Sexual dimorphism in immunity and longevity among the oldest old

The role of the Y chromosome in human diseases has been overlooked for many years, and a great paradigm shift regarding its biological importance has only occurred recently. There are a limited number of human studies that have examined the impact of the Y chromosome on lifespan. In 2023, Teoli et al. demonstrated transcriptome changes in men with different karyotypes (46,XX, 46,XY, 47,XXY and 47,XYY) showing that the number of Y chromosomes negatively affects longevity. The authors claimed that an additional Y chromosome leads to increased expression of transposable elements (TEs), thereby disrupting genomic stability. Overall, the results obtained in this study support the hypothesis regarding the toxicity of the Y chromosome ( 66 ). The length of the Y chromosome is significantly shorter than the X chromosome, estimated to be approximately 62 MB. The Y chromosome contains 693 genes, of which 106 are protein-coding ( 67 ). Genes linked to the Y chromosome are called “Holandric”, and in contrast to X-linked genes, Y-linked genes do not directly regulate either innate or adaptive immunity, but they may indirectly influence immune system functions. In 2009, Sezgin et al. studied 3,727 males in a cohort of HIV-positive patients to investigate whether major Y haplogroups are associated with HIV infection and progression. They genotyped 11 common Y haplogroups in European Americans and African Americans and found that American men with haplogroup I lineage exhibited faster disease progression and a higher mortality rate at seven years post-infection. Additionally, individuals with the Y-I haplogroup tended to experience more rapid T cell depletion, which resulted in delayed responses to viral load suppression following antiretroviral therapy ( 68 ). These results suggest that the Y chromosome is implicated in the regulation of T cells. The Y chromosome is essential for male health, and mosaic loss of chromosome Y (mLOY) has been shown to have a strong correlation with aging. Namely, mLOY has been observed to increase with advancing age, with detection rates of 40% in males aged 70 years and 57% in males aged 93 years and among centenarians ( 69 – 72 ). Therefore, mLOY is considered a potential marker of biological age and is associated with certain age-related diseases, such as neurodegenerative disorders ( 73 ), cardiovascular diseases ( 74 ), macular degeneration ( 75 ), and Alzheimer’s disease ( 76 ). Furthermore, in aging men, mLOY has been linked to cancer with an advanced aggressive phenotype ( 77 , 78 ). The Y chromosome is known not to code for immune-related genes; therefore, mLOY serves as a suitable model to study the indirect effects of the Y chromosome on immunity. In 2019, Thompson et al. addressed the question of whether the loss of a Y chromosome from circulating leukocytes has a direct functional effect and whether loss of Y chromosome in leukocytes serves as an indicator of broader genomic instability. A study was conducted involving the analysis of 205,011 men aged between 40 and 70 years, which revealed that hematopoietic stem and progenitor cells exhibited the highest enrichment for cell types associated with variants linked to LOY. These observations suggest that LOY may have a direct impact on hematopoietic stem cells rather than on more differentiated white blood cell types. Additionally, by examining PBMC from nine male donors aged 64 to 89 at a single-cell resolution, they demonstrated dysregulation of the TCL1A gene in B-lymphocytes with LOY ( 70 ). Notably, expression of TCL1A in T and B cells was shown to disrupt cycle transitions and hinder adequate DNA damage response, leading to genomic instability ( 79 ). This provides proof of the concept that LOY may be functionally active in these cells. Other evidence suggesting that the Y chromosome may be implicated in immune regulation was published by Dumanski et al. ( 80 ). By analyzing PBMCs from men aged 64 to 94 with prostate cancer or Alzheimer’s disease, they demonstrated that LOY is associated with dysregulation of autosomal gene expression in leukocytes. It is important to note that LOY exhibited cellular specificity; patients with Alzheimer’s disease were preferentially affected by LOY in NK cells, whereas in patients with prostate cancer, LOY was observed in CD4+ T cells and granulocytes.

