Immulectin-3 mediates parasitoid larvae encapsulation by binding to host hemocytes and larval surfaces.

Background: Parasitoids evade host immunity, ultimately causing host death. However, the limited understanding of the mechanisms underlying the host's encapsulation response to parasitoids hinders our ability to comprehend the strategies parasitoids use to overcome host immune defenses.

Results: Based on our previous study, immulectin-3 in Ostrinia furnacalis (OfIML-3) was speculated to function in encapsulating the larvae of Macrocentrus cingulum. OfIML-3 was expressed mainly in the fat body and secreted into the plasma, and its expression could be induced at both the messenger RNA (mRNA) and protein levels by wasp larvae. Approximately 88% of wasp larvae were completely encapsulated by hemocytes after transplantation into naive O. furnacalis larvae; however, this percentage significantly decreased to 37% after the expression of OfIML-3 was knocked down by RNA interference. In the homozygous mutant strain of IML-3 (OfIML-3), the proportion of encapsulated wasp larvae further decreased to 7%. This indicates that OfIML-3 plays a core role in encapsulation. Furthermore, higher levels of OfIML-3 were detected on hemocytes after wasp larvae were transplanted into naive O. furnacalis larvae, and on the surface of the wasp larvae. In the OfIML-3 strain, less OfIML-3 was detected on both surfaces, indicating that the binding of OfIML-3 to both hemocytes and wasp larvae is a crucial step in the encapsulation process.

Conclusion: Our findings suggest that OfIML-3 likely acts as a recognition factor to promote encapsulation of parasitoids by the host. This elucidates the potential mechanisms of encapsulation of closely related species by insects and enhances our insight into parasitoid-host coevolution. © 2025 Society of Chemical Industry.