A Thermally Stable Protein EPP1 of Corn Borer Ostrinia furnacalis Regulates Hemocytic Encapsulation

Encapsulation is a vital cellular immune reaction of host insects against endoparasitoids; however, how encapsulation is regulated is still unclear. Utilizing a cell line, SYSU-OfHem C, derived from larval hemocytes of the Asian corn borer Ostrinia furnacalis to assay for encapsulation response, an encapsulation-promoting protein (OfEPP1) was isolated from the plasma of O. furnacalis larvae. OfEPP1 is a novel secretory protein, which exists only in O. furnacalis to date. The OfEpp1 gene is intronless and encodes a protein containing several groups of short repetitive sequences and a high proportion of proline residues (18.3%). OfEPP1 is a thermally stable protein that is mainly expressed in fat bodies, and its accumulation could be induced by the injection of foreign objects (Sephadex beads). Eukaryotically expressed recombinant OfEPP1 promoted hemocytes to encapsulate Sephadex beads, while prokaryotically expressed protein did not, indicating that posttranscriptional modification affects the function of OfEPP1. The encapsulation-promoting function of OfEPP1 could be neutralized by the addition of polyclonal antibodies against OfEPP1 or disrupted by the injection of dsRNA targeting OfEpp1. Eukaryotically expressed OfEPP1 promoted the aggregation, but not spreading, of both granulocytes and plasmatocytes. Immunocytochemistry analysis showed that eukaryotically expressed OfEPP1 could bind to the surface of hemocytes. Therefore, we speculate that OfEPP1 possibly promotes hemocytic encapsulation by binding to the surface of hemocytes as a ligand to induce their aggregation. This study provides evidence clarifying the mechanism of encapsulation in insects.