The field of sex-based biology has made significant progress over the last decade, particularly in the areas of healthy aging and lifespan. Extreme longevity, particularly reaching the age of 100 years, is an exceptionally rare trait in the human population, exhibiting significant variations in prevalence between genders. Women generally exhibit greater survival rates than men across all age groups, including centenarians ( 1 , 2 ), with the gender gap in life expectancy ranging from 4.2 to 6.2 years ( 3 ). Genetic factors and immune responses play a crucial role in achieving longevity; however, there is limited information regarding the mechanisms that regulate the differences in biological aging between men and women. From birth, females benefit from genetic advantages in their chromosomal constitution, which is associated with a preponderance of longer telomeres ( 4 ). Furthermore, women typically maintain this phenotype better throughout the aging process; however, in the centenarian group, no significant difference in telomere length between men and women was observed ( 5 ). A strong immune system is a key factor in determining lifespan, and sex plays an important role in its composition and activity. In humans, sex is genetically determined by the presence of X and Y chromosomes, with females possessing an XX pair and males having an XY pair. The sex-based diversity in immunity between males and females is regulated by several mechanisms, including X chromosome inactivation, mosaicism, skewing, and dimorphism in the expression of X chromosome-encoded genes, as well as regulatory genes on the Y chromosome ( 6 ). Generally, women exhibit stronger innate and adaptive immune responses than men. It is known that young women are more susceptible to autoimmune diseases, while men are more frequently affected by bacterial, parasitic, fungal, and viral infections ( 7 , 8 ). An additional factor that may contribute to gender differences in the immune system is the gut microbiota. Composition of gut microbiota varies between males and females providing sex-specific differences in immune responses ( 9 ). Additionally, various environmental factors may induce different immune responses in males and females, thereby influencing longevity in distinct ways. With age, immune dysregulation manifests differently between the sexes, potentially contributing to the rarity of men surviving to age 100. Although elderly females generally experience poorer health, men of the same age tend to have fewer physical disabilities ( 10 ). Numerous studies have reported age-related changes in the composition and functions of immune cells that are sex-specific, as well as differences in secretome production and the activation of signaling pathways ( 11 ). However, the majority of such research has been conducted using various animal models, which cannot adequately replicate the human immune microenvironment. Consequently, the data obtained from these studies cannot always be correlated with human responses, leading to a significant lack of human data. Similarly, there is no suitable animal model that can sufficiently represent phenomena observed in human centenarians. In this review, we focus exclusively on human data. Given that sex hormones influence a broad spectrum of biological processes and significantly affect sexual dimorphism in various aspects, we chose not to address this topic in our review. Instead, we opted to reference well-written recent reviews on the subject ( 12 ). We discuss the interplay between age and sex in both innate and adaptive immunity, as well as the contribution of sex chromosomes, gut microbiome, and several environmental factors to these effects, with particular emphasis on the limited studies involving centenarians.

The gut microbiome has been identified as a potential contributor to the development of a favorable health phenotype that facilitates extreme longevity, as observed in centenarians. This section aims to elucidate the most recent evidence that positions the gut microbiome as a potential protective factor in the pursuit of extreme longevity, particularly through its association with centenarians. Several studies have examined the microbiome of centenarians from various geographic regions, including South Korea ( 233 ), Italy ( 234 , 235 ), Japan ( 236 ), and China ( 237 , 238 ). However, it is well established that factors such as ethnicity, habitat, and lifestyle significantly influence the gut microbiome, which complicates the ability to draw universal conclusions. In 2024, Chen et al conducted a comprehensive fecal metagenomic analysis involving eight long-lived populations from three countries: Japan, China, and Italy. The objective of this study was to investigate the associations between microbial composition and longevity-related traits in a large cohort. This approach will facilitate the identification of lifespan-associated microbiomes that are consistent across various ethnic populations from several geographic locations. The analysis revealed that several species were significantly enriched in the microbiota of long-lived individuals, including nonagenarians and centenarians, across multiple cohorts, namely Eisenbergiella tayi , Methanobrevibacter smithii , Hungatella hathewayi , and Desulfovibrio fairfieldensis . Further analysis revealed that the identified populations are engaged in various functional processes. E. tayi is involved in protein N-glycosylation, while D. fairfieldensis and M. smithii contribute to the biosynthesis of vitamin K2 and 3-dehydroquinate/chorismate, respectively. Additionally, H. hathewayi plays a role in the purine nucleobase degradation. Despite the conduct of a well-designed descriptive study, the focus on sex-specific aspects was overlooked ( 239 ). In the same year, Luan et al. sought to elucidate the influences and contributions of sex on the gut microbiome in the context of healthy aging, using a cohort of healthy centenarians from Hainan. Their findings indicated that the composition and functional patterns of the microbiome exhibited significant variations between sexes, with 31 species identified as being enriched in males and 7 species enriched in females ( 240 ). Among species with a predominance of males, three probiotic Lactobacillus species have been identified. It is noteworthy that a higher abundance of certain bacteria in men’s gut microbiome, including Eggerthella lenta , Clostridium hathewayi , Clostridium difficile , Anaerotruncus colihominis , and Actinomyces viscosus , has been linked to unhealthy aging. In contrast, species associated with healthy aging were predominantly identified in female centenarians, including Prevotella copri , Eubacterium rectale , Prevotella stercorea , Roseburia inulinivorans , and Coprococcus comes . The study identified exceptions in the enrichment of Dorea longicatena , which has been associated with poorer cognitive performance ( 241 ), as well as Lachnospiraceae bacteria , which exhibit both beneficial and detrimental effects. On one hand, these bacteria are characterized as commercial probiotics; on the other hand, they are known to provide immunity-stimulating and homeostasis-maintaining effects ( 242 ). These findings suggest a significant difference in microbiome composition and functionality between sexes, which may speculatively contribute to the observed gender longevity gap.

