Constitutive differences in Cu/Zn superoxide dismutase mRNA levels and activity in hemocytes of Biomphalaria glabrata (Mollusca) that are either susceptible or resistant to Schistosoma mansoni (Trematoda)

Abstract

Genetic strains of the snail Biomphalaria glabrata vary in their resistance to the parasite Schistosoma mansoni. Phagocytic cells (hemocytes) circulating in the hemolymph of B. glabrata play an essential role in the snail's innate immune response. Hemocytes of resistant B. glabrata kill S. mansoni in vitro via a mechanism which involves a respiratory burst. Reactive oxygen species (ROS), which are products of the respiratory burst, can act as mediators of both oxidative damage and of immune-related intracellular signaling pathways. One specific ROS, hydrogen peroxide (H2O2), has been shown to be involved in hemocyte-mediated sporocyst killing. We tested the hypothesis that Cu/Zn superoxide dismutase (SOD), a cytosolic enzyme that catalyzes the conversion of superoxide anion to H2O2, is somehow different between resistant and susceptible snail strains. We report a hemocyte transcript with all the features of a typical cytosolic Cu/Zn SOD (GenBank accession numbers AY505496 and AY505497). The amount of Cu/Zn SOD mRNA in hemocytes from resistant snails was double that of hemocytes from susceptible snails, and this correlated directly with an increased Cu/Zn SOD enzymatic activity in resistant hemocytes. Additional experiments determined that in vitro interaction/encapsulation of sporocysts did not influence Cu/Zn SOD mRNA levels in hemocytes from either snail strain. Thus, resistance in this host-parasite system does not appear to depend on a transcriptional response of hemocyte Cu/Zn SOD, but may be due, at least in part, to a constitutively elevated enzymatic level of Cu/Zn SOD.