A secretory PLA 2 associated with tobacco hornworm hemocyte membrane preparations acts in cellular immune reactions

We report on a secretory phospholipase A 2 (sPLA 2 ) associated with membrane‐enriched fractions prepared from hemocytes of the tobacco hornworms, Manduca sexta . Virtually no PLA 2 activity was detected in serum of immunologically naive or bacterially challenged hornworms. PLA 2 activity was detected in cytosolic and membrane‐enriched fractions prepared from hemocytes. PLA 2 activity in the cytosolic fraction (1.2 pmol/mg/h) was approximately 4‐fold greater than in the membrane‐enriched fraction. The cytosol‐associated PLA 2 activity was strongly inhibited in reactions conducted in the presence of the specific cytosolic PLA 2 inhibitor methylarachidonyl fluorophosphate (MAFP) but not in the presence of the sPLA 2 inhibitor p ‐bromophenacyl bromide (BPB). Conversely, the membrane‐associated PLA 2 activity was inhibited in reactions conducted in the presence of BPB but not in the presence of MAFP. While the cytosol‐associated PLA 2 was independent of calcium, the membrane‐associated sPLA 2 required calcium for full catalytic activity. Hornworms treated with either BPB, MAFP or the glucocorticosteroid dexamethasone were severely impaired (by 50 to 80% relative to controls) in their ability to form nodules in reaction to bacterial challenge. However, the immune‐impairing influence of the inhibitors was reversed by treating larvae with arachidonic acid, a precursor for eicosanoid biosynthesis. We infer that the biological significance of the sPLA 2 (as well as the previously characterized cytosolic PLA 2 ) relates to hydrolysis of polyunsaturated fatty acids from cellular phospholipids. Moreover, this enzyme may be the target of immunity‐impairing factors from the bacterium Xenorhabdus nematophila . The fatty acids serve as precursors for the generation of eicosanoids responsible for mediating and coordinating cellular immune reactions to infection. Arch. Insect Biochem. Physiol. 60:105–115, 2005. Published 2005 Wiley‐Liss, Inc.