Alterations in sympathetic innervation of thymus and spleen in aged mice

Sympathetic Motor Neuron Dysfunction is a Missing Link in Age-Associated Sympathetic Overactivity

Overactivity of the sympathetic nervous system is a hallmark of aging. The cellular mechanisms behind this overactivity remain poorly understood, with most attention paid to likely central nervous system components. In this work, we hypothesized that aging also affects the function of motor neurons in the peripheral sympathetic ganglia. To test this hypothesis, we compared the electrophysiological responses and ion-channel activity of neurons isolated from the superior cervical ganglia of young (12 weeks), middle-aged (64 weeks), and old (115 weeks) mice. These approaches showed that aging does impact the intrinsic properties of sympathetic motor neurons, increasing spontaneous and evoked firing responses. A reduction of M current emerged as a major contributor to age-related hyperexcitability. Thus, it is essential to consider the effect of aging on motor components of the sympathetic reflex as a crucial part of the mechanism involved in sympathetic overactivity.

Neuroanatomy of lymphoid organs: Lessons learned from whole-tissue imaging studies

Decades of extensive research have documented the presence of neural innervations of sensory, sympathetic, or parasympathetic origin in primary and secondary lymphoid organs. Such neural inputs can release neurotransmitters and neuropeptides to directly modulate the functions of various immune cells, which represents one of the essential aspects of the body's neuroimmune network. Notably, recent studies empowered by state-of-the-art imaging techniques have comprehensively assessed neural distribution patterns in BM, thymus, spleen, and LNs of rodents and humans, helping clarify several controversies lingering in the field. In addition, it has become evident that neural innervations in lymphoid organs are not static but undergo alterations in pathophysiological contexts. This review aims to update the current information on the neuroanatomy of lymphoid organs obtained through whole-tissue 3D imaging and genetic approaches, focusing on anatomical features that may designate the functional modulation of immune responses. Moreover, we discuss several critical questions that call for future research, which will advance our in-depth understanding of the importance and complexity of neural control of lymphoid organs.

Consequences of spinal cord injury on the sympathetic nervous system

Spinal cord injury (SCI) damages multiple structures at the lesion site, including ascending, descending, and propriospinal axons; interrupting the conduction of information up and down the spinal cord. Additionally, axons associated with the autonomic nervous system that control involuntary physiological functions course through the spinal cord. Moreover, sympathetic, and parasympathetic preganglionic neurons reside in the spinal cord. Thus, depending on the level of an SCI, autonomic function can be greatly impacted by the trauma resulting in dysfunction of various organs. For example, SCI can lead to dysregulation of a variety of organs, such as the pineal gland, the heart and vasculature, lungs, spleen, kidneys, and bladder. Indeed, it is becoming more apparent that many disorders that negatively affect quality-of-life for SCI individuals have a basis in dysregulation of the sympathetic nervous system. Here, we will review how SCI impacts the sympathetic nervous system and how that negatively impacts target organs that receive sympathetic innervation. A deeper understanding of this may offer potential therapeutic insight into how to improve health and quality-of-life for those living with SCI.

Age-Related Variation in Sympathetic Nerve Distribution in the Human Spleen

Introduction: The cholinergic anti-inflammatory pathway (CAIP) has been proposed as an efferent neural pathway dampening the systemic inflammatory response via the spleen. The CAIP activates the splenic neural plexus and a subsequent series of intrasplenic events, which at least require a close association between sympathetic nerves and T cells. Knowledge on this pathway has mostly been derived from rodent studies and only scarce information is available on the innervation of the human spleen. This study aimed to investigate the sympathetic innervation of different structures of the human spleen, the topographical association of nerves with T cells and age-related variations in nerve distribution.

Materials and Methods: Spleen samples were retrieved from a diagnostic archive and were allocated to three age groups; neonates, 10–25 and 25–70 years of age. Sympathetic nerves and T cells were identified by immunohistochemistry for tyrosine hydroxylase (TH) and the membrane marker CD3, respectively. The overall presence of sympathetic nerves and T cells was semi-automatically quantified and expressed as total area percentage. A predefined scoring system was used to analyze the distribution of nerves within different splenic structures.

Results: Sympathetic nerves were observed in all spleens and their number appeared to slightly increase from birth to adulthood and to decrease afterward. Irrespective to age, more than halve of the periarteriolar lymphatic sheaths (PALSs) contained sympathetic nerves in close association with T cells. Furthermore, discrete sympathetic nerves were observed in the capsule, trabeculae and red pulp and comparable to the total amount of sympathetic nerves, showed a tendency to decrease with age. No correlation was found between the number of T cells and sympathetic nerves.

Conclusion: The presence of discrete sympathetic nerves in the splenic parenchyma, capsule and trabecular of human spleens could suggest a role in functions other than vasoregulation. In the PALS, sympathetic nerves were observed to be in proximity to T cells and is suggestive for the existence of the CAIP in humans. Since sympathetic nerve distribution shows interspecies and age-related variation, and our general understanding of the relative and spatial contribution of splenic innervation in immune regulation is incomplete, it remains difficult to estimate the anti-inflammatory potential of targeting splenic nerves in patients.

