Sexual Dimorphism in Immunometabolism and Autoimmunity: Impact on Personalized Medicine

Abstract

Immune cells play essential roles in metabolic homeostasis and thus, undergo analogous changes in normal physiology (e.g., puberty and pregnancy) and in various metabolic and immune diseases. An essential component of this close relationship between the two is sex differences. Many autoimmune diseases, such as systemic lupus erythematous and multiple sclerosis, feature strikingly increased prevalence in females, whereas in contrast, infectious diseases, such as Ebola and Middle East Respiratory Syndrome, affect more men than women. Therefore, there are fundamental aspects of metabolic homeostasis and immune functions that are regulated differently in males and females. This can be observed in sex hormone-immune interaction where androgens, such as testosterone, have shown immunosuppressive effects whilst estrogen is on the opposite side of the spectrum with immunoenhancing facilitation of mechanisms. In addition, the two sexes exhibit significant differences in metabolic regulation, with estrous cycles in females known to induce variability in traits and more pronounced metabolic disease phenotype exhibited by males. It is likely that these differences underlie both the development of metabolic and autoimmune diseases and the response to current treatment options. Sexual dimorphism in immunometabolism has emerged to become an area of intense research, aiming to uncover sex-biased effector molecules in the various metabolic tissues and immune cell types, identify sex-biased cell-type-specific functions of common effector molecules, and understand whether the sex differences in metabolic and immune functions influence each other during autoimmune pathogenesis. In this review, we will summarize recent findings that address these critical questions of sexual dimorphism in immunometabolism as well as their translational implications for the clinical management of autoimmune diseases.

1. Introduction

Sexual dimorphism, or biological differences between male and female (the sexes of a species), can be noted throughout countless developmental, pathological, and physiological processes which humans go through^1–3. Sex disparity in the manifestation of autoimmune disease represents one of the most remarkable and unexplained examples of the biological differences between men and women^3–6. According to the American Autoimmune Related Diseases Association (AARDA), there are 80–100 different autoimmune diseases ranging from the rare disorders such as Asheron’s Syndrome to common disorders such as type 1 diabetes. Notably, rheumatic diseases including systemic lupus erythematosus (SLE, female: male 9:1) and Sjögren’s syndrome (SS, female: male 20-9:1) are chronic systemic autoimmune disorders that predominantly affect women. Other common autoimmune diseases have moderately skewed ratios between the sexes, i.e., multiple sclerosis (MS, female: male 2-3:1). It is important to note that there are few known autoimmune diseases that are exceptions - these diseases processes are ankylosing spondylitis (AS, male: female 2-3:1), type 1 diabetes (male: female 3:2), and psoriasis (male: female 2:1)^{4, 7, 8}.

To better understand the sexually dimorphic basis of autoimmune etiology, sex as a biological variable has become, in the last decade, a standard of research design and analysis in vertebrate animal and human studies - backed by peer-review literature that the consideration of sex is critical to the interpretation, validation, and generalizability of research findings^{9, 10}.Though mechanisms have been put forward in order to elucidate sex bias in immune processes, its molecular underpinnings and their translation into disease phenotype have yet to fully come to fruition^{3, 11–13}.

One intriguing mechanism for sex-biased autoimmunity that emerged from recent study is sexual dimorphism in immunometabolism, which describes the changes in intracellular metabolic pathways in immune cells that alter their function. Fundamental metabolic pathways are essential for mammalian cells to produce energy, precursors for biosynthesis of macromolecules, and reducing power in redox regulations. There is a growing interest in the role of immunometabolism as a critical regulator of the fate and homeostatic function of immune cells. Changes in metabolic pathways within immune cells can be triggered by events of nutrient loss or anoxia, and by immune signals and regulation. Other than energy production and biosynthesis, distinct metabolic pathways can govern the phenotype and function of immune cell subtypes.

Systemic and cellular metabolism of specific immune cell populations highlight novel targets for immune-based therapies. Further understanding of sex differences in immunometabolic regulation will guide personalized medicine for immune-associated diseases. This review aims to highlight key discoveries and unanswered questions in sexual dimorphism in immunometabolism, paving the way for future studies that explore new prevention and treatment strategies for autoimmune diseases.

2. Sexual dimorphism in the immune system

Sex differences in autoimmune diseases can be incompletely elucidated by known differences in the immune system^{3, 21, 58–60}. The following sections will outline observed sexual dimorphism in the immune system and their molecular basis.

2.1. Sexual dimorphism in innate and adaptive immunity

Sex differences in humans are exhibited by both the innate and adaptive immune systems. During an innate immune response, Toll-like receptors (TLRs) are able to sense bacterial and viral components and provoke the stimulation of the cell in order to eliminate the infection^{14, 15}. In addition, TLRs are found to regulate development of dendritic cells (DCs) and initiate antigen-specific adaptive immune responses as they bridge the innate and adaptive immunity¹⁵. In the context of autoimmunity, TLR dysregulation is central to disease pathogenesis because when inappropriately activated by self-components, a sustained or exacerbated TLR stimulation can lead to an overproduction of proinflammatory mediators, resulting in sterile inflammation and autoimmunity¹⁵.

