During pregnancy, it is evolutionary advantageous for inflammatory immune responses that might lead to fetal rejection to be reduced and anti-inflammatory responses that promote transfer of maternal antibodies to the fetus to be increased. Hormones modulate the immunological shift that occurs during pregnancy. Estrogens, including estradiol and estriol, progesterone, and glucocorticoids increase over the course of pregnancy and affect transcriptional signaling of inflammatory immune responses at the maternal-fetal interface and systemically. During pregnancy, the reduced activity of natural killer cells, inflammatory macrophages, and helper T cell type 1 (Th1) cells and production of inflammatory cytokines, combined with the higher activity of regulatory T cells and production of anti-inflammatory cytokines, affects disease pathogenesis. The severity of diseases caused by inflammatory responses (e.g., multiple sclerosis) is reduced and the severity of diseases that are mitigated by inflammatory responses (e.g., influenza and malaria) is increased during pregnancy. For some infectious diseases, elevated inflammatory responses that are necessary to control and clear a pathogen have a negative consequence on the outcome of pregnancy. The bidirectional interactions between hormones and the immune system contribute to both the outcome of pregnancy and female susceptibility to disease.

Studies of the 1918 H1N1 influenza pandemic, the H5N1 avian influenza outbreak, and the 2009 H1N1 pandemic illustrate that sex and pregnancy contribute to severe outcome from infection, suggesting a role for sex steroids. To test the hypothesis that the sexes respond differently to influenza, the pathogenesis of influenza A virus infection was investigated in adult male and female C57BL/6 mice. Influenza infection reduced reproductive function in females and resulted in greater body mass loss, hypothermia, and mortality in females than males. Whereas lung virus titers were similar between the sexes, females had higher induction of proinflammatory cytokines and chemokines, including TNF-α, IFN-γ, IL-6, and CCL2, in their lungs than males. Removal of the gonads in both sexes eliminated the sex difference in influenza pathogenesis. Manipulation of testosterone or dihydrotestosterone concentrations in males did not significantly impact virus pathogenesis. Conversely, females administered high doses of estradiol had a ≥10-fold lower induction of TNF-α and CCL2 in the lungs and increased rates of survival as compared with females that had either low or no estradiol. The protective effects of estradiol on proinflammatory cytokines and chemokines, morbidity, and mortality were primarily mediated by signaling through estrogen receptor α (ERα). In summary, females suffer a worse outcome from influenza A virus infection than males, which can be reversed by administration of high doses of estradiol to females and reflects differences in the induction of proinflammatory responses and not in virus load.

Epidemiological evidence from influenza outbreaks and pandemics reveals that morbidity and mortality are often higher for women than men. Sex differences in the outcome of influenza are age-dependent, often being most pronounced among adults of reproductive ages (18-49 years of age) and sometimes reflecting the unique state of pregnancy in females, which is a risk factor for severe disease. Small animal models of influenza virus infection illustrate that inflammatory immune responses also differ between the sexes and impact the outcome of infection, with females generating higher proinflammatory cytokine and chemokine responses and experiencing greater morbidity and mortality than males. Males and females also respond differently to influenza vaccines, with women initiating higher humoral immune responses but experiencing more adverse reactions to seasonal influenza vaccines than men. Small animal models further show that elevated immunity following vaccination in females leads to greater cross-protection against novel influenza viruses in females compared with males. Sex steroid hormones, including estradiol and testosterone, as well as genetic differences between the sexes may play roles in modulating sex differences in immune responses to influenza virus infection and vaccination. Future studies must elucidate the pathways and cellular responses that differ between the sexes and determine how best to use this knowledge to inform public health policy-makers about prophylaxis and therapeutic treatments of influenza virus infections to ensure adequate protection in both males and females.

ABSTRACT17␤-Estradiol (E2) treatment limits the pathology associated with pulmonary diseases caused by pathogens, allergens, and asthma, partly by reducing the production of proinflammatory cytokines and chemokines. To test the hypothesis that E2 protects against influenza A virus (IAV) infection by altering the recruitment and activity of innate immune cells and T cells, chemokine concentrations were measured and innate and adaptive immune cells were enumerated from the lungs of E2-and placebotreated ovariectomized female C57BL/6 mice following infection. Females treated with E2 experienced less morbidity but had similar lung virus titers to placebo-treated females. Females treated with E2 had lower induction of CCL2 but higher CCL3 and CXCL1 responses in their lungs than placebo-treated females. Pulmonary recruitment of neutrophils, NK cells, macrophages, and dendritic cells was increased following infection, but only neutrophil numbers were greater in E2-treated than placebotreated females. Neutrophils enhance the responses of influenza virus-specific CD8 T cells to promote virus clearance and improve the outcome of infection. Total numbers of virus-specific CD8 T cells were not altered by treatment with E2, but the proportion of gamma interferon (IFN-␥)-and tumor necrosis factor alpha (TNF-␣)-producing, virus-specific CD8 T cells was increased. Neutrophil depletion in E2-treated females increased morbidity, reduced pulmonary production of chemoattractants for neutrophils, and reduced IFN-␥ production by virus-specific CD8 T cells. Neutrophils mediate both inflammation and tissue repair during IAV infection and are regulated by E2 to improve the outcome of influenza in females. IMPORTANCESevere influenza is associated with excessive inflammation that leads to tissue damage. We demonstrate that estradiol (E2) is a potent anti-inflammatory hormone that reduces the severity of influenza A virus infection in females. Treatment of female C57BL/6 mice with E2 does not affect virus replication but rather alters the production of chemokines, pulmonary recruitment of neutrophils, and the cytokine responses of virus-specific CD8 T cells to protect females against severe influenza.

