Fig. 3. Cellular sources of IFN-I during viral infections.
TLR7- and TLR9-expressing pDCs detect RNA and DNA viruses and provide an initial source of IFN-I. However, pDC-mediated IFN-I production is limited and transient, indicating that the majority IFN-I required for controlling viral replication is derived from additional cellular sources such as cDCs, macrophages, monocytes, B cells, and stromal cells that utilize TLRs, RLRs, and DNA sensors for viral detection. The expression of RLRs and DNA sensors is often low but highly inducible in response of IFN-I, suggesting that initial IFN-I by pDCs may be essential for RLR- and DNA sensor-mediated responses.