Importantly, TLR pathways exhibit sexual dimorphism. Souyris and collogues have shown sex-biased expression of genes from the TLR pathway, including increased expression of TLR7 in females compared to males in B cells and myeloid cells^{15, 16}. It has been shown that peripheral blood lymphocytes (PBLs) from women produce higher amounts of IFN-α after stimulation by TLR7 and TLR9 ligands^{16, 17}. Similarly, upon TLR7 stimulation, human female plasmacytoid dendritic cells (pDCs)¹⁸ produce higher amounts of IFN-α than their male counterpart, in addition to increased levels of IRF5 at basal state in females compared to males^{17, 19}.

While peripheral blood mononuclear cells (PBMCs) from men produce less IFN-α after TLR7 stimulation, upon TLR9 stimulation, they produce higher levels of the anti-inflammatory cytokine IL-10 than their female counterparts^20–22. Additionally, male neutrophils have higher levels of TLR4 and produce more TNF than female neutrophils both at basal state and after stimulation with LPS²³, a TLR4 ligand^{15, 24}. Consequently, the increased reactivity of male neutrophils to LPS and resultant increased secretion of proinflammatory cytokines justifies increased risk for septic shock in males²⁵.

In addition to innate immunity, the human adaptive immune system shows strong evidence of sexual dimorphism^{15, 26}. Varying differences are found with immune cell counts dependent on cell type. For examples, higher counts of the cluster of differentiation-4 (CD4⁺) T cells and increased CD4⁺/CD8⁺ ratio are found in females versus males^{15, 18, 27, 28}. Differences in cell function are also observed as CD4⁺ T cells in females produce higher levels of IFN-γ, and proliferate quicker than CD4⁺ T cells from men²². Male activated CD4⁺ T cells have a greater tendency for IL-17α production versus females^{22, 29}. Although B cell counts of the two sexes appear to be comparable, the concentration of serum immunoglobulins (Ig) differs between the sexes^{16, 30}.

In summary, the two sexes exhibit significant differences in both innate and adaptive immunity, which is thought to underlie the observed sex bias in autoimmune diseases (Figure 1).

Figure 1: Summary of sexually dimorphic factors which contribute to sex bias in immune-associated diseases.

Females and males differ in regulation of both innate and adaptive immunity, including female-biased TLR7 expression, type I - IFN activity, CD4⁺ T cell count (left) and male-biased IL-10 production and CD8⁺ T cell count (right). The sexual dimorphism of immunological factors is consistent with the sex bias observed in shown disease processes, where incidence rates, prevalence, susceptibility to and even prognosis of single diagnoses are different for male and females in most cases.

2.2. Causes of sexual dimorphism in immunity

Numerous factors could hypothetically contribute to sex differences in immune cell functions, but several have stood out in the last few years - sex hormones^{9, 10, 12}, sex chromosomes^{9, 10, 12}, epigenetics^{12, 31–33}, and environmental factors^{12, 32, 34}. It is highly important to note that established sex differences in immune cell function change with age and are altered during puberty and pregnancy and parturition^{18, 33, 35}. These changes are associated with lifespan milestones where hormone levels within the body are changed significantly and confirm that sex hormones as well as their regulators play a role in immune responses¹². Androgens, such as testosterone (T), have shown immunosuppressive effects whilst estrogen is on the opposite side of the spectrum with immunoenhancing facilitation of mechanisms (Table 1)^{18, 36–38}.

Table 1: Hormonal effects on immune processes^{11, 26–28, 42–50}.

The table summarizes the influence of estrogen, prolactin and testosterone on different cell types of the immune system.

Estrogen		Prolactin		Testosterone
Cell Type	Effect	Cell Type	Effect	Cell Type	Effect
B cell	Retarded B cell maturation Increased plasma cell and autoantibody producing cells Increased expression in CD22, SHP-1, and BCL-2	B cell	Increased induction of CD40 Decreased B cell receptor mediated activation threshold Increased IgM and IgG secretion Increased JAK2 expression via B cell autoreactivity. Increased STAT phosphorylation and upregulation Decreased B cell apoptosis related to increased BAFF production and BCL-2 expression.	B Cell	Increased B cells and decreased IgM and lymphopoiesis
DCs	Retarded DC maturation Altered regulation of cytokine and chemokine expression (IL-6, IL-10, IL-12, IL-23, CCL2, and TGFβ	DCs	Increased expression of CD80/86 via enhanced MHC-II Increased maturation of APCs	DCs	Decreased MHC-2 and CD86 Decreased proinflammatory cytokines and TLR-mediated activation Increased anti-inflammatory cytokines
Macrophage	Altered chemotaxis and phagocytic activity Increased induction of IL-6 and TNFα	Macrophage	Increased in TNFα, IFNγ, IL-1β, and IL-12 Increased secretion of MCP Controversial increase of IL-10 contingent upon concentration	Macrophage	Decreased in TNFα, TLR4, as well as eosinophil mediated chemokines Increase in M2 and decreased MCP-1
Neutrophils	Increased induction of TNFα, IL-1β, and IL-6	Granulocytes	Increased regulation of IRF-1 and iNOS Increased activation of MAPK pathways via STAT1	Neutrophils	Increase in granulopoiesis and IL-10 and TGFβ concentrations Decrease in ROS and proinflammatory cytokines and chemokines
Th1	Increased IFNγ expression Increase in Th1 bias	NKCs	Increased secretion of IFNγ Increased proliferation and cytotoxic activity	Th1	Decrease in Th1 bias
Th2	Decrease in Th2 bias	T cell	Increased adhesion of ECs by LFA-1 and VLA-4	Th2	Increase in Th2 bias
Treg	Increase in regulation of FOXP3 and CTLA-4			T cell	Increased apoptosis and decreased proliferation
				Mast cell	Increased IL-6 production

