Muramyl peptides and serotonin interact at specific binding sites on macrophages and enhance superoxide release

Conjunctival epitheliopathy induced by topical exposure to bacterial peptidoglycan, muramyl dipeptide

Noninfectious exudative conjunctivitis can be experimentally produced in rabbits by application of the apoptogenic bacterial cell wall peptidoglycan, muramyl dipeptide (MDP) to the ocular surface. The purpose of this study was to investigate the acute conjunctival cytopathology induced by unilateral ocular surface exposure to MDP. Hematoxylin and eosin staining assessed bilateral tear cytopathology and conjunctival histopathology. The caspases levels in conjunctival tissue and tears were measured in standard assays utilizing p-nitroanaline tagged caspase-specific substrates. Immunofluorescent antibody identified intracellular caspase-3, nuclear factor-κβ (NF-κβ), and oxidative DNA damage (8-OHdG; 8-oxo-2'-deoxyguanosine) in tear and conjunctiva cells. DNA extracted from conjunctival tissues and pooled tear fluids were visualized by ethydium bromide agarose gel electrophoresis. Onset of ipsilateral conjunctivitis was due to an epitheliopathy characterized by loss of conjunctival epithelial cell adherence, exuviation of conjunctival epithelial cells, and neutrophil infiltration. Caspase-3 levels were significantly higher in exuviated cells in ipsilateral than contralateral tear (p's ≤ 0.001) collected at 3-5 h post MDP. Significantly higher caspase-2, -3, -6, -8 and -9 (p's ≤ 0.03) levels were detected in ipsilateral than contralateral conjunctival tissue at 5 h. Polymeric DNA was detected in ipsilateral but not contralateral conjunctival tissue and tears. Caspase-3, NF-κβ, and 8-OHdG positive neutrophils were detected in bilateral conjunctiva and tear. The caspase-3/NF-κβ epithelial cells and polymeric DNA in conjunctival tissue and shedding of caspase positive cells and polymeric DNA into ipsilateral tears support MDP induction of acute programmed cell death in vivo. The results suggest that ipsilateral exudative conjunctivitis is due to acute caspase-mediated conjunctival epitheliopathy induced by topical exposure to the bacterial peptidoglycan MDP.

Tryptophan-derived serotonin-kynurenine balance in immune activation and intestinal inflammation

Endogenous tryptophan metabolism pathways lead to the production of serotonin (5‐hydroxytryptamine; 5‐HT), kynurenine, and several downstream metabolites which are involved in a multitude of immunological functions in both health and disease states. Ingested tryptophan is largely shunted to the kynurenine pathway (95%) while only minor portions (1%–2%) are sequestered for 5‐HT production. Though often associated with the functioning of the central nervous system, significant production of 5‐HT, kynurenine and their downstream metabolites takes place within the gut. Accumulating evidence suggests that these metabolites have essential roles in regulating immune cell function, intestinal inflammation, as well as in altering the production and suppression of inflammatory cytokines. In addition, both 5‐HT and kynurenine have a considerable influence on gut microbiota suggesting that these metabolites impact host physiology both directly and indirectly via compositional changes. It is also now evident that complex interactions exist between the two pathways to maintain gut homeostasis. Alterations in 5‐HT and kynurenine are implicated in the pathogenesis of many gastrointestinal dysfunctions, including inflammatory bowel disease. Thus, these pathways present numerous potential therapeutic targets, manipulation of which may aid those suffering from gastrointestinal disorders. This review aims to update both the role of 5‐HT and kynurenine in immune regulation and intestinal inflammation, and analyze the current knowledge of the relationship and interactions between 5‐HT and kynurenine pathways.

Uptake, recognition and responses to peptidoglycan in the mammalian host

Microbiota, and the plethora of signalling molecules that they generate, are a major driving force that underlies a striking range of inter-individual physioanatomic and behavioural consequences for the host organism. Among the bacterial effectors, one finds peptidoglycan, the major constituent of the bacterial cell surface. In the steady-state, fragments of peptidoglycan are constitutively liberated from bacterial members of the gut microbiota, cross the gut epithelial barrier and enter the host system. The fate of these peptidoglycan fragments, and the outcome for the host, depends on the molecular nature of the peptidoglycan, as well the cellular profile of the recipient tissue, mechanism of cell entry, the expression of specific processing and recognition mechanisms by the cell, and the local immune context. At the target level, physiological processes modulated by peptidoglycan are extremely diverse, ranging from immune activation to small molecule metabolism, autophagy and apoptosis. In this review, we bring together a fragmented body of literature on the kinetics and dynamics of peptidoglycan interactions with the mammalian host, explaining how peptidoglycan functions as a signalling molecule in the host under physiological conditions, how it disseminates within the host, and the cellular responses to peptidoglycan.

The Effects of Serotonin in Immune Cells

Serotonin [5-hydroxytryptamine (5-HT)] plays an important role in many organs as a peripheral hormone. Most of the body’s serotonin is circulating in the bloodstream, transported by blood platelets and is released upon activation. The functions of serotonin are mediated by members of the 7 known mammalian serotonin receptor subtype classes (15 known subtypes), the serotonin transporter (SERT), and by covalent binding of serotonin to different effector proteins. Almost all immune cells express at least one serotonin component. In recent years, a number of immunoregulatory functions have been ascribed to serotonin. In monocytes/macrophages, for example, serotonin modulates cytokine secretion. Serotonin can also suppress the release of tumor necrosis factor-α and interleukin-1β by activating serotonin receptors. Furthermore, neutrophil recruitment and T-cell activation can both be mediated by serotonin. These are only a few of the known immunomodulatory roles of serotonin that we will review here.

