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Keywords:

  • innate immunity;
  • melatonin;
  • neuroimmunomodulation

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Monocytes and macrophages
  5. Dendritic cells
  6. Polymorphonuclear granulocytes
  7. Neutrophils
  8. Eosinophils
  9. Basophils and mast cells
  10. Natural killer cells
  11. Concluding remarks
  12. Acknowledgements
  13. Financial disclosure and conflict of interest
  14. References

Melatonin is the major secretory product synthesized and secreted by the pineal gland and shows both a wide distribution within phylogenetically distant organisms from bacteria to humans and a great functional versatility. In recent years, a considerable amount of experimental evidence has accumulated showing a relationship between the nervous, endocrine, and immune systems. The molecular basis of the communication between these systems is the use of a common chemical language. In this framework, currently melatonin is considered one of the members of the neuroendocrine–immunological network. A number of in vivo and in vitro studies have documented that melatonin plays a fundamental role in neuroimmunomodulation. Based on the information published, it is clear that the majority of the present data in the literature relate to lymphocytes; thus, they have been rather thoroughly investigated, and several reviews have been published related to the mechanisms of action and the effects of melatonin on lymphocytes. However, few studies concerning the effects of melatonin on cells belonging to the innate immunity have been reported. Innate immunity provides the early line of defense against microbes and consists of both cellular and biochemical mechanisms. In this review, we have focused on the role of melatonin in the innate immunity. More specifically, we summarize the effects and action mechanisms of melatonin in the different cells that belong to or participate in the innate immunity, such as monocytes–macrophages, dendritic cells, neutrophils, eosinophils, basophils, mast cells, and natural killer cells.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Monocytes and macrophages
  5. Dendritic cells
  6. Polymorphonuclear granulocytes
  7. Neutrophils
  8. Eosinophils
  9. Basophils and mast cells
  10. Natural killer cells
  11. Concluding remarks
  12. Acknowledgements
  13. Financial disclosure and conflict of interest
  14. References

Melatonin (N-acetyl-5-methoxytryptamine) was first isolated by Lerner et al. [1] from the bovine pineal gland and is the main secretory product synthesized by this organ. Melatonin is the main chronobiotic hormone that regulates the circadian rhythms and seasonal changes in vertebrate physiology via its daily nocturnal increase in the blood [2, 3]. Its biosynthesis from tryptophan involves four well-defined intracellular steps catalyzed by tryptophan hydroxylase (EC 1.14.16.4, TPH), aromatic amino acid decarboxylase (EC 4.1.1.28, AADC), arylalkylamine-N-acetyltransferase (EC 2.3.1.87, AA-NAT), and hydroxyindole-O-methyltransferase (EC 2.1.1.4, HIOMT) [4]. The remarkable functional versatility of melatonin is reflected in its wide distribution within phylogenetically distant organisms from plants [5], bacteria [6], and to humans [7]. Additionally, melatonin shows a remarkable functional versatility exhibiting antioxidant [8-12], oncostatic [13, 14], antiaging [15, 16], and immunomodulatory [17-19] effects. With regard to the melatonin mechanisms of action, four models have been described: (i) via an interaction with membrane receptors [20], (ii) by binding to nuclear receptors [21], [21], (iii) an interaction with cytoplasmic proteins [22], and (iv) via direct, receptor-independent actions [23, 24]. In this context, it is interesting to note that the large spectrum of functions of melatonin can be divided into chronobiotic and nonchronobiotic functions. Chronobiotic effects are mediated by the daily rhythm of melatonin in the circulation due to nocturnal pineal synthesis, whereas the melatonin produced by other cells, such as gastrointestinal and immunocompetent cells, is independent of the light/dark cycle and exerts nonchronobiotic effects [25].

Numerous studies involving a definition of relationships between nervous, endocrine, and immune systems have shown one of the most noteworthy discoveries in modern biology that these systems use a common chemical language for intra- and intersystem communication (Fig. 1) [26]. In this framework, currently pineal-synthesized melatonin is considered one of the members of the complex neuroendocrine-immunological network, and the existence of a bidirectional communication between the pineal gland and the immune system is accepted. In this context, a number of in vivo and in vitro studies have documented that melatonin plays a fundamental role in neuroimmunomodulation [17-19, 27, 28], and a direct correlation between melatonin production and the circadian and seasonal variations in the immune system has been documented [29, 30]. Reciprocally, immunological signals produced by the immunocompetent cells are perceived by the pineal gland and provide a feedback for the regulation of pineal function. Thus IFN-γ [31], IL-12 [32], TNF-α [33], granulocyte–macrophage colony-stimulating factor (GM-CSF), and granulocyte colony-stimulating factor (G-CSF) [34] have several effects on pineal function. In this context, it is interesting to note what is called immune–pineal axis. Briefly, pathogen- or danger-associated molecular patterns activate the transcription factor nuclear factor kappa B (NF-κB) both in the pinealocytes and macrophages. NF-κB blocks melatonin synthesis in pinealocytes while inducing melatonin synthesis in macrophages. In addition, melatonin reduces NF-κB activation in pineal gland and immunocompetent cells. This coordinated shift between pineal and extrapineal production of melatonin driven by NF-κB has been termed the immune–pineal axis [35-37].

image

Figure 1. Simplified model of the interaction between the neuroendocrine and the immune systems (modified from Guerrero and Reiter, 1992). BALT, bronchus-associated lymphoid tissue; GALT, gut-associated lymphoid tissue.

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The neuroimmunomodulatory effect of melatonin on the immune system is supported by the existence of specific melatonin receptors in immune organs as well as immunocompetent cells; these melatonin receptors are located both in plasma membrane and in nucleus of the cells. Using the melatonin agonist 2-[125I]-iodomelatonin, specific binding sites for melatonin have been located and characterized in plasma membranes of the several types of immunocompetent cells of different species including birds [38-40], rodents [41-44], and human lymphocytes [44, 45]. Moreover, functional studies have shown that human lymphocyte membrane receptors are coupled to a G protein, and via these receptors, melatonin inhibits forskolin-stimulated cyclic AMP (cAMP) production and cyclic GMP (cGMP) production and diacylglycerol (DAG) production [45]. In immunocompetent cells, specific nuclear melatonin-binding sites have been reported in several species, including humans [18, 46, 47].

Using the official nomenclature suggested by the IUPHAR committee [48], the expression of both MT1 and MT2 receptors has been reported in organs and immune cells of different species including humans [49-53]. The nuclear melatonin receptors belong to the RZR/ROR subfamily of nuclear receptors, which includes the products of three genes: splicing variants of RORα (RORα1, RORα2, RORα3, and RZRα), which differ in the N-terminal domain, RZRβ, and RORγ [54]. RZRα, RORα1, and RORα2 are present in Jurkat cells [51], RORα1 and RORα2 in U937 cells [51], RZRα, RORα1, and RORα2 in different subpopulations of human lymphocytes [53], and RORα in both thymus and spleen of mice [50].