Testosterone influences the immune system by altering T-helper 1 (T_h1) response and the action of CD8⁺ cells whilst down-regulating natural killer (NK) cell response and production of TNFα²². Furthermore, testosterone is found to increase the production of anti-inflammatory cytokines such as IL-10²⁰. Consistently, the presence of testosterone leads to higher production of T_h1 by peripheral blood cells, signified by a higher T_h1:T_h2 ratio in men³⁹. Further sexual dimorphic behavior was shown in immune cell subtypes in a humanized mouse model (DRAG mouse - HLA-DRA,HLA-DRB1*0401⁴⁰) of inflammation where exogenous supplementation of estradiol (E2) in castrated male mice led to an surge in autoimmunity by amplifying Major Histocompatibility Complex II (MHC2) expression and moderating B cell function (Table 1)³⁹. The regulation of immune response of estrogen can be seen by the impairment of B cells and skewing of T_h1 response^{1, 39} and has been confirmed in rheumatoid arthritis (RA) mouse model (DRAG mouse- HLA-DR4/DQ8⁴¹). A summary of the effect of sex hormones on immune cells can be found in Table 1.

The X chromosome encodes the largest number of immune related genes⁹, and a large portion of these genes escape from X chromosome inactivation leading to female-biased expression¹². The human males produce two types of sex chromosomes, X and Y. Hence, the gametes produced by them are also of two types- one bearing X chromosome and the other bearing the Y chromosome. Thus, human males are said to be heterogametic, and deleterious recessive alleles in X-linked genes (i.e., TLR7, FOXP3, CD4⁺, and IRAK1) are more likely to cause immune phenotypes in males than in females^{9, 10}. TLR7 and IRAK1 proteins play critical roles in pathogen recognition and induction of a proinflammatory immune response, ensuing in type I IFN production and induction of the IFN inducible genes^{15, 16}. The TLR7 gene escapes X inactivation, leading to gene dosage effects⁹ that may be relevant for the recognition of both viral and self-RNA-related antigens during autoimmune pathogenesis^51–53. Additionally, the X chromosome contains a large amount of microRNAs associated with the immune system, further contributing to sex differences in metabolic and immune function¹². Sex differences in immune response are suggestive that sex-specific treatments would be efficacious for clinical and acute care treatment within these population groups.

3. Metabolic regulation of the immune system and its sexual dimorphism

Immune and metabolic functions closely regulate each other at a systemic level, which suggests that crosstalk, as well as, cross-inhibition plays a role in the regulation of their sexually dimorphic functions¹². Therefore, immunometabolism, the study of the multilayered interactions between immune and metabolic systems, has emerged as an exciting and important area of scientific investigation. It is expected that a better understanding of sex differences in immunometabolic regulation will help guide personalized, sex-specific treatment of autoimmune diseases.

3.1. Concept of immunometabolism

The immune system encompasses a heterogeneous populace of cells that are relatively quiescent in the steady state but share the ability to rapidly respond to infection and inflammation⁸. The ability to rapidly and effectively mount an inflammatory reaction requires considerable energy expense and is accompanied by metabolic changes. Metabolism consists of exceedingly interconnected, and complicated biochemical pathways within the human body^{11, 45, 54}. The major metabolic pathways are: glycolysis, where glucose is oxidized in order to generate ATP, albeit in a relatively inefficient manner; citric acid cycle (CCA) cycle, a nexus for multiple nutrients inputs that is used for efficient ATP generation; the pentose phosphate pathway (PPP), allowing diversion of intermediates from glycolysis towards the production of nucleotide and amino acid precursors; fatty acid oxidation, allowing the conversion of fatty acids into downstream products for energy generation; fatty acid synthesis, generating lipids for cellular growth and proliferation; amino acid metabolic pathways, using amino acids for protein synthesis and signaling regulation²⁸.

Cells use intricate mechanisms to sense levels of metabolites produced by these metabolic pathways and activate signaling pathways accordingly to maintain metabolic homeostasis. Of these mechanisms, one central metabolic regulator of immunity is the mechanistic target of rapamycin (mTOR) - AMP kinase (AMPK) pathway^{28, 51, 55, 56}. mTOR is the catalytic subunit of mTOR complex (mTORC-) 1 and 2 which sense amino acids and growth factors and promote mRNA translation⁵⁷. Additionally, mTORC1/2 signaling contributes to lipid synthesis and cell growth⁵⁷. Intriguingly, in the immune system, mTOR signaling facilitates events critical for T cell and monocyte differentiation, suggesting immunometabolic crosstalk. Nutrient deprivation signals to AMP kinase, which promotes catabolism of free fatty acids (FFA) and inhibits mTOR activity, thus limiting immune cell activation⁵⁷.

