**2. Sex hormones and the immune system**

Many differences have been identified between men and women with respect to the immune responses. In general, women show higher immunoglobulin levels, but lower activity of cell‐ mediated immune reactions than men [3]. These differences result in the effects of two major groups of biological factors: endocrine (sex hormones) and genetic (X‐chromosome) [4]. Unique immunological peculiarities are observed in women during pregnancy.

Sex hormones affect the immune system in various ways. Immune cells, such as T and B lymphocytes, monocytes, macrophages, natural killer (NK) cells, dendritic cells (DC), express receptors for sex hormones although the lymphoid cells are not their main target. Estrogen effects are mediated through two isoforms of estrogen receptors (ER)—ERα and ERβ. Two isoforms have been identified for the progesterone receptor (PR) as well—PRA and PRB, while androgen receptor has no variants and binds both testosterone and 5α‐ dihydrotestosterone [5–8].

The regulation of T helper 1 (Th1)‐type cytokine production by estrogens, appears to be dose dependent. Some authors report increased production of IFN‐γ and IL‐2 by low, "physiolog‐ ical" doses of estrogens, while others find them unchanged [9, 10]. Biphasic secretion of TNF‐ α has also been described, with stimulation by low doses and inhibition by high doses of estrogens [11]. Increased production of IL‐4 by T lymphocytes has been registered after incubation with progesterone [12].

pathological process in MS. Female gender is now regarded as an independent risk factor for the development of the disease, with female:male ratio 3:1, and even higher (3.2:1) in subjects with MS onset before age 20 [1, 2]. Despite this incidence, women do not have poorer prognosis than men, suggesting a biological mechanism underlying these divergences. Pregnancy, the most potent disease‐modifying factor in MS, is a physiological condition characterized with significant hormonal and immunological changes, which raises the idea that sex hormones are implicated in some aspects of the autoimmune process. Many of these observations have been

The purpose of this chapter is to provide an updated, summarized overview of currently published scientific information about the role of sex hormones in the pathogenesis and clinical course of multiple sclerosis and to outline the perspectives to use this knowledge for control

An advanced search was conducted, based on the following key words in different combina‐ tions: "multiple sclerosis", "experimental autoimmune encephalomyelitis", "sex hormones", "pregnancy", "cytokines", "estriol", "estradiol", "progesterone", "testosterone", disability", and "MRI". The relevant scientific works (original articles, book chapters, and systematic reviews) published in English, in electronic database (PubMed, MEDLINE, and Medscape) have been retrieved and summarized. The search period was unrestricted. The following inclusion criteria have been determined: (1) subjects, suffering from multiple sclerosis; (2) studies on EAE; (3) assessment of sex hormones and cytokines; (4) brain imaging findings in relation to hormonal concentrations. Case report articles were excluded. A relationship between the concentrations of sex hormones, cytokines, physical disability, and the course of

Many differences have been identified between men and women with respect to the immune responses. In general, women show higher immunoglobulin levels, but lower activity of cell‐ mediated immune reactions than men [3]. These differences result in the effects of two major groups of biological factors: endocrine (sex hormones) and genetic (X‐chromosome) [4].

Sex hormones affect the immune system in various ways. Immune cells, such as T and B lymphocytes, monocytes, macrophages, natural killer (NK) cells, dendritic cells (DC), express receptors for sex hormones although the lymphoid cells are not their main target. Estrogen effects are mediated through two isoforms of estrogen receptors (ER)—ERα and ERβ. Two isoforms have been identified for the progesterone receptor (PR) as well—PRA and PRB, while androgen receptor has no variants and binds both testosterone and 5α‐ dihydrotestosterone

The regulation of T helper 1 (Th1)‐type cytokine production by estrogens, appears to be dose dependent. Some authors report increased production of IFN‐γ and IL‐2 by low, "physiolog‐

Unique immunological peculiarities are observed in women during pregnancy.

confirmed and elaborated in experimental autoimmune encephalomyelitis (EAE).

of the disease activity.

258 Trending Topics in Multiple Sclerosis

the disease has been searched.

[5–8].

