23.1Introduction

Over the last decades much evidence of communication between the immune, nervous, and endocrine systems has accumulated. This communication has a solid molecular basis. The messengers are hormones, neuropeptides, neurotransmitters, cytokines, and their receptors. These messengers have endocrine action (at a distance), paracrine action (on the neighboring cells), and autocrine action (on the cells themselves). The integrated bidirectional communication of the three systems, now called the immune-neuroendocrine system, regulates the adaptive response to stress [1]. Stressful situations, such as the ones induced by an inflammatory or infectious process, the activation of autoimmunity, trauma, surgery, and emotional events, trigger a series of reactions that activate the immune-neuroendocrine system.

The immuno-neuroendocrine system includes the hypothalamic-pituitary-adrenal axis, the hypothalamic-pituitary-gonadal axis, the hypothalamic-pituitary-thyroid axis, prolactin,/and growth hormone. The autonomic nervous system, comprised of the sympathetic and parasympathetic nervous systems, also participates in the stress response through its sympathetic limb [2].

The interactions of the stress response systems support the concept that the immune-neuroendocrine system plays an important role in modulating host susceptibility and resistance to inflammatory disease. Recent studies have demonstrated that a disruption or an abnormal response of the immune-neuroendocrine communication may be associated with susceptibility to or severity of autoimmune diseases, including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), and Sjögren’s syndrome (SS). The immune-neuroendocrine system may influence the activity of lymphoid organs and cells via endocrine and local autocrine/paracrine pathways or alter the function of different cell types in target organs involved in SS [3].

The aim of this chapter is to analyze the role of the immune-neuroendocrine system in the pathogenesis and clinical picture of SS and the hormonal treatment as a new perspective in this disease.

23.2Immune-Neuroendocrine System in Sjögren Syndrome

During a stressful situation such as one involving inflammation, activated immune system cells release proinflammatory cytokines (IL-1-b, TNF-a, IL-6) that reach the liver via the blood stream. In the liver, these proinflammatory cytokines stimulate the production of acute phase reactants. The cytokines also traverse the blood– brain barrier and exert direct stimulation on the hypothalamus, leading to the production of corticotropin releasing hormone (CRH) and arginine-vasopressin. These hormones, in turn, activate anterior pituitary cells to release adrenocorticotrophin hormone (ACTH), b-endorphins, prolactin, and the melanocyte stimulation hormone. These anterior pituitary hormones then exert endocrine effects on distant organs. As an example, ACTH, produced under the direction of CRH,

induces the production of corticosteroids by the adrenal glands. Stressful events also activate the autonomic nervous system, leading to the release of adrenaline and noradrenaline and resultant increases in blood glucose, heart rate, blood pressure, and the state of alertness; inhibition of the immune system; and decreases in reproductive function [2].

The impact of stress following major and minor life events before the onset of primary SS (pSS) has been investigated [4]. A disproportionate percentage of pSS patients report the occurrence of negative stressful life events within 1 year of disease onset, compared to patients with lymphoma and healthy controls. Coping strategies were defective and the overall social support was lower in patients with pSS. After SS is established, nearly half of the patients suffer from anxiety and 32% have depression [5]. Conversely, up to 52% of patients with chronic fatigue syndrome report mucosal dryness, and salivary gland biopsies demonstrate findings consistent with SS in 32% of cases [6].

23.3Hypothalamus-Pituitary-Adrenal Axis

Several neuroendocrine functions appear to be impaired in SS. The HPA axis produces basal ACTH and cortisol levels that are significantly lower than those of healthy controls, and pSS patients have correspondingly diminished responses of the pituitary and adrenal glands in response to ovine CRH [3, 7]. The lack of response to CRH suggests an important defect in the hypothalamic influence over the anterior pituitary. However, another study on both the basal state and following stimulation with CRH, TRH, and LHRH demonstrated that women with pSS without glucocorticoid treatment have intact cortisol synthesis but decreased serum concentrations of dehydroepiandrosterone sulphate (DHEAS) and increased cortisol/ DHEAS ratios compared with healthy controls. These findings suggest a constitutional or disease-mediated influence on adrenal steroid synthesis [8]. Other authors have found an increase of IL-6 in the serum and saliva of patients with pSS and a slight increase of tumor necrosis factor (TNF) levels. Stimulation of host IL-6 increases the protective host responses such as acute phase response and HPA axis. IL-6 stimulates CRH, ACTH, and cortisol synthesis [9]. According with these findings, patients with SS have not only HPA hypoactivity, but also varying patterns of dysfunction of other hormonal axes [10].

23.4Hypothalamus-Pituitary-Gonadal Axis

The role of androgens and estrogens in SS has been of interest since the 1980s. The first demonstration of the role of sex hormones in SS was derived from experimental models. In 1984, a synthetic androgen, nandrolone decanoate that has strong immunosuppressive but only weak androgenic effects was used in B/W mice, an

experimental model of SLE and SS. Three weekly injections of this androgen therapy reduced the development of mononuclear infiltrates in the submandibular gland, thereby preventing the destruction of glandular tissue [11].

