Novel insights into the sexual dimorphism-associated immune response

In their recent article in Science, Chi et al. [1] characterized a mechanism that regulates tissue/organ-specific immunity in mice. Their findings revealed that the androgen type 2 innate lymphoid cell-dendritic cell axis played a prominent role in a microbiota- and sex hormone-dependent manner. These observations implicate sexual immune dimorphism as a molecular, cellular and tissue/organ-specific regulatory hub for the (im)balance between health and disease.

Biological sex is defined as the anatomical, biological, chromosomal and physiological characteristics of a species and is mainly related to reproductive functions. Biological sex also affects immune system function, including both innate and adaptive immune responses, which are evolutionarily conserved across species [2] and change throughout life. Sex chromosome genes and sex hormones (i.e., estrogens, progesterone, testosterone and androgens) contribute to the differential regulation of immune responses between sexes [2] (Fig. 1).

Fig. 1.

Sexual dimorphism-associated tissue immunoregulation. Overview of the impact of sexual dimorphism on immune system/adaptive immunity via the androgen-type 2 innate lymphoid cell (ILC2)-dendritic cell (DC) axis. Pathophysiological susceptibility (from antiviral and antitumoral immunity to allergic and autoimmune diseases) is dependent on the specific type of tissue or organ (and prevalent in the skin and lung) and is affected by the dual action of sex hormones (androgens and estrogens) and the microbiota during the vital transition (aging), with interactions with environmental variables such as age, circadian rhythms, body mass index (BMI) and/or diet. This figure was created with Biorender.com

Environmental factors, including nutritional status, diet, vital light-dark rhythms (i.e., circadian and infradian rhythms) and the microbiota, shape the development and function of the immune system differently in males and females, which can result in differences in susceptibility to autoimmune diseases, malignancies and infectious diseases and affect the outcome of vaccination [3] (Fig. 1).

The immune system is driven by external and internal threats. For instance, the immune system defends against environmental challenges, such as pathogenic infections (e.g., bacteria, viruses, fungi and parasites), and detects and eliminates harmful cells that can lead to malignancy. Optimal immune homeostasis is achieved when a threat is eliminated with high efficiency and at low cost to the tissue, organ and/or species [2, 3] (Fig. 1).

The nature and strength of immune responses differ between women and men, resulting in sex-specific differences in the prevalence, manifestation and outcome of malignancies and autoimmune and infectious diseases [2, 3]. While women are generally able to mount a stronger immune response to infections, vaccinations and some malignancies, they also suffer more from inflammatory and autoimmune diseases. Although several mechanisms have been proposed to explain this phenomenon, our knowledge of the key players involved in shaping sex-specific immunity remains largely unclear [2, 3]. A better understanding of these processes would help optimize prevention and treatment strategies for women and men as a first step toward personalized medicine.

In an exciting paper published in Science, Chi et al. [1] identified novel key players involved in the regulation of tissue-specific immune bias. Their findings underscore for the fact that type 2 innate lymphoid cell (ILC2) control of tissue-resident dendritic cells (DCs) plays a pivotal role in shaping sex-specific immune differences in the skin and lung, with the cooperation of sex hormones and the microbiota mediating the intensity of the immune response (Fig. 1).

The authors initially examined the lymphoid landscape of sex-biased immunity in major barrier tissues (skin, lung and intestine) of sexually mature male and female mice. Consistent with a previous report [3], the number of classical T-cell subsets in the skin and lungs was greater in females than in males at homeostasis, with no significant differences in lymph nodes or in the small intestine. This result indicates that immune sex bias is associated with specific tissues. This finding was corroborated in female and male germ-free mice, where immune sex differences in the phenotype and the number of T cells in the skin were affected in a microbiota-independent manner. These findings are in accordance with defined gene expression profiles in human skin associated with sex differences [4]. Although the microbiota was not necessary for the observed differences between males and females in the skin, inoculation of adult germ-free mice with a complex microbiota was linked to type 17/T_reg immunity in females, suggesting that the microbiota can shape sex-specific immunity in females. These differences were also reproduced in male and female mice born with natural, wild-derived microbiota and pathogens [5].

The authors then investigated whether responses to other microbiota populations (e.g., Staphylococcus epidermis, Corynebacterium accolens and Candida albicans) were differentially affected by biological sex. They found that T-cells, keratinocytes, and T-cell responses were differentially affected by sex, with enhanced immune responses observed in females compared with males.

As sex-specific differences could result from sex-specific hormones and/or sex-related chromosomes, they analyzed male and female mice without gonads before sexual maturation and during the adult stage. Ovary removal in females had no effect on skin lymphoid composition, whereas castration in males restored the number and frequency of IL-17-A-producing cells and T_reg cells to the levels found in females. These findings provide evidence that sex-related immune differences in the skin are derived from male sex hormones.

As DCs are the primary sensors and mediators of tissue immunity, including T-cell responses to the microbiota, the authors next investigated whether the differences in lymphocytes between male and female mice could be due to quantitative and/or functional distinctions in the skin DC network. They found that while the overall composition of the DC network was broadly comparable between males and females, the absolute number of all resident DC subsets (with the exception of CD11b^hi DC2 cells) was greater in the skin of females than in that of males. The authors then employed 3’ single-cell RNA sequencing (scRNA-seq) to identify qualitative differences between male and female DCs. Flow cytometry analysis demonstrated that the overall composition of the skin DC network was comparable between males and females. However, the transcriptional profile of DCs from males and females differed in each subset analyzed, with increased expression of genes associated with activation and differentiation in females. To corroborate potential functional differences in skin DCs between sexes, the researchers monitored DC migration from the tissue to lymphoid organs [6, 7] and found that DCs from females had a greater ability to migrate to draining lymph nodes than did those from males, accompanied by higher levels of costimulatory molecules (CD40 and CD86) and a maturation marker (CD83). In addition, DCs from males exhibited a reduced capacity for MHC class I antigen presentation in comparison with those from females. Accordingly, females exhibited a denser and more activated DC network in the skin than males. The authors postulated that within the skin, DCs and T-cells are regulated by sex hormones and that male sex hormones exert a negatively impact both cell subsets.

