Thymic hormone-containing cells VI. Immunohistologic evidence for the simultaneous presence of thymulin, thymopoietin and thymosin alpha 1 in normal and pathological human thymuses

Thymus-derived hormonal and cellular control of cancer

The thymus gland is a central lymphoid organ in which developing T cell precursors, known as thymocytes, undergo differentiation into distinct type of mature T cells, ultimately migrating to the periphery where they exert specialized effector functions and orchestrate the immune responses against tumor cells, pathogens and self-antigens. The mechanisms supporting intrathymic T cell differentiation are pleiotropically regulated by thymic peptide hormones and cytokines produced by stromal cells in the thymic microenvironment and developing thymocytes. Interestingly, in the same way as T cells, thymic hormones (herein exemplified by thymosin, thymulin and thymopoietin), can circulate to impact immune cells and other cellular components in the periphery. Evidence on how thymic function influences tumor cell biology and response of patients with cancer to therapies remains unsatisfactory, although there has been some improvement in the knowledge provided by recent studies. Herein, we summarize research progression in the field of thymus-mediated immunoendocrine control of cancer, providing insights into how manipulation of the thymic microenvironment can influence treatment outcomes, including clinical responses and adverse effects of therapies. We review data obtained from clinical and preclinical cancer research to evidence the complexity of immunoendocrine interactions underpinning anti-tumor immunity.

Thymus, undernutrition, and infection: Approaching cellular and molecular interactions

Undernutrition remains a major issue in global health. Low protein-energy consumption, results in stunting, wasting and/or underweight, three deleterious forms of malnutrition that affect roughly 200 million children under the age of five years. Undernutrition compromises the immune system with the generation of various degrees of immunodeficiency, which in turn, renders undernourished individuals more sensitive to acute infections. The severity of various infectious diseases including visceral leishmaniasis (VL), influenza, and tuberculosis is associated with undernutrition. Immunosuppression resulting from protein-energy undernutrition severely impacts primary and secondary lymphoid organs involved in the response to related pathogens. The thymus—a primary lymphoid organ responsible for the generation of T lymphocytes—is particularly compromised by both undernutrition and infectious diseases. In this respect, we will discuss herein various intrathymic cellular and molecular interactions seen in undernutrition alone or in combination with acute infections. Many examples illustrated in studies on humans and experimental animals clearly revealed that protein-related undernutrition causes thymic atrophy, with cortical thymocyte depletion. Moreover, the non-lymphoid microenvironmental compartment of the organ undergoes important changes in thymic epithelial cells, including their secretory products such as hormones and extracellular matrix proteins. Of note, deficiencies in vitamins and trace elements also induce thymic atrophy. Interestingly, among the molecular interactions involved in the control of undernutrition-induced thymic atrophy is a hormonal imbalance with a rise in glucocorticoids and a decrease in leptin serum levels. Undernutrition also yields a negative impact of acute infections upon the thymus, frequently with the intrathymic detection of pathogens or their antigens. For instance, undernourished mice infected with Leishmania infantum (that causes VL) undergo drastic thymic atrophy, with significant reduction in thymocyte numbers, and decreased levels of intrathymic chemokines and cytokines, indicating that both lymphoid and microenvironmental compartments of the organ are affected. Lastly, recent data revealed that some probiotic bacteria or probiotic fermented milks improve the thymus status in a model of malnutrition, thus raising a new field for investigation, namely the thymus-gut connection, indicating that probiotics can be envisioned as a further adjuvant therapy in the control of thymic changes in undernutrition accompanied or not by infection.

Histochemical and molecular overview of the thymus as site for T-cells development

The thymus represents the primary site for T cell lymphopoiesis, providing a coordinated set for critical factors to induce and support lineage commitment, differentiation and survival of thymus-seeding cells. One irrefutable fact is that the presence of non-lymphoid cells through the thymic parenchyma serves to provide coordinated migration and differentiation of T lymphocytes. Moreover, the link between foetal development and normal anatomy has been stressed in this review. Regarding thymic embryology, its epithelium is derived from the embryonic endodermal layer, with possible contributions from the ectoderm. A series of differentiating steps is essential, each of which must be completed in order to provide the optimum environment for thymic development and function. The second part of this article is focused on thymic T-cell development and differentiation, which is a stepwise process, mediated by a variety of stromal cells in different regions of the organ. It depends strongly on the thymic microenvironment, a cellular network formed by epithelial cells, macrophages, dendritic cells and fibroblasts, that provide the combination of cellular interactions, cytokines and chemokines to induce thymocyte precursors for the generation of functional T cells. The mediators of this process are not well defined but it has been demonstrated that some interactions are under neuroendocrine control. Moreover, some studies pointed out that reciprocal signals from developing T cells also are essential for establishment and maintenance of the thymic microenvironment. Finally, we have also highlighted the heterogeneity of the lymphoid, non-lymphoid components and the multi-phasic steps of thymic differentiation. In conclusion, this review contributes to an understanding of the complex mechanisms in which the foetal and postnatal thymus is involved. This could be a prerequisite for developing new therapies specifically aimed to overcome immunological defects, linked or not-linked to aging.