Immunofluorescent characterization of innervation and nerve-immune cell neighborhood in mouse thymus

The central nervous system impacts the immune system mainly by regulating the systemic concentration of humoral substances, whereas the peripheral nervous system (PNS) communicates with the immune system specifically according to local "hardwiring" of sympathetic/parasympathetic (efferent) and sensory (afferent) nerves to the primary and secondary lymphoid tissue/organs (e.g., thymus spleen and lymph nodes). In the present study, we use immunofluorescent staining of neurofilament-heavy to reveal the distribution of nerve fibers and the nerve-immune cell neighborhood inside the mouse thymus. Our results demonstrate (a) the presence of an extensive meshwork of nerve fibers in all thymic compartments, including the capsule, subcapsular region, cortex, cortico-medullary junction and medulla; (b) close associations of nerve fibers with blood vessels (including the postcapillary venules), indicating the neural control of blood circulation and immune cell dynamics inside the thymus; (c) the close proximity of nerve fibers to various subsets of thymocytes (e.g., CD4⁺, CD8⁺ and CD4⁺CD8⁺), dendritic cells (e.g., B220⁺, CD4⁺, CD8⁺ and F4/80⁺), macrophages (Mac1⁺ and F4/80⁺) and B cells. Our novel findings concerning thymic innervation and the nerve-immune cell neighborhood in situ should facilitate the understanding of bi-directional communications between the PNS and primary lymphoid organs. Since the innervation of lymphoid organs, including the thymus, may play essential roles in the pathogenesis and progression of some neuroimmune, infectious and autoimmune diseases, better knowledge of PNS-immune system crosstalk should benefit the development of potential therapies for these diseases.

Neuro-Immune Cell Units: A New Paradigm in Physiology

The interplay between the immune and nervous systems has been acknowledged in the past, but only more recent studies have started to unravel the cellular and molecular players of such interactions. Mounting evidence indicates that environmental signals are sensed by discrete neuro-immune cell units (NICUs), which represent defined anatomical locations in which immune and neuronal cells colocalize and functionally interact to steer tissue physiology and protection. These units have now been described in multiple tissues throughout the body, including lymphoid organs, adipose tissue, and mucosal barriers. As such, NICUs are emerging as important orchestrators of multiple physiological processes, including hematopoiesis, organogenesis, inflammation, tissue repair, and thermogenesis. In this review we focus on the impact of NICUs in tissue physiology and how this fast-evolving field is driving a paradigm shift in our understanding of immunoregulation and organismal physiology.

The spleen as a neuroimmune interface after spinal cord injury

Traumatic spinal cord injury (SCI) causes widespread damage to neurons, glia and endothelia located throughout the spinal parenchyma. In response to the injury, resident and blood-derived leukocytes orchestrate an intraspinal inflammatory response that propagates secondary neuropathology and also promotes tissue repair. SCI also negatively affects autonomic control over peripheral immune organs, notably the spleen. The spleen is the largest secondary lymphoid organ in mammals, with major roles in blood filtration and host defense. Splenic function is carefully regulated by neuroendocrine mechanisms that ensure that the immune responses to infection or injury are proportionate to the initiating stimulus, and can be terminated when the stimulus is cleared. After SCI, control over the viscera, including endocrine and lymphoid tissues is lost due to damage to spinal autonomic (sympathetic) circuitry. This review begins by examining the normal structure and function of the spleen including patterns of innervation and the role played by the nervous system in regulating spleen function. We then describe how after SCI, loss of proper neural control over splenic function leads to systems-wide neuropathology, immune suppression and autoimmunity. We conclude by discussing opportunities for targeting the spleen to restore immune homeostasis, reduce morbidity and mortality, and improve functional recovery after SCI.

Intrinsic and Extrinsic Thymic Adrenergic Networks: Sex Steroid-Dependent Plasticity

The thymus is sexually differentiated organ providing microenvironment for T-cell precursor differentiation/maturation in the major histocompatibility complex-restricted self-tolerant T cells. With increasing age, the thymus undergoes involution leading to the decline in efficacy of thymopoiesis. Noradrenaline from thymic nerve fibers and "(nor)adrenergic" cells is involved in the regulation of thymopoiesis. In rodents, noradrenaline concentration in thymus and adrenoceptor (AR) expression on thymic cells depend on sex and age. These differences are suggested to be implicated in the development of sexual diergism and the age-related decline in thymopoiesis. The programming of both thymic sexual differentiation and its involution occurs during the critical early perinatal period and may be reprogrammed during peripubertal development. The thymic (re)programming is critically dependent on circulating levels of gonadal steroids. Although the underlying molecular mechanisms have not yet been elucidated fully, it is assumed that the gonadal steroid action during the critical perinatal/peripubertal developmental periods leads to long-lasting changes in the efficacy of thymopoiesis partly through (re)programming of "(nor)adrenergic" cell networks and AR expression on thymic cells.

Loss of Sympathetic Nerves in Spleens from Patients with End Stage Sepsis

The spleen is an important site for central regulation of immune function by noradrenergic sympathetic nerves, but little is known about this major region of neuroimmune communication in humans. Experimental studies using animal models have established that sympathetic innervation of the spleen is essential for cholinergic anti-inflammatory responses evoked by vagal nerve stimulation, and clinical studies are evaluating this approach for treating inflammatory diseases. Most data on sympathetic nerves in spleen derive from rodent studies, and this work has established that remodeling of sympathetic innervation can occur during inflammation. However, little is known about the effects of sepsis on spleen innervation. Our primary goals were to (i) localize noradrenergic nerves in human spleen by immunohistochemistry for tyrosine hydroxylase (TH), a specific noradrenergic marker, (ii) determine if nerves occur in close apposition to leukocytes, and (iii) determine if splenic sympathetic innervation is altered in patients who died from end stage sepsis. Staining for vesicular acetylcholine transporter (VAChT) was done to screen for cholinergic nerves. Archived paraffin tissue blocks were used. Control samples were obtained from trauma patients or patients who died after hemorrhagic stroke. TH + nerves were associated with arteries and arterioles in all control spleens, occurring in bundles or as nerve fibers. Individual TH + nerve fibers entered the perivascular region where some appeared in close apposition to leukocytes. In marked contrast, spleens from half of the septic patients lacked TH + nerves fibers and the average abundance of TH + nerves for the septic group was only 16% of that for the control group (control: 0.272 ± 0.060% area, n = 6; sepsis: 0.043 ± 0.026% area, n = 8; P < 0.005). All spleens lacked cholinergic innervation. Our results provide definitive evidence for the distribution of noradrenergic nerves in normal human spleen and the first evidence for direct sympathetic innervation of leukocytes in human spleen. We also provide the first evidence for marked loss of noradrenergic nerves in patients who died from sepsis. Such nerve loss could impair neuroimmunomodulation and may not be limited to the spleen.