Based on published data, it is clear that the majority of the present data refer to lymphocytes. A number of reviews have been published related to the mechanisms of action and the effects of melatonin on lymphocytes [18, 19, 44, 55, 56], perhaps because the lymphocytes are considered the most important cells of the immune system. However, few studies concerning the effects of melatonin on cells belonging to the innate immunity have been reported. Innate immunity (also called natural or native immunity) provides an early line of defense against microbes and consists of both cellular and biochemical mechanisms that are in place even before infection and are poised to respond rapidly to infections [57]. This review is focused on the role of melatonin on cells involved in the innate immune response. More specifically, we will review the effects and actions of melatonin on the following cells that belong to or participate in the innate immunity: monocytes, macrophages, dendritic cells, neutrophils, eosinophils, basophils, mast cells, and natural killer (NK) cells.

Monocytes and macrophages

  1. Top of page
  2. Abstract
  3. Introduction
  4. Monocytes and macrophages
  5. Dendritic cells
  6. Polymorphonuclear granulocytes
  7. Neutrophils
  8. Eosinophils
  9. Basophils and mast cells
  10. Natural killer cells
  11. Concluding remarks
  12. Acknowledgements
  13. Financial disclosure and conflict of interest
  14. References

Monocytes and macrophages belong to the mononuclear phagocyte system that consists of cells whose primary function is phagocytosis [58]. Monocytes and macrophages arise from colony-forming unit granulocyte–monocyte (CFU-GM) progenitors that differentiate first into monoblasts, then promonocytes, and finally monocytes [59]. Monocytes leave the bone marrow and circulate in the bloodstream, and when blood monocytes are recruited into tissues, they become macrophages. Thus, macrophages are monocytes differentiated that reside in various tissues, including lung, liver, and brain [60]. Monocytes are cells with bean-shaped nuclei and finely granular cytoplasm containing lysosomes, phagocytic vacuoles, and cytoskeletal filaments. Monocytes are heterogeneous, and at least two subsets of blood monocytes exist, which are distinguishable by cell surface proteins and kinetics of migration into tissues. One population is identified as being inflammatory because it is rapidly recruited from the blood into inflamed tissue. The other type may be the source of tissue-resident macrophages and some dendritic cells [61].

Several cytokines have been shown to participate in the development of monocytes and granulocytes. For example, stem cell factor (SCF), IL-3, IL-6, IL-11, and GM-CSF have all been shown to promote the development of myeloid lineage cells from CD34+ stem cells, especially those in the early stages of differentiation. Another cytokine, macrophage colony-stimulating factor (M-CSF), acts at the later stages of development and is lineage specific, inducing maturation into macrophages [62]. The 24-hr melatonin rhythm has an effect on circadian rhythmicity of CFU-GM proliferation in rat bone marrow cell cultures [63]. Further, the expression of the activity of CFU-GM in rat bone marrow cell cultures depends on the time when pinealectomy is performed or melatonin is added [63]. Also of interest is the high levels of melatonin in bone marrow cells [64, 65].

It has been reported that exogenous administration of melatonin stimulates monocyte production in both bone marrow and the spleen from mice [66]. The action of melatonin on monocyte production may be partly due to its direct action on melatonin receptors [67] or may be a result of an increase in monocyte sensitivity to stimulants such as IL-3, IL-4, IL-6, or GM-CSF [68, 69]. Monocytes have both membrane and nuclear melatonin receptors. Human monocytes express membrane melatonin receptors, and this expression depends on their state of maturation [67]. Moreover, it appears that in vitro monocyte differentiation and maturation negatively influence human monocyte membrane melatonin receptor expression [67]. Finally, gene expression studies have demonstrated the expression of MT1 receptors in both human monocytes and in the human monocytic cell line U937 [51, 53]. Nuclear receptors RZRα and RORα2 are present in human monocytes [53], and the expression of RORα1 and RORα2 in U937 cells is stimulated by IFN-γ [51].

Two important functions of monocytes are the secretion of cytokines and the production of reactive oxygen species (ROS). Melatonin activates and stimulates IL-1 secretion by human monocytes [70] via protein kinase C [70]. Also in human monocytes, melatonin stimulates the production of IL-6 [71] and IL-12 [72]. The stimulation of IL-6 production by IFN-γ has also been reported in U937 cells [51], and both in human monocytes and in U937 cells, nuclear melatonin receptors are involved [51, 71]. Melatonin inhibits the production of IL-10 in IL-2-stimulated human monocytes [73] and TNF-α production in LPS-stimulated human monocytes [74] and THP-1-derived human monocytes [75].

Melatonin reportedly stimulates the production of ROS in human monocytes [70]. However, in the U937 cells and using the cytokinesis-block micronucleus technique to study DNA damage by ROS, melatonin diminished hydrogen peroxide (H2O2)-induced micronuclei production [76]. In U937 cells, melatonin increased, rather than decreased, ROS production [77]. This report suggested that a melatonin–calmodulin interaction may promote this effect [77]. More precisely, the authors postulate that melatonin might bind calmodulin, thus inducing the release of sequestered Ca2+-independent PLA2. Subsequently, this enzyme moves to the membrane and releases large amounts of arachidonic acid. This liberated arachidonic acid feeds 5-lipoxygenase to produce free radicals and 5-HETE [78].

Normally, circulating monocytes are short lived and undergo spontaneous apoptosis [79]. Several reports suggest that melatonin protects monocytes from apoptosis. Thus, melatonin prevented apoptosis induced by ultraviolet irradiation in U937 cells, and this effect seems to be mediated by the protection of mitochondria [80]. The same authors demonstrated that melatonin interferes with the intrinsic pathway of apoptosis at the mitochondrial level. Thus, in response to an apoptogenic stimulus, melatonin allows mitochondrial translocation of the pro-apoptotic protein Bax, but impairs its activation/dimerization. The downstream apoptotic events, such as cytochrome c release, caspase 9 and caspase 3 activation, and nuclear vesiculation, are equally impaired, indicating that melatonin interferes with Bax activation within mitochondria [81]. Interestingly, melatonin induces a strong relocalization of Bcl-2, the main Bax antagonist, to mitochondria, suggesting that Bax activation may in fact be antagonized by Bcl-2 at the mitochondrial level [81]. These effects are mediated by the interaction of melatonin with MT1/MT2 plasma membrane receptors and the activation of extracellular signal-regulated protein kinase (ERK) and mitogen-activated protein kinase (MAPK) pathway [81-83].