Thymic reticulo-epithelial cells

The reticulo-epithelial (RE) cellular network of the thymic stromal cellular microenvironment plays a vital role in neuroendocrine regulation and lymphoid cell homing and development. Transmission electronmicroscopic observations have confirmed that there are four functional subtypes of medullar RE cells: undifferentiated; squamous; villous; and cystic. Immunocytochemical observations have shown that the secreted thymic hormones, thymosin alpha1 and thymopoietin (and its short form, thymopentin or TP5), are both produced by RE cells. Thymic RE cells also produce numerous cytokines, including IL-1 and -6, G-CSF, macrophage-CSF and GM-CSF that likely are important during the various stages of thymocyte activation and differentiation. The coexistence of pituitary hormone and neuropeptide secretion, such as growth hormone, prolactin, adrenocorticotopic hormone and thyroid-stimulating hormone, among many others, and the production of a number of interleukins and growth factors, as well as the expression of receptors for all, by the same RE cell, is an unique molecular biological phenomenon. The thymic RE cell network represents an important cellular and humoral microenvironment in the neuroendocrine homeopathic regulatory mechanisms of the multicellular organism.

Thymic epithelial cells in age-dependent involution

Aging involves morphological and functional alterations within the microenvironment of the thymus where heterogenous populations of thymic epithelial cells (TEC) play the main roles. The studies performed to date on thymic involution signalize a disturbed interaction between individual thymic compartments that disrupt thymocyte-TEC interactions and, as a sequele, disturb differentiation of both TEC and thymocytes. The process of aging affects the various subsets of TEC at different periods of life. Changes in different subsets of TEC are documented on the basis of their phenotypical characteristics, involving morphological analysis and immunocytochemistry. The character and kinetics of changes in TEC are typical for individual subsets and probably sex-dependent. In the course of life, the involutionary changes, expressed by disorganised thymic structure and function, are accompanied by changes in medullary TEC, manifested by alterations in the differentiation process of the cells. In parallel, at the same stage of individual life, the aging process induces increased proliferative and secretory activity of subseptal TEC, which seem to functionally replace medullary TEC. Structural and phenotypic modifications of TEC are locally controlled by complex sets of different factors and seem to represent a morphological adaptation of the gland to the process of aging. Microsc. Res. Tech. 62:488-500, 2003.

Vertebrate thymus and the neurotrophin system

An immunomodulary role has been proposed for growth factors included in the family of neurotrophins. This is supported by the presence of both neurotrophins and neurotrophin receptors in the immune organs and some immunocompetent cells, the in vitro and in vivo effects of the neurotrophins on the immune cells, and the structural changes of lymphoid organs in mice deficient in neurotrophins and their receptors. The current data strongly indicate that neurotrophins regulate the biology of thymic stromal cells and T cells, including survival, and are involved in the thymic organogenesis. This review compiles the available data about the occurrence and distribution of neurotrophins and their signaling receptors (Trk proteins and p75(NTR)) in the vertebrate thymus and the possible contribution of these molecules to the thymic microenvironment and, therefore, to the T cells differentiation.

Metallothioneins (I+II) and thyroid-thymus axis efficiency in old mice: role of corticosterone and zinc supply

Thymic atrophy or thymus absence causes depressed thyroid-thymus axis (TTA) efficiency in old, young propyl-thiouracil (PTU) (experimental hypothyroidism) and in young-adult thymectomised (Tx) mice, respectively. Altered zinc turnover may be also involved in depressed TTA efficiency. Zinc turnover is under the control of zinc-bound metallothioneins (Zn-MTs) synthesis. Thyroid hormones, corticosterone and nutritional zinc affect Zn-MT induction. Zn-MT releases zinc in young-adult age during transient oxidative stress for prompt immune response. In constant oxidative stress (ageing and liver regeneration after partial hepatectomy), high liver Zn-MTs, low zinc ion bioavailability and depressed TTA efficiency appear. This last finding suggested that MT might not release zinc during constant oxidative stress leading to impaired TTA efficiency. The aim of this work/study is to clarify the role of Zn-MTs (I+II) in TTA efficiency during development and ageing. The main results are (1) Old and PTU mice display high corticosterone, enhanced liver MTmRNA, low zinc and depressed TTA efficiency restored by zinc supply. Increased survival and no significant increments in basal liver Zn-MTs proteins occur in old and PTU mice after zinc supply. (2) Lot of zinc ions bound with MT in the liver from old mice than young (HPLC). (3) Young-adult Tx mice, evaluated at 15 days from thymectomy, display high MTmRNA and nutritional-endocrine-immune damage restored by zinc supply or by thymus grafts from old zinc-treated mice. (4) Young-adult Tx mice, but evaluated at 40 days from thymectomy, display natural normalisation in MTmRNA and nutritional-endocrine-immune profile with survival similar to normal mice. (5) Stressed (constant dark for 10 days) mice overexpressing MT display low zinc, depressed immunity, reduced thymic cortex, high corticosterone, altered thyroid hormones turnover showing a likeness with old mice. These findings, taken altogether, show that corticosterone is pivotal in MTs induction under stress. MTs bind preferentially zinc ions in constant oxidative stress, but with no release of zinc from MT leading to impaired TTA efficiency. Zinc supply restores the defect because zinc has no interference in affecting pre-existing Zn-MTs protein concentrations in old and PTU mice. Therefore, free zinc ions are available for TTA efficiency after zinc supply. Thymus from old zinc-treated mice induces the same restoring effect when transplanted in Tx recipients. However, Tx mice display natural normalisation in MTmRNA and in nutritional-endocrine-immune profile in the long run. Therefore, Zn-MTs (I+II) are crucial in zinc homeostasis for endocrine-immune efficiency during the entire life assuming a role of potential and novel 'biological clock of ageing'.