Peripubertal ovariectomy influences thymic adrenergic network plasticity in adult rats

The study investigated the influence of peripubertal ovariectomy on the thymic noradrenaline (NA) concentration, and the thymocyte NA content and β2- and α1-adrenoceptor (AR) expression in adult 2- and 11-month-old rats. In control rats, the thymic NA concentration increased with age. This increase reflected rise in the density of catecholamine (CA)-containing fluorescent nerve fibers and cells and their CA content. Additionally, the average β2- and α1-AR thymocyte surface density changed in the opposite direction with age; the density of β2-AR decreased, whereas that of α1-AR increased. Ovariectomy diminished the thymic NA concentration in 2-month-old rats. This reflected the decrease in the density of fluorescent nerve fibers, and CA content in fluorescent nerve fibers and non-lymphoid cells, since the thymocyte NA content was increased in ovariectomized (Ox) rats. Estrogen supplementation prevented the ovariectomy-induced changes. In Ox rats, the density of CA-synthesizing nerve fibers and non-lymphoid cells diminished with age. To the contrary, NA content in thymocytes increased with age, but it did not exceed that in 11-month-old controls. Additionally, ovariectomy diminished the average thymocyte surface density of β2-ARs, but it increased that of α1-ARs in 2-month-old-rats (due to estrogen, and estrogen and progesterone deficiency, respectively). These changes, despite of the rise in circulating estrogen level post-ovariectomy, remained stable with age. This most likely reflected a decreased sensitivity to estrogen action, as a consequence of the hormone misprinting in peripubertal age. The analysis of thymocyte proliferation in culture suggested that age- and ovariectomy-induced alterations in thymocyte NA synthesis and AR expression altered NA autocrine/paracrine action on thymocytes. In conclusion, the study indicates that the ovarian hormone deficiency in peripubertal age affects ovarian steroid-dependent remodeling of thymic adrenergic regulatory network in adult rats.

Close interactions between sympathetic neural fibres and follicular dendritic cells network are not altered in Peyer's patches and spleen of C57BL/6 mice during the preclinical stage of 139A scrapie infection

During preclinical stage of prion diseases, secondary lymphoid organs seem to play an important role in prion amplification prior the invasion of the associated peripheral nervous system. In mice, it was shown that the relative positioning of follicular dendritic cells (FDC) and sympathetic nervous system (SNS) affects the velocity of neuroinvasion following scrapie inoculation. In this study, we checked if scrapie infection, by oral or intraperitoneal route, could influence this neuroimmune interface between FDC and tyrosine hydroxylase (TH) positive neural fibres within Peyer's patches (PP) and spleen of the C57BL/6 mouse strain. We concluded that, in vivo, scrapie 139A and ME7 strains do not modify FDC-SNS neuroimmune interface. However, age seems to alter this neuroimmune interface and thus could influence the neuroinvasion in prion pathogenesis.

Chronic thoracic spinal cord injury impairs CD8+ T-cell function by up-regulating programmed cell death-1 expression

Background

Chronic spinal cord injury (SCI) induces immune depression in patients, which contributes to their higher risk of developing infections. While defects in humoral immunity have been reported, complications in T-cell immunity during the chronic phase of SCI have not yet been explored.

Methods

To assess the impact of chronic SCI on peripheral T-cell number and function we used a mouse model of severe spinal cord contusion at thoracic level T9 and performed flow cytometry analysis on the spleen for T-cell markers along with intracellular cytokine staining. Furthermore we identified alterations in sympathetic activity in the spleen of chronic SCI mice by measuring splenic levels of tyrosine hydroxylase (TH) and norepinephrine (NE). To gain insight into the neurogenic mechanism leading to T-cell dysfunction we performed in vitro NE stimulation of T-cells followed by flow cytometry analysis for T-cell exhaustion marker.

Results

Chronic SCI impaired both CD4⁺ and CD8⁺ T-cell cytokine production. The observed T-cell dysfunction correlated with increased expression of programmed cell death 1 (PD-1) exhaustion marker on these cells. Blocking PD-1 signaling in vitro restored the CD8⁺ T-cell functional defect. In addition, we showed that chronic SCI mice had higher levels of splenic NE, which contributed to the T-cell exhaustion phenotype, as PD-1 expression on both CD4⁺ and CD8⁺ T-cells was up-regulated following sustained exposure to NE in vitro.

Conclusions

These studies indicate that alteration of sympathetic activity following chronic SCI induces CD8⁺ T-cell exhaustion, which in turn impairs T-cell function and contributes to immune depression. Inhibition of the exhaustion pathway should be considered as a new therapeutic strategy for chronic SCI-induced immune depression.

