Sexual dimorphism in hemocyte responses to Staphylococcus aureus infection in Drosophila melanogaster

Abstract

Biological sex, determined by a combination of genetic, reproductive, and hormonal factors, significantly influences immune responses. This phenomenon is strikingly evident in the higher incidence of autoimmune diseases in females and the increased susceptibility to infectious diseases in males, as observed with COVID-19. Despite these disparities, less than 10% of immunological studies report their findings by sex, leaving the underlying mechanisms of this dimorphism unexplored.

Staphylococcus aureus is a gram-positive bacterium responsible for numerous nosocomial infections, disproportionately affecting men. Drosophila melanogaster is an excellent model organism for studying innate immune responses due to its available genetic resources, highly conserved immune pathways, and professional phagocytes similar to mammalian macrophages. Since S. aureus is primarily controlled through phagocytosis, in this thesis we employed Drosophila melanogaster to investigate sex-specific responses to the S. aureus challenge, mainly focusing on hemocytes (the insect equivalent of macrophages) as a foundation for the sexually dimorphic outcomes of this infection. We utilised various laboratory techniques, including the generation of transgenic lines, pathogen microinjection and survival assays, bacterial load quantification and a flow cytometry-based assay for the determination of phagocytic activity.

Our findings revealed that analogous to humans, males are more susceptible to S. aureus infection than females, accompanied by higher bacterial loads, especially in the early phase of infection. Additionally, females exhibited a higher proportion of phagocytic hemocytes compared to males. We implemented a genetic ablation protocol to eliminate Drosophila hemocytes, resulting in significantly increased mortality rates in both sexes and higher S. aureus proliferation compared to controls. Interestingly, the depletion of hemocytes eliminated this dimorphism, indicating that both sexes rely on heavily on hemocytes to control the infection. Taking advantage of Drosophila's unique cell-autonomous sex determination cascade, we generated male flies with genetically feminised hemocytes.

Remarkably, these sex-switched hemocyte-carrying individuals demonstrated significantly improved survival rates to the S. aureus challenge, nearly matching those of control females. A similar trend was observed in bacterial proliferation, suggesting that female hemocytes are more efficient than their male counterparts, conferring a survival advantage in S. aureus infection. Lastly, we challenged Drosophila lines, which had been selected for survival against wasp parasitism during larval development.

Intriguingly, the sex dimorphism in survival and bacterial load disappeared in these individuals, implying that survival to parasitism at the juvenile stages leads to the loss of dimorphism and a trade-off with resistance to S. aureus infection in adulthood.

The research uncovers macrophages as an important source of sexually dimorphic susceptibility to S. aureus infection. It highlights the importance of considering "sex" as a variable in experimental design and its implications for personalised medicine.