In vivo effects of growth hormone on thymus function in aging mice

Intrathymic somatotropic circuitry: consequences upon thymus involution

Growth hormone (GH) is a classic pituitary-derived hormone crucial to body growth and metabolism. In the pituitary gland, GH production is stimulated by GH-releasing hormone and inhibited by somatostatin. GH secretion can also be induced by other peptides, such as ghrelin, which interacts with receptors present in somatotropic cells. It is well established that GH acts directly on target cells or indirectly by stimulating the production of insulin-like growth factors (IGFs), particularly IGF-1. Notably, such somatotropic circuitry is also involved in the development and function of immune cells and organs, including the thymus. Interestingly, GH, IGF-1, ghrelin, and somatostatin are expressed in the thymus in the lymphoid and microenvironmental compartments, where they stimulate the secretion of soluble factors and extracellular matrix molecules involved in the general process of intrathymic T-cell development. Clinical trials in which GH was used to treat immunocompromised patients successfully recovered thymic function. Additionally, there is evidence that the reduction in the function of the somatotropic axis is associated with age-related thymus atrophy. Treatment with GH, IGF-1 or ghrelin can restore thymopoiesis of old animals, thus in keeping with a clinical study showing that treatment with GH, associated with metformin and dehydroepiandrosterone, could induce thymus regeneration in healthy aged individuals. In conclusion, the molecules of the somatotrophic axis can be envisioned as potential therapeutic targets for thymus regeneration in age-related or pathological thymus involution.

Somatotrope GHRH/GH/IGF-1 axis at the crossroads between immunosenescence and frailty

Immunosenescence, characterized by complex modifications of immunity with age, could be related to frailty syndrome in elderly individuals, leading to an inadequate response to minimal aggression. Functional decline (i.e., the loss of ability to perform activities of daily living) is related to frailty and decreased physiological reserves and is a frequent outcome of hospitalization in older patients. Links between immunosenescence and frailty have been explored and 20 immunological parameters, including insulin-like growth factor-1 (IGF-1), thymopoeisis, and telomere length, were shown to be affected in elderly patients with functional decline. A strong relationship between IGF-1 and thymic ouput was evidenced. IGF-1, a mediator of growth hormone (GH), was subsequently shown to induce interleukin-7 secretion in cultured primary human thymic epithelial cells. We are exploring the stress hypothesis in which an acute stressor is used as the discriminator of frailty susceptibility. GH can counteract the deleterious immunosuppressive effects of stress-induced steroids. Under nonstress conditions, the immunosenescent system preserves physiological responses, while under stress conditions, the combination of immunosenescence and a defect in the somatotrope axis might lead to functional decline.

Role of thymulin on the somatotropic axis in vivo

AIMS

There is clear evidence for the existence of a bi-directional thymus-somatotropic axis and several studies suggest that the thymic peptide thymulin may be involved in this communication. We undertook to assess the impact of serum thymulin immunoneutralization in C57BL/6 mice and that of neonatal thymulin gene therapy (NTGT) in nude mice on body weight (BW) gain and on the histomorphometric profile of the somatotrope population.

MAIN METHODS

Immunoneutralization of thymulin was done from postnatal day 1 to 35 by i.p. injections of rabbit anti-thymulin serum (α-FTS) and normal rabbit serum (NRS) in controls. NTGT was implemented in nudes using an adenoviral vector expressing a synthetic gene for thymulin (RAd-FTS). On postnatal day 1, heterozygous (nu/+) and homozygous (nu/nu) pups received a single bilateral i.m. injection either RAd-FTS or RAd-GFP (a control vector expressing green fluorescent protein). BW gain was recorded and at the end of the study the pituitaries were immunostained for growth hormone (GH). Serum GH and thymulin were determined by radioimmunoassay and bioassay, respectively.

KEY FINDINGS

Thymulin immunoneutralization induced a significant decrease in BW gain, serum GH and somatotrope cell density as well as an increase in somatotrope cell size. NTGT markedly increased BW gain, serum thymulin (P<0.01) and somatotrope cell and volume density in nu/nu mice.

SIGNIFICANCE

Our results suggest that thymulin plays a relevant physiological role on the thymus-somatotropic axis in mice.

Thymulin-based gene therapy and pituitary function in animal models of aging

Thymulin is a thymic hormone exclusively produced by the thymic epithelial cells. After its discovery and initial characterization in the 1970s, it was demonstrated that thymulin production and secretion is strongly influenced by the neuroendocrine system. Conversely, a growing core of information, to be reviewed here, points to thymulin as a hypophysiotropic peptide. Additionally, thymulin was shown to possess anti-inflammatory and analgesic properties in the brain. In recent years, a synthetic DNA sequence coding for a biologically active analog of thymulin, metFTS, was constructed and cloned in different adenoviral vectors. These include bidirectional regulatable Tet-Off vector systems that simultaneously express metFTS and green fluorescent protein and that can be downregulated reversibly by the addition of the antibiotic doxycycline. A number of recent studies suggest that thymulin gene therapy may be a suitable therapeutic strategy to prevent some of the endocrine and reproductive alterations that typically appear in congenitally athymic (nude) mice, taken as a suitable model of neuroendocrine and reproductive aging. The present article briefly reviews the literature on the physiology of the thymulin-pituitary axis as well as on the new molecular tools available to exploit the therapeutic potential of thymulin.

T-cell dysfunction in HIV-1 infection: targeting the inhibitors

Since AIDS emerged almost three decades ago, there have been considerable advances in the field of antiretroviral chemotherapy for those chronically infected with HIV-1. However, this therapy is noncurative and as our understanding of HIV-1 immunopathogenesis increases, it is becoming apparent that further therapeutic interventions are required to reverse the devastating effects of HIV-1 infection worldwide. While viral clearance remains the principle goal of HIV-1 treatment, this article describes immunotherapeutic options that target the immunological effects of the virus, to reduce its presence in the body and counteract viral-induced T-cell dysfunction and inhibition. Such approaches may augment existing antiretroviral therapy to overturn virus-induced T-cell anergy in the infected host, improving levels of immune control that reduce viremia and decrease the rate of transmission.

