Human plasmacytoid dendritic cells mount a distinct antiviral response to virus-infected cells

Plasmacytoid dendritic cells (pDCs) can rapidly produce interferons and other soluble factors in response to extracellular viruses or virus mimics such as CpG-containing DNA. pDCs can also recognize live cells infected with certain RNA viruses, but the relevance and functional consequences of such recognition remain unclear. We studied the response of primary DCs to the prototypical persistent DNA virus, the human cytomegalovirus (CMV). Human pDCs responded poorly to free CMV but strongly to live CMV-infected fibroblasts, in a process that involved integrin-mediated adhesion, transfer of viral DNA to pDCs and its recognition through TLR9. Compared to transient polyfunctional responses to CpG or free influenza virus, pDC response to CMV-infected cells was long-lasting, dominated by the production of type I (IFN-I) and type III (IFN-III) interferons, and lacked diversification into functionally distinct populations. Similarly, pDC activation by influenza-infected lung epithelial cells was highly efficient, prolonged and dominated by interferon production. Prolonged pDC activation by CMV-infected cells facilitated the activation of natural killer cells that are critical for CMV control. Finally, patients with CMV viremia harbored phenotypically activated pDCs and increased levels of IFN-I and IFN-III in circulation. Thus, recognition of live infected cells is a common mechanism of virus detection by pDCs that elicits a unique antiviral response program.