The thymus-neuroendocrine axis: physiology, molecular biology, and therapeutic potential of the thymic peptide thymulin

Defining the identity and the niches of epithelial stem cells with highly pleiotropic multilineage potency in the human thymus

Thymus is necessary for lifelong immunological tolerance and immunity. It displays a distinctive epithelial complexity and undergoes age-dependent atrophy. Nonetheless, it also retains regenerative capacity, which, if harnessed appropriately, might permit rejuvenation of adaptive immunity. By characterizing cortical and medullary compartments in the human thymus at single-cell resolution, in this study we have defined specific epithelial populations, including those that share properties with bona fide stem cells (SCs) of lifelong regenerating epidermis. Thymic epithelial SCs display a distinctive transcriptional profile and phenotypic traits, including pleiotropic multilineage potency, to give rise to several cell types that were not previously considered to have shared origin. Using here identified SC markers, we have defined their cortical and medullary niches and shown that, in vitro, the cells display long-term clonal expansion and self-organizing capacity. These data substantively broaden our knowledge of SC biology and set a stage for tackling thymic atrophy and related disorders.

Thymodepressin-Unforeseen Immunosuppressor

The paper summarizes the available information concerning the biological properties and biomedical applications of Thymodepressin. This synthetic peptide drug displays pronounced immunoinhibitory activity across a wide range of conditions in vitro and in vivo. The history of its unforeseen discovery is briefly reviewed, and the current as well as potential expansion areas of medicinal practice are outlined. Additional experimental evidence is obtained, demonstrating several potential advantages of Thymodepressin over another actively used immunosuppressor drug, cyclosporin A.

Comparison of Thymulin Activity with Other Measures of Marginal Zinc Deficiency

Activity of the immunoregulatory peptide thymulin reflects differences in zinc status. This study compared thymulin activity with four other zinc status measures in rats fed zinc at either 5 or 25 ppm. Rats fed the lower zinc showed the following results compared with rats with adequate zinc intake: serum thymulin activity 61% lower, serum zinc 31% lower, serum extracellular superoxide dismutase 18% lower, serum 5'-nucleotidase activity 26% lower, and liver metallothionein 28% lower. Thus, thymulin activities showed more sensitivity to restricted zinc intake than did four other parameters.

Protective Effect of PBCA Nanoparticles Loaded with Thymulin Against the Relapsing-Remitting Form of Experimental Autoimmune Encephalomyelitis in Mice

Relapsing-remitting experimental autoimmune encephalomyelitis (rEAE) in mice is a model that closely resembles relapsing-remitting multiple sclerosis in humans. This study aims to investigate a new approach to modulation of the inflammatory response in rEAE mice using a thymic peptide thymulin bound to polybutylcyanoacrylate (PBCA) nanoparticles. PBCA nanoparticles were used to prolong the presence of thymulin in the blood. Cytokine levels in blood were measured by ELISA; NF-κB and SAPK/JNK cascade activation, as well as Hsp72 and p53 protein expression, were measured by Western blotting. Animal health statuses were estimated using severity scores. Results showed that the cytokine response in rEAE was multi-staged: an early phase was accompanied by an increase in plasma interferon-γ, while the interleukin (IL)-17 response was markedly increased at a later stage. The stages were attributed to rEAE induction and maintenance phases. Thymulin significantly alleviated symptoms of rEAE and lowered plasma cytokine levels both in early and later stages of rEAE, and decreased NF-κB and SAPK/JNK cascade activation. Thymulin modulated NF-kappaB pathway activity via site-specific phosphorylation of RelA/p65 protein (at Ser276 and Ser536). The effect of nanoparticle-bound thymulin was more pronounced than the effect of free thymulin. Therefore, PBCA-thymulin can be considered a prospective treatment for this pathology.

