Sympathetic innervation of murine thymus and spleen: a comparative histofluorescence study

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.

3D anatomy of autonomic innervations in immune organs of a non-human primate and the human

Direct neural inputs to immune organs have been observed for decades, with their functions in neuroimmune regulation being increasingly appreciated. However, the current knowledge of such neural structures, particularly those in primate immune organs, remains incomplete. In this study, we comprehensively assessed the 3D anatomy of autonomic (i.e., sympathetic and parasympathetic) innervations in the immune organs of the rhesus macaque monkey and the human for the first time. Aided with the advanced technique of whole-tissue immunolabeling and lightsheet fluorescence imaging, we revealed the densely organized sympathetic architecture in the parenchyma of the adult monkey and human spleens. On the other hand, only sparse, if any, sympathetic inputs were observed inside the lymph nodes, Peyer's patches, or thymus. In contrast, there were minimal parasympathetic innervations in the parenchyma of these examined immune organs. Together, this work has documented the unique patterns of autonomic innervations in different immune organs of a non-human primate and the human, serving as an essential reference for future research on neuroimmune regulation in the field.

Neuronal regulation of B-cell immunity: Anticipatory immune posturing?

The brain may sense, evaluate, modulate, and intervene in the operation of immune system, which would otherwise function autonomously in defense against pathogens. Antibody-mediated immunity is one arm of adaptive immunity that may achieve sterilizing protection against infection. Lymphoid organs are densely innervated. Immune cells supporting the antigen-specific antibody response express receptors for neurotransmitters and glucocorticoid hormones, and they are subjected to collective regulation by the neuroendocrine and the autonomic nervous system. Emerging evidence reveals a brain-spleen axis that regulates antigen-specific B cell responses and antibody-mediated immunity. In this article, we provide a synthesis of those studies as pertinent to neuronal regulation of B cell responses in secondary lymphoid organs. We propose the concept of defensive immune posturing as a brain-initiated top-down reaction in anticipation of potential tissue injury that requires immune protection.

Neural control of the spleen as an effector of immune responses to inflammation: mechanisms and treatments

Immune system responses are a vital defense mechanism against pathogens. Inflammatory mediators finely regulate complex inflammatory responses from initiation to resolution. However, in certain conditions, the inflammation is initiated and amplified, but not resolved. Understanding the biological mechanisms underlying the regulation of the immune response is critical for developing therapeutic alternatives, including pharmaceuticals and bioelectronic tools. The spleen is an important immune effector organ since it orchestrates innate and adaptive immune responses such as pathogen clearance, cytokine production, and differentiation of cells, therefore playing a modulatory role that balances pro- and anti-inflammatory responses. However, modulation of splenic immune activity is a largely unexplored potential therapeutic tool that could be used for the treatment of inflammatory and life-threatening conditions. This review discusses some of the mechanisms controlling neuroimmune communication and the brain-spleen axis.

Neuroimmune axis of cardiovascular control: mechanisms and therapeutic implications

Cardiovascular diseases (CVDs) make a substantial contribution to the global burden of disease. Prevention strategies have succeeded in reducing the effect of acute CVD events and deaths, but the long-term consequences of cardiovascular risk factors still represent the major cause of disability and chronic illness, suggesting that some pathophysiological mechanisms might not be adequately targeted by current therapies. Many of the underlying causes of CVD have now been recognized to have immune and inflammatory components. However, inflammation and immune activation were mostly regarded as a consequence of target-organ damage. Only more recent findings have indicated that immune dysregulation can be pathogenic for CVD, identifying a need for novel immunomodulatory therapeutic strategies. The nervous system, through an array of afferent and efferent arms of the autonomic nervous system, profoundly affects cardiovascular function. Interestingly, the autonomic nervous system also innervates immune organs, and neuroimmune interactions that are biologically relevant to CVD have been discovered, providing the foundation to target neural reflexes as an immunomodulatory therapeutic strategy. This Review summarizes how the neural regulation of immunity and inflammation participates in the onset and progression of CVD and explores promising opportunities for future therapeutic strategies.

Glucocorticoid resistance and β2-adrenergic receptor signaling pathways promote peripheral pro-inflammatory conditions associated with chronic psychological stress: A systematic review across species

Activation of the HPA-axis and SNS are widely accepted to link chronic stress with elevated levels of peripheral pro-inflammatory markers in blood. Yet, empirical evidence showing that peripheral levels of glucocorticoids and/or catecholamines mediate this effect is equivocal. Recent attention has turned to the possibility that cellular sensitivity to these ligands may contribute to inflammatory mediators that accompany chronic stress. We review current evidence for the association of chronic stress with glucocorticoid receptor (GR) and β-adrenergic receptor (β-AR) signaling sensitivity. Across 15 mouse, 7 primate, and 19 human studies, we found that chronic stress reliably associates with downregulation in cellular GR sensitivity, alterations in intracellular β-AR signaling, and upregulation in pro-inflammatory biomarkers in peripheral blood. We also present evidence that alterations in GR and β-AR signaling may be specific to myeloid progenitor cells such that stress-related signaling promotes release of cells that are inherently less sensitive to glucocorticoids and differentially sensitive to catecholamines. Our findings have broad implications for understanding mechanisms by which chronic stress may contribute to pro-inflammatory phenotypes.

