The immune system as a regulator of thyroid hormone activity

Inhaling Benzene, Toluene, Nitrogen Dioxide, and Sulfur Dioxide, Disrupts Thyroid Function in Captive American Kestrels (Falco sparverius)

Research investigating the effects of air contaminants on biota has been limited to date. Captive adult female American kestrels (Falco sparverius) were exposed to a mixture of benzene (0.6 ppm), toluene (1 ppm), nitrogen dioxide (NO₂; 2 ppm) and sulfur dioxide (SO₂; 5.6 ppm), in a whole-body inhalation chamber. Thyroid axis responses to meet metabolic demands were examined through thyroid histology, plasma thyroxine (T₄), and triiodothyronine (T₃), and hepatic outer ring deiodination (T₄-ORD). Plasma free (F) T₃ and T₄ were measured at baseline, and at 9 days and 18 days of exposure, whereas total (T) T₃ and TT₄, thyroid histology and hepatic T₄-ORD were determined at the final 18 day exposure. Inhalation of these contaminants significantly suppressed plasma FT₄ and TT₄, and depleted follicular colloid and increased epithelial cell height at 18 days, and significantly altered the temporal pattern of plasma FT₄. Significant histological changes in the follicular colloid:epithelial cell height ratio indicated sustained T₄ production and release by the thyroid glands. There was no effect on plasma FT₃, TT₃, or hepatic T₄-ORD. We hypothesize that contaminant-related activation of the hypothalamus-pituitary-thyroid axis in the kestrels increased elimination of plasma T₄ through Phase II enzymes. Further research is required to test this hypothesis in wild birds.

MECHANISMS IN ENDOCRINOLOGY: Impact of isolated TSH levels in and out of normal range on different tissues

Routine treatment of thyroid cancer (TC) includes long-term suppression of TSH. The necessity of this treatment in low- and intermediate-risk patients as well as the extent of TSH suppression is currently under discussion. A literature search was performed to illustrate the role of TSH in extrathyroidal cells and to identify potential reasons for different effects of exogenously suppressed and endogenously low TSH levels. Although adverse effects of subnormal and supranormal TSH blood levels on heart and brain have not been consistently found, studies show a clear negative effect of suppressed TSH levels on bone mineral density. Experimental data also support an important role of TSH in the immune system. The ability of levothyroxine (l-T4) to regulate TSH levels and triiodothyronine levels in a physiological manner is limited. Reduction of circadian changes in TSH levels, decrease of thyroid hormone-binding proteins, prevention of potential compensatory increases of TSH levels (e.g., in old age), and unresponsiveness of TSH-producing cells to TRH on l-T4 treatment might cause adverse effects of suppressed TSH levels. In view of the adverse effects of aggressive TSH suppression, achieving the suggested levels of TSH between 0.9 and 1 mU/l in the treatment of low-to-intermediate risk TC patients appears justified.

Splenic Leukocytes Traffic to the Thyroid and Produce a Novel TSHβ Isoform during Acute Listeria monocytogenes Infection in Mice

The thyroid stimulating hormone beta-subunit (TSHβ) with TSHα form a glycoprotein hormone that is produced by the anterior pituitary in the hypothalamus-pituitary-thyroid (HPT) axis. Although TSHβ has been known for many years to be made by cells of the immune system, the role of immune system TSH has remained unclear. Recent studies demonstrated that cells of the immune system produce a novel splice variant isoform of TSHβ (TSHβv), but little if any native TSHβ. Here, we show that within three days of systemic infection of mice with Listeria monocytogenes, splenic leukocytes synthesized elevated levels of TSHβv. This was accompanied by an influx of CD14⁺, Ly6C⁺, Ly6G⁺ cells into the thyroid of infected mice, and increased levels of intrathyroidal TSHβv gene expression. Adoptive transfer of carboxyfluorescein succinimidyl ester (CFSE)-labeled splenic leukocytes from infected mice into non-infected mice migrated into the thyroid as early as forty-eight hours post-cell transfer, whereas CFSE-labeled cells from non-infected mice failed to traffic to the thyroid. These findings demonstrate for the first time that during bacterial infection peripheral leukocytes produce elevated levels of TSHβv, and that spleen cells traffic to the thyroid where they produce TSHβv intrathyroidally.

