IMMUNE-RESPONSE IN INSECTS - THE ROLE OF PHENOLOXIDASE IN DEFENSE REACTIONS IN RELATION TO MELANIZATION AND SCLEROTIZATION

It is well known that activated prophenoloxidase (proPO) plays an impo rtant role in cuticular melanization and sclerotization. In addition, studies dealing with immune response of insects suggest that phenoloxi dase (PO) is also critical in the defense reactions of insects against invaders. proPO is activated by elicitors derived from microbial cell wall components such as peptidoglycan, beta-1,3-glucan, and lipopolys accharide (LPS). According to our recent studies we proposed a model c larifying the role of PO in both cellular and humoral immune responses . LPS triggers Ceratitis capitata hemocytes via induced protein tyrosi ne phosphorylation to release biologically active molecules, including p47 and proPO-activators. Furthermore, hemocytes in response to LPS f acilitate clearance of LPS from the hemocoel of medfly. The effector m olecules involved in the LPS clearance are hemocyte surface-associated p47 (mp47), soluble p47 (sp47), activated proPO, and tyrosine. A simi lar LPS clearance system in the integument of medfly in vitro was also demonstrated. According to our data, the proposed mechanism for LPS c learance from hemocoel and from integument is the crosslinking of LPS to p47 or certain integumental proteins via the intermediacy of reacti ve tyrosine derivatives generated by PO activity, as is the case for c uticular protein-chitin crosslinks during sclerotization. We also demo nstrated that metabolites of the eumelanin biosynthesis and not melani n itself or N-acetyldopamine (NADA), the key precursor of sclerotizing agent, were necessary for the immune responses by hemocytes and integ ument. (C) 1996 Wiley-Liss, Inc.