mTOR function is regulated by the protein kinase B (PKB/Akt), which is known to play a critical role in cell growth, metabolism, proliferation, and survival. PKB/Akt activation is controlled by a complex stepwise progression that involves phosphoinositide-3-kinase (PI3K)^58–60. Stimulated receptors incite class 1A PI3Ks that triggers the activation of PI3K and conversion by its catalytic domain of phosphatidylinositol (3,4)-bisphosphate (PIP2) lipids to phosphatidylinositol (3,4,5)-trisphosphate (PIP3). Subsequently, PKB/Akt binds to PIP3, permitting PDK1 to access and phosphorylate T308 in the activation loop, leading to partial activation of PKB/Akt⁵⁸. Successively this activate mTOR-complex 1 (mTORC1) by phosphorylating and inhibiting tuberous sclerosis protein 2 (TSC2)⁵⁹.

mTORC1 substrates are found to further phosphorylate ribosomal protein-S6 (RPS6), promoting protein synthesis and cellular proliferation⁵⁸. Depletion of energy leads to inactivation of mTORC1, activation of AMPK, forkhead box transcription family-O (FOXO), and promotes constitution of mTORC2 that leads to phosphorylation of Akt^58–60. Akt can also be activated without PI3K; which appears to be advantageous in situations like nutrition deprivation, where insulin/insulin growth factor signaling is not optimal⁵⁹. An applied example of this can be seen when CD3/CD28 ligation activates CD4⁺ T cells, leading to signaling through PI3K/Akt/mTOR. PI3K/Akt/mTOR signaling subsequently leads to activation of glycolysis and mitochondrial oxidative phosphorylation (OXPHOS), resulting in CD4⁺ activation^{7–8, 19–23,60} (Table 2).

Table 2: Immunometabolic pathways in immune cells.

Components of the inflammatory response where ‘inducers, sensors, mediators, effectors, and outcomes’ are associated with specific metabolic processes. Herein, inducers of inflammation activate ‘mediator’ signaling, resulting in modulation of ‘effector’ metabolic pathways and leading to cellular outcomes such as activation, proliferation, and cytokine production.

Cell Type	Inducers	Mediators	Effectors	Outcome	Reference
Activated CD4+ T Cell	CD3/CD28	PI3K/Akt/mTOR ERK/MAPKc-MycHIF-1α	Glycolysis, Mitochondrial OXPHOS	Activation, Proliferation, Cytokine production	7–8, 19–23
Activated Dendritic Cell	PAMPs	PI3K/AktHIF-1α	Glycolysis	Presentation, Cytokine production	7
B Cell	PAMPs	PI3K/Akt	Glycolysis	Activation, Proliferation	7, 20
Memory CD8+ T Cell	IL-15	AMPK	FAO	Survival, Quiescence	7, 24–28
Naive CD4+ T Cell	IL-7	PI3K/Akt	Mitochondrial OXPHOS, FAO	Survival	7–8, 22–23, 28–30
Neutrophil	PAMPs,	HIF-1α	Glycolysis	ROS	7
Resting Dendritic Cell	Growth factors (GM-CSF, FLT3)	-	FAO	Growth, Survival Activation	7, 31–32

In addition to CD4⁺ T cells, PI3K/Akt/mTOR/AMPK regulates immunometabolic functions in a variety of immune cells. A summary of immunometabolic pathways regulating immune cell function can be found in Table 2.

The direct regulation of immune processes by metabolism can further be observed within various immune cell types where they switch between distinctive metabolic pathways to respond to changes in a dynamic immune response⁵⁷. For example, an inflammatory M1 macrophage uses the glycolysis pathway to support phagocytosis and inflammatory cytokine production, and utilizes the pentose phosphate pathway to support nucleotide and ROS production⁵⁷. Another depiction of this specific pathway reliance can be observed in regulatory T cells when utilizing the CCA pathway instead of FFA oxidation because an suppressive function is needed versus the generation of T_reg cells in response to tolerogenic stimuli⁵⁷.

3.2. Sexual dimorphism in immunometabolism

While historically metabolism has been studied with the assumption that basic cellular machineries operate in the same way in males and females, it has been recently accepted that the two sexes exhibit significant differences in metabolic regulation. Estrous cycles in females are known to induce variability in traits, and males can exhibit more pronounced metabolic disease phenotype than females⁵⁷.

Similarly, sex-biased regulation of immunometabolism is supported by the finding that the sex steroid regulator sex hormone-binding globulin (SHBG) regulates the tissue availability of sex steroids and influences E2 signaling in lymphocytes, which possibly underlies the female bias in multiple sclerosis⁶¹. At the intersection of immune and metabolic functions, SHBG also contributes to pathogenesis of metabolic diseases such as obesity and metabolic syndrome⁶².

In addition, in research of the immunometabolic alterations in diabetes, it was found that sex hormones regulate visceral adipose tissue mesenchymal stromal cells and their production of IL-33, which could account for differences in regulatory T cells (Tregs) in basal or obese state between males and females^{63, 64}.