The consideration of gender in human biomedical research, particularly in the contexts of immunity and aging, is gradually gaining prominence. Although the majority of the reviewed studies are descriptive, correlative, and associative in nature, our understanding of sexual dimorphism has become clearer on one hand, while simultaneously revealing additional factors that remain inadequately explored and warrant further investigation. For example, males exhibit greater susceptibility to infections from birth through adulthood, suggesting that sex chromosomes play a significant role in the sexual dimorphism observed in innate immunity. However, the molecular mechanisms that modulate adaptive immunity and contribute to the increased vulnerability of males remain largely unknown. The sex-based regulation of immune responses ultimately influences the development of age-related diseases and life expectancy. A more comprehensive understanding of these mechanisms could facilitate the development of improved diagnostic and therapeutic strategies, thereby positively influencing healthy aging and longevity. The aging process is characterized by alterations in immune system function. Given that the immune systems of young men and women exhibit significant differences, they experience distinct age-related immunological changes and aging consequences in a sex-specific manner. These sex-specific alterations in immunity during aging may lead to varying predispositions to health disorders and differing therapeutic outcomes. Therefore, sexual dimorphism should be regarded as a critical variable in the prophylactic and therapeutic management of age-associated diseases. Furthermore, sexual dimorphism also influences immune responses to various occupational activities, lifestyle choices, and individual characteristics, potentially affecting healthy aging and longevity. From this perspective, women appear to be more susceptible and may experience more severe responses to environmental factors. The gut microbiome has emerged as a potential factor associated with the establishment of a favorable health phenotype, exhibiting a significant gender-specific influence. However, the mechanisms by which the gut microbiome contributes to conditions conducive to successful aging remain unclear. Consequently, future research should prioritize investigating the causal relationship between sexually dimorphic immunity and the microbiota while taking into account the aforementioned confounding factors. It is evident that men and women pursue distinct trajectories in their quest for extreme longevity. The primary objective of the field of longevity research is not only to extend lifespan but also to promote a healthy life characterized by an active, productive, and mentally positive lifestyle. Consequently, this research domain is becoming increasingly multifaceted and necessitates more specialized investigations that take gender differences into account. Notably, while over 80% of the world’s centenarians are female, male centenarians tend to exhibit better health and are less susceptible to environmental factors. There is numerous data regarding the impact of genetic factors on the likelihood of women reaching extreme lifespan limits compared to men; however, the significance of longevity-associated genetic factors is highly variable and contingent upon ethnicity. It is clear that our understanding of human extreme longevity is largely associative and requires further investigation, including larger human cohorts across various countries, while considering genetic, medical, and social dimensions. In conclusion, there is a pressing need for additional clinical applications to address the existing gap in human data concerning sexual dimorphism, immunity, and healthy aging. This research should particularly focus on the integration of bioinformatics methodologies capable of effectively managing and analyzing the highly dynamic and complex omics datasets commonly encountered in clinical studies. Furthermore, the incorporation of sex-specific thresholds in the analysis should be consistently implemented to prevent the oversight of significant gender-related findings.

Given the diverse nature of the approaches and findings in the field of longevity, the following sections will primarily focus on the sexual dimorphism observed in centenarians. We will highlight the broad heterogeneity of factors that may influence centenarians lifespan, including genetic, physical, economic, social, and cultural aspects, as well as lifestyle choices. The prevalence of centenarians has increased globally over the past few decades, reaching approximately 600,000 individuals by 2024 ( 125 ). In 2016, Buettner et al. found that a larger number of centenarians was associated with specific geographical areas known as blue zones, where they proposed to discover the healthiest lifestyles that contribute to vitality and longevity ( 126 ). However, the distribution of centenarians in 2024 ( Figure 4A ) clearly indicates that the density of the world’s longest-lived individuals also strongly correlates with the development status of the country and cultural patterns. Life expectancy is greatly higher in developed countries compared to developing countries, where advancements in preventive healthcare, treatments, social care, and overall quality of life are much more advanced. Japan stands out as the country with the highest number of centenarians, boasting approximately 97 individuals per 100,000 people. This phenomenon can be attributed not only to the unique Japanese diet and lifestyle but also, and more importantly, to economic success and robust health insurance systems that led to notably reduced mortality rates from ischemic heart disease and cancer ( 127 ). Global distribution of centenarians and sex-specific longevity effects. (A) The density is represented as the number of centenarians per 100,000 people for the year 2024, specifically in countries with populations exceeding 10 million inhabitants. Data is marked as NA (not analyzed) for countries with populations below 10 million inhabitants. (B) Sex-specific positive and negative effects on centenarian longevity mediated by genetic and environmental factors. France is a second leading country in terms of longevity, often referred to as the “French paradox”. It is the concept that highlights