Functional human TSHβ splice variant produced by plasma cell may be involved in the immunologic injury of thyroid in the patient with Hashimoto's thyroiditis

Hashimoto's Thyroiditis (HT) is the most common cause of hypothyroidism in areas of the world where iodine levels are sufficient. However, the pathogenesis of HT has not been completely elucidated. The first functional human TSHβ splice variant was supposed to be involved in the pathology of Hashimoto's thyroiditis. The question remains as to which kind of intrathyroid cells expresses functional TSHβ splice variant and whether there are expression variations of functional TSHβ splice variant in the injured thyroid of patient with HT. For the answer to this question, immune-injured thyroids were obtained from 30 patients with HT. Localization study of functional TSHβ splice variant in injured thyroid was done by immunofluorescence double staining. Transcription and translation level of functional TSHβ splice variant were detected by using qRT-PCR and semi-quantitative immunohistochemistry method, respectively. The correlation between expression level of functional TSHβ splice variant and degree of thyroid follicles damage was assessed. It was firstly identified that functional TSHβ splice variant was predominately expressed by plasma cells infiltrated around follicles and germinal center in injured thyroid of patient with HT. Of particular interest, the TSHβ splice variant was expressed at significantly higher levels in the thyroid tissues of patients with HT than that in the normal thyroid tissues, furthermore, expression level of TSHβ splice variant was positive related with the degree of follicles damage in thyroid of patient with HT. These findings defined the immune-derived functional TSHβ splice variant that resided in the thyroid of patient with HT, which exerted the unique effects on the pathogenesis of HT, meanwhile, we considered these findings to have significant implications for understanding immune-endocrine interactions in a number of ways.

Molecular basis of thyrotropin and thyroid hormone action during implantation and early development

BACKGROUND

Implantation and early embryo development are finely regulated processes in which several molecules are involved. Evidence that thyroid hormones (TH: T4 and T3) might be part of this machinery is emerging. An increased demand for TH occurs during gestation, and any alteration in maternal thyroid physiology has significant implications for both maternal and fetal health. Not only overt but also subclinical hypothyroidism is associated with infertility as well as with obstetric complications, including disruptions and disorders of pregnancy, labor, delivery, and troubles in early neonatal life.

METHODS

We searched the PubMed and Google Scholar databases for articles related to TH action on ovary, endometrium, trophoblast maturation and embryo implantation. In addition, articles on the regulation of TH activity at cellular level have been reviewed. The findings are hereby summarized and critically discussed.

RESULTS

TH have been shown to influence endometrial, ovarian and placental physiology. TH receptors (TR) and thyrotropin (thyroid-stimulating hormone: TSH) receptors (TSHR) are widely expressed in the feto-maternal unit during implantation, and both the endometrium and the trophoblast might be influenced by TH either directly or through TH effects on the synthesis and activity of implantation-mediating molecules. Interestingly, due to the multiplicity of mechanisms involved in TH action (e.g. differential expression of TR isoforms, heterodimeric receptor partners, interacting cellular proteins, and regulating enzymes), the TH concentration in blood is not always predictive of their cellular availability and activity at both genomic and nongenomic level.

CONCLUSIONS

In addition to the known role of TH on the hormonal milieu of the ovarian follicle cycle, which is essential for a woman's fertility, evidence is emerging on the importance of TH signaling during implantation and early pregnancy. Based on recent observations, a local action of TH on female reproductive organs and the embryo during implantation appears to be crucial for a successful pregnancy. Furthermore, an imbalance in the spatio-temporal expression of factors involved in TH activity might induce early arrest of pregnancy in women considered as euthyroid, based on their hormonal blood concentration. In conclusion, alterations of the highly regulated local activity of TH may play a crucial, previously underestimated, role in early pregnancy and pregnancy loss. Further studies elucidating this topic should be encouraged.

