Oligonucleotide motifs that disappear during the evolution of influenza virus in humans increase alpha interferon secretion by plasmacytoid dendritic cells.

CpG motifs in an A/U context have been preferentially eliminated from classical H1N1 influenza virus genomes during virus evolution in humans. The hypothesis of the current work is that CpG motifs in a uracil context represent sequence patterns with the capacity to induce an immune response, and the avoidance of this immunostimulatory signal is the reason for the observed preferential decline. To analyze the immunogenicity of these domains, we used plasmacytoid dendritic cells (pDCs). pDCs express pattern recognition receptors, including Toll-like receptor 7 (TLR7), which recognizes guanosine- and uridine-rich viral single-stranded RNA (ssRNA), including influenza virus ssRNA. The signaling through TLR7 results in the induction of inflammatory cytokines and type I interferon (IFN-I), an essential process for the induction of specific adaptive immune responses and for mounting a robust antiviral response mediated by IFN-α. Secretion of IFN-α is also linked to the activation of other immune cells, potentially amplifying the effect of an initial IFN-α secretion. We therefore also examined the role of IFN-α-driven activation of NK cells as another source of selective pressure on the viral genome. We found direct evidence that CpG RNA motifs in a U-rich context control pDC activation and IFN-α-driven activation of NK cells, likely through TLR7. These data provide a potential explanation for the loss of CpG motifs from avian influenza viruses as they adapt to mammalian hosts. The selective decrease of CpG motifs surrounded by U/A may be a viral strategy to avoid immune recognition, a strategy likely shared by highly expressed human immune genes.