Infection, immunity and the neuroendocrine response

The Central Nervous (CNS) and Immune Systems (IS) are the two major adaptive systems which respond rapidly to numerous challenges that are able to compromise health. The defensive response strictly linking innate to acquired immunity, works continuously to limit pathogen invasion and damage. The efficiency of the innate response is crucial for survival and for an optimum priming of acquired immunity. During infection, the immune response is modulated by an integrated neuro-immune network which potentiates innate immunity, controls potential harmful effects and also addresses metabolic and nutritional modifications supporting immune function. In the last decade much knowledge has been gained on the molecular signals that orchestrate this integrated adaptive response, with focus on the systemic mediators which have a crucial role in driving and controlling an efficient protective response. These mediators are also able to signal alterations and control pathway dysfunctions which may be involved in the persistence and/or overexpression of inflammation that may lead to tissue damage and to a negative metabolic impact, causing retarded growth. This review aims to describe some important signalling pathways which drive bidirectional communication between the Immune and Nervous Systems during infection. Particular emphasis is placed on pro-inflammatory cytokines, immunomodulator hormones such as Glucocorticoids (GCs), Growth hormone (GH), Insulin-like Growth Factor-1 (IGF-1), and Leptin, as well as nutritional factors such as Zinc (Zn). Finally, the review includes up-to-date information on this neuroimmune cross-talk in domestic animals. Data in domestic animal species are still limited, but there are several exciting areas of research, like the potential interaction pathways between mediators (i.e. cytokine-HPA regulation, IL-6-GCS-Zn, cytokines-GH/IGF-1, IL-6-GH-Leptin and thymus activity) that are or could be promising topics of future research in veterinary medicine.

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

Thymulin is a thymic hormone exclusively produced by the thymic epithelial cells. It consists of a nonapeptide component coupled to the ion zinc, which confers biological activity to the molecule. After its discovery in the early 1970s, thymulin was characterized as a thymic hormone involved in several aspects of intrathymic and extrathymic T cell differentiation. Subsequently, it was demonstrated that thymulin production and secretion is strongly influenced by the neuroendocrine system. Conversely, a growing core of information, to be reviewed here, points to thymulin as a hypophysotropic peptide. In recent years, interest has arisen in the potential use of thymulin as a therapeutic agent. Thymulin was shown to possess anti-inflammatory and analgesic properties in the brain. Furthermore, an adenoviral vector harboring a synthetic gene for thymulin, stereotaxically injected in the rat brain, achieved a much longer expression than the adenovirally mediated expression in the brain of other genes, thus suggesting that an anti-inflammatory activity of thymulin prevents the immune system from destroying virus-transduced brain cells. Other studies suggest that thymulin gene therapy may also be a suitable therapeutic strategy to prevent some of the endocrine and metabolic alterations that typically appear in thymus-deficient animal models. The present article briefly reviews the literature on the physiology, molecular biology, and therapeutic potential of thymulin.

The peptide molecular links between the central nervous and the immune systems

The central nervous system (CNS) and the immune system were for many years considered as two autonomous systems. Now, the reciprocal connections between them are generally recognized and very well documented. The links are realized mainly by various immuno- and neuropeptides. In the review the influence of the following immunopeptides on CNS is presented: tuftsin, thymulin, thymopoietin and thymopentin, thymosins, and thymic humoral factor. On the other side, the activity in the immune system of such neuropeptides as substance P, neurotensin, some neurokinins, enkephalins, and endorphins is discussed.

Growth hormone modulates thymocyte development in vivo through a combined action of laminin and CXC chemokine ligand 12

Previous evidence indicates that GH modulates thymic cell migration. In this study we approached this issue in vivo, studying thymocyte migration in GH transgenic animals and in normal mice treated intrathymically with GH. Extracellular matrix and chemokines are involved in thymocyte migration. In this respect, thymocyte adhesion to laminin was higher in GH-treated animals than controls, and the numbers of migrating cells in laminin-coated Transwells was higher in GH-transgenic and GH-injected mice. Additionally, CXC chemokine ligand 12 (CXCL12)-driven migration was higher in GH-Tg and GH-treated animals compared with controls. Interestingly, although CXCR4 expression on thymocytes did not change in GH-Tg mice, the CXCL12 intrathymic contents were higher. We found that CXCL12, in conjunction with laminin, would additionally enhance the migration of thymocytes previously exposed to high concentrations of GH in vivo. Lastly, there was an augmentation of recent thymic emigrants in lymph nodes from GH-Tg and GH-injected animals. In conclusion, enhanced thymocyte migration in GH transgenic mice as well as GH-injected mice results at least partially from a combined action of laminin and CXCL12. Considering that GH is presently being used as an adjuvant therapeutic agent in immunodeficiencies, including AIDS, the concepts defined herein provide important background knowledge for future GH-based immune interventions.

Insights into thymic aging and regeneration

The deterioration of the immune system with progressive aging is believed to contribute to morbidity and mortality in elderly humans due to the increased incidence of infection, autoimmunity, and cancer. Dysregulation of T-cell function is thought to play a critical part in these processes. One of the consequences of an aging immune system is the process termed thymic involution, where the thymus undergoes a progressive reduction in size due to profound changes in its anatomy associated with loss of thymic epithelial cells and a decrease in thymopoiesis. This decline in the output of newly developed T cells results in diminished numbers of circulating naive T cells and impaired cell-mediated immunity. A number of theories have been forwarded to explain this 'thymic menopause' including the possible loss of thymic progenitors or epithelial cells, a diminished capacity to rearrange T-cell receptor genes and alterations in the production of growth factors and hormones. Although to date no interventions fully restore thymic function in the aging host, systemic administration of various cytokines and hormones or bone marrow transplantation have resulted in increased thymic activity and T-cell output with age. In this review, we shall examine the current literature on thymic involution and discuss several interventional strategies currently being explored to restore thymic function in elderly subjects.

Growth hormone in T-lymphocyte thymic and postthymic development: a study in HIV-infected children

OBJECTIVES

Growth hormone (GH) plays a role in thymic function, and recombinant GH may stimulate thymopoiesis in HIV-infected individuals. We performed immunologic analyses in 26 antiretroviral-treated children matched for age, pubertal status, clinical parameters, and antiretroviral exposure who did or did not show an impaired response to GH-release stimulation tests with arginine + GH-releasing hormone.

RESULTS

The following abnormalities were found in GH-deficient compared with GH-nondeficient children after >4 years of therapy: CD4 count ( P = .02) and percentage ( P = .03), CD4 as percentage of normal cells for age ( P = .003), serum interleukin-7 concentration ( P = .02), and thymic volume ( P = .01). Naive CD4 (4+62+RA+ and 4+CCR7+RA+) and CD8 (8+CCR7+RA+) lymphocytes were lower in GH-deficient children ( P = .003; P = .007; and P = .02, respectively). Postthymic pathways were also impaired in GH-deficient children. Thus, central memory (4+CCR7+RA-) CD4+ cells were reduced ( P = .006), whereas effector memory (4+CCR7-RA-) CD4+ cells ( P = .002) and late effector CD8+ lymphocytes (8+CCR7-RA+ and 8+27-28-) ( P = .009 and P = .002, respectively) were increased in these children.