Thyroid signaling in immune organs and cells of the teleost fish rainbow trout (Oncorhynchus mykiss)

Thyroid hormones are involved in modulating the immune system in mammals. In contrast, there is no information on the role played by these hormones in the immune system of teleost fish. Here we provide initial evidence for the presence of active thyroid signaling in immune organs and cells of teleosts. We demonstrate that immune organs (head kidney and spleen) and isolated leukocytes (from head kidney and peripheral blood) of the rainbow trout (Oncorhynchus mykiss) express both thyroid receptor α (THRA) and β (THRB). Absolute mRNA levels of THRA were significantly higher than those of THRB. THRA showed higher expression in immune organs and isolated immune cells compared to the reference organ, liver, while THRB showed the opposite. In vivo exposure of trout to triiodothryronine (T3) or the anti-thyroid agent propylthiouracil (PTU) altered THR expression in immune organs and cells. Effect of T3 and PTU over the relative expression of selected marker genes of immune cell subpopulations was also studied. Treatments changed the relative expression of markers of cytotoxic, helper and total T cells (cd4, cd8a, trb), B lymphocytes (mIgM) and macrophages (csf1r). These findings suggest that the immune system of rainbow trout is responsive to thyroid hormones.

Pain and thyroid hormones

The role of endocrine systems in chronic pain mechanisms is slowly getting increasing experimental and clinical consideration. Many painful conditions appear to be directly and/or indirectly induced, reduced or, in some cases, modulated by hormones. We have done much work in trying to understand the relationship between hormones and pain, with particular attention to the hypothalamus-pituitary-gonadal axis. To expand our knowledge of this field, we have directed our attention to another axis, the hypothalamus-pituitary-thyroid (HPT). The literature on thyroid functions is vast but very few studies have focused on the HPT axis and pain. The few available data are considered in the present review to stimulate interest in the possible interactions between the HPT axis and pain.

Iodine deficiency and subclinical hypothyroidism are common in cystic fibrosis patients

BACKGROUND

Disorders of thyroid function have been inconsistently described in cystic fibrosis (CF) patients and in CF transmembrane regulator protein knockout animals. The literature lacks reports on iodine status of CF individuals. We hypothesize, that iodine deficiency is common in CF and account for abnormal thyroid function in CF patients.

METHODS

We investigated 129 children, adolescents, and adults with CF, who were living in the northern part of Bavaria/Germany. Malnutrition and lung function were analyzed. Urinary iodine excretion, TSH (thyroid-stimulating hormone), and ft4 (free thyroxine) were measured and set in relation to population-based, age-adjusted reference ranges.

RESULTS

Subclinical hypothyroidism (normal fT4, elevated TSH) was found in 11.6% of subjects, and iodine deficiency in 83.7%. No correlations were found with age, BMI, status of malnutrition, or lung function.

CONCLUSION

Dramatic iodine deficiency was found in our cohort of CF patients. This condition can cause subclinical hypothyroidism; therefore, an individual iodine supplementation program is necessary and should be started immediately.

Nongenomic effects of thyroid hormones on the immune system cells: New targets, old players

It is now widely accepted that thyroid hormones, l-thyroxine (T(4)) and 3,3',5-triiodo-l-thyronine (T(3)), act as modulators of the immune response. Immune functions such as chemotaxis, phagocytosis, generation of reactive oxygen species, and cytokine synthesis and release, are altered in hypo- and hyper-thyroid conditions, even though for many immune cells no clear correlation has been found between altered levels of T(3) or T(4) and effects on the immune responses. Integrins are extracellular matrix proteins that are important modulators of many cellular responses, and the integrin αvβ3 has been identified as a cell surface receptor for thyroid hormones. Rapid signaling via this plasma membrane binding site appears to be responsible for many nongenomic effects of thyroid hormones, independent of the classic nuclear receptors. Through the integrin αvβ3 receptor the hormone can activate both the ERK1/2 and phosphatidylinositol 3-kinase pathways, with downstream effects including intracellular protein trafficking, angiogenesis and tumor cell proliferation. It has recently become clear that an important downstream target of the thyroid hormone nongenomic pathway may be the mammalian target of rapamycin, mTOR. New results demonstrate the capability of T(3) or T(4) to induce in the short time range important responses related to the immune function, such as reactive oxygen species production and cell migration in THP-1 monocytes. Thus thyroid hormones seem to be able to modulate the immune system by a combination of rapid nongenomic responses interacting with the classical nuclear response.

