Trans R Soc Trop Med Hyg 2014; 108: 385–387 doi:10.1093/trstmh/tru079
EDITORIAL
Sexual dimorphism in biomedical research: a call to analyse by sex Katie L. Flanagan* Department of Immunology, Monash University, Prahran, Melbourne, Victoria 3181, Australia *Corresponding author: E-mail: [email protected]
Received 10 April 2014; accepted 23 April 2014 Keywords: Immunity, Infections, MicroRNAs, Sex hormones, Vaccines, X chromosome
encodes for a number of inflammatory pathway genes that can only be expressed in males.9 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,10 which is thought to contribute to greater inflammatory responses among females.11,12 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.13 Males and females differ in their miRNA expression,14 even in embryonic stem cells;15 oestrogens16 and testosterone17 can regulate miRNAs expression, although testosterone is thought to work via its conversion to oestradiol;18 and the X chromosome is particularly enriched in miRNAs involved in immunity.13 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.19 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.20 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,21,22 whereas testosterone is Th1 skewing and immunosuppressive.23–26 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;27–29 they are more susceptible to certain bacterial infections such as invasive pneumococcal disease,30 group A streptococcal pharyngitis,31 and enterohaemorrhagic E. coli 31 and campylobacter diarrhoea;32 and pulmonary tuberculosis is more prevalent among adult males.33 By contrast, females suffer more Mycoplasma pneumoniae and Bordetella pertussis infections.31 In general, viral infections are more prevalent
# The Author 2014. Published by Oxford University Press on behalf of Royal Society of Tropical Medicine and Hygiene. All rights reserved. For permissions, please e-mail: [email protected].
385
Downloaded from http://trstmh.oxfordjournals.org/ at UQ Library on June 16, 2014
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.1–3 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,4 male fetuses produce more androgens5 and have higher IgE levels,6 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.7 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;8 while the Y chromosome
K. L. Flanagan
386
It is likely that we are missing important scientific information by not investigating more comprehensively how males and females differ in immunological and clinical trials. We are entering an era in which there is increasing discussion regarding personalised medicine. Therefore, it is quite reasonable to imagine that females and males might benefit differently from certain interventions such as vaccines, immunotherapies and drugs. The mindset of the scientific community needs to shift. We are in an era where we can analyse the microbiome in various body compartments,47 can analyse the genome and every gene transcribed by it,48 we can analyse the epigenome,49 the metabolome,50 the proteome,51 and multiple cytokines and chemokines in a single small sample,52 and yet we still fail to allow sufficient sample size to do a separate analysis by sex. I appeal to readers of this article to take heed and start to turn the tide in the direction whereby analysis by sex becomes the norm for all immunological and clinical studies.53 The knowledge gained would be of huge scientific and clinical importance.
Funding: None. Competing interests: None declared. Ethical approval: Not required
References 1 Editorial: Putting gender on the agenda. Nature 2010;465:665. 2 Zucker I, Beery AK. Males still dominate animal studies. Nature 2010; 465:690. 3 Kim AM, Tingen CM, Woodruff TK. Sex bias in trials and treatment must end. Nature 2010;465:688–9. 4 Goldenberg RL, Andrews WW, Faye-Petersen OM et al. The Alabama Preterm Birth Study: intrauterine infection and placental histologic findings in preterm births of males and females less than 32 weeks. Am J Obstet Gynecol 2006;195:1533–7. 5 Barry JA, Kay AR, Navaratnarajah R et al. Umbilical vein testosterone in female infants born to mothers with polycystic ovary syndrome is elevated to male levels. J Obstet Gynaecol 2010;30:444–6. 6 Bergmann RL, Schulz J, Gunther S et al. Determinants of cord-blood IgE concentrations in 6401 German neonates. Allergy 1995;50:65–71. 7 Khulan B, Cooper WN, Skinner BM et al. Periconceptional maternal micronutrient supplementation is associated with widespread gender related changes in the epigenome: a study of a unique resource in the Gambia. Hum Mol Genet 2012;21:2086–101. 8 Fish EN. The X-files in immunity: sex-based differences predispose immune responses. Nat Rev Immunol 2008;8:737–44. 9 Charchar FJ, Bloomer LD, Barnes TA et al. Inheritance of coronary artery disease in men: an analysis of the role of the Y chromosome. Lancet 2012;379:915–22. 10 Carrel L, Willard HF. X-inactivation profile reveals extensive variability in X-linked gene expression in females. Nature 2005;434:400–4. 11 Prothero KE, Stahl JM, Carrel L. Dosage compensation and gene expression on the mammalian X chromosome: one plus one does not always equal two. Chromosome Res 2009;17:637–48.
