The immunotoxicity of Cd exposure to gypsy moth larvae: An integrated analysis of cellular immunity and humoral immunity

A new insight into Cd exposure-induced hemocyte reduction in Lymantria dispar larvae: Involvement of the ROS-ATF6-ER stress-apoptosis pathway

Hemocytes are important targets for heavy metal-induced immunotoxicity in insects. This study aimed to investigate the mechanism by which cadmium (Cd) exposure affects the hemocyte count in Lymantria dispar larvae. The results showed that the number of larval hemocytes was significantly decreased under Cd exposure, accompanied by a significant increase in the apoptosis rate and the expression of Caspase-3. The endoplasmic reticulum (ER) of hemocytes in the Cd-treated group showed irregular swelling. Expression levels of ER stress indicator genes (CHOP, Bip1, Bip2, Bip3, and Bip4) were significantly higher in the Cd-treated group. Among the three pathways that potentially mediate ER stress, only the key genes in the ATF6 pathway (ATF6, S1P-1, S1P-2, and WFS1) exhibited differential responses to Cd exposure. Cd exposure significantly increased the levels of reactive oxygen species (ROS) and the expression of oxidative stress-related genes (CNCC, P38, and ATF2) in hemocytes. Studies using inhibitors confirmed that apoptosis mediated the decrease in hemocyte count, ER stress mediated apoptosis, ATF6 pathway mediated ER stress, and ROS or oxidative stress mediated ER stress through the activation of the ATF6 pathway. Taken together, the ROS-ATF6-ER stress-apoptosis pathway is responsible for the reduction in the hemocyte count of Cd-treated L. dispar larvae.

Changes in the Biology and Susceptibility of Weevil (Cylas formicarius) to the Insecticide Spinetoram as a Response to Cadmium Contamination

The sweet potato weevil Cylas formicarius is a notorious underground pest in sweet potato (Ipomoea batatas L.). However, little is known about the effects of cadmium (Cd) stress on weevil biology and resistance to pesticides and biotic agents. Therefore, we fed sweet potato weevils with Cd-contaminated sweet potato and assessed adult food intake and survival and larval developmental duration and mortality rates, as well as resistance to the insecticide spinetoram and susceptibility to the entomopathogenic fungus Beauveria bassiana. With increasing Cd concentration, the number of adult weevil feeding holes, adult survival and life span, and larval developmental duration decreased significantly, whereas larval mortality rates increased significantly. However, at the lowest Cd concentration (30 mg/L), adult feeding was stimulated. Resistance of adult sweet potato weevils to spinetoram increased at low Cd concentration, whereas Cd contamination did not affect sensitivity to B. bassiana. Thus, Cd contamination affected sweet potato weevil biology and resistance, and further studies will investigate weevil Cd accumulation and detoxification mechanisms.

Toxic effect of Cd burden on the gut microflora and immune responses of wolf spider Pardosa pseudoannulata

Spiders are dominant predators in the paddy ecosystem, but the immunotoxicity induced by environmental contaminants like heavy metals is still largely unknown. The gut microbiota-host interaction was the basic immune mechanism discovered in the arthropods. Here, we investigated the gut microflora and immune responses of wolf spider Pardosa pseudoannulata under Cd burden. Cd exposure was identified to shape the gut microbial community structure of spiders, with increased levels of Firmicutes and pathogens, and decreased levels of Proteobacteria and Bacteroidota. The alteration of microbiota-derived immune messengers like peptidoglycan (PGN) was also observed. ELISA and hemolymph metabolomic analysis showed that the activities of immune effectors phenoloxidase (PO) and lysozyme (LZM) and the abundance of tyrosine derivates were decreased, which indicated the suppression of Cd on the melanization immune response of spiders. Correlation analysis revealed a close relationship between the impaired immune system and the disordered microbiota. This study provides insight into the underlying mechanisms of the gut microflora-immune system interaction of P. pseudoannulata in response to Cd burden.

Transfer of Cd along the food chain: The susceptibility of Hyphantria cunea larvae to Beauveria bassiana under Cd stress

Heavy metal can affect the bio-control efficiency of entomopathogenic fungi on pests, but this has not been studied in the food chain. Here, the food chain of soil-Fraxinus mandshurica-Hyphantria cunea was constructed to investigate the effect of cadmium (Cd) exposure on the susceptibility of H. cunea larvae to Beauveria bassiana (Bb) and to analyze the corresponding mechanism through larval innate immunity and energy metabolism. Cd through the food chain synergistically enhanced the susceptibility of H. cunea larvae to Bb. Cellular immunity-related parameters decreased when the Cd treatment group was compared with the control group and when the combined treatment group of Cd and Bb was compared with the Bb treatment group. Cd exposure induced hormesis on pathogen recognition and signal transduction genes of humoral immunity, but reduced the expression of effector genes. The expression of the 13 humoral immunity-related genes in the combined treatment group was lower than in the Bb treatment group. Cd exposure decreased the energy storage of H. cunea larvae before Bb infection and aggravated the disorder level of energy metabolism after Bb infection. Taken together, disturbance of innate immunity and energy metabolism improves the susceptibility of H. cunea larvae to Bb in the Cd-polluted food chain.

