Insects defense mechanisms are based on cellular and humoral innate immune response systems. One of the most important mechanisms of humoral defense is induction of antimicrobial peptides and proteins in response to infection with bacteria or inoculation of bacteria products (lipopolisaccharide-LPS, peptidoglycan) into the body cavity. Cecropins, insect defensins, glicine-rich and proline-rich peptides, lysozyme, hemolin and others have been identified among inducible peptides/proteins in challenged insects.

It is known that P. aeruginosa is an opportunistic human pathogen responsible for many types of infectious diseases. Bacteria secrete several extracellular proteolytic enzymes that have been implicated as virulence factors. Among them there are: protease IV, alkaline protease (aeruginolysin), two elastases , namely Las A (staphylolysin) and Las B (pseudolysin).

It is known that the insect-pathogenic strain P. aeruginosa impaired cellular defenses of the greater wax moth G. mellonella larvae by hemocyte breakdown. It was also demonstrated that P. aeruginosa serine protease IV degraded in vitro and in vivo apolipophorin-III (apolp-III) from hemolymph of G. mellonella larvae, while lysozyme appeared to be resistant for degradation.

In this study we tested the expression level and activity of peptides in hemolymph of G. mellonella larvae infected with P. aeruginosa in different periods of time. Significant increase of peptides activity 18 h after infection was detectable while after 30 h postinjection time only trace of activity was noted. Decrease of antimicrobial activity was correlated with significant decrease of peptides of M_r 4-6 kDa revealed by Tris- tricine SDS/PAGE.

In the following experiments we demonstrated that antimicrobial peptides in hemolymph of G. mellonella was degraded in vitro by P. aeruginosa metalloprotease - elastase B. When immune hemolymph preincubated with enzyme fraction was separated by Tris-tricine SDS/PAGE, peptide bands with molecular mass 4-5 kDa were not observed in comparison to control immune hemolymph. Peptides degradation was inhibited by 1 mM EDTA, additionally demonstrating that digestion was catalysed by metalloprotease. By using bioautography it was also demonstrated that in elastase B treated hemolymph samples antimicrobial activity was absent.

The experimental evidence reported here indicates that peptides proteolytic degradation in G. mellonella hemolymph may be caused by P. aeruginosa elastase B. This protease seems to participate in the virulence against insect immune response.

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