Downloaded from http://trstmh.oxfordjournals.org/ at UQ Library on June 16, 2014
among males, while the clinical outcomes are often more severe among females due to a more pronounced immune response leading to increased immunopathology.34 A greater male susceptibility to viral infections in childhood may reverse to a female preponderance in adulthood,31 although females are more susceptible to certain viruses such as herpes infections from childhood.35 Males are generally more susceptible to parasitic infections, including Leishmania spp., Plasmodium falciparum and P. vivax, Schistosoma mansoni and certain filarial infections.36 Females seem more susceptible to Toxoplasma gondii infection36 and possibly to Giardia lamblia and Entamoeba histolytica. 37 The protective effects of breast feeding may not be the same in males and females, since breast feeding protects females but not males against severe acute lung disease.38 These effects cannot simply be explained by gender differences, but seem to be true effects of sex. Sex differences have been described for almost every commercially available vaccine in use.39 Females mount higher antibody responses to certain vaccines such as measles, hepatitis B, influenza and tetanus vaccines, while males have better antibody responses to yellow fever, pneumococcal polysaccharide and meningococcal A and C vaccines.40 However, the data are conflicting with some studies showing sex effects, whereas other studies show none. Post-vaccination clinical attack rates also vary by sex with females experiencing less influenza41,42 and males experiencing less pneumococcal disease after vaccination.43 Females experience more adverse events (AEs) to certain vaccines, such as oral polio vaccine44 and influenza vaccine,45 while males have more AEs to other vaccines such as yellow fever vaccine,40,46 suggesting the sex effect varies according to the vaccine given. The existing data hint at higher vaccine-related AEs in infant males progressing to a female preponderance from adolescence, suggesting a hormonal effect, but this has not been confirmed in a systematic review. If male and female immune systems behave in opposing directions with one pro-inflammatory/Th2 and one anti-inflammatory/ Th1 , then clearly analysing them together may well cause effects and responses to be cancelled out. Separate analysis by sex would detect effects that were not seen in the combined analysis. Furthermore, a dominant effect in one of the sexes might be wrongly attributed to both sexes. For drug and vaccine trials this could have serious implications. Given the huge body of evidence that males and females are immunologically divergent, why do most scientific studies fail to analyse by sex? Traditionally in science the sexes have been regarded as being equal and the main concern has been to recruit an equivalent number of males and females into studies. Adult females are often not enrolled into drug and vaccine trials because of the potential interference of hormones of the menstrual cycle or risk of pregnancy; thus, most data come from trials conducted in males only. Similarly, the majority of animal studies are conducted in males, although many animal studies fail to disclose the sex of the animals used.2 Analysing data by sex adds the major disadvantage that sample sizes would need to double in order to have sufficient power to detect significant sex effects. This potentially means double the cost and double the time to conduct the study, in a time when research funding is limited and hard to obtain. Furthermore, since the funders don’t request analysis by sex, and the journals do not ask for it, it is not a major priority in today’s highly competitive research environment.
Transactions of the Royal Society of Tropical Medicine and Hygiene
12 Spolarics Z. The X-files of inflammation: cellular mosaicism of X-linked polymorphic genes and the female advantage in the host response to injury and infection. Shock 2007;27:597–604. 13 Sharma S, Eghbali M. Influence of sex differences on microRNA gene regulation in disease. Biol Sex Differ 2014;5:3. 14 Langevin SM, Stone RA, Bunker CH et al. MicroRNA-137 promoter methylation in oral rinses from patients with squamous cell carcinoma of the head and neck is associated with gender and body mass index. Carcinogenesis 2010;31: 864–70. 15 Ciaudo C, Servant N, Cognat V et al. Highly dynamic and sex-specific expression of microRNAs during early ES cell differentiation. PLoS Genetics 2009; 5(8):e1000620. 16 Klinge CM. miRNAs and estrogen action. Trends Endocrinol Metab 2012;23:223–33. 17 Waltering KK, Porkka KP, Jalava SE et al. Androgen regulation of micro-RNAs in prostate cancer. Prostate 2011;71:604–14.
