Abstract
Injection of a culture supernatant of Serratia marcescens into the bloodstream of the silkworm Bombyx mori increased the number of freely circulating immunosurveillance cells (hemocytes). Using a bioassay with live silkworms, serralysin metalloprotease was purified from the culture supernatant and identified as the factor responsible for this activity. Serralysin inhibited the in vitro attachment of both silkworm hemocytes and murine peritoneal macrophages. Incubation of silkworm hemocytes or murine macrophages with serralysin resulted in degradation of the cellular immune factor BmSPH-1 or calreticulin, respectively. Furthermore, serralysin suppressed in vitro phagocytosis of bacteria by hemocytes and in vivo bacterial clearance in silkworms. Disruption of the ser gene in S. marcescens attenuated its host killing ability in silkworms and mice. These findings suggest that serralysin metalloprotease secreted by S. marcescens suppresses cellular immunity by decreasing the adhesive properties of immunosurveillance cells, thereby contributing to bacterial pathogenesis.
Highlights
Serratia marcescens induced an acute increase in immune cell numbers in silkworms
We recently demonstrated that bacterial and fungal cell wall components induce the activation of an insect cytokine named paralytic peptide (PP)2 in the silkworm hemolymph [9] and that active PP promotes the expression of adhesion molecules in hemocytes [10]
Decrease in Silkworm Hemocyte Number by Injury Stimulation and Suppression of This Phenomenon by the S. marcescens Culture Supernatant—When silkworms were injured by needles, the number of hemocytes freely circulating in the bloodstream decreased after 0.5 h (Fig. 1A)
Summary
Serratia marcescens induced an acute increase in immune cell numbers in silkworms. Results: Serralysin metalloprotease was identified as a factor that inhibited immune cell adhesion and bacterial clearance. Injection of a culture supernatant of Serratia marcescens into the bloodstream of the silkworm Bombyx mori increased the number of freely circulating immunosurveillance cells (hemocytes). Disruption of the ser gene in S. marcescens attenuated its host killing ability in silkworms and mice These findings suggest that serralysin metalloprotease secreted by S. marcescens suppresses cellular immunity by decreasing the adhesive properties of immunosurveillance cells, thereby contributing to bacterial pathogenesis. In the fruit fly Drosophila melanogaster, mutation of the gene encoding a transmembrane protein mediating bacterial phagocytosis (Eater) leads to an increase in the number of circulating hemocytes [17] Based on these reports, we hypothesized that S. marcescens decreases the adhesive abilities of immune cells and exerts its virulence via the suppression of host cellular immunity. We purified factors that increase the cell density of silkworm hemocytes from the culture supernatant of S. marcescens
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