sPLA2 behaves like a prophylactic agent and mediates cellular and humoral immune responses in Plutella xylostella

Most immune effectors are inducible to microbial pathogen infection while some are already present to act as prophylactic immunity against as yet unseen infection. This study identified secretory phospholipase A2 (sPLA2) as a prophylactic factor in diamondback moth, Plutella xylostella. Western blotting using a polyclonal antibody raised against other lepidopteran sPLA2 reacted specifically with ∼25 kDa protein, which was present at approximately 0.4 mM in the plasma of naïve larvae. Interrogation of P. xylostella transcriptomes revealed an open‐reading frame for sPLA2 (Px‐sPLA2), exhibiting high homology with other Group III sPLA2s. Px‐sPLA2 was expressed in all developmental stages. In the larval stage, bacterial challenge induced its expression in hemocytes and fat body but not in gut or epidermis. RNA interference (RNAi) suppressed Px‐sPLA2 messenger RNA level and sPLA2 activity in plasma. An inhibition zone assay showed that Px‐sPLA2 exhibited antibacterial activities against different species, because specific RNAi knockdown impaired the activity. The RNAi treatment also suppressed the cellular immune response assessed by hemocyte nodule formation and humoral immune response assessed by antimicrobial peptide gene expression. Finally, benzylideneacetone (BZA, a specific sPLA2 inhibitor) treatment inhibited plasma sPLA2 activity of naive larvae in a dose‐dependent manner. An addition of BZA significantly increased the bacterial virulence of an entomopathogen, Bacillus thuringiensis. These results suggest that Px‐sPLA2 is an immune‐associated factor of P. xylostella and its relatively high level of concentration in the plasma of naive larvae strongly suggests its role as a prophylactic factor in defending against pathogens at early infection stages. Secretory PLA2 (sPLA2) of Plutella xylostella is identified using bioinformatics tools. RNA interference of sPLA2 expression leads to immunosuppression in cellular and humoral immune responses. Research Highlights Secretory PLA2 (sPLA2) gene is identified using bioinformatic tools in Plutella xylostella. RNA interference of sPLA2 expression led to immunosuppression. Naïve larvae keep ∼0.4 mM of sPLA2 in plasma.