Thymic carcinoma of the thymic hormone secretory type in a cow

An 8-year-old Holstein cow had tumor nodules and enlarged lymph nodes in the mediastinum, and metastatic tumor masses in the pelvic cavity. The neoplastic cells were characterized by squamous features and intracytoplasmic vacuoles carrying microvilli, some of which contained periodic acid Schiff-positive globular cores, but tubular structures or goblet cells were absent. Many neoplastic cells stained positively for keratin, and occasional cells were positive for thymosin. The presence of secretory granules in the cytoplasm was confirmed by electron microscopy. This neoplasm was considered to be of thymic hormone-secreting epithelial cell origin.

Congenital hypothyroidism and immunodeficiency: evidence for an endocrine-immune interaction

Bi-directional interactions between the endocrine and immune systems have been well described, particularly in relation to the growth hormone and adrenal axes. The possible effects of the thyroid gland on the immune system have not been clearly elucidated. This report describes a patient with congenital hypothyroidism and immune deficiency characterized by severe and persistent lymphopenia. The clinical course was punctuated by recurrent episodes of respiratory symptoms (in association with bronchiectasis) and diarrhea. The child ultimately died from overwhelming respiratory infection. It is proposed that the prolonged deficiency of thyroid hormone may be directly related to the impairment of the cellular immune system.

Neuroendocrine control of the thymus

Thymocytes undergo a complex process of differentiation, largely dependent on interactions with the thymic microenvironment, a tridimensional cellular network formed by epithelial cells, macrophages, dendritic cells, and fibroblasts. One key cellular interaction involves the TCR-CD3 complex expressed by thymocytes with MHC-peptide complexes present on microenvironmental cells. Additionally, thymic epithelial cells (TEC) interact with thymocytes via soluble polypeptides such as thymic hormones and interleukins, as well as through extracellular matrix (ECM) ligands and receptors. Such types of heterotypic interactions are under neuroendocrine control. For example, thymic endocrine function, represented by thymulin production, is up-regulated, both in vivo and in vitro, by thyroid and pituitary hormones, including prolactin and growth hormone. We also showed that these peptides enhance the expression of ECM ligands and receptors, as well as the degree of TEC-thymocyte adhesion. In addition, we studied the thymic nurse cell complex, used herein as an in vitro model for ECM-mediated intrathymic T-cell migration. We observed that T-cell migration is also hormonally regulated as ascertained by the thymocyte entrance into and exit from these lymphoepithelial complexes. Taken together these data clearly illustrate the concept that neuroendocrine circuits exert a pleiotropic control on thymus physiology. Lastly, the intrathymic production of classic hormones such as prolactin and growth hormone suggests that, in addition to endocrine circuits, paracrine and autocrine interactions mediated by these peptides and their respective receptors may exist in the thymus, thus influencing both lymphoid and microenvironmental compartments of the organ.

Modulation of dye-coupling and proliferation in cultured rat thymic epithelium by factors involved in thymulin secretion

Cultures of rat thymic epithelium were used to measure the effect of thymulin secretagogues on dye-coupling and proliferation. Dye-coupling was assessed after the injection of lucifer yellow dextran which cannot permeate the connexin pore of gap junctions and the smaller, permeant cascade blue. In addition to gap junctional communication, larger intercellular bridges were demonstrated by the transfer of lucifer yellow dextran between cells. The extent of intercellular communication was found to be influenced by both cell density and the number of passages. In control cultures, intercellular communication was reduced in cell groups of low (< 20 cells/group) or high cell densities (> 100 cells/group) compared with groups of 20-60 cells. The highest coupling indices were found in subcultures 20-30. Taking these factors into account, significant decreases in coupling index were observed after pretreatment of test cultures with factors known to influence the secretion of thymulin (5 U/ml interleukin 1 (alpha and beta), 1 microM progesterone, 1 microM oestrogen, 1 microM testosterone, 1 ng/ml adrenocorticotropic hormone, 100 nM rat growth hormone) but 7.5 ng/ml thymulin had no effect on dye-coupling. The nonspecific gap junction uncoupler, octanol, abolished dye-coupling. Cellular proliferation, as measured by the uptake of tritiated thymidine, showed that the same factors that reduced coupling also increased proliferation. None of these factors affected the number of multinucleate cells present, except interleukin-1beta which caused a significant reduction in the average number of nuclei per cell. Thus rat thymic epithelium in vitro provides a model for the study of the direct action of factors on cells of the thymic microenvironment.

The possible role of copper ions in atherogenesis: the Blue Janus

It has been proposed that the oxidative modification of low density lipoprotein (LDL) is a key event in human atherogenesis. Copper ions can catalyse the oxidative modification of LDL in vitro and there is some evidence that they may also participate in the oxidation of LDL within the arterial wall. However, copper ions also form an intrinsic constituent of superoxide dismutase and caeruloplasmin, enzymes that may be involved in preventing oxidative injury. Atherosclerotic lesions frequently contain considerable quantities of extracellular matrix molecules. These may contribute to the expansion of the arterial neointima, causing luminal narrowing. They may also play a beneficial role by stabilising the plaque. Copper is an essential component of lysyl oxidase, an enzyme involved in the biosynthesis of collagen, which is a major constituent of the extracellular matrix. The impact of alterations in body copper status on atherogenesis is therefore difficult to predict. Experimental and epidemiological data are conflicting and therefore do not provide a clear resolution of this issue. We have reviewed the biochemical and cellular effects of copper ions that may play a role in atherogenesis.

