Innate immune responses in the silkworm, Bombyx mori

—Phagocytosis of hemocytes and inducible synthesis of antibacterial proteins—

Abstract

Insects have developed powerful innate immune system, but lack acquired immune function for antibody production and immunological memory similar to that present in vertebrates. Insect immune system is divided into two major reaction types, cellular and humoral immune responses.

Actual protective responses were shown in the silkworm larvae, Bombyx mori, which had be anaesthetized on a frozen thermal gel and subsequently injected with various inocula. As a cellular response, phagocytic activity of hemocytes was observed 3 hr after injection of India ink, followed by accumulation of the ink carbon particles in hemocytes for successive 1 day. When polystyrene beads were injected as an inoculum, the uptake of beads into hemocytes was observed evidently. Chemical protection induced by humoral immune response was also examined, using larvae injected with heat-killed bacteria and bacterial lipo-polysaccharide (LPS). Antibacterial activities in hemolymph were detected constantly 6–24 hr after injection, which were assayed by the agar cup-plate method. In addition, hemolymph samples immunized for 6, 12 and 24 hr were analyzed by native-polyacrylamide gel electrophoresis, demonstrating that inducible antibacterial proteins were found in these hemolymph samples. Based on the present study, the topic of insect immune responses is thought to be accepted as a useful teaching material in high school.