With the last decades seeing a growing interest in immunometabolism research, the recent recognition of sex differences being a fundamental feature of immunometabolism calls for attention from the scientific community. A better understanding of sexual dimorphism in immunometabolism will provide scientific basis to develop sex-based precision medicine for immune and metabolic diseases.

4. Metabolic alterations in autoimmune disease and immunometabolism as a fundamental mechanism for sexual dimorphism in autoimmunity

With mounting evidence supporting the metabolic regulation of immune functions, it is not surprising that metabolic alterations in autoimmune disease have been documented^{2, 7, 65–67}. The findings highlighting major metabolic alterations in autoimmunity are summarized below.

4.1. Systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disease that is characterized by chronic inflammation^{2, 8, 68}; often illustrated by the involvement of multiple organs and clinical displays of nephritis, vasculitis and pathogenic autoantibodies such as anti-double stranded DNA (dsDNA)^{69, 70}. In addition to altered function of immune cells^{1, 11} including CD4⁺ T Cells^{26, 71, 72}, dendritic cells (DC)⁷³, macrophages^{8, 73–78}, and neutrophils⁷⁶, metabolic systems play an integral role in checkpoints that control immune cell fate and function^{28, 54, 79, 80}. Therefore, it is crucial to examine the relationship between mitochondrial dysfunction, oxidative stress, and abnormal metabolism that involves glucose, lipid and amino acid metabolism of immune cells to understand the underlying pathogenic mechanisms of SLE^{13, 52, 81}. Notably, metabolite intermediates that are produced within mitochondria have been found to serve as inflammatory signals (e.g., succinate in myeloid cells)^{21, 52}. A sentential breakthrough by Frauwirth and colleagues⁸² highlighted the activation of CD28 by glycolysis in T cells, leading to a large push in researchers looking to elucidate the regulation of T cells by metabolic substrates^{11, 45, 54, 71, 83}. It is now understood that resting T cells are influenced by mitochondrial oxidative phosphorylation (OXPHOS) and that antigen-mediated stimulation and acquisition of effector functions elicit a striking metabolic reprogramming, shown an upregulation of glucose use followed by the activation of mitochondria-independent glycolysis as the major source of building blocks necessary to cope with considerable proliferation as well as production of effector molecules^{13, 29, 83}.

Glucose is a fundamental energy source for most cells and aids cellular proliferation, development and survival^{28, 72}. It is known that activated T cells enhance glucose metabolism in order to meet requirements of cellular proliferation and differentiation. Subsequently, glucose deficiency leads to decreased levels of ATP and AMP-activated protein kinase (AMPK) activation⁸⁴, which in the normal setting has a positive homeostatic effect on signaling pathways that compensate for cellular ATP. This can be shown in the activation of AMPK promoting GLUT4 transcription and translocation to promote glucose intake⁷². Conversely, AMPK negatively modulates key proteins in ATP-consuming reactions such as mTORC2^{28, 51, 56}, glycogen synthase, sterol regulatory element binding protein 1 (SREBP-1) and tuberous sclerosis 2 (TSC2), leading to inhibition of gluconeogenesis as well as glycogen, lipid, and protein synthesis⁷². It is important to note that GLUT1 overexpression in CD4⁺ T cells has an influence on Treg cell expansion, which has led to the concept that there is a difference in glucose metabolism for regulatory and effector T cells. GLUT1 is induced by HIF1α, which ultimately aids in Th17 differentiation^{22, 72, 85}. Additionally, the increase in GLUT1 expression and glucose uptake occurs in a PI3K/Akt-dependent manner, allowing cells to maintain their mitochondrial potential and ATP homeostasis⁷². Correspondingly, in the absence of sufficient extrinsic signals, cell surface GLUT1 expression decreases, resulting in diminished glucose uptake, drop in mitochondrial membrane potential and ATP synthesis, and cell death⁷². Since this decline in viability occurs in the presence of appropriate glucose and oxygen, it suggests that growth factor signaling is indispensable for maintenance of metabolic homeostasis in naïve CD4⁺ T cells^{52, 72, 83}.

Yet, the inhibition of AMPK and the downstream mTORC1 activation by Roquin-1 promotes a lupus-prone phenotype⁷¹. Roquin-1 blocks AMPK activation, allowing the function of mTORC1 and mTORC2, which are known to impact T helper follicular (Tfh) cell differentiation⁸⁶. In addition, recent studies have found that retention of activated mTORC1 during asymmetric cell division in CD8⁺ T cells presents the daughter cell with effector functions, whereas the mTORC1-low daughter cell acquires memory properties. It is likely that a similar asymmetric distribution of mTORC1 exists between effector and memory CD4⁺ T cells⁵⁵. Dysregulation of Tfh, CD8⁺ and CD4⁺ T cell differentiation altogether may underlie the lupus-prone phenotype induced by Roquin-1.

Similarly, mTORC1 activation was observed in CD4⁺ T cells from several strains of lupus-prone mice. Interestingly, treatment of these mice with 2-Deoxy-D-glucose (2-DG) and metformin normalized mTORC1 activation concomitant with disease reversal⁵².