Over 100 million women use progesterone therapies worldwide. Despite having immunomodulatory and repair properties, their effects on the outcome of viral diseases outside of the reproductive tract have not been evaluated. Administration of exogenous progesterone (at concentrations that mimic the luteal phase) to progesterone-depleted adult female mice conferred protection from both lethal and sublethal influenza A virus (IAV) infection. Progesterone treatment altered the inflammatory environment of the lungs, but had no effects on viral load. Progesterone treatment promoted faster recovery by increasing TGF-β, IL-6, IL-22, numbers of regulatory Th17 cells expressing CD39, and cellular proliferation, reducing protein leakage into the airway, improving pulmonary function, and upregulating the epidermal growth factor amphiregulin (AREG) in the lungs. Administration of rAREG to progesterone-depleted females promoted pulmonary repair and improved the outcome of IAV infection. Progesterone-treatment of AREG-deficient females could not restore protection, indicating that progesterone-mediated induction of AREG caused repair in the lungs and accelerated recovery from IAV infection. Repair and production of AREG by damaged respiratory epithelial cell cultures in vitro was increased by progesterone. Our results illustrate that progesterone is a critical host factor mediating production of AREG by epithelial cells and pulmonary tissue repair following infection, which has important implications for women’s health.

A mouse model was used to determine if protective immunity to influenza A virus infection differs between the sexes. The median lethal dose of H1N1 or H3N2 was lower for naïve females than males. After a sublethal, primary infection with H1N1 or H3N2, females and males showed a similar transient morbidity, but females generated more neutralizing and total anti-influenza A virus antibodies. Immunized males and females showed similar protection against secondary challenge with a homologous virus, but males experienced greater morbidity and had higher lung viral titers after infection with a lethal dose of heterologous virus. Females develop stronger humoral immune responses and greater cross protection against heterosubtypic virus challenge.

BackgroundBoth coxsackievirus B3 (CVB3) and influenza A virus (IAV; H1N1) produce sexually dimorphic infections in C57BL/6 mice. Gonadal steroids can modulate sex differences in response to both viruses. Here, the effect of sex chromosomal complement in response to viral infection was evaluated using four core genotypes (FCG) mice, where the Sry gene is deleted from the Y chromosome, and in some mice is inserted into an autosomal chromosome. This results in four genotypes: XX or XY gonadal females (XXF and XYF), and XX or XY gonadal males (XXM and XYM). The FCG model permits evaluation of the impact of the sex chromosome complement independent of the gonadal phenotype.MethodsWild-type (WT) male and female C57BL/6 mice were assigned to remain intact or be gonadectomized (Gdx) and all FCG mice on a C57BL/6 background were Gdx. Mice were infected with either CVB3 or mouse-adapted IAV, A/Puerto Rico/8/1934 (PR8), and monitored for changes in immunity, virus titers, morbidity, or mortality.ResultsIn CVB3 infection, mortality was increased in WT males compared to females and males developed more severe cardiac inflammation. Gonadectomy suppressed male, but increased female, susceptibility to CVB3. Infection with IAV resulted in greater morbidity and mortality in WT females compared with males and this sex difference was significantly reduced by gonadectomy of male and female mice. In Gdx FCG mice infected with CVB3, XY mice were less susceptible than XX mice. Protection correlated with increased CD4+ forkhead box P3 (FoxP3)+ T regulatory (Treg) cell activation in these animals. Neither CD4+ interferon (IFN)γ (T helper 1 (Th1)) nor CD4+ interleukin (IL)-4+ (Th2) responses differed among the FCG mice during CVB3 infection. Infection of Gdx FCG mice revealed no effect of sex chromosome complement on morbidity or mortality following IAV infection.ConclusionsThese studies indicate that sex chromosome complement can influence pathogenicity of some, but not all, viruses.

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