The importance of ovary-immune interactions in the context of age and disease

Another rich source of information on the interaction of immune modulation and hormone levels comes from studies of altered ovarian function. PCOS is an ovarian condition affecting premenopausal, reproductive-age women. The clinical presentation is multi-faceted but typically consists of obesity, amenorrhea or oligomenorrhea, insulin resistance, hirsutism, depression, and anxiety. PCOS can also present as infertility in patients who are attempting to get pregnant. Signature findings of PCOS include ovaries with multiple cysts and having high levels of androgens in circulation. Additionally, since patients with PCOS may exhibit increased adiposity, and adipose tissue displays immune activity and function, systemic inflammation has been studied in the context of PCOS. Increased levels of inflammatory cytokines, immune cells, and inflammatory markers are associated with PCOS (Xiao, 2019; Orio, 2005), specifically, an imbalance of pro- and anti-inflammatory cytokines (reviewed in Zhai and Pang, 2022 ). Levels of circulating Treg cells were decreased and Th17 cells were increased in patients with PCOS (Yang et al., 2021). Interestingly, ovarian hormone levels were not correlated with elevated systemic levels of leukocytes or C-reactive protein in these patients (Orio, 2005), however, CRP and homocysteine were negatively correlated with leukocyte telomere length in patients with PCOS (Pedroso, 2015). Hyperandrogenism has been associated with inflammation and pyroptosis in a mouse model of PCOS, and is mediated by ER stress, such that tauroursodeoxycholic acid treatment (attenuate ER stress) reduced inflammatory factors and pyroptosis (Xiang, 2023). Insulin resistance, hyperandrogenism, and obesity ( Conway et al., 1992 ) place PCOS patients at increased risk of cardiovascular disease and hypertension, (Wu, 2020) underscoring the broad systemic effects of ovarian malfunctioning. Similar to ovarian dysfunction seen in PCOS, ovarian hyperactivity also has systemic effects. In premenopausal women of reproductive age, ovarian stimulation for fertility treatments can result in a rare condition known as ovarian hyperstimulation syndrome (OHSS). In brief, the protocol for ovarian induction typically includes the administration of gonadotropins followed by a final administration of human choriogonadotropin (hCG) for the final stages of maturation of oocytes (Gallos, 2017). This surge of hormones results in the activation of ovarian processes but can, in rare cases, result in hyperactivation. While the mechanisms leading to the development of OHSS are not well defined, the primary inciting event may be increased permeability in vasculature. In a rat model of hyperstimulation, elevated levels of VEGFA mRNA were induced by hCG ( Gómez and l., Simón, C., Remohí, J. & Pellicer, A. , 2002 ). Human endothelial cells cultured from umbilical veins, exposed to fertility hormones showed increased capillary permeability ( Villasante et al., 2007 ), specifically, through increased expression of VEGFA. IL-6, a cytokine known to be elevated with angiogenic activity, is elevated in abdominal and follicular fluid of OHSS patients (Geva, 1997; Farkas, 2020). Furthermore, OHSS has been linked to an increased risk of vascular complications such as cerebral venous thrombosis and ischemic stroke, among others ( Sachar and Rajamani, 2016 ). In view of the low prevalence of OHSS, it is unknown whether the origin of this disease process is a pre-existing susceptibility or if it is due to an aberrant ovarian process. However, it does provide another model where reproductive hormones and their modulation of immune activity, might provide insight on vascular dysfunction and permeability and their implications for cardiovascular diseases. POI is a condition in which ovarian function declines before the age of 40, and is often accompanied by alterations in immune system activation (Liu, 2020) and dysfunction ( Hoek et al., 1993 ). Bioinformatic analysis associates differential expressions of seven immune-related genes ( Cd200 ; Cd274 ; Cd28 , Nrp1 , Cd276 , Cd40lg , and Cd47 ) with the pathogenesis of POI, indicating that it is likely an immune-driven pathology (Zhang, 2024). Pro-inflammatory cytokines, such as MIP-1α and IP-10, are elevated in blood of POI patients (Liu, 2020), while functional monocytes and DCs are less abundant ( Hoek et al., 1993 ). Likewise, elevated levels of infiltrating monocytes and macrophages are seen in follicular fluid (Liu, 2023). In clinical studies, patients with POI have reduced Tregs and increased activated T cells in circulation (Kobayashi, 2019). Human placenta-derived mesenchymal stem cell transplantation into a mouse model of POI renews ovarian function concomitantly with increased levels of Tregs and reduced levels of pro-inflammatory cytokines (Yin, 2018). A systematic review and meta -analysis reported that the POI patient population had a significantly higher risk of cardiovascular events and coronary heart disease as compared to postmenopausal women ( Behboudi-Gandevani et al., 2023 ). Ovarian-immune interactions are also exhibited in systemic conditions associated with immune dysfunction such as rheumatologic diseases. Conditions such as systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), spondyloarthritis, MS, and Behcet’s disease exhibit reduced anti-Mullerian hormone (AMH). AMH can be used as a measure of ovarian reserve, an estimate of early onset of menopause, suggesting a potential bidirectional impact of the ovaries and the immune system (Henes, 2015; Sepúlveda, 2016). Moreover, antral follicle count was also reduced in patients with SLE (Henes, 2015; Morales-Martínez, 2021; Lawrenz, 2011). However, some conflicting studies have shown that there is no change in ovarian reserve in patients with RA (Peña-Lizola, 2021). Moreover, alterations in ovarian function and hormone levels can also modulate autoimmune diseases. In the case