Consistent with mouse model studies, mTORC1 activation has been demonstrated in CD4⁺ T cells of SLE patients and has been proposed to serve as a biomarker of autoimmune inflammation^{56, 72}. Treatment with rapamycin, which inhibits mTOR and enhances Treg suppressive function, is effective in SLE patients and in lupus-prone New Zealand mixed (NZW/NZW F1) mice^{28, 51, 56, 72, 87}. Therefore, targeting of mTORC, the critical player integrating environmental cues, nutrient levels and immune response output, is promising in treatment of SLE (Figure 2).

Figure 2: Potential metabolic pathways of intervention in autoimmune diseases.

Schematic representing important metabolic targets in SLE treatment in efforts to correct the immunometabolic alterations, as AMPK and mTORC are mechanistically critical for SLE pathogenesis (green). In a similar fashion, pyruvate metabolism, OXPHOS, and CCA are highlighted as targets for MS (blue). Low energy level is associated with both SS (pink) and SLE which can be linked to the antiapoptotic effects of adiponectin mediated by phosphorylation of AMPK (blue box).

Although most research has been conducted on the influence of glucose metabolism in SLE T cells^{11, 28, 72, 81}, glucose is also important to other immune cell types. It has been shown that B cells in the lupus-prone NZW/NZW F1 mice exhibit a highly glycolytic phenotype⁵². However, their mechanistic actions remain unsettled⁷².

4.2. Sjögren’s syndrome

Sjögren’s syndrome (SS) is a systemic autoimmune disease that is characterized by infiltration of lymphocytes into the exocrine glands, inflammation, tissue damage, and dysfunctional glandular secretion^{84, 88, 89}. Destruction of the lacrimal and salivary glands, which typically occurs in patients with SS, results in ocular dryness (keratoconjunctivitis sicca) and oral dryness (xerostomia)⁸⁴. Patients with SS often have extra-glandular complications such as non-erosive polyarthritis, arthralgias, vasculitis, and chronic fatigue⁸⁴. Furthermore, patients with SS have an increased incidence of progression to various non-Hodgkin lymphomas, which may influence the rate of morbidities^{84, 89}. The pathogenesis of SS is mediated by complex mechanisms involving infiltration by lymphocytes (mainly T and B cells) of target organs during a dysregulated adaptive immune response^{84, 88}. In the T- and B-cell-containing ectopic lymphoid structures in the salivary and lacrimal glands, hyperactivated B cells produce autoantibodies, e.g., anti-SSA/Ro and -SSB/La, against small RNA molecules and rheumatoid factors^{84, 88, 89}. Activation of B cells by follicular helper T (Tfh) cells is crucial for the clonal selection and affinity maturation^{71, 86}.

Since metabolic aberrations of immune cells drive immune regulation in mammals, it was postulated that the “immune” phenotype of salivary gland epithelial cell (SGEC) undergoes similar control by their metabolism and may actively shape the autoimmune response in SS^{30, 84}. SGEC are secretory cells with constant high energy demands and their metabolic machinery is expected to suit their lifestyle⁸⁴. Disturbances of this process may be enforced by insufficient energy supply, endoplasmic reticulum (ER) stress or even chronic stress, leading to metabolic reprogramming and eventually immunogenic cell death characterized by the release of cellular autoantigens⁸⁴. Differential adiponectin production by SGEC in SS indicates a low energy phenotype and the antiapoptotic effects of adiponectin mediated by phosphorylation of AMPK provide a robust paradigm for the interconnection between metabolism and immune functions of SGEC in the context of glandular lesion in SS⁸⁴ (Figure 2).

4.3. Multiple sclerosis

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the central nervous system (CNS)⁴³. The incursion of the brain by activated immune cells across the endothelial cells (ECs) of the blood brain barrier is due to the loss of immune self-tolerance⁴³. MS is characterized by inflammation, demyelination, nonspecific reactive changes of glial cells, and neuronal loss. From a pathological perspective, the presence of perivascular lymphocytic infiltrates are indicative of the disease process, with consequent macrophage degradation of myelin sheaths that surround neurons⁴³. The MS predominance ratio of female to male has increased within the last few decades from 2.3–3 – 5:1. This is suggestive that the presence of hormone receptors associated with immune cells and sex hormones (androgens, estrogens, progesterone, and prolactin) have a great influence on immune system function and disease progression^{38, 43, 47, 49, 87, 90}.

Biochemical studies regarding MS have established the notion of defective pyruvate metabolism, in addition to increased sera concentrations of citric acid cycle (CCA) acid such as α-ketoglutarate (AKG)⁹¹ and citrate (Figure 2). AKG is one of the most important nitrogen transporters in metabolic pathways, produced by oxidative decarboxylation via isocitrate dehydrogenase as well as oxidative deamination of glutamate via glutamate dehydrogenase⁹¹. MS patients are found to have elevated serum and cerebrospinal fluid pyruvate levels, as well as antibodies that were reactive with triose phosphate isomerase and GAPDH and inhibit glycolytic activity of GAPDH^{42, 47}. In regard to altered OXPHOS, there is a striking reduction of ATP synthase and increased activation of mitochondrial electron transport chain⁴⁷. Therefore, correcting deficiencies in pyruvate metabolism is critical to managing MS clinically (Figure 2).

