Sex-specific 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 that 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. Authors SummaryOur paper deals with a fundamental question in biology, how does the sex of an organism influence its anatomy and physiology. In particular, we focus on sex-differences in immunity and stem cell function. We establish the Drosophila larva as a model for analysing sex-based differences in cellular immunity. Cellular immunity in Drosophila is based on the production of multiple types of mature immune cells from blood progenitors and takes place in the fly hematopoietic organ, the larval lymph gland. We find that sex controls the number of various cell types in the lymph gland (progenitors, and specific varieties of mature blood cells). These sex-differences vary by cell type and change depending on whether flies are raised under homeostatic or infection conditions. We provide insight into the mechanisms that mediate these sex differences, identifying a possible role for the hematopoietic niche and insulin signaling. Taken together our work not only serves as an initial characterization of baseline sex-differences in fly hematopoiesis and cellular immunity but also identifies important areas for future exploration.