of MS, a neurodegenerative autoimmune disease of the central nervous system, where the myelin encasing neuronal axons is degraded, patients experience reduced incidence of relapse during pregnancy (Vukusic, 2004). However, in the immediate postpartum phase, patients were at an increased risk for relapse (Vukusic, 2004). Parity was inversely associated with risk of disease-related events in MS (Ponsonby, 2012), and female patients have an increased risk for inflammatory-characterized lesions compared to men (Pozzilli, 2003), underscoring the interaction between disease pathogenesis and ovarian hormones. The ovaries are also indicated in the pathogenesis of another neuroinflammatory condition, ischemic stroke. Sex differences exist in stroke outcomes, where women are at reduced risk of stroke compared to men at younger ages but are at increased risk at older ages (Reeves, 2008). While 17β-estradiol is required for brain health in young individuals and may contribute to reduced risk for stroke in this group, the Women’s Health Initiative demonstrated that HT after menopause worsened the incidence of cardiovascular disease and fatal ischemic stroke ( Girijala et al., 2017 ; Rossouw, 2002; Mozaffarian, 2016). Furthermore, OVX of young, normally cycling animals worsens stroke outcomes as compared to ovary-intact females ( Branyan et al., 2023 ; Selvamani and Sohrabji, 2010 ), which is improved by estradiol treatment. Remarkably, OVX of middle-aged, acyclic (reproductive senescent) females improved stroke outcomes ( Branyan et al., 2023 ), and was made worse with estradiol treatment. Collectively, these studies suggest that the reproductively senescent ovary might play a different role in stroke injury, separate from sex hormone signaling. One possibility is that the senescent ovary may have depleted levels of ovarian hormones but may act as a depot for immune cells. Tregs were increased in the ovaries of reproductive senescent animals after stroke, while younger females had lower ovary levels of this cell type, suggesting that Tregs were mobilized from the younger ovary after stroke ( Branyan et al., 2023 ). Reproductive senescent females that had undergone OVX had improved stroke outcomes and increased abundance of Tregs, MHC II + cells, and M2 macrophages in the post-stroke brain, likely mobilized from other immune depots ( Branyan et al., 2023 ).

Immune cell modulation is vital to reproductive function and is critical for the processes of ovulation, menstruation, fetal implantation, and pregnancy. For example, in pregnancy, innate immunity, including activation of complement, is widely upregulated while adaptive immunity is suppressed (Richani, 2004; Derzsy et al., 2010 ). While these adaptations should preserve pregnancy, in some cases, dysfunction or excessive activation of the innate immune system, such as complement overactivity, has been associated with pregnancy complications and adverse outcomes. These complications include outcomes such as preeclampsia and fetal loss in both humans and animals ( Derzsy et al., 2010 ; Pierik, 2020). This instance of immune activity in pregnancy is just one of many examples by which reproductive-immune interactions impact health outcomes. Precise hormonal regulation of the ovarian cycle is crucial to pre-serving the ovarian reserve, or the remaining capacity of the ovary to provide oocytes capable of fertilization (see Fig. 1 ). Ovarian reserve is finite from birth and declines with ongoing ovulatory cycles. Here, we will outline each of these steps and how hormonal regulation supports normal cyclicity and development to provide context to later discussed disease states. At puberty, the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary gland initiates proliferation of granulosa cells and marks the transition of primordial follicles to primary follicles ( Nilsson et al., 2007 ). During maturation, two layers of thecal cells develop; this addition indicates that the follicle is now in its secondary phase. The theca has two distinct layers: the theca interna, which contains endocrine cells and a large amount of endoplasmic reticulum, and the theca externa, a fibrous connective tissue layer ( Logan et al., 2002 ; O’Shea et al., 1978 ). The theca interna produces androgens that support follicular maturation and are converted to 17β-estradiol by granulosa cells. The theca and granulosa cells are the main source of sex hormones. Additionally, conditional ablation of macrophages in mice demonstrated that M2 macrophages present in the theca layer secrete angiogenic factors, such as vascular endothelial growth factor (VEGF), which promote and support vascularization for steroidogenic activity (Turner, 2011). In mammalian models ( Cui et al., 2011 ; Maeda, 2007), macrophages are shown to release cytokines, such as tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6), to regulate follicle growth and selection. Interestingly, excessive androgen production of the theca layer can lead to the PCOS phenotype (Nelson, 2001). Therefore, remodeling follicles in the ovarian cycle relies on both endocrine and immune regulation for proper reproductive function. As FSH continues to stimulate follicles, antrum formation occurs, creating a large fluid-filled space within the dominant tertiary follicle ( Cox and Takov, 2024 ). Granulosa cells, expressing the chemokine CXCL10, recruit proangiogenic natural killer (NK) cells to the follicular fluid which support maturation of the oocyte. This was observed in patients that responded well to in vitro fertilization (IVF) ( Fainaru et al., 2010 ). The dominant follicle undergoes rapid proliferation of theca and granulosa cells, while smaller follicles will undergo follicular atresia ( Cox and Takov, 2024 ). Granulosa cells will continue to increase 17β-estradiol, triggering a surge in LH, resulting in ovulation and the release of the oocyte down the fallopian tube. The corpus luteum, which forms from the dominant follicle post-ovulation, secretes significant