Fat bodies responsible for the degradation of encapsulated foreign substances in the body cavity of Mythimna separata larvae.

In the immune systems of insects, the fat body synthesizes most of the immune proteins found in the hemolymph. We investigated a novel role for the fat body in immune processing of encapsulated parasitoid larvae using fat bodies from Mythimna separata (Ms) larvae. When first instar larvae of Cotesia kariyai (Ck) were transplanted into Ms larvae, time-dependent melanization and encapsulation occurred, and the surface area of Ck larvae began to decrease 24 h post-transplantation. Histological analysis revealed physical contact between encapsulated Ck larvae and the fat body of the host. In vitro co-culture experiments showed that the size of Ck larvae decreased significantly when incubated with fat bodies, and this effect could be inhibited by leupeptin, a serine protease inhibitor. Immunofluorescence staining detected the presence of trypsin in the fat body, and qPCR analysis demonstrated upregulation of trypsin mRNA following co-culture. These findings suggest that at least Ms fat bodies are involved in the enzymatic degradation of encapsulated foreign substances via trypsin expression and secretion, and highlight a novel effector function of insect fat bodies in the later stages of innate immune defense.