Control of immune cell trafficking through inter-organ communication

Cell migration is a cardinal feature of the immune system. Immune cell trafficking is orchestrated principally by chemokines and adhesion molecules, which guide the cells to the right place and at the right time to efficiently induce immune responses. Recent studies have demonstrated that signals from other organ systems influence the expression of and responsiveness to these guidance cues and consequentially immune cell migration. Neuronal inputs control entry and exit of immune cells to and from lymphoid and non-lymphoid tissues. The circadian clock helps establish diurnal variations in immune cell distribution among tissues. Nutritional status also alters immune cell homing to the bone marrow. In this review, we summarize the current knowledge about inter-organ control of immune cell trafficking and discuss the physiological and pathological significance of these mechanisms.

Adrenergic regulation of immune cell function and inflammation

The sympathetic nervous system integrates the functions of multiple organ systems by regulating their autonomic physiological activities. The immune system is regulated both locally and systemically by the neurotransmitters epinephrine and norepinephrine secreted by the adrenal gland and local sympathetic neurons. Immune cells respond by activation of adrenergic receptors, primarily the β2-adrenergic receptor, which signal through heterotrimeric G-proteins. Depending upon the cell type, adrenergic signaling regulates a variety of functions in immune cells ranging from cellular migration to cytokine secretion. Furthermore, due to the diurnal oscillation of systemic norepinephrine levels, various immune functions follow a circadian rhythmic pattern. This review will highlight recent advances in our understanding of how the sympathetic nervous system regulates both innate and adaptive immune functions and how this regulation is linked to circadian rhythms.

Adrenergic Signaling in Circadian Control of Immunity

Circadian rhythms govern a multitude of physiologic processes, both on a cell-intrinsic level and systemically, through the coordinated function of multi-organ biosystems. One such system-the adrenergic system-relies on the catecholamine neurotransmitters, adrenaline and noradrenaline, to carry out a range of biological functions. Production of these catecholamines is under dual regulation by both neural components of the sympathetic nervous system and hormonal mechanisms involving the hypothalamus-pituitary-adrenal axis. Importantly, both neural and hormonal arms receive input from the body's central clock, giving rise to the observed rhythmic variations in catecholamine levels in blood and peripheral tissues. Oscillations in catecholamine signals have the potential to influence various cellular targets expressing adrenergic receptors, including cells of the immune system. This review will focus on ways in which the body's central master clock regulates the adrenergic system to generate circadian rhythms in adrenaline and noradrenaline, and will summarize the existing literature linking circadian control of the adrenergic system to immunologic outcomes. A better understanding of the complex, multi-system pathways involved in the control of adrenergic signals may provide immunologists with new insight into mechanisms of immune regulation and precipitate the discovery of new therapeutics.

Neuroimmune interactions in cardiovascular diseases

Our body is continuously in contact with external stimuli that need a fine integration with the internal milieu in order to maintain the homoeostasis. Similarly, perturbations of the internal environment are responsible for the alterations of the physiological mechanisms regulating our main functions. The nervous system and the immune system represent the main interfaces between the internal and the external environment. In carrying out these functions, they share many similarities, being able to recognize, integrate, and organize responses to a wide variety of stimuli, with the final aim to re-establish the homoeostasis. The autonomic nervous system, which collectively refers to the ensemble of afferent and efferent neurons that wire the central nervous system with visceral effectors throughout the body, is the prototype system controlling the homoeostasis through reflex arches. On the other hand, immune cells continuously patrol our body against external enemies and internal perturbations, organizing acute responses and forming memory for future encounters. Interesting to notice, the integration of the two systems provides a further unique opportunity for fine tuning of our body's homoeostasis. In fact, the autonomic nervous system guides the development of lymphoid and myeloid organs, as well as the deployment of immune cells towards peripheral tissues where they can affect and control several physiological functions. In turn, every specific immune cell type can contribute to regulate neural circuits involved in cardiovascular function, metabolism, and inflammation. Here, we review current understanding of the cross-regulation between these systems in cardiovascular diseases.

The β2-adrenergic receptor controls inflammation by driving rapid IL-10 secretion

The mammalian nervous system communicates important information about the environment to the immune system, but the underlying mechanisms are largely unknown. Secondary lymphoid organs are highly innervated by sympathetic neurons that secrete norepinephrine (NE) as the primary neurotransmitter. Immune cells express adrenergic receptors, enabling the sympathetic nervous system to directly control immune function. NE is a potent immunosuppressive factor and markedly inhibits TNF-α secretion from innate cells in response to lipopolysaccharide (LPS). In this study, we demonstrate that NE blocks the secretion of a variety of proinflammatory cytokines by rapidly inducing IL-10 secretion from innate cells in response to multiple Toll-like receptor (TLR) signals. NE mediated these effects exclusively through the β2-adrenergic receptor (ADRB2). Consequently, Adrb2^-/- animals were more susceptible to L. monocytogenes infection and to intestinal inflammation in a dextran sodium sulfate (DSS) model of colitis. Further, Adrb2^-/- animals rapidly succumbed to endotoxemia in response to a sub-lethal LPS challenge and exhibited elevated serum levels of TNF-α and reduced IL-10. LPS-mediated lethality in WT animals was rescued by administering a β 2-specific agonist and in Adrb2^-/- animals by exogenous IL-10. These findings reveal a critical role for ADRB2 signaling in controlling inflammation through the rapid induction of IL-10. Our findings provide a fundamental insight into how the sympathetic nervous system controls a critical facet of immune function through ADRB2 signaling.