Reiterated from the glucose metabolism of SLE above, the metabolism of glucose in the setting of MS is the same. The foundation of glucose metabolism starts with glucose entering cells via GLUT transporters and being phosphorylated by hexokinase⁴⁹. The product glucose 6-phosphate can be metabolized via glycolysis - producing pyruvate, ATP, and NADH, where pyruvate enters the mitochondria and is metabolized via the CCA cycle and OXPHOS^{13, 29, 83, 92}. It is important to mention that pyruvate can also be reduced to lactate-by-lactate dehydrogenase and released into extracellular space via monocarboxylate transporters (MCTs). Additionally, glucose 6-phosphate can be taken through PPP or converted to glycogen via glycogenesis in astrocytes in the brain²⁸. However, unlike the role of insulted glucose metabolism in SLE, the role of glucose metabolism in MS is still incompletely understood.

4.4. Immunometabolism as a fundamental mechanism for sexual dimorphism in autoimmunity

It has been discussed that maintaining metabolic homeostasis is critical in the prevention of autoimmunity. A bulk of metabolism, including energy balance - glucose and lipid metabolism, are regulated in a sexually dimorphic manner and successively influence the pathogenesis of autoimmune disease. However, the fundamental question of why sex differences in autoimmunity exist remains unanswered. To address this question, researchers such as Pagenkopf and colleagues^{3, 6} have focused on immunometabolic functions of transcriptional cofactors that provide an evolutionary validation for sexual dimorphism in autoimmunity. An example of this can be highlighted within the female-biased gene network that has been described in human skin that is associated significantly with the susceptibility to female-biased autoimmunity. An upstream regulator of this gene network, vestigial family member-3 (VGLL3), exhibits female-biased expression in healthy human skin and is further upregulated in autoimmune diseases including SLE, SS and systemic sclerosis⁶. In secondary studies from this group, their results demonstrated that energy deficiency is a critical trigger that upregulates VGLL3 and that female-biased expression of VGLL3 helps cells adapt to metabolic stress. Intriguingly, when placental mammals evolved, the need to feed a developing embryo posed significant challenge to metabolic pathways⁹³. Therefore, the finding that VGLL3 helps non-placental tissue such as the skin adapt to energy stress provides an evolutionary rational for the selection of its increased expression in females. This study further identifies nutritional deficiency as a trigger that can turn this evolutionary strength into weakness by causing autoimmune pathogenesis, and highlights the importance of maintaining metabolic homeostasis in prevention of autoimmunity³.

5. Translational Implications

It can be inferred that immunometabolism in regard to autoimmune disease since its inception has primarily focused on glycolysis, the CCA cycle, OXPHOS, and free fatty acids (FFA) synthesis and oxidation. This is based on the findings of pathways associated with the energy needs of cell growth, membrane rigidity, cytokine production and proliferation. Seemingly translational immunometabolism is suggestive of a repositioning of metabolic drugs that exploit new targets.

Novel drugs which modulate metabolic processes have the potential to correct the aberrant immune responses and be used to treat autoimmune disease patients (Figure 2). Looking at SLE specifically, strategies targeting mTOR activation, including use of rapamycin, could be promising ways to diminish the disease severity in SLE patient populations^{28, 51, 55, 56}.

Additionally, tuning of FFA pathways, including that seen in glucocorticoid (prednisone) treatment, has been directly linked to leptin reduction through inhibition of mTOR in SLE patient populations^{51, 56, 68}. Also, the complex interaction among mitochondrial, and mTOR signaling pathways and their ability to control the chemotaxis of neutrophils suggest metabolic options to restore normal neutrophil functions in SLE⁵¹. Based on the finding that macrophage polarization follow distinct metabolic pathways, the translation of metabolic shifts to disease has gained importance, especially for diseases that clearly lean toward either phenotype (M1 vs M2)⁵¹. Notably, the influence of macrophage polarization has met with relative success clinically for ovarian carcinoma, showing that therapeutically targeting macrophage metabolism might be a viable option in the future for SLE and MS⁵¹.

Symptomatically, fatigue and low energy states are commonly reported amongst patients hindering with SLE and MS. Defining fatigue can be tricky, where varying definitions can be grouped according to type (i.e., subjective, physiological, and/or performance). Herein this review, we define fatigue as insufficient cellular capacity or system-wide energy to maintain the original level of activity and/or processing by using normal resources. Furthermore, physiological processes have been described to play a role in fatigue that include oxygen/nutrient supply and metabolism - which are exaggerated by inflammation. Effects contributing to fatigue are associated with enhanced inflammation and increased cytokine expression amongst others⁹⁴. In addition, with nutritional deficiency as an autoimmune trigger, it is reasonable to assume that nutritional monitoring strategies can be employed to develop in order to prevent and/or treat autoimmune disorders.

This is not without considering the impact the biological sex has on personal immunity. Clear differences in male and female immunity contribute to variations in disease predisposition, severity, and drug responses (Figure 1). Additional co-factors that influence sex hormones, such as environment stress and toxin exposure, could also impact immunometabolic responses and autoimmune pathogenesis in a dynamic manner (i.e., change with age and events in life). One prominent example of sex-specific drug response is the impact of gender on immune checkpoint inhibitors-induced autoimmunity⁸⁰. Using the example of MS, Golden and colleagues⁹⁵ were able to elucidate the efficacious clinical benefit of discussing sex differences with treatment options of patients. Taking these observations to the laboratory bench allowed researchers to describe the mechanisms underlying sex differences, and to investigate therapeutics based on findings. Continually examining sex differences in the same “bedside to bench to bedside” fashion will bare endless novel therapeutics and treatment strategies following the identification of sex-specific disease drivers. In addition, sexual dimorphic studies will allow us to design sex-stratified treatment strategies that maximize efficiency and minimize side effects in both male and female patients.

