The activation of caspases in immunocompetent cells is an important infection factor of the pathogenic fungus Conidiobolus coronatus (Entomophthorales: Ancylistaceae)

Abstract

Apoptosis is a mechanism commonly used by pathogenic fungi to inhibit the host’s immune response. One opportunistic pathogen is Conidiobolus coronatus, which causes fungal infection in mammals and insects. In a study, larvae of Galleria mellonella were exposed to the pathogen for 24 h. After this exposure, some larvae were used for hemolymph collection (F24), while others were incubated for an additional 24 h (F48). The activity of caspase-9 and caspase-3-like proteins in hemocytes was measured using a colorimetric method. The changes in caspase concentration were calculated using ELISA tests. Immunocytochemical analyses were employed to show changes in the levels of the examined proteins in both their pro- and active forms. Fluorescence microscopy was used to detect changes in cultured hemocytes and flow cytometry analysis was conducted to detect both forms of caspases in freshly collected hemocytes. To evaluate the effect of fungal infection, caspase inhibitors (Z-DEVD-FMK and Z-LEHD-FMK) were injected into the larvae, and their impact on insect development and resistance to fungal infection was determined. The exposure of larvae to the entomopathogen increased the detection levels, concentrations, and activity of both caspase-like proteins in hemocytes during fungal infection. The research has indicated that inhibition of these proteins disrupts larval development and increases resistance to infection. These results suggest that apoptosis might be an important mechanism for a pathogen to inhibit the insect immune response. Given the similarities between insects’ and mammals’ innate immune responses, the presented results may indicate a potential mechanism of fungal pathogenicity in both groups.