Immune response in the hemocyte after phagocytosis influences the infection with Beauveria brongniartii to the silkworm, Bombyx mori

Abstract

Among strains of the silkworm, Bombyx mori, large differences exist in susceptibility to the entomopathogenic fungus Beauveria brongniartii when it is injected into the larval hemocoel. This infection route bypasses the integument, suggesting that internal factors, such as those for cellular and humoral defenses, may correlate with resistance to B. brongniartii. Here, we investigate causes of this resistance. We injected the conidial suspensions into larvae of susceptible and resistant strains; the difference in susceptibility resulted from the difference in the speed and quantity of multiplication of hyphal bodies in the hemocoel. Phenoloxidase activity of the hemolymph declined after hyphal bodies increased in the larval hemocoel of both strains. There were no differences in the mortality rates when the larvae were injected with conidial compared with hyphal body suspensions in both strains; therefore, the mechanism of silkworm resistance to B. brongniartii was likely independent of differences between conidia and hyphal bodies. Furthermore, the germination rate and germ tube length of the conidia and multiplication of the hyphal bodies within the larval hemolymph in vitro did not differ significantly between strains. In addition, granulocytes of the susceptible and resistant strains phagocytosed fluorescent latex beads. We transformed B. brongniartii to express green fluorescent protein; the transformed hyphal bodies were phagocytosed by the larval granulocytes of both strains. Then the hyphal bodies grew from the granulocytes in the susceptible strain but not in the resistant strain; therefore, the abilities of the strains to suppress the fungus within the granulocytes differed.