Abstract
The insect immune response is initiated by the recognition of invading microorganisms. Peptidoglycan recognition proteins (PGRPs) function primarily as pattern recognition receptors by specifically binding to peptidoglycans expressed on microbial surfaces. We cloned a full-length cDNA for a PGRP from the Asian corn borer Ostrinia furnacalis (Guenée) and designated it as PGRP1. PGRP1 mRNA was mainly detected in the fat bodies and hemocytes. Its transcript levels increased significantly upon bacterial and fungal challenges. Purified recombinant PGRP1 exhibited binding activity to the gram-positive Micrococcus luteus, gram-negative Escherichia coli, entomopathogenic fungi Beauveria bassiana, and yeast Pichia pastoris. The binding further induced their agglutination. Additionally, PGRP1 preferred to bind to Lys-type peptidoglycans rather than DAP-type peptidoglycans. The addition of recombinant PGRP1 to O. furnacalis plasma resulted in a significant increase in phenoloxidase activity. The injection of recombinant PGRP1 into larvae led to a significantly increased expression of several antimicrobial peptide genes. Taken together, our results suggest that O. furnacalis PGRP1 potentially recognizes the invading microbes and is involved in the immune response in O. furnacalis.
Highlights
Insects lack the acquired immune system possessed by vertebrates and mainly rely on a rapid and effective innate immune system to defend against microbial infections [1,2]
The results from the qPCR assay indicated that the mRNA abundance of O. furnacalis PGRP1 increased significantly in the larvae challenged by E. coli or B. bassiana conidia (Figure 2C)
The results showed that the recombinant PGRP1 caused increased agglutination of E. coli, M. luteus, and
Summary
Insects lack the acquired immune system possessed by vertebrates and mainly rely on a rapid and effective innate immune system to defend against microbial infections [1,2]. Some PGRPs perform amidase activities by hydrolyzing the bond between the N-acetylmuramyl group in the glycan strand and the L-alanine in the stem peptide of peptidoglycan and generating non-immunogenic peptidoglycan fragments [11]. These PGRPs are classified as catalytic PGRPs and act as immune modulators. They only can recognize and bind to, but not cleave, bacterial peptidoglycans [21] They function as sensors for ligand-dependent signaling to activate the prophenoloxidase (PPO) cascade or produce antimicrobial peptides (AMPs) via the Toll or Imd pathway [23,24]. The injection of recombinant PGRP1 into larvae led to a significant increase in the synthesis of some antimicrobial peptides (AMPs), including attacin, cecropin-4, gloverin-1, and moricin-4
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