CRISPR-based functional analysis of chicken IRF9 reveals distinct modulation of dsRNA stimulated innate immune pathways

The chicken immune system is distinct from mammalian models due to its reduced immune gene repertoire, yet it retains the ability to mount a highly effective immune response. In mammals, interferon regulatory factor 9 (IRF9) is a key transcriptional regulator of the type I interferon (IFN) pathway, stimulating the expression of hundreds of antiviral genes. Although IRF9 was previously thought to be absent in chickens, current chicken reference genome annotation (bGalGal1.mat.broiler.GRCg7b) lists a putative chicken IRF9. To investigate the function of this gene in chickens, we utilized a clustered regularly interspace short palindromic repeats (CRISPR) based transcriptional modulation platform to elucidate the role of the putative chicken IRF9 in the innate immune response. We analyzed the transcriptomes of IRF9 repressed cells stimulated with double stranded RNA at 0, 0.5, 1, and 6 h post-stimulation. Gene set enrichment analysis revealed that IRF9 repression resulted in the enrichment of pathways associated with regulating the type I IFN response, including the retinoic acid inducible gene I like (RIG-I like) receptor pathway and the Toll-like receptor pathway. Furthermore, concurrent transcriptional repression of type I IFN modulator IRF7 with transcriptional activation of IRF9 failed to rescue the expression of downstream IFN-stimulated genes. These results suggest chicken IRF9 plays a distinct regulatory role from canonical mammalian IRF9 in the type I IFN response and demonstrate a need for functional evidence-based classification of chicken IRFs.