Neuroimmune connections in ovine pharyngeal tonsil: potential site for prion neuroinvasion

Recent studies have established the involvement of nasal-associated lymphoid tissues, mainly the pharyngeal tonsil, in prion pathogenesis. However, the mechanisms of the associated neuroinvasion are still debated. To determine potential sites for prion neuroinvasion inside the ovine pharyngeal tonsil, the topography of heavy (200 kDa) and light (70 kDa) neurofilaments and of glial fibrillar acidic protein has been semi-quantitatively analysed inside the various compartments of the tonsil. The results show that the most innervated areas are the interfollicular area and the connective tissue located beneath the respiratory epithelium. The existence of rare synapses between follicular dendritic cells and nerve fibres inside the germinal centre indicates that this mechanism of neuroinvasion is possible but, since germinal centres of lymphoid follicles are poorly innervated, other routes of neuroinvasion are likely. The host PRNP genotype does not influence the pattern of innervation in these various tonsil compartments, unlike ageing during which an increase of nerve endings occurs in a zone of high trafficking cells beneath the respiratory epithelium. A minimal age-related increase of innervation inside the lymphoid follicles has also been observed. An increase in nerve fibre density around the lymphoid follicles, in an area rich in mobile cells such as macrophages and dendritic cells capable of capturing and conveying pathogen prion protein (PrPd), might ensure more efficient infectivity, not in the early phase but in the advanced phase of lymphoinvasion after the amplification of PrPd; alternatively, this area might even act as a direct site of entry during neuroinvasion.

Age-associated remodeling of neural and nonneural thymic catecholaminergic network affects thymopoietic productivity

Ageing is associated with a progressive decline in thymic cytoarchitecture followed by a less efficient T cell development and decreased emigration of naïve T cells to the periphery. These thymic changes are linked to increased morbidity and mortality from infectious, malignant and autoimmune diseases in old age. Therefore, it is of paramount importance to understand the thymic homeostatic processes across the life span, as well as to identify factors and elucidate mechanisms driving or contributing to the thymic involution. Catecholamines (CAs) derived from sympathetic nerves and produced locally by thymic cells represent an important component of the thymic microenvironment. In young rats, they provide a subtle tonic suppressive influence on T cell development acting via β(2)- and α(1)-adrenoceptors (ARs) expressed on thymic nonlymphoid cells and thymocytes. In the face of thymic involution, a progressive increase in the thymic noradrenaline level, reflecting a rise in the density of noradrenergic nerve fibers and CA-synthesizing cells, occurs. In addition, the density of β(2)- and α(1)-AR-expressing thymic nonlymphoid cells and the α(1)-AR thymocyte surface density also exhibit a pronounced increase with age. The data obtained from studies investigating effects of AR blockade on T cell development indicated that age-related changes in CA-mediated thymic communications, certainly those involving α(1)-ARs, may contribute to diminished thymopoietic efficiency in the elderly. Having in mind thymic plasticity in the course of ageing, and broadening possibilities for pharmacological modulation of CA signaling, we here present and discuss the progress in research related to a role of CAs in thymic homeostasis and age-related decay in the thymic naïve T cell output.

Sympathetic innervation of the spleen in male Brown Norway rats: a longitudinal aging study

Aging leads to reduced cellular immunity with consequent increased rates of infectious disease, cancer, and autoimmunity in the elderly. The sympathetic nervous system (SNS) modulates innate and adaptive immunity via innervation of lymphoid organs. In aged Fischer 344 (F344) rats, noradrenergic (NA) nerve density in secondary lymphoid organs declines, which may contribute to immunosenescence with aging. These studies suggest there is SNS involvement in age-induced immune dysregulation. The purpose of this study was to longitudinally characterize age-related change in sympathetic innervation of the spleen and sympathetic activity/tone in male Brown Norway (BN) rats, which live longer and have a strikingly different immune profile than F344 rats, the traditional animal model for aging research. Splenic sympathetic neurotransmission was evaluated between 8 and 32 months of age by assessing (1) NA nerve fiber density, (2) splenic norepinephrine (NE) concentration, and (3) circulating catecholamine levels after decapitation. We report a decline in NA nerve density in splenic white pulp (45%) at 15 months of age compared with 8-month-old (M) rats, which is followed by a much slower rate of decline between 24 and 32 months. Lower splenic NE concentrations between 15 and 32 months of age compared with 8M rats were consistent with morphometric findings. Circulating catecholamine levels after decapitation stress generally dropped with increasing age. These findings suggest there is a sympathetic-to-immune system dysregulation beginning at middle age. Given the unique T-helper-2 bias in BN rats, altered sympathetic-immune communication may be important for understanding the age-related rise in asthma and autoimmunity.

Sympathetic modulation of immunity: relevance to disease

Optimal host defense against pathogens requires cross-talk between the nervous and immune systems. This paper reviews sympathetic-immune interaction, one major communication pathway, and its importance for health and disease. Sympathetic innervation of primary and secondary immune organs is described, as well as evidence for neurotransmission with cells of the immune system as targets. Most research thus far has focused on neural-immune modulation in secondary lymphoid organs, has revealed complex sympathetic modulation resulting in both potentiation and inhibition of immune functions. SNS-immune interaction may enhance immune readiness during disease- or injury-induced 'fight' responses. Research also indicate that dysregulation of the SNS can significantly affect the progression of immune-mediated diseases. However, a better understanding of neural-immune interactions is needed to develop strategies for treatment of immune-mediated diseases that are designed to return homeostasis and restore normal functioning neural-immune networks.

