Immune and cellular impacts in the autogenous Aedes caspius larvae after experimentally-induced stress: Effects of Bacillus thuringiensis infection

Insects possess effective defense mechanisms against pathogens via induction of antimicrobial immune and oxidative stress responses. In this study, immune impact and histological damages in the gastric caeca have been investigated in the 3rd instar larvae of the autogenous Aedes caspius upon infection with Bacillus thuringiensis (Bt). Data showed a significant increase in phenoloxidase (PO) activity by 1.23 folds at 4 h post-infection which then reduced to the normal level at 8 h post-infection and until larval death. Besides, the nitric oxide (NO) titer was significantly increased by 1.4 folds at 4 h post-infection, then, reduced down to its normal level at 8 h post-infection, after which, it was significantly decreasing by time until being hardly detected at 44 h post-infection compared to that of control mosquitoes. Moreover, percentages of cellular apoptosis were significantly elevating from 6 to 48 h post-infection. Consequently, cytological damages in the epithelium and the microvilli of the gastric caeca were observed at 48 h post-infection. Finally, larval body sizes were significantly smaller prior to death (at 48 h post-infection). Taken together, these data suggest further modes of action of Bt as inhibiting the antibacterial immune responses, inducing cellular apoptosis prior to damaging the epithelium of gastric caeca. This may explain – partially at least – the irresistibility and high pathogenicity of Bt against mosquito vector, which may help in understanding, and hence, overcoming the developing resistance by some mosquito vectors to some mosquitocidal bacteria. This may help in improving the biocontrol measures against mosquito vectors.