As described above, macrophages are cells derived from circulating monocytes, long-lived resident macrophages are present in all tissues, and they have different names in the different locations [60]. The in vivo administration of melatonin influences the number of macrophages (CD68+ cells) in the spleen of experimental animals [84]. Thus, long-term administration of melatonin under conditions of natural illumination had an immunosuppressive effect that was manifested by the depopulation of the marginal zones, white pulp and all the zones of the red pulp, parenchyma loosening, and denudation of the reticular stroma of the organ. However, long-term melatonin administration under conditions of artificial darkening has an immunostimulatory effect, as evidenced by the increased inflow of immunocompetent cells into the spleen, their migration from the white pulp into the marginal zones, and emigration into peripheral blood flow, concomitant with the increase in the number of lymphoid nodules. The number of CD68+ cells was increased in splenic periarterial lymphoid sheaths and decreased in B-dependent zones of the spleen [84]. Macrophages express a number of cell surface molecules, including major histocompatibility complex class I and class II (MHC-I and MHC-II), CD68 (a adhesion molecule), CD14 (the receptor for lipopolysaccharide), CD64 (a high-affinity receptor for IgG), CD16 (a low-affinity receptor for IgG involved in the antibody-dependent cellular cytotoxicity or ADCC), CR1 and CR3 (complement receptors), and co-stimulatory molecules B7-1 and B7-2 [62]. By the expression of both MHC class I and class II molecules and co-stimulatory molecules, macrophages serve as antigen-presenting cells (APCs) that display antigens to and activate T lymphocytes [85]. Administration of melatonin to mice enhances antigen presentation by splenic macrophages to T cells [86]. This effect is consistent with an increase in the expression of MHC class II molecules and production of IL-1 and TNF-α [86]. One study examined the response of macrophages/microglia to multiple injections of melatonin into the pineal gland and different regions of the brain. The study concluded that CR3, both MCH-I and MHC-II, and CD4 antigens are upregulated following melatonin administration [87]. However, melatonin downregulated the expression of the co-stimulatory molecule B7-1, but not B7-2 on macrophages [88].

Activated macrophages are a major source of cytokines including IFN-γ, IL-1, IL-12, and TNF-α, as well as complement proteins and prostaglandins [85, 89]. A significant increase in IL-12 production by melatonin-stimulated macrophages and in the human myeloid monocytic cell line THP-1 has been reported [90]. Interestingly, a significant reduction in IL-12 production was observed in macrophages previously activated with LPS and then treated with melatonin [90]. Melatonin induced a significant decline in TNF-α production in both humans [91] and mouse macrophages [92]. Likewise, melatonin suppressed the production of IL-6 in Prevotella intermedia (the main agent causing periodontal disease) LPS-activated RAW 264.7 macrophages [93] and decreased Toll-like receptor type 3 (TLR3)-mediated TNF-α expression in respiratory syncytial virus-activated RAW 264.7 macrophages [94]. Moreover, melatonin also inhibited Toll-like receptor type 4 (TLR4)-mediated TNF-α, IL-1β, IL-6, IL-8, and IL-10 expression in the same macrophage cell line stimulated by LPS [95]. However, melatonin pretreatment had no effect on TNF-α, IL-1β, and IL-6 production in both LPS-stimulated rat and murine macrophages [96, 97].

Activated macrophages also produce free radicals, including superoxide anion (inline image) and nitric oxide (NO) [85]. Peroxynitrite is a toxic oxidant formed from the coupling of the inline image and NO, and it produces DNA damage. DNA single-strand breakage and activation of the nuclear enzyme poly (ADP ribose) synthetase (PARS) trigger an energy-consuming and an inefficient repair cycle, which contributes to peroxynitrite-induced cellular injury [98]. In vivo studies, in a nonseptic shock model induced by zymosan in rats, have demonstrated that melatonin significantly reduces peroxynitrite formation, prevents the appearance of DNA damage, and prevents the PARS activation [98]. Furthermore, in vitro studies using J774 macrophages showed that melatonin inhibits the development of DNA single-strand breakage in response to peroxynitrite [99] and reduced the lipid peroxidation (LPO), inline image production, and DNA fragmentation in naphthalene-induced oxidative stress [100]. One report documented that the administration of tryptophan (the precursor of melatonin biosynthesis) to rats enhanced the detoxification of inline image in peritoneal macrophages [101]. However, contradictory results have also been published. Thus, it has been reported that melatonin pretreatment had no effect on H2O2 and inline image by LPS-stimulated murine macrophages [96].

A significant rise in NO production in melatonin-stimulated synovial macrophages has been reported [90]. Interestingly, a nonsignificant increase in NO production was measured in synovial macrophages previously activated with LPS and then treated with melatonin [90]. In THP-1 cells, melatonin increased NO production [90]. However, a subsequent investigation showed that melatonin inhibited the expression of inducible nitric oxide synthase (iNOS) and, consequently, NO production in LPS-activated murine macrophages [102]. The inhibition of iNOS expression was associated with reduction in the activation of the NF-κB [102]. Likewise, melatonin pretreatment significantly reduced the generation of NO in LPS/IFN-γ-stimulated murine macrophages [103]. In this study, Western blot and reverse transcription–polymerase chain reaction (RT-PCR) techniques showed that melatonin decreased the expression of iNOS at both the protein and mRNA levels [103]. In the same report, melatonin significantly attenuated the nitration of cytoplasmic IκB-α, inhibited the degradation of IκB-α, and blocked the translocation of p65/RelA into the nuclei [103]. Furthermore, melatonin reduced the activation of iNOS and the consequent NO production in LPS-activated RAW 264.7 macrophages [104]. In the same macrophage cell line, melatonin attenuated LPS-induced iNOS transcriptional activation by inhibiting p300 histone acetyltransferase (HAT) activity, thereby suppressing p52 acetylation, binding, and transactivation [105]. Likewise, melatonin suppressed the production of NO in P. intermedia LPS-activated RAW 264.7 macrophages at both transcription and translation levels [93]. In this investigation, melatonin blocked NF-κB signaling through the inhibition of nuclear translocation and DNA-binding activity of NF-κB p50 subunit and suppressed STAT1 signaling [93]. Finally, melatonin decreased TLR3-mediated iNOS expression in respiratory syncytial virus-activated RAW 264.7 cells [94] and also attenuated LPS-induced upregulation of iNOS mediated by activation of TLR4 in the same macrophage cell line [95].

Cyclooxygenase-2 (COX-2), the inducible isoform of COX, is the key enzyme that catalyzes the two sequential steps in the biosynthesis of prostaglandins (PGs) from arachidonic acid. This enzyme plays a critical role in the inflammatory response, and its overexpression has been associated with several pathologies including neurodegenerative diseases and cancer [106]. Melatonin prevented both COX-2 activation and PGE2 production in LPS-activated RAW 264.7 cells without affecting COX-1 protein levels [104]. Also in RAW 264.7 macrophages, melatonin suppressed COX-2 transcriptional activation by inhibiting p300 HAT activity, thereby suppressing p52 acetylation, binding, and transactivation [105]. Melatonin also inhibited COX-2 expression and the NF-κB activation in the same macrophage cell line stimulated with fimbriae of Porphyromonas gingivalis, a oral anaerobe [107]. Finally, it has also been reported that melatonin decreased the production of PGF by murine macrophages [102].

