IMMUNOCYTOCHEMICAL LOCALIZATION OF BACILLUS-THURINGIENSIS INSECTICIDAL CRYSTAL PROTEINS IN INTOXICATED INSECTS

The damage of different insecticidal crystal proteins (ICPs) from Bacillus thuringiensis to the midgut of Manduca sexta, Plutella xylostella, and Leptinotarsa decemlineata larvae was determined by light microscopic observations. During the intoxication process, the distribution of the different ICPs in the larval body was monitored with specific antibodies. In lepidopteran and in coleopteran insects, histopathological changes include disruption of the brush border, vacuolization of the cytoplasm, hypertrophy of the epithelial cells, and disintegration of the cell. After ingestion by the insect larvae, the ICPs rapidly accumulate in the peritrophic membrane. However, the binding to the peritrophic membrane does not correlate with toxicity, since the coleopteran-specific toxin (CryIIIA) is also retained in the peritrophic membrane of lepidopteran insects, while the lepidopteran-specific toxin CryIA(b) binds to the peritrophic membrane of the Colorado potato beetle larvae. In contrast, ICPs bind to the microvilli of the midgut epithelial cells of susceptible insects only, confirming a correlation between toxicity and binding to the brush border membrane. In the lepidopteran larvae, the lepidopteran-specific toxic ICPs initially accumulate at the apical microvilli of the epithelial cells in the anterior part of the midgut. In the Colorado potato beetle larvae, CryIIIA is primarily retained by the microvilli of the epithelial cells from the posterior part of the midgut. During the intoxication process, internalization of the ICPs into midgut epithelial cells is not detected, even several hours after toxin ingestion. Apparently, the ICPs are retained within the gut since they were not detected in other organs such as the Malpighian tubules. © 1992.