Leptin is an adipose-derived endocrine with a cardinal function in ordinance of energy homeostasis and metamorphosis, including appetency and famishment. It besides plays an of import function in the ordinance of immune responses and perchance in the autoimmune diseases. Leptin has proinflammatory effects on T cell populations, switching the T assistant balance towards a Th1 phenotype, by bring oning release of proinflammatory cytokines, and by exciting macrophage and natural slayer cell map, the phenomena that happens besides in multiple induration ( MS ) . In add-on, leptin or leptin receptor-deficient mice are less affected by or are protected against the development of assortment of autoimmune diseases including experimental autoimmune encephalomyelitis ( carnal theoretical account of MS ) . Given the immunoregulatory belongingss of leptin it would be interesting to measure whether antagonizing its activity could hold curative benefits for patients with MS. The nexus between leptin and MS may therefore offer a new curative scheme for MS. This reappraisal will discourse recent findings about the function of leptin in oncoming, development, clinical manifestations and result of MS and applications of a possible leptin adversary for MS patient and schemes for planing such an adversary.
In 1994, leptin was discovered by Friedman and co-workers as the merchandise encoded by the ob cistron through the survey of corpulent mice [ 8 ] . The ob/ob or corpulent mouse is a mutant mouse enduring from a complex syndrome chiefly characterised by inordinate feeding, which consequences in deeply corpulent mice. Leptin is a protein playing as both endocrine and cytokine consisting of 167 aminic acids and is an I±-helical-bundle cytokine [ 9 ] . The construction of leptin is extremely similar to other members of this big cytokine household including growing endocrine, interleukins such as interleukin-6 ( IL-6 ) , IL-11, IL-12, granulocytes settlement exciting factor ( G-CSF ) and leukemia inhibitory factor ( LIF ) [ 10, 11 ] . Leptin is preponderantly produced by adipocytes and its go arounding degree positively correlates with white adipose tissue mass. Administration of leptin to ob/ob mice additions basal metamorphosis and reduces nutrient consumption, taking to a pronounced rapid weight loss [ 12-14 ] .
Leptin interacts with leptin receptor, besides known as Ob-R which is encoded by the dubnium cistron in human and has a individual transmembrane-spanning sphere. Ob-R has besides been designated as CD295 ( bunch of distinction 295 ) [ 15 ] and belongs to the category I cytokine receptor superfamily [ 16 ] . Six isoforms of leptin receptor has been identified ( Ob-Ra, B, degree Celsius, vitamin D, vitamin E and degree Fahrenheit ) : one long ( Ob-Rb ) , four short ( Ob-Ra, degree Celsius, vitamin D and degree Fahrenheit ) , and one secreted ( Ob-Re ) [ 17, 18 ] , which are merchandises of alternate messenger RNA splice, and differ in the length of their intracellular dress suits but portion indistinguishable extracellular-binding spheres. [ 19 ] . Leptin binds to the ventromedial karyon of the hypothalamus, known as the “ appetite Centre ” [ 20 ] . Ob-Rb is present in a figure of hypothalamic karyon. The long isoform Ob-Rb has a long intracellular sphere in human and is responsible for most of the known effects of leptin through its complete intracellular tail, at which the signalling of four different tracts affecting JAK-STAT, MAPK, PI3K and AMPK can happen [ 18 ] . Ob-Rb is besides expressed by endothelial cells, CD34+ hematopoietic bone marrow precursors, monocytes/macrophages, T and B cells [ 11, 21-27 ] . db/db mice have a omission in the long isoform of the leptin receptor and therefore resistant to leptin [ 28 ] .
The short signifier ( Ob-Ra ) is much more widely expressed, frequently at higher degrees compared to long signifier, and is expressed in different variety meats such as in the choroid rete, kidney, cells of the immune system, lung and liver [ 11 ] . The short isoforms is believed to hold