Entomopathogenic fungal infection leads to temporospatial modulation of the mosquito immune system

Abstract

Alternative methods of mosquito control are needed to tackle the rising burden of mosquito-borne diseases while minimizing the use of synthetic insecticides, which are threatened by the rapid increase in insecticide resistance in mosquito populations. Fungal biopesticides show great promise as potential alternatives because of their ecofriendly nature and ability to infect mosquitoes on contact. Here we describe the temporospatial interactions between the mosquito Aedes aegypti and several entomopathogenic fungi. Fungal infection assays followed by the molecular assessment of infection-responsive genes revealed an intricate interaction between the mosquito immune system and entomopathogenic fungi. We observed contrasting tissue and time-specific differences in the activation of immune signaling pathways and antimicrobial peptide expression. In addition, these antifungal responses appear to vary according to the fungal entomopathogen used in the infection. Enzyme activity-based assays coupled with gene expression analysis of prophenoloxidase genes revealed a reduction in phenoloxidase (PO) activity in mosquitoes infected with the most virulent fungal strains at 3 and 6d post-fungal infection. Moreover, fungal infection led to an increase in midgut microbiota that appear to be attributed in part to reduced midgut reactive oxygen species (ROS) activity. This indicates that the fungal infection has far reaching effects on other microbes naturally associated with mosquitoes. This study also revealed that despite fungal recognition and immune elicitation by the mosquito, it is unable to successfully eliminate the entomopathogenic fungal infection. Our study provides new insights into this intricate multipartite interaction and contributes to a better understanding of mosquito antifungal immunity.

Aedes aegypti (Diptera: Culicidae) Immune Responses with Different Feeding Regimes Following Infection by the Entomopathogenic Fungus Metarhizium anisopliae

“…Thioester-containing proteins (TEPs) are known to bind to foreign surface molecules and are thus considered critical for pathogen recognition [29]. TEPs have been shown to be involved in antifungal immunity in mosquitoes [30,31].…”

“…The Toll pathway can regulate the expression of several antimicrobial peptides, including DEF1 (defensin A and C), CEC1 (cecropin A), and GAM1 (gambicin) [37][38][39][40][41] The IMD pathway is another major signaling pathway that plays an essential role in mosquito immunity. This pathway has some downstream components overlapping with that of the Toll pathway in eliciting an immune response and has also been shown to confer immunity to fungal infection [30]. Like REL1 in the Toll pathway, REL2 plays a central role in IMD signaling as a downstream transcription factor [42].…”

Peptidoglycan Recognition Proteins (PGRPs) Modulates Mosquito Resistance to Fungal Entomopathogens in a Fungal-Strain Specific Manner

“…To compare the levels of expression of these important pathogen recognition receptors, we also measured two molecules that have been previously found to be elicited in response to fungal infection, CLSP2 and TEP22 Ramirez et al, 2018a). Our results indicate a significant elicitation of CLSP2 at 48 h PI in I. javanica-infected mosquitoes and at 60 and 72 h in mosquitoes infected with either fungi.…”

“…Briefly, fungal cultures of Beauveria bassiana (MBC 076) and Isaria javanica (ARSEF 5874) were grown on ¼ strength Sabouraud dextrose agar and yeast extract (SDAY) medium at 26 • C for 15-days. Spore oil formulations were prepared as previously reported (Ramirez et al, 2018a). Briefly, fungal spores were harvested from 15-day old cultures with soy bean oil as a carrier.…”

“…Fungal-based biopesticides are currently being considered as alternative tools to synthetic insecticides for the control of mosquito populations (Alkhaibari et al, 2016;Deng et al, 2019;Lovett et al, 2019). During the fungal infection process, a complex network of molecular interactions occurring between the invading fungi and the arthropod host determines the outcome of infection (Butt et al, 2016;Ramirez et al, 2018a). In this regard, pathogen recognition is one of the most critical aspects of immune activation and essential for a proper host defense Lu and St Leger, 2016).…”