Identification of delta- and mu-type opioid receptors on human and murine dendritic cells

Opioid Use, Gut Dysbiosis, Inflammation, and the Nervous System

Opioid use disorder (OUD) is defined as the chronic use or misuse of prescribed or illicitly obtained opioids and is characterized by clinically significant impairment. The etiology of OUD is multifactorial as it is influenced by genetics, environmental factors, stress response and behavior. Given the profound role of the gut microbiome in health and disease states, in recent years there has been a growing interest to explore interactions between the gut microbiome and the central nervous system as a causal link and potential therapeutic source for OUD. This review describes the role of the gut microbiome and opioid-induced immunopathological disturbances at the gut epithelial surface, which collectively contribute to OUD and perpetuate the vicious cycle of addiction and relapse.

The novel mechanism of anticancer effect on gastric cancer through inducing G0/G1 cell cycle arrest and caspase-dependent apoptosis in vitro and in vivo by methionine enkephalin

Background

Gastric cancer (GC) is the second cause of cancer-related deaths. Methionine enkephalin (MENK), an endogenous opioid peptide, has immunological and antitumor activity.

Purpose

The aim of this work was to investigate whether MENK could exhibit activity against human GC in vitro and in vivo.

Materials and methods

Human GC cells were treated with MENK. Cell viability, colony formation, cell morphology, cell cycle, and apoptosis were assessed. The effects of MENK on gene expression of OGFr, Bax, BCL-2, caspase-3, PARP, Ki67, cyclin D1, c-myc, survivin were quantifed by qRT-PCR. Western blot was used to analyze the effects of MENK on protein expression of OGFr, Bax, BCL-2, caspase-3, PARP. The anti-tumor activity of MENK in gastic carcinoma was also investigated with animal experiments.

Results

The results indicate that MENK could significantly inhibit the growth of human GC cells SGC7901 and HGC27 in a concentration- and time-dependent manner, decrease the number of cell colonies, and arrest cell cycle in the G0/G1 phase by causing a decrease in Ki67, cyclin D1, and c-myc mRNA. Furthermore, MENK could induce tumor cell apoptosis associated with the upregulation of Bax, a corresponding downregulation of BCL-2 and survivin, and activation of caspase-3 and PARP. Moreover, MENK upregulated the expression of opioid receptors (OGFr) in SGC7901 and HGC27 cells. The interaction between MENK and OGFr in SGC7901 and HGC27 cells appears to be essential for the antitumor activity of MENK.

Conclusion

We conclude that MENK may be a potential drug for the treatment of GC.

Effect of chronic morphine administration on circulating dendritic cells in SIV-infected rhesus macaques

We studied the effect of chronic morphine administration on the circulating dendritic cell population dynamics associated with SIV infection using rhesus macaques. Animals were either first infected with SIV and then given chronic morphine, or visa versa. SIV infection increased the numbers of myeloid DCs (mDCs), but morphine treatment attenuated this mDC expansion. In contrast, morphine increased the numbers of plasmacytoid DCs (pDCs) in SIV-infected animals. Finally, chronic morphine administration (no SIV) transiently increased the numbers of circulating pDCs. These results show that chronic morphine induces a significant alteration in the available circulating levels of critical antigen-presenting cells.

Clinical evaluation of the efficacy of methylnaltrexone in resolving constipation induced by different opioid subtypes combined with laboratory analysis of immunomodulatory and antiangiogenic effects of methylnaltrexone

Background

Opioid-induced constipation (OIC) is one of the major symptoms in palliative care with a prevalence of 30-50%. Methylnaltrexone for the treatment of OIC is significantly more effective than placebo, but only in about fifty percent of the patients regardless of dose increase. Dose increases cause increased toxicity without additional efficacy, and are therefore not recommended.

While methylnaltrexone is a μ-receptor antagonist, only a few opioids are solely μ-receptor agonists. Therefore, the response to methylnaltrexone may be determined by the receptor-profile of a specific opioid. In addition, methylnaltrexone may also affect the immune system and angiogenesis as was found in pre-clinical studies. Primary aim of this study is to determine differences in the efficacy of methylnaltrexone prescribed to resolve opioid induced constipation between three commonly used opioid subtypes: morphine sulphate, oxycodone and fentanyl. Secondary aim is to explore potential immunomodulatory and antiangiogenic effects of methylnaltrexone.