6. Conclusions

There is a growing amount of academic literature on immunometabolism that provides novel insights into autoimmune pathogenesis. When adding in additional contributory factors such as sex and its biological implications the complexity of the topic grows. Notably, the biological sex effects the production, maturation, differentiation, metabolism and ultimately the functioning of cells, in both physiology and pathology of the immune system. Taken together the topics covered can shed light on how sex-specific metabolic reprogramming therapeutic can be implored to enhance outcomes in autoimmune diseases.

Highlights.

• Susceptibility and progression of autoimmune diseases exhibit sex differences, which necessitates sex-specific prevention and treatment strategies

• Fundamental aspects of immune functions and metabolic homeostasis are regulated differently in males and females, underlying sex differences in autoimmune diseases

• Recent discoveries in the area of immunometabolism, the regulation of immune responses by metabolic processes, represent exciting opportunities to combat autoimmune diseases

• Based on the metabolic regulation of immune cell functions, sex-specific metabolic reprogramming therapeutics can be implored as novel approaches to enhance outcomes in autoimmune diseases in a personalized manner

Abbreviations

AAM

Alternatively activated macrophage

5-HT

5-Hydroxytryptamine (Serotonin)

AARDA

American Autoimmune Related Diseases Association, Inc

AKG

α-ketoglutarate

AMPK

AMP-activated protein kinase

APC

Antigen-presenting cell

AS

Ankylosing spondylitis

ATP

Adenosine triphosphate

BCR

B cell receptor

CAM

Classically activated macrophage

CCA

Citric acid cycle / Krebs cycle

CD

Cluster of differentiation

CNS

Central nervous system

COX5b

Cytochrome c oxidase subunit 5b

cTfh

Circulating-Tfh

dsDNA

Anti-double-stranded DNA

E2

Estradiol

EC

Endothelial cell

ERK

Extracellular signal-regulated kinase

FADH₂

Fuel oxidative phosphorylation

FAO

Fatty-acid oxidation

FFA

Free fatty acid

FLT3

Fms-related tyrosine kinase 3 ligand

FOXO

Forkhead box transcription factors-O

FOXP

Forkhead box protein-P

GH

Growth hormone

GM-CSF

Granulocyte-macrophage colony stimulating factor

HIF-1α

Hypoxia-inducible factor 1-alpha

HIV

Human immunodeficiency virus

HLA

Human leukocyte antigen

Ig-

Immunoglobulin

IL

Interleukin

ILC

Innate lymphoid cells

iNOS

Inducible nitric oxide synthase

IRAK-1

Interleukin-1 receptor associated kinase

IRF-1

Interferon regulatory factor 1

IRS

Insulin receptor substrate

LFA-1

Lymphocyte function-associated antigen 1

MAPK

Mitogen-activated protein kinase

MCL-2

Macrophage C-type lectin 2

MCP

Monocyte chemoattractant protein

MCP-1

Monocyte chemoattractant protein 1

MCT

Monocarboxylate transporters

MHC

Major Histocompatibility Complex (1/2)

MS

Multiple sclerosis

mTOR

Mechanistic target of rapamycin

mTORc

Mechanistic target of rapamycin complex

NADH

Nicotinamide adenine dinucleotide

NK

Natural killer

NO

Nitric oxide

NOD

Nucleotide oligomerization domain

OXPHOS

Oxidative phosphorylation

P4

Progesterone

PAMP(s)

Pathogen-associated molecular pattern

PBL

Peripheral blood lymphocytes

PDK-1

Phosphoinositide-dependent kinase 1

PGC1β

Peroxisome proliferator-activated receptor beta

PI3K

Phosphoinositide 3-kinase

PIP2

Phosphatidylinositol (3,4)-bisphosphate

PIP3

Phosphatidylinositol (3,4,5)-trisphosphate

PKB/Akt

Protein kinase B

PPP

Pentose phosphate pathway

PRR(s)

Pattern recognition receptors

PVM

Perivascular macrophage

RA

Rheumatoid arthritis

ROS

Reactive oxygen species

RPS6

Ribosomal protein-S6

SGEC

Salivary gland epithelial cell

SHBG

Sex hormone-binding globulin

SLE

Systemic lupus erythematosus

SREBP-1

Sterol regulatory element binding protein 1

SS

Sjögren’s syndrome

STAT1

Signal transducer and activation of transcription factor 1

T

Testosterone

TB

Tuberculosis

TCA

Tricarboxylic acid

TCR

T cell receptor

Tfh

T-follicular helper cells

Th

T-helper (1/2)

TLR

Toll-like receptors

TSC2

Tuberous sclerosis 2

VGLL3

Vestigial family member 3

VLA-4

Very late antigen 4