Sexual dimorphism describes differences between males and females across various factors, including but not limited to behavior and immunity (1). Sex hormones are one contributor to this dimorphism, where males have higher testosterone and lower estrogen levels, while females have higher estrogen and lower testosterone levels, with age being a major factor in this ratio (2). Sex hormone signaling via sex hormone receptors is a major transcription pathway that influences cellular function, namely cytokine production, cell proliferation, and reactivity (3).

Sex differences in immune function between males and females are also influenced by genetics, with several key genes involved in immunity expressed on the X chromosome. This includes receptors such as TLR7, TLR8, and ACE2 (4, 5), as well as FOXP3, which controls regulatory T cell production (6). Further, the X-chromosome encodes 10% of all miRNAs, including miRNA-18 and 19 which are associated with sex-biased immune response (7, 8).

Functional programming of innate and adaptive immune cells depends on epigenetic remodeling, which alters the regulatory landscape of the genome and controls gene expression (9, 10). These changes influence immune cell identity and can predict how the cell will respond to exogenous stimuli (11). For example, T-cell exhaustion has been linked to epigenetic reprogramming that leads to changes in differentiation trajectory (12, 13). Autoimmune disease is associated with altered cytokine production and immune cell reactivity due to epigenetic changes that are caused by environmental influences, genetic variants, and medication (14). Additionally, certain infections, including SARS-CoV-2 and malaria, as well as immunizations, such as influenza and BCG vaccines, induce changes in the epigenomes of hematopoietic stem cells and monocytes (15–18).

This mini review will summarize recent data on sex hormone receptor-mediated epigenetic remodeling and immune function modulation, with implications for understanding immune function changes in transgender individuals following gender affirming hormone therapy (GAHT). Although not covered in this mini-review, the sex hormone progesterone also plays a vital role in immunity, particularly the promotion of maternal-fetal tolerance during pregnancy, through expanding regulatory T cells and regulation of reactivity of other immune cells (19).

Sex hormones influence various physiological systems including neurological, reproductive, musculoskeletal, and immune systems (20, 21). Testosterone, which is a type of androgen, is associated with masculinizing effects, spermatogenesis, and is a modulator of immune response (7, 22). Estrogen is dominantly expressed in females, is associated with feminizing effects, and is a driver of several diseases such as cancer, and a promoter of immune function (7, 23).

Epigenetics literally means ‘above DNA’ and refers to the study of molecular interactions that influence DNA structure, compaction, and function (35). Epigenetic marks can be ‘written’, ‘erased’, and ‘read’ by specific nuclear proteins to regulate gene expression in a range of physiological processes (36, 37). Three major epigenetic modifications are (i) DNA methylation, where a methyl (-CH₃) group is added to the cytosine nucleotide within a cytosine-guanine sequence (CpG dinucleotide) (38–40); (ii) histone post-translational modifications, such as acetylation and methylation (41); and (iii) non-coding RNAs, which are transcribed RNAs that are not translated into proteins, but can regulate gene expression through mediation of chromatin structure (42).

Most of the genome (over 95%) does not code for proteins. Much of this noncoding landscape plays an important role in regulating the transcription of coding genes (43). Indeed, hormone receptors such as ESR1 bind large swaths of the noncoding genome, influencing the activity of coding genes (44).

Sex differences are observed in immune responses to infections, with females generally exhibiting hyper reactivity than males (7). This is particularly evident in influenza infections and following influenza vaccination, where females produce higher neutralizing antibodies and inflammatory cytokines (64–66). Post-influenza vaccination, females show elevated levels of inflammatory markers such as leptin, or interleukin-receptor agonist (IL-1RA) (64, 67). Higher circulating estradiol concentrations in females reduce other proinflammatory cytokines like tumor necrosis factor (TNF)-α and chemokine ligand (CCL)-2, primarily mediated via the ERα, thereby lowering influenza-related morbidity and mortality (68). Conversely, males, influenced by the immunosuppressive effects of testosterone, generally exhibit weaker responses to influenza vaccination, with lower antibody production, especially in those with high serum testosterone concentrations (64, 66, 69). These sex differences extend to COVID-19, where males typically experience more severe outcomes, partly due to higher transmembrane protease, serine 2 or TMPRSS2 expression facilitating viral entry (70). Conversely, the effects of estradiol on angiotensin-converting enzyme 2 and angiotensin II receptor type 1 signaling in females reduce the severity of COVID-19 infection (71, 72). Males with long COVID cognitive symptoms show higher levels of the neuroinflammation-linked chemokine CCL11 compared to females (73), suggesting increased susceptibility to certain post-COVID neuroinflammatory effects. Together, these findings highlight the complex relationship between sex hormones and immune responses in viral infections.