CONCLUSIONS

Growth hormone plays a role in thymic and postthymic pathways, and defective GH production may be associated with incomplete immunoreconstitution. Immunomodulant agents (including GH) could be useful in patients with defective GH production.

Thymulin and the neuroendocrine system

Thymulin is a thymic hormone exclusively produced by the thymic epithelial cells. It consists of a nonapeptide component coupled to the ion zinc, which confers biological activity to this molecule. After its discovery in the early 1970, thymulin was characterized as a thymic hormone involved in several aspects of intra- and extrathymic T-cell differentiation. Subsequently, it was demonstrated that thymulin production and secretion is strongly influenced by the neuroendocrine system. Conversely, an emerging core of information points to thymulin as a hypophysotropic peptide. Here we review the evidence supporting the hypothesis that thymulin is an important player in the hypophyso-thymic axis.

Growth hormone accelerates immune recovery following allogeneic T-cell-depleted bone marrow transplantation in mice

OBJECTIVE

To test in a murine model whether recombinant human growth hormone can promote immune recovery after allogeneic T-cell-depleted bone marrow transplantation.

MATERIALS AND METHODS

Lethally irradiated (8.5 Gy) BALB/c mice (H2(d)) were transplanted with 5 x 10(6) T cell-depleted bone marrow cells from C57BL/6 mice (H2(b)). Recipient mice were injected intraperitoneally with recombinant human growth hormone (20 microg/dose/day) or saline for the first 4 weeks after transplantation. These animals were followed for phenotypic and functional immune recovery.

RESULTS

Administration of human recombinant growth hormone improved the CD4(+) T-cell counts in peripheral blood on day +14 (44+/-14 vs 33+/-7/microL blood, p<0.05) and day +21 (281+/-109 vs 187+/-76/microL blood, p<0.01) compared with the saline control. These differences were no longer significant by day +28 despite continued growth hormone administration. Similar effects were also observed on CD8(+) T cells and B220(+) B cells. The improvements in peripheral T-cell counts were at least partially as a result of enhanced thymopoiesis because there was an increase in total thymocytes after treatment with growth hormone. T-cell-depleted bone marrow recipients treated with growth hormone rejected the third-party grafts faster than those treated with saline control (median survival time: 20 days vs 26 days, p<0.05).

CONCLUSIONS

These data demonstrated that recombinant human growth hormone can accelerate phenotypic and functional immune reconstitution following allogeneic T-cell-depleted bone marrow transplantation in mice.

In vivo effects of growth hormone on thymic cells

Increasing evidence has placed the thymus as a target for neuroendocrine control. Herein we review the pleiotropic effects of growth hormone (GH) on this primary lymphoid organ, with emphasis on data derived from in vivo experiments. A series of results strongly indicate that GH enhances thymocyte proliferation in both rodents and humans. Moreover, in vivo treatment with GH enhances interleukin (IL)-6 production by mouse thymocytes, and ex vivo experiments show that production of other cytokines, such as IL-1 and GM-CSF, is also augmented. In a second vein, GH exerts a modulatory role on thymic hormone production, particularly the secretion of thymulin. In GH-treated animals as well as GH-transgenic mice, thymulin secretion is enhanced. In acromegalic patients we found higher levels of thymulin secretion, whereas the opposite was seen in dwarf mice and GH receptor knockout animals. Developing T cell migration is also under GH influence. Recombinant GH was found to increase human T cell engraftment in the thymus of SCID mice. Moreover, ex vivo thymocyte traffic into and out of thymic nurse cell complexes is enhanced after GH treatment. Lastly, we show that thymocyte export in vivo is modulated by GH, which favors the homing of CD4(+) recent thymic emigrants towards lymph nodes. In conclusion, the possibility that GH improves in vivo thymic functions, including thymocyte proliferation and migration, points to this molecule as a potential therapeutic adjuvant in T cell associated immunodeficiencies.

Human CD26 expression in transgenic mice affects murine T-cell populations and modifies their subset distribution

CD26 is a type II transmembrane glycoprotein with dipeptidyl peptidase (DPPIV) activity, constitutively expressed in different cell types and contributing to T-cell activation by acting as costimulatory molecule. Although data suggest an important role for CD26 within the immune system, the physiologic function of this molecule is still unknown. To investigate the role of CD26 in vivo we have produced transgenic mice expressing the human molecule in T cells. Human CD26 (huCD26) is constitutively expressed in all thymocytes and peripheral T lymphocytes of these transgenic mice and is endowed with an enhanced DPPIV activity. CD26 transgene expression induces major phenotypic changes to T-cell populations within the thymus and in peripheral blood. After the onset of sexual maturity, huCD26 expression induces an age-related overreduction of thymus cellularity accompanied by a relative impairment of thymocyte proliferation following lectin stimulation. Also the peripheral blood T-cell pool is reduced in huCD26 transgenic mice and this is accompanied by an increase of the apoptotic rate of CD4+ and CD8+ subpopulations. Taken together these data suggest that CD26 interferes with transduction pathway(s) needed for the maturation of T cells and plays an important role in T lymphocyte homeostasis in peripheral blood.

The thymus gland: a target organ for growth hormone

Increasing evidence has placed hormones and neuropeptides among potent immunomodulators, in both health and disease. Herein, we focus on the effects of growth hormone (GH) upon the thymus. Exogenous GH enhances thymic microenvironmental cell-derived secretory products such as cytokines and thymic hormones. Moreover, GH increases thymic epithelial cell (TEC) proliferation in vitro, and exhibits a synergistic effect with anti-CD3 in stimulating thymocyte proliferation, which is in keeping with the data showing that transgenic mice overexpressing GH or GH-releasing hormone exhibit overgrowth of the thymus. GH also influences thymocyte traffic: it increases human T-cell progenitor engraftment into the thymus; augments TEC/thymocyte adhesion and the traffic of thymocytes in the lymphoepithelial complexes, the thymic nurse cells; modulates in vivo the homing of recent thymic emigrants, enhancing the numbers of fluroscein isothiocyanate (FITC)+ cells in the lymph nodes and diminishing them in the spleen. In keeping with the effects of GH upon thymic cells is the detection of GH receptors in both TEC and thymocytes. Additionally, data indicate that insulin-like growth factor (IGF)-1 is involved in several effects of GH in the thymus, including the modulation of thymulin secretion, TEC proliferation as well as thymocyte/TEC adhesion. This is in keeping with the demonstration of IGF-1 production and expression of IGF-1 by TEC and thymocytes. Also, it should be envisioned as an intrathymic circuitry, involving not only IGF-1, but also GH itself, as intrathymic GH expression is seen both in TEC and in thymocytes, and that thymocyte-derived GH could enhance thymocyte proliferation. Finally, the possibility that GH improve thymic functions, including thymocyte proliferation and migration, places this molecule as a potential therapeutic adjuvant in immunodeficiency conditions associated with thymocyte decrease and loss of peripheral T cells.

