Characterization of conserved viral leader RNA sequences that stimulate innate immunity through TLRs.

Viruses of the order Mononegavirales encompass life-threatening pathogens with single-stranded segmented or nonsegmented negative-strand RNA genomes. The RNA genomes are characterized by highly conserved sequences at the extreme untranslated 3' and 5' termini that are most important for virus infection and viral RNA synthetic processes. The 3' terminal genome regions of negative-strand viruses such as vesicular stomatitis virus, Sendai virus, or influenza virus contain a high number of conserved U and G nucleotides, and synthetic oligoribonucleotides encoding such sequences stimulate sequence-dependent cytokine responses via TLR7 and TLR8. Immune cells responding to such sequences include NK cells, NK/T cells, plasmacytoid, and myeloid dendritic cells, as well as monocytes and B cells. Strong Th1 and pro-inflammatory cytokine responses are also induced upon in vivo application of oligoribonucleotides. It appears possible that the presence of highly conserved untranslated terminal regions in the viral genome fulfilling fundamental functions for the viral replication may enable the host to induce directed innate immune defense mechanisms, by allowing pathogen detection through essential RNA regions that the virus cannot readily mutate.