MyD88 is a signaling adapter molecule used by all TLRs except TLR3 as well as by the receptors for IL-1 and IL-18. The study of MyD88 -/mice has allowed to demonstrate a crucial role of this molecule for pDC IFN-I production in response to in vitro stimulations by virus type stimuli as well as in vivo during viral infections [80] . TLR9 appears to be the only MyD88-associated receptor necessary for IFN-I production during HSV-2 infection [83] , which is consistent with the known presence of unmethylated CpG DNA sequences in the genome of herpesviruses. In contrast, during HSV-1 or MCMV infections, IFN-I production shows only a partial dependence on TLR9 [39,40, 84, 85] . This could be in part explained by the fact that other cells than pDCs contribute to IFN-I production, in a manner that is independent of MyD88 [39, 84, 86] but which may rely on intracellular detection of viral replication by cytoplasmic sensors for viral DNA [87] or RNA (as documented for EBV [88] ). However, we have also recently shown a partial redundancy between TLR9 and TLR7 in the MyD88dependent pDC activation for IFN-I production during MCMV infection in vivo [89] . Indeed, in TLR9 -/mice, TLR7 can promote sufficient residual pDC IFN-I production to allow a better control of viral infection and an enhanced resistance to viral-induced lethality as compared to MyD88 -/or TLR7 -/-TLR9 -/animals. The ability of pDCs to respond to a DNA virus through TLR7 is a strong argument in favor of their ability to engulf apoptotic bodies from dying infected cells for delivery of viral or host mRNA to TLR7-containing endosomes. Of note, TLR7 has recently been reported to be responsible for the activation of IFN-I production by cDCs upon infections with phagosomal bacteria [90] . Thus, TLR7 functions are clearly not restricted to the recognition of, and responses to, RNA viruses, but also extend to defense against herpesviruses and phagosomal bacteria. Interestingly, a cross-regulation between TLR9 and TLR7 has recently been demonstrated in mouse cDCs, whereby TLR9 outcompetes TLR7 for association with the scaffolding molecule UNC93B1 which is required for the trafficking of all endosomal TLRs from the endoplasmic reticulum to the endosomes. Thus, in wildtype animals, TLR responses in cDCs are biased towards DNA-but against RNA-sensing [91] . It will be interesting to determine whether this mechanism is also at play in pDCs, which have a particularly strong expression of TLR7 and an exquisite sensitivity to its specific ligands.