To establish a causal relationship, the authors investigated whether the negative impact of androgen signaling on DC homeostasis was direct or indirect. They generated a mouse model in which Ar^fl/fl mice were crossed with Il7r^cre mice to eliminate the ability of lymphoid cells to respond to androgen signaling. This abolished the observed sex differences in DC numbers between Il7r^cre Ar^fl/fl female and Il7r^cre Arf^l/Y male mice, indicating that androgen-induced sex differences are dependent on androgen receptor (AR) signaling in lymphocytes.

To discover the potential targets(s) of AR signaling, they performed scRNA-seq profiling on skin lymphoid cells in males and females. These findings revealed differences in the ILC2 population, a dominant population of skin lymphoid cells, and associated gene expression, suggesting that ILC2s might be direct targets of androgens in the skin and sensors of skin DC homeostasis and function. To analyze the relationship between ILCs and DC homeostasis, the authors compared the DC networks of mice with an intact, wild-type (WT) immune system with those of mice without an adaptive immune system (Rag1^-/-) or those lacking both innate and adaptive lymphoid cells (Rag2^-/-γc^-/-). No differences were found in DC composition or number between the WT and Rag1^-/- mice, whereas the DC skin network was severely disrupted in the Rag2^-/-γc^-/- mice compared with the WT and Rag1^-/- mice. These observations suggest that the absence of ILCs profoundly impairs DC homeostasis and that DC sex bias is no longer observed, supporting the idea of a functional link between ILC2, androgen signaling and a specific skin subset of DCs.

They then performed adoptive transfer experiments to determine whether ILC2 was sufficient to control the skin DC network. The results demonstrated that the direct delivery of ILC2 into the skin of mice with a disrupted DC network was sufficient to restore the cDC1 compartment. In addition, they investigated the mechanism(s) by which ILC2 might control DC homeostasis, focusing on the potential of ILC2 to control DC subsets in a GM-CSF-dependent manner [8, 9]. The results showed that ILC2-derived GM-CSF was sufficient to promote the local accumulation of cDC1s in the skin.

Chi et al. [1] demonstrated that sex dimorphism is an essential variable in both homeostasis and pathology. However, significant gaps in our understanding of the precise mechanisms that mediate sex-specific immune responses and how this affects the outcome of human disease remain. Future objectives should answer the following questions: (i) Could the novel findings of Chi et al. [1] be applied to human settings involving sex dimorphism, the immune system and inflammatory response-associated diseases in a tissue/organ-dependent manner? (ii) Can the endocrine capacity be reprogrammed in other tissues/organs, such as the skin, to modulate immune system responses? (iii) What is the immune threshold in each tissue/organ and what is their degree of dependence/independence in relation to sex hormones? (iv) What is the impact of sex hormone-induced changes in the microbiota and the control of hormone levels by the microbiota, and what are the contributions of sex hormone-induced changes to differences in inflammatory disease intensity and tropism? (v) How is hormonal control of tissue immunity maintained in the chronic state based on the complex etiology of inflammatory disorders during aging? (vi) Is the DC network of the skin a potential therapeutic target for sex hormones as adjuvants or regulators of local immunity? (vii) How do local sex hormones influence tissue pathology in an environmental context depending on age/age, development and other environmental conditions? (viii) Could microbiota transplantation and healthy dietary habits be used as targets to influence the tissue immune system? (ix) What type of mechanism(s) and specific factors do tissues utilize to specifically regulate and modulate the availability and homeostasis of female and male sex hormones? (x) Could ILC2 levels be increased to boost immunity in pregnant females to protect the fetus? (xi) What specific factors produced by ILC2 influence the DC network to achieve DC homeostasis? (xii) Are there alternative mechanisms of DC network control other than ILC2 in skin and other tissues, and what is the degree of dependence on sex hormones and other hormones? (xiii) Should transgender individuals be assessed to better understand the effects of sex hormones (including artificial hormones) on the immune system through replacement therapies? (xiv) Should the microbiota and sexual immune dimorphism during life be investigated jointly to prevent age-related diseases and their tropisms in males, females, and transgender individuals? (xv) Could ambitious multidimensional and multifaceted research on sexual immune dimorphism and its variables change public health strategies and policies (i.e., diagnostics, prognostics, and vaccination campaigns) aimed at addressing autoimmune diseases, malignancies, infectious diseases and inflammaging-related disorders? Future studies should evaluate biological sex as a vital immunological player in experimental design, interpretation and reporting in both human and animal research, always including both sexes in all empirical contexts [10].

Overall, the findings from Chi et al. [1] show that male sex hormones in mice reduce the number of ILC2s, which limits the function of DCs by reducing the production of ILC2-GM-CSF, a common DC growth factor. A reduction in the number and antigen-presenting capacity of DCs in the skin of male mice causes less skin inflammation than that in female mice.

Competing interests

The author declares no competing interests.