amounts of progesterone to prepare the uterine lining for potential fertilization ( Gibson and Mahdy, 2024 ). If fertilization does not occur, the corpus albicans forms from the degenerated corpus luteum. This scar tissue no longer produces hormones leading to a drop in progesterone that initiates menstruation. This allows the endometrium to shed and be renewed for the next cycle. This cycle continues until declining hormone production and depleted ovarian reserve result in reproductive senescence. If fertilization occurs, the ovaries support early pregnancy by producing hormones that protect the developing embryo. Pregnancy involves significant immunological events, divided into three distinct immunological phases corresponding to each trimester of typical human pregnancy. In the first trimester, the maternal immune system is in a pro-inflammatory state to facilitate fetal implantation. During this phase, trophoblasts break the uterine wall and communicate to immune cells through cytokine secretion, notably IL-8 and monocyte chemoattractant protein-1(MCP-1) ( Trophoblast as modulators of the immune response, 2009 ). NK cells and macrophages are the primary immune cells present during this process and are critical for successful implantation of the fetus. They facilitate trophoblast invasion and support angiogenesis, particularly of the spiral arteries ( Trophoblast as modulators of the immune response, 2009 ). After initial implantation, B regulatory cells (Bregs), producing anti-inflammatory IL-10, are elevated in maternal blood and are believed to promote maternal-fetal tolerance into the second trimester (Rolle, 2013). In the second trimester, there is a remarkable reduction in inflammatory activity. This reduction is attributed to regulatory B cell (Breg) suppression of CD4 + T cells that secrete TNF-α (Rolle, 2013). In mice, B cell suppression is thought to be critical to maternal-fetal tolerance and is linked to T cell suppression through trophoblast glycans and LYN-dependent Siglec signaling (Rizzuto, 2022). While some studies suggest that B cell suppression occurs independently of regulatory T cells (Tregs) (Rizzuto, 2022), patient decidua samples from spontaneous abortions had low levels of Tregs indicating that Tregs are crucial to maternal-fetal tolerance (Sasaki, 2004). Progesterone from the ovaries elevates levels of Tregs and enhances immune suppression during midterm pregnancy for continued fetal acceptance (Mao, 2010). Interestingly, in mice this expansion of Tregs was noted not only in the uterus but also systemically (Mao, 2010). This elevation in systemic Treg levels is associated with suppression of the inflammatory disease process of experimental autoimmune encephalomyelitis in mice, in which Tregs are elevated by estradiol and Breg signaling ( Bodhankar et al., 2012 ), suggesting a possible explanation for pregnancy-related alleviation of multiple sclerosis (MS). Towards the end of pregnancy in both humans and mice, there is once again an increased inflammatory state which is believed to facilitate fetal expulsion ( Zhao et al., 2007 ). The immune microenvironment in female reproduction is robust and effective, but the role of the ovary as an immunomodulator remains to be fully understood. Immune cell types play critical roles in the menstrual cycle and reproduction. Additionally, immune cells are known to express receptors to ovarian and non-ovarian hormones (summarized in Table 1 ), indicating their potential to respond systemically to reproductive hormonal pulses ( Buskiewicz et al., 2016 ). In this section, we briefly discuss what is known about these cell types in the female reproductive tract and their functions in ovarian health and disease. Macrophages are abundant within the ovary and play many roles in reproductive function. In rat ovaries, macrophages are present in the thecal layer before and during ovulation ( Brännström et al., 1993 ). A macrophage depletion mouse model observed that these cells are required for the development and maintenance of a functional corpus luteum after ovulation (Care, 2013). Furthermore, without macrophages, secretion of progesterone, necessary for preparing the uterus for implantation, decreases (Care, 2013). Progesterone is also neuroprotective and immunomodulatory in preclinical studies, including a transgenic mouse model of Alzheimer’s disease (Carroll, 2007), and acute neural injuries like stroke ( Gibson et al., 2009 ) and traumatic brain injury ( Pettus et al., 2005 ). However, there are mixed results in clinical trials (Skolnick, 2014), and further research is needed to clarify the therapeutic potential of progesterone. Differentiation and proliferation of monocytes/macrophages are induced by colony-stimulating factor-1 (CSF-1), which has been correlated to different stages of pregnancy, specifically with placental development ( Arceci et al., 1989 ). In mice, decidua and trophoblasts increase expression of CSF-1, this recruits macrophages to aid in the development of the placenta ( Arceci et al., 1989 ). Additionally, CSF-1 is increased in the mouse uterus and levels of CSF-1 are further increased after addition of 17β-estradiol and progesterone (Pollard, 1987), indicating the hormone-responsive nature of these innate immune cells. In animal models, reduced levels of CSF-1 protein and mRNA were associated with pregnancy loss (Gorivodsky, 1999). Furthermore, pharmacologic depletion of macrophages in mice reduced ovarian estradiol secretion and expression of VEGFA and transforming growth factor (TGF-β) (Li, 2022). However, transgenic depletion of monocytes in a Cx3cr1-Dtr transgenic Wistar rat model resulted in reduced ovarian macrophages but did not significantly alter ovarian function (Sominsky, 2021). Neutrophils or polymorphonuclear cells (PMNs) are widely considered to be first responders in the immune activation cascade. PMNs, along with macrophages, are localized to different regions in the ovary during the menstrual cycle. Specifically, PMNs