Age-associated loss of bone marrow hematopoietic cells is reversed by GH and accompanies thymic reconstitution

Deterioration of the thymus gland during aging is accompanied by a reduction in plasma GH. Here we report gross and microscopic results from 24-month-old Wistar-Furth rats treated with rat GH derived from syngeneic GH3 cells or with recombinant human GH. Histological evaluation of aged rats treated with either rat or human GH displayed clear morphologic evidence of thymic regeneration, reconstitution of hematopoietic cells in the bone marrow, and multiorgan extramedullary hematopoiesis. Quantitative evaluation of formalin-fixed, hematoxylin and eosin-stained sections of bone marrow from aged rats revealed at least a 50% reduction in the number hematopoietic bone marrow cells, compared with that of young 3-month-old rats. This age-associated decline in bone marrow leukocytes, as well as the increase in bone marrow adipocytes, was significantly reversed by in vivo treatment with GH. Restoration of bone marrow cellularity was caused primarily by erythrocytic and granulocytic cells, but all cell lineages were represented and their proportions were similar to those in aged control rats. On a per-cell basis, GH treatment in vivo significantly increased the number of in vitro myeloid colony forming units in both bone marrow and spleen. Morphological evidence of enhanced extramedullary hematopoiesis was observed in the spleen, liver, and adrenal glands from animals treated with GH. These results confirm that GH prevents thymic aging. Furthermore, these data significantly extend earlier findings by establishing that GH dramatically promotes reconstitution of another primary hematopoietic tissue by reversing the accumulation of bone marrow adipocytes and by restoring the number of bone marrow myeloid cells of both the erythrocytic and granulocytic lineages.

Role of oestrogen receptors alpha and beta in immune organ development and in oestrogen-mediated effects on thymus

Oestrogens affect the development and regulation of the immune system. To determine the role of oestrogen receptors alpha (ER-alpha) and beta (ER-beta) on the development of the immune system, male ER-alpha (ERKO) and ER-beta (BERKO) mice, as well as alphabeta-double knockout (DERKO) mice, were studied. Deletion of ER-alpha led to hypoplasia of both thymus and spleen. Interestingly, a higher frequency of immature double CD4+ CD8+ thymocytes was found in ER-alpha(-) mice compared with ER-alpha(+) mice. Female oophorectomized BERKO mice given oestradiol (E2) displayed a similar degree of thymic atrophy compared with the wild-type strain but showed only limited involution of thymus cortex and no alteration of thymic CD4/CD8 phenotype expression. Our data demonstrate that expression of ER-alpha, but not ER-beta, is mandatory in males for development of full-size thymus and spleen, whereas expression of ER-beta is required for E2-mediated thymic cortex atrophy and thymocyte phenotype shift in females. A potential background for the above findings may be down-regulated activity in the growth hormone/insulin-like growth factor-1 (GH/IGF-1) axis in males lacking ER-alpha and suppressed sensitivity of females lacking ER-beta to E2-mediated suppression of IGF-1.

Is there a role for growth hormone upon intrathymic T-cell migration?

Intrathymic T-cell differentiation is essentially driven by the thymic microenvironment, a tridimensional network formed by thymic epithelial cells and to a lesser extent, dendritic cells, macrophages, fibroblasts, and extracellular matrix components. Thymocyte migration throughout the thymus is partially dependent on extracellular-matrix (ECM)-mediated interactions. Herein we investigated the putative role of growth hormone (GH) upon events related to intrathymic T-cell migration. We demonstrated that GH upregulates the expression of ECM ligands and receptors in distinct preparations of cultured thymic epithelial cells TECs). We also showed that adhesion of thymocytes to thymic epithelial cells was significantly increased by GH treatment, an effect that could be consistently abrogated when TECs were treated to antifibronectin, anti-VLA5, antilaminin, or anti-VLA6 antibodies before addition of thymocytes to the cultures. We also studied thymic nurse cells (TNCs), lymphoepithelial complexes that can be isolated ex vivo from the thymus. In this system, we had previously demonstrated that ECM ligands and receptors control both inward and outward thymocyte traffic. We then showed that GH enhances thymocyte release from TNCs, as well as the reconstitution of these lymphoepithelial complexes. Lastly, we evaluated the in vivo influence of GH on thymocyte exit. This was done by means of intrathymic injection of GH plus fluorescein isothiocyanate (FITC), and further analysis of recent thymic emigrants (FITC+ cells) in peripheral lymphoid organs, as defined by CD4/CD8-based cytofluorometric phenotyping. The proportions of FITC+ T cells appeared augmented in lymph nodes in GH-treated mice, as compared to controls. Taken together, these data indicate that GH stimulates intrathymic T-cell traffic, an effect that is at least partially mediated by extracellular matrix-mediated interactions.

Arginine supplementation is well tolerated but does not enhance mitogen-induced lymphocyte proliferation in elderly nursing home residents with pressure ulcers

BACKGROUND

Immune function declines with age, increasing risk for infection and delaying wound healing. Arginine enhances immune function and healing of standardized wounds in healthy elderly persons. The purpose of this study was to determine what level of arginine supplementation was orally and metabolically tolerated and effective in enhancing immune function in elderly persons with pressure ulcers.

METHODS

Residents with one or more pressure ulcers were recruited from two local nursing homes. Subjects were randomized to receive 0 g (n = 10; age, 82 +/- 3 years), 8.5 g (n = 11; 81 +/- 3 years), or 17 g (n = 11; 87 +/- 2 years) of supplemental arginine each day for 4 weeks. Oral tolerance, ie, absence of nausea, vomiting, abdominal distention, or diarrhea, was assessed daily. Metabolic tolerance was assessed weekly by evaluating serum electrolytes. Lymphocyte proliferation to phytohemagglutinin and interleukin 2 production were measured at baseline and after 4 weeks of supplementation as indicators of immune function.

