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

Abstract

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-<i>Of</i>Hem C, derived from larval hemocytes of the Asian corn borer <i>Ostrinia furnacalis</i> to assay for encapsulation response<i>,</i> an encapsulation-promoting protein (<i>Of</i>EPP1) was isolated from the plasma of <i>O. furnacalis</i> larvae. <i>Of</i>EPP1 is a novel secretory protein, which exists only in <i>O. furnacalis</i> to date. The <i>OfEpp1</i> gene is intronless and encodes a protein containing several groups of short repetitive sequences and a high proportion of proline residues (18.3%). <i>Of</i>EPP1 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 <i>Of</i>EPP1 promoted hemocytes to encapsulate Sephadex beads, while prokaryotically expressed protein did not, indicating that posttranscriptional modification affects the function of <i>Of</i>EPP1. The encapsulation-promoting function of <i>Of</i>EPP1 could be neutralized by the addition of polyclonal antibodies against <i>Of</i>EPP1 or disrupted by the injection of dsRNA targeting <i>OfEpp1.</i> Eukaryotically expressed <i>Of</i>EPP1 promoted the aggregation, but not spreading, of both granulocytes and plasmatocytes. Immunocytochemistry analysis showed that eukaryotically expressed <i>Of</i>EPP1 could bind to the surface of hemocytes. Therefore, we speculate that <i>Of</i>EPP1 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.