Gender-related Histological Changes in the Thymus Gland After Pinealectomy and Short-term Melatonin Treatment in Rats

Objectives

The aim of this study was to investigate the effects of pinealectomy and melatonin treatment on the rat thymus gland characteristics, taking into consideration possible gender differences.

Materials and methods

Thirty adult Wistar rats of both sexes were divided into three groups. Group C and group PX served as control groups and included sham-pinealectomized and pinealectomized animals that were treated with 10% ethanol solution (0,1ml/daily, subcutaneous). Animals from third group (group PXM) underwent pinealectomy and seven days after surgery started receiving melatonin dissolved in 10% ethanol solution (3mg/kg/daily, subcutaneous). All animals were treated for 4 weeks.

Results

Volume density of the thymus cortex showed statistically significant (p<0,05) decrease while the volume density of the thymus medulla was increased in the pinealectomized compared to the sham-pinealectomized female rats. Numerical density of macrophages as well as the distribution of blood vessels showed no gender differences. The numerical density of lymphocytes was statistically significantly decreased in female in comparison to the male pinealectomized rats. Melatonin treatment was proved to cause reverse effects in the sense that the results from the melatonin treated group corresponded to the results obtained from the control group of animals.

Conclusion

The results of this study suggest that the pinealectomy causes gender-related changes in the rat thymus. Short-term melatonin treatment showed reverse effect, equally in both sexes.

β2 -Adrenoceptors inhibit neutrophil extracellular traps in human polymorphonuclear leukocytes

This study tests the hypothesis that in isolated human polymorphonuclear leukocytes (PMN) adrenergic ligands can affect neutrophil extracellular trap (NET) formation. We have previously shown that, in PMN, adrenaline (A), through the activation of adrenergic receptors (AR), reduces stimulus-dependent cell activation; we have, therefore, planned to investigate if AR are involved in NET production. PMN were obtained from venous blood of healthy subject. The ability of adrenergic ligands to affect reactive oxygen species (ROS) production, NET production, and cell migration was investigated in cells cultured under resting conditions or after activation with N-formyl-methionyl-leucyl-phenylalanine (fMLP), LPS, or IL-8. Stimuli-induced NET production measured as ROS, microscopic evaluation, and elastase production was reverted by A and this effect was blocked by the selective β₂ -AR antagonist ICI-118,551. The stimulus-induced ROS generation and migration was prevented by A and by isoprenaline (ISO), and these effects were counteracted only by ICI-118,551 and not by the other two selective ligands for the β₁ and β₃ -AR. Finally, the presence of the β-ARs on PMN was confirmed, by means of microscopy and flow cytometry. The data of the present study suggest that adrenergic compounds, through the interaction of mainly β₂ -AR, are able to affect neutrophil functions. These data are suggestive of a possible therapeutic role of β₂ -AR ligands (in addition to their classical use), promoting the possible therapeutic relevance of adrenergic system in the modulation of innate immunity proposing their possible use as anti-inflammatory drugs.

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.

Autonomic nervous system and immune system interactions

The present review assesses the current state of literature defining integrative autonomic-immune physiological processing, focusing on studies that have employed electrophysiological, pharmacological, molecular biological, and central nervous system experimental approaches. Central autonomic neural networks are informed of peripheral immune status via numerous communicating pathways, including neural and non-neural. Cytokines and other immune factors affect the level of activity and responsivity of discharges in sympathetic and parasympathetic nerves innervating diverse targets. Multiple levels of the neuraxis contribute to cytokine-induced changes in efferent parasympathetic and sympathetic nerve outflows, leading to modulation of peripheral immune responses. The functionality of local sympathoimmune interactions depends on the microenvironment created by diverse signaling mechanisms involving integration between sympathetic nervous system neurotransmitters and neuromodulators; specific adrenergic receptors; and the presence or absence of immune cells, cytokines, and bacteria. Functional mechanisms contributing to the cholinergic anti-inflammatory pathway likely involve novel cholinergic-adrenergic interactions at peripheral sites, including autonomic ganglion and lymphoid targets. Immune cells express adrenergic and nicotinic receptors. Neurotransmitters released by sympathetic and parasympathetic nerve endings bind to their respective receptors located on the surface of immune cells and initiate immune-modulatory responses. Both sympathetic and parasympathetic arms of the autonomic nervous system are instrumental in orchestrating neuroimmune processes, although additional studies are required to understand dynamic and complex adrenergic-cholinergic interactions. Further understanding of regulatory mechanisms linking the sympathetic nervous, parasympathetic nervous, and immune systems is critical for understanding relationships between chronic disease development and immune-associated changes in autonomic nervous system function.

Mouse models of sepsis elicit spontaneous action potential discharge and enhance intracellular Ca2+ signaling in postganglionic sympathetic neurons

Sepsis is a severe systemic inflammatory disorder that rapidly activates the sympathetic nervous system to enhance catecholamine secretion from postganglionic sympathetic neurons and adrenal chromaffin cells. Although an increase in preganglionic drive to postganglionic sympathetic tissues has been known to contribute to this response for quite some time, only recently was it determined that sepsis also has direct effects on adrenal chromaffin cell Ca2+ signaling and epinephrine release. In the present study, we characterized the direct effects of sepsis on postganglionic sympathetic neuron function. Using the endotoxemia model of sepsis in mice, we found that almost a quarter of postganglionic neurons acquired the ability to fire spontaneous action potentials, which was absent in cells from control mice. Spontaneously firing neurons possessed significantly lower rheobases and fired a greater number of action potentials at twice the rheobase compared to neurons from control mice. Sepsis did not significantly affect voltage-gated Ca2+ currents. However, global Ca2+ signaling was enhanced in postganglionic neurons isolated from 1 to 24 h endotoxemic mice. A similar increase in the amplitude of high-K+-stimulated Ca2+ transients was observed during the cecal ligation and puncture model of sepsis. The enhanced excitability and Ca2+ signaling produced during sepsis likely amplify the effect of increased preganglionic drive on norepinephrine release from postganglionic neurons. This is important, as sympathetic neurons are integral to the anti-inflammatory autonomic reflex that is activated during sepsis.

