Sex differences in the regulation and function of cellular immunity in Drosophila.

Sex differences in development and physiology are prevalent in animals. One physiological system with pronounced differences between the sexes is the immune system: the immune response in humans differs between sexes and results in differential susceptibility of males and females to autoimmune diseases, malignancies, and infectious diseases. However, much remains to be discovered about the mechanisms underlying these sex-based differences in immunity. Here, we use the Drosophila hematopoietic organ, the lymph gland, as a model to investigate sex differences in cellular immunity and determine the underlying mechanisms. We find that, in line with their smaller body size, males have smaller lymph glands than females that contain fewer blood progenitors and produce less immune cells. Single cell RNA-seq analysis of the lymph gland showed that they expressed sex determination genes and identified substantial sex-specific differences in gene expression. By manipulating the sexual identity of different cell types in the lymph gland we show that a subset of these sex differences are controlled by organ-intrinsic mechanisms involving the hematopoietic niche. Importantly, we find a differential response between males and females to changes in insulin signaling, an important regulator of the immune response in the niche. Finally, we provide evidence for differences in the cellular immune response following infection between males and females. Overall, our results provide mechanistic insight into how sex differences in immunity are established.