Neuroimmune connections in jejunal and ileal Peyer’s patches at various bovine ages: potential sites for prion neuroinvasion

During preclinical stages of cattle orally infected with bovine spongiform encephalopathy (BSE), the responsible agent is confined to ileal Peyer's patches (IPP), namely in nerve fibers and in lymph follicles, before reaching the peripheral and central nervous systems. No infectivity has been reported in other bovine lymphoid organs, including jejunal Peyer's patches (JPP). To determine the potential sites for prion neuroinvasion in IPP, we analyzed the mucosal innervation and the interface between nerve fibers and follicular dendritic cells (FDC), two dramatic influences on neuroinvasion. Bovine IPP were studied at three ages, viz., newborn calves, calves less than 12 months old, and bovines older than 24 months, and the parameters obtained were compared with those of JPP. No differences in innervation patterns between IPP and JPP were found. The major difference observed was that, in calves of less than 12 months, IPP were the major mucosal-associated lymphoid organ that possessed a large number of follicles with extended FDC networks. Using a panel of antibodies, we showed that PP in 24-month-old bovines were highly innervated at various strategic sites assumed to be involved in the invasion and replication of the BSE pathogen: the suprafollicular dome, T cell area, and germinal centers. In PP in calves of less than 12 months old, no nerve fibers positive for the neurofilament markers NF-L (70 kDa) and NF-H (200 kDa) were observed in contact with FDC. Thus, in view of the proportion of these protein subunits present in neurofilaments, the innervation of the germinal centers can be said to be an age-dependent dynamic process. This variation in innervation might influence the path of neuroinvasion and, thus, the susceptibility of bovines to the BSE agent.

Long-term beta-adrenergic receptor blockade increases levels of the most mature thymocyte subsets in aged rats

Age-related increase in the density of thymic noradrenergic fibres and noradrenaline (NA) concentration is proposed to be associated with thymic involution and altered thymopoiesis. To test this hypothesis thymocyte differentiation/maturation and thymic structure were studied in 18-month-old male Wistar rats subjected to 14-day-long propranolol (P) blockade of beta-adrenoceptors (beta-ARs). The treatment primarily resulted in changes in the T-cell receptor (TCR)-dependent stages of thymopoiesis, which led to an increase in both the relative and absolute numbers of the most mature single positive (SP) CD4(+)CD8(-) (including cells with the CD4(+)CD25(+) regulatory phenotype) and CD4(-)CD8(+) TCRalphabeta(high) thymocytes. Accordingly, in the thymi of these rats an increase in both numerical density and absolute number of medullary thymocytes encompassing mainly the most mature SP cells was found. These findings, together with an increase in the thymocyte surface expression of the regulatory molecule Thy-1 (CD90) (implicated in negative regulation of TCRalphabeta-dependent thymocyte selection thresholds) in the same rats, may suggest increased positive/reduced negative thymocyte selection. Collectively, the results indicate that a decline in thymic efficiency in generating both conventional and regulatory T cells, and consequently in immune function, in aged rats may be, at least partly, attenuated by long-term blockade of beta-ARs with P.

Transneuronal mapping of the CNS network controlling sympathetic outflow to the rat thymus

The thymus is a primary immune organ that is essential for the development of functional T cells. The thymus receives sympathetic innervation, and thymocytes and thymic epithelial cells express functional adrenergic receptors. In this study, we employed retrograde, transneuronal virus tracing to identify the CNS cell groups that regulate sympathetic outflow to the thymus. Pseudorabies virus (PRV) was injected into the thymus, and the pattern of PRV infection in sympathetic regulatory centers of the CNS was determined at 72 and 120 h post-inoculation. PRV infection within the CNS first appeared within the spinal cord at 72 h post-inoculation and was confined to neurons within the intermediolateral cell column at levels T1-T7. At 120 h post-inoculation infection had spread within the spinal cord to include the central autonomic nucleus, intercalated cell nucleus and light infection within the cells of the lateral funiculus. Within the brain, PRV positive cells were found within nuclei of the medulla oblongata, pons and hypothalamus. Infection in the hypothalamus was observed within the arcuate nucleus, dorsal, lateral, and posterior hypothalamus and in all parvicellular subdivisions of the paraventricular hypothalamic nucleus. None of the infected animals exhibited labeling of the dorsal motor nucleus of the vagus. In summary, this study provides the first anatomic map of CNS neurons involved in control of sympathetic outflow to the thymus.

Beta2-adrenergic receptors mediate the differential effects of catecholamines on cytokine production of PBMC

We determined characteristics of beta2-adrenergic receptors (beta2R) on peripheral blood mononuclear cells (PBMC) and cytokine production after mitogenic stimulation and coincubation with catecholamines. PBMCs were stimulated with interleukin-2 (IL-2), tetanus toxoid (TT), anti-CD3 antibody, or phytohemagglutinin (PHA). The cytokines interferon-gamma (IFN-gamma), IL-4, and IL-6 were determined by ELISA following coincubation with high-dose (10(-5) M) and low-dose (10(-9) M) epinephrine (EPI) and norepinephrine (NE). Intracellular IFN-gamma and IL-4 were studied by FACS analysis. The beta2R density was investigated using a radioligand binding assay. The stimuli induced various cytokine profiles in PBMCs. Synthesis of IFN-gamma was induced by all mitogens and could be suppressed by catecholamines (26%-85% reduction). In PHA-stimulated PBMCs, IL-4 synthesis was decreased by high-dose catecholamines (24%-28% reduction). Adding a beta-blocking agent attenuated most catecholamine effects. A highly significant negative correlation between the density of beta2R with IFN-gamma and IL-6 levels of PHA-activated PBMCs (r = -0.88 to -0.96, p < 0.01-< 0.001) was observed. The results indicate that the density of beta2R on PBMC plays a role in mediating the differential catecholamine effects on cytokine production of PBMC. Furthermore, changes in cytokine expression induced by catecholamines favor Th2 responses.