The Immunoendocrine Thymus as a Pacemaker of Lifespan

The thymus develops from an endocrine area of the foregut, and retains the ancient potencies of this region. However, later it is populated by bone marrow originated lymphatic elements and forms a combined organ, which is a central part of the immune system as well as an influential element of the endocrine orchestra. Thymus produces self-hormones (thymulin, thymosin, thymopentin, and thymus humoral factor), which are participating in the regulation of immune cell transformation and selection, and also synthesizes hormones similar to that of the other endocrine glands such as melatonin, neuropeptides, and insulin, which are transported by the immune cells to the sites of requests (packed transport). Thymic (epithelial and immune) cells also have receptors for hormones which regulate them. This combined organ, which is continuously changing from birth to senescence seems to be a pacemaker of life. This function is basically regulated by the selection of self-responsive thymocytes as their complete destruction helps the development (up to puberty) and their gradual release in case of weakened control (after puberty) causes the erosion of cells and intercellular material, named aging. This means that during aging, self-destructive and non-protective immune activities are manifested under the guidance of the involuting thymus, causing the continuous irritation of cells and organs. Possibly the pineal body is the main regulator of the pacemaker, the neonatal removal of which results in atrophy of thymus and wasting disease and its later corrosion causes the insufficiency of thymus. The co-involution of pineal and thymus could determine the aging and the time of death without external intervention; however, external factors can negatively influence both of them.

Effect of the herbal formulation Jianpijiedu on the TCRVβCDR3 repertoire in rats with hepatocellular carcinoma and subjected to food restriction combined with laxative

The aim of this study was to investigate the effects of the Chinese herbal formulation Jianpijiedu (JPJD) in a rat model of orthotopic hepatocellular carcinoma (OHC). The tumor-bearing rats underwent food restriction combined with laxative (FRL) treatment in order to model the nutritional and digestive symptoms of patients with hepatocellular carcinoma. In addition, the study aimed to elucidate the effect of JPJD on the T cell receptor Vβ-chain complementarity-determining region 3 (TCRVβCDR3) repertoire and the underlying mechanism. The FRL rat model was established by alternate-day food restriction and the oral administration of Glauber's salt (sodium sulfate), based on which the OHC model was then established. Subsequently, the FRL-OHC induced animals received JPJD or thymopentin-5 (TP5) for 17 days. Differences in the TCRVβCDR3 repertoire in the rat thymus, liver and hepatocellular carcinoma tissues were analyzed by polymerase chain reaction. Compared with the FRL-OHC model animals without any treatment, those treated with JPJD exhibited significantly inhibited hepatocellular carcinoma growth (P<0.05), reduced weight loss (P<0.01) and stable visceral indices (P<0.05). Furthermore, the JPJD treatment appeared to improve Simpsons diversity index (Ds) values and the quasi-Gaussian distribution rate of the TCRVβCDR3 repertoire in the thymus, liver and hepatocellular carcinoma tissues. However, no anti-hepatoma effects were evident in the rats treated with TP5. In addition, TP5 increased the Ds values and the quasi-Gaussian distribution rate of the TCRVβCDR3 repertoire in hepatocellular carcinoma tissues compared with those in the JPJD-treated group. The anti-hepatoma effects of JPJD in FRL-OHC-induced animals may be due to the promotion of the Ds values of the TCRVβCDR3 repertoire.

[THYMIC HORMONES, ANTIOXIDANT ENZYMES AND NEUROGENESIS OF BULBUS OLFACTORIUS IN RATS WITH PARKINSONISM: THE EFFECT OF MELATONIN]

The adult rats received both neurotoxin 6-hidroxidophamine and neurotoxin and melatonin. It was investigated a link between the disturbances of the brain antioxidant enzymes activity and thymic endocrine function, as possible pathogenic factors of parkinsonism, with changes in the number of neural stem cells (NSC) in the bulbus olfactorius. Rats with motor asymmetry in the apomorphine test and significant damage of the dopaminergic neurons in the-substantia nigra have decreased levels of superoxide dismutase, catalase and glutathione peroxidase activities in striatum (1.3-1.4 times) and blood thymulin content (8 times) compared to control group. On the contrary, examined indices were not changed in rats without motor asymmetry and correspondingly partly damaged neurons. The number of nestin(+)-cells in the bulbus olfactorius of rats without motor asymmetry increased from 91.2% to 99.3% and remained unchanged after melatonin administration course (10 mg/kg during 18 days). Melatonin administration resulted in the decrease in the number of nestin(+)-cells along with significant elevation of the decreased antioxidant enzymes activity and blood thymulin content in rats with circulatory movements. Possibilities of the enhancement of NSC differentiation in bulbus olfactorius into neuronal direction in such animals has been discussed. The conclusion about the potential use of melatonin as a neuroprotector in parkinsonism therapy has been made.

