Cecropin-2

Cecropins and defensins are important antimicrobial peptides in insects and are inducible after injection of immune triggers. In this study, we cloned the cDNAs of two antimicrobial peptides (AMPs), cecropin-2 (BdCec-2) and defensin (BdDef) from Bactrocera dorsalis (Hendel), a serious pest causing great economic losses to fruits and vegetables. The BdCec-2 sequence of 192bp encodes a protein of 63 amino acids residues with a predicted molecular weight of 6.78kD. The 282bp cDNA of BdDef encodes a protein of 93 residues with a predicted molecular weight of 9.81kD. Quantitative real-time PCR analyses showed that BdCec-2 and BdDef had similar expression profiles among development stages, the highest mRNA levels of these two AMP genes were observed in the adult stage. Among different adult body segments and tissues, both genes had similar transcriptional profiles, the highest mRNA levels appeared in abdomen and fat body, which was consistent with the reported fact that fat body was the main organ synthesizing AMPs in insects. The expression of BdCec-2 and BdDef were up-regulated after challenge with peptidoglycans from Escherichia coli (PGN-EB) and Staphylococcus aureus (PGN-SA), respectively, suggesting their antimicrobial activity against Gram-negative and Gram-positive microorganisms. These results describe for the first time the basic properties of the cecropin-2 and defensin genes from B. dorsalis that probably play an important role in the defense response against invading microbes.

The β-glucan recognition proteins (βGRPs) play a significant role as important pattern recognition proteins (PRPs) for recognizing conserved surface determinants of pathogens and trigger complex signaling pathways in invertebrates. In the present study, a full-length cDNA 1793bp encoding 479 amino acids and βGRP1 was obtained from Plutella xylostella by reverse transcription PCR (RT-PCR) (designed as P×βGRP1) which showed significant similarities with other insect's βGRPs. The transcription level was constitutively expressed and upregulated by microbial induction in all life stages of P. xylostella. Tissue distribution showed P×βGRP1 to be mainly expressed in fat body as detected by quantitative real-time PCR (qRT-PCR) and immunohistochemistry. Subsequent to knock down the P×βGRP1 expression and the transcripts of Toll-like receptor, cecropin 1 and cecropin 2 decreased in P. xylostella. Meanwhile, the bacterial colonies increased and the expression of four AMP genes decreased on injection of anti-P×βGRP1 into Bombyx mori. The results demonstrated that P×βGRP1 can play a vital role in response to the expression of AMP genes in P. xylostella.

20-hydroxyecdysone (20E) increases its titre level during the wandering stage and influences innate immunity in many holometabolous insects. However, the function of 20E as an immune-activator or -suppressor needs to be determined. Here, the transcriptome of the peptidoglycan-challenged fat body of the cotton bollworm, Helicoverpa armigera, was analysed using Illumina sequencing technology. Overall, 32 073 unigenes were assembled with a mean length of 643 nucleotides. Gene expression dynamics in the fat body during the wandering stage and of peptidoglycan-challenged individuals were investigated by the digital gene expression system. Pattern recognition receptors [such as peptidoglycan recognition protein B (PGRP B), PGRP S2 precursor, C-type lectin 5, hemolin and β-1,3-glucan recognition protein 2a] and antimicrobial peptides (namely attacin, gloverin, gloverin precursor, gloverin-like, cecropin 2, cecropin D, cecropin D-like and i-type lysozyme) significantly increased their mRNA levels during the wandering stage. 20E treatment significantly induced the expression of these genes. Antibacterial activities were also enhanced during the wandering stage and after 20E injections. Bacillus subtilis peptidoglycan induced the expression of PGRP D, PGRP B, PGRP S2 precursor, gloverin, gloverin precursor, gloverin-like, cecropin 2, cecropin D and lebocin-like genes. These results demonstrate that 20E acts by enhancing humoral immunity in H. armigera.