are localized to the theca and follicular wall during ovulation in human samples ( Brannstrom et al., 1994 ). Menstruation is inherently pro-inflammatory due to the shedding of endometrial tissue. Apoptosis of endometrial cells results in the chemoattraction of PMNs to the uterus in both human endometrial tissue and a mouse model of menstruation (Armstrong, 2017). Furthermore, the cyclical nature of sex hormones affects immune cell activation in the reproductive tract. In mice, 17β-estradiol suppresses PMN activation during ovulation and progesterone recruits PMNs in the post-ovulation period, highlighting the phase-dependent actions of PMNs (Salinas-Muñoz, 2018). Dendritic cells (DCs) are antigen-presenting cells that can modulate innate and adaptive immunity. Ovarian-resident and recruited DCs are important for ovarian functions in mice such as folliculogenesis, ovulation, and development of a functional corpus luteum (Cohen-Fredarow, 2014). In women with rising estradiol release, DCs mature in the ovary (Fainaru, 2012). Critical for their role in ovulation, DCs help maintain balance in immune activation during the menstrual cycle. In mice, depletion of DCs results in decreased expression of anti-inflammatory markers ( Tnfaip6 and Adcyap1 ) and increased expression of pro-inflammatory markers (e.g. Il1b, Tnf, Il6, Ccl2, Cxcl2 ) (Cohen-Fredarow, 2014). DC dysfunction along with dysfunction of monocytes is associated with premature ovarian failure in humans ( Hoek et al., 1993 ), underlining their importance to functioning reproductive-immune interactions. Mast cells, while typically studied in relation to histamines and allergic reactions, exhibit a wide range of immune actions and even play a significant role in neuroimmunoendocrine signaling. The presence of mast cells in the rat ovary is altered throughout the menstrual cycle, increased by gonadotropin releasing hormone (GnRH) and decreased by prolactin (Laoharatchatathanin, 2023). Interestingly, in the human ovary, mast cell abundance changes with age such that postmenopausal ovaries have increased abundance of s100 + autonomic nerve fibers and diminished levels of mast cells ( Heider et al., 2001 ). In a review on mast cells in the neuroendocrine system, Theoharides discusses the myriad impacts of mast cell immunoactivation ( Theoharides, 2017 ). Mast cells express neurohormonal receptors and secrete hormones such as adrenocorticotropic hormone (ACTH), corticotropin-releasing hormone (CRH), endorphins, making them critical responders to alterations in homeostasis ( Theoharides, 2017 ). Mast cells not only express estrogen receptor alpha (ERa) (Laoharatchatathanin, 2023; Guo, 2021) but also respond to 17β-estradiol via degranulation (Zhu, 2018), activation of the NLRP3 inflammasome, and secretion of IL-1beta, pro-inflammatory markers in patients with endometriosis (Guo, 2021). This inflammatory response mirrors that seen in patients with ovarian insufficiency (Navarro-Pando, 2021), suggesting mast cells might contribute to development or progression of ovarian dysfunction and disease. B cells are present in the ovary but are not as abundant as other resident immune cell populations. However, systemically, B cells have been shown to play a significant role in the reproductive system. In pregnant women, B cells are reduced in circulation to preserve fetal tolerance, while Bregs are increased in the placenta and periphery to modulate inflammatory responses to the developing fetus (Fettke, 2016; Lima, 2016). These Bregs express IL-35 and IL-10, known anti-inflammatory immune-modulating factors (Fettke, 2016; Liu, 2019). B cells are responsive to ovarian hormones, expressing receptors for progesterone and 17β-estradiol, among others. In a mouse model of multiple sclerosis (MS), Bregs mediate one of the neuroprotective effects of estradiol through the induction of Tregs, highlighting their ability to inhibit adaptive immune responses ( Bodhankar et al., 2012 ). On the other hand, in PCOS, B cells mediate pathogenesis such that they are increased in plasma from patients with PCOS, and their activation contributes to the development of the metabolic phenotype (Xiao, 2019; Ascani, 2023). Localized to the cortex and medulla of the ovary, in addition to circulating in the periphery, T cells are crucial to human reproductive function (Wagner, 2020; Knapik et al., 2022 ). CD4 + effector and CD8 + cytotoxic T cells participate in the process of ovulation in women (Qi, 2023). In fact, in the aging mouse ovary, there is a decreased naive CD4 + T cell population, which could limit the ovary’s ability to produce effector T cells for future ovulation (Ma, 2020). T cells are responsive to sex hormones. Low 17β-estradiol levels, like those seen in menopause, stimulate pro-inflammatory T cell responses, such as increasing interferon gamma (IFN-gamma) and Th1 cell activation ( Alanazi et al., 2024 ; Cenci, 2003; Fox et al., 1991 ). However, high 17β-estradiol levels are associated with Th2 and Treg responses, inducing expansion of Th2 and Tregs, increasing FOXP3 expression, and increasing secretion of IL-10 ( Zang et al., 2002 ; Tai, 2008; Prieto and Rosenstein, 2006 ). Tregs, specifically, are important in many aspects of reproduction, including implantation of the embryo and for sustaining fetal tolerance during pregnancy ( Alanazi et al., 2024 ; Tai, 2008; Jasper et al., 2006 ; Heitmann, 2017). In pregnancy, progesterone inhibits Th1 and Th17 responses, while activating immunomodulatory T cell responses ( Alanazi et al., 2024 ). Hence, T cells are vital actors in the coordination of immune responses throughout ovarian and reproductive cycles. The complex ovarian immune landscape combined with the responsiveness of immune cells to ovarian hormones creates an area of research ripe for further investigation. Most importantly, ovarian changes throughout the lifespan may impact physiology not only at the local level, but also at a distant site or adjacent system (summarized in Fig. 2 ).