Age effects on macrophage function vary by tissue site, nature of stimulant, and exercise behavior

We explored the effects of aging on macrophage function in male BALB/c mice from three age groups: young (2 months), middle-aged (12 months), and old (21 months). Macrophages were collected from alveoli, peritonea, and spleens of each age group. Cells were cultured in vitro with LPS or LPS+IFN-gamma and assayed for production of IL-1, IL-12, NO, and TNF-alpha. Using herpes simplex virus-1, age-related changes in intrinsic antiviral resistance (plaque assay) and extrinsic antiviral resistance (NO and TNF-alpha production) were determined in alveolar and/or peritoneal macrophages. Effects of chronic exercise on age-related macrophage changes were examined. In vitro, macrophages from the alveoli and spleen of older mice generally produced more cytokine and NO compared to younger counterparts. Conversely, macrophages from the peritonea of older mice generally produced less cytokine and NO in vitro compared to younger counterparts. Alveolar macrophages from both old and young mice showed higher intrinsic antiviral resistance to HSV-1 compared to middle-aged mice, while peritoneal macrophages from young mice showed reduced intrinsic resistance compared to those from both middle-aged and old mice. When challenged with HSV-1, a trend towards decreased peritoneal macrophage production of TNF-alpha and decreased alveolar macrophage production of IL-12 with advancing age was found. Chronic moderate exercise tended to reverse age-associated changes in macrophage function in old mice.

Is thymus redundant after adulthood?

Thymus is considered to involute with age with a decline in thymic function. However, this generality is not universally and incontrovertibly true. Many studies performed in animals and men have proved to the contrary that thymic activity and function appear to be well maintained in the old age and may be indispensable for T cell reconstitution in different immunological settings. During some clinical situations where T cell pool needs to be regenerated, renewal of thymic activity and mass has been observed in an otherwise dormant thymic remnant. New studies have revealed a dynamic interplay between postnatal thymus output and peripheral T cell pool. Moreover, age-related loss of thymic function appears to be only quantitative and not qualitative. This review, thus, focuses on the different conditions that lead to thymic involution and attempts to bring about the emerging notion and the clinical relevance of continuous thymic activity well beyond the adulthood to optimise the function of the immune system in the context of cancer and infectious diseases.

Expression of the winged helix/forkhead gene, foxn4, during zebrafish development

The winged-helix/forkhead transcription factor gene, foxn4, is expressed in the nervous system of developing and adult zebrafish. Prominent expression sites include the olfactory placode, the basal layer of the olfactory epithelium, the neuroepithelium of the developing retina, the germinal zone of the differentiated eye, regions of motoneuron development in the neural tube and periventricular regions of the brain. The adult thymus is the only major site of foxn4 expression outside of the nervous system.

Age-dependent changes in the nervous and endocrine control of the thymus

The immune system, especially the thymus, undergoes age-related modifications leading to structural and functional changes in the lymphoid organs and immunocompetent cells. Nevertheless, the consequences of thymic involution in the peripheral pool of T-cells are still a matter of controversy. The control of the thymic function is very complex and involves intrathymic signals, the autonomic nervous system, and the endocrine system. Both thymocytes and thymic stromal cells express receptors for a wide range of hormones, as well as for neurotransmitters and neuropeptides, thus affecting thymocytes maturation. This review summarizes the age-dependent variations in the extrathymic components of the thymic microenvironment, i.e., vegetative nerves and hormones, and the possible effects of those changes in the immune function.

Sympathetic nerve destruction in spleen in murine AIDS

In susceptible strains of mice, the LP-BM5 mixture of murine retroviruses induces the fatal immunodeficiency disease known as murine acquired immunodeficiency syndrome (murine AIDS or MAIDS). We have previously reported that murine AIDS produces a profound depletion of splenic norepinephrine (NE). Here, we demonstrate that NE depletion is limited to the spleen, a major site affected by LP-BM5 infection. NE depletion in the spleen is first observed at two weeks following LP-BM5 inoculation, concurrent with the onset of splenomegaly, and continues through 12 weeks post-infection. Neuroanatomical studies revealed that the reduction in NE is due to destruction of splenic sympathetic nerve fibers. Administration of the NE reuptake blocker desipramine did not prevent LP-BM5-induced NE depletion, suggesting that destruction is not caused by excess release and reuptake of NE. Elucidating the mechanism of MAIDS-induced sympathetic nerve destruction may provide insight into autonomic and peripheral neuropathies reported in people with AIDS.

Psychosocial stress augments tumor development through beta-adrenergic activation in mice

Housing conditions affect behavioral and biological responses of animals. We investigated the effect of same-sex-grouped (G), crowded (GC) and isolated (I) conditions on the growth of B16 melanoma or Meth A fibrosarcoma implanted in the footpad of syngeneic male C57BL / 6 or BALB / c mice. Differential housing altered host resistance to tumor growth. The host responses to stress were reflected in thymic atrophy, which was lowest in the G mice, highest in the GC mice and intermediate in the I mice. The GC condition was a more stressful social environment than the I condition in both male C57BL / 6 and BALB / c mice. Reflecting the extent of psychosocial stress, tumor growth was augmented in the order of GC, I and G condition, and a negative mass correlation between tumor and thymus was observed, thus clearly indicating that the host resistance to tumors was attenuated by psychosocial stress. Furthermore, the stress-enhanced tumor growth and thymus atrophy were completely abrogated by the oral administration of the non-selective beta-adrenergic antagonist, propranolol. On the contrary, the chronic administration of corticosterone significantly induced the atrophy of thymus and spleen without affecting tumor growth. These results suggest an interrelationship among psychosocial stress, tumor growth and beta-adrenergic activation.