As we have mentioned previously, melatonin has different effects on ROS, cytokines, and NO production. In this context, it is important to note that cytokines, iNOS, and COX-2 are proteins that are coded by genes regulated by NF-κB, which is the pivotal transcription factor that regulates the time course of the innate immune response, thereby controlling the on/off timing of genes related to the proinflammatory and recovery phases of the innate immune response [35]. On the other hand, it has been described that melatonin reduces NF-κB activation in several cells and organs, such as T cells [108], neurons [109], liver [110], kidney [111], lung [112], and heart [113], and in different experimental models of inflammatory diseases, such as inflammatory bowel disease [114], colitis [115], chronic gastric ulceration [116], diabetic neuropathy [117], spinal cord trauma [118], and fulminate hepatic failure [110]. In summary, most of the studies in the literature indicate that the anti-inflammatory effect of melatonin is mediated by the inhibition of NF-κB activation. However, several studies have shown that melatonin is able to activate NF-κB [119-121]. This apparent controversial result could be reconciled through a better understanding of the role of different subunits of NF-κB in each phase of the defense response [35]. Finally, it is both interesting and important to note that NF-κB drives the synthesis of melatonin in RAW 264.7 macrophages by inducing the transcription of the AA-NAT gene and that macrophage-synthesized melatonin modulates the function of this cell in an autocrine manner [122].

Toll-like receptors (TLRs) are an evolutionarily conserved family of pattern recognition receptors expressed on many cell types that recognize products of a wide variety of microbes. Mammalian TLRs are involved in responses to a variety of molecules that are expressed by microbial, but not by healthy mammalian cells, although they are also involved in response to endogenous molecules whose expression or location indicates cell damage. TLRs are found on the cell surface and on intracellular membranes and are, thus, able to recognize microbes in different cellular locations. TLR recognition of microbial ligands results in the activation of several signaling pathways and ultimately transcription factors, which induce the expression of genes whose products are important for inflammatory and antiviral responses [57, 123, 124]. Currently, it is clear that TLRs play a critical role in both the physiology and the pathophysiology of the inflammation [124, 125]. Melatonin decreased TLR-3-mediated downstream gene expression in respiratory syncytial virus-infected macrophages via inhibition of NF-κB activation [94]. However, melatonin did not influence TLR3 at either the protein or the mRNA levels or MyD88 transcription [94]. In LPS-stimulated RAW 264.7 macrophages, melatonin attenuated the expression of MyD88, and although it had no effect on TLR4-mediated phosphorylation of c-Jun N-terminal kinase (JNK), p38, and ERK, melatonin did significantly reduce the activation of NF-κB [95]. In the same report, melatonin inhibited TLR4-mediated Akt phosphorylation and attenuated the elevation of interferon (IFN)-regulated factor-3 (IRF3), which was involved in TLR4-mediated TRIF-dependent signaling pathway [95].

One of the major physiological functions of macrophages relates to their phagocytic capacity [85]. The effects of external administration of l-tryptophan on the synthesis of serotonin and melatonin, as well as on the immune function of Wistar rats, have been examined. The results of these investigations showed a significant increase in nocturnal circulating melatonin levels and a rise in the phagocytic activity of rat peritoneal macrophages, very probably through its conversion to the immunoregulatory molecules serotonin and melatonin [101, 126]. Interestingly, rat peritoneal macrophages synthesize melatonin from tryptophan, and this process is regulated by IFN-α, IFN-γ, LPS, and phorbol myristate acetate (PMA) [127]. Moreover, as we have described above, NF-κB drives the synthesis of melatonin in RAW 264.7 macrophages by inducing the transcription of the AA-NAT gene [122]. Conversely, melatonin increased the phagocytic capacity in rat testicular macrophages; this effect appears to be related to a rise in intracellular free calcium concentration [128].

The macrophages are regulated by different molecules that bind to specific receptors expressed by these cells [85]. Importantly, membrane melatonin receptors in mouse peritoneal macrophages and the signal transduction processes of melatonin through these receptors involve in a pertussis toxin-sensitive pathway and the inhibition of adenylyl cyclase [43]. Synovial macrophages also have melatonin receptors [129]. On the other hand, bone marrow macrophages possess type 1 κ-opioid receptors [130, 131], and melatonin exerts its stimulatory effects on hematopoiesis via the induction of T-helper-cell-derived opioid cytokines (MIOS system) [132].

Also of importance is that melatonin regulates gene expression in macrophages [133]. RAW 264.7 macrophages were treated with LPS or melatonin plus LPS for 24 h, and gene expression was investigated both by microarray analysis and by real-time RT-PCR. LPS induced the upregulation of 1073 genes and the downregulation of 1144 genes. Melatonin pretreatment with LPS-stimulated cells resulted in the downregulation of 241 genes and upregulation of 164 genes. Interestingly, among genes related to macrophage-mediated immunity, LPS increased the expression of seven genes and reduced the expression of one gene, and these changes were attenuated by melatonin. Thus, these results show that melatonin may have a suppressive effect on LPS-induced expression of genes involved in the function of macrophages. In addition, these results suggest that melatonin possesses anti-inflammatory properties and is involved in the regulation of immunity- and defense-related genes that act as key mediators in various inflammatory processes [133].

Finally, it is interesting to note that concanavalin A (Con A)-primed macrophages oxidize melatonin by a mechanism dependent on myeloperoxidase (MPO) and independent of IFN-γ [134].

Dendritic cells

  1. Top of page
  2. Abstract
  3. Introduction
  4. Monocytes and macrophages
  5. Dendritic cells
  6. Polymorphonuclear granulocytes
  7. Neutrophils
  8. Eosinophils
  9. Basophils and mast cells
  10. Natural killer cells
  11. Concluding remarks
  12. Acknowledgements
  13. Financial disclosure and conflict of interest
  14. References

Dendritic cells are the most important APCs for activating naїve T cells, and they play major roles in innate responses to infections and in linking innate and adaptive immune responses. Except for bone marrow, dendritic cells are found in virtually all primary and secondary lymphoid tissues, as well as in skin, mucosa, and blood, and in different regions, they are known with different names, such as Langerhans cells or interdigitating dendritic cells. Dendritic cells are derived from CD34+ cells present in the bone marrow, and both GM-CSF and TNF-α are involved in their development. There are at least two types of dendritic cells: the CD1 type, which is bone marrow derived and found in lymphoid tissues, and plasmacytoid dendritic cells or DC2, which are high producers of IFN-α. Their derivation is unclear, but they may be myeloid or lymphoid [135, 136].

No reports on the effect of melatonin on dendritic cell biology have been published. It has been postulated that the pineal gland may be a central regulator of the cytokine network, and in this context, melatonin could stimulate IL-2 release by T lymphocytes and IL-12 by dendritic cells, whereas both IL-2 and IL-12 would inhibit melatonin release [137]. More recently, it has been reported that pinealectomy abolished the circadian rhythm in trafficking of dendritic cells in hamsters [138].

Polymorphonuclear granulocytes

  1. Top of page
  2. Abstract
  3. Introduction
  4. Monocytes and macrophages
  5. Dendritic cells
  6. Polymorphonuclear granulocytes
  7. Neutrophils
  8. Eosinophils
  9. Basophils and mast cells
  10. Natural killer cells
  11. Concluding remarks
  12. Acknowledgements
  13. Financial disclosure and conflict of interest
  14. References

Polymorphonuclear (PMN) granulocytes arise from progenitors that mature in the bone marrow. They are released into the blood as short-lived (2–3 days), essentially end-stage, cells. They constitute 65–75% of the white blood cells in the peripheral blood and have a multilobed nucleus. PMN granulocytes are also found in tissues and use the process of diapedesis to exit the blood. Granulocytes act as early soldiers in the response to stress, tissue damage, or pathogen invasion. Because of their function in phagocytosis, they possess granules whose unique staining characteristics are used to categorize the cells as neutrophils, eosinophils, or basophils [139]. Interestingly, the production of granulocytes by the bone marrow exhibits a diurnal rhythm, and pinealectomy in chickens shifted the acrophase of this rhythm [140]. Administration of melatonin restored the normal rhythm of granulocytes [140].

