Eiger pleiotropically regulates the immunity of Ostrinia furnacalis larvae.

The tumor necrosis factor (TNF) receptor superfamily is a set of essential inflammatory cytokines. In Drosophila, as a homolog of TNF, Eiger was first identified, which is involved in innate immunity signaling pathways. However, in Lepidoptera, the functions of Eiger and its molecular regulatory mechanisms on immune responses are unknown. In this study, Eiger was cloned from Ostrinia furnacalis. OfEiger has a conserved structural domain of the TNF family. The expression of OfEiger increased with the age of O. furnacalis larvae, and was the highest in the hemocytes. After O. furnacalis larvae were injected with Pseudomonas aeruginosa or Micrococcus luteus, the expression of OfEiger was significantly upregulated. The expression levels of several genes in the immune pathways of O. furnacalis larvae were downregulated at 60 h post-injection of double-stranded Eiger (dsEiger), including IMD, Toll, nitric oxide (NO), and mitogen-activated protein kinase (MAPK) signaling pathways. Moreover, the expression levels of the 4 antimicrobial peptides (AMPs), OfLebocin, OfAttacin, OfGloverin, and OfMoricin, were downregulated. In addition, the phagocytosis of Escherichia coli by hemocytes was reduced in O. furnacalis larvae after dsEiger injection. Injections of P. aeruginosa or M. luteus into O. furnacalis larvae following OfEiger RNA interference reduced the larvae's survival rate and increased the expression levels of OfMyD88 and OfJNK. Still, they suppressed the expression of OfNOS1 and AMP genes and inhibited phenoloxidase (PO) activity in O. furnacalis larvae. In conclusion, OfEiger is a vital insect immune regulator, which synergistically regulates IMD, Toll, NO, and MAPK signaling pathways and adjusts PO activity, cellular immunity, AMPs, and other effectors.