Immunomodulatory effects mediated by serotonin

Serotonin (5-HT) induces concentration-dependent metabolic effects in diverse cell types, including neurons, entherochromaffin cells, adipocytes, pancreatic beta-cells, fibroblasts, smooth muscle cells, epithelial cells, and leukocytes. Three classes of genes regulating 5-HT function are constitutively expressed or induced in these cells: (a) membrane proteins that regulate the response to 5-HT, such as SERT, 5HTR-GPCR, and the 5HT₃-ion channels; (b) downstream signaling transduction proteins; and (c) enzymes controlling 5-HT metabolism, such as IDO and MAO, which can generate biologically active catabolites, including melatonin, kynurenines, and kynurenamines. This review covers the clinical and experimental mechanisms involved in 5-HT-induced immunomodulation. These mechanisms are cell-specific and depend on the expression of serotonergic components in immune cells. Consequently, 5-HT can modulate several immunological events, such as chemotaxis, leukocyte activation, proliferation, cytokine secretion, anergy, and apoptosis. The effects of 5-HT on immune cells may be relevant in the clinical outcome of pathologies with an inflammatory component. Major depression, fibromyalgia, Alzheimer disease, psoriasis, arthritis, allergies, and asthma are all associated with changes in the serotonergic system associated with leukocytes. Thus, pharmacological regulation of the serotonergic system may modulate immune function and provide therapeutic alternatives for these diseases.

Debug Your Bugs - How NLRs Shape Intestinal Host-Microbe Interactions

The host's ability to discriminate friend and foe and to establish a precise homeostasis with its associated microbiota is crucial for its survival and fitness. Among the mediators of intestinal host-microbe interactions, NOD-like receptor (NLR) proteins take center stage. They are present in the epithelial lining and innate immune cells that constantly monitor microbial activities at the intestinal barrier. Dysfunctional NLRs predispose to intestinal inflammation as well as sensitization to extra-intestinal immune-mediated diseases and are linked to the alteration of microbial communities. Here, we review advances in our understanding of their reciprocal relationship in the regulation of intestinal homeostasis and implications for intestinal health.

Muramyl peptides augment cytotoxic effect of tumor necrosis factor-alpha in combination with cytotoxic drugs on tumor cells

We have demonstrated that biologically active muramyl peptides, in particular, glucosaminylmuramyl dipeptide (GMDP), augmented in vitro cytotoxic activity of tumor necrosis factor-alpha (TNF-alpha) against murine fibrosarcoma L929 cells. The introduction of GMDP resulted in cytotoxic effect characteristic for substantially higher dose of cytokine. Even more potent was the combination of GMDP, TNF-alpha and Actinomycin D (ActD). According to clonogenic and MTT assays 100% L929 cells could be killed in culture with low doses of TNF-alpha and ActD if GMDP was present. When cisplatin was substituted for ActD similar results were obtained. GMDP also enhanced cytotoxicity of TNF-alpha and cisplatin against human breast carcinoma MCF7 and histiocytic lymphoma U937 cells. Normal cells, namely human peripheral blood leucocytes and murine peritoneal macrophages, were resistant to selected doses of TNF-alpha/cisplatin/GMDP.

Neuroendocrine immune interactions in health and disease

The purpose of this paper is to review ways in which the neurohormonal system can interact with the immune system and to outline the main mechanisms which are involved in this interaction. Experimental as well as clinical evidence is presented to support the existence of a close interaction and bi-directional communication between the central nervous and immune systems. The role of major endocrine mechanisms and hormones is discussed. The evidences from experimental work to support the roles of the nervous system with neurotransmitters, the endocrine system with hormones, and the immune system with cytokines are presented. Aging, depression and cancer have a high degree of co-association and share mechanisms which result in cellular immune deficiency. Hormone therapy, zinc replenishment, antidepressants, immunomodulators like MDP act on these pathways to upregulate and improve cellular immunity. The authors believe that the central nervous system (CNS)-immune interaction is an important new frontier to be considered for new combination therapy in diseases with cellular immune deficiency such as cancer particularly in the aged with depression.

Serotonin upregulates the activity of phagocytosis through 5-HT1A receptors

1 In this study, we investigated whether serotonin could regulate the in vitro activity of phagocytosis through 5-hydroxytryptamine or serotonin (5-HT(1A)) receptors. 2 Mouse peritoneal macrophages were cultured with serotonin and the activity of phagocytosis was assessed by the uptake of zymosan and latex particles added to the culture media. Specific binding of [(3)H]8-OH-DPAT and immunohistochemistry using an affinity-purified anti-5-HT(1A)-receptor antibody were assayed in the macrophages. In addition, we took advantage of the availability of pharmacological inhibitors of nuclear factor-kappaB (NF-kappaB) to explore its role in the regulation of the 5-HT(1A) receptor. 3 Serotonin increased the in vitro activity of phagocytosis in a dose-dependent manner. The 5-HT(1A) receptor agonist (+/-)-8-hydroxy-2-(di-n-propyl-amino)-tetralin (R(+)-8-OH-DPAT) reproduced these effects. Serotonin- or R(+)-8-OH-DPAT-induced increases in phagocytosis were blocked by the 5-HT(1A) receptor antagonist WAY100635 and the NF-kappaB inhibitor pyrrolidinedithiocarbamate. Moreover, mouse peritoneal macrophages expressed specific binding sites for [(3)H]8-OH-DPAT when cultivated in the presence of zymosan or latex beads. Immunohistochemistry confirmed the expression of the 5-HT(1A) receptor protein in the macrophages. 4 These results show that serotonin can upregulate the activity of peritoneal macrophages through 5-HT(1A) receptors.