the significant role of moderate wine consumption in promoting longevity, particularly through its positive therapeutic effects on ischemic heart disease, metabolic syndrome, cognitive decline, depression, and cancer ( 128 ). Next, countries such as Canada, Italy, Greece, Spain, and Portugal boast more than 30 centenarians per 100,000 inhabitants. Following closely are countries like the US, Australia, Sweden, Germany, England, Belgium, Chile, and Cuba, which have more than 20 centenarians per 100,000 people. Conversely, countries with the fewest super-agers are primarily located on the African continent. Africa faces serious health challenges, characterized by a wide range of diseases and limited access to essential medicines ( 129 ). As a result, African countries have conducted little or no research on centenarians. Therefore, when summarizing the geographical distribution of the world’s extreme long-livers, it becomes evident that one substantial factor contributing to longevity may be the balance between a particular lifestyle and quality of life. In general, more than eighty percent of centenarians are women, despite the paradoxical fact that men tend to have better health at this age ( 130 ). This observation of the gender paradox remains difficult to explain. The prevalence of females among centenarians has been observed in all countries, with the exception of Mexico, where the number of centenarians slightly favors men over women. Intriguingly, in 2017, the ratio of male to female centenarians was radically different, indicating that there were three times as many women as men. However, it has been reported that in Mexico, females face more unfavorable conditions compared to men, particularly in terms of loneliness, limitations in physical and social activities, and health issues ( 131 ). More than 25% of the variability in human lifespan is estimated to be genetic ( 132 ); however, it seems unlikely that this genetic phenotype is solely hereditary ( 133 ). In the last decade, several studies have identified numerous genes and genetic loci that promote longevity, including APOE , TOMM40 , HLA-DQA/DRB1 , LPA , SHC1 , and insulin/IGF1 signaling, which are considered sex-independent genetic risk factors ( 134 – 136 ). However, the genetic contribution to longevity traits has been shown to be moderated by gender, with females exhibiting a stronger genetic background. Additionally, genetic influence appears to be highly specific to ethnicity. Thus, the analysis of non-synonymous SNPs in DNA repair genes among German and French centenarians revealed a significant enrichment of the functionally relevant SNP rs1776180 in female centenarians. This SNP was associated with increased EXO1 expression, positioning it as a novel candidate gene for longevity in women ( 137 ). In the same context, a sex-specific genome-wide association study (GWAS) and polygenic risk score analysis were conducted using a cohort of Chinese centenarians to identify longevity loci specific to males and females. The longevity-associated genes that were most significant for women from the northern and southern regions of China included ZFP69B , PGGT1B , FAM19A1 , FSTL5 , NOTCH4 , KIR3DX1 , CYP1B1-AS1 , TBX3 , KIAA1715 , FRK , and LOC1720 . Notably, a deeper comparative analysis of the Chinese women-specific loci with available datasets revealed only two longevity-associated genes that were identified to be common between Chinese and U.S. women, as well as between Chinese and European women: LINC00871 and ABCG2 , respectively ( 138 ). In another study, FOXO1A was detected to be more strongly associated with longevity in Chinese females than in males ( 139 ). In contrast, specific genes associated with longevity in Chinese male centenarians include MTHFD1 , KCHA5 , CSMD1 , IMPG1 , LRRFIP1 , PKHD1 , NUDT12 , SYDE2 , BRD1 , SLC39A11 , and STK10 ( 138 ). However, unlike the cohort of women, no common gene associations have been published between Chinese and European or U.S. men. Additionally, there is evidence that inhibited IL6 gene expression is significantly associated with longevity in Italian male centenarians ( 140 ). Controversial data have been published regarding the FOXO3A gene. Several studies indicate that FOXO3A is associated with longevity in both sexes among individuals of German, Japanese, French, and Chinese descent. However, some data suggest a predominant association of FOXO3A specifically with the lifespan of Italian men ( 139 , 141 – 143 ). An analysis of sex-specific pathways involved in the regulation of human lifespan reveals a significant activation of metabolic pathways in female centenarians, while immune- and inflammatory-related pathways are predominantly activated in male centenarians ( 138 ). Polymorphisms in the coding regions of mitochondrial DNA play a pivotal role in human longevity across various ethnicities, exhibiting a sex-unbiased effect ( 144 – 146 ). However, some sex-specific genetic influences have also been reported. Notably, mitochondrial haplogroup F has been significantly associated with exceptional longevity exclusively in Chinese females. The authors suggested that haplogroup F is linked to higher levels of HDL and a functional SNP variant (m.13928G>C) ( 147 ). It appears that ethnicity significantly influences variations in results and should be taken into account, particularly in the investigation of genetic factors, which is currently under intense scrutiny. For example, genes such as TOMM40 , APOC1 , and APOE , which have previously been shown to correlate significantly with longevity in European women, have not been replicated in studies involving African American and Hispanic women. This discrepancy is likely due to the fact that different ethnicities may exhibit a higher prevalence of certain alleles compared to others ( 148 ). DNA methylation represents a crucial epigenetic modification that facilitates the interaction between genetic and environmental factors, and it has demonstrated potential in elucidating the mechanisms that contribute to exceptional healthy longevity. The epigenetic clock, which relies on DNA methylation patterns, is widely recognized and utilized to estimate chronological age or to identify biological age. In recent study, the epigenetic clock was calibrated using data from 336 Japanese participants. An analysis of the epigenetic