The disease states outlined in the previous section increase in prevalence with age. The following section will discuss alterations that occur in ovarian aging which affects fertility and reproductive health, as well as increases risk for developing other diseases such as osteoporosis, cardiovascular disease, and dementia (Panay, 2020). There are several factors that play into ovarian aging and vary based on an individual’s lifestyle, health, environment, and reproductive history, which precludes a singular definition for “ovarian aging”. However, research has characterized several pathological findings that contribute to ovarian aging. The term “menopause” covers a complex combination of physiological processes and creates an event at which reproductive capacity has declined. Generally, spontaneous menopause is recognized as an event of age-related hormonal changes affecting reproductive capacity. Spontaneous menopause is defined as the end of menses for 12 consecutive months, occurring on average at 51 years of age ( Talaulikar, 2022 ). Leading up to this stage, oocytes are depleted through ovulation and atresia, decreasing secretion of inhibin B (Borelli-Kjær, 2024). This results in an increased level of FSH which in turn depletes follicles and results in low production of 17β-estradiol and progesterone from the ovary ( González-Pérez et al., 2024 ). Decreased steroid hormone production and quality and quantity of follicles are two critical components of reproductive health in aging (Tian, 2024). For some women with POI, this loss can occur before the age of 40 (Panay, 2020). Therefore with an average life expectancy of 80 years of age for women, about 50% of a woman’s lifetime can be spent in this state of low hormone production CDC, N. C. f. H. S., ( CDC, 2022 ). The importance of understanding the drivers of ovarian aging earlier will be crucial for targeting menopause-associated health consequences and alleviating the risk for associated diseases. With aging comes a natural increase in peripheral proinflammatory stress, commonly referred to as “inflammaging”. Isola et al. reviewed reproductive aging across patient and mice studies and found cytokines such as IL-6, IL-8, and TNF-α, that are secreted by macrophages present in the ovary, become exaggerated prior to reproductive failure (Isola, 2024). In patients seeking IVF, higher levels of TNF-α in follicular fluid is related to reduced oocyte quality and fertility. In reproductively senescent mice, this prolonged pro-inflammatory response resulted in the presence of multinucleated macrophage giant cells (MNGCs), commonly found at sites of chronic inflammation (Briley, 2016). Macrophage activity is needed to cope with the accumulation of debris, from follicular atresia and tissue remodeling due to continual ovarian cycles. MNGCs, which are the result of macrophage fusion, accumulate in the aging ovary of mice due to the increase in phagocytic needs (Xu, 2022); ( Foley et al., 2021 ). This prolonged inflammatory state seen in ovarian aging and associated with reproductive decline is paralleled in POI. Increased expression of pro-inflammatory cytokines and imbalance of TH17 and Treg cells was observed in patient serum samples and furthered progression of POI (Nouri, 2024). This feed-forward inflammatory loop may be exacerbated by other molecular events, such as iron overload in response to oxidative stress, which is seen in ovaries of ER-alpha (ERα) deficient mice ( Schröder et al., 2024 ) and leads to ferroptosis. Ferroptosis, a specialized form of cell death, can increase a proinflammatory environment by release of damage-associated molecular patterns and cytokines that contribute to immune cell invasion and the decline of fertility in aging animals ( Schröder et al., 2024 ). Mitochondria are of particular interest in the context of aging, in view of their critical roles in energy production, cellular senescence, and involvement with ROS. In rat luteal cells treated with Prostaglandin F, ROS are required to induce cell death of the corpus luteum in the absence of fertilization (Tanaka, 2000). However, excessive ROS is associated with development of chronic inflammation and is a risk factor for cardiovascular and neurological diseases ( Chelombitko, 2018 ). Mitochondrial DNA, which encodes components of the oxidative phosphorylation chain, accumulates mutations over time. Women as early as 39 years of age had 1.5 times lower levels of mitochondrial DNA copy number in cumulus cells surrounding oocytes, and a decrease in ATP production within the oocyte (Korolkova, 2019). Notably, mutant mitochondrial DNA (mtDNA) in oocytes resulted in follicular atresia and affected the quantity and quality of oocytes in the ovarian reserve (Korolkova, 2019), consistent with the decline in fertility with age. Aging oocytes in rodent models also exhibit mitochondria with elongated dysmorphology and swollen vacuoles, suggesting weak bio-energetic profiles (Simsek-Duran, 2013). With aging, there was reduced antioxidant enzymes in cumulus cells, follicular fluid, and granulosa cells which are important components to protecting the oocyte ( Mihalas et al., 2017 ). Increase in ROS species can cause damage to metaphase II mouse oocyte chromosomes, induce spindle defects and meiotic instability ( Zhang et al., 2006 ). Studies of mitochondrial DNA in cumulus cells of women who spontaneously conceived versus women who used assisted reproductive technologies (ART) found the latter group had increased