Immunosuppressive Activities of Novel PLA Inhibitors from , an Entomopathogenic Bacterium.

Eicosanoids mediate both cellular and humoral immune responses in insects. Phospholipase A (PLA) catalyzes the first committed step in eicosanoid biosynthesis. It is a common pathogenic target of two entomopathogenic bacteria, and . The objective of this study was to identify novel PLA inhibitors from and determine their immunosuppressive activities. To identify novel PLA inhibitors, stepwise fractionation of culture broth and subsequent enzyme assays were performed. Eight purified fractions of bacterial metabolites were obtained. Gas chromatography and mass spectrometry (GC-MS) analysis predicted that the main components in these eight fractions were 2-cyanobenzoic acid, dibutylamine, 2-ethyl 1-hexanol, phthalimide (PM), dioctyl terephthalate, docosane, bis (2-ethylhexyl) phthalate, and 3-ethoxy-4-methoxyphenol (EMP). Their synthetic compounds inhibited the activity of PLA in hemocytes of a lepidopteran insect, , in a dose-dependent manner. They also showed significant inhibitory activities against immune responses such as prophenoloxidase activation and hemocytic nodulation of larvae, with PM and EMP exhibiting the most potent inhibitory activities. These immunosuppressive activities were specific through PLA inhibition because an addition of arachidonic acid, a catalytic product of PLA, significantly rescued such suppressed immune responses. The two most potent compounds (PM and EMP) showed significant insecticidal activities after oral administration. When the compounds were mixed with (Bt), they markedly increased Bt pathogenicity. This study identified eight PLA inhibitors from bacterial metabolites of and demonstrated their potential as novel insecticides.