Regeneration of cultured midgut cells after exposure to sublethal doses oftoxin from two strains of Bacillus thuringiensis

Toxin from two strains of Bacillus thuringiensis (Bt), AA 1-9 and HD-73, ca used dose-dependent destruction of cultured midgut cells from Heliothis vir escens larvae. HD-73 toxin was more effective although, at the doses used, not all cells were killed. After 2 days of exposure to 0.8 pg/mul AA 1-9 or 0.06 pg/mul HD-73, columnar and goblet cell numbers declined to ca 20% of controls. In contrast, stem and differentiating cells increased to 140-200% of controls. The dynamic of depletion and replacement depended on toxin ty pe and concentration. Two days after toxin was washed out, ratios of cell t ypes returned to approximate control levels, suggesting rapid population co rrections in vitro. Regulation of the ratio of cell types in each populatio n, and the rate of proliferation and differentiation of stem cells was indu ced by the cultured midgut cells themselves. Controls and cells treated wit h toxin from Bt strain AA 1-9 were stained using a polyclonal antibody to L epidopteran midgut differentiation factor 1 (MDF1). With Bt toxin, 1.5 time s more cells stained for MDF1, suggesting increased synthesis of this diffe rentiation factor during increased stem cell differentiation. The response of cultured midgut cells to Bt toxin injury is similar to injured vertebrat e tissues dependent on stem cells for replacement and healing. Published by Elsevier Science Ltd.