Heterogeneity of epithelial cells in the rat thymus

The morphological heterogeneity of the thymic epithelium has been well documented both at the light and electron microscopic level. Immunohistochemistry has revealed four broad classes of epithelial cells (EC): subcapsule/perivascular, cortical, medullary EC, and medullary Hassall's corpuscles. Ultrastructural analysis has revealed further heterogeneity. In the cortex, four EC subtypes have been described ultrastructurally: subcapsular/perivascular, "pale," "intermediate," and "dark" EC. These subtypes are also present in the medulla. Two additional EC subtypes are restricted to the medulla: an undifferentiated subtype, and a subtype displaying signs of high metabolic activity. Based on the morphological features of the epithelium, it has been hypothetized that the thymic EC subtypes represent a process of differentiation.

Plasticity of neuroendocrine-thymus interactions during aging

Thymic regrowth and reactivation of thymic endocrine activity may be achieved even in old animals by different endocrinological or nutritional manipulations such as, (a) intrathymic transplantation of pineal gland or treatment with melatonin, (b) implantation of a growth hormone (GH) secreting tumor cell line or treatment with exogenous GH, (c) castration or treatment with exogenous luteinizing hormone-releasing hormone (LH-RH), (d) treatment with exogenous thyroxine or triiodothyronine, and (e) nutritional interventions such as arginine or zinc supplementation. These data strongly suggest that thymic, involution is a phenomenon secondary to age-related alterations in neuroendocrine-thymus interactions and that it is the disruption of such interactions in old age that is responsible for age-associated dysfunction. With regard to the mechanisms involved in hormone-induced thymic reconstitution, it is at present, difficult to draw any definitive conclusions. The effect of GH, thyroid hormones, and LH-RH may be due to the presence on thymic epithelial cells supposed to produce thymic peptides, of the specific hormone receptors. Melatonin or other pineal factors may also act through specific receptors, but experimental evidence is still lacking. The role of zinc, whose turnover is usually reduced in old age, is diverse. The effects range from the reactivation of zinc-dependent enzymes, required for both cell proliferation and apoptosis, to the reactivation of thymulin, a zinc-dependent thymic hormone. The role of zinc may even be more crucial. According to recent preliminary data obtained both in animal and human studies, it appears that the above reported endocrinological manipulations capable of restoring thymic activity in old age, may act also by normalizing the altered zinc pool.

Pituitary hormones modulate cell-cell interactions between thymocytes and thymic epithelial cells

The thymic microenvironment plays a key role in the intrathymic T-cell differentiation. It is composed of a tridimensional network of epithelial cells whose physiology is controlled by extrinsic circuits such as neuroendocrine axes. Herein we show that the expression of extracellular matrix ligands and receptor by cultured thymic epithelial cells is upregulated by prolactin (PRL) and growth hormone (GH), the latter apparently occurring via insulin-like growth factor I (IGF-I). Thymocyte release from the lymphoepithelial complexes, thymic nurse cells, as well as the reconstitution of these complexes are enhanced by PRL, GH or IGF-I. Treatment of a mouse thymic epithelial cell line with these hormones induced an increase in thymocyte adhesion, an effect significantly prevented in the presence of antibodies to fibronectin, laminin or respective receptors VLA-5 and VLA-6. Our data suggest that the in vitro changes in thymocyte/thymic epithelial cell interactions induced by pituitary hormones are partially mediated by the enhancement of extracellular matrix ligands and receptors.

Aging, immunity and neuroendocrine hormones

Recent evidence indicates that the neuroendocrine and immune systems are intimately integrated into one system that provides a complex homeostatic network. Disruption of one system by extrinsic factors such as stress or antigenic exposure usually has consequences on the other. With advancing age, a progressive disruption can be observed in both systems which may have profound implications with respect to age-associated pathologies, including autoimmunity. In this review evidence is summarized which supports the hypothesis that neuroendocrine factors influence the age-associated decline of the immune system.

Immunomodulation of peripheral lymphocytes by hormones of the hypothalamus-pituitary-thyroid axis

The aim of this review is to provide a comprehensive examination of the current literature describing the immunoregulatory effects on the peripheral immune system by the hormones that comprise the hypothalamic-pituitary-thyroid (HPT) axis. This article discusses the effects of the HPT axis hormones on the peripheral lymphoid tissues and the immune responses mediated by the cells that comprise these lymphoid tissues. Neuroendocrine dysfunction in the HPT axis, either naturally or experimentally induced, and the resulting immune dysfunction are also discussed. Emphasis in this article is placed on the most recent study findings and those that provide a unique or novel way of evaluating HPT hormone effects on the immune system. Our knowledge of the immunoregulatory effects of the hormones that comprise the HPT axis has grown tremendously in the last 10 years. As can be seen in this review, the immunoregulatory effects of the HPT axis hormones are quite diverse and influence most, if not all, aspects of immune system physiology. The continued exploration of the bidirectional circuitry between the immune and neuroendocrine systems may allow for development of appropriate prophylactic procedures that prevent dysfunction in both systems.

Thyroid and thymic endocrine function and survival in severely traumatized patients with or without head injury

OBJECTIVE

Functional links among the brain, endocrine and immune system have been described previously. An impairment of both immunological defence mechanisms and thyroid hormone turnover was present in trauma conditions. An investigation on the relevance of thymulin and thyroid hormones in multiple trauma patients with or without head injury has been performed. The role of these hormones as predictive factors for patients outcome was also evaluated.

DESIGN

Plasma thymulin levels and plasma thyroid hormone concentrations were tested in multiple trauma patients 24 h after admission to the Intensive Care Unit (ICU) and again after 5 and 10 days.

SETTING

Department of Immunology Ctr. INRCA, IInd ICU, S. Matteo Hospital Pavia and ICU "Umberto I" Hospital, Ancona.