19 Kuijper EA, Ket JC, Caanen MR, Lambalk CB. Reproductive hormone concentrations in pregnancy and neonates: a systematic review. Reprod Biomed Online 2013;27:33–63. 20 Kuiri-Hanninen T, Seuri R, Tyrvainen E et al. Increased activity of the hypothalamic-pituitary-testicular axis in infancy results in increased androgen action in premature boys. J Clin Endocrinol Metab 2011; 96:98–105. 21 Kovats S. Estrogen receptors regulate an inflammatory pathway of dendritic cell differentiation: mechanisms and implications for immunity. Horm Behav 2012;62:254–62. 22 Miller L, Hunt JS. Sex steroid hormones and macrophage function. Life Sci 1996;59:1–14. 23 D’Agostino P, Milano S, Barbera C et al. Sex hormones modulate inflammatory mediators produced by macrophages. Ann N Y Acad Sci 1999;876:426–9. 24 Liva SM, Voskuhl RR. Testosterone acts directly on CD4+ T lymphocytes to increase IL-10 production. J Immunol 2001;167:2060–7. 25 Olsen NJ, Kovacs WJ. Gonadal steroids and immunity. Endocr Rev 1996;17: 369–84. 26 Rettew JA, Huet-Hudson YM, Marriott I. Testosterone reduces macrophage expression in the mouse of toll-like receptor 4, a trigger for inflammation and innate immunity. Biol Reprod 2008; 78:432–7. 27 Angele MK, Schwacha MG, Ayala A, Chaudry IH. Effect of gender and sex hormones on immune responses following shock. Shock 2000;14:81–90. 28 Ghuman AK, Newth CJ, Khemani RG. Impact of gender on sepsis mortality and severity of illness for prepubertal and postpubertal children. J Pediatr 2013;163:835–40 e1. 29 Marriott I, Bost KL, Huet-Hudson YM. Sexual dimorphism in expression of receptors for bacterial lipopolysaccharides in murine macrophages: a possible mechanism for gender-based differences in endotoxic shock susceptibility. J Reprod Immunol 2006;71:12–27. 30 Kaltoft Zeuthen N, Konradsen HB. Epidemiology of invasive pneumococcal infections in children aged 0–6 years in Denmark: a 19-year nationwide surveillance study. Acta Paediatr Suppl 2000;89:3–10. 31 Eshima N, Tokumaru O, Hara S et al. Age-specific sex-related differences in infections: a statistical analysis of national surveillance data in Japan. PLoS One 2012;7:e42261.
33 Neyrolles O, Quintana-Murci L. Sexual inequality in tuberculosis. PLoS Med 2009;6(12):e1000199. 34 Klein SL. Sex influences immune responses to viruses, and efficacy of prophylaxis and treatments for viral diseases. Bioessays 2012;34: 1050–9. 35 Fleming DM, Cross KW, Cobb WA, Chapman RS. Gender difference in the incidence of shingles. Epidemiol Infect 2004;132:1–5. 36 Klein SL. Hormonal and immunological mechanisms mediating sex differences in parasite infection. Parasite Immunol 2004;26: 247–64. 37 Perch M, Sodemann M, Jakobsen MS et al. Seven years’ experience with Cryptosporidium parvum in Guinea-Bissau, West Africa. Ann Trop Paediatr 2001;21:313–8. 38 Klein MI, Bergel E, Gibbons L et al. Differential gender response to respiratory infections and to the protective effect of breast milk in preterm infants. Pediatrics 2008;121:e1510–6. 39 Klein SL, Poland GA. Personalized vaccinology: one size and dose might not fit both sexes. Vaccine 2013;31:2599–600. 40 Cook IF. Sexual dimorphism of humoral immunity with human vaccines. Vaccine 2008;26:3551–5. 41 Vila-Corcoles A, Rodriguez T, de Diego C et al. Effect of influenza vaccine status on winter mortality in Spanish community-dwelling elderly people during 2002–2005 influenza periods. Vaccine 2007; 25:6699–707. 42 Wang CS, Wang ST, Chou P. Efficacy and cost-effectiveness of influenza vaccination of the elderly in a densely populated and unvaccinated community. Vaccine 2002;20:2494–9. 43 Wagner C, Popp W, Posch M et al. Impact of pneumococcal vaccination on morbidity and mortality of geriatric patients: a case-controlled study. Gerontology 2003;49:246–50. 44 Nzolo D, Ntetani Aloni M, Mpiempie Ngamasata T et al. Adverse events following immunization with oral poliovirus in Kinshasa, Democratic Republic of Congo: preliminary results. Pathog Glob Health 2013; 107:381–4. 45 Klein SL, Hodgson A, Robinson DP. Mechanisms of sex disparities in influenza pathogenesis. J Leukoc Biol 2012;92:67–73. 46 Lindsey NP, Schroeder BA, Miller ER et al. Adverse event reports following yellow fever vaccination. Vaccine 2008;26:6077–82. 47 Kim BS, Jeon YS, Chun J. Current status and future promise of the human microbiome. Pediatr Gastroenterol Hepatol Nutr 2013;16:71–9. 