Methods

In this multi-center, prospective, parallel group trial we will evaluate the efficacy of methylnaltrexone in resolving OIC occurring as a side effect of the most common opioid subtypes: morphine, oxycodone and fentanyl. In total 195 patients with OIC despite prophylactic laxatives will receive methylnaltrexone every other day up to fourteen days. Patients will report its effect in a laxation diary. Group allocation is based on the opioid type the patient is using. At the start and end of the study period patients complete the Bowel Function Index questionnaire. A subgroup of the patients will donate blood for analysis of immunomodulatory- and anti-angiogenic effects of methylnaltrexone.

Discussion

In this study we aim to determine the efficacy of methylnaltrexone per opioid subtype to reduce constipation. We expect that the outcome of this study will improve the clinical use of methylnaltraxone.

Trial registration

This trial is registered at clinicaltrials.gov: NCT01955213 and in the Dutch trial register: NTR4272.

Opioids and clonidine modulate cytokine production and opioid receptor expression in neonatal immune cells

OBJECTIVE

Opioids and clonidine, used in for sedation, analgesia and control of opioid withdrawal in neonates, directly or indirectly activate opioid receptors (OPRs) expressed in immune cells. Therefore, our objective is to study how clinically relevant concentrations of different opioids and clonidine change cytokine levels in cultured whole blood from preterm and full-term infants.

STUDY DESIGN

Using blood from preterm (≤ 30 weeks gestational age (GA), n=7) and full-term ( ≥ 37 weeks GA, n=19) infants, we investigated the changes in cytokine profile (IL-1β, IL-6, IL-8, IL-10, IL-12p70 and TNF-α), cyclic adenosine monophosphate (cAMP) levels and μ-, δ- and κ- opioid receptor (OPR) gene and protein expression, following in-vitro exposure to morphine, methadone, fentanyl or clonidine at increasing concentrations ranging from 0 to 1 mM.

RESULT

Following lipopolysaccharide activation, IL-10 levels were 146-fold greater in cultured blood from full-term than from preterm infants. Morphine and methadone, but not fentanyl, at >10(-5) M decreased all tested cytokines except IL-8. In contrast, clonidine at <10(-9) M increased IL-6, while at >10(-5) M increased IL-1β and decreased TNF-α levels. All cytokine changes followed the same patterns in preterm and full-term infant cultured blood and matched increases in cAMP levels. All three μ-, δ- and κ-OPR genes were expressed in mononuclear cells (MNC) from preterm and full-term infants. Morphine, methadone and clonidine, but not fentanyl, at >10(-5)M decreased the expression of μ-OPR, but not δ- or κ-OPRs.

CONCLUSION

Generalized cytokine suppression along with downregulation of μ-OPR expression observed in neonatal MNC exposed to morphine and methadone at clinically relevant concentrations contrast with the modest effects observed with fentanyl and clonidine. Therefore, we speculate that fentanyl and clonidine may be safer therapeutic choices for sedation and control of opioid withdrawal and pain in neonates.

Induction on differentiation and modulation of bone marrow progenitor of dendritic cell by methionine enkephalin (MENK)

Methionine enkephalin (MENK), the endogenous neuropeptide, is known to exert direct effects on the neuroendocrine and the immune systems and participates in regulation of various functions of cells related to both the innate and adaptive immune systems. Dendritic cells (DCs) play important role in initiating and regulating T cell responses. The aim of this work is to investigate the effects of MENK on differentiation, maturation, and function of DCs derived from murine bone marrow progenitors (BM-derived DCs). Our result showed that MENK could induce BM-derived DCs to polarize predominantly to mDC subtype, rather than pDC both in vivo and in vitro, and this was in favor of Th1 response. BM-derived DCs, after treatment with MENK, up-regulated the expressions of MHC class II and key costimulatory molecules. Result by RT-PCR showed MENK could increase expressions of delta and kappa receptors on BM-derived DCs. Also MENK promoted BM-derived DCs to secret higher levels of proinflammatory cytokines of IL-12p70, TNF-α. Furthermore, differentiated BM-derived DCs treated with MENK displayed higher activity to induce allogeneic T cell proliferation and MENK also inhibited tumor growth in vivo and induced apoptosis of tumor cells in vitro. Thus, it is concluded that MENK could be an effective inducer of BM-derived DCs and might be a new therapeutic agent for cancer, as well as other immune handicapped disease. Also we may consider MENK as a potential adjuvant in vaccine preparation.