ages of Japanese centenarians and supercentenarians revealed that all participants exhibited younger epigenetic ages compared to their chronological ages, accompanied by negative epigenetic-age acceleration ( 149 ). Similar findings were observed in a cohort of French centenarians ( 150 ). These results imply that healthy longevity may possess an epigenetic basis. However, no evidence of sexual dimorphism was detected in either study. In contrast, the epigenetic age predictors for the old oldest men from Italy and Greece demonstrated lower correlations with chronological age and exhibited positive epigenetic age acceleration when compared to women from the same regions ( 151 ). Additionally, a study conducted by Sun et al. provided a comprehensive comparison of the DNA methylome between the longest-lived men and women in the Chinese population. Although the average methylation levels of the examined CpGs were comparable, women exhibited DNA hypermethylation and hypomethylation in specific chromosomal regions and genes, particularly on chromosome 17, which harbors numerous disease-associated genes ( 152 ). During the COVID-19 pandemic, older individuals were more susceptible to infection and had faced the highest risk of death. Due to the differing responses to infections between the two genders described above, older females exhibit greater resilience to COVID-19 than males. However, in the context of centenarians, women have a higher mortality rate, which may be attributed to immune changes resulting from lifelong stress ( 153 ). In general, there is statistical evidence that centenarians born before 1919 were more resilient to COVID-19 compared to other elderly individuals. This phenomenon has been speculatively attributed to a specific immune response developed following the Spanish flu epidemic of 1918 ( 154 , 155 ). Long noncoding RNAs (lncRNAs) serve as critical regulatory elements and have been shown to significantly impact various biological processes as well as the mechanisms associated with human diseases. However, the roles of lncRNAs in the aging process and their correlation with sex remain largely underexplored. In the study conducted by Jiang et al., the authors investigated centenarian-specific lncRNAs within a cohort of long-lived individuals from both the Northern and Southern regions of Hainan Province in China. The study identified 11 lncRNAs that were overexpressed in centenarians compared to control cohorts, with 8 of these lncRNAs being proposed as candidates for promoting healthy aging and longevity. Nonetheless, the study did not address the issue of sexual dimorphism in this context ( 156 ). Cognition and physical capacity are essential functions that serve as important indicators of healthy aging. Consequently, age-related cognitive and physical changes in centenarians have been examined from various perspectives. For instance, mitochondrial dysfunction associated with oxidative stress and, consequently, age-related neurodegeneration was investigated from a gender perspective. Mitochondrial functions such as basal respiration, ATP production, reserve capacity, and maximal respiration were assessed in PBMC from Danish centenarians. A significant inverse correlation between cognitive performance and mitochondrial function was observed only in male centenarians, suggesting a stronger relationship between cognitive ability and mitochondrial functional capacity in men ( 157 ). In contrast, cognitive impairments such as difficulties with orientation, calculation, and memory in centenarian women were associated with poor sleep quality and shorter nighttime sleep. One potential explanation for this observation may be that women generally have a higher susceptibility to sleep disorders, and the consequences of this factor are more pronounced in female centenarians than in their male counterparts ( 158 ). Physical independence is another age-related indicator in which women experience greater challenges than men. For instance, poor nutritional status has been linked to increased functional limitations among Chinese female centenarians ( 159 ), while men were not significantly affected. Additionally, low levels of total cholesterol have a mediating effect on body mass index and blood pressure. This factor was strongly correlated with a higher frequency of disabilities exclusively among Chinese female centenarians ( 160 ). Psychology is an additional discipline that can significantly contribute to longevity by emphasizing the importance of mental health, which may differ between the sexes. Generally, among centenarians, anxiety, low self-efficacy, and a lack of optimism have been shown to correlate with poorer subjective health and feelings of desperation ( 161 ). Building on this concept, personality, character, and temperament profiles were compared between Greek male and female centenarians. Female centenarians exhibited higher levels of adaptability and were more reward-dependent, while male centenarians displayed a more optimistic attitude. From a sociodemographic perspective, men were less likely to report experiences of widowhood and more likely to mention having first-degree relatives who are centenarians, as well as engaging in negative habits such as smoking ( 162 ). In conclusion, human longevity is a multifaceted trait influenced significantly by both environmental factors and genetic predispositions. The investigation of genetic mechanisms that regulate human longevity has the potential to influence healthy aging by delaying or preventing age-related diseases. However, it is evident that favorable genotypes are not consistently replicated across populations and cannot fully account for the gender longevity gap. Therefore, additional factors that influence human lifespan, such as environmental stimuli, lifestyle, and immunity, must also be considered (summarized in Figure 4B ). In general, the basic biological mechanisms associated with favorable biological signatures for extreme longevity are still in their early stages of understanding. To enhance the healthy lifespan of the increasing population of older adults, further research on the health status of centenarians should be conducted, with a focus on large-scale studies across different countries. These studies should take into account various genetic backgrounds, ethnicities, and cultural traits.

