SUPEROXIDE ANION GENERATION IN DROSOPHILA DURING MELANOTIC ENCAPSULATION OF PARASITES

Quinoid precursors of melanin and/or reactive oxygen species (ROS) gen erated during melanogenesis have been implicated as cytotoxic molecule s in the immune responses of insects against their internal metazoan p arasites. No study has yet identified the killing components produced in conjunction with melanotic encapsulation responses, or explained ho w cytotoxic molecules generated in the open circulatory system of an i nsect can selectively destroy foreign tissues. Strains of Drosophila m elanogaster with differing immune capabilities against the wasp parasi toid Leptopilina boulardi were examined for superoxide anion (O-2(-.)) formation during parasitization. Elevated levels of O-2(-.) were prod uced by immune reactive (R-strain) hosts during melanotic encapsulatio n of the parasitoid, but not by susceptible (S-strain) hosts in which the parasitoid developed unmolested. Both a superoxide dismutase (SOD) -deficient strain (cSOD(n108), red/TM(3)/Sb Ser) and a catalase (CAT)- deficient strain (Cat(n1)) also produced melanotic capsules and elevat ed levels of O-2(-.) when infected, but these reactions were unsuccess ful and the parasitoids survived, indicating that neither the quinoid precursors of melanin nor O-2(-.) per se were cytotoxic. Immune incomp etence in SOD-deficient and CAT-deficient hosts is attributed in part to defects in hydrogen peroxide (H2O2) metabolism, and/or the inabilit y of these metalloenzyme-deficient strains to initiate the metal-media ted reductive cleavage of H2O2 required for the production of the cyto toxic hydroxyl radical ((OH)-O-.). The role proposed for O-2(-.) in Dr osophila cellular immunity is one of potentiating the formation of (OH )-O-.. Melanin, which contains both oxidizing and reducing components, may serve a dual role in producing O-2(-.) and sequestering redoxacti ve metal ions, thereby confining the production of ROS. Host-parasite susceptibility in the Drosophila-Leptopilina system may be determined by the ability of the parasitoid to modulate hemocyte activity and pre vent both effective melanotic encapsulation and the generation of cyto toxic levels of ROS.