number of de novo mitochondrial DNA variants in oocytes (Mertens, 2024). It is worth noting that offspring of the ART group had increased mtDNA variants, suggesting a frequently overlooked trans-generational component to ovarian aging ( Schröder et al., 2024 ). Prolonged inflammation and oxidative stress in aging can result in fibrosis (Castello, 2010). In fact, fibrosis in tissue is often considered to be a sign of aging and is also observed in aging ovaries (Briley, 2016; McCloskey, 2020). In mice, aging ovaries exhibit increased deposition of extracellular matrix components and collagen accompanied by a decrease in hyaluronan, resulting in increased ovarian stiffness (Amargant, 2020). Ovaries from vasoactive intestinal peptide knock out mice exhibit increased collagen deposition, elevated ROS, and increased levels of IL-1β (Castagnola, 2025). Several studies have implicated a maladaptive action of TGF-β1 in aging, which is activated by cytokines, and promotes fibroblast proliferation, fibroblast-to-myofibroblast conversion, and extracellular matrix production and deposition (Orisaka, 2023). Treatment with Pirfenidone, primarily used to treat idiopathic pulmonary fibrosis, increased follicle count and hormone levels as well as sustained cyclicity in mice for up to 12 months of age ( Amargant et al., 2024 ), suggesting that targeting fibrosis may serve to extend reproductive health. Another significant contributor to declining function in aging is related to epigenetic modifications, including DNA methylation, chromatin remodeling, and non-coding RNA (ncRNA) regulation. A notable example of the latter in the ovary is the long non-coding RNA X-chromosome inactive specific transcript (XIST). XIST is a long non-coding RNA (lncRNA) expressed specifically by the inactive X-chromosome and localized to the X chromosome inactivation center (Brown, 1991). Expression of XIST is abundant throughout follicular development in the ovary ( Ernst et al., 2018 ). XIST regulates autophagy via binding to premature microRNAs, resulting in oocyte loss and formation of the postnatal pool of primordial follicles in the ovary in mice (Zhou, 2021). In diseases of the ovary, such as PCOS, XIST upregulation modulates inflammatory pathways acting as a competing endogenous RNA (ceRNA), or a microRNA sponge (Berenji, 2024). Interestingly, after birth as well as in ovarian aging, XIST is downregulated (Zhou, 2021; Tamura, 2017). XIST plays a critical role in other disease processes such as stroke in which XIST is differentially regulated at different stages of injury in both patients with a previous ischemic event and in a mouse model of Middle Cerebral Artery occlusion (Wang, 2021). Patient serum levels of XIST negatively correlated with post-stroke impairment, and XIST modulates inflammation, through Kruppel-like transcription factor 4, as well as angiogenesis, acting as a post-stroke neuroprotectant (Wang, 2021). Since ovarian XIST expression changes with age, and evidence shows that XIST can modulate disease processes via modulation of inflammation, this may provide a mechanistic target for ovarian contribution to age-related disease. Overall, this section highlights some of the better-known mechanisms that contribute to ovarian aging. The following section describes approaches to target ovarian aging.

Since cellular senescence includes a phenotype like that of the aging ovary, some studies have assessed whether known senolytics would ameliorate ovarian aging phenotypes. ABT263, a recently identified senolytic (Chang, 2016), reduced senescent oocytes and increased follicle count and oocyte maturation in young mice (Yan, 2024). In 16-month-old mice, ABT263 reduced fibrosis, but it also reduced ovarian reserve (Xia, 2024). Similarly, senolytics like Dasatinib, Quercetin, and Fisetin were unable to improve ovarian reserve but were able to reduce lipofuscin accumulation and infiltration of macrophages in female mice (Garcia, 2024). Lastly, nicotinamide mononucleotide (NMN), a supplement and precursor of NAD+, that has been identified as a senolytic, was able to improve fertility and oocyte quality in addition to reducing ROS and improving mitochondrial health in aging animals (Bertoldo, 2020; Yang, 2020). Thus, senolytic therapies may target different aspects of ovarian aging, and it remains to be determined how these therapies may subsequently affect cardiovascular and neurologic disease. Mitochondrial dysfunction and oxidative stress are hallmarks of aging and cellular dysfunction. Moreover, both mitochondrial dysfunction and oxidative stress exacerbate immune system activation, and vice versa. ( de Faas and Vos, 1866 ; Hanaford and Johnson, 2022 ; Morris et al., 2022 ). Current research has focused on targeting oxidative stress in infertility and other ovarian conditions, such as PCOS, centered on the use of dietary vitamin supplements, an approach that tends to have minimal side effects on patients. In animal models, supplementation with vitamin C preserves ovarian reserve and volume and increases antioxidant activity in the ovary, reducing oxidative stress and senescence (Abdollahifar, 2019; Olaniyan, 2019; Sapmaz, 2023). Melatonin is another supplement shown to have beneficial effects on ovarian function, reducing oxidative stress, increasing levels of antioxidants, reducing senescence markers, and improving mitochondrial function in both patient and animal studies (Lin, 2020; Song, 2016). Moreover, in an aging mouse model, melatonin preserved ovarian