Comparison of stimulating effect on subpopulations of lymphocytes in human peripheral blood by methionine enkephalin with IL-2 and IFN-γ

The aim of this study was to investigate the effects of mechanisms of methionine enkephalin (MENK) on lymphocytes in human peripheral blood. We detected CD4+T cells, CD8+T cells, CD4+CD25+ regulatory T cells (Treg), dendritic cells (DCs), natural killer cells (NK), NKT cells and γδT cells before and after treatment with 10 (-12) M MENK, in cell culture by FCM and RT-PCR. Our findings show that MENK stimulating expansion of lymphocyte subpopulationns by inhibiting CD4+CD25+ regulatory T cells (Treg), which is unique discovery of our study. We may use MENK as a drug to treat cancer patients, whose immune systems are damaged by chemotherapy or radiotherapy.

Role of the mu-opioid receptor in opioid modulation of immune function

Endogenous opioids are synthesized in vivo to modulate pain mechanisms and inflammatory pathways. Endogenous and exogenous opioids mediate analgesia in response to painful stimuli by binding to opioid receptors on neuronal cells. However, wide distribution of opioid receptors on tissues and organ systems outside the CNS, such as the cells of the immune system, indicate that opioids are capable of exerting additional effects in the periphery, such as immunomodulation. The increased prevalence of infections in opioid abuser-based epidemiological studies further highlights the immunosuppressive effects of opioids. In spite of their many debilitating side effects, prescription opioids remain a gold standard for treatment of chronic pain. Therefore, given the prevalence of opioid use and abuse, opioid-mediated immune suppression presents a serious concern in our society today. It is imperative to understand the mechanisms by which exogenous opioids modulate immune processes. In this review, we will discuss the role of opioid receptors and their ligands in mediating immune-suppressive functions. We will summarize recent studies on direct and indirect opioid modulation of the cells of the immune system, as well as the role of opioids in exacerbation of certain disease states.

Endogenous opioid peptides in regulation of innate immunity cell functions

Endogenous opioid peptides comprise a group of bioregulatory factors involved in regulation of functional activity of various physiological systems of an organism. One of most important functions of endogenous opioids is their involvement in the interaction between cells of the nervous and immune systems. Summary data on the effects of opioid peptides on regulation of functions of innate immunity cells are presented.

Opioid drug abuse and modulation of immune function: consequences in the susceptibility to opportunistic infections

Infection rate among intravenous drug users (IDU) is higher than the general public, and is the major cause of morbidity and hospitalization in the IDU population. Epidemiologic studies provide data on increased prevalence of opportunistic bacterial infections such as TB and pneumonia, and viral infections such as HIV-1 and hepatitis in the IDU population. An important component in the intravenous drug abuse population and in patients receiving medically indicated chronic opioid treatment is opioid withdrawal. Data on bacterial virulence in the context of opioid withdrawal suggest that mice undergoing withdrawal had shortened survival and increased bacterial load in response to Salmonella infection. As the body of evidence in support of opioid dependency and its immunosuppressive effects is growing, it is imperative to understand the mechanisms by which opioids exert these effects and identify the populations at risk that would benefit the most from the interventions to counteract opioid immunosuppressive effects. Thus, it is important to refine the existing animal model to closely match human conditions and to cross-validate these findings through carefully controlled human studies. Better understanding of the mechanisms will facilitate the search for new therapeutic modalities to counteract adverse effects including increased infection rates. This review will summarize the effects of morphine on innate and adaptive immunity, identify the role of the mu opioid receptor in these functions and the signal transduction activated in the process. The role of opioid withdrawal in immunosuppression and the clinical relevance of these findings will also be discussed.

