Sex differences in resident immune cell phenotype underlie more efficient acute inflammatory responses in female mice

Females are protected against mortality arising from severe sepsis; however, the precise mechanisms that confer this survival advantage in females over males are unclear. Resident leukocytes in resting tissues have a significant influence on circulating cytokine levels and recruitment of blood leukocytes during acute inflammatory responses. Whether the phenotype of resident leukocytes is distinct in females is unknown. In the present study, we show that the numbers of leukocytes occupying the naive peritoneal and pleural cavities is higher in female than in male mice and rats, comprising more T and B lymphocytes and macrophages. The altered immune cell composition of the female peritoneum is controlled by elevated tissue chemokine expression. Female resident macrophages also exhibit greater TLR expression and enhanced phagocytosis and NADPH oxidase-mediated bacterial killing. However, macrophage-derived cytokine production is diminished by proportionally more resident immunomodulatory CD4+ T lymphocytes. Ovarian hormones regulate macrophage phenotype, function, and numbers, but have no significant impact on T-lymphocyte populations in females. We have identified a fundamental sex difference in phenotype of resident leukocytes. We propose that the distinct resident leukocyte population in females allows aggressive recognition and elimination of diverse infectious stimuli without recruitment of circulating neutrophils or excessive cytokine production.