PATIENTS

45 patients were evaluated including 14 multiple trauma patients without head injury and 31 multiple trauma patients with head injury at various level of coma, graded according to the Glascow Coma Score (GCS).

INTERVENTIONS

Routine protocol interventions were performed in all head injured patients.

MEASUREMENTS AND RESULTS

Thymulin and triiodothyronine (T3) levels were reduced, and reverse triiodothyronine (rT3) increased in all traumatized patients, but multiple trauma patients with head injury and GCS < or = 5 had the lowest levels of thymulin and T3 and the highest levels of rT3. No difference in plasma thyroxine (T4) and thyrotropin (TSH) levels was observed among injured patients. The analysis of predictive factors for the outcome has assigned to thymulin the highest score (29.6%) compared with the score for T3 (19.3%) and rT3 (26.3%). The total relative risk (delta %) calculated on the basis of T3 or rT3 rises significantly when thymulin relative risk is added.

CONCLUSIONS

Thymulin is markedly reduced in multiple trauma patients with head injury and it represents a predictive factor for the outcome better than the one deriving from the single measurements restricted to thyroid hormones.

Functional gap junctions in thymic epithelial cells are formed by connexin 43

A multiparametric study was carried out to investigate the presence and possible role of communicating junctions in the thymus, particularly in the thymic epithelium, the major component of the thymic microenvironment. The presence of direct cell-cell communication mediated by gap junctions was demonstrated in human and murine thymic epithelial cells (TEC) by means of in situ and in vitro immunohistochemical labeling as well as in vitro fluorochrome injection and double whole-cell patch clamp experiments. Moreover, both immuno- and Northern blot studies revealed that the gap junction protein connexin 43 and its mRNA were present in TEC. Importantly, we showed that thymic endocrine activity, as ascertained by thymulin production, could be specifically down-modulated in vitro by a gap junction inhibitor, octanol. We also investigated the existence of gap junctions between TEC and thymocytes. In thymic nurse cells we were able to detect cell-cell communication, although only a minor percentage of epithelial/thymocyte pairs were coupled in a given moment. In contrast, intercellular communication was not detected between cultured phagocytic cells of the thymic reticulum and the respective rosetting thymocytes. We suggest that gap junctions formed by connexin 43 may represent a novel (and rather cell type-specific) pathway for intrathymic cellular communication, including TEC/TEC as well as possible TEC/thymocyte interactions.

The effects of pregnancy on the mouse thymic epithelium

Changes in the murine thymus during pregnancy were studied using immunocytochemistry with monoclonal antibodies against thymic epithelial, neuroendocrine, and thymulin-producing cells, fibroblasts, blood vessels and connective tissue components. Extensive alterations occur in mid-pregnancy. The medulla was greatly enlarged in the involuted thymus, and there were greater numbers of epithelial cells. These epithelial cells had an altered distribution forming large structures surrounding spherical masses of mononuclear cells, lacked epithelial cells and often contained a central blood vessel with fibroblasts and connective tissue. We have called these structures 'medullary epithelial rings' (MERs). To our knowledge these structures have not been described before. Late in pregnancy the loss of the central mononuclear cells leaves collapsed structures in a smaller medulla that nevertheless retains many epithelial cells. In virgins and early-pregnancy, there are cortical channels free of epithelial cells that are very infrequent later in pregnancy. This may reflect the loss of steroid-sensitive thymocytes from the cortex. The influence of sex-steroids, neurological impulses and immune activity in causing the changes are discussed, as are the possible consequences in pregnancy of a reduced, thymocyte-depleted cortex and an enlarged medulla that shows great complexity and activity.

Mixed interleukins and thymosin fraction V synergistically induce T lymphocyte development in hydrocortisone-treated aged mice

Analysis of the role of interleukins in T cell ontogeny in vitro indicates that the regulation of T cell development involves interleukins (ILs) as well as thymic hormones (THs). In order to assess their respective roles in T lymphocyte development in vivo, chemically thymectomized mice were treated with ILs and THs. After 2 days of hydrocortisone treatment, aged mice showed acute thymic involution (weight was less than 30% of control) and reduced spleen size (less than 80% of control) with progressive recovery to 8 days. After 2 days of hydrocortisone treatment, adult mice were injected for 5 days with mixed buffy coat interleukins (BC-IL; 50 units IL2 equivalence), purified IL2 (50 units), rIL1 beta (4 ng), and thymosin fraction V (TF5; 100 micrograms). The animals were sacrificed and spleens and thymuses were analyzed for weight, cellularity, T cell number, subsets, and function as determined by proliferative responses to concanavalin A and ILs. BC-IL treatment increased the recovery of spleen and thymus weights and cellularity with corresponding augmentation of number and function of T lymphocytes; neither IL1 or IL2 or their combination had this effect. TF5 had no effect alone but strongly potentiated the effect of BC-IL on T lymphocyte function. These data indicate that BC-IL in combination with thymic peptides potently promotes T lymphocyte development. The combination may be therapeutically relevant for immunorestoration.

