Sex-specific regulation of macrophage function determines outcome to acute bacterial infection

Sex differences in infection outcome are widespread across sexually reproducing animals. The immune mechanisms generating these differences remain incompletely understood, in part because it is challenging to decompose systemic effects from cell-intrinsic regulation in mammalian model systems and clinical data. Sex-biased infection outcomes are observed in taxa lacking adaptive immunity, suggesting that innate immune cells, such as macrophages, have the potential to drive dimorphisms. It is largely unknown whether macrophage-intrinsic sex identity is causal for infection susceptibility, or for sex differences in other, homeostatic functions. Here, we address this question using Drosophila melanogaster, an in vivo model of innate immunity where sex is established, and can be manipulated, cell-autonomously. We show that during infection by the bacterium Staphylococcus aureus, adult male flies succumb faster than females, with more rapid early bacterial proliferation. Hemocyte ablation reveals that survival is hemocyte-dependent in both sexes, and flow cytometry indicates a higher fraction of actively phagocytic hemocytes in females. Critically, genetically feminizing male hemocytes abolishes the dimorphism in survival and bacterial burden in S. aureus infection, bringing feminized males to the equivalent load and mortality risk as females. Transcriptomic analysis of whole carcasses shows strongly sex-biased responses during S. aureus infection, whereas hemocyte-specific RNA-sequencing reveals minimal sex differences in induced responses but substantial, sustained baseline transcriptomic divergence, including female-biased expression of bactericidal mechanisms linked to ROS generation and lysozyme production. Together, these findings demonstrate that the sex identity of innate immune cells is sufficient to shape infection outcome.