**2. Sex hormones and the immune system**

Studies on Th2 cytokine production (IL‐4 and IL‐10) do not reveal any effect of estrogens in normal conditions. No differences have been found between fertile and postmenopausal women in regard to IL‐4 levels [13–15]. No differences in IL‐10 production have been detected between women and men [10]. On the other hand, an enhancing effect of estrogen on IL‐10 production has been found in T lymphocytes from patients with MS, suggesting potentially different regulatory pathways in autoimmune diseases [16, 17]. Consistent with these findings are the results from experiments, showing that estradiol at 10–100 nM inhibits lipopolysac‐ charide (LPS)‐induced TNF‐α production from human peripheral blood mononuclear cells (PBMCs) but is stimulatory in the absence of LPS. These data illustrate the importance of cellular context for the effect of estrogens on T cells—cytokine secretion [18].

In vitro, naïve T cells stimulated with CNS autoantigens in the presence of testosterone produce higher levels of IL‐5 and IL‐10, but decreased levels of IFN‐γ [19]. Testosterone can also reduce the in vitro production of proinflammatory cytokines, such as TNF‐α and IL‐1β by human macrophages and monocytes [20, 21].

Concentrations of IL‐1β producing monocytes have been found higher in men than in women [10]. The influence of estrogens on IL‐1 production from monocytes and macrophages seems to be biphasic as well. Progressive inhibition of IL‐1 transcription with increasing concentra‐ tions of estrogen and progesterone has been described in cultured peripheral monocytes [22]. The study conducted by Kramer et al. [23] demonstrated that 17‐β‐estradiol can mediate release of IL‐1, IL‐6, and TNF‐α from activated monocytes and/or macrophages through modulation of CD16 expression, with low doses being stimulatory for CD16‐expression and cytokine secretion, respectively. Clear evidence for dose‐dependent effects of estrogens on cytokine secretion was revealed by Matalka [24]. At preovulatory concentrations, estradiol significantly enhanced IFN‐γ, IL‐12, and IL‐10 secretion from stimulated whole blood cells. Concentrations, similar to those in pregnancy, have caused increased production of IL‐10 and reduced IL‐12, IFN‐γ levels and IFN‐γ/IL‐10 ratio. In the same concentration, estradiol has been shown to increase IL‐10 secretion and decrease expression of TNF‐α mRNA in proteoli‐ pid protein (PLP)‐activated peripheral blood mononuclear cells isolated from healthy subjects [17].

During pregnancy, ovarian secretion of female sex hormones gradually reaches the peak of physiological levels. At the same time, a shift from Th1 toward Th2‐type immune responses is observed. Marzi et al. [25] have studied the antigen and mitogen‐stimulated cytokine production by PBMC obtained from healthy women during pregnancy and postpartum, and has established decreased secretion of IL‐2 and IFN‐γ, and increased IL‐4 and IL‐10 expression in the last trimester of pregnancy [25]. Another study in healthy pregnant women has found reduced serum levels of IL‐12 and TNF‐α together with high estrogen and progesterone levels during pregnancy compared with puerperium [26]. Similar changes have been observed in pregnant women with MS [27].

Hormonal influences on the function of B lymphocytes have been assessed by the analysis of immunoglobulin levels. Higher immunoglobulin levels in women than in men are a part of the sex differences in immune responses [3, 4]. Kanda et al.'s [28] study has shown that estrogens increase IgG and IgM production in both males and females directly, and through a potentiating effect of IL‐10, released from monocytes [28]. The effect of testosterone appears to be opposite, as it inhibits IgM and IgG production both directly and indirectly by reducing the production of IL‐6 by monocytes [29].

Progesterone and estrogens have been shown to influence the activity of NK cells. Increased activity of these cells has been reported in postmenopausal women and men compared with fertile females in the luteal phase of the cycle [30]. Partially in line with these data are the results from experiments, investigating the direct effect of sex hormones on NK cells activity. High‐ dose estrogen elicits a suppressive effect, whereas progesterone, testosterone, and estrogen at physiological concentrations have no effect in vitro on established cell lines [31, 32].

There are evidence for estrogen impact on DC functions. Exposure of the immature cells to estrogen has increased their IL‐6, IL‐8, and MCP‐1 production, but most importantly, has enhanced their capacity to stimulate T lymphocytes [33–35]. Another study has revealed the ability of estrogen to enhance DC proinflammatory cytokine production in animal models [36].

These findings demonstrate that the interactions between sex hormones and the immune system are extremely complex and variable during different physiological states. The depend‐ ence of hormonal effects on the concrete cellular context and local cytokine milieu suggests that some specifics might be present in pathological conditions and especially in autoimmune diseases.
