TLR9 and viral infectious disease: Constitutive expression of Toll-like receptor (TLR)7 and TLR9, along with interferon (IFN) response factor 7, enables pDCs to specialize in sensing microbial nucleic acids and uniquely equips them for contributing to defense against viral infections (Dalod et al., 2002; Honda et al., 2005; Ito et al., 2005; Smit et al., 2006), through production of high levels of type I IFNs (IFN-α/β) (Cella et al., 1999; Asselin-Paturel et al., 2001; Björck, 2001; Dzionek et al., 2001; Nakano et al., 2001).