Identification of immunoreactive forms of thymosin alpha 1 in serum and supernatants by combining HPLC and RIA

Thymosin alpha 1 (T alpha 1) is a biologically active peptide, originally isolated from the thymus and currently undergoing clinical trials as an immunomodulator in cancer patients, in individuals with chronic active hepatitis, and as an immunoenhancer of vaccines in immunocompromised individuals. Absorption of rabbit antibody to thymosin alpha 1 with a synthetic C-14 fragment of T alpha 1 results in an antiserum with increased affinity for the amino terminal region of T alpha 1 and the precursor protein prothymosin alpha (ProT alpha). Using HPLC methodologies, the predominant form of immunoreactivity in serum and thymus was T alpha 1 not the precursor. Using this assay we detected a decline in mouse serum T alpha 1 following irradiation but not thymectomy, an observation consistent with the existence of an important radiation sensitive lymphoid source of serum T alpha 1. The secretion of authentic T alpha 1 but not the precursor into culture medium by thymic epithelial cells as well as in mitogen-stimulated peripheral blood lymphocytes was also demonstrated by HPLC/RIA. HPLC analysis by molecular weight sizing columns demonstrated that unlike thymic epithelial cells or peripheral blood lymphocytes, the immunoreactive T alpha 1 (IRT alpha 1) form in the supernatants from tumor cells such as MCF-7 breast carcinoma was of a lower molecular weight than authentic T alpha 1. These studies suggest that the authentic form of T alpha 1 is the major immunoreactive form in normal serum and that it is secreted by the medullary thymic epithelial cells as well as by peripheral blood lymphocytes. An additional immunoreactive form, secreted by tumor cells has also been identified and is the subject of future studies.

Neuroendocrine-thymus interactions. I. In vitro modulation of thymic factor secretion by thyroid hormones

Several in vivo experimental and clinical studies suggest that the production of thymic hormones, such as thymulin (Zn-FTS), is modulated by thyroid hormones. It was not determined in these studies, however whether such modulation is exerted directly on the thymic epithelial cells which synthesize and secrete thymic hormones. In order to discriminate between direct and indirect modulation, the effect of thyroid hormones on the in vitro production of thymulin by whole thymic organ culture, as detected by the rosette inhibition assay, has been investigated. Donors of thymuses were young 6N-propyl-2 thiouracil (PTU)-treated hypothyroid Balb/c mice and normal littermates. Thymuses from hypothyroid mice were shown to produce concentrations in vitro nearly undetectable of thymic hormone, when compared to thymuses from normal mice. The in vitro addition of triiodothyronine (T3) caused a complete recovery of the thymic hormone production by thymuses from hypothyroid mice and an increased synthesis even by normal thymuses over control values. The complete blockade of in vitro thymic hormone production with cycloheximide, which inhibits mRNA and protein synthesis but not thyroid hormone permissive actions, suggests that the T3 induced increment of thymic hormone level in the supernatant is due to de novo synthesis. Furthermore, the number of thymulin-producing cells, as detected by immunofluorescence using a specific antithymulin monoclonal antibody, which is quite low in thymuses from hypothyroid mice, is completely regained after in vitro incubation with T3. These findings support the idea that the modulation of thyroid hormones on thymic endocrine activity is directly exerted at thymic level.

Differential diagnosis between undifferentiated tumor and thymoma by electron microscopy and immunohistochemical labelling

This study of a particular case of tumor posed and resolved problems of differential diagnosis between an undifferentiated tumor and a thymoma by using electron microscopy in association with immunocytochemical methods. The first step was the distinction between an epithelial and a mesenchymal tumor, which was done by electron microscopy and immunofluorescence observation with anti-keratin antibody. The second step, a new approach to this problem, was the distinction between an epithelial tumor of thymic origin and another tumor located in the mediastinal lodge. A clear distinction was made by observation in immunofluorescence using anti-thymulin monoclonal antibody. This double approach permits differential diagnosis, excludes neoplasms of germ-cell origin, malignant lymphomas and leukemias, as well as mesenchymal tumors, and affirms the thymic origin of the tumor observed. A second type of cell observed in this tumor with a peculiar aspect, different from all types of epithelial cells observed in normal thymus, is discussed.

Immunohistochemical studies on a human thymic epithelial cell subset defined by the anti-cytokeratin 18 monoclonal antibody

Two monoclonal antibodies respectively recognizing cytokeratins (CK) 18 and 19 were applied to the human thymic epithelium (in vivo and in vitro) in normal and pathological conditions, including 12 thymomas. We observed that in both normal and hyperplastic thymuses (from patients with myasthenia gravis) virtually the entire epithelial network was CK19-positive as were the majority of cells growing in culture. In four thymomas, however, the expression of cytokeratin 19 was not detected by immunofluorescence. On the other hand, CK18 was expressed by a discrete subset of medullary thymic epithelial cells in normal and in hyperplastic thymuses. Among the thymomas a large majority was either negative or contained few isolated CK18-positive cells scattered within the tumour. Conversely, in the two undifferentiated epithelial thymomas, virtually all the tumoral network was strongly labeled with the anti-CK18 monoclonal antibody. The present investigation thus not only defines the human thymic epithelial cell subset on the basis of differential cytokeratin expression but also indicates that anti-CK antibodies with single cytokeratin specificities can be regarded as useful tools to study the heterogeneity of thymomas.

Thymotaxin: a thymic epithelial peptide chemotactic for T-cell precursors

The embryonic thymus is seeded by invading hemopoietic precursor cells that differentiate intrathymically into T lymphocytes. We have recently reported that avian thymic epithelial cells secrete chemotactic peptides, which provoke oriented migration of hemopoietic precursor cells in vitro. The established rat thymic epithelial cell line IT-45 R1 produced a polypeptide that resolves as a single band in the region of 11 kDa on NaDodSO4/polyacrylamide gels. This molecule, which we have named thymotaxin, induced a chemotactic response in a subpopulation of hemopoietic cells from juvenile rat bone marrow. Responding cells were generated by short-term coculture of rat bone marrow hemopoietic cells with mouse bone marrow stroma in a steroid-free medium. Cells selected in a chemotactic chamber have a lymphoid or blast cell morphology. The phenotype of the responding cells is Thy-1+, CD4- [corrected] and CD8-. In contrast, CD8 T-lymphocyte differentiation antigen was expressed after coculture with embryonic thymic monolayers, suggesting that the responding cells correspond to the precursors colonizing the thymus.