In humans, the role of sex steroid hormones in SS syndrome was studied for the first time in five patients with hypogonadism and Klinefelter’s syndrome, three of whom had SS and two had SLE. All patients had low serum testosterone but high LH levels, and reduced total T lymphocytes and suppressor/cytotoxic T lymphocytes. The erythrocyte sedimentation rate (ESR) was above normal in all patients, and all had high titers of antinuclear antibodies (ANA) and rheumatoid factor (RF). After 60 days of testosterone therapy, serum testosterone levels had increased and LH levels had decreased in comparison with placebo group. The numbers of total T lymphocytes and suppresor/cytotoxic T lymphocytes normalized, titers of ANA and RF decreased, and the ESR declined in all patients. After therapy, the patients’ SS and SLE entered clinical remissions [12].

Dihydrotestosterone, estrogen, and testosterone were studied in the minor salivary glands of women with SS and normal controls, using the Peroxidase– Antiperoxidase method. In normal controls, estrogen was positive in the epithelial cells of duct, but testosterone and dihydrotesterone were negative. However, estrogen, testosterone, and dihydrotestosterone were positive in the labial minor salivary glands of the SS patients, suggesting an influence of sex hormones in SS [13].

The incidence of SS in women increases from menarche and is highest near the menopause. However, the prevalence of SS in men increases with older age (e.g., age > 70 years). Previous studies have shown that androgens reduce the development of experimental SS. These observations suggest that androgens may play an immunosuppressive and “protective” role for SS in young men [14, 15].

Women with primary and secondary SS have low levels of testosterone. The function of the meibomian glands and the secretion of tears are regulated in part by androgens. Thus, androgen deficiency might be associated with eye dryness [16]. Meibomian gland tissue contains androgen receptor mRNA, androgen receptor protein within acinar epithelial cell nuclei, and types 1 and 2 5[alpha]-reductase mRNAs. Androgens deficiency may lead to meibomian gland dysfunction, altered lipid profiles in meibomian gland secretions, tear film instability, and evaporative dry eye [17].

Activation of the cysteine-rich secretory protein-3 (CRISP-3) gene has been identified in the minor salivary glands of women with SS. This gene is expressed under normal conditions in the testicles and salivary glands of men, and is regulated by androgens. The CRISP-3 gene is important in cellular growth, differentiation, proliferation, and apoptosis. The finding of CRISP-3 gene activation and expression by mononuclear cells infiltrating the labial minor salivary glands of SS women patients was unexpected, especially in light of the androgen deficiency that is characteristic of pSS. Activation of the CRISP-3 gene within the minor salivary glands of pSS patients is consistent with an immune-neuroendocrine molecular communication [18].

The serum concentration of androgens correlates with parameters of SS activity such as serum IgG concentrations, the ESR, and inflammatory infiltrates within minor salivary glands. In contrast, negative correlations exist between dihydrotestosterone and C-reactive protein concentrations. No correlation has been reported between estrogen concentrations and pSS disease activity, but higher levels of disease activity are associated with increasing testosterone concentrations [19].

These controversial results had led to other investigations. In fact, only DHEA and DHEAS but not testosterone are affected in SS. Low serum levels of DHEAS in pSS might be the common denominator and a shared pathogenetic factor contributing to salivary gland involvement in pSS or sSS. Thus, androgen levels, normally low in women at the time of menopause and adrenopause, are decreased in SS patients compared with ageand sex-matched healthy controls [8]. In support of this hypothesis, it has been demonstrated that the epithelial cells of the healthy human labial salivary glands house a well-organized intracrine machinery capable of converting the DHEAS prohormone to its most active sex steroid metabolites, dihydrotestosterone and 17b-estradiol. In contrast, the salivary glands of patients with SS demonstrate derangements of the intracrine enzymes and a deficiency of androgens. This dysfunction, together with low serum levels of DHEAS, may explain the local androgen deficiency observed in salivary glands of patients with SS [20]. In support to these findings, the expression of CRISP-3 within glandular tissue and salivary in pSS patients is weak, indicating that the expression level of DHEA-regulated CRISP-3 is pathologically low in association with low salivary levels of DHEA [21] (Fig. 23.1). Moreover, the upregulation of integrins by androgens in tubuloepithelial cells and in labial salivary glands in SS is defective, also contributing to defective outside-in signaling, acinar atrophy, and ductal cell hyperplasia [22]. Other studies confirm that women with SS are androgen-deficient [23].

Estrogens increase the inflammatory infiltrate in salivary glands of normal mice in a manner that is indistinguishable from human SS [24]. In contrast, in another murine model for SS, estrogen deficiency was associated with inflammation and destruction of salivary and lacrimal glands. These effects, which appeared to be mediated by functional alterations of T lymphocytes, disappeared with estrogen treatment. The dysfunction of regulatory T cells by estrogen deficiency may play a crucial role on acceleration of autoimmune lesions. A murine model of SS suggested that Fas-mediated apoptosis controls the action of estrogens on epithelial cells [25]. One model of estrogen deficiency in rodents is the aromataseknockout (ArKO) mouse. These animals have an elevated B lymphopoiesis in bone marrow and develop a severe autoimmune exocrinopathy that resembles SS [26].

In humans, a possible role for estrogen in the induction or acceleration of SS has been described. Some patients who received estrogen therapy developed full-blown SS 3 years after starting the treatment [27]. It is of interest that normal salivary epithelium constitutively expresses the same functional estrogen receptors that appear to mediate immunomodulatory effects [28].