48 McHale CM, Zhang L, Thomas R et al. Analysis of the transcriptome in molecular epidemiology studies. Environ Mol Mutagen 2013;54: 500–17. 49 Huang B, Jiang C, Zhang R. Epigenetics: the language of the cell? Epigenomics 2014;6:73–88. 50 Fang ZZ, Gonzalez FJ. LC-MS-based metabolomics: an update. Arch Toxicol 2014: [Epub ahead of print]. 51 Legrain P, Rain JC. Twenty years of protein interactions studies for biological functions deciphering. J Proteomics 2014: S18743919(14)00160-2 [Epub ahead of print]. 52 Sachdeva N, Asthana D. Cytokine quantitation: technologies and applications. Front Biosci 2007;12:4682–95. 53 Klein SL. Immune cells have sex and so should journal articles. Endocrinology 2012;153:2544–50.
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18 Morgan CP, Bale TL. Early prenatal stress epigenetically programs dysmasculinization in second-generation offspring via the paternal lineage. J Neurosci 2011;31:11748–55.
32 Strachan NJ, Watson RO, Novik V et al. Sexual dimorphism in campylobacteriosis. Epidemiol Infect 2008;136:1492–5.
EDITORIAL
Sexual dimorphism in biomedical research: a call to analyse by sex Katie L. Flanagan* Department of Immunology, Monash University, Prahran, Melbourne, Victoria 3181, Australia *Corresponding author: E-mail: [email protected]
Received 10 April 2014; accepted 23 April 2014 Keywords: Immunity, Infections, MicroRNAs, Sex hormones, Vaccines, X chromosome
encodes for a number of inflammatory pathway genes that can only be expressed in males.9 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,10 which is thought to contribute to greater inflammatory responses among females.11,12 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.13 Males and females differ in their miRNA expression,14 even in embryonic stem cells;15 oestrogens16 and testosterone17 can regulate miRNAs expression, although testosterone is thought to work via its conversion to oestradiol;18 and the X chromosome is particularly enriched in miRNAs involved in immunity.13 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.19 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.20 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,21,22 whereas testosterone is Th1 skewing and immunosuppressive.23–26 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;27–29 they are more susceptible to certain bacterial infections such as invasive pneumococcal disease,30 group A streptococcal pharyngitis,31 and enterohaemorrhagic E. coli 31 and campylobacter diarrhoea;32 and pulmonary tuberculosis is more prevalent among adult males.33 By contrast, females suffer more Mycoplasma pneumoniae and Bordetella pertussis infections.31 In general, viral infections are more prevalent
# The Author 2014. Published by Oxford University Press on behalf of Royal Society of Tropical Medicine and Hygiene. All rights reserved. For permissions, please e-mail: [email protected].
385
Downloaded from http://trstmh.oxfordjournals.org/ at UQ Library on June 16, 2014
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.1–3 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,4 male fetuses produce more androgens5 and have higher IgE levels,6 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.7 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;8 while the Y chromosome
K. L. Flanagan
386
It is likely that we are missing important scientific information by not investigating more comprehensively how males and females differ in immunological and clinical trials. We are entering an era in which there is increasing discussion regarding personalised medicine. Therefore, it is quite reasonable to imagine that females and males might benefit differently from certain interventions such as vaccines, immunotherapies and drugs. The mindset of the scientific community needs to shift. We are in an era where we can analyse the microbiome in various body compartments,47 can analyse the genome and every gene transcribed by it,48 we can analyse the epigenome,49 the metabolome,50 the proteome,51 and multiple cytokines and chemokines in a single small sample,52 and yet we still fail to allow sufficient sample size to do a separate analysis by sex. I appeal to readers of this article to take heed and start to turn the tide in the direction whereby analysis by sex becomes the norm for all immunological and clinical studies.53 The knowledge gained would be of huge scientific and clinical importance.