Neutrophils

  1. Top of page
  2. Abstract
  3. Introduction
  4. Monocytes and macrophages
  5. Dendritic cells
  6. Polymorphonuclear granulocytes
  7. Neutrophils
  8. Eosinophils
  9. Basophils and mast cells
  10. Natural killer cells
  11. Concluding remarks
  12. Acknowledgements
  13. Financial disclosure and conflict of interest
  14. References

Neutrophils, also called PMN leukocytes, are the most abundant population of circulating white blood cells and mediate the earliest phases of inflammatory reactions; they derive from CFU-GM progenitor cells. The cytokines SCF, IL-3, IL-6, IL-11, and GM-CSF promote the growth and development of neutrophil precursors, whereas certain cytokines are important for differentiation of CFU-GM progenitors into mature neutrophils. In this context, the pineal gland and melatonin seem to have an effect on circadian rhythmicity of CFU-GM proliferation in rat bone marrow cell cultures [63]. Furthermore, the expression of the activity of CFU-GM in rat bone marrow cell cultures depends on the time when pinealectomy is performed or melatonin is administered [63]. Neutrophils contain granules of two types. The majority, called specific granules, are filled with enzymes such as lysozyme, collagenase, and elastase. The remainder of the granules, called azurophilic granules, are lysosomes containing enzymes and other microbicidal substances, including defensins and cathelicidins. These granules fuse with ingested organisms to form phagolysosomes, which eventually kill the invading organisms. In response to a bacterial infection, the number of circulating granulocytes and their function are increased [139, 141]. Neutrophils express a number of myeloid antigens, including CD13, CD15, CD16, and CD89. Two important physiological functions of these neutrophils include pronounced phagocytic capacity and production of ROS.

Melatonin regulates the respiratory burst of PMA-stimulated human neutrophils, showing a concentration-dependent dual effect [142]. However, the same group later documented that melatonin increased the intensity of respiratory burst in PMA-stimulated human neutrophils [143]. On the contrary, melatonin reduced the inline image production and the subsequent LPO in Candida albicans-stimulated human neutrophils [144] and lowered ultraviolet light-induced ROS in a dose-dependent manner in IL-3-stimulated human neutrophils [145]. The incubation of HL-60-derived human neutrophils with increasing concentrations of melatonin resulted in a modest dose-dependent reduction in PMA-stimulated inline image production [75]. Similar results were obtained when HL-60 cells were stimulated with N-formyl methionyl-leucyl-phenylalanine (fMLP) [75]. New in vivo experiments in animal models showed that oral administration of melatonin reduced inline image levels in heterophils from the ring dove Streptopelia risoria; these are the major phagocytic cells from peripheral blood in these birds [146]. Interestingly, l-tryptophan, the precursor in the anabolic pathway of melatonin, administered orally at night, modified the oxidative metabolism of ring dove heterophils [147]. Moreover, in vitro experiments showed that melatonin inhibited the production of inline image [148-150] and superoxide dismutase (SOD) activity in ring dove heterophils [149]. Additionally, there is a correlation between the circadian rhythm of melatonin and inline image levels in these cells [151]. Finally, in cyclosporine (CsA)-stimulated rat neutrophils, melatonin protected the cells from oxidative damage [152].

A correlation between the circadian rhythm of melatonin and the number and circadian phagocytic activity of heterophils from ring dove S. risoria, and rat neutrophils has been reported [151, 153, 154]. In pinealectomized juvenile female fowls, heterophil counts were increased compared with those in unoperated birds [155]. Similarly, a study conducted in ring dove (S. risoria) showed that pinealectomy induced a significant increase in the number of total white blood cells and altered different stages of the phagocytic process [156]. Functional pinealectomy by constant light exposure lowered the average circadian level of phagocytosis, whereas in vivo administration of melatonin significantly increased circadian levels of this process, suggesting a physiological role of melatonin in the maintenance of neutrophil phagocytosis [157]. In addition, both in vivo and in vitro experiments in ring dove showed that melatonin increases heterophil phagocytic activity [146, 148, 150]. Oral administration of l-tryptophan, the precursor of melatonin, increased the phagocytic capacity of heterophils in the same experimental animal model [147].

The phagocytic function of neutrophils in a local inflammatory site includes the passage of neutrophils through intact capillary walls into surrounding damaged or infected tissue. This biological phenomenon, called diapedesis, is a complex process that includes chemoattraction, rolling adhesion, tight adhesion, and transmigration; this activity involves numerous molecules in both the neutrophils and the endothelial cells [158-160]. Melatonin prevented l-selectin shedding in human neutrophils [143] and reduced the increases in the vascular permeability induced by leukotriene B4 [161]. Moreover, it has been described that melatonin inhibits leukocyte rolling and adhesion to rat microcirculation [162]. In this context, it is very important to note that endothelial cells are targets for melatonin [163, 164].

The antioxidant properties of melatonin and its effects on neutrophil infiltration have been investigated in numerous experimental animal models. In rats, LPO in the liver induced by ischemia–reperfusion was abolished by melatonin; moreover, the indole counteracted the decrease in the concentrations of reduced glutathione, the increase in the concentrations of oxidized glutathione, prevented the loss of the activity of the glutathione reductase, and reversed partially the infiltration of neutrophils [165]. Also in rats, in a model of liver injury induced by α-naphthylisocyanate, the administration of melatonin reduced significantly the serum levels of both alanine aminotransferase and aspartic acid aminotransferase and the rise in hepatic LPO, reversed the drop in the liver microsomal membrane fluidity and neutrophil infiltration, and limited hepatocyte apoptosis [166]. In another rat model, that is, nonseptic shock model induced by zymosan, pretreatment with melatonin reduced peroxynitrite formation and prevented neutrophil infiltration [167]. In a model of peritonitis-induced septic shock with multiple organ dysfunction syndrome, melatonin attenuated hyporeactivity to norepinephrine and delayed hypotension, reduced plasma indices of hepatic and renal dysfunction, diminished plasma NO and IL-1β concentration, decreased aortic inline image levels, reduced neutrophil infiltration in the lungs and liver, and promoted survival [168]. After experimental spinal cord trauma, melatonin administration markedly reduced the nitrosamine and PAR formation, neutrophil infiltration, an apoptosis and significantly ameliorated the recovery of limb function [118]. Escherichia coli-induced pyelonephritis was associated with a reduction in renal glutathione levels and an increase in LPO levels, MPO activity, ROS production, and the collagen content of the renal tissues. Similarly, serum TNF-α, lactate dehydrogenase activity, and creatinine levels were elevated. These biochemical indices, as well as histopathological alterations, were reversed in animals treated with melatonin [169]. Likewise, both in a rat and mouse model of lung injury induced by radiation and bleomycin, respectively, melatonin protects against acute lung injury by several mechanisms, including the inhibition of neutrophil infiltration [170, 171]. Finally in septic human newborns, melatonin reduced serum LPO and improved survival [172]. In summary, the protective effects of melatonin in these pathological models of disease can be ascribed to its ability to inhibit neutrophil infiltration, to balancing the oxidant–antioxidant status, and to the regulation of the generation of inflammatory mediators.