Effects of catecholamines on thymocyte apoptosis and proliferation depend on thymocyte microenvironment

The present study, through quantification of tyrosine hydroxylase (TH) expression and catecholamine (CA) content in the presence and in the absence of α-methyl-p-tyrosine (AMPT), a TH inhibitor, in adult thymic organ (ATOC) and thymocyte culture, demonstrated that thymic cells produce CAs. In addition, in ATOC an increase in β2-adrenoceptor (AR) mRNA expression and β2-AR thymocyte surface density was registered. Furthermore, AMPT (10(-4)M), as propranolol (10(-4)M), augmented thymocyte apoptosis and diminished thymocyte proliferation in ATOC. Propranolol exerted these effects acting on CD3(high) thymocytes. However, in thymocyte cultures, propranolol (10(-6)M) acting on the same thymocyte subset exerted the opposing effect on thymocyte apoptosis and ConA-stimulated proliferation. This suggested that, depending on thymocyte microenvironment, differential effects can be induced through the same type of AR. Additionally, arterenol (10(-8) to 10(-6)M), similar to propranolol, diminished apoptosis, but increased ConA-stimulated thymocyte proliferation in thymocyte culture. However, differently from propranolol, arterenol affected manly CD3- thymocyte subset, which harbors majority of α1-AR+thymocytes. Additionally, arterenol showed a dose-dependent decrease in efficiency of thymocyte apoptosis and proliferation modulation with the rise in its concentration. Considering greater affinity of arterenol for α1-ARs than for β2-ARs, the previous findings could be attributable to increased engagement of β2-ARs with the rise of arterenol concentration. Consistently, in the presence of propranolol (10(-6)M), a β-AR blocker, the arterenol (10(-8)M) effects on thymocytes were augmented. In conclusion, thymic endogenous CAs, acting through distinct AR types and, possible, the same AR type (but in different cell microenvironment) may exert the opposing effects on thymocyte apoptosis/proliferation.

Neural regulation of gastrointestinal inflammation: role of the sympathetic nervous system

The sympathetic innervation of the gastrointestinal (GI) tract regulates motility, secretion and blood flow by inhibiting the activity of the enteric nervous system (ENS) and direct vasoconstrictor innervation of the gut microvasculature. In addition to these well-established roles, there is evidence that the sympathetic nervous system (SNS) can modulate GI inflammation. Postganglionic sympathetic neurons innervate lymphoid tissues and immune cells within the GI tract. Furthermore, innate and adaptive immune cells express receptors for sympathetic neurotransmitters. Activation of these receptors can affect a variety of important immune cell functions, including cytokine release and differentiation of helper T lymphocyte subsets. This review will consider the neuroanatomical evidence of GI immune cell innervation by sympathetic axons, the effects of blocking or enhancing SNS activity on GI inflammation, and the converse modulation of sympathetic neuroanatomy and function by GI inflammation.

Cellular and nerve fibre catecholaminergic thymic network: steroid hormone dependent activity

The thymus plays a critical role in establishing and maintaining the peripheral T-cell pool. It does so by providing a microenvironment within which T-cell precursors differentiate and undergo selection processes to create a functional population of major histocompatibility complex-restricted, self-tolerant T cells. These cells are central to adaptive immunity. Thymic T-cell development is influenced by locally produced soluble factors and cell-to-cell interactions, as well as by sympathetic noradrenergic and endocrine system signalling. Thymic lymphoid and non-lymphoid cells have been shown not only to express beta- and alpha(1)- adrenoceptors (ARs), but also to synthesize catecholamines (CAs). Thus, it is suggested that CAs influence T-cell development via both neurocrine/endocrine and autocrine/paracrine action, and that they serve as immunotransmitters between thymocytes and nerves. CAs acting at multiple sites along the thymocyte developmental route affect T-cell generation not only numerically, but also qualitatively. Thymic CA level and synthesis, as well as AR expression exhibit sex steroid-mediated sexual dimorphism. Moreover, the influence of CAs on T-cell development exhibits glucocorticoid-dependent plasticity. This review summarizes recent findings in this field and our current understanding of complex and multifaceted neuroendocrine-immune communications at thymic level.

The role of the sympathetic nervous system in the thymus in health and disease

The existence of a network of immunoneuroendocrine interactions that results in the reciprocal modulation of the classical functions of each system is well established at present. Most of the evidence derives from studies on secondary lymphoid organs, such as the spleen and lymph nodes. In this article, several aspects relevant to understand the role of the sympathetic nervous system in the establishment of these interactions in the thymus are discussed. At present, the sympathetic innervation of the thymus, the expression of adrenergic receptors in thymic cells, particularly of β-adrenergic receptors, and the effect of sympathetic neurotransmitters, although mainly derived from in vitro or pharmacological studies, seem to be relatively well studied. However, other aspects, such as the relevance that immune-sympathetic interactions at the thymic level may have for certain diseases, specially autoimmune or other diseases that primarily involve the activation of the immune system, as well as how the integration of sympathetic and hormonal signals at local levels may affect thymic functions, certainly deserve further investigation.