Contrasting responses of human gingival and periodontal ligament fibroblasts to bacterial cell-surface components through the CD14/Toll-like receptor system

We compared human periodontal ligament fibroblasts with human gingival fibroblasts isolated from the same donor to examine interleukin-8 (IL-8) responses of the cells to Salmonella lipopolysaccharide, a water-soluble peptidoglycan from Staphylococcus epidermidis and the synthetic muramyldipeptide, with special reference to the possible involvement of the CD14/Toll-like receptor (TLR) system of the cells in the responses. Human gingival fibroblasts expressed CD14 on their surfaces and strongly expressed CD14 mRNA, while human periodontal ligament fibroblasts showed considerably lower levels of expression in both respects. Both cells expressed mRNA of TLR-related molecules, i.e. TLR2, TLR4, MD-2 and MyD88, although human periodontal ligament fibroblasts expressed TLR2 more strongly than human gingival fibroblasts. Human gingival fibroblasts exhibited a stronger IL-8 response than human periodontal ligament fibroblasts to lipopolysaccharide, while human periodontal ligament fibroblasts exhibited a response comparable to, or slightly stronger than, that of human gingival fibroblasts to S. epidermidis peptidoglycan and muramyldipeptide. The IL-8 responses of both cells to lipopolysaccharide and S. epidermidis peptidoglycan were completely inhibited by antihuman CD14 monoclonal antibody (MAb). The responses of both cells to lipopolysaccaride were significantly inhibited by antihuman TLR4 MAb, while those to S. epidermidis peptidoglycan were inhibited by antihuman TLR2 MAb. In contrast, muramyldipeptide activated both types of cells in a TLR2- and TLR4-independent manner, although the activities of muramyldipeptide on human gingival fibroblasts, but not human periodontal ligament fibroblasts, were significantly inhibited by anti-CD14 MAb.

Effects of fluoxetine on the activity of phagocytosis in stressed mice

We studied the effects of 1, 2, 5, 10 and 20 mg/kg of fluoxetine on the activity of phagocytosis in mice subjected to a chronic auditory stressor. Both the in vitro and in vivo activity of phagocytosis, measured using the zymosan-particle uptake method and the carbon clearance test, respectively, were reduced after 2, 4, 8 and 16 days of stress exposure. A partial recovery on the in vivo activity of phagocytosis was found on day 16th. Daily treatment with fluoxetine partially reversed the adverse effects of stress in a dose-dependent manner on both parameters but did not significantly affect the activity of phagocytosis in unstressed mice. Significant differences appeared when fluoxetine was administered at 2 mg/kg. Maximum effect was reached at 5 mg/kg.

Role of blood platelets in infection and inflammation

Blood platelets are here presented as active players in antimicrobial host defense and the induction of inflammation and tissue repair in addition to their participation in hemostasis. Megakaryopoiesis is inhibited after acute infection with viruses or bacteria. In contrast, chronic inflammation is often associated with reactive thrombocytosis. Platelets can bind and internalize pathogens and release microbicidal proteins that kill certain bacteria and fungi. By making cell-cell contacts with leukocytes and endothelial cells, platelets assist white blood cells in rolling, arrest and transmigration. On stimulation by bacteria or thrombin, platelets release the content of their alpha-granules, which include an arsenal of bioactive peptides, such as CC-chemokines and CXC-chemokines and growth factors for endothelial cells, smooth muscle cells and fibroblasts. Thus, integral to innate immunity, the tiny little platelets may become bombshells when irritated by pathogens.

Role of central 5-HT(2) receptors in fluoxetine-induced decreases in T lymphocyte activity

Previous studies have demonstrated that fluoxetine administration decreases mitogen-induced T lymphocyte proliferation. The present studies were carried out to determine which receptor subtype(s) was involved and whether these effects on lymphocyte responses were centrally or peripherally mediated. Two hours following administration of the 5-HT(1A) agonist 8-OH-DPAT (1 mg/kg), there was no change in lymphocyte proliferation responses, whereas the 5-HT(2) agonist DOI (2.5 mg/kg) significantly decreased (80%) proliferation. Similarly, pretreatment with the 5-HT(2) antagonists ritanserin (5 mg/kg, 30 min) or ketanserin (5 mg/kg, 1 h) was found to completely antagonize the effects of fluoxetine on lymphocyte proliferation. Consistent with central 5-HT(2) receptor involvement, microinjection of DOI (50 microg) resulted in a decrease in lymphocyte proliferation similar to that observed following systemic administration. Furthermore, central administration of ketanserin (20 microg) prevented the suppressive effects of systemic fluoxetine. Collectively, these results suggest that decreases in mitogen-induced lymphocyte proliferation following acute fluoxetine administration was due to indirect effects of fluoxetine following the activation of central 5-HT(2) receptors.

Synergistic effect of muramyldipeptide with lipopolysaccharide or lipoteichoic acid to induce inflammatory cytokines in human monocytic cells in culture

An analog of 1alpha,25-dihydroxyvitamin D3, 22-oxyacalcitriol (OCT), differentiated human monocytic THP-1 and U937 cells to express membrane CD14 and rendered the cells responsive to bacterial cell surface components. Both THP-1 and U937 cells expressed Toll-like receptor 4 (TLR4) on the cell surface and TLR4 mRNA in the cells, irrespective of OCT treatment. In contrast, OCT-treated U937 cells scarcely expressed TLR2 mRNA, while OCT-treated THP-1 cells expressed this transcript. Muramyldipeptide (MDP) by itself exhibited only a weak ability to induce secretion of inflammatory cytokines such as interleukin-8 (IL-8) in the OCT-differentiated THP-1 cells but showed marked synergistic effects with Salmonella lipopolysaccharide (LPS) or lipoteichoic acid (LTA) from Staphylococcus aureus, both of which exhibited strong activities. Combinatory stimulation with LPS plus LTA did not show a synergistic effect on OCT-differentiated THP-1 cells. Similar results were observed in OCT-differentiated U937 cells, although combination experiments were carried out only with MDP plus LPS. Anti-CD14 monoclonal antibody (MAb) MY4, anti-TLR4 MAb HTA125, and the synthetic lipid A precursor LA-14-PP almost completely inhibited the IL-8-inducing activities of LTA as well as LPS on OCT-treated THP-1 cells, but these treatments increased MDP activity. OCT-treated THP-1 cells primed with MDP exhibited enhanced production of IL-8 upon stimulation with LPS, while the cells primed with LPS showed no change in production upon stimulation with MDP. MDP up-regulated mRNA expression of an adapter molecule to TLRs, MyD88, to an extent similar to that for LPS in OCT-treated THP-1 cells. These findings suggested that LTA as well as LPS activated human monocytic cells in a CD14- and TLR4-dependent manner, whereas MDP exhibited activity in a CD14-, TLR4-, and probably TLR2-independent manner and exhibited synergistic and priming effects on the cells for cytokine production in response to various bacterial components.