The infiltration of neutrophils into a damaged or swollen tissue requires a chemotactic agent. Humans treated with melatonin showed an increased neutrophil chemotactic response to a physiological chemoattractant and an elevated expression of intracellular chemokines [173]. Moreover, peritoneal leukocytes were increased after an intraperitoneal melatonin injection in rats [173].

Among the different physiological actions of neutrophils are their microbicidal and cytotoxic activities [139]. With regard to this function, it has been reported in an in vitro model of co-culturing bovine neutrophils and mammary epithelial cells that melatonin reduced neutrophil-induced cytotoxicity in a dose-dependent manner [174]. Interestingly, melatonin and its kynurenine-like oxidation product N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK) decreased the microbicidal activity of human neutrophils [175]. Furthermore, melatonin almost completely inhibited hypochlorous acid (HClO) formation [175]. This group also showed that melatonin and AFMK inhibited the production of TNF-α and IL-8 by LPS-stimulated human neutrophils [176]. Melatonin is normally oxidized by activated neutrophils [177, 178]. This oxidation is a MPO-catalyzed process and produces AFMK and N1-acetyl-5-methoxykynuramine (AMK) [177, 178]. One study showed that the rate of oxidation of melatonin is dependent on the H2O2 concentration, is not affected by SOD, and is quickly terminated by sodium cyanide [179]. However, a subsequent study showed contrary results. Thus, the oxidation of melatonin by MPO to AFMK was a process highly dependent on inline image but not on H2O2. Moreover, it did not require HClO, singlet oxygen, or the hydroxyl radical. In addition, oxidation of melatonin was partially inhibited by catalase or SOD [180]. Finally, it has been reported that taurine, the most abundant free amino acid in leukocytes, undergoes an oxidation to taurine chloramines by HClO, and this compound oxidizes melatonin to 2-hydroxymelatonin and AFMK [181].

Neutrophils not only play a critical role as a first line of defense against bacterial and fungal infections, but also contribute to tissue injury associated with autoimmune and inflammatory diseases. The inflammatory activity of neutrophils must be regulated with exquisite precision and timing, a task mainly achieved through a complex network of mechanisms, which influence neutrophil survival. Neutrophils have the shortest life span among leukocytes and usually die via apoptosis. The life span of neutrophils can be dramatically modulated by a large variety of agents such as cytokines, pathogens, danger-associated molecular patterns, as well as by pharmacological manipulation. The precise control of the neutrophil death program provides a balance between their defense functions and safe clearance and a wide range of inflammatory pathologies. Thus, apoptosis is essential for neutrophil functional shutdown, removal of emigrated neutrophils, and timely resolution of inflammation [182]. Melatonin has been shown to attenuate delayed neutrophil apoptosis in human acute pancreatitis [183]. Moreover, melatonin reversed CD18 expression and respiratory burst activity in neutrophils of these patients. Another study showed that melatonin alleviated human neutrophil apoptosis induced by Ca++ [184]. The results of this study suggest that melatonin reduced caspase-9 and caspase-3 activities induced by Ca2+, which was produced through the inhibition of both mitochondrial permeability transition pore (mPTP) and Bax activation [184]. Subsequently, the same authors showed that melatonin delayed endoplasmic reticulum stress-induced apoptosis of neutrophils from elderly humans [185], and they also proved that the protective effects resulting from melatonin administration on leukocyte apoptosis likely depend on antioxidant properties of melatonin, as this protection was shown to be receptor independent [186]. Interestingly, it has recently been shown that melatonin stimulates ROS production in tumor cells making them undergo apoptosis, while it prevents apoptosis in healthy cells. Thus, melatonin enhanced H2O2-induced apoptosis in the human promyelocytic leukemia HL-60 cells, while when healthy leukocytes were exposed to H2O2, melatonin increased the viability of the cells [187].

Eosinophils

  1. Top of page
  2. Abstract
  3. Introduction
  4. Monocytes and macrophages
  5. Dendritic cells
  6. Polymorphonuclear granulocytes
  7. Neutrophils
  8. Eosinophils
  9. Basophils and mast cells
  10. Natural killer cells
  11. Concluding remarks
  12. Acknowledgements
  13. Financial disclosure and conflict of interest
  14. References

Eosinophils derive from a progenitor (colony-forming unit eosinophils or CFU-Eo) and progress through development stages similar to those of neutrophils. Three cytokines are important in the development of eosinophils: GM-CSF, IL-3, and IL-5. As stated previously, melatonin seems to have an effect on circadian rhythmicity of CFU-GM proliferation in rat bone marrow cell cultures and that the expression of the activity of CFU-GM depends on the time when pinealectomy is performed or when melatonin is substituted [63]. In a study related to the immunotherapy of cancer, several patients with solid tumor received IL-2 alone or in combination with melatonin. Both treatments significantly enhanced mean eosinophil numbers, but this increase was significantly higher in patients receiving IL-2 plus melatonin. The authors suggest that T-helper lymphocyte type 2 (Th2), which is the source of IL-5, may be the target of melatonin action [188]. Also of interest is that eosinophil production exhibits a diurnal variation, with a peak at night [189].

Eosinophils are capable of phagocytosis followed by killing, although this is not their main function. Cytoplasmic granules of eosinophils contain a large amount of a major basic protein, which is toxic. Organisms that are too large to be phagocytosed, such as parasites, can be exposed and damaged by this eosinophil protein; however, eosinophils can also damage host tissues. Some eosinophils are normally present in peripheral tissues, especially in mucosal linings of the respiratory, gastrointestinal, and genitourinary tracts, and their numbers are increased by recruitment from the blood during inflammation [189].

There is little information about melatonin and eosinophils, and the main data derive from studies related to immunotherapy of cancer. Thus, as noted above, patients with cancer treated with IL-2 and melatonin had a significantly higher number of eosinophils [188, 190]. In an asthmatic experimental model in rats, a pathology that involves neutrophils, melatonin administration decreased eosinophil number in the bronchoalveolar lavage fluid [191]. Moreover, melatonin expression in nasal mucosa of patients with polypous rhinosinusopathy was significantly increased and, in these areas, there was a rise in the number of eosinophils [192]. However, contrary results have also been reported. Thus, in a study designed to determine the effects of melatonin on rat endometrial morphology and embryo implantation in rats, the highest number of eosinophils was detected in pinealectomized animals [193]. Curiously, melatonin has been immunocytochemically identified in eosinophilic leukocytes in submucosa of human stomach [194].