Extrapituitary production of anterior pituitary hormones: an overview

Protein hormones from the anterior pituitary gland have well-established endocrine roles in their peripheral target glands. It is, however, now known that these proteins are also produced within many of their target tissues, in which they act as local autocrine or paracrine factors, with physiological and/or pathophysiological significance. This emerging concept is the focus of this brief review.

Pathogenesis of infertility and recurrent pregnancy loss in thyroid autoimmunity

Thyroid autoimmunity is the most prevalent autoimmune state that affects up to 4% of women during the age of fertility. A growing body of clinical studies links thyroid autoimmunity as a cause of infertility and adverse pregnancy outcomes that includes miscarriage or preterm deliveries. Importantly, these adverse effects are persistent in euthyroid women. In the current review we elaborate on the pathogenesis that underlies infertility and increased pregnancy loss among women with autoimmune thyroid disease. Such mechanisms include thyroid autoantibodies that exert their effect in a TSH-dependent but also in a TSH-independent manner. The later includes quantitative and qualitative changes in the profile of endometrial T cells with reduced secretion of IL-4 and IL-10 along with hypersecretion of interferon-γ. Polyclonal B cells activation is 2-3 time more frequent in thyroid autoimmunity and is associated with increased titers of non-organ specific autoantibodies. Hyperactivity and Increased migration of cytotoxic natural killer cells that alter the immune and hormonal response of the uterus is up to 40% more common in women with thyroid autoimmunity. Lack of vitamin D was suggested as a predisposing factor to autoimmune diseases, and was shown to be reduced in patients with thyroid autoimmunity. In turn, its deficiency is also linked to infertility and pregnancy loss, suggesting a potential interplay with thyroid autoimmunity in the context of infertility. In addition, thyroid autoantibodies were also suggested to alter fertility by targeting zona pellucida, human chorionic gonadotropin receptors and other placental antigens.

Seasonal variation in plasma thyroid hormone concentrations in coastal versus inland populations of juvenile American alligators (Alligator mississippiensis): influence of plasma iodide concentrations

Thyroid hormones, essential for normal growth and health, are associated with changes in temperature, photoperiod, and reproduction. Iodide, a necessary element for thyroid hormone production, varies in diet, and is more abundant in estuarine environments, which could alter thyroid hormone variation. However, associations between thyroid hormone concentrations in animals from marine versus freshwater environments, which could become more pertinent with rising sea levels associated with global climate change, are not well studied. To determine the importance of dietary iodide in seasonal variation of plasma thyroid hormone concentrations, we analyzed seasonal variation of plasma thyroxine (T(4)) and triiodothyronine (T(3)) concentrations in juvenile alligators from an estuarine habitat (Merritt Island National Wildlife Refuge; MI) and a freshwater habitat (Lake Woodruff National Wildlife Refuge; LW) and compared these results to plasma inorganic iodide (PII) concentrations. Alligators from MI did not display seasonal variation in plasma T(4), but exhibited a seasonal pattern in plasma T(3) concentrations similar to alligators from LW. Plasma thyroid hormone concentrations were consistently higher at MI than at LW. PII concentrations were correlated with plasma T(4) and T(3) concentrations in juvenile alligators from LW but not MI. The data on plasma T(4) and T(3) concentrations suggest altered iodide metabolism in estuarine alligators. Differences in thyroid hormone concentrations between the populations could be due to differences in dietary iodide, which need to be further evaluated.

Thyroid hormones as modulators of immune activities at the cellular level

BACKGROUND

Increasing evidence suggests that thyroid hormones, L-thyroxine (T(4)) and 3,3',5-triiodo-L-thyronine (T(3)), are modulators of the immune response. In monocytes, macrophages, leukocytes, natural killer cells, and lymphocytes, a wide range of immune functions such as chemotaxis, phagocytosis, generation of reactive oxygen species (ROS), and cytokine synthesis and release are altered under hypo- and hyperthyroid conditions.