Effects of GnRH immunization on the reproductive axis and thymulin

The bidirectional regulation of thymulin in the reproductive-endocrine function of the hypothalamic-pituitary-gonadal (HPG) axis of rats immunized against GnRH remains largely unclear. We explored the alterations in hormones in the HPG axis in immunized rats to dissect the repressive effect of immunization on thymulin, and to clarify the interrelation of reproductive hormones and thymulin in vivo. The results showed that, in the first 2 weeks of booster immunization, thymulin was repressed when reproductive hormones were severely reduced. The self-feedback regulation of thymulin was then stimulated in later immune stages: the rising circulating thymulin upregulated LH and FSH, including GnRH in the hypothalamus, although the levels of those hormones were still significantly lower than in the control groups. In astrocytes, thymulin produced a feedback effect in regulated GnRH neurons. However, in the arcuate nucleus (Arc) and the median eminence (ME), the mediator of astrocytes and other glial cells were also directly affected by reproductive hormones. Thus, in immunized rats, the expression of glial fibrillary acidic protein was distinctly stimulated in the Arc and ME. This study demonstrated that thymulin was downregulated by immunization against GnRH in early stage. Subsequently, the self-feedback regulation was provoked by low circulating thymulin. Thereafter, rising thymulin levels promoted pituitary gonadotropins levels, while acting directly on GnRH neurons, which was mediated by astrocytes in a region-dependent manner in the hypothalamus.

Immunomodulation of Homeopathic Thymulin 5CH in a BCG-Induced Granuloma Model

The present study analyzed the immune modulation mechanisms of thymulin 5CH in a granuloma experimental model. Male adult Balb/c mice were inoculated with BCG into the footpad to induce granuloma, which was quantitatively evaluated. The phenotypic characterization of phagocyte, T- and B-lymphocyte populations in the peritoneum, and local lymph node was done by flow cytometry. During all experimental periods, thymulin 5CH and vehicle (control) were given ad libitum to mice, diluted into the drinking water (1.6 × 10⁻¹⁷ M). After 7 days from inoculation, thymulin-treated mice presented reduction in the number of epithelioid cytokeratine-positive cells (P = 0.0001) in the lesion, in relation to young phagocytes. After 21 days, the differentiation of B1 peritoneal stem cells into phagocytes reached the peak, being higher in thymulin-treated mice (P = 0.0001). Simultaneously, the score of infected phagocytes in the lesion decreased (P = 0.001), and the number of B1-derived phagocytes, CD4+ and CD8+ T lymphocytes in the local lymph node increased in relation to control (P = 0.0001). No difference was seen on the CD25+ Treg cells. The results show that thymulin 5CH treatment is able to improve the granuloma inflammatory process and the infection remission, by modulating local and systemic phagocyte differentiation.

Thymus hormones as prospective anti-inflammatory agents

IMPORTANCE OF THE FIELD

Inflammatory diseases are characterized by severe immune imbalances, leading to excessive or inappropriate release of mediators, which, in turn, result in massive damage to organs and systems. Effective means to control inappropriate immune reactions are often life-critical needs. Available data on the role of thymus-derived hormones in inflammation show their great potential.

AREAS COVERED IN THIS REVIEW

The review aims to systematize information for the last two decades on immune system regulation by thymic peptide hormones, with a primary focus on the role of these hormones in the systemic inflammatory response and inflammatory diseases. Anti-inflammatory potential of three thymic hormones - thymulin, thymosin-alpha, and thymopoietin - is discussed, reviewing recently published clinical and experimental studies.

