Anti-virus effects of interferon regulatory factors (IRFs) identified in ascidian Ciona savignyi.

Interferon regulatory factors (IRFs) are key transcription factors that function in the immune system via the interferon (IFN) pathway. In the current study, we identified and characterized three IRFs (CsIRFL1, CsIRFL2, and CsIRFL3) from ascidian Ciona savignyi. Phylogenetic analysis showed that CsIRFL1 was clustered with two IRFs from Ciona robusta and shrimp IRF apart from the vertebrate IRFs, whereas CsIRFL2 and CsIRFL3 were grouped with an unnamed protein from Oikopleura dioica into a sub-branch highly identifying with the vertebrate IRF4, IRF8, and IRF9. Gene expression analysis revealed that CsIRFL1 and CsIRFL2 expressed in all the examined adult tissues (stomach, intestines, eggs, hemocytes, gonad, heart, and pharynx) and predominantly in hemocytes. However, the expression of CsIRFL3 was undetectable in the tested adult tissues. Furthermore, in situ hybridization showed that CsIRFL1 and CsIRFL2 mainly expressed in immunocytes within hemolymph, including phagocytes, macrophage-like cells, morula cells, and amoebocytes, suggesting CsIRFL1 and CsIRFL2 were involved in ascidian immune responses. We then performed LPS and poly(I:C) challenge assay and found that CsIRFL1 highly expressed in the cultured hemocytes following LPS infection for 24 h. After viral analogue poly(I:C) stimulation, the expression of CsIRFL2 was dramatically upregulated from 12 to 24 h. Meanwhile, two critical components of the IFN signaling pathways, STAT and TBK1, showed the increased expression as well after poly(I:C) induction, indicating that CsIRFL2 and IFN pathways genes were activated under the infection of viral analogue. Thus, our findings suggested that CsIRFL2 was a potential transcriptional regulatory factor that participated in regulating the ascidian anti-virus immune response.

Chitin-based barrier immunity and its loss predated mucus-colonization by indigenous gut microbiota.

Mammalian gut microbiota are integral to host health. However, how this association began remains unclear. We show that in basal chordates the gut space is radially compartmentalized into a luminal part where food microbes pass and an almost axenic peripheral part, defined by membranous delamination of the gut epithelium. While this membrane, framed with chitin nanofibers, structurally resembles invertebrate peritrophic membranes, proteome supports its affinity to mammalian mucus layers, where gut microbiota colonize. In ray-finned fish, intestines harbor indigenous microbes, but chitinous membranes segregate these luminal microbes from the surrounding mucus layer. These data suggest that chitin-based barrier immunity is an ancient system, the loss of which, at least in mammals, provided mucus layers as a novel niche for microbial colonization. These findings provide a missing link for intestinal immune systems in animals, revealing disparate mucosal environment in model organisms and highlighting the loss of a proven system as innovation.