RESULTS

Supplemental arginine significantly increased plasma arginine levels and was orally and metabolically tolerated with no complaints of abdominal distress or no clinically relevant changes in electrolyte levels among groups. Lymphocyte proliferation and interleukin 2 production were significantly different between nursing homes. When data from nursing homes were considered individually, arginine supplementation did not enhance the proliferative response. In subjects from nursing home 2 only, there was a 38% and 75% decrease (p < .05) in lymphocyte proliferation with 8.5 and 17 g of supplemental arginine, respectively. Interleukin 2 production was no different among supplementation groups.

CONCLUSIONS

Pharmacologic doses of arginine were well tolerated but did not enhance lymphocyte proliferation or interleukin 2 production in nursing home residents with pressure ulcers. CLINICAL RELEVANCY: Enteral formulas supplemented with pharmacologic levels of arginine are frequently administered to elderly persons. This study demonstrates that the very old can tolerate these nitrogen loads if baseline renal function is normal and fluid intake is encouraged. Further research needs to be completed investigating the effect of arginine supplementation on immune function in this population before recommending arginine use.

Neuroendocrine control of thymus physiology

The thymus gland is a central lymphoid organ in which bone marrow-derived T cell precursors undergo differentiation, eventually leading to migration of positively selected thymocytes to the peripheral lymphoid organs. This differentiation occurs along with cell migration in the context of the thymic microenvironment, formed of epithelial cells, macrophages, dendritic cells, fibroblasts, and extracellular matrix components. Various interactions occurring between microenvironmental cells and differentiating thymocytes are under neuroendocrine control. In this review, we summarize data showing that thymus physiology is pleiotropically influenced by hormones and neuropeptides. These molecules modulate the expression of major histocompatibility complex gene products by microenvironmental cells and the extracellular matrix-mediated interactions, leading to enhanced thymocyte adhesion to thymic epithelial cells. Cytokine production and thymic endocrine function (herein exemplified by thymulin production) are also hormonally controlled, and, interestingly in this latter case, a bidirectional circuitry seems to exist since thymic-derived peptides also modulate hormonal production. In addition to their role in thymic cell proliferation and apoptosis, hormones and neuropeptides also modulate intrathymic T cell differentiation, influencing the generation of the T cell repertoire. Finally, neuroendocrine control of the thymus appears extremely complex, with possible influence of biological circuitry involving the intrathymic production of a variety of hormones and neuropeptides and the expression of their respective receptors by thymic cells.

Arginine enhances In vivo immune responses in young, adult and aged mice

Arginine supplementation enhances in vitro lymphocyte proliferation in healthy adult humans and in rodent models. Studies examining the effect of arginine supplementation on in vivo immune responses are lacking. The purpose of this study was to determine whether arginine supplementation could enhance in vivo immune responses in adult mice and reverse known age-associated alterations in immune function of young and aged mice. Mice (1, 10 and 33 mo old) were fed a 2% arginine or an isonitrogenous diet for 2 wk. Delayed-type hypersensitivity to 2,4-dinitrofluorobenzene-challenged ears and changes in popliteal lymph node weights to injected sheep red blood cells were measured. The mean percentage of increase in ear thickness in challenged vs. unchallenged ears was 27, 35 and 24% with arginine supplementation and 7, 12 and 0% with the isonitrogenous diet in the 1-, 10- and 33-mo-old mice, respectively (P </= 0.05 for each age). Across all ages, the mean differences in popliteal lymph node weights were 1.9 +/- 0.3 vs. 1.0 +/- 0.3 mg with the arginine and isonitrogenous diets, respectively (P </= 0. 05). Only the 33-mo-old mice fed the isonitrogenous diet did not respond to this immune challenge. These findings suggest that arginine supplementation may enhance in vivo immune responses and/or reverse age-associated changes.

Age-associated increased interleukin-6 gene expression, late-life diseases, and frailty

Interleukin-6 (IL-6) is a proinflammatory cytokine that is normally tightly regulated and expressed at low levels, except during infection, trauma, or other stress. Among several factors that down-regulate IL-6 gene expression are estrogen and testosterone. After menopause or andropause, IL-6 levels are elevated, even in the absence of infection, trauma, or stress. IL-6 is a potent mediator of inflammatory processes, and it has been proposed that the age-associated increase in IL-6 accounts for certain of the phenotypic changes of advanced age, particularly those that resemble chronic inflammatory disease [decreased lean body mass, osteopenia, low-grade anemia, decreased serum albumin and cholesterol, and increased inflammatory proteins such as C-reactive protein (CRP) and serum amyloid A]. Furthermore, the age-associated rise in IL-6 has been linked to lymphoproliferative disorders, multiple myeloma, osteoporosis, and Alzheimer's disease. This overview discusses the data relating IL-6 to age-associated diseases and to frailty. Like the syndrome of inappropriate antidiuretic hormone, it is possible that certain clinically important late-life changes are due to an inappropriate presence of IL-6.

Studies on the gonadotropin-releasing activity of thymulin: changes with age

We assessed the ability of thymulin, a zinc-dependent nonapeptide produced by the thymic epithelial cells, to influence the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from dispersed anterior pituitary (AP) cells from young, adult, and senescent female rats. Perifusion of young and senescent AP cells with thymulin doses of 10(-6) to 10(-5) M gave a significant stimulatory response for LH but not FSH. Gonadotropin release was always lower in the senescent cells. AP cells from both age groups incubated with 10(-8) to 10(-3) M thymulin showed a time- and dose-dependent response for both gonadotropins, with a maximal stimulation at 10(-7) M. Preincubation of thymulin with an antithymulin serum completely quenched the secretagogue activity of the hormone. Coincubation of thymulin with the secretagogue gonadotropin-releasing hormone (GnRH) revealed a synergistic effect on LH release and an additive effect on the release of FSH. The calcium chelator EGTA blocked the gonadotropin-releasing activity of thymulin in AP cells. The cAMP enhancers, caffeine, NaF, and forskolin significantly increased the thymulin-stimulated release of gonadotropins. The inositol phosphate enhancer LiCl potentiated the action of thymulin on gonadotropins. It is concluded that the gonadotropin-releasing activity documented here for thymulin is an age- and receptor-dependent effect mediated in part by calcium, cAMP, and inositol phosphates.