Colonization Resistance of Symbionts in Their Insect Hosts

The symbiotic microbiome is critical in promoting insect resistance against colonization by exogenous microorganisms. The mechanisms by which symbionts contribute to the host's immune capacity is referred to as colonization resistance. Symbionts can protect insects from exogenous pathogens through a variety of mechanisms, including upregulating the expression of host immune-related genes, producing antimicrobial substances, and competitively excluding pathogens. Concordantly, insects have evolved fine-tuned regulatory mechanisms to avoid overactive immune responses against symbionts or specialized cells to harbor symbionts. Alternatively, some symbionts have evolved special adaptations, such as the formation of biofilms to increase their tolerance to host immune responses. Here, we provide a review of the mechanisms about colonization resistance of symbionts in their insect hosts. Adaptations of symbionts and their insect hosts that may maintain such symbiotic relationships, and the significance of such relationships in the coevolution of symbiotic systems are also discussed to provide insights into the in-depth study of the contribution of symbionts to host physiology and behavior.

Susceptibility of Hyphantria cunea larvae to Beauveria bassiana under Cd Stress: An integrated study of innate immunity and energy metabolism

Biological control is widely used for integrated pest management. However, there are many abiotic factors that can affect the biocontrol efficiency. In this study, we investigated the susceptibility of Hyphantria cunea larvae to Beauveria bassiana under Cd stress, and the corresponding mechanism was analyzed around innate immunity and energy metabolism. The results showed that mortality of H. cunea larvae treated with Cd and B. bassiana was significantly higher than those treated with B. bassiana alone, and the combined lethal effect exhibited a synergistic effect. Compared with the single fungal treatment group, the total hemocyte count in the combined Cd and fungal treatment group decreased significantly, accompanied by a decrease in phagocytosis, encapsulation, and melanization activity. The expression levels of three phagocytosis-related genes, one encapsulation-promoting gene, and one melanization-regulating gene were significantly lower in the combined treatment group than those in the single fungal treatment group. Furthermore, pathogen recognition ability, signal transduction level, and immune effector expression level were weaker in the combined treatment group than those in the single fungal treatment group. The expression levels of 14 key metabolites and 7 key regulatory genes in glycolysis and tricarboxylic acid cycle pathways were significantly lower in the combined treatment group than those in the single fungal treatment group. Taken together, the weakness of innate immunity and energy metabolism in response to pathogen infection resulted in an increased susceptibility of H. cunea larvae to B. bassiana under Cd pre-exposure. Microbial insecticide is a preferred strategy for pest control in heavy metal-polluted areas. AVAILABILITY OF DATA AND MATERIAL: All the data that support the findings of this study are available in the manuscript.

The susceptibility of Hyphantria cunea larvae to microbial pesticides Bacillus thuringiensis and Mamestra brassicae nuclear polyhedrosis virus under Cd stress

Heavy metal pollution is an abiotic factor that can affect the efficiency of pest control. In this study, two microbial pesticides, Bacillus thuringiensis and Mamestra brassicae nuclear polyhedrosis virus (MbNPV), were used to treat Hyphantria cunea larvae with Cd pre-exposure, and the humoral and cellular immunity of H. cunea larvae with Cd exposure were evaluated. The results showed that Cd exposure increased the susceptibility of H. cunea larvae to microbial pesticides B. thuringiensis and MbNPV, and the lethal effect of Cd exposure and microbial pesticides on H. cunea larvae was synergistic. Cd exposure significantly decreased the expression of pathogen recognition genes (GNBP1 and GNBP3), signal transduction genes (Relish, Myd88, Tube, and Imd), and antimicrobial peptide gene (Lebocin) in the humoral immunity of H. cunea larvae compared with the untreated larvae. Parameters of cellular immunity, including the number of hemocytes, phagocytic activity, melanization activity, encapsulation activity, and the expression of three phagocytic regulatory genes (HEM1, GALE1, GALE2), were also found to decrease significantly in Cd-treated larvae. TOPSIS analysis showed that humoral immunity, cellular immunity, and total immunity levels of H. cunea larvae with Cd exposure were weaker than those in untreated larvae. Correlation analysis showed that the mortality of two microbial pesticides investigated in H. cunea larvae was negatively correlated with the humoral and cellular immunity of larvae. Taken togther, Cd exposure results in immunotoxic effects on H. cunea larvae and the use of microbial pesticides are an effective strategy for pest control in heavy metal-polluted areas.