Basophils and mast cells

  1. Top of page
  2. Abstract
  3. Introduction
  4. Monocytes and macrophages
  5. Dendritic cells
  6. Polymorphonuclear granulocytes
  7. Neutrophils
  8. Eosinophils
  9. Basophils and mast cells
  10. Natural killer cells
  11. Concluding remarks
  12. Acknowledgements
  13. Financial disclosure and conflict of interest
  14. References

Basophils mature from a progenitor (colony-forming unit basophil/mast cell or CFU-BM); however, less is known about the stages of mast cell development, although they are probably derived from the same CFU-BM progenitor as basophils. In human basophils and mast cells, SCF (also known as kit ligand) induces the most consistent effects on growth and differentiation. Basophils represent <1% of the cells in the peripheral circulation and have basophilic-staining cytoplasmic granules. Although they are normally not present in tissues, basophils may be recruited to some inflammatory sites. Mast cells are found only in tissues, especially in skin and mucosal epithelium, and they are often adjacent to small blood vessels and nerves. Mast cells are much larger than peripheral blood basophils, the granules are less abundant, and the nucleus is more prominent. There are two types of mast cells, designated mucosal or connective tissue, depending on their location. Mucosal mast cells require T cells for their proliferation, whereas connective tissue mast cells do not. Both types have granules that contain effector molecules. After degranulation, which is effected by cross-linkage of cell surface IgE bound to cells via the high-affinity receptor for IgE, basophils and mast cells release heparin, histamine, and other effector molecules involved in the symptoms of allergic diseases [195, 196].

Although basophils and mast cells have some distinct morphologic and functional characteristics, both cells share a number of phenotypic and functional features. Thus, they both contain basophilic-staining cytoplasmic granules, express the high-affinity IgE receptor (FcεRI), express receptors for IgG, and release a number of chemical mediators, such as histamine and cytokines, which participate in immune and inflammatory responses [195, 196].

There are no publications related to the effects of melatonin on basophil function. It has been suggested, however, that as the chick pineal gland contains histamine, that amine, which likely originates from the pineal mast cells, may function as a regulator of pineal physiology [197]. Also mast cells are speculated to be involved in the age-related pineal calcification, which may influence pineal melatonin biosynthesis [198].

A new concept developed over recent decades is the inclusion of melatonin in the diffuse neuroendocrine system. For many years, melatonin was considered exclusively a hormone of the pineal gland. In recent years, melatonin has been identified also in extrapineal tissues, such as retina, Harderian gland, gut mucosa, cerebellum, airway epithelium, liver, kidney, adrenals, thymus, thyroid, pancreas, ovary, carotid body, placenta, and endometrium, as well as in non-neuroendocrine cells like mast cells, NK cells, eosinophils, platelets, and endothelial cells [199-201]. Taking into account the large number of melatonin-producing cells in many organs, the wide spectrum of biological activities of melatonin, and especially its role in the regulation of biological rhythms, it can be considered as a key paracrine and/or autocrine signal molecule for the local coordination of intercellular relationships both in physiology and in pathology [194, 202, 203]. The melatonin gene has been found to be expressed in RBL-2H3 rat mast cell line [204].

Several studies related to the role of melatonin in the function of mast cells have been published. One in vivo study claimed that melatonin decreased the number of mast cells in the testis of the frog Rana esculenta when stimulated with 17β-estradiol, a hormone that promotes mast cell accumulation in the frog testis [205]. In addition, melatonin interfered with the stimulatory effects of estradiol on mast cell number in short-term-cultured testes [205]. Another study reported that melatonin had a protective effect on degeneration of rat bladder epithelium due to chronic water avoidance stress [206]. In this in vivo study, melatonin decreased the number of mast cells in the mucosa and also reverted the rise in LPO and the reduction in the glutathione levels [206]. In the same experimental model, melatonin treatment reduced mast cell degranulation [207]. In a study related to the contractile responses of cisplatin-treated rat detrusor smooth muscle, melatonin pretreatment also limited the number of mast cells [208].

A major biochemical study related to melatonin and mast cells was carried out by our group. In this investigation, we demonstrated that both resting and stimulated RBL-2H3 cells synthesized and released melatonin [209]. We also reported that the necessary enzymatic machinery for the synthesis of melatonin is present in mast cells and that these cells possess both the MT1 and MT2 melatonin membrane receptors [209]. Based on these results, we suggested that the melatonin probably has a regulatory effect on inflammatory reactions mediated by mast cells [209]. We have further observed that melatonin is secreted by mast cells in the late phase of secretion and that melatonin inhibits the activation of mast cells, probably by an autocrine mechanism (M.D. Maldonado and J.R. Calvo, unpublished data).

Natural killer cells

  1. Top of page
  2. Abstract
  3. Introduction
  4. Monocytes and macrophages
  5. Dendritic cells
  6. Polymorphonuclear granulocytes
  7. Neutrophils
  8. Eosinophils
  9. Basophils and mast cells
  10. Natural killer cells
  11. Concluding remarks
  12. Acknowledgements
  13. Financial disclosure and conflict of interest
  14. References

Natural killer cells comprise the third major lymphocyte subset, that is, 10–15% of circulating lymphocytes; these cells play important roles in innate immune responses mainly against intracellular viruses and bacteria. The term ‘NK’ derives from the fact that these cells are capable of performing their killing function without the need for clonal expansion and differentiation. These cells are usually larger than typical lymphocytes and display less nuclear material and more cytoplasm than small lymphocytes. They possess electron-dense peroxidase-negative granules and a well-developed Golgi apparatus. Mature NK cells do not express somatically rearranged antigen receptors. Some NK cells express FcγRIII (CD16), while others express CD56, an adhesion molecule. NK cells also express the CD2 molecule and the β-chain of the IL-2 receptor (CD122), which allows resting NK cells to respond directly to IL-2.

NK cells distinguish infected, stressed, and transformed cells, and NK cell activation is regulated by a balance between signals that are generated from activating receptors and inhibitory receptors [210]. Most NK cells express inhibitory receptors that recognize MHC class I molecules, which are cell surface proteins normally expressed on virtually all healthy cells in the body, whereas activating receptors on NK cells recognize a heterogeneous group of ligands, some of which may be expressed on normal cells and others of which are expressed mainly on cells that have undergone stress, are infected with microbes, or are transformed [211-213].

NK cells also can kill infected cells that have been coated with antibody molecules. This death delivery mechanism, known as ADCC, occurs via binding of the antibody to the Fcγ receptor CD16. After activation, NK cells produce cytokines, such as IFN-γ, that can influence the proliferation, activation, and differentiation of other cell types [214, 215].

NK cells probably develop extrathymically, and recent data suggest that they can develop from stem cells in lymph nodes. NK cells arise from triple-negative CD3CD4CD8 precursors that are CD56+, but they do not express CD34 or CD5. By comparison, T cells develop from triple-negative precursors that are CD34+CD5+CD56+. It is likely that T cells and NK cells arise from a common triple-negative precursor with the phenotype CD7+CD34+CD5+CD56+. Recent evidence suggests that the cytokine receptor that determines lineage specificity is the α-chain of the IL-2 receptor (CD25). Once CD25 is upregulated, the cell is designated to become a T cell. The cytokines most important in the early development of NK cells are IL-15 and IL-17. Flt ligand and c-kit also facilitate NK cell expansion. Several cytokines, including IL-2, IL-12, IL-15, IL-17, IL-18, and type I interferons, have been shown to promote the growth, differentiation, activation, and survival of mature NK cells [215, 216].