It is important to understand and consider the role of gender in the entity of changes that affect the immune system during aging to be able to increase a probability of reaching an advanced age with strong health. In this section of the review, we examine the variations in immune composition between young men and women, as well as the changes that occur during the aging process. The immune system begins to develop very early in intrauterine life, and differences in lymphocyte composition between males and females are present already at that stage. Around two decades ago, the first evidence of sex-based differences in immune system development emerged through studies that tracked male and female cohorts from cord blood at birth up to 18 years of age ( 13 , 14 ). It was shown that a total lymphocyte number was similar between girls and boys in infancy and was followed by a gradual and significant decline into adulthood. However, the distribution of individual cell subpopulations between genders was slightly different. Thus, girls contained a higher level of CD4+ T cells and higher CD4/CD8 T cell ratios. In contrast, boys exhibited a higher abundance of CD8+ T cells and NK cells. The numbers of B cells and regulatory T (Treg) cells were comparable between both genders of children. Recent studies have provided greater clarity regarding the propagation of specific B cell and T cell subpopulations. Using cord blood as a model, Bous et al. compared sixteen male and twenty-one female newborns. They found that the frequencies of transitional and naïve B cells were similar; however, populations of innate B cells, class switched memory B cells, late memory B cells, plasmablasts, and B2 cells were more prevalent in the cord blood of male newborns. Among T cell populations, only naïve thymus-negative T helper (Th) cells were significantly more abundant in male cord blood, while the other twenty T cell subpopulations showed similar frequencies in both sexes ( 15 ). Throughout development and aging, the composition of immune cells in adults changes, with some immunological differences between genders persisting throughout life, while others only become evident with age-related changes. In the context of healthy aging, centenarians appear to experience a slower rate of aging, making their repertoire of immune cells particularly intriguing. Seminal studies have provided important data on gender- and age-related differences in immune cell populations. For instance, the levels of circulating blood monocytes were found to be higher in young men compared to young women, and these levels were observed to increase in aged individuals and centenarians for both genders ( 16 – 20 ). However, there is evidence that monocytes from females exhibit a more activated phenotype than those from males ( 21 ). The majority of studies discovered a higher prevalence of B cells in women than in men ( 22 – 24 ); however, there is some controversial data in this context ( 16 , 20 , 25 , 26 ). In general, an age effect has been observed for both genders, resulting in a slight decrease in B cell numbers ( 27 , 28 ), with the male cohort showing a greater reduction trend ( 22 – 24 , 29 ). Nevertheless, despite the seemingly controversial cell density, the activation of B cells and their transcriptional profiles notably differ between the sexes during aging or in response to pathogens ( 20 , 25 ). The density of dendritic cells (DC) has been found to be similar in both young and elderly men and women ( 16 , 20 ); however, it is reported to be slightly impaired in centenarians ( 20 ). While sex-specific phenotypic differences in DC have been observed in in vitro studies, they indicate that plasmacytoid DC isolated from females produce higher levels of interferon compared to those isolated from males ( 30 ). A remarkable gender-related increase in the number of natural killer (NK) cells was observed in young men. With aging, the level of NK cells drastically enhanced for both sexes, reaching its peak in centenarians ( 16 , 20 , 23 , 24 , 31 ). Even though both elderly men and women were able to maintain high levels of NK cells, a greater significance was still noted in men. Notably, women have not only a lower overall number of NK cells, but they also have a relatively smaller percentage of mature NK cells ( 22 ). The activity and distribution of T cell subsets vary significantly between sexes and are known to be differentially affected by aging. Females have higher CD4+ T cell levels and higher CD4/CD8 ratios than males, whereas males have higher CD8+ T cell frequencies ( 16 , 26 , 32 ). Young men exhibited higher levels of regulatory T cells with enhanced immunosuppressive functions compared to women of the same age ( 33 , 34 ). With aging, the majority of T cell subpopulations decrease in both genders, with a more pronounced reduction observed in men ( 32 ). In contrast, the number of Treg cells was found to be enriched in both genders over a lifetime ( 16 , 35 , 36 ). In centenarians, in addition to the ‘younger’ genetic profile of naive T cells ( 37 ), a twofold increase in T cells with a cytotoxic phenotype was observed ( 20 ), which may ultimately influence longevity. It is also important to note that a decreased level of Treg cells was reported in centenarians compared to an older human cohort ( 36 ). In summary, there exists considerable variation in the predominance of immune cells between genders, and the composition of immune cells undergoes significant alterations with aging (as summarized in Figure 1 ). Generally, females exhibit stronger innate and adaptive immune responses compared to males. Analyses of the cellular landscape indicate that women demonstrate more active dendritic cells, a higher absolute number of B and CD4+ T cells. Consequently, innate immune cells exhibit more effective phagocytosis and antigen presentation, enhanced production of inflammatory cytokines, and elevated levels of circulating antibodies. The illustration depicts the composition of immune cells in males and females, as well as the alterations that occur throughout their lifespan.