5-HT-moduline controls serotonergic activity: implication in neuroimmune reciprocal regulation mechanisms

The serotonergic neurotransmission is known as a neuromodulatory system exerting its activity in the central nervous system (CNS) as well as at the periphery. The anatomical and morphological organization of the system based on a marked centralization of the cellular bodies and the large, almost ubiquitary, presence of axonal projections of the neurons is in good agreement with this modulatory role. Furthermore, a very high number of varicosities located along the axonal branches are capable of releasing serotonin (5-HT). The amine stimulates a number of different specific receptor types which allows 5-HT to exert different activities on its various cellular targets. Among these receptors, the 5-HT1B subtypes play a particular role as they are autoreceptors located on 5-HT neurons terminals and heteroreceptors located on non-serotonergic terminals where they control the release of the neurotransmitter. 5-HT-moduline, an endogenous tetrapeptide, regulates the efficacy of these 5-HT1B receptors, hence, is able to control the serotonergic activity in a synchronous manner for the various varicosities from a single neuron and thus may favour the differential effect of that neuron on distinct cerebral functions. Accordingly, the peptide allows the 'fine tuning' of the cerebral activity by the serotonergic system to elaborate the response given by the brain to a particular stimulus, that is, stress situations. At the periphery, the serotonergic system also appears to possess a regulatory activity via 5-HT1B receptors. In particular, the receptors located on immunocompetent cells control their activity and are themselves regulated by 5-HT-moduline likely originating from adrenal medulla and released after acute stress. The serotonergic system appears to play a major role in the reciprocal signalling existing between the neuronal and the immune system. The participation of 5-HT-moduline is likely in physiological functions as well as in pathological disorders affecting central and peripheral activities.

Specific binding of glucosaminylmuramyl peptides to histones

Intracellular N-acetylglucosaminylmuramyl peptide-binding proteins of murine macrophages and myelomonocytic WEHI-3 cells were characterized. SDS-PAGE and Western blotting revealed proteins with molecular masses of 18, 32 and 34 kDa retaining the ability to specifically bind glucosaminylmuramyl dipeptide. The inhibition analysis demonstrated that only biologically active muramyl peptides but not inactive analogs or fragments of glucosaminylmuramyl dipeptide could inhibit glucosaminylmuramyl dipeptide-binding to these proteins. Purification of these proteins and sequencing of peptides obtained after in-gel trypsin digestion enabled us to identify the above mentioned proteins as histones H1 and H3. These findings suggest that nuclear histones might be target molecules for muramyl peptides.

The interaction of immunomodulatory muramyl dipeptide with peripheral 5-HT receptors: overview of the current state

Immunomodulator muramyl dipeptide (MDP) exerts also pronounced neuropharmacological activities which are probably mediated by an interaction with 5-HT receptors. Some of these effects are considered as undesirable by its clinical use. More precise information concerning MDP effects on 5-HT receptors with respect to their many subtypes could result from studies using isolated organs in vitro. Earlier conducted studies of this type provided data that are concisely overviewed and reinterpreted here from the view of current 5-HT receptor classification. Since new 5-HT receptor types have emerged recently, new studies are under way. The results might contribute to the development of novel immunomodulatory drugs devoid of adverse effects.

Role of serotonin in the immune system and in neuroimmune interactions

Serotonin (5-HT) is one of the most extensively studied neurotransmitters of the central nervous system. 5-HT is, however, also present in a variety of peripheral tissues including in constituents of the immune system. The function of 5-HT in the immune system has received increasing attention since about 1984, but has been reviewed only once, in 1985. In recent years, modern techniques of molecular biology such as reverse-transcriptase polymerase chain reaction and targeted gene disruption have made it possible to study new important aspects of 5-HT in the immune system. In the first part of the review, we explore whether 5-HT is involved in interactions between the central nervous and immune systems. It emerges that 5-HT may mediate interactions of these two systems by four different pathways. In the second part, we dissect the functional roles of 5-HT in the immune system. We describe the distribution of 5-HT receptors and the 5-HT transporter on immune cells and estimate which levels 5-HT may attain in the extracellular space in physiological conditions and under pathological circumstances such as inflammation, thrombosis, and ischemia. At these 5-HT concentrations, four major functions for 5-HT emerge. These include T cell and natural killer cell activation, delayed-type hypersensitivity responses, production of chemotactic factors, and natural immunity delivered by macrophages. Finally, we discuss promising future avenues to further advance knowledge of the role of 5-HT in the immune system and in neuroimmune interactions.