Functional modulation of the pathway between dendritic cells (DCs) and CD4+T cells by the neuropeptide: methionine enkephalin (MENK)

MENK, the endogenous neuropeptide, is suggested to be involved in the regulatory loop between the immune and neuroendocrine systems, with modulation of various functions of cells related to both the innate and adaptive immune systems. Our present research findings show that MENK serves as an immune modulator to the pathway between DCs and CD4+T cells. We studied changes of DCs in key surface molecules, the activity of acid phosphatases (ACPs), the production of IL-12, and the effects on murine CD4+T cell expansion and their cytokine production by MENK alone, and in combination with interleukin-2 (IL-2) or interferon-γ (IFN-γ). In fact, we found that MENK could markedly induce the maturation of DCs through the addition of surface molecules such as MHC class II, CD86, and CD40 on murine DCs, the production of IL-12, and the down-regulation of ACP inside DCs, (which occurs when phagocytosis of DCs is decreased, and antigen presentation increased with maturation). We also found that MENK alone or in combination with IL-2 or IFN-γ, could markedly up-regulate both CD4+T cell expansion and the CD4 molecule expression in vivo and in vitro and that MENK alone, or MENK+IL-2, could enhance the production of interferon-γ from CD4+T cells. Moreover, MENK alone, or MENK+IFN-γ, could enhance the production of IL-2 from CD4+T cells. It is therefore concluded that MENK can exert positive modulation to the pathway between dendritic cells and CD4+T cells.

Inducible expression of functional mu opioid receptors in murine dendritic cells

Opioids are known to exert direct effects on the immune system, and the expression of functional opioid receptors has been reported on several immune cell types. Dendritic cells (DCs) are important inducers and regulators of immune responses. In this study, we investigated whether murine dendritic cells express functional mu opioid receptors (MOR). RT-PCR analysis and double immunofluorescence staining revealed the expression of MOR in activated murine dendritic cells. We also studied the dynamic expression of MOR messenger RNA in murine dendritic cells in response to different Toll-like receptor ligands. Functionally, treatment of DCs with endomorphin 1 (EM1), a specific agonist of MOR, can inhibit the forskolin-induced formation of cyclic adenosine monophosphate level in activated DCs. Moreover, EM1 treatment resulted in less activation of p38 MAPK and more activation of ERK signaling in lipopolysaccharide-stimulated DCs. Consistently, treatment of DCs with EM1 altered cytokine production by increasing IL-10 and decreasing IL-12 and IL-23. Our results suggest that MOR is inducibly expressed on activated DCs and functionally mediates EM1-induced effects on DCs. Thus, dendritic cells might be involved in crosstalk between the neuroendocrine and the immune system.

Non-peptidic delta-opioid receptor antagonists suppress mitogen-induced tryptophan degradation in peripheral blood mononuclear cells in vitro

Opioid receptors are expressed not only on neuroendocrine cells but also on immunocompetent cells such as lymphocytes, monocytes and macrophages. micro-Opioid receptor agonists were found to exert immunosuppressive effects, whereas delta-opioid receptor agonists have been shown to act as immunostimulants. delta-Opioid receptor agonists stimulate T and B cells and activate granulocytes and monocytes, conversely, immunostimulation can be blocked by the non-peptidic delta-opioid receptor antagonist (NTI). We investigated the impact of NTI and of the two structurally related compounds HS-378 and HS-459 on degradation of tryptophan and formation of neopterin in mitogen-stimulated human peripheral blood mononuclear cells (PBMC). Both these biochemical pathways were found to be suppressed by all three opioid receptor antagonists, HS-378 and HS-459 exhibiting slightly greater potency than NTI. The suppression of tryptophan degradation suggests that the tested delta-opioid antagonists are able to influence the serotonergic system via a non-opioid action.

Opioid receptor blockade increases the number of lymphocytes without altering T cell response in draining lymph nodes in vivo

A number of studies have been dedicated to estimate the consequences on immunity of the clinical use of opioids by focusing on mitogen-induced polyclonal proliferation of T cells from blood or spleen. Here we examined, under physiological conditions, the contribution of endogenous opioids in the development of a CD4(+) T cell response within draining lymph nodes. We show in OVA-primed DO11.10 mice that delta-opioid receptors were up-regulated upon T cell activation in vivo and that opioid receptor neutralization increased the number of specific anti-OVA T lymphocytes without promoting their capacity to proliferate. The sensitivity to Fas-mediated apoptosis of T lymphocytes and the synthesis of homeostatic lymphoid chemokines were not either affected suggesting that opioids operate mainly before the entry of T lymphocytes into lymph nodes.

