Insect cytokine paralytic peptide activates innate immunity via nitric oxide production in the silkworm Bombyx mori.

Insect cytokine paralytic peptide (PP) upregulates the expression of immune-related genes and contributes to host defense in the silkworm Bombyx mori. The present findings demonstrated that PP promotes nitric oxide (NO) production and induces the expression of NO synthase. A pharmacologic NO synthase inhibitor suppressed the PP-dependent (i) induction of immune-related genes, (ii) activation of p38 mitogen-activated protein kinase, and (iii) killing delay of silkworm larvae by Staphylococcus aureus. The upstream mechanism of NO synthesis in insect immunity has been unknown, and the present results suggest for the first time that an insect cytokine induces NO and contributes to self-defense.