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.

Neuroimmunomodulation: Neuroendocrine Regulation of Immunity: The Effects of Noradrenaline inXenopus Laevis, the South African Clawed Toad

A functional association between the peripheral nervous and the immune system in Xenopus laevis, the South African clawed toad, is demonstrated. This association involves the neurotransmitter noradrenaline (NA), produced and released by the sympathetic nerves of the spleen. Chemical sympathectomy prior to immunization reduces splenic NA, and decreases thymus-dependent (TD), but increases thymus-independent (TI), antibody responses. Immune challenge with representatives of the three antigen classes affects splenic NA levels differentially. Thus, the modulatory effect of NA on immunity will depend on the immunogen used. Carrier-priming of helper function in TD responses stimulates a transitory NA release in the spleen, while subsequent immunization activates a more prolonged release. The two types of challenge differ in the antigenic dose given. The effects of NA also depend on the time when it is applied. If used early in the in vivo TD response, antibody production is increased, but if given later, suppressor function is stimulated, thus decreasing antibody production. NA increases both amplifying and suppressing T cell functions in TD responses through stimulation of the alpha 2 adrenoceptor. Alpha 2 adrenoceptor stimulation decreases, and beta adrenoceptor stimulation increases, anti-TNP reactivity. Since an alpha 2 receptor agonist does not affect lectin-stimulated T cell mitogenesis, while a beta receptor agonist depresses it, NA appears to up-regulate T cell functions by affecting their maturation, rather than their clonal expansion.

EFFECTS OF BETA-ADRENOCEPTOR BLOCKADE ON THE PHENOTYPIC CHARACTERISTICS OF THYMOCYTES AND PERIPHERAL BLOOD LYMPHOCYTES

The study revealed that beta-adrenoceptor blockade with propranolol (0.40 mg/100 g/day, s.c.) in adult male DA rats: (i) increased the thymocyte proliferation and apoptosis, (ii) caused disturbances in kinetics of T cell differentiation leading to distinguishable changes in relative proportion of thymocytes at distinct maturational steps and to an expansion of the most mature single positive (CD4+, CD8+) thymocyte pool, (iii) affected the relative proportion of neither CD4+ nor CD8+ peripheral blood lymphocytes (PBL), and (iv) augmented the relative number of CD8+CD25+ cells. Thus, the results suggest the role of beta-adrenoceptors in fine-tuning of T cell maturation, and, possibly, distribution and activation of distinct PBL subsets.

Sympathetic nervous system control of anti-influenza CD8+ T cell responses

Despite the longstanding appreciation of communication between the nervous and the immune systems, the nature and significance of these interactions to immunity remain enigmatic. Here, we show that 6-hydroxydopamine-mediated ablation of the mouse peripheral sympathetic nervous system increases primary CD8(+) T cell responses to viral and cellular antigens presented by direct priming or cross-priming. The sympathetic nervous system also suppresses antiviral CD4(+) T cell responses, but this is not required for suppressing CD8(+) T cell responses. Adoptive transfer experiments indicate that enhanced CD8(+) responses do not result from permanent alterations in CD8(+) T cell function in sympathectomized mice. Rather, additional findings suggest that the sympathetic nervous system tempers the capacity of antigen-presenting cells to activate naïve CD8(+) T cells. We also show that antiviral CD8(+) T cell responses are enhanced by administration of a beta(2) (but not beta(1) or alpha) adrenergic antagonist. These findings demonstrate a critical role for the sympathetic nervous system in limiting CD8(+) T cell responses and indicate that CD8(+) T cell responses may be altered in patients using beta-blockers, one of the most widely prescribed classes of drugs.

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.

Role of nonsynaptic communication in regulating the immune response

The discovery of nonsynaptic communication in the 1960s and 1970s was an important milestone in investigating the function of the nervous system, and it revolutionized our view about information transmission between neurons. In addition, nonsynaptic communication has a practical importance not only within the nervous system, but in the communication between the peripheral nervous system and other organ systems. Nonsynaptic communication takes place in different immune organs, which are innervated by sympathetic nerve terminals. In addition, the function of microglia, one of the immunocompetent cell types of the brain, can also be affected by neurotransmitters released from axon varicosities. The various functions of immune cells are modulated by released neurotransmitters without any direct synaptic contact between nerve endings and targeted immune cells requiring only functional neurotransmitter receptors on immune cells. Here, we briefly overview the role of the various receptor subtypes mediating nonsynaptic modulation of the function of immunocompetent cells both in the periphery and in the central nervous system.

Ontogeny of intrinsic innervation in the human thymus and spleen

The ontogeny of the innervation of human lymphoid organs has not been studied in detail. Our aim was to assess the nature and distribution of parenchymal nerves in human fetal thymus and spleen. We used the peroxidase immunohistochemical technique with antibodies specific to neuron-specific enolase (NSE), neurofilaments (NF), PGP9.5, S100 protein, and tyrosine hydroxylase (TH) and evaluated our results with image analysis. In human fetal thymus, NSE-, NF-, S100-, PGP9.5-, and TH-positive nerves were identified associated with large blood vessels from 18 gestational weeks (gw) onwards, increasing in density during development. Their branches penetrated the septal areas at 20 gw, reaching the cortex and the corticomedullary junction between 20 and 23 gw. Few nerve fibers were seen in the medulla in close association with Hassall's corpuscles. In human fetal spleen, NSE-, NF-, S100-, PGP9.5-, and TH-positive nerve fibers were localized in the connective tissue surrounding the splenic artery at 18 gw. Perivascular NSE-, NF-, S100-, PGP9.5-, and TH-positive nerve fibers were seen extending into the white pulp, mainly in association with the central artery and its branches, increasing in density during gestation. Scattered NSE-, NF-, S100-, PGP9.5-, and TH-positive nerve fibers and endings were localized in the red pulp from 18 gw onward. The predominant perivascular distribution of most parenchymal nerves implies that thymic and splenic innervation may play an important functional role during intrauterine life.

