INTRODUCTION
Females exhibit a survival advantage throughout the lifespan compared to males in many diseases. These differences in clinical outcome are particularly evident in early life as male neonates exhibit a significantly worse prognosis across a range of pathological states1,2. In particular, male neonates have higher rates of infection compared to females and experimental, clinical and epidemiological studies indicate improved prognosis for females after a septic challenge3. The disparities in outcome in neonates is most robustly described for prematurely born neonates, especially at the borders of viability4. Sex-specific differences in immune function have been described in neonates5 and sex hormones modulate immune responses6. Investigating molecular responses to sex-specific factors will help define novel targets for more effective prevention and treatment of immune-related diseases in neonates.
Estrogens are the primary group of sex hormones in females, with 17β-estradiol (E2) being the most potent and abundant. E2 interacts with estrogen receptors (ERs) which shuttle between the cytoplasm and nucleus in response to changes in its concentration. The two main ERs, α and β, transduce opposing actions in target tissues7. Their classical actions are genomic, however rapid effects are also recognised8. Non-classical ER signalling also occurs in immune cells, enabling protein–protein interactions between ERs and independent transcription factors, including NF­κB9. ER-α and ER-β exhibit differential expression among immune cell subsets which may outline the differential effects of estrogens on immune function independent of circulating concentrations6. Fetuses are chronically exposed to high levels of steroid hormones from placental production. Concentrations of E2 in umbilical cord blood (UCB) range from 2 to 150 nM, which are higher than peak concentrations of E2 during the menstrual cycle18. In newborn females, E2 concentrations are higher than in males and do not fall to prepubertal values until the age of approximately 1 year19.
High expression of intracellular sex steroid receptors have been described in newborn monocytes10. CD11b is a receptor on the cell surface that is important for monocyte migration to sites of inflammation. Although baseline CD11b expression is reported to be similar to that of adults, neonates are unable to upregulate CD11b expression to the same magnitude following lipopolysaccharide (LPS/endotoxin) stimulation11. E2 has been shown to promote differentiation of CD11b+ dendritic cells from bone marrow progenitors12 and sex differences in CD11b expression in microglia following ischaemia have been described13.
The key to recognising LPS on the immune cell surface and initiating the downstream inflammatory signalling is toll-like receptor 4 (TLR4). E2 has been shown to enhance the expression of pattern recognition receptors, including TLR4, on the surface of peritoneal macrophages in rodents correlating to LPS tolerance14.
Reactive oxygen species (ROS) formation is an essential mechanism by which monocytes kill invading microorganisms15. Excessive oxidative stress is however involved in the pathology of neonatal diseases including brain and lung injury4,16. Several studies have highlighted the protective effects of E2 on mitigating oxidative tissue damage through anti-inflammatory mechanisms17.
Various studies have examined the effects of estrogens on the immune system6, however to the best of our knowledge, the effects of E2 treatment on monocyte cell surface receptor expression and mononuclear cell ROS production have not been compared between male and female neonates. In this study, we investigated quantitative differences in CD11b, TLR4, ER-α and ER-β levels in male and female CD14+ monocytes from UCB and adult whole blood following E2, ICI 182,780 (ER antagonist), and LPS treatment in various combinations. We also explored ROS production in mononuclear cells isolated from UCB and adult peripheral blood under conditions of E2 and LPS stimulation. Overall, these preliminary data indicate that E2 reduces the LPS effect on ROS production in female versus male UCB and this may suggest that endocrinological responses on the immune system underlie, in part, sex-specific differences in inflammation in neonates.