SUMMARY

Hyperthyroidism decreases the proinflammatory activities of monocytes and macrophages, whereas enhancement of phagocytosis and increased levels of ROS may occur during hypothyroidism. The expression of proinflammatory molecules such as macrophage inflammatory protein-1α and interleukin-1β increases in hypothyroidism. However, in Kupffer cells, proinflammatory activities such as the respiratory burst, nitric oxide synthase activity, and tumor necrosis factor-α expression may result from increased T(3) levels. Thyroid hormones also affect natural killer cell activity and cell-mediated immune responses. Still, for many immune cells no clear correlation has been found so far between abnormally high or low T(3) or T(4) levels and the effects observed on the immune responses.

CONCLUSIONS

In this review we outline the contributions of thyroid hormones to different aspects of innate and adaptive immune responses. The relationship between thyroid hormones and immune cells is complex and T(3) and T(4) may modulate immune responses through both genomic and nongenomic mechanisms. Future studies of the molecular signaling mechanisms involved in this cross-talk between thyroid hormones and the immune system may support development of new strategies to improve clinical immune responses.

A novel thyroid stimulating hormone beta-subunit isoform in human pituitary, peripheral blood leukocytes, and thyroid

Thyroid stimulating hormone (TSH) is produced by the anterior pituitary and is used to regulate thyroid hormone output, which in turn controls metabolic activity. Currently, the pituitary is believed to be the only source of TSH used by the thyroid. Recent studies in mice from our laboratory have identified a TSHbeta isoform that is expressed in the pituitary, in peripheral blood leukocytes (PBL), and in the thyroid. To determine whether a human TSHbeta splice variant exists that is analogous to the mouse TSHbeta splice variant, and whether the pattern of expression of the splice variant is similar to that observed in mice, PCR amplification of RNAs from pituitary, thyroid, PBL, and bone marrow was done by reverse-transcriptase PCR and quantitative realtime PCR. Human pituitary expressed a TSHbeta isoform that is analogous to the mouse TSHbeta splice variant, consisting of a 27 nucleotide portion of intron 2 and all of exon 3, coding for 71.2% of the native human TSHbeta polypeptide. Of particular interest, the TSHbeta splice variant was expressed at significantly higher levels than the native form or TSHbeta in PBL and the thyroid. The TSHalpha gene also was expressed in the pituitary, thyroid, and PBL, but not the BM, suggesting that the TSHbeta polypeptide in the thyroid and PBL may exist as a dimer with TSHalpha. These findings identify an unknown splice variant of human TSHbeta. They also have implications for immune-endocrine interactions in the thyroid and for understanding autoimmune thyroid disease from a new perspective.

Virus infection activates thyroid stimulating hormone synthesis in intestinal epithelial cells

The small intestine has been shown to be an extra-pituitary site of thyroid stimulating hormone (TSH) production, and previous in vivo studies have shown that TSH synthesis localizes within areas of enteric virus infection within the small intestine; however, the cellular source of intestinal TSH has not been adequately determined. In the present study, we have used the murine MODE-K small intestinal epithelial cell line to demonstrate both at the transcriptional level and as a secreted hormone, as measured in a TSHbeta-specific enzyme-linked assay, that epithelial cells in fact respond to infection with reovirus serotype 3 Dearing strain by upregulating TSH synthesis. Moreover, sequence analysis of a PCR-amplified TSHbeta product from MODE-K cells revealed homology to mouse pituitary TSHbeta. These findings have direct functional implications for understanding a TSH immune-endocrine circuit in the small intestine.