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.

Characterization of thymocyte phenotypic alterations induced by long-lasting beta-adrenoceptor blockade in vivo and its effects on thymocyte proliferation and apoptosis

Adult male Wistar rats were subjected to propranolol (P, 0.40 mg/100 g/day) or saline (S) administration (controls) over 14 days. The expression of major differentiation molecules on thymocytes and Thy-1 (CD90) molecules, which are shown to adjust thymocyte sensitivity to TCRalphabeta signaling, was studied. In addition, the sensitivity of thymocytes to induction of apoptosis and concanavalin A (Con A) signaling was estimated. The thymocytes from P-treated (PT) rats exhibited an increased sensitivity to induction of apoptosis, as well as to Con A stimulation. Furthermore, P treatment produced changes in the distribution of thymocyte subsets suggesting that more cells passed positive selection and further differentiated into mature CD4+ or CD8+ single positive (SP) TCRalphabeta(high) cells. These changes may, at least partly, be related to the markedly increased density of Thy-1 surface expression on TCRalphabeta(low) thymocytes from these rats. The increased frequency of cells expressing the CD4+25+ phenotype, which has been shown to be characteristic for regulatory cells in the thymus, may also indicate alterations in thymocyte selection following P treatment. Inasmuch as positive and negative selections play an important role in continuously reshaping the T-cell repertoire and maintaining tolerance, the hereby presented study suggests that pharmacological manipulations with beta-AR signaling, or chemically evoked alterations in catecholamine release, may interfere with the regulation of thymocyte selection, and consequently with the immune response.

Interdisciplinary research: noradrenergic regulation of adaptive immunity

To understand the complexity of mechanisms involved in the regulation of adaptive immunity by the sympathetic neurotransmitter norepinephrine and adrenergic receptor stimulation, there must be a rich history of basic science and clinical findings upon which to form hypotheses for testing, as well as a rich supply of individuals trained in two or more disciplines. This review is intended to offer a tour of the past, present, and future discoveries that have been made in the area of adrenergic regulation of adaptive immunity, as well as share a vision of how our field of study will progress years from now, given that every individual who contributes to the interdisciplinary nature of our research is valued. And finally, this review will discuss how the lessons from the past can help us to attain a vision of interdisciplinary research for the future.

Amygdaloid kindled seizures can induce functional and pathological changes in thymus of rat: role of the sympathetic nervous system

The present study sought to determine the effects of long-term kindled seizures of the basal amygdala upon immune function in rat, utilizing the thymus, as a principal target for study. Histopathology from kindled Sprague-Dawley rats revealed the presence of epithelial cell thymoma in 70% of these rats. The results revealed an increased rate of apoptosis and proliferation in thymic epithelial cells. Analysis of thymocytes indicated a decrease in the ratio of CD4 to CD8 positive T cells and reduced proliferative response to T-cell mitogens. To determine whether these effects were mediated through the sympathetic nervous system, animals were treated with guanethidine, which blocked the development of epithelial cell thymomas, while mifepristone treatment, employed to determine the possible role of the hypothalamic-pituitary axis, was ineffective in attenuating thymoma development. Thus, the present study demonstrated that functional and pathological changes in the thymus during kindled seizures are mediated through the sympathetic nervous system.

beta2-Adrenoceptor-mediated suppression of human intestinal mast cell functions is caused by disruption of filamentous actin dynamics

Previous studies indicated potent inhibitory effects of beta2-adrenoceptor (beta2AR) activation on the immunological mediator release of mast cells (MC). Here, we studied effects of beta2AR agonists on human MC mediator release, and in particular on MC proliferation, adhesion, and migration. MC were isolated from human intestinal mucosa, purified, and cultured in the presence of stem cell factor (SCF). beta2AR activation by epinephrine, norepinephrine, and salbutamol suppressed the IgE receptor-dependent release of histamine, lipid mediators, and TNF-alpha, and inhibited SCF-dependent MC proliferation and migration. Moreover, beta2-adrenergic stimulation interfered with MC adhesion to fibronectin and human endothelial cells. Using fluorescent phallacidin, we found that beta2AR activation reduced the amount of filamentous actin (F-actin) within minutes, whereas MC stimulation by either IgE receptor cross-linking or SCF caused F-actin accumulation. Interestingly, this activation-induced F-actin increase was abolished by previous beta2-adrenergic stimulation. Finally, we demonstrated that disruption of the F-actin cytoskeleton by latrunculin B mimicked the effects of beta2AR agonists on MC adhesion and migration. Our results argue for an important role of F-actin interference in beta2AR-mediated MC inhibition. Furthermore, the data support the concept of neuroimmune interactions regulating intestinal MC distribution, density, and functionality in vivo.