Aging and T-cell-mediated immunity

Changes in the T-lymphocyte compartment represent the most critical component of immunological aging. Recent studies have demonstrated that the age-related decline in T-cell-mediated immunity is a multifactorial phenomenon affecting T-cell subset composition as well as several proximal events such as protein tyrosine phosphorylation, generation of second messengers, calcium mobilization and translocation of protein kinase C, and distal events such as lymphocyte proliferation and cytokine production of the T-cell activation pathway. Age-related T-cell immune deficiency is preceded by thymic involution and is influenced by several intrinsic as well as extrinsic factors. Further, the role of monocytes and macrophages in T-cell activation changes with advancing age. This brief review will summarize the current knowledge of the cellular as well as molecular aspects of immunodeficiency of T cells due to aging, some of the paradoxes of aging as related to T-cell-mediated immunity, and possible factors which contribute to this paradox. Finally, experimental approaches will be suggested that might resolve these controversies and that might provide insights into the diverse and complex mechanisms that contribute to immunodeficiency of T cells. Ultimately these studies may suggest possible therapeutic interventions to enhance immune function in the elderly.

Thymulin stimulates prolactin and thyrotropin release in an age-related manner

Thymulin is a Zn-bound nonapeptide produced by the thymic epithelial cells (TEC) whose secretion is modulated by prolactin (PRL) and thyroid hormones, among other hormones. We assessed the ability of thymulin to influence the release of PRL and thyroid stimulating hormone (TSH) from dispersed anterior pituitary (AP) cells from young, middle-aged and senescent Sprague-Dawley female rats. Perifused and incubated AP cells were used in different sets of experiments and PRL and TSH release was measured by radioimmunoassay. Perifusion of young and senescent AP cells with thymulin doses ranging from 10(-8) to 10(-5) M gave a logarithmic dose response pattern for both hormones. Supernatants from TEC lines also showed PRL and TSH secretagogue activity. Hormone release was always lower in the senescent cells. AP cells incubated with 10(-8) to 10(-3) M thymulin showed a time- and dose-dependent response for both hormones, the latter being bell-shaped with a maximum at 10(-7) M thymulin. Preincubation of thymulin with an anti-thymulin serum completely quenched the secretagogue activity of the thymic hormone. Coincubation of thymulin with TRH revealed a synergistic release of PRL and TSH in AP cells from all age groups. The calcium chelator EGTA but not the calcium ionophore A23187, blocked the hormone-releasing activity of thymulin in AP cells. The cAMP enhancers, caffeine, NaF and forskolin, significantly increased the thymulin-stimulated release of PRL and TSH, while trifluoperazine, a protein kinase C inhibitor, had no effect. The inositol phosphate enhancer LiCl, potentiated the action of thymulin on PRL and TSH. The present results suggest that the TRH-like activity documented here for thymulin is a receptor-mediated effect which appears to involve calcium, cAMP and inositol phosphates. Senescence but not middle age, appears to impair AP cell responsiveness to thymulin.

Insulin growth factor-I inhibits apoptosis in hematopoietic progenitor cells. Implications in thymic aging

A decline in plasma concentrations of both growth hormone and IGF-I occurs during aging of humans and rodents, and this is accompanied by involution of the thymus gland. Exogenous growth hormone induces the synthesis of IGF-I, which acts on bone marrow-derived hematopoietic progenitors of the myeloid and lymphoid lineages to promote their replication and survival. The increase in survival of these cells is caused by the ability of IGF-I to inhibit their apoptotic death. In contrast to the multipotential colony-stimulating-factor IL-3, inhibition of apoptosis by IGF-I requires the activation of the critical intracellular effector PI 3-kinase. These data establish that hematopoietic progenitors can use more than one intracellular signaling pathway in order to maintain their survival. The data also extend the original hypothesis that IGF-I shares with the colony-stimulating factors the properties of promoting DNA synthesis and inhibiting programmed cell death. Collectively, these data establish that hematopoietic progenitor cells are important targets for IGF-I, and this is likely to be important in understanding thymic aging.

Growth hormone receptors and immunocompetent cells

Growth hormone plays a significant role in regulation of the humoral and cellular immune responses in physiological as well as pathological situations. This role is exerted via the existence of specific receptors on cells of the immune system. Using flow cytofluorometry and biotinylated bovine GH, we have recently analyzed the expression of GH receptors (GHRs) in murine lymphoid organs. GHRs are widely expressed in all murine hematopoietic tissues and in fetuses, newborns, and 3- and 7-week-old animals. In the bone marrow, all hematopoietic lineages express variables levels of GH receptors, whereas in the thymus, this expression is mainly seen in CD4-, CD8-, CD4+CD8+, and CD8+ subpopulations. At the periphery, 50% of splenocytes and peripheral blood lymphocytes and 20% of lymph node cells are GHR positive, with a wider receptor expression on B cells and macrophages (approximately 50%) than on T cells (approximately 20%), where the labeling is seen on both CD4+ and CD8+ cell subsets. Interestingly, the proportion of GHRs bearing CD4+ and CD8+ splenocytes is increased after T-cell activation with Con A or anti-CD3. Finally, all peripheral T cells expressing GHRs also express prolactin receptors. These data provide a molecular basis to study the factors controlling GHR expression and regulation of immune function.

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.

Immunologic aspects of osteoporosis

Osteoporosis is a major cause of morbidity in older people. There are a large number of risk factors for the development of osteoporosis. However, these risk factors eventually must mediate their effects through modulation of bone remodeling. A variety of compounds including hormones and nutrients modulate bone remodeling. In addition to these well-characterized substances, the immune system plays a role in bone remodeling through pro-inflammatory cytokines. Specifically, interleukin-1 (IL-1), IL-11, interferon-g are known to influence osteoclasts and osteoblasts. Recently, the cytokine IL-6 has joined ranks with these cytokines as a bone reactive agent. IL-6 has been shown to increase with age and menopause. Additionally, murine models suggest that IL-6 plays a central role in bone resorption. Finally, in vitro studies demonstrate that IL-6 induces osteoclast activity. In this review, we will discuss the pathogenesis of osteoporosis in the context of aging and IL-6.

Effects of growth hormone and insulin-like growth factor I on T- and B-lymphocytes and immune function

The concept of a neuro-endocrine-immune axis was proposed more than 50 years ago. Growth hormone (GH), a central component of this axis has many functions at both a molecular and cellular level, including thymocyte proliferation, stimulation of the cytotoxic activity of natural killer cells and induction of lymphocyte proliferation. Binding of GH to its receptors on lymphocytes stimulates the production of insulin-like growth factor I (IGF-I), which mediates the effects of GH on cell proliferation. Other effects of GH on the immune system appear to be direct, such as priming monocytes for enhanced production of hydrogen peroxide in response to phorbol esters, and stimulating neutrophils to secrete superoxide anions associated with enhanced phagocytic activity. Many of the effects of GH are shared by IGF-I. Despite these observations, and the fact that GH is produced and secreted in immunological tissues such as the thymus and spleen, immune deficiency is not characteristic of GH deficiency in humans. The question remains as to whether GH and IGF-I could be used as immunotherapy. Currently, both agents have been used in adults to diminish wasting due to acquired immunodeficiency syndrome, and GH has been shown to stimulate CD8+ cell counts. However, they had little impact on CD4+ cell counts, which may be due to IGF-I and GH resistance in these individuals. The use of GH and IGF-I as immunotherapies merits further study.

