Hormonal and sex chromosomal regulation of influenza A virus pathogenesis in C57BL/6 mice

Data from influenza pandemics and outbreaks reveal that sex is a determinant of morbidity and mortality associated with influenza A virus infection of young (i.e., 18-44 years of age) adults. While gender roles associated with each sex may influence exposure and infection outcome, I hypothesized that biological differences between the sexes impact the immune response to and outcome of infection. Using a mouse model, male and female C57BL/6 mice were intranasally inoculated with a mouse-adapted influenza A virus, resulting in greater morbidity and mortality in females than males. Influenza severity in females did not correlate with differences in virus replication, but with excessive lung inflammation. To determine whether genetic sex contributes to sex differences during influenza infection, 'four core genotype' mice were utilized. The 'four core genotypes' include XY and XX gonadal males and XX and XY gonadal females. Regardless of their sex chromosome complement, gonadal females died sooner than gonadal male mice following infection, indicating the importance of sex hormones determining influenza outcome.;To establish whether sex hormones affect influenza outcome, mice were gonadectomized (gdx), to mitigate endogenous sex steroid hormone production. Gonadectomy prior to influenza infection eliminated the sex difference in influenza outcome due to increased mortality of gdx males, whereas gdx females experienced severe disease similar to gonadally intact females. To test the hypothesis that elevated concentrations of estradiol (E2) reduces inflammatory responses, thereby protecting females against influenza, gdx females were treated with E2 or placebo. E2 significantly improved survival from influenza, not by altering virus replication, but by modulating pulmonary inflammatory responses---reducing IFN-gamma and CCL2 and increasing CCL3 and CXCL1, which suggested that E2 may modulate immune cell recruitment and activity. Immune cells enumerated from the lungs of E2 and placebo-treated females following infection revealed greater neutrophil recruitment and cytokine production from influenza-specific CD8+ T cells in the lungs of E2-treated females. The protective effects of E2 during influenza infection are dependent on neutrophils as their depletion increased morbidity and reversed the chemokine profile associated with protection. Future studies are required to determine whether E2 modulates the function of neutrophils and CD8+ T cells directly.;These studies are the first to demonstrate that gonadal sex, is a more important predictor of influenza pathogenesis than genetic sex. My studies also illustrate the significant role of E2 in modulating the immune response and outcome of infection with a pathogen of public health importance.