MDP(Lysyl)GDP, a nontoxic muramyl dipeptide derivative, inhibits cytokine production by activated macrophages and protects mice from phorbol ester- and oxazolone-induced inflammation

High levels of pro-inflammatory cytokines and nitric oxide are proposed to orchestrate pathophysiologic mechanism(s) associated with various inflammatory dermatoses. This study examines whether a water soluble 3-O-[N-acetylmuramyl-L-lysyl-D-iso]-2-di-on-glycine [MDP(Lysyl)GDP], a nontoxic and nonpyrogenic derivative of muramyl dipeptide (MDP), can inhibit the in vitro production of inflammatory mediators by lipopolysaccharide- or interferon-gamma-activated macrophages, and whether such an inhibitory effect can translate into in vivo protection of mice from irritant and allergic contact dermatitis. Thioglycollate-elicited peritoneal macrophages cultured in medium alone or in medium supplemented with MDP(Lysyl)GDP (1-100 microg per ml) expressed neither mRNA transcripts for inducible nitric oxide synthase, interleukin-1beta, and tumor necrosis factor-alpha, nor cytokine proteins and nitric oxide activity. Incubation of the cells with either lipopolysaccharide or interferon-gamma for 6 h resulted in a significant induction of inducible nitric oxide synthase, interleukin-1beta, and tumor necrosis factor-alpha mRNA, and the accumulation of high levels of monokines and nitrites in cultures by 24 h. Co-incubation of the macrophages with lipopolysaccharide or interferon-gamma and MDP(Lysyl)GDP (1-100 microg per ml) resulted in a concentration-dependent suppression of the steady-state mRNA transcripts for inducible nitric oxide synthase, tumor necrosis factor-alpha, and interleukin-1beta, induced by lipopolysaccharide, but not by interferon-gamma. In mouse models of phorbol ester- and oxazolone-induced ear inflammation, topical application of MDP(Lysyl)GDP significantly suppressed ear swelling in a dose-dependent manner. Likewise, oral treatment with MDP(Lysyl)GDP at days -3, -2, and -1 before elicitation with oxazolone also significantly inhibited ear inflammation. Taken together, our findings suggest that MDP(Lysyl)GDP has the potential to be a therapeutic application in the treatment of inflammatory conditions in which overproduction of pro-inflammatory mediators are implicated to play a pathogenic role.

Bacterial products and the control of ingestive behavior: clinical implications

Bacterial products such as lipopolysaccharides (LPS) and muramyl peptides are delivered in the course of infections. They trigger the host's acute phase responses to bacterial infections and are probably involved in the accompanying hypophagia because LPS and muramyl dipeptide (MDP, the minimal immunologically active muramyl peptide) reduce food intake after parenteral administration in animals. LPS and MDP inhibit feeding synergistically through separate but interacting mechanisms. The hypophagic effects of LPS and MDP are presumably mediated by the combined actions of interleukin-1, tumor necrosis factor, and other cytokines. More work is required to understand the interactions between these cytokines, and between bacterial products and cytokines, before cytokine antagonists can be used for treatment of the hypophagia during bacterial infections. As the hypophagia seems to be an early mechanism of host defense, a treatment should be carefully considered. If an intervention is indicated because of a patient's poor condition, inhibitors of eicosanoid synthesis and glucocorticoids may hold more promise for therapy because such substances block LPS and MDP hypophagia. Although LPS can reduce food intake by direct action on the brain, presently available evidence indicates that systemic LPS acts primarily in the periphery to generate a neural signal that is transmitted to the brain and inhibits feeding through the vagus. The exact site where LPS acts on peripheral nerves remains to be identified. LPS hypophagia is conditionable, but conditioning cannot solely account for LPS hypophagia under most test conditions. Whether MDP hypophagia is also conditionable and mediated by vagal afferents is not yet known. All in all, the putative mediators and mechanisms of LPS and MDP hypophagia suggest some options for a treatment of the hypophagia during bacterial infection, but present knowledge about the mechanisms and interactions of the involved substances is still fragmentary and requires further investigation.

Oral administration of muramyl dipeptide into mice modulates cell proliferation, immunoglobulin synthesis and cytokine mRNA levels in gut associated lymphoid tissues

Muramyl peptides (MDPs) are synthetic immunostimulants capable of potentiating a multitude of immune functions following parenteral administration into a host. The parent molecule MDP was also found to exert certain activities when applied via the oral route. Thus, we have studied the effect of oral treatment of mice with MDP on the lymphoproliferative responses, immunoglobulin secretion and cytokine induction in gut-associated lymphoid tissues (GALT) employing lymphocyte transformation test, ELISA and RT-PCR, respectively. Cells derived from Peyer's patches (PP) of mice orally primed with MDP were found to have enhanced proliferative responses to different mitogens and to secrete significantly more IgG, IgM and IgA immunoglobulins than cells from unprimed mice. These effects were not observed with cells derived from mesenteric lymph nodes (MLN) or spleens of MDP-primed mice. However, oral administration of MDP resulted in the up-regulation of interleukin-6 (IL-6) mRNA in MLN and down-regulation of IL-4 and tumour necrosis factor-alpha (TNF-alpha) mRNAs in MLN, spleens and PP. These studies suggest that selective modulations of GALT responses by orally administered MDP are achievable and imply that these agents may be useful in the therapy and prophylaxis of allergic diseases.

Further evidence on the interaction of muramyl dipeptide with the serotonergic system

The mode of interaction between muramyl dipeptide (MDP), a compound with immunopharmacological activities, and 5-hydroxtryptamine (5-HT, serotonin) was studied in isolated nerve-smooth muscle preparations of the carp stomach. Application of exogenous 5-HT evoked direct smooth muscle contractions; electric neurogenic stimulation evoked twitches due to release of 5-HT from nerve endings. Contractions evoked by a high concentration of 5-HT (3-30 microM) were resistant to atropine and potentiated in the presence of MDP. Isamoltan (5-HTID antagonist) decreased the amplitude of contractions, whereas ketanserin (5-HT2 antagonist) and MDL 72,222 (5-HT3 antagonist) had no effect. The addition of low concentrations (0.1-1.5 microM) of 5-HT did not contract the preparation but caused a decrease in the amplitude of neurogenic twitches, which might be due to the presynaptic inhibition of serotonin release. This effect of 5-HT was not changed by isamoltan or ketanserin, but it was largely reduced in the presence of 5-HT3 antagonists tropisetron and MDL 72,222. This inhibitory effect of 5-HT on twitch amplitude was potentiated by MDP. The interaction of MDP with the serotonergic system thus involved not only potentiation of the postsynaptic effect of higher 5-HT concentrations, which might have been mediated via the 5-HT1 subsystem, but also presynaptic inhibition. MDP enhancement of 5-HT's inhibitory effect, mediated via 5-HT3 receptors, might represent a new feature in mutual 5-HT-MDP interactions.

