Investigating the Link Between the Immune System and Sex Hormones Across the Menstrual Cycle in PMDD and Healthy Controls

The immune system is strongly intertwined with the reproductive system. This is seen when an immune dysfunction interferes with fertility, for example in endometriosis (Miller et al., 2017), and when patients experience symptom relief in immunological disorders during major hormonal transition periods, such as pregnancy (Amin et al., 2011). While immunological changes during major hormonal transition periods are somewhat studied, for example in pregnancy (see Robinson & Klein, 2012), changes in inflammatory cytokines and immune cells across the female menstrual cycle remain poorly understood. Two major sex hormones estradiol (E2) and progesterone (P4) fluctuate in a systematic manner across the menstrual cycle of premenopausal females. E2 and P4 levels start low in the early follicular phase. E2 increases across the follicular phase, reaches a first peak shortly before ovulation with a subsequent sudden drop and rises again to a second moderate peak in the mid-luteal phase, before decreasing towards the end of the cycle. P4 remains low throughout the follicular phase and starts rising after ovulation has occurred to reach its peak in the mid-luteal phase before decreasing again. Beyond reproductive functions, these two sex hormones seem to affect the immune system (Bouman et al., 2005), suggested to provide evolutionary benefits (Alvergne & Högqvist Tabort, 2018). While research investigating immune cells across the menstrual cycle is scarce, there are relatively more studies regarding inflammatory markers, such as cytokines and acute-phase proteins. However, the specific ways in which immune cells and cytokines vary across the menstrual cycle and their relation to sex hormone fluctuations remain to be elucidated. Sex hormones can act upon the immune system via sex hormone receptors that are expressed by innate and adaptive immune cells (Hoffmann et al., 2023). Specifically, E2 exerts its effect via nuclear and membrane-bound estrogen receptors (ERs) (Khan & Ahmed, 2016; Hoffmann et al., 2023). Nuclear ERα and ERβ either directly bind to estrogen response elements in gene promoter regions or serve as cofactors with other transcription factors leading to long-lasting, genomic changes in immune cells. Membrane-bound ERs impact downstream kinase pathways, resulting in rapid, non-genomic changes in immune cells (Hoffmann et al., 2023). P4 too exerts slow, genomic effects on the immune system through nuclear progesterone receptors (PRs) alpha and beta (PRα and PRβ), as well as rapid, non-genomic effects via membrane-bound PRs. Additionally, P4 shows a strong affinity to glucocorticoid receptors (GRs) (Hoffmann 2023). Although the effects of E2 and P4 are commonly perceived as anti-inflammatory (Nadkarni & McArthur, 2013; Zwahlen & Stute, 2023; respectively), both E2 and P4 can exhibit dual roles, possessing both anti- and pro-inflammatory effects. Additionally, they may act antagonistic or in synergy to one another, depending on factors such as immune cell and cytokine subtypes or hormone concentration (for estrogen, see Straub, 2007). This complexity contributes to contradictory research findings, highlighting the intricate and multifaceted relationship between sex hormones and the immune system. Critically, changes in the immunological profile have been linked to the experience of mood symptoms. This association has been demonstrated in healthy females with no history of a mood disorder or experience of premenstrual symptoms (Puder et al., 2006), but also in females with Premenstrual Syndrome (PMS) (Granda et al., 2021). It is, thus, not far-streched to expect inflammatory changes across the menstrual cycle in subjects with Premenstrual Dysphoric Disorder (PMDD). Affected females experience distressing mental and physical symptoms (e.g., depressed mood) during the premenstrual phase, which vanish shortly after menstruation (Hantsoo & Epperson, 2015). Whether the specific effects of fluctuating E2 and P4 across the female menstrual cycle on the immune system exert an impact on depressive symptoms remains unclear. With this study we aim to investigate the link between endogenous sex hormones (E2 and P4) and inflammatory markers (immune cells and inflammatory cytokines and acute-phase proteins) by exploring their relationships across the menstrual cycle focusing on the periovulatory phase, characterised by high E2 and low P4, and the premenstrual phase, characterised by declining but still elevated levels of E2 and P4 in healthy females and females with PMDD. A secondary aim of this study is to establish an inflammatory profile that turns individuals more susceptible to the experience of premenstrual symptoms.