Viral infection switches non-plasmacytoid dendritic cells into high interferon producers

Type I interferons (IFN-I) are important cytokines linking innate and adaptive immunity(1). Plasmacytoid dendritic cells make high levels of IFN-I in response to viral infection and are thought to be the major source of the cytokines in vivo(2). Here, we show that conventional non-plasmacytoid dendritic cells taken from mice infected with a dendritic-cell-tropic strain of lymphocytic choriomeningitis virus make similarly high levels of IFN-I on subsequent culture. Similarly, non-plasmacytoid dendritic cells secrete high levels of IFN-I in response to double-stranded RNA (dsRNA), a major viral signature(3), when the latter is introduced into the cytoplasm to mimic direct viral infection. This response is partially dependent on the cytosolic dsRNA-binding enzyme protein kinase R-4 and does not require signalling through toll-like receptor (TLR) 3, a surface receptor for dsRNA(5). Furthermore, we show that sequestration of dsRNA by viral NS1 (refs 6,7) explains the inability of conventional dendritic cells to produce IFN-I on infection with influenza. Our results suggest that multiple dendritic cell types, not just plasmacytoid cells, can act as specialized interferon-producing cells in certain viral infections, and reveal the existence of a TLR-independent pathway for dendritic cell activation that can be the target of viral interference.