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.

Endocrine characterization of primary mediastinal lymphoma

Because of the characteristic presentation of primary mediastinal large cell lymphoma (PMLCL) in young females its known origin from thymic B-cells, there might be a role in lymphomagenesis for estrogen receptors as well as for known neuroendocrine thymic mediators. A retrospective review of all patients with diffuse large cell lymphoma seen at our institution from January 1985 to January 1994 revealed 75 consecutive cases with a diagnosis of PMLCL. Through retrieval from our pathologic archives and requests to outside pathologists, we recovered and analyzed 17 biopsy specimens for the presence of the following hormone receptors: estrogen, beta endorphin, prolactin, T3, growth hormone, and leutinizing hormone. None of the specimens stained for any of the reagents. The most plausible explanation is that PMLCL tissues are devoid of these hormonal receptors and thus these receptors do not seem to play a role in the pathogenesis of PMLCL.

Thymulin, zinc and insulin-like growth factor-I (IGF-I) activity before and during recombinant growth hormone (rec-GH) therapy in children and adults with GH deficiency

Plasma thymulin (active and total) levels; IGF-I and zinc concentrations were evaluated in 9 children and in 8 adults with GH-deficiency (GHD) before and after 3-6 months of recombinant-GH treatment. Before therapy, GH deficient children had lower plasma active thymulin levels (1.0 +/- 0.3 log-2), not due to a peripheral defect in zinc saturation since plasma zinc levels were within the normal range, and total thymulin levels (1.3 +/- 0.3 log-2) than in the age-matched control group. GH therapy significantly increased active thymulin (3rd month: 3.0 +/- 0.2 log-2, 6th month: 4.0 +/- 0.2 log-2), total thymulin (3rd month: 3.3 +/- 0.3 log-2, 6th month: 4.3 +/- 0.2 log-2) and IGF-I levels (3rd month: 283.3 +/- 7.2 micrograms/L, 6th month: 411.2 +/- 44.2 micrograms/L, vs basal: 144.3 +/- 11.5 micrograms/L); at the 6th month of therapy, thymulin levels (active and total) were comparable to those found in controls. A positive correlation existed between zinc and plasma IGF-I levels (r = 0.66, p < 0.05). In adults with GHD, plasma active (1.9 +/- 0.3 log-2) and total thymulin levels (3.9 +/- 0.1 log-2), significantly lower (p < 0.01 and 0.05, respectively) than in controls before treatment, increased after GH therapy (active thymulin, 3rd month: 3.0 +/- 0.2 log-2, 6th month: 4.4 +/- 0.3 log-2; total thymulin, 3rd month: 3.9 +/- 0.3 log-2, 6th month: 4.7 +/- 0.2 log-2), being at 6th month of therapy no more different from the values recorded in the age-matched control group. In conclusion, children and adults with GHD have a marked impairment of the thymic endocrine activity, which can be restored by six months of GH treatment. The effects of GH on thymic functions may be mediated by IGF-I, through the modulation of zinc turnover, suggesting the possible existence of an interplay among GH, zinc, IGF-I and thymulin both in children and adults with GHD.

Effects of growth hormone and estrogen on T lymphocytes in older women

OBJECTIVE

To assess the effect on peripheral blood T lymphocytes of recombinant human growth hormone administered to healthy older women.

DESIGN

Prospective, open study.

SETTING

Veterans Administration clinical research unit and community surrounding Palo Alto, California.

PARTICIPANTS

Thirty-three women were recruited in two age groups: 20 to 40 years (n = 13) and 70 years or older (n = 24). Subjects were healthy, community-dwelling volunteers.

INTERVENTIONS

Recombinant human growth hormone at a dose of 0.025 mg/kg body weight/day was administered to the older subjects by daily subcutaneous injection over a 6-month study period.

MAIN OUTCOME MEASURES

Mean percentage and number of peripheral blood CD45RA + ("naive") T cells, mean counts per minute (CPM) of [3H]-thymidine incorporation following stimulation of peripheral blood mononuclear cells with phytohemaglutinin (T cell proliferation).

RESULTS

Before therapy, mean percentage and number of peripheral blood CD45RA + T cells and T cell proliferative responses were significantly reduced in older compared with younger women. The fraction of older women with CD45RA + T cell levels or T cell proliferative responses in the young range was significantly decreased in those who were receiving estrogen (1/10) compared with those who were not (9/14). After treatment with growth hormone, there were no significant changes in the mean CD45RA + T cell levels or proliferative responses of the older women.

CONCLUSIONS

The results suggest that T cell changes associated with the age-related decline in secretion of growth hormone cannot be reversed by growth hormone therapy during the eighth decade.

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.

Interdependence of the endocrine and immune systems

The cross-talk involving the endocrine and immune systems is now largely established. These systems actually use similar ligands and receptors to establish a physiological intra- and inter-system communication circuitry, which apparently plays a relevant role in homeostasis (reviewed in Blalock, 1992). Accordingly, classical hormones such as prolactin (PRL), growth hormone (GH) and even glucocorticoids (GC) can be produced by cells of the immune system, whereas a variety of cytokines, originally described as being produced by cells of the immune system, are synthesized and released by a variety of endocrine glands and nervous tissue. Moreover, specific receptors for such distinct molecular families can be detected in both the immune and endocrine systems.

