An Entomopathogenic Bacterium, \u3ci\u3eXenorhabdus nematophila\u3c/i\u3e, Inhibits Hemocytic Phospholipase A2 (PLA2) in Tobacco Hornworms, \u3ci\u3eManduca sexta\u3c/i\u3e

The entomopathogenic bacterium, Xenorhabdus nematophila, induces immunodepression in target insects and finally leads to lethal septicemia of the infected hosts. A hypothesis has been raised that the bacteria inhibit eicosanoid-biosynthesis pathway to interrupt immune signaling of the infected hosts. Here, we show direct evidence that X. nematophila inhibits the activity of phospholipase A2 (PLA2), the initial step in the eicosanoid-biosynthesis pathway. Inhibition of PLA2 was dependent on both incubation time with X. nematophila and the bacterial concentration in in vitro PLA2 preparations of Manduca sexta hemocytes. While living bacteria inhibited PLA2 activity, heat-killed X. nematophila rather increased PLA2 activity. X. nematophila secreted PLA2 inhibitor(s), which were detected in the organic, but not aqueous, extract of the bacterial culture medium. The PLA2 inhibitory activity of the organic extract was lost after heat treatment. These results clearly indicate that X. nematophila inhibits PLA2 activity and thereby inhibits eicosanoid biosynthesis, which leads to immunodepression of the infected hosts