Funding: None. Competing interests: None declared. Ethical approval: Not required
References 1 Editorial: Putting gender on the agenda. Nature 2010;465:665. 2 Zucker I, Beery AK. Males still dominate animal studies. Nature 2010; 465:690. 3 Kim AM, Tingen CM, Woodruff TK. Sex bias in trials and treatment must end. Nature 2010;465:688–9. 4 Goldenberg RL, Andrews WW, Faye-Petersen OM et al. The Alabama Preterm Birth Study: intrauterine infection and placental histologic findings in preterm births of males and females less than 32 weeks. Am J Obstet Gynecol 2006;195:1533–7. 5 Barry JA, Kay AR, Navaratnarajah R et al. Umbilical vein testosterone in female infants born to mothers with polycystic ovary syndrome is elevated to male levels. J Obstet Gynaecol 2010;30:444–6. 6 Bergmann RL, Schulz J, Gunther S et al. Determinants of cord-blood IgE concentrations in 6401 German neonates. Allergy 1995;50:65–71. 7 Khulan B, Cooper WN, Skinner BM et al. Periconceptional maternal micronutrient supplementation is associated with widespread gender related changes in the epigenome: a study of a unique resource in the Gambia. Hum Mol Genet 2012;21:2086–101. 8 Fish EN. The X-files in immunity: sex-based differences predispose immune responses. Nat Rev Immunol 2008;8:737–44. 9 Charchar FJ, Bloomer LD, Barnes TA et al. Inheritance of coronary artery disease in men: an analysis of the role of the Y chromosome. Lancet 2012;379:915–22. 10 Carrel L, Willard HF. X-inactivation profile reveals extensive variability in X-linked gene expression in females. Nature 2005;434:400–4. 11 Prothero KE, Stahl JM, Carrel L. Dosage compensation and gene expression on the mammalian X chromosome: one plus one does not always equal two. Chromosome Res 2009;17:637–48.
Downloaded from http://trstmh.oxfordjournals.org/ at UQ Library on June 16, 2014
among males, while the clinical outcomes are often more severe among females due to a more pronounced immune response leading to increased immunopathology.34 A greater male susceptibility to viral infections in childhood may reverse to a female preponderance in adulthood,31 although females are more susceptible to certain viruses such as herpes infections from childhood.35 Males are generally more susceptible to parasitic infections, including Leishmania spp., Plasmodium falciparum and P. vivax, Schistosoma mansoni and certain filarial infections.36 Females seem more susceptible to Toxoplasma gondii infection36 and possibly to Giardia lamblia and Entamoeba histolytica. 37 The protective effects of breast feeding may not be the same in males and females, since breast feeding protects females but not males against severe acute lung disease.38 These effects cannot simply be explained by gender differences, but seem to be true effects of sex. Sex differences have been described for almost every commercially available vaccine in use.39 Females mount higher antibody responses to certain vaccines such as measles, hepatitis B, influenza and tetanus vaccines, while males have better antibody responses to yellow fever, pneumococcal polysaccharide and meningococcal A and C vaccines.40 However, the data are conflicting with some studies showing sex effects, whereas other studies show none. Post-vaccination clinical attack rates also vary by sex with females experiencing less influenza41,42 and males experiencing less pneumococcal disease after vaccination.43 Females experience more adverse events (AEs) to certain vaccines, such as oral polio vaccine44 and influenza vaccine,45 while males have more AEs to other vaccines such as yellow fever vaccine,40,46 suggesting the sex effect varies according to the vaccine given. The existing data hint at higher vaccine-related AEs in infant males progressing to a female preponderance from adolescence, suggesting a hormonal effect, but this has not been confirmed in a systematic review. If male and female immune systems behave in opposing directions with one pro-inflammatory/Th2 and one anti-inflammatory/ Th1 , then clearly analysing them together may well cause effects and responses to be cancelled out. Separate analysis by sex would detect effects that were not seen in the combined analysis. Furthermore, a dominant effect in one of the sexes might be wrongly attributed to both sexes. For drug and vaccine trials this could have serious implications. Given the huge body of evidence that males and females are immunologically divergent, why do most scientific studies fail to analyse by sex? Traditionally in science the sexes have been regarded as being equal and the main concern has been to recruit an equivalent number of males and females into studies. Adult females are often not enrolled into drug and vaccine trials because of the potential interference of hormones of the menstrual cycle or risk of pregnancy; thus, most data come from trials conducted in males only. Similarly, the majority of animal studies are conducted in males, although many animal studies fail to disclose the sex of the animals used.2 Analysing data by sex adds the major disadvantage that sample sizes would need to double in order to have sufficient power to detect significant sex effects. This potentially means double the cost and double the time to conduct the study, in a time when research funding is limited and hard to obtain. Furthermore, since the funders don’t request analysis by sex, and the journals do not ask for it, it is not a major priority in today’s highly competitive research environment.