Morphine reciprocally regulates IL-10 and IL-12 production by monocyte-derived human dendritic cells and enhances T cell activation

We evaluated the effect of morphine on human dendritic cells (DCs). Interestingly, immature DCs were found to express all 3 (mu, kappa, delta) opioid receptors on the cell surface. Chronic morphine treatment (10(-8) to 10(-12) M) during the development of DCs from monocytes augmented LPS-induced upregulation of HLA-DR, CD86, CD80, and CD83 and increased the T cell stimulatory capacity of DCs, which could be inhibited by naloxone, an opioid receptor antagonist. The change in surface phenotype was paralleled by a p38 MAPK-dependent decrease in IL-10 and increase in IL-12 secretion. Our data indicate that morphine exerts an immunostimulatory effect by modulating LPS-induced DC maturation.

Morphine reciprocally regulates IL-10 and IL-12 production by monocyte-derived human dendritic cells and enhances T cell activation

We evaluated the effect of morphine on human dendritic cells (DCs). Interestingly, immature DCs were found to express all 3 (mu, kappa, delta) opioid receptors on the cell surface. Chronic morphine treatment (10(-8) to 10(-12) M) during the development of DCs from monocytes augmented LPS-induced upregulation of HLA-DR, CD86, CD80, and CD83 and increased the T cell stimulatory capacity of DCs, which could be inhibited by naloxone, an opioid receptor antagonist. The change in surface phenotype was paralleled by a p38 MAPK-dependent decrease in IL-10 and increase in IL-12 secretion. Our data indicate that morphine exerts an immunostimulatory effect by modulating LPS-induced DC maturation.

The distinctive effects of acute and chronic psychological stress on airway inflammation in a murine model of allergic asthma

BACKGROUND

Psychological stress has long been recognized to be associated with asthma symptoms. There appear to be individual differences in the susceptibility to even the same kind of stress, and furthermore, stress responses are different between the types of the stress, acute and chronic, even in the same person. However, the mechanisms linking stress to asthma are not well defined. Psychological stress upregulates the expression of endogenous opioids. The opioids stimulate the hypothalamus-pituitary-adrenal axis and sympathetic and adrenomedullary system, through the activation of mu-opioid receptor (MOR) to release stress hormones, such as cortisol and catecholamines, respectively. These hormones can modulate immune responses via the induction of Th1 immunity.

METHODS

Female BALB/c and C57BL/6, wild and MOR-deficient, mice sensitized with ovalbumin (OVA) were exposed to OVA with or without either acute or chronic restraint stress. Airway inflammation was evaluated by the measurement of the number of inflammatory cells and cytokine contents in bronchoalveolar lavage fluids.

RESULTS

In BALB/c mice, but not in C57BL/6 mice, the number of total cells, eosinophils and lymphocytes in the acute stress group were significantly decreased compared with those in the non-acute stress group. In contrast, chronic stress significantly increased the cell numbers and the contents of IL-4 and IL-5 in both mouse strains. Furthermore, these exacerbations were abolished in MOR-deficient mice.

CONCLUSIONS

These results suggest that acute stress modifies the allergic airway responses distinctively depending on the genetic background, and MOR is involved in the chronic psychological stress-induced exacerbation of allergic airway inflammation.

Effect of novel non-peptidic delta opioid receptor antagonists on human T and B cell activation

The effects of the antagonist naltrindole (NTI) on cells of the immune system have been largely studied although the mechanisms of action are still unclear. The aim of this study is to evaluate, in vitro, the immunomodulatory activity of four new delta-selective opioid compounds structurally related to naltrindole. The effects at different concentrations of these opioid antagonists on proliferative response were studied on normal human peripheral blood mononuclear cells (PBMC) stimulated with different stimuli: mitogens, the antigen PPD, the anti-CD3 monoclonal antibodies (mAb), the superantigen Staphylococcus aureus Cowan strain 1 (SAC) and alloantigens in the mixed lymphocyte cultures (MLR). The immunomodulatory capacity of these compounds was evaluated by determining the interleukin-2 (IL-2) release in mitogen activated PBMC. The present study shows that all the new delta opioid antagonists at 10(-5) M concentration are immunosuppressive. The inhibitory action is also evident at lower concentrations when anti-CD3 mAb and SAC were used as stimulators. In addition, the production of IL-2 was inhibited by the opioid treatment, but this might not be the only mechanism of action.

