Abstract
Antimicrobial peptides (AMPs) and lysozymes are the main effectors of the insect immune system, and they are involved in both local and systemic responses. Among local responses, midgut immune reaction plays an important role in fighting pathogens that reach the insect body through the oral route, as do many microorganisms used in pest control. Under this point of view, understanding how insects defend themselves locally during the first phases of infections caused by food-borne pathogens is important to further improve microbial control strategies. In the present study, we analyzed the transcriptional response of AMPs and lysozymes in the midgut of Spodoptera exigua (Lepidoptera: Noctuidae), a polyphagous pest that is commonly controlled by products based on Bacillus thuringiensis (Bt) or baculovirus. First, we comprehensively characterized the transcripts encoding AMPs and lysozymes expressed in S. exigua larval midgut, identifying 35 transcripts that represent the S. exigua arsenal against microbial infection. Secondly, we analyzed their expression in the midgut after ingestion of sub-lethal doses of two different pore-forming B. thuringiensis toxins, Cry1Ca and Vip3Aa, and the S. exigua nucleopolyhedrovirus (SeMNPV). We observed that both Bt toxins triggered a similar, wide and in some cases high transcriptional activation of genes encoding AMPs and lysozymes, which was not reflected in the activation of the classical systemic immune-marker phenoloxidase in hemolymph. Baculovirus ingestion resulted in the opposed reaction: Almost all transcripts coding for AMPs and lysozymes were down-regulated or not induced 96 hours post infection. Our results shed light on midgut response to different virulence factors or pathogens used nowadays as microbial control agents and point out the importance of the midgut immune response contribution to the larval immunity.
Highlights
Antimicrobial peptides (AMPs) and lysozymes are small proteins produced by a wide range of organisms which have direct antimicrobial activity against pathogens like bacteria, fungi and viruses [1,2]
All the transcripts identified after transcriptome sequence mining were amplified using cDNA synthesized from midgut RNA, revealing that these antimicrobial proteins are actively transcribed in S. exigua midgut in normal conditions
This work has addressed the response of local immune system effectors in S. exigua after ingestion of different virulence factors (Bt toxins such Cry1Ca and Vip3Aa) and a viral pathogen (SeMNPV) used in microbial control agricultural practices
Summary
Antimicrobial peptides (AMPs) and lysozymes are small proteins produced by a wide range of organisms (from bacteria to humans) which have direct antimicrobial activity against pathogens like bacteria, fungi and viruses [1,2] In eukaryotes, they are part of the innate immune system and they act as chemical effectors to arrest microbial infections. Since the discovery of the first insect AMP [5], the role of antimicrobial proteins in humoral response has been broadly investigated [2], their contribution to the local response was discovered later [6] and to date it has only been studied in model organisms [3,7,8] Besides their functions in the direct clearance of invading microbes in immune response, antimicrobial proteins like AMPs and lysozymes exert other important functions as immuno-modulatory activities [9] and maintenance of the endosymbiotic bacteria homeostasis [10]
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