Age-related changes in noradrenergic sympathetic innervation of the rat spleen is strain dependent

Previous findings from our laboratory revealed an age-related decline in noradrenergic (NA) sympathetic innervation of the spleen in male Fischer 344 (F344) rats. The purpose of this study was to determine whether other rat strains also progressively lose NA sympathetic nerves in the aging spleen. Sympathetic innervation of spleens from 3- and 21-month-old male F344, Brown Norway (BN), BN X F344 (BNF(1)), and Lewis rats was examined using fluorescence histochemistry to localize catecholamines combined with morphometric analysis and using high-performance liquid chromatography with electrochemical detection for measuring norepinephrine (NE). Neurochemistry revealed a significant age-related decline in NE concentrations in spleens from F344 and Lewis rats. In contrast, there was no effect of age on splenic NE concentrations in BN or BNF(1) rats. Consistent with neurochemical analysis, fluorescence histochemistry revealed a striking decline in NA innervation of spleens from old F344 and Lewis rats not observed in the other two strains. However, in BN and BNF(1) rats, nerve fibers were diminished in distal portions of the spleen but not in the hilar regions. Morphometric analysis confirmed neurochemical and histological findings, revealing approximately 65-70% loss in NA nerve density in spleens from F344 and Lewis rats. These findings indicate that age-related changes in sympathetic innervation of the rat spleen are strain-dependent. Whether the loss of sympathetic nerves in spleens from F344 and Lewis rats is associated with age-related changes in the splenic microenvironment remains to be determined. The functional significance of altered sympathetic innervation of the spleen with advancing age is discussed.

Occurrence of adrenergic nerve fibers in human thymus during immune response

The adrenergic nerve fibers (ANF), the neuropeptide Y-like immunoreactive nerve fibers (NPY-NF) and the noradrenaline (NA) amount were studied in the human thymus in subjects previously treated or not treated with interferon therapy with the aim to identify the changes due to the interferon therapy. This therapy has been used in patients affected by multiple sclerosis (MS). Biochemical and morphological methods were used associated with quantitative analysis of images. The whole thymuses were removed during autopsies in young and adult patients not treated with interferon. Moreover, samples of thymus were removed from patients, either young or adult who had previously been treated with interferon therapy, and subjected, for diagnostic reasons, to thymic biopsy. All samples of thymus were weighed, measured and dissected. Thymic slices were stained with Eosin-orange for detection of the microanatomical details, or with Bodian's reaction for recognition of nervous structures. Histofluorescence microscopy was used for detection of ANF, and immunofluorescence microscopy for recognition of NPY-like immunoreactive structures. All morphological results were subjected to quantitative analysis of images. Noradrenaline contained in thymic structures was measured by biochemical methods. Our results only concerned the effects of the therapy and suggested that treatment with interferon therapy induces many changes in the thymic structures: (1) The protein content of thymus is significantly increased; (2) the NA content in the thymus is also significantly increased; (3) NPY-like immunoreactive structures in the thymus are significantly increased; (4) occurrence of NPY-like immunoreactivity is particularly and significantly increased both in thymic microenvironment and in structures resembling nerve fibers; (5) ANF are significantly increased in the same thymic structures in which NPY-like immunoreactivity is also increased (i.e. thymic microenvironment and structures resembling nerve fibers). The morphological and biochemical changes observed can also explain the immunological changes induced in the thymus after immunostimulating therapy.

Effects of interleukin-2 on the expression of corticotropin-releasing hormone in nerves and lymphoid cells in secondary lymphoid organs from the Fischer 344 rat

This study examined the influence of interleukin (IL)-2 on corticotropin releasing hormone (CRH) immunoreactivity in the Fischer 344 (F344) rat spleen. Rats were given either vehicle or 1, 10, 25, 50, 100, or 200 ng of human recombinant (hr)IL-2 by intraperitoneal (i.p.) injection, and were sacrificed 0.5, 1, 4, 12, or 24 h after treatment. Spleens and mesenteric lymph nodes were prepared for immunocytochemistry to localize CRH. In spleens from vehicle-treated animals, CRH immunoreactivity was present in several types of cells of the immune system, but CRH(+) nerves were not observed in either spleens or lymph nodes from vehicle-treated animals. Treatment with IL-2 induced CRH expression in nerves in the spleen in a dose- and time-dependent manner. CRH(+) nerves were not found in the mesenteric lymph nodes after IL-2 treatment, instead a dramatic time- and dose-dependent accumulation of CRH(+) cells (resembling small lymphocytes and large granular mononuclear cells) in the cortex and medulla. These findings indicate that IL-2 stimulates the synthesis of CRH in nerves that innervate the F344 rat spleen, and promote the appearance of CRH(+) immunocytes into draining mesenteric lymph nodes.

The NPY effects on murine leukocyte adherence and chemotaxis change with age. Adherent cell implication

The two-way communication between the nervous and immune system is currently well-known, but the age-related changes in this communication have been scarcely studied. In the present work, we have investigated the in vitro effects of neuropeptide Y (NPY) at concentrations ranging from 10(-13) to 10(-7) M on the adherence and chemotaxis capacities of spleen, axillary node, thymus and peritoneum leukocytes from BALB/c mice. The NPY effect on these functions was examined on cells from animals of four different ages, i.e. young (12+/-2 weeks old), adult (24+/-2 weeks old), mature (50+/-2 weeks old) and old (72+/-2 weeks old). In young animals, NPY stimulates the adherence of leukocytes from spleen, axillary nodes and thymus and inhibits it in cells from peritoneum. In adult animals NPY inhibits the adherence of leukocytes from thymus. These effects disappear with ageing in all locations. Chemotaxis is stimulated by this neuropeptide at all ages in cells from axillary nodes and peritoneum, but this effect is absent in old mice. NPY exerts an inhibitory effect on the chemotaxis of leukocytes from thymus at all ages studied. These NPY effects on leukocytes seem to be carried out through adherent cells.