In autoimmune disease, sexual dimorphism leads to stronger, estrogen-driven immune responses in females, increasing their overall susceptibility (74, 75). For example, in juvenile idiopathic arthritis (JIA), females are more susceptible to chronic inflammation with three to six females for every male patient are affected due to increased activation of immature neutrophil-related genes, leading to enhanced neutrophil activation that may impact treatment effectiveness, including responses to Interleukin (IL)-1 receptor antagonists (76). Similarly, females are more prone to developing multiple sclerosis (MS), with a 3:1 ratio compared to males, partly due to estrogen-enhanced IL-17 expression, which drives autoimmune pathogenesis by boosting pro-inflammatory cytokine production and T cell activity (75, 77, 78). Females also experience more severe skin inflammation, particularly in psoriasis, due to estrogen amplifying the inflammatory response by increasing cytokine production and immune cell activity (79). While genetic differences between males and females are a factor in this dimorphism, most differences are attributable to sex hormones receptors as well (80).

Sex hormone receptor signaling in immune cells influences various functions such as cell proliferation, reactivity, and overall function (81). Table 2 summarizes studies on how sex hormone signaling affects immune cells.

At least 1.5 million people in the United States are transgender (~0.5%), of whom 90% have considered or are undergoing GAHT (113), with transgender women having a disproportionately higher rate of HIV infection (114). A cross-sectional study comparing transgender women and men receiving GAHT to cis-men and women, revealed that GAHT influences both the proportions in the circulation and the transcriptome of regulatory T (Treg) cells (115). Interestingly, sex hormones and chromosomes may collectively influence some cell proportions. Peckham et al. found that CD19+ CD27+ IgD- classical-switched memory B cells were sensitive to estrogen only in the XX karyotype, decreasing in transgender women following GAHT, but in post-menopausal cis-women following HRT, but not influenced by estrogen GAHT in transgender women (116). Collectively, these findings show that GAHT leads to a unique impact on different immune cell subtypes. The long-term effect of GAHT on the immune system has not been extensively studied which has clinical implications given the known sex-specific prevalence and risks in many inflammatory, infective and autoimmune diseases. To address this significant knowledge gap, we have longitudinally profiled the immune system of transgender women and transgender men newly commencing GAHT. A significant advantage to this approach is it allows us to dissect the contribution of sex hormone action relative to sex chromosomes in immune function, which is usually difficult to separate from the underlying genetic differences between males and females. Using longitudinal GAHT cohorts, we have shown that this therapy can influence epigenetic marks in blood cells affecting the immune response. A notable example of this is differentially methylated CpG site in the promoter of IL-21, which gained DNA methylation after 12 months of masculinizing GAHT, but lost DNA methylation after 12 months of feminizing GAHT (117). We further elucidated the effects of feminizing GAHT in the metabolome wherein a cyproterone-acetate specific decrease in glutamine levels, an important amino acid related to immune cell metabolism, has been observed (118). Altogether, these findings suggest that GAHT influences the changes within the immune cell in a transcriptional level. Recently, Lakshmikanth et al., showed that masculinizing GAHT increased chromatin accessibility at canonical NFkB binding sites in T and NK cells after 12 months of therapy together with the promotion of monocyte responsiveness, together with downstream upregulation of NF-kB and interferon-γ production in NK cells (119). Modulation of interferon signaling by masculinizing GAHT was previously reported, with a decline in IFN-I production by plasmacytoid dendritic cells through the regulation of TLR7/8 (120). This testosterone-associated reduction in IFN-I responses was in line with lower TLR7 responses in males compared to females (121). Therefore, approaches that study immune cell phenotypes, including membrane and nuclear receptors, and downstream chromatin remodeling will provide insights into how sex hormones signal in primary human cells, and potentially explain the contribution of sex hormones to sexual dimorphism in inflammation and development of complex immune diseases.

Epigenetic remodeling through sex hormone signaling may have wide-ranging impacts on the immune profile of transgender men and women. We highlight that most of our knowledge about how sex hormone receptors remodel chromatin come from cancer studies, and that human studies in the context of hormone change are warranted. Considering that AR and ERs are expressed in a range of tissues and immune cell types, changes in circulating sex hormone concentrations can influence cell responses in the circulation, as well as progenitor populations in the bone marrow. Based on the literature we presented, estrogen and testosterone do not simply promote or inhibit immune responses, but changes in their concentrations would lead to a unique immune cell phenotype in the context of GAHT. Profiling the effects of GAHT on epigenetic remodeling will not only provide insight into the role of sex hormones in immune function and the development of complex immune diseases, but will help inform the healthcare of transgender people on GAHT.