after stimulation, which although modest, is consistent with our previous finding of lower costimulatory markers in older subjects; this correlated with responsiveness to vaccination against influenza [41
]. Induction of type I IFN is a key feature of DC function; thus, the deficiencies shown here, although only 2–4-fold lower in these populations in vitro, might be greatly amplified in vivo and impair responses at numerous levels, including maturation of DCs [42
], upregulation of TLR7, stimulation of allogenic CD4+ and CD8+ T cells [43
], activation of natural killer cell [45
], and enhancing mature lymphocyte survival and B cell class switching [46
]. In support of that is our previous result showing that lower levels of IFN-α production by DC correlate with reduced efficiency of vaccination in older donors [21
Cells recognize and respond to RNA viruses through nucleic acid sensors, which activate downstream signaling, IFN-responsive genes, and establish a positive feedback loop. Deficiency of TLR3 or RIG-I in murine models or human viral infections shows impaired induction of host responses and enhanced viral replication [47
]. We have identified significant age-related reduction in upregulation of TLR7 in DCs after WNV infection and reduced production of IFN from pDCs stimulated with agonists of TLR7, consistent with our previous study showing increased susceptibility to WNV by TLR7-deficient mice [13
]. Reduction of the IFN-dependent induction of TLR7 may be critical to the reduced immune responses observed in older patients. Thus, triggering the TLR7 pathway, as has previously been shown to enhance host responses to hepatitis C virus [49
] and reduction in HIV replication [50
], may provide a potential therapeutic approach in WNV infection or in older patients.