Selected human T cell lines respond to thymopoietin with intracellular cyclic GMP elevations

Ten established human cell lines were tested for their responsiveness to thymopoietin by measuring their intracellular cyclic nucleotide levels. Three T cell lines (CCRF-CEM, MOLT-4 and CCRF-HSB-2) responded to thymopoietin with elevations of intracellular cGMP but not cAMP; seven other human cell lines did not respond to thymopoietin (three T cell lines, three B cell lines and one erythropoietic stem cell line). Interestingly, only one cell line (MOLT-4) was also responsive to the closely related polypeptide splenin, and this reactivity was restricted to human and not bovine splenin. The detection of human cell lines with distinctive patterns of response to immunoregulatory peptides should provide support for understanding the immunopharmacological mechanisms by which these molecules act.

c-fos expression interferes with thymus development in transgenic mice

To study the function of the proto-oncogene c-fos in hematopoietic tissues, transgenic mice were generated that express c-fos from the H2-Kb promoter in several organs. These H2-c-fos mice have enlarged spleens and hyperplastic thymuses containing an increased number of thymic epithelial cells. The exogenous c-fos expression specifically affects T cell development in the thymus, thereby increasing the fraction of mature thymocytes. Results obtained with bone marrow radiation chimeras suggest that the altered distribution of T cell subsets is not a direct effect of c-fos expression within the T cell lineage. No changes in the proportion of hematopoietic cell lineages are seen in the spleen, and these mice do not develop lymphoid malignancies. B and T cell function, however, is impaired, and H2-c-fos mice are immune deficient. It appears that c-fos specifically stimulates the proliferation of thymic epithelial cells, and may thus indirectly affect T cell development.

A cloned rat thymic epithelial cell line established from serum-free selective culture

A serum-free system has been developed for selective growth and long-term culture of rat thymic epithelial cells. The growth media is a modification of McKeehan's WAJC 404, plus insulin, cholera toxin, dexamethasone, and epidermal growth factor. Cultures have been continuously passaged and maintained for over 6 mo., and a cloned cell line, TEA3A1, has been established. These cells are epithelial, judging by morphology and ultrastructure, and are positive for A2B5 and thymosin alpha 1 markers for thymic endocrine cells.

Modulation of thymic endocrine function by thyroid and steroid hormones

The thymic epithelium is responsible for the secretion of thymic hormones that intervene in some steps of intra- and extra-thymic T cell differentiation. In the present paper, we studied the in vivo and in vitro influences of thyroid and steroid hormones on the secretion of thymulin, one of the chemically-defined thymic hormones. Triodothyronine injected in young mice or applied into supernatants of cultured thymic epithelial cells resulted in an increase of thymulin synthesis and secretion, respectively evaluated by the analyses of thymulin containing cells and thymulin levels. The influence of steroids was investigated in vivo by steroid depletion (adrenalectomy and/or castration) and in vitro by the addition of various steroids and/or their specific antagonists to cultured thymic epithelial cells. Surgical ablation of adrenals and/or gonads induced a transient depletion of circulating thymulin and, by feedback mechanism, an increase in the numbers of thymulin containing cells. From the in vitro data we could conclude that the various steroid hormones (that exhibited a stimulatory effect) can act directly on the thymic epithelium, probably via specific receptors. The bulk of data above described represents a strong evidence for the physiological involvement of the neuroendocrine network, on the hormonal function of the thymic epithelium.

Thymic hormone containing cells--IX. Steroids in vitro modulate thymulin secretion by human and murine thymic epithelial cells

We investigated the in vitro effects (kinetics and dose-response) of adrenal and sexual steroid hormones on the secretion of thymulin, a thymic hormone, by human thymic epithelial cells in primary cultures as well as in a rat epithelial cell line. We demonstrated that all steroids tested, in a range of physiological doses, stimulated thymulin production to various extents. Progesterone and estradiol, however, were revealed to be the most efficient. Specific steroid antagonists abrogated the steroid-induced stimulation of thymulin production. These findings confirm our previous in vivo results and demonstrate that steroid hormones can act directly on thymic epithelial cells to modulate their endocrine production.

Age-related involution and terminal disorganization of the human thymus

The terminal involution pattern of the human thymus was studied based on autopsy cases (both sexes, age range 63-91 years). Large sections through the entire thymic fat body were examined with the help of both conventional histological and immunohistochemical techniques. The findings demonstrate that thymic atrophy in old humans (a) goes far beyond the degree of involution observed in small rodents; (b) results in a system of thin, branching, in part interrupted, non-keratinizing epithelial plates containing no typical Hassall bodies; (c) concerns all components of the thymus except fat tissue which progressively replaces original thymic structures; and (d) involves various types of disorganization of individual lobules with T and B lymphocytes often located outside rather than within epithelial remnants. Effects of low-level radiation on this final regression of the human thymus are unknown.