Muramyl dipeptide mimicry in the regulation of murine macrophage activation by serotonin

Muramyl peptides (MPs) are regulators of macrophage function. That the activities of MPs may be mediated by serotonin (5-HT) is supported by earlier work that demonstrated specific binding sites for MPs on macrophages that competitively bind 5-HT. Both mediators were also shown to enhance the production of superoxide anion (an antibacterial agent) by these cells. We now report on two additional macrophage activation phenomena affected by 5-HT: phagocytosis and induction of tumor necrosis factor alpha (TNF) mRNA. Serotonin acts as a muramyl peptide-like agonist by increasing phagocytosis of tubercle bacilli by murine peritoneal macrophages, and as a partial agonist/antagonist in the induction of mRNA for tumor necrosis factor. These observations provide further evidence for a serotonergic involvement in some of the physiological responses to MPs.

Sleep, sleep deprivation and infectious disease: studies in animals

Common perceptions that the desire for sleep is increased during mild infectious diseases like colds and 'the flu' have fostered beliefs that sleep promotes recovery from infectious disease and that lack of sleep increases susceptibility to infections. However, until recently, the relationship between infectious disease and vigilance received relatively little systematic study. At present, several model systems provide evidence that infectious disease is accompanied by alterations in sleep. Indeed, increased sleepiness, like fever and anorexia, may be viewed as a facet of the acute phase response to infectious challenge. Recent studies also suggest that sleep, sleep deprivation and infectious disease may be related via mechanisms of the immune system (Fig. 1). Data are now accumulating to address questions such as whether immune processes alter sleep, whether sleep or sleep deprivation influences immune competence, and whether sleep facilitates recovery from infectious disease.

Serotonin, serotoninergic agents and their antagonists suppress humoral immune reaction in vitro

Serotonin (5-HT), its derivative 5-methoxy-tryptamine (MeOT) and ergot-alkaloid dihydroergosine (DHESN) suppressed the immune reaction of mouse spleen cells against sheep erythrocytes in vitro (PFC assay). Ketanserin and propranolol, antagonists of 5-HT, also caused suppression. However, cells incubated with propranolol 20 min before 5-HT or MeOT produced as many plaques as the non-treated cells. Preincubation with ketanserin resulted in similar interference, but the reversal was not complete. It is concluded that serotonin and related agents probably affect several types of immunocompetent cells participating in successive stages of the PFC reaction. The antagonists probably interfered via 5-HT receptors, adrenergic receptors or both. Immunosuppression observed with serotoninergic agents and their antagonists draws attention to possible side effects of such drugs.

Biological properties of bacterial peptidoglycan

Synthetic and natural muramyl peptides have a variety of biological actions in mammals, including the abilities to enhance sleep and body temperature. Although muramyl peptides can be detected constitutively in mammalian organisms, no biochemical synthetic pathways are known for muramyl peptides in mammals. However, muramyl peptides are well known as components of bacterial cell wall peptidoglycan (synonym: murein). Isolated bacterial cell walls elicit host responses similar to those produced by bacterial infections or by purified muramyl peptides. Mammalian cells which phagocytize bacteria can digest bacterial cell walls and release biologically active muramyl peptides. The released muramyl peptides then express some or all of the biological effects observed with synthetic muramyl peptides. Also, cell-free systems consisting of isolated bacterial cell walls and lysozyme produce substances with similar biological activities.

Pharmacokinetics of an immunomodulator peptidoglycan monomer in mice after intravenous administration

A 14C labeled low molecular weight immunomodulator, peptidoglycan monomer (14C-PGM), was injected intravenously (i.v.) into mice. At various time intervals thereafter (15 min-6 h), radioactivity in the urine, whole blood, plasma, kidneys, liver, spleen, lungs, intestines and the brain of the mice was determined. Shortly after injection, 14C-PGM was very rapidly excreted from the organism, so that 1 h following administration, 80% of the radioactivity was found in the urine (62% as unchanged PGM and the rest as the metabolites pentapeptide and disaccharide). At the same time, around 2% of the injected material was found in the blood. Six hours after injection, equal quantities were found in the intestines, liver and blood (0.5%), slightly less in the kidneys, lungs and spleen (0.2-0.3%) and the least quantity in the brain (0.04%). However, the dynamics of retention in the organs was evidently different. In the kidneys, lungs and spleen, radioactivity steadily decreased over the studied period. In the liver following an initial decrease, radioactivity remained the same 3 and 6 h after injection. On the other hand, in the intestines and brain PGM seemed to accumulate rather than disappear following i.v. administration. This fact should be considered when explaining different biological activities of low molecular weight bacterial peptidoglycans.

Alteration of NK- and ADCC-activities in rats genetically selected for low or high platelet serotonin level

By selective breeding we have recently obtained two discrete sublines of rats that differ in serotonin content in their platelets. As both serotonin and platelets may influence, or even take part, in immune reactions, we tested in this work the natural cytotoxicity in rats with constitutionally different platelet serotonin levels (PSL). Rats with low platelet serotonin level (mean +/- SD, 1.26 +/- 0.14 micrograms 5HT/mg protein; 81% vs. controls) had significantly higher (P less than 0.001) natural killer (NK) activity (mean +/- SD, 9.1 +/- 3.9%) than control rats with average PSL (1.57 +/- 0.18 micrograms 5HT/mg protein). On the contrary, rats with constitutionally high PSL (2.42 +/- 0.21 micrograms 5HT/mg protein, 154% vs. controls) had somewhat lower (P less than 0.02) NK activity (4.1 +/- 1.7%) than control animals (5.7 +/- 1.9%). Antibody-dependent cellular cytotoxicity (ADCC) against nucleated targets of the RCH line, detecting lymphoid effectors, as well as ADCC against chicken red blood cells (CRBC), detecting predominantly non-lymphoid effectors, were also significantly higher (P less than 0.001) in rats with low PSL (19.6 +/- 6.8% vs. 6.6 +/- 3.1% in controls for lymphoid effectors, and 71.8 +/- 6.1% vs. 48.7 +/- 8.8% in control rats for non-lymphoid effectors). However, no significant alteration of either ADCC was determined in rats with high PSL. The results suggest in vivo regulation of natural cytotoxicity by serotonin.