Transactions of the Royal Society of Tropical Medicine and Hygiene
12 Spolarics Z. The X-files of inflammation: cellular mosaicism of X-linked polymorphic genes and the female advantage in the host response to injury and infection. Shock 2007;27:597–604. 13 Sharma S, Eghbali M. Influence of sex differences on microRNA gene regulation in disease. Biol Sex Differ 2014;5:3. 14 Langevin SM, Stone RA, Bunker CH et al. MicroRNA-137 promoter methylation in oral rinses from patients with squamous cell carcinoma of the head and neck is associated with gender and body mass index. Carcinogenesis 2010;31: 864–70. 15 Ciaudo C, Servant N, Cognat V et al. Highly dynamic and sex-specific expression of microRNAs during early ES cell differentiation. PLoS Genetics 2009; 5(8):e1000620. 16 Klinge CM. miRNAs and estrogen action. Trends Endocrinol Metab 2012;23:223–33. 17 Waltering KK, Porkka KP, Jalava SE et al. Androgen regulation of micro-RNAs in prostate cancer. Prostate 2011;71:604–14.
19 Kuijper EA, Ket JC, Caanen MR, Lambalk CB. Reproductive hormone concentrations in pregnancy and neonates: a systematic review. Reprod Biomed Online 2013;27:33–63. 20 Kuiri-Hanninen T, Seuri R, Tyrvainen E et al. Increased activity of the hypothalamic-pituitary-testicular axis in infancy results in increased androgen action in premature boys. J Clin Endocrinol Metab 2011; 96:98–105. 21 Kovats S. Estrogen receptors regulate an inflammatory pathway of dendritic cell differentiation: mechanisms and implications for immunity. Horm Behav 2012;62:254–62. 22 Miller L, Hunt JS. Sex steroid hormones and macrophage function. Life Sci 1996;59:1–14. 23 D’Agostino P, Milano S, Barbera C et al. Sex hormones modulate inflammatory mediators produced by macrophages. Ann N Y Acad Sci 1999;876:426–9. 24 Liva SM, Voskuhl RR. Testosterone acts directly on CD4+ T lymphocytes to increase IL-10 production. J Immunol 2001;167:2060–7. 25 Olsen NJ, Kovacs WJ. Gonadal steroids and immunity. Endocr Rev 1996;17: 369–84. 26 Rettew JA, Huet-Hudson YM, Marriott I. Testosterone reduces macrophage expression in the mouse of toll-like receptor 4, a trigger for inflammation and innate immunity. Biol Reprod 2008; 78:432–7. 27 Angele MK, Schwacha MG, Ayala A, Chaudry IH. Effect of gender and sex hormones on immune responses following shock. Shock 2000;14:81–90. 28 Ghuman AK, Newth CJ, Khemani RG. Impact of gender on sepsis mortality and severity of illness for prepubertal and postpubertal children. J Pediatr 2013;163:835–40 e1. 29 Marriott I, Bost KL, Huet-Hudson YM. Sexual dimorphism in expression of receptors for bacterial lipopolysaccharides in murine macrophages: a possible mechanism for gender-based differences in endotoxic shock susceptibility. J Reprod Immunol 2006;71:12–27. 30 Kaltoft Zeuthen N, Konradsen HB. Epidemiology of invasive pneumococcal infections in children aged 0–6 years in Denmark: a 19-year nationwide surveillance study. Acta Paediatr Suppl 2000;89:3–10. 31 Eshima N, Tokumaru O, Hara S et al. Age-specific sex-related differences in infections: a statistical analysis of national surveillance data in Japan. PLoS One 2012;7:e42261.
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