The immune response is modulated by a multitude of factors, with sex chromosomes serving as critical determinants. The X chromosome contains a high density of immune-related genes that play significant roles in both innate and adaptive immune responses. As aging progresses, the composition and activity of immune cells undergo substantial changes, which may be driven by modifications in immunoregulatory gene expression. In this section, we examine the variations in the expression of immuno-associated genes located on the X chromosome throughout the aging process (summarized in Figure 2 and Table 1 ). Expression of X-linked genes associated with immunity: ↑ - expression is up-regulated in the older group; ↓ - expression is down-regulated in the older group; and ↑↓ - up- or down-regulated gene expression depending on the cell type. AD, group with Alzheimer’s disease; NI, not identified. Expression of immune-associated X-linked genes in young and old groups. PS, induced pluripotent stem cells; HMSCs, human mesenchymal stem Cells; ↑, expression is upregulated in the case group; ↓, expression is downregulated in the case group; RNA-seq, bulk RNA sequencing; scRNA-seq, single-cell RNA sequencing. The human X chromosome possesses a unique biology and is present in both males and females. It spans approximately 155 MB in length ( 38 ) and contains more than 1000 genes, 897 of which are protein-coding genes, as reviewed by Barbara R. Migeon ( 39 ). Due to the fact that women have two X chromosomes while men have only one, the expression of X-linked genes must be equalized between the two sexes. To achieve this, the majority of genes on one of the two X chromosomes in females undergo random inactivation through transcriptional silencing. However, 10-15% of these genes can escape X-inactivation, acting as potential contributors to sexually dimorphic traits, leading not only to phenotypic variability among females but also to clinical abnormalities in patients with an abnormal number of X chromosomes ( 40 ). Even though the expression is considered balanced between males and females and detailed comparisons were not conducted in most studies, many genes exhibit sex-biased regulatory targeting patterns, which may lead to different effects. The inhibitory expression effect was determined for several X-linked genes. One of these is DDX3X (DEAD-box helicase 3 X-linked), which plays a role in antiviral activity and the induction of type I interferon (IFN) ( 41 ). In 2004, Tao Lu performed a transcriptional profiling study using postmortem samples from the human frontal cortex of 30 individuals aged 26 to 106 years. This study revealed a decrease in DDX3X expression with age ( 42 ). Supporting these findings, a study conducted by Y. Zheng and colleagues in 2020 demonstrated a significant decrease in DDX3X expression in memory B cells from the peripheral blood of older individuals compared to younger individuals (healthy donors). Similarly, DDX3X expression was notably reduced in dendritic cells; however, this analysis did not pass multiple comparison corrections ( 43 ). These results suggest a potential trend of inhibited DDX3X expression in immune cells with age. Expression of transcription factor TSC22D3 which encodes a glucocorticoid (GC)-induced anti-inflammatory leucine zipper (GILZ) protein and has immunosuppressive functions ( 44 ) was detected to be inhibited in CD8+ T cells ( 45 ) and monocytes in aged group older than 60 years; however, at the same time, it has risen in B cells ( 43 ). Another gene, SH2D1A , which is predominantly expressed in T, NK, and some B cells, has been shown to be downregulated in CD4+ and CD8+ T cells with aging. Similarly, the expression of XIAP (X-linked inhibitor of apoptosis), which plays a role in both innate and adaptive immunity by modulating TNF-receptor signaling and regulating inflammasome activity, was also reduced in CD8+ T cells of the elderly. The CYBB gene, also known as cytochrome b-245 beta chain or NOX2 , is involved in the production of reactive oxygen species (ROS) by phagocytes. In the older group (ages 60-80), the expression of CYBB was detected to be significantly downregulated in B cells, dendritic cells, and monocytes compared to the younger group (ages 20-45) ( 43 ). IL13RA is also classified as an X-chromosome-regulated gene. IL13RA1 mediates the JAK1, STAT3, and STAT6 signaling pathways induced by IL-13 and IL-4, which play critical roles in the initiation and regulation of innate immune responses and adaptive immunity ( 46 ). Older individuals exhibit a reduction in IL13RA1 expression in monocytes ( 43 ). The CFP gene encodes the complement factor properdin involved in regulating the natural immune system’s alternative pathway. It was found to be down-regulated in skin infiltrated immune cells ( 47 ), but was detected to be up-regulated in monocytes of the elderly group ( 43 ). The expression of MID2 (midline 2) was found to be decreased in the muscle tissue of elderly individuals (ages 65-84) compared to that of younger individuals (ages 18-28) ( 48 ). TLR7 and TLR8 are other important immune-associated genes that are mapped to the X chromosome. As reviewed by Tianhao Duan et al. , toll-like receptors (TLRs) bridge innate and adaptive immunity by influencing antigen-presenting