IRF-5 and IRF-8 govern the response of Human Plasmacytoid Dendritic Cells following TLR9 ligation

Synthetic oligonucleotides (ODNs) containing CpG motifs stimulate human plasmacytoid dendritic cells (pDCs) to produce type-1 interferons (IFN) and pro-inflammatory cytokines. Previous studies demonstrated that interferon regulatory factors (IRFs) played a central role in mediating CpG-induced pDC activation. This work explores the inverse effects of IRF-5 and IRF-8 on CpG dependent gene expression. Results from RNA interference and microarray studies indicate that IRF-5 up-regulates TLR9-driven gene expression whereas IRF-8 down-regulates the same genes. Several findings support the conclusion that IRF-8 inhibits TLR9 dependent gene expression by directly blocking the activity of IRF-5. First, the inhibitory activity of IRF-8 is only observed when IRF-5 is present. Second, proximity ligation analysis shows that IRF-8 and IRF-5 co-localize within the cytoplasm of resting human pDC and co-translocate to the nucleus after CpG stimulation. Taken together, these findings suggest that two transcription factors with opposing functions control TLR9 signaling in human pDCs. CAL-1 cells were transfected with siRNA targeting IRF-5 (IRF-5si) and left unstimulated (n=4, technical repeats) or stimulated with K-type CpG ODN (n=4, technical repeats). CAL-1 cells were transfected with siRNA targeting IRF-8 (IRF-8si) and left unstimulated (n=4, technical repeats) or stimulated with K-type CpG ODN (n=4, technical repeats). CAL-1 cells were transfected with control siRNA (Contsi) and left unstimulated (n=4, technical repeats) or stimulated with K-type CpG ODN (n=4, technical repeats).