Serotonergic aspects of the response of human platelets to immune-adjuvant muramyl dipeptide

The human platelet serotonergic responses of aggregation and uptake were shown to be modulated by muramyl peptides. Muramyl dipeptide inhibited serotonin uptake in a temperature dependent and stereospecific manner. It also blocked the binding to platelets of [3H]imipramine, a well-known inhibitor of serotonin uptake. Muramyl dipeptide decreased serotonin2 (5-HT2) mediated platelet aggregation, and blocked the binding of a 5-HT2-specific ligand: lysergic acid diethylamide. Since muramyl peptides are released upon degradation of bacteria, the findings offer a possible mechanism for neuro-immune modulation by emphasizing the interaction between 5-HT (a neurotransmitter) and muramyl peptides (immuno-adjuvants).

Role of biological membranes in slow-wave sleep

Two involvements of cellular membranes in slow-wave sleep (SWS) are discussed. In the first the endoplasmic reticulum (ER) is focussed upon, and in the second, the plasmalemma, where specific binding sites (receptors?) for promoters of slow-wave sleep are believed to be located. The study concerning the ER focuses on an enzyme in the brain, glucose-6-phosphatase, which, although present at low levels, manifests greatly increased activity during SWS compared to the waking state. The work on the plasmalemma has to do with the specific binding of muramyl peptides, inducers of slow-wave sleep, to various cells, and membrane preparations of various sorts, including those from brain tissue. Such cells as macrophages from mice, B-lymphocytes from human blood, and cells from a cell line (C-6 glioma) have been examined in this context.

Serotonin binding sites. II. Muramyl dipeptide binds to serotonin binding sites on myelin basic protein, LHRH, and MSH-ACTH 4-10

Previously, we reported the existence of structurally similar serotonin binding sites on myelin basic protein, LHRH, and MSH-ACTH 4-10. We now report that the adjuvant peptide, muramyl dipeptide (N-acetyl-muramyl-L-Ala-D-isoGln) also binds to these sites. This observation may help to explain previous observations of serotonin-like activity by muramyl peptides, including the promotion of slow-wave sleep and fever induction. The observation may also provide an important link between the immune system and the nervous system that may explain the role of muramyl dipeptide adjuvants in causing autoimmune diseases to serotonin-regulated proteins and their receptors, as well as the alterations in serotonin levels that are often observed in autoimmune diseases. The observation provides concrete evidence for a dual-antigen hypothesis for the induction of autoimmune diseases by an adjuvant-peptide complex. Application of such a mechanism for induction of autoimmunity may be of importance in understanding a number of postinfectious and postvaccinal neuropathies, and suggests a possible etiology for autism, in which many patients have high blood serotonin levels, autoimmune reactions to myelin basic protein, and antibodies to serotonin binding sites. Finally, the observation suggests that glycopeptides may act as neurotransmitters.

HLA class II-restricted binding of muramyl peptides to B lymphocytes of normal and narcoleptic subjects

The propensity for narcolepsy, a clinical sleep disorder of unknown etiology, is virtually totally included within the HLA-DR2,DQw1 (DRw15,DQw6) phenotype. The disorder is characterized by decreased sleep latency, early onset of rapid eye movement sleep, and a paucity of nocturnal slow-wave sleep. Muramyl peptides, naturally occurring bacterial cell wall peptidoglycans, potently enhance the duration and amplitude of slow-wave sleep in animals, bind to murine mononuclear cells, and exhibit a major histocompatibility complex-restricted immunoadjuvant effect in mice. We examined the binding of muramyl peptides to peripheral blood mononuclear leukocytes of HLA-typed normal (n = 13) and narcoleptic (n = 10) subjects. Muramyl peptides bound specifically and with high affinity to normal B- but not T-lymphocyte-enriched preparations. There was no significant specific binding to B-cell-enriched preparations from narcoleptic patients. Furthermore, B-lymphocyte-enriched preparations of normal individuals who had the HLA-DR2,DQw1 phenotype (n = 8) exhibited a lower level of specific binding than those of normals who did not have this phenotype (n = 5, p less than 0.001). These observations are an additional indication of the relevance of muramyl peptides to slow-wave sleep and provide a basis for a better understanding of the relation between narcolepsy and the MHC at the biochemical level.

Unification of the findings in schizophrenia by reference to the effects of gestational zinc deficiency

The hypothesis presented here suggests that schizophrenia is caused by the action of gestational zinc deficiency on genetically susceptible foetuses. The psychosis seen is suggested to be due to a combination of dietary or otherwise induced zinc deficiency and lack of zinc releasing capacity in the hippocampus. A non genetic but transmissable immune defect is suggested to be relevant to psychosis and to the nonmendelian pattern of inheritance of the disorder.