The thyrotropin-releasing hormone (TRH)-immune system homeostatic hypothesis

Decades of research have established that the biological functions of thyrotropin-releasing hormone (TRH) extend far beyond its role as a regulator of the hypothalamic-pituitary-thyroid axis. Gary et al. [Gary, K.A., Sevarino, K.A., Yarbrough, G.G., Prange, A.J. Jr., Winokur, A. (2003). The thyrotropin-releasing hormone (TRH) hypothesis of homeostatic regulation: implications for TRH-based therapeutics. J Pharmacol Exp Ther 305(2):410-416.] and Yarbrough et al. [Yarbrough, G.G., Kamath, J., Winokur, A., Prange, A.J. Jr. (2007). Thyrotropin-releasing hormone (TRH) in the neuroaxis: therapeutic effects reflect physiological functions and molecular actions. Med Hypotheses 69(6):1249-1256.] provided a functional framework, predicated on its global homeostatic influences, to conceptualize the numerous interactions of TRH with the central nervous system (CNS) and endocrine system. Herein, we profer a similar analysis to interactions of TRH with the immune system. Autocrine/paracrine cellular signaling motifs of TRH and TRH receptors are expressed in several tissues and organs of the immune system. Consistent with this functional distribution, in vitro and in vivo evidence suggests a critical role for TRH during the developmental stages of the immune system as well as its numerous interactions with the fully developed immune system. Considerable evidence supports a pivotal role for TRH in the pathophysiology of the inflammatory process with specific relevance to the "cytokine-induced sickness behavior" paradigm. These findings, combined with a number of documented clinical actions of TRH strongly support a potential utility of TRH-based therapeutics in select inflammatory disorders. Similar to its global role in behavioral and energy homeostasis a homeostatic role for TRH in its interactions with the immune system is consonant with the large body of available data. Recent advances in the field of immunology provide a significant opportunity for investigation of the TRH-immune system homeostatic hypothesis. Moreover, this hypothesis may provide a foundation for the development of TRH-based therapeutics for certain medical and psychiatric disorders involving immune dysfunction.

Neuroprotective gene profile in the brain of magnesium-deficient mice

BACKGROUND

Magnesium (Mg) deficiency may lead to serious metabolic, biological and organic dysfunctions, and cause various clinical disorders. In the current study, we explore endothelial cell activation, inflammation and cell death induced in the brain of adult mice by Mg-deficient diet.

METHODS AND RESULTS

Neither TNFalpha, substance P, sTNFRI, sTNFRII proteins (ELISA), nor TNFalpha, adherence molecules and prolactin mRNAs, nor NK1R (immunohistochemistry on brain sections) were up-regulated. No inflammatory infiltrates and no apoptotic cells were observed. Using cDNA assay, we showed a neuroprotective, anti-apoptotic and neurotrophic gene expression profile in the brain at early stage of hypomagnesemia. As a model for neuronal injury, mild sound stimulation of Mg-deficient mice without convulsive seizures triggers neither the release of substance P, nor the development of an inflammatory process or cell death in the brain.

CONCLUSION

Our results suggest that Mg-deficiency in mice favours the development of a neuroprotective environment in the brain.

Infection, immunity, and hormones/endocrine interactions

Infections and stress, immune responses, and hormones are interconnected, ensuring immune competence to deal with immediate threat of overwhelming infection and metabolic collapse. Emergence of cytokines as key signal mediators and appreciation of autocrine-paracrine influences of hormones have helped explain how signals are transmitted and responses evoked. This has led to possibilities of creating therapies that might be used to enhance protective signals and dampen signals emanating from host and invading organism interaction that might otherwise be detrimental. Correcting certain metabolic abnormalities, such as hyperglycemia and metabolic acidosis, benefits the host by decreasing morbidity and mortality.

Estrogen replacement regimen and brain infusion of lipopolysaccharide differentially alter steroid receptor expression in the uterus and hypothalamus

The regimen of estrogen replacement can alter the consequences of estrogen therapy and stressors. To determine the long-term effects and interaction of these systems on the brain and periphery, adult female rats were infused with lipopolysaccharide (LPS) into the fourth ventricle of the brain for 4 weeks, and ovariectomized rats were administered either constant or pulsed regimens of estrogen replacement (17beta-estradiol) until sacrifice at 8 weeks. Constant, but not pulsed, estrogen replacement reduced ERalpha and increased HSP90, HSP70, and PR(B) uterine protein levels. Both estrogen regimens increased ERbeta, HSP27, and PR(A) uterine proteins. Both regimens reduced hypothalamic levels of ERalpha, but not ERbeta, HSP, or PR. No changes were observed in the hippocampus. Long-term brain infusion of LPS activated microglia and reduced body weight, but did not alter corticosterone or nitrotyrosine levels. LPS infusion into intact rats suppressed uterine weight, increased ERalpha and decreased HSP90 in the uterus. LPS did not alter uterine weight in ovariectomized rats treated with constant or pulsed estrogen. Together, these data suggest the timing of estrogen replacement and neuroinflammatory stressors can profoundly affect uterine and hypothalamic steroid receptor expression and may be important parameters to consider in the post-menopausal intervention with estrogen.