reserve (Tamura, 2017; Yang, 2021) and improved fertility and pregnancy rates in humans (Tamura, 2008). Combining melatonin with other supplements, like magnesium resulted in reduced pro-inflammatory markers and increased antioxidant activity in women with PCOS (Alizadeh, 2021; Mousavi, 2022). Coenzyme Q10, or CoQ10, is another endogenous antioxidant that has beneficial effects on mitochondrial function and ovarian reserve, normalizing the redox balance of the ovaries (Abdulhasan, 2017; Ben-Meir, 2015). Additionally, CoQ10 improves metabolic status and lipid levels in patients, reducing negative cardiac outcomes in patients with PCOS (Izadi, 2019; Izadi et al., 2019). Metformin, a drug typically used in patients with diabetes mellitus, has been shown to have broad health benefits. As a single therapy, metformin improved mitochondrial function and when combined with resveratrol, a natural antioxidant found in fruits, metformin reduced apoptosis in the ovary and increased antioxidant activity in PCOS ovarian tissue (Furat Rencber, 2018). Metformin combined with oral contraceptives reduced inflammation and insulin resistance in patients (Kumar, 2018), mitigating risk factors for other conditions such as cardiovascular and cerebrovascular disease. Critically, many of these studies have been conducted in animal models of PCOS or in patients with ovarian dysfunction. Future investigation could assess mitochondrial dysfunction and oxidative stress within the context of natural or spontaneous menopause. In a somewhat crude approach to target the ovary in disease and aging, some studies have examined the consequences of transplanting young ovaries into older rodents. Young ovarian transplantation significantly increases lifespan ( Mason et al., 2009 ; Kagawa, 2010) and reduces age-related negative cardiac outcomes such as fibrosis, thrombosis, and cardiomyopathy in reproductive senescent mice (Mason, 2011). Additionally, young ovaries improved cognition, renal function, and immune responses, such as increased naïve T cell population in aging animals ( Parkinson et al., 2017 ; Peterson et al., 2017 ). Of note, in these studies, ovarian transplant was conducted after removing the aging ovary, leaving open the question of whether the presence of a young ovary could counteract the effects of an aging ovary. Ovarian transplantation has been conducted in human patients, primarily to treat patients who have reduced fertility due to either cancer therapies or ovarian insufficiency. In a review on these studies, Shespari et. al. showed that these surgeries have successfully improved reproductive function and cycles (Sheshpari, 2019). Moreover, mathematical modeling revealed the potential for ovarian cryopreservation and autotransplantation to extend reproductive lifespan, delaying spontaneous menopause ( Johnson et al., 2024 ). Thus, ovaries are vital for reproductive health through both endocrine and immune actions. Immune cells not only actively participate in the well-studied reproductive role of the ovaries, but the work reviewed here reveals roles for immuno-ovarian interaction in the pathogenesis of various systemic diseases. Gaining a better understanding of the reciprocal relationship between the ovary and the immune system, as was briefly introduced in this review, would allow for the identification of potential therapeutic targets for non-reproductive health issues that are exacerbated with age and disease.

The ovary, the paired female gonad, functions as an endocrine organ that stores oocytes and produces hormones, including 17β-estradiol, progesterone, testosterone, and inhibin. These hormones are crucial to the development of secondary sex characteristics and to the regulation folliculogenesis, menstruation, fertility, and pregnancy. Unlike male gonads, which produce sperm through spermatogenesis for life, the ovary lacks germline stem cells. Instead, primordial germ cells undergo cell division to form primordial follicles, which consist of an oocyte with a singular supportive layer of granulosa cells. By around 11–12 weeks of gestation in humans, this process of meiosis begins forming the ovarian reserve prior to birth ( Gondos et al., 1986 ) This finite reserve of oocytes will deplete through continual ovarian cycles after puberty until reproductive senescence occurs. A critical role of the ovary is to preserve oocytes through development and maturity via endocrine and immune regulation. While the endocrine (hormones, cellular structures) components of the ovary are well recognized, this review will focus on the immuno-regulatory functions of and within the ovary by first outlining ovarian-resident immune cells and how their function supports reproductive health. This will be followed by a discussion of ovarian diseases and the important modulatory role that the ovary plays in other inflammatory disease processes, including cardiovascular, neurologic, and autoimmune diseases. Lastly, ovarian alterations around menopause will be covered, along with a discussion of current research on therapeutic approaches to alleviate pathological ovarian and systemic inflammatory changes in disease and aging. This review aims to focus attention on the immune role of the ovary in health, disease, and aging, highlighting the importance of considering sex differences in not only an endocrine context, but also in immune interactions. Further consideration of the ovary in immune processes is a promising target for therapeutics in both disease and aging.