The effects of chronic stress on thymus innervation in the adult rat

Various stressors induce changes in the immune system. However, it has not yet been analyzed how stressors affect thymus innervation. To examine whether chronic stress alters the morphology of the thymus by changing the nerve components of the thymus, adult male rats, 9-weeks old, were exposed to forced swimming during 21 successive days. The animals were sacrificed by decapitation after the last session and their thymuses were used for analysis of (i) the thymus compartments, (ii) distribution patterns of monoamine-containing nerve profiles and (iii) distribution patterns of acetylcholinesterase (AChE)-containing nerve profiles. Our results show that chronic stress in rats reduces the volume of both thymus cortex and medulla, numbers of thymocytes in the deep cortex and medulla and the density of fluorescent nerve profiles, whereas it increases density of fluorescent cells. The distribution patterns of nerve profiles containing monoamine and AChE were not affected. These changes indicate that chronic stress affects thymus development and T cell maturation by altering the sympathetic nerve component.

Autonomic innervation of immune organs and neuroimmune modulation

1. Increasing evidence indicates the occurrence of functional interconnections between immune and nervous systems, although data available on the mechanisms of this bi-directional cross-talking are frequently incomplete and not always focussed on their relevance for neuroimmune modulation. 2. Primary (bone marrow and thymus) and secondary (spleen and lymph nodes) lymphoid organs are supplied with an autonomic (mainly sympathetic) efferent innervation and with an afferent sensory innervation. Anatomical studies have revealed origin, pattern of distribution and targets of nerve fibre populations supplying lymphoid organs. 3. Classic (catecholamines and acetylcholine) and peptide transmitters of neural and non-neural origin are released in the lymphoid microenvironment and contribute to neuroimmune modulation. Neuropeptide Y, substance P, calcitonin gene-related peptide, and vasoactive intestinal peptide represent the neuropeptides most involved in neuroimmune modulation. 4. Immune cells and immune organs express specific receptors for (neuro)transmitters. These receptors have been shown to respond in vivo and/or in vitro to the neural substances and their manipulation can alter immune responses. Changes in immune function can also influence the distribution of nerves and the expression of neural receptors in lymphoid organs. 5. Data on different populations of nerve fibres supplying immune organs and their role in providing a link between nervous and immune systems are reviewed. Anatomical connections between nervous and immune systems represent the structural support of the complex network of immune responses. A detailed knowledge of interactions between nervous and immune systems may represent an important basis for the development of strategies for treating pathologies in which altered neuroimmune cross-talking may be involved.

Cocaine enhances susceptibility to endotoxemic shock in a subset of rats

OBJECTIVE

We hypothesized that the sympathomimetic cocaine may alter cardiovascular and inflammatory responses and enhance susceptibility to endotoxemia due to innate differences in patterns of sympathetic and cardiovascular responsiveness.

DESIGN

Prospective study.

SETTING

Experimental animal laboratory.

SUBJECTS

Fifty-six conscious, instrumented albino rats.

INTERVENTIONS

Rats were instrumented for determination of arterial pressure and intravenous drug administration and, in some rats, for cardiac output. After recovery, rats were given cocaine (5 mg/kg i.v., twice daily with 4-6 trials) to identify one of two hemodynamic response patterns: a) an increase in systemic vascular resistance with cardiac depression (vascular responders) or b) smaller increases in systemic vascular resistance and no change or an increase in cardiac output (mixed responders). At least 1 month after characterizing response patterns to cocaine, animals were pretreated with cocaine (5 mg/kg i.v.) or an equivalent bolus of vehicle (0.9% saline) while recording hemodynamics. Five minutes later, Escherichia coli lipopolysaccharide (serotype O55:B5, 20 mg/kg i.v.) was administered for 15 mins.

MEASUREMENTS AND MAIN RESULTS

Hemodynamic responses, pupillary diameter, and serum cytokines were determined at several time points. Lipopolysaccharide administration (5-40 mg/kg) without cocaine produced dose-dependent depressor responses with recovery typically within 2 hrs. Although 87% of rats survived a single 20 mg/kg dose of lipopolysaccharide when given alone, pretreatment of vascular responders with cocaine before lipopolysaccharide resulted in greater increases in systemic vascular resistance and pupillary mydriasis and lethality in five of six vascular responders, whereas only one of six mixed responders died. Pretreatment with the alpha1-adrenoceptor antagonist prazosin (0.1 mg/kg i.v.) before cocaine and lipopolysaccharide attenuated hemodynamic responses and improved survival among vascular responders. Serum interleukin-6 and interleukin-10 were elevated in rats treated with cocaine and lipopolysaccharide compared with rats treated with lipopolysaccharide alone, whereas serum tumor necrosis factor-alpha was reduced by cocaine pretreatment. Moreover, serum interleukin-1beta, tumor necrosis factor-alpha, and interleukin-6 were elevated in nonsurvivors compared with survivors after cocaine and lipopolysaccharide administration.

CONCLUSIONS

We conclude that cocaine enhances susceptibility and worsens outcome from endotoxic shock by augmenting sympathetic activity, particularly in vascular responders, and that alpha-adrenoceptors mediate the altered inflammatory responses.

Effects of 6-hydroxydopamine on the development of the immune system in chickens

It has been suggested that the sympathetic nervous system communicates with lymphocytes expressing cell surface receptors for neurotransmitters such as norepinephrine (NE), on the basis of the finding that neurotransmitters modify immune responses in mammalian species. We confirmed that chicken lymphocytes in the brusa of Fabricius, thymus and spleen expressed beta-adrenergic receptor (beta-AR) mRNA from embryonic day (E) 10 and that intracellular cAMP level was elevated by NE, suggesting that lymphocytes express functional beta-AR on their surface at an early embryonal stage. To clarify whether the nervous system is involved in the development of the immune system, the effects of 6-hydroxydopamine (6-OHDA), one of sympathectomizing agents, on chicken lymphocytes was investigated. A single injection of 6-OHDA at a dose of 400 microg into a chicken embryo was carried out at E7 or 14 (as referred to E7 group and E14 group, respectively). NE level and the relative proportion of Bu-1a(+), CD4(+) and CD8(+) cells in the spleen of 3-week-old chickens were not altered by 6-OHDA treatment. However, the proliferative responses and expression of IL-2 mRNA in spleen cells cultured with pokeweed mitogen were reduced in E7 group compared with those of control. Furthermore, in CD8(+) spleen cells of E14 group of 3-week-old chickens, the expression of beta-AR mRNA and the relative increase of intracellular cAMP stimulated with NE were significantly decreased. These results suggest that the sympathetic nervous system affects the development of the immune system.

The human spleen during physiological stress

Many mammals have the ability to autotransfuse a large quantity of red blood cells from the spleen into the active circulation during times of stress. This enhancement of the oxygen transport system has benefited the athletic mammal, that is, the thoroughbred horse, fox and greyhound in an improved aerobic performance. The role of the spleen in sequestering 50% of the total red cell volume in seals and horses, during times of inactivity, dramatically reduces the viscosity of the blood and therefore the work of the heart. In comparison, the human spleen contains only a small percentage of red blood cells, and has been primarily thought of as a lymphoid organ. The aim of this review is to emphasise the similarities between the human spleen and that of several athletic mammalian species during acute physiological stress. In the athletic mammalian model the expulsion of blood from the spleen is facilitated via the sympathetic nervous system resulting in contraction of smooth muscle within the splenic capsule. In comparison, the lack of smooth muscle contained within the human splenic capsule has meant that active contraction of the spleen has historically been viewed as unlikely, although evidence of contractile proteins within the red pulp have suggested otherwise. Exercise results in haemoconcentration, which has been attributed solely to a reduction in plasma volume. Indirect calculation of plasma volume changes utilise haemoglobin and haematocrit and assume that the circulating red cell volume remains constant. However, several studies have suggested that the human spleen could account for 30% of the increase in haematocrit. This would result in a substantial overestimation of the reduction in plasma volume, indicating that the expulsion of red blood cells from the spleen must not be overlooked when utilising these equations.

Golli-myelin basic proteins delineate the nerve distribution of lymphoid organs

The golli-myelin basic proteins (MBPs) have been known to mark the nerve fiber extensions in both the peripheral nervous system (PNS) and the central nervous system. In this paper, we show that the nerve fibers revealed by neurofilament (NF) antibody staining in thymus and spleen, colocalized with golli in the capsular, trabecular (tr), and vasculature (v) systems. In the thymus, the density of these fibers was greater in the medulla than in the cortex. In the spleen, the golli immunoreactive fibers were seen within the capsule (ca), trabeculae, and along the artery tree, as well as the fine nerve fiber networks in the periarteriolar lymphoid sheath (PALS). Golli immunoreactivity appeared to colocalize with ER-TR7, a putative marker of connective tissue in lymphoid organs. However, further examination by Western blot analysis and immunohistochemistry performed on golli "knock out" mice showed that the antigens recognized by these two antibodies were different. The reason for the apparent colocalization of golli and ER-TR7 appears to be due to the close physical association of nerve fibers with connective tissue in these organs. These results suggest that golli immunoreactivity can visualize the distribution of nerve fibers in these lymphoid organs.

Effect of 6-hydroxydopamine on host resistance against Listeria monocytogenes infection

Recent studies have shown that immunocompetent cells bear receptors of neuropeptides and neurotransmitters and that these ligands play roles in the immune response. In this study, the role of the sympathetic nervous system in host resistance against Listeria monocytogenes infection was investigated in mice pretreated with 6-hydroxydopamine (6-OHDA), which destroys sympathetic nerve termini. The norepinephrine contents of the plasma and spleens were significantly lower in 6-OHDA-treated mice than in vehicle-treated mice. The 50% lethal dose of L. monocytogenes was about 20 times higher for 6-OHDA-treated mice than for vehicle-treated mice. Chemical sympathectomy by 6-OHDA upregulated interleukin-12 (IL-12) and tumor necrosis factor-alpha (TNF-alpha) production in enriched dendritic cell cultures and gamma interferon (IFN-gamma) and TNF-alpha production in spleen cell cultures, whereas chemical sympathectomy had no apparent effect on phagocytic activities, listericidal activities, and nitric oxide production in peritoneal exudate cells and splenic macrophages. Augmentation of host resistance against L. monocytogenes infection by 6-OHDA was abrogated in IFN-gamma(-/-) or TNF-alpha(-/-) mice, suggesting that upregulation of IFN-gamma, IL-12, and TNF-alpha production may be involved in 6-OHDA-mediated augmentation of antilisterial resistance. Furthermore, adoptive transfer of spleen cells immune to L. monocytogenes from 6-OHDA-treated mice resulted in untreated naive recipients that had a high level of resistance against L. monocytogenes infection. These results suggest that the sympathetic nervous system may modulate host resistance against L. monocytogenes infection through regulation of production of IFN-gamma, IL-12, and TNF-alpha, which are critical in antilisterial resistance.