WHAT THE READER WILL GAIN

Our analysis revealed the regulation of inflammatory processes via thymic hormones that could be prospective for therapeutic application. This regulation may be mediated through thymic hormone effects on peripheral immune cell activities and bidirectional coupling between thymic hormones and the hypothalamic-pituitary-adrenal axis.

TAKE-HOME MESSAGE

In view of the role of thymic hormones in immune and neuroendocrine systems, they could be suitable as therapeutic agents for inflammation.

Thymulin and zinc (Zn2+)-mediated inhibition of endotoxin-induced production of proinflammatory cytokines and NF-kappaB nuclear translocation and activation in the alveolar epithelium: unraveling the molecular immunomodulatory, anti-inflammatory effect of thymulin/Zn2+ in vitro

The immunomodulatory potential of thymulin and zinc (Zn(2+)) in the perinatal alveolar epithelium is not well characterized. In an in vitro model of fetal alveolar type II epithelial cells (FATEII), we have investigated the exhibition of an anti-inflammatory activity of this peptide hormone. Thymulin selectively ameliorated, in a dose-dependent manner, the endotoxin (ET/LPS [lipopolysaccharide])-induced release of IL-1beta, but not IL-6 or TNF-alpha. Furthermore, Zn(2+), an anti-inflammatory antioxidant, which is required for the biological activity of thymulin, independently reduced the secretion of IL-1beta, TNF-alpha and, to a lesser extent, at a supraphysiologic dose (1 mM), IL-6. The underlying cellular and molecular pathways associated with the anti-inflammatory effect of thymulin and Zn(2+) in the alveolar epithelium are not well established. Further in this study, the role of cyclic AMP (cAMP) in the anti-inflammatory effect of thymulin was investigated, in addition to unraveling the possible involvement of the NF-kappaB pathway. Interestingly, thymulin upregulated, in a dose- and time-dependent manner, the release of the nucleotide cAMP. To understand whether the inhibitory effect of thymulin on cytokine release is cAMP-dependent, Forskolin, a labdane diterpene known to elevate intracellular cAMP, was shown to reduce the secretion of IL-1beta and TNF-alpha, but not IL-6, an effect mimicked by dibutyryl-cAMP (dbcAMP), an analog of cAMP. Alveolar epithelial cells treated with thymulin markedly showed a downregulation of the nuclear translocation of RelA (p65), the major transactivating member of the NF-kappaB family, in addition to NF-kappaB(1) (p50) and c-Rel (p75), an effect mildly substantiated with Zn(2+). Furthermore, thymulin/Zn(2+) reduced, in a dose-dependent manner, the DNA-binding activity of NF-kappaB (RelA/p65). These results indicate that the anti-inflammatory effect of thymulin, which is mediated by cAMP, is NF-kappaB-dependent and involves the downregulation of the release of proinflammatory cytokines, particularly IL-1beta, an effect synergistically amplified, at least in part, by Zn(2+). The molecular regulation of thymulin via a NF-kappaB-dependent pathway is critical to understanding the anti-inflammatory alleviating role of this nonapeptide in regulating proinflammatory signals.

Thymus as a target tissue of glucocorticoid action: what are the consequences of glucocorticoids thymectomy?

Glucocorticoids represent the most powerful endogenous anti-inflammatory and immunosuppressive effectors, interfering with virtually every step of immunoinflammatory responses. Glucocorticoids are often the most effective therapy in the prevention or suppression of inflammation and other immunologically mediated processes, but their use is limited by systemic side effects induced by the over-production of reactive oxygen species, causing dysregulation of physiological processes. The thymus is an organ with both endocrine and immune functions. Glucocorticoids induce thymocyte apoptosis, causing a profound reduction in thymic mass and volume and inducing hormonal thymectomy. The clinical aspects of glucocorticoid thymectomy are not under enough investigation. These unwanted systemic side effects may be the consequence of prolonged therapeutic application of glucocorticoids and prolonged or chronic activation of the hypothalamic-pituitary adrenal axis, which may lead to increased and prolonged secretion of glucocorticoids. This review will discuss the metabolic effects of glucocorticoids in the context of thymic physiology asthe primary sex hormone-responsive organ.