It is interesting that melatonin stimulates the production of IL-2 and IL-12 [71, 72]. Numerous articles, however, have demonstrated that melatonin increases the number of NK cells under a variety of conditions. One in vivo study investigated the effect of exogenously administered melatonin on the hemopoietic and immune cell populations of the bone marrow and spleen in healthy young adult male mice. The finding was that melatonin administration increased the number of NK cells both in bone marrow and in spleen [66]. In addition, studies performed in patients with cancer documented that immunological treatment with IL-2 plus melatonin induced a significant rise in the number of NK cells [13, 217]. Likewise, melatonin administration to both normal and leukemic mice resulted in a quantitative and functional enhancement of NK cells [218]. The elevated NK cell number and function brought about by melatonin was attributed partly to the increased production of cytokines by melatonin-stimulated T-helper cells; these cytokines included IL-2, IL-6, IL-12, and IFN-γ [66, 219].

Aging is a biological process that causes a decline in the immune response. The age-associated deterioration in the immune system, which is referred to as immunosenescence, contributes to an increased susceptibility to infectious diseases, autoimmunity, cancer, and degenerative diseases in the elderly [220, 221]. In immunosenescence, innate, cellular, and humoral immunity all exhibit additional deterioration [220, 221]. In a similar manner, the levels of pineal and circulating melatonin decrease with age [222-224], and this phenomenon obviously coincides with the age-related decline of immune function. This fact and the antioxidant and immuno-enhancing properties of melatonin are the basis for suggesting a potential application of melatonin as a ‘replacement therapy’ to limit or reverse age-related immunosenescence [225, 226]. The functional competence of NK cells also decreases with advancing age [225], and in this condition, melatonin stimulates both the production of NK cells and the release of various cytokines from these cells [225, 227]. However, there are also some contrasting data. Thus, the long-term melatonin administration in old mice had no effect on the NK cell number [228]. In another study, rats with mammary tumors possessed a reduced number of NK cells, and the melatonin administration was ineffective in stimulating an increase in the number of these cells [229]. Likewise, in another investigation, changes in the number of splenic CD57+ and CD68+ cells (NK cells and macrophages, respectively) after melatonin administration to the animals kept on different illumination regimens were examined. In this case, long-term administration of melatonin under conditions of natural illumination had an immunosuppressive effect that was manifested by the depopulation of the marginal zones, white pulp, and all the zones of the red pulp; loss of parenchyma; and denudation of the reticular stroma of the spleen [84]. However, prolonged melatonin administration under conditions of dark exposure in the laboratory reportedly had an immunostimulatory effect, as evidenced by the elevated inflow of immunocompetent cells into the spleen, their migration from the white pulp into the marginal zones, and emigration into peripheral blood, concomitant with the rise in the number of lymphoid nodules. The number of CD57+ and CD68+ cells also was increased in splenic periarterial lymphoid sheaths and reduced in B-dependent zones of the organ [84].

Regarding the effect of melatonin on the cytotoxic activity of NK cells, several reports with different results have been published. Thus, pinealectomized mice showed a reduced NK cell activity, but the administration of melatonin restored NK cell activity [230]. However, chronic treatment with melatonin failed to reverse the impairment of the immune response in the pinealectomized animals [230]. Likewise, it has been reported that melatonin suppressed the activity of human NK cells in vitro [231]. To make matters more confusing, exogenous melatonin administration allegedly enhanced the ADCC in mice [232], while in another study, melatonin enhanced ADCC in summer, but not in winter [233]. More recently, it was found that intraperitoneal injection of melatonin recovered the reduced NK cell activity of traumatized rats [234]. Finally, exogenous melatonin increased the NK cell activity in aged mice [235] and recovered the diminished NK cell activity induced by the immunotoxicity of cadmium [236] and lead [237] in mice. In addition, it has been suggested that the oncostatic actions of melatonin include a direct augmentation of NK cell activity [238].

The presence of melatonin in NK cells has been reported [194], as well as the expression of the melatonin nuclear receptors RZRα, RORα2, membrane receptor MT1, and the HIOMT in human NK cells [53].

Concluding remarks

  1. Top of page
  2. Abstract
  3. Introduction
  4. Monocytes and macrophages
  5. Dendritic cells
  6. Polymorphonuclear granulocytes
  7. Neutrophils
  8. Eosinophils
  9. Basophils and mast cells
  10. Natural killer cells
  11. Concluding remarks
  12. Acknowledgements
  13. Financial disclosure and conflict of interest
  14. References

A large amount of evidence has been accumulated suggesting a direct link between the pineal gland/melatonin and the immune system; this evidence indicates a bidirectional interaction where melatonin influences immune system, while immune signals also affect pineal function (Fig. 1) [27]. Based on the information published about the proposed relationships between melatonin and the immune system, it is clear that the majority of the published data specifically involve lymphocytes. A number of reviews have been published which relate to the mechanisms of action and the effects of melatonin on lymphocytes [17, 18, 44, 56]. However, there are few studies concerning the effects of melatonin on cells belonging to the innate immune response. Innate immunity (also called natural or native immunity) provides the early line of defense against microbes and consists of both cellular and biochemical mechanisms [57]. In this review, we have focused on the role of melatonin in the innate immunity. More specifically, we have summarized the effects and actions of melatonin on the different cells that belong to or participate in the innate immune response, including monocytes, macrophages, dendritic cells, neutrophils, eosinophils, basophils, mast cells, and NK cells (Fig. 2). Currently, the role of melatonin in the regulation of immune function has generated significant interest because of the potential clinical applications, and it is increasingly clear that melatonin is involved in infectious diseases [239-242], some types of cancer [13, 218, 238, 240], immunosenescence [225-227], autoimmunity [129, 243-245], and inflammation [246-250]. It is hoped that these and new areas of research, such as autoinflammatory diseases [251, 252], will be exploited in the near future, and a better understanding of the immunomodulatory role of melatonin of both specific and innate immune responses will be clarified.

image

Figure 2. Summary of the actions of melatonin on cells involved in the innate immune response.

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Financial disclosure and conflict of interest

  1. Top of page
  2. Abstract
  3. Introduction
  4. Monocytes and macrophages
  5. Dendritic cells
  6. Polymorphonuclear granulocytes
  7. Neutrophils
  8. Eosinophils
  9. Basophils and mast cells
  10. Natural killer cells
  11. Concluding remarks
  12. Acknowledgements
  13. Financial disclosure and conflict of interest
  14. References

The authors do not have a financial relationship with any commercial entity that has an interest in the subject of this manuscript.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Monocytes and macrophages
  5. Dendritic cells
  6. Polymorphonuclear granulocytes
  7. Neutrophils
  8. Eosinophils
  9. Basophils and mast cells
  10. Natural killer cells
  11. Concluding remarks
  12. Acknowledgements
  13. Financial disclosure and conflict of interest
  14. References