cells and inflammatory cytokines ( 49 ). Generally, older adults exhibit dysfunctional activity of TLRs ( 50 ), which may contribute to increased infection-related morbidity and mortality, as well as the development of age-associated neurodegenerative diseases ( 51 ). Notably, the expression of TLR7 has been shown to be reduced in monocytes from individuals over 60 years of age ( 43 ). Cribbs et al. examined the expression of TLR7 and TLR8 genes in various brain regions of young (20-59 years), aged ( 60 – 99 ), and individuals with Alzheimer’s disease (74-95 years), revealing a robust upregulation of these genes in the aged population ( 52 ). It is important to note that both TLR8 and TLR7 can evade X chromosome inactivation in female immune cells, specifically in monocytes, CD4+ T cells, and dendritic cells, resulting in sex-biased expression. Consequently, the co-expression of genes from both the active and inactive X chromosomes may lead to increased TLR8/TLR7 protein levels in female cells, potentially influencing their response to viruses and bacteria, as well as the risk of developing inflammatory and autoimmune diseases. However, it is also worth mentioning that a similar effect has also been observed in men with Klinefelter syndrome, who possess 47,XXY chromosomes ( 53 – 55 ). In contrast, the expression of several X-linked genes is activated during the aging process. For instance, the inducible chemokine receptor CXCR3 has been shown to be upregulated in CD4+ and CD8+ T cells in older individuals ( 43 , 45 ). Another gene, WAS , which encodes the WASP actin nucleation-promoting factor and is a member of the Wiskott-Aldrich syndrome (WAS) family of proteins, is involved in the transduction of signals from receptors on the cell surface to the actin cytoskeleton. With aging, the expression of WAS is enhanced in most immune cell populations, namely in B cells, CD4+ and CD8+ T cells, dendritic cells, monocytes, and NK cells ( 43 ), however, the data are limited for individuals older than 80-years. PQBP1 gene encodes polyglutamine binding protein-1, which functions as an intracellular receptor that recognizes pathogens and neurodegenerative proteins ( 56 ). This gene was detected to be upregulated in the monocytes and CD4+ T cells of elderly individuals (aged 60-80) compared to younger individuals (aged 20-45) ( 43 ). The interleukin 2 receptor subunit gamma ( IL2RG ) serves as an important signaling component for many interleukin receptors and was significantly increased in CD4+ T cells, CD8+ T cells, and NK cells of senior participants ( 43 ). Additionally, the expression of other X-chromosome-regulated genes, such as biglycan ( BGN ), was found to be enhanced in the skin fibroblasts of the older group (aged 70-76) compared to the younger group (aged 35-48) ( 47 ). In 2022, Liu et al. conducted a transcriptome analysis of human mesenchymal stem cells (hMSCs) isolated from healthy young males (ages 19–25) and elderly males (ages 75–92) to investigate why younger individuals possess a higher regenerative capacity than their older counterparts. This analysis revealed several X-linked genes that are differentially expressed between the two groups. Specifically, the expression of genes such as IL13RA1 , OTUD5 (which regulates interferon signaling), and PRKX (involved in monocyte-macrophage function) was inhibited in cells from old individuals. Conversely, the genes MID1 (Midline-1) and HMGB3 (high mobility group box protein 3) were found to be overexpressed ( 57 ). There is increasing evidence that X-linked genes influence the aging process and age-related diseases. Meyer et al. generated induced pluripotent stem cell (iPSC)-derived neural progenitor cells and neurons from aged healthy donors (ages 60-92) and aged donors with Alzheimer’s disease (AD) (ages 60-87). The iPSC-derived neural cells from AD patients exhibited accelerated neural differentiation and reduced progenitor cell renewal. These neurons showed upregulation of the X-linked genes TSC22D3 , HMGB3 , and SYP . Additionally, a similar phenotype was observed in AD iPSC-derived neural progenitor cells, which demonstrated significantly impaired expression of several X-linked genes, including DDX3X , TLR7 , TLR8 , BTK , ACE2 , WAS , CYBB , CFP , and IL13RA . Overexpression was identified solely for the IL1RAPL1 gene, which is implicated in dendrite differentiation ( 58 ) and SYP gene which has been shown to be responsible for intellectual disability ( 59 ). These findings suggest that the dysregulation of X-linked gene networks may create favorable conditions for the development of Alzheimer’s disease and impact the healthy aging process. Other X-linked immune-related genes, such as IRAK1 , MAPK3K7IP3 , MIR223 , NOX1 , BM X, and ELF4 , were not identified as differentially expressed between the young and elderly groups. Research indicates that certain genes, such as IRAK1 and MIR223 , play a role in the regulation of inflammatory and infectious responses. These genes exhibit polymorphisms that are associated with sex-specific differences in immune responses. Furthermore, these variations may be influenced by inherited traits and the unique X-chromosome mosaicism present in females, which contributes to the regulation of immune responses as aging ( 59 – 61 ).