Lung cancer-derived bombesin-like peptides down-regulate the generation and function of human dendritic cells

Development of tumors is regulated by tumor-derived neuroendocrine factors, including bombesin-like peptides (BLP). We have evaluated neuroendocrine regulation of dendritic cell (DC) maturation and function by both tumor-derived and purified bombesin (BOM), neuromedin B (NMB), gastrin-releasing peptide (GRP), and a BOM antagonist D-Phe-bombesin (DPB). BOM, NMB and GRP dose-dependently inhibited maturation of DC assessed as down-regulation of CD40, CD80 and CD86 expression on DC. BOM and GRP also inhibited interleukin-12 (IL-12) production by DC and their ability to activate T cells. DPB partly abrogated immunosuppressive effect of tumor cells on DC. These data are a first evidence for the role of BLP in the regulation of DC maturation and function, demonstrating that BLP inhibit DC maturation and longevity in the lung cancer microenvironment. This suggests a new mechanism of tumor escape and provides new targets for the immunopharmacological correction of immune effectors in cancer.

Anti-inflammatory properties of the mu opioid receptor support its use in the treatment of colon inflammation

The physiologic role of the mu opioid receptor (MOR) in gut nociception, motility, and secretion is well established. To evaluate whether MOR may also be involved in controlling gut inflammation, we first showed that subcutaneous administration of selective peripheral MOR agonists, named DALDA and DAMGO, significantly reduces inflammation in two experimental models of colitis induced by administration of 2,4,6-trinitrobenzene sulfonic acid (TNBS) or peripheral expansion of CD4(+) T cells in mice. This therapeutic effect was almost completely abolished by concomitant administration of the opioid antagonist naloxone. Evidence of a genetic role for MOR in the control of gut inflammation was provided by showing that MOR-deficient mice were highly susceptible to colon inflammation, with a 50% mortality rate occurring 3 days after TNBS administration. The mechanistic basis of these observations suggests that the anti-inflammatory effects of MOR in the colon are mediated through the regulation of cytokine production and T cell proliferation, two important immunologic events required for the development of colon inflammation in mice and patients with inflammatory bowel disease (IBD). These data provide evidence that MOR plays a role in the control of gut inflammation and suggest that MOR agonists might be new therapeutic molecules in IBD.

Anti-inflammatory properties of the mu opioid receptor support its use in the treatment of colon inflammation

The physiologic role of the mu opioid receptor (MOR) in gut nociception, motility, and secretion is well established. To evaluate whether MOR may also be involved in controlling gut inflammation, we first showed that subcutaneous administration of selective peripheral MOR agonists, named DALDA and DAMGO, significantly reduces inflammation in two experimental models of colitis induced by administration of 2,4,6-trinitrobenzene sulfonic acid (TNBS) or peripheral expansion of CD4(+) T cells in mice. This therapeutic effect was almost completely abolished by concomitant administration of the opioid antagonist naloxone. Evidence of a genetic role for MOR in the control of gut inflammation was provided by showing that MOR-deficient mice were highly susceptible to colon inflammation, with a 50% mortality rate occurring 3 days after TNBS administration. The mechanistic basis of these observations suggests that the anti-inflammatory effects of MOR in the colon are mediated through the regulation of cytokine production and T cell proliferation, two important immunologic events required for the development of colon inflammation in mice and patients with inflammatory bowel disease (IBD). These data provide evidence that MOR plays a role in the control of gut inflammation and suggest that MOR agonists might be new therapeutic molecules in IBD.

Expression of functional kappa-opioid receptors on murine dendritic cells

Endogenous and exogenous opioids are known to exert direct effects on the immune system and the expression of functional opioid receptors has been reported for several immune cell types. Since dendritic cells are important inducers and regulators of immune responses, we investigated whether murine dendritic cells express functional kappa-opioid receptors. FACScan analysis and radioligand binding studies revealed the expression of kappa-opioid receptors by murine dendritic cells, which by RT-PCR were also shown to express kappa-opioid mRNA. In a primary allogenic mixed-lymphocyte reaction the kappa-agonists dynorphin A and U50488H suppressed the capacity of dendritic cells to induce T-cell proliferation in a concentration-dependent manner. Preincubation with the kappa-specific antagonist nor-binaltrophimine abolished the observed effect, indicating specificity. In contrast, antigen uptake by dendritic cells as well as phenotypic maturation of dendritic cells were not influenced by the kappa-agonists dynorphin A and U50488H. In summary our data demonstrate that dendritic cells express functional kappa-opioid receptors and that specific agonists exert a direct effect on these cells. Therefore, dendritic cells might be involved in the interaction of the neuroendocrine hormones and the immune system.