Sexual dimorphism in biomedical research: a call to analyse by sex.

Despite the huge body of evidence that males and females have very different immune systems and respond differently to immune challenge, few biomedical studies consider sex in their analyses. This editorial discusses the underlying biology behind immunological sex differences, their effects on immunity to infections and vaccines, and the reasons why sex differences are frequently overlooked in biomedical research. Readers are urged to design their future studies in order to analyse by sex, and to analyse existing datasets by sex. The information gained is likely to be of considerable importance in our current understanding of immune mechanisms. Sex refers to the intrinsic characteristics that distinguish males from females, whereas gender refers to the socially determined behaviour, roles or activities that males and females adopt. Male and female immune systems are not equal, leading to clear sexual dimorphism in response to infections and vaccination. In 2010, Nature featured a series of articles aimed at raising awareness regarding the inherent sex bias in modern day biomedical research and yet little has changed since that time. They comment that modern day medical practice is less evidencebased for women than for men due to a sex bias towards the study of males in biomedical research. They further suggest that journals and funders should insist on studies being conducted in both sexes, or that authors should state the sex of animals used in their studies. Unfortunately this was not widely adopted. This editorial will discuss the literature regarding sex differences in immunity to infections and vaccines and urge the readership to consider sex in their future biomedical studies. Even before they are born, intrauterine differences begin to differentially shape male and female immune systems. The male intrauterine environment is more inflammatory than that of females, male fetuses produce more androgens and have higher IgE levels, all of which lead to sexual dimorphism before birth. Furthermore, male fetuses have been shown to undergo more epigenetic changes than females with decreased methylation of many immune response genes, probably due to physiological differences. The X chromosome contains numerous immune response genes, such as toll-like receptors 7 and 8, multiple cytokine receptors, genes involved in T cell and B cell activity, and transcriptional and translational regulatory factors; while the Y chromosome encodes for a number of inflammatory pathway genes that can only be expressed in males. Females have two X chromosomes, one of which is inactivated, usually leading to expression of the wild type gene. X inactivation is incomplete or variable, which is thought to contribute to greater inflammatory responses among females. The immunological X and Y chromosome effects will begin to manifest in utero leading the sex differences in immunity from birth, which continue throughout life. MicroRNAs (miRNAs) regulate physiological processes, including cell growth, differentiation, metabolism and apoptosis. Males and females differ in their miRNA expression, even in embryonic stem cells; oestrogens and testosterone can regulate miRNAs expression, although testosterone is thought to work via its conversion to oestradiol; and the X chromosome is particularly enriched in miRNAs involved in immunity. All these sex-differential miRNA factors likely contribute to sex differences in the prevalence, pathogenesis and outcome of infections and vaccination. Females are born with higher oestriol concentrations than males, while males have more testosterone. Shortly after birth, male infants undergo a transient activation of the hypothalamo-pituitary-gonadal axis, characterised by a testosterone surge, while the female effect is variable. This so called ‘mini-puberty’ peaks at approximately 3 months of age. Once puberty begins, the ovarian hormones such as oestrogen dominate in females, while testicular-derived androgens dominate in males. Many immune cells express sex hormone receptors, allowing the sex hormones to influence immunity. Very broadly, oestrogens are Th2 biasing and pro-inflammatory, whereas testosterone is Th1 skewing and immunosuppressive. Thus sex steroids undoubtedly play a major role in sexual dimorphism in immunity throughout life. Numerous sex differences in susceptibility to infections have been described in the literature. Males fare worse during sepsis; they are more susceptible to certain bacterial infections such as invasive pneumococcal disease, group A streptococcal pharyngitis, and enterohaemorrhagic E. coli and campylobacter diarrhoea; and pulmonary tuberculosis is more prevalent among adult males. By contrast, females suffer more Mycoplasma pneumoniae and Bordetella pertussis infections. In general, viral infections are more prevalent