Interactions of autonomic nervous, neuroendocrine, and immune systems in rheumatoid arthritis

In general, it is assumed that the two pathways (i.e., HPA axis and sympathetic nervous system) probably act cooperatively to maintain homeostasis. The previously mentioned studies clearly point to a disturbance in the interaction between the ANS, the HPA axis, and the immune system in chronic rheumatic diseases (Fig. 2). Even early on in the course of RA, these changes can be observed. Along with the results obtained in animal models, an important role of neuroendocrine interactions in the pathogenesis of RA is proposed. Further studies are required to establish the exact contribution of the ANS in the initiation and perpetuation of RA. To date, it is quite obvious that neuropeptides play a part in the orchestration of the various molecules (e.g., cytokines) exerting modulatory effects on immune cells. One can speculate that therapeutic implications are likely to result from investigations on the ANS-immune interactions. Based on early observations that blocking catecholamine actions ameliorate symptoms of RA, it is quite promising to follow this avenue in investigating ANS-immune interactions of various time points of the disease. Conversely, further studies are required to determine the contribution of the HPA axis to the onset of RA. Results from ongoing studies are eagerly awaited so as to establish new therapeutic options. In the future,it may be possible to interfere with the inflammatory process in RA by an exactly timed neuroendocrine intervention right at or even before the onset of disease. Therapy with steroids in RA might be better planned based on the genetically determined reactivity of an individual's HPA axis. In this respect, a recent report by Masi et al is of special interest. Based on the current literature on the disturbances in the neuroendocrine, immune, and microvascular systems found in early RA, the authors hypothesize that an imbalance in the interactive homeostasis of these systems develops during a long preclinical phase and eventually leads to the outbreak of the disease in genetically predisposed individuals. This interesting hypothesis includes the perspective that individuals prone to develop RA may be identified in a preclinical phase and treated prophylactically. In any event, results from all these studies are promising in two ways: to gain more insight in the pathogenic process of RA and to establish novel therapies to help the patients bear their burden of a chronic rheumatic disease.

Occurrence of adrenergic nerve fibers and of noradrenaline in thymus gland of juvenile and aged rats

Adrenergic nerve fibers (ANF) were studied in juvenile, adult and old rats by biochemical and morphological methods and by quantitative analysis of images (QAI). After chemical sympathectomy with neurotoxin 6-OH dopamine, the greater part of ANF disappeared. The whole thymus was drawn in juvenile normal or sympathectomized rats, in adult normal or sympathectomized rats and in old normal or sympathectomized rats. Thymuses from the above-mentioned animals were weighed, measured and dissected. Thymic slices were stained with eosin orange for the detection of the microanatomical details and with Bodian's method for the recognition of the whole nerve fibers. Histofluorescence microscopy was used for staining of ANF while immunofluorescence microscopy was employed for staining of neuropeptide Y (NPY)-like immunoreactivity. Biochemical dosage of proteins and of noradrenaline amount was performed. Finally, all morphological results were subjected to QAI. Our results suggest that: (1) total innervation of the thymus increases with age; (2) ANF do not change with age; (3) the content of noradrenaline in the thymus increases with age; and (4) NPY-like immunoreactive structures in the thymus decrease with age. Biochemical results are in accordance with the morphological ones and both are confirmed by means of QAI. The probable function of sympathetic innervation of rat thymus is also discussed.

Alterations in sympathetic noradrenergic innervation in lymphoid organs with age

Previous work in our laboratories has shown a selective age-related loss of sympathetic noradrenergic (NA) nerves from the spleen and lymph nodes of rats. In contrast, NA nerve fiber density is dramatically increased in the thymus of aged mice and rats. We hypothesize that the diminished splenic NA innervation in old rats is due to cumulative oxidative metabolic autodestruction of NA nerve terminals by high concentrations of norepinephrine released during specific time periods of immunologic reactivity. These old animals show a loss of NK cell activity, diminished T-cell proliferation, and diminished cell-mediated immunity. To assess the plasticity of NA nerve fibers in the aged rat spleen, male Fischer 344 rats at 24 months of age were treated for 2 months with daily injections of 0.25 or 1.0 mg/kg/day of L-deprenyl, followed by a 10-day drug washout period. The L-deprenyl-treated rats showed a remarkable sprouting of NA nerves into the splenic white pulp, with regrowth into appropriate vascular and parenchymal compartments. Rats with enhanced NA nerve regrowth showed increased IL-2 and IFN-gamma production and NK cell activity compared with vehicle injected or uninjected age-matched control rats. We proposed that L-deprenyl stimulates NA nerve regrowth into secondary lymphoid organs, which leads to partial recovery of measures of immunosenescence.

Age-associated alterations in sympathetic neural interactions with the immune system

We have examined age-related alterations in sympathetic noradrenergic (NA) innervation in primary and secondary lymphoid organs from mouse and rat. As the thymus involuted with age, the density of NA innervation and norepinephrine (NE) concentration increased markedly. Total thymic NE was not altered significantly with age, suggesting that NA innervation is maintained as the thymus involutes. In the rat spleen, NA innervation and NE concentration were diminished with age. Enhanced antibody responses and in vitro proliferation to a T-dependent protein antigen were observed following selective destruction of NA nerve fibers with the neurotoxin 6-hydroxydopamine (6-OHDA), demonstrating that the diminished NA innervation in the aged spleen is capable of signaling the immune system. Plasticity of NA nerves in old rats was demonstrated following lesioning with 6-OHDA and in intact rats treated with L-deprenyl, a monoamine oxidase B inhibitor. These age-related alterations in NA innervation of lymphoid organs occur concurrently with age-associated changes in immune function. Understanding the functional relationship between these two physiological systems in aging will contribute to a greater understanding of sympathetic nervous system regulation of immune function.