Reduced ability of hypothalamic and pituitary extracts from old mice to stimulate thymulin secretion in vitro

There is substantial evidence that growth hormone (GH) is particularly important in the control of the age-related decline of thymus function. It was therefore of interest: (a) to assess the overall capacity of tissue extracts from mediobasal hypothalamus (MBH), anterior pituitary (AP) and testis, obtained from young (3 months, Yc), middle-aged (13 months, MAc) and old (18 months, Oc) intact C57BL/6 mice to stimulate in vitro the release of thymulin, a Zn-bound immunoregulatory thymic peptide, from pure cultures of mouse thymic epithelial cells (TEC); (b) to perform the same evaluation utilizing MBH, AP and testicular extracts from mice of the same age-range but treated for 45 days with a sc dose of ovine GH (2 micrograms/g body wt) known to stimulate thymulin secretion in vivo. Pituitary hormones were measured by heterologous rat RIAs, whereas thymulin release was estimated by a rosette assay. Untreated animals showed a significant age-dependent increase in the AP content of follicle stimulating hormone but not in other AP hormones. In both control and treated animals, pituitary GH content decreased significantly with age. MBH extracts from C57BL/6 males evidenced thymulin-releasing activity on mouse TEC lines. This activity was maximal in the MBH from young animals and declined with the age of the MBH donors. The thymulin-releasing activity of MBHs from GH-treated mice was higher than that of the control animals and showed a less pronounced decline with age. AP extracts from the same animals showed a higher thymulin-releasing activity than did MBH preparations. This activity showed a progressive age-associated reduction in the APs from untreated mice, whereas in the GH-treated group, an age-related decline was only seen in the old donors. Control testicular extracts had little effect on thymulin release whereas GH treatment induced a definite thymulin-release inhibiting activity in the testicular homogenates of our animals which increased progressively with the age of the testis donors. We conclude that the MBH, AP and testis of the young mouse contain factors able to affect directly the endocrine activity of the thymic epithelium. The amount of these substances declines with age and seems to be modulated by GH.

Reversibility of the thymic involution and of age-related peripheral immune dysfunctions by zinc supplementation in old mice

With advanced ageing the zinc pool undergoes progressive reduction as shown by the low zinc plasma levels and the negative crude zinc balance, both in humans and in rodents. It has been suggested that such zinc deficiency might be involved in many age-related immunological dysfunctions, including thymic failure. The relevance of zinc for good functioning of the entire immune system is, at present, well documented. In particular, zinc is required to confer biological activity to one of the best-known thymic peptides, thymulin, which is responsible for cell-mediated immunity. In deep zinc deficiencies, in humans and other animals, the low thymulin levels are due not to a primary failure of the thymus, but to a reduced peripheral saturation of thymic hormones by zinc ions. In aged mice both a reduced peripheral saturation of the hormone and a decreased production by the thymus were present. Oral zinc supplementation in old mice (22 months old) for 1 month induced a complete recovery of crude zinc balance from negative (-1.82) to positive values (+1.47), similar to those of young animals (+1.67). A full recovery of thymic functions with a regrowth of the organ and a partial restoration of the peripheral immune efficiency, as measured by mitogen responsiveness (PHA and ConA) and natural killer cell (NK) activity, were observed after zinc supplementation. These findings clearly pin-point for relevance of zinc for immune efficiency and suggest that the age-related thymic involution and peripheral immunological dysfunctions are not intrinsic and irreversible events but are largely dependent on the altered zinc pool.

The role of interleukin-6 in certain age-related diseases

Interleukin-6 (IL-6) is a pro-inflammatory cytokine with a wide range of functions. Perhaps the most important physiologically is its role as a mediator of the acute phase inflammatory response. Normally, there is little measurable IL-6 in the circulation, but levels increase abruptly to nanogram amounts during an inflammatory process. During aging, it has been proposed that the tight regulation of IL-6 gene expression becomes less effective and levels are measurable even when there is no evidence for inflammation. Several investigators have identified this cytokine as being involved in the pathogenesis of various disease processes and we have suggested that certain age-associated diseases are directly related. Among these are late-life lymphoma and myeloma, osteoporosis and possibly Alzheimer's disease.

Control of thymus physiology by peptidic hormones and neuropeptides

Several immune-neuroendocrine interactions are known to occur both in primary and secondary lymphoid organs. Here, Mireille Dardenne and Wilson Savino review data obtained in humans and experimental animals showing that peptidic hormones and neuropeptides can influence the thymus pleiotropically. These mediators modulate thymic epithelial cells (TECs), affecting thymulin secretion, cytokeratin expression, production of extracellular matrix (ECM) proteins and cell growth. Interestingly, such neuroendocrine influences appear to be part of a bidirectional circuitry, since thymic-derived peptides also regulate the release of hormones from the pituitary and hypothalamus.

Growth hormone inhibits normal B-cell differentiation and neutrophils' chemotaxis in vitro

In acromegalic patients we have previously described a low ability of B-lymphocytes to differentiate into plasma cells under PWM stimulation, and a decreased chemotaxis of polymorphonuclear leukocytes (PMN) towards N-formylmethionylphenilalanine (FMP). In this study we examined the effect of exogenous GH over these immune functions in normal cells. PMN were purified by dextran sedimentation, incubated with recombinant human GH (0 to 20 ng/ml) and subjected to stimulation with FMP. PBMC were cultured with or without PWM, in the presence of GH (between 0 and 100 ng/ml). Plasma cells were determined as hemolysis plaque forming cells and also by immunofluorescence. GH, in a dose-dependent way, decreased directed migration of PMN (5 ng/ml: 1.787 +/- 148 microns; 10 ng/ml: 1.581 +/- 221 microns; 20 ng/ml: 1.569 +/- 149 microns, all as mean +/- S.E.M.), when compared to similar values of untreated PMN (0 ng/ml 2.085 +/- 139 microns). GH treatment did not modify spontaneous migration. Net migration showed the same pattern as directed migration. GH decreased dose-dependently the PWM-driven differentiation of B-lymphocytes into plasma cells to 60% of the basal level. Although not significantly, GH tended to increase spontaneous B-cell differentiation. These results could account for the already described defect in B-cell differentiation and PWN chemotaxis in acromegaly, emphasizing the relationship between the endocrine and immune systems.

Concentrations of thymulin in unextracted serum from pigs, sheep and cattle as measured by ELISA

These studies were conducted to develop an ELISA for measurement of thymulin concentrations in unextracted blood serum (or plasma) from domestic animal species (pigs, sheep and cows). This assay was quite variable (intraassay C.V. of 13.3 and 6.4% at 12.6 and 50.5 pg/mL and interassay C.V. of 24.2%). Serial dilutions of serum from these species produced inhibition curves parallel to the reference standard, suggesting that there were no substances in serum causing non-specific interference in the assay. In addition, none of the other thymic peptides tested resulted in problematic displacement of thymulin binding to the antiserum. Using this assay, it was found that somatotropin (ST) treatment had no effect on serum thymulin concentrations in either pigs or cows. Chromatographic separation of thymulin activity in sheep serum showed three peaks with approximate MW estimates of 95, 80 and 1 kDa. Serum thymulin concentrations in a sheep injected with thymulin was cleared from blood with a half-life (t1/2) of 10.3 +/- 0.6 min. Serum thymulin concentrations increased between birth and 6 mo old in pigs. These data indicate that a rapid and reliable ELISA has been developed to measure thymulin in blood of these domestic animals. This assay should be of value in the study of thymulin function and factors regulating its secretion.