Unexpected thymic hyperplasia in transgenic mice harboring a neuronal promoter fused with simian virus 40 large T antigen

The hypothalamic peptide growth hormone-releasing factor (GRF) regulates the secretion and production of growth hormone from the anterior pituitary (M. C. Gelato and G. R. Merriam, Annu. Rev. Physiol. 48:569-591). To study GRF gene regulation, transgenic mice were generated that harbor the human GRF promoter fused to the coding sequences from the simian virus 40 early region. These mice had normal hypothalamic functions but unexpectedly suffered from severe thymic hyperplasia. Immunohistochemical analysis revealed that large T antigen was expressed in the thymic epithelial cells. These cells have endocrine properties and are known to produce thymic hormones [corrected]. The thymic hyperplasia was the apparent consequence of inappropriate production of T-cell maturation factors by epithelial cells and could involve increased self renewal of apparently normal T stem cells in the thymus.

Unexpected thymic hyperplasia in transgenic mice harboring a neuronal promoter fused with simian virus 40 large T antigen

The hypothalamic peptide growth hormone-releasing factor (GRF) regulates the secretion and production of growth hormone from the anterior pituitary (M. C. Gelato and G. R. Merriam, Annu. Rev. Physiol. 48:569-591). To study GRF gene regulation, transgenic mice were generated that harbor the human GRF promoter fused to the coding sequences from the simian virus 40 early region. These mice had normal hypothalamic functions but unexpectedly suffered from severe thymic hyperplasia. Immunohistochemical analysis revealed that large T antigen was expressed in the thymic epithelial cells. These cells have endocrine properties and are known to produce thymic hormones [corrected]. The thymic hyperplasia was the apparent consequence of inappropriate production of T-cell maturation factors by epithelial cells and could involve increased self renewal of apparently normal T stem cells in the thymus.

Natural killer cells in murine muscular dystrophy. IV. Characterization of Percoll fractionated splenic and thymic natural killer cells and natural killer-sensitive thymocyte targets

The Natural Killer (NK) activity in the thymus and NK-sensitive thymocyte targets of dystrophic mice was investigated. Dystrophic and normal mouse thymocytes or spleen cells were layered on discontinuous Percoll gradients (5 or 10% increments, respectively) between 40 and 70% and centrifuged at 1700 g for 30 min. All fractions were tested for either NK activity or used a 51Cr-labeled NK-sensitive targets in a 6-hr 51Cr release assay. The density interface between the 50% (1.060 g/ml) and 60% (1.075 g/ml) Percoll fractions of either dystrophic or normal mouse spleen cells and the 40% (1.050 g/ml) and 50% (1.060 g/ml) Percoll fractions of either dystrophic or normal mouse thymocytes were found to contain the largest proportion of NK activity using YAC-1 lymphoma tumor cells as targets. In addition, the NK activity in dystrophic mouse spleen cells and thymocytes was significantly greater when compared with normal mouse controls. Target binding cell studies revealed that these Percoll fractions of dystrophic mouse spleen cells and thymocytes had greater numbers of conjugate-forming cells when compared with normal control groups. Cell depletion experiments using either anti-Thy 1.2, anti-asialo-GM 1 or anti-NK-1 plus complement treatment revealed that the cell responsible for NK activity in the 50% Percoll fraction interface of dystrophic mouse spleen cells was asialo-GM 1 positive. NK-1 positive, and partially Thy 1.2 positive. However, the cells displaying NK-activity in the thymus of normal or dystrophic mice were found to be highly Thy-1.2 positive and peanut agglutinin (PNA) negative. The density interface between the 60% (1.075 g/ml) and 65% (1.081 g/ml) Percoll fractions of either normal or dystrophic mouse thymocytes contained the largest proportion of NK-sensitive target cells. Interestingly, the 60% Percoll fraction of dystrophic mouse thymocyte targets was significantly more susceptible to NK-mediated lysis than that of the normal mouse thymocyte population. Cell depletion experiments revealed that the NK-sensitive thymocyte population was similar in both mice, that is, Thy-1.2 positive, cortisone sensitive, PNA positive, Dolichos biflorus (DBA) negative and asialo GM-1 negative. The results indicate that there are density differences between splenic and thymic NK cells. In addition, there are density and phenotypic differences between thymic NK cells and thymic NK-sensitive target cells. The findings support the hypothesis that there are different populations of NK cells.

Immunohistochemical localization of thymopoietin with an antiserum to synthetic Cys-thymopoietin28-39

Thymopoietin-containing cells in the thymus were identified immunohistochemically using murine antiserum generated by immunization with synthetic Cys-thymopoietin28-39 (Cys-TP28-39). human thymopoietin, This antiserum, previously shown to react with both bovine and human thymopoietin, gave reactivity restricted to cortical and medullary epithelial cells of bovine and human thymus. Monoclonal antibodies with reactivity restricted to native bovine thymopoietin did not react with tissue sections of bovine thymus; most likely the epitopes recognized by monoclonal antibodies are not expressed on the inactive precursor forms of thymopoietin within thymic epithelial cells.

Analysis of thymic epithelial cell proliferation in vitro by combining bromodeoxyuridine and keratin labeling in an immunofluorescence assay

A simple method of analyzing thymic epithelial cell (TEC) proliferation has been developed by combining bromodeoxyuridine (BrDU) and keratin labeling in an immunofluorescence assay. The first reagent specifically visualizes the cells entering the S phase of the cell cycle, whereas the second immunostaining reveals which of the proliferating BrDU-positive cells actually belong to the epithelial lineage. This method, besides being rapid and free of radioactivity, appears to be reliable in view of the minor variations in the percentages of BrDU+ TEC observed in several distinct experiments. Thus, BrDU/keratin immunolabeling appears to represent a useful tool for the analysis of in vitro TEC proliferation.