Since the ovaries can have such extensive impacts, this next section will consider the literature reporting the effects of ovarian removal on various physiological processes in both human and animal models. In humans, oophorectomy, surgical removal of the ovaries, is performed for various reasons at any time in a patient’s life, before or after natural menopause. Bilateral oophorectomy is typically performed alongside the surgical removal of the uterus, known as a hysterectomy. Patients may undergo premenopausal oophorectomy for conditions such as ovarian carcinoma, familial genetic risk of breast cancer or ovarian cancer, severe endometriosis, or as part of gender-affirming care. When the ovaries are removed before spontaneous menopause, the patient enters a surgically induced menopause-like state. The onset of menopause in these patients is considered premature or early menopause if it is before the age of 45 years. These patients also experience symptoms like spontaneous menopause such as hot flashes, sexual dysfunction, vaginal dryness, anxiety, depression, and worsened suicidality, among others (Tucker, 2016; Epidemiology and Physiology, 1990 ). To alleviate these symptoms, patients are typically recommended hormone therapy (HT) in the form of vaginal estradiol, transdermal estradiol, or oral estradiol conjugates. Although HT is recommended to all premenopausal patients who experience surgically induced menopause, many patients do not elect to take HT, potentially resulting in increased menopausal symptoms and worsened quality of life (Tucker, 2016; Chubaty et al., 2011 ). Additionally, undergoing bilateral oophorectomy before spontaneous menopause increases the risk of all-cause mortality, chronic kidney disease, parkinsonism, and cardiovascular disease ( Shuster et al., 2010 ; Kattah, 2018; Gierach, 2014; Rocca, 2008), diseases which have known associations with immune dysfunction ( Kurts et al., 2013 ; Tansey, 2022; Wang, 2025). Risk of all-cause mortality is inversely related to the patient’s age at the time of oophorectomy, with risk being greater in patients who received the surgery at a younger age (before the age of 35 years). This association continues until the age of 45 years, after which risk of all-cause mortality decreases with bilateral oophorectomy (Gierach, 2014; Kurts et al., 2013 ; Tuesley, 2020). These findings are particularly intriguing because it implies that presence of ovaries in later life stages might be contributing to increased mortality risk. Much less is known about postmenopausal oophorectomy and subsequent outcomes. Postmenopausal patients may undergo bilateral oophorectomy, typically with hysterectomy, for reasons such as benign diseases of the uterus or ovaries, cancer (e.g. colon, endometrial), or to reduce risk of ovarian cancer (Jiang, 2023; Mai, 2022; Kotsopoulos, 2012). In these patients, there is a decrease in systemic levels of testosterone and 17β-estradiol (Mai, 2022; Stanczyk, 2019). Postmenopausal removal of the ovaries also results in reduced risk of breast cancer (Kotsopoulos, 2012). While premenopausal oophorectomy is correlated with declining cognition, postmenopausal oophorectomy was not associated with changes in cognition according to one study (Kurita, 2016). However, in another study, bilateral oophorectomy was associated with an increased risk of dementia and Parkinson’s Disease, although, the risk was worse in patients who received the surgery prior to the age of 50 (Evans, 2016). Furthermore, postmenopausal oophorectomy showed no association with risk of Alzheimer’s Disease (Kurita, 2016), highlighting the complex association of the presence of ovaries with diseases associated with aging and inflammation. Ovariectomy (OVX) in animal models is often used as an experimental method to model menopause conditions or to measure the impact of ovarian hormones on the progression and modulation of various diseases ( Wang et al., 1999 ; Bramlett and Dietrich, 2001 ; Fang, 2018; Branyan et al., 2023 ). In reproductive diseases like PCOS, a disease considered to be primarily hormone-driven, OVX reduces reactive oxygen species (ROS) production and dyslipidemia in rats ( Possible Approach to Postmenopausal Polycystic Ovary Syndrome, 2021 ). On the other hand, OVX alone can result in metabolic dysfunction, resulting in fatty liver and dyslipidemia, weight gain, and mitochondrial dysfunction (Costa-Beber, 2021; Malinská, 2021; Sutham, 2018), conditions associated with increased levels of systemic inflammation ( de Faas and Vos, 1866 ; Kim, 2021; Paragh et al., 2014 ). In young rats, OVX alone results in increased cognitive impairment, increased depressive behaviors, and increased neuroinflammation (Fang, 2018). Moreover, removal of the ovaries can impact immune modulation and result in a low estrogen phenotype. After OVX, T cells were activated, and pro-inflammatory cytokines, like IFN-gamma, were increased, resulting in bone loss (Cenci, 2003). The effects of OVX in young animals result in worsened ischemic and traumatic brain injury ( Wang et al., 1999 ; Bramlett and Dietrich, 2001 ; Selvamani and Sohrabji, 2010 ; Liu et al., 2009 ) and increased histopathological markers of neurodegenerative disease (Zhou, 2023; Farkas, 2022). Altogether, animal studies using OVX as a model have shown the ovary to be a key regulator of pathology. Another animal model of menopause is chemical depletion of pre-antral follicles using 4-vinylcyclohexene diepoxide (VCD), which causes gradual ovarian failure. The VCD model of gradual menopause results in outcomes such as insulin resistance and metabolic syndrome, much like OVX surgery, and estrogen treatment prevents some of these sequelae ( Romero-Aleshire et al., 2009 ). A comparison of VCD-induced ovarian failure, OVX and age-related estropause, revealed differences in vaginal cytology. For example, vaginal lavage in 4-month-old VCD treated mice exhibited reduced neutrophils compared to older (reproductively senescent) mice, with OVX mice displaying an intermediate number of neutrophils compared to the other 2 groups (Zucon Bacelar, 2023). VCD could be a potential model for studies investigating the effects of retained ovarian stroma on immune function and activity in a menopausal background, but the differences in each model highlight the need for further investigation of the immune impact of retained postmenopausal ovaries.