Muramyl peptides bind specifically to rat brain membranes

The capability of immunoactive muramyl peptides to bind specifically to rat brain membranes has been discovered. The reaction of an N-acetylglucosaminylmuramyl dipeptide analog having an additional C-terminal lysine residue (GMDP-Lys) with Bolton-Hunter reagent or N-hydroxysuccinimidyl-4-azidosalicylate afforded two acylated derivatives, GMDP-Lys-(Hp) and GMDP-Lys(Azs). Their iodination resulted in radioactive derivatives (spec. act. approximately 2000 Ci/mmol) whose binding to rat brain membranes is characterized by Kd approximately 3 nM and Bmax approximately 10 fmol/mg membrane protein. Binding could be inhibited by muramyl dipeptide (MDP), GMDP-Lys, and GMDP, while fragments of the latter, N-acetylglucosamine dipeptide and disaccharide, were ineffective.

Photoaffinity labelling of macrophages and B-lymphocytes using 125I-labelled aryl-azide derivatives of muramyldipeptide

Two 125I-labelled aryl-azide derivatives of MDP have been synthesized. Photoaffinity labelling experiments demonstrated cell surface binding sites for muramylpeptides on activated B-lymphocytes and intracellular muramylpeptides binding protein(s) of 40-45 kD in rat alveolar macrophages.

Alteration of sleep in rabbits by Staphylococcus aureus infection

Abundant evidence suggests that sleep might be altered during infectious disease, although the relationship between sleep and infectious disease has never been examined systematically. To address this issue, we determined the effects of Staphylococcus aureus infection on rabbit sleep. Rabbits inoculated intravenously with S. aureus demonstrated the expected physiological changes consistent with a state of infectious disease (e.g., lymphopenia, neutrophilia, and fever), as well as time-dependent changes in sleep patterns. The sleep changes were characterized initially by increases in (i) the time spent in slow-wave sleep, (ii) the electroencephalographic slow-wave amplitudes during slow-wave sleep, and (iii) the duration of individual bouts of slow-wave sleep. At 20 to 36 h after inoculation, sleep responses fell to levels below corresponding control values for 6 to 12 h. At 6 to 10 h after inoculation, rapid-eye-movement sleep was suppressed and remained at low levels throughout the remainder of the 48-h recording period. These effects of bacterial infection on sleep were attenuated by antibiotic (cephalothin) therapy. Inoculation with killed bacteria produced similar changes in sleep and other physiological parameters, although significantly higher numbers of organisms were required to produce equivalent responses. We postulate that changes in sleep may represent an adaptive response of the host to infectious disease.

Immunological studies on paroxetine, a novel anti-depressant drug

Paroxetine is a novel and selective neuronal 5-hydroxy-tryptamine uptake inhibitor with anti-depressant activity. Paroxetine was examined for its ability to induce adverse immunological reactions, either as a consequence of a specific immune response or by a direct or indirect effect on the immune system. Paroxetine did not react in vitro with protein amino or thiol groups, suggesting that it lacks the capacity to form potentially immunogenic hapten protein conjugates. No anti-paroxetine antibody was detected in plasma or serum samples from patients and rats following oral administration over prolonged periods, or from epicutaneously exposed guinea pigs, or from rabbits given paroxetine in Freund's adjuvant, suggesting that paroxetine does not have the capacity to elicit humoral immune responses. Guinea pigs epicutaneously exposed to paroxetine did not develop contact sensitivity, suggesting that it does not have the capacity to elicit cell-mediated immune responses. These results suggest that paroxetine lacks intrinsic immunogenicity. Anti-SRBC antibody plaque-forming cell responses in mice were unaffected by oral administration of paroxetine, and paroxetine had no significant effect on ex vivo and in vitro murine macrophage phagocytosis of opsonized SRBC or on ex vivo murine splenocyte mitogen responses, suggesting that paroxetine does not exert modulatory effects on the immune system or on macrophage function. These findings, together with the results of pre-clinical safety evaluation studies, suggest that paroxetine is unlikely to have immunotoxic effects.

Binding sites for peptidoglycan on mouse lymphocytes

Binding of peptidoglycan (PG), a B-cell mitogen and polyclonal activator, to mouse lymphocytes was studied using rosetting with PG-sensitized erythrocytes and a direct binding assay with 125I-labeled PG. Thirty-four percent of splenic lymphocytes formed PG rosettes, 62% of which were inhibited by preincubation of lymphocytes with free PG. Less than 1 or 3% of spleen cells formed rosettes with uncoated or albumin-coated red cells. The formation of rosettes was not inhibited by 0.1% azide and was not dependent on the presence of complement or immunoglobulins. The 125I-PG bound both specifically and nonspecifically to the lymphocytes. The binding was completed within 15-20 min, was proportional to the cell concentration, and was not inhibited by 0.1% azide or treatment of lymphocytes with formalin. The cells had one set of specific binding sites of low affinity (KD = 1.2-4.6 X 10(-7) M +/- 9% SE, based on competitive) experiments. The binding, however, was complex, probably involving interaction of multiple binding sites on PG with the cell surface. The EC50 (920 micrograms/ml) was similar to the optimal lymphocyte-activating concentration of PG (400-1000 micrograms/ml). The binding correlated with the ability of different PG preparations to stimulate lymphocytes, since only high Mr PG (not low Mr PG preparations, muramyl dipeptide (MDP), or PG pentapeptide) had the ability to specifically bind to lymphocytes, to compete with PG binding, and to stimulate lymphocytes. Also, low Mr PG preparations, MDP, or PG pentapeptide did not inhibit the mitogenic stimulation of lymphocytes by high Mr PG. These results indicate the presence of specific binding sites for PG on the surface of murine lymphocytes and suggest that the binding of PG to these binding sites is involved in lymphocyte activation by PG.

A novel muramyl peptide derivative stimulates tumoricidal activity of macrophages and antibody production by B cells

A novel analog of MDP, the 3'-iodo-4'-azido-L-phenylalanine methyl ester derivative of N-acetyl-L-alanyl-D-isoglutamine, has been prepared. This compound is capable of activating macrophages to the tumoricidal state and increasing the specific immune response of B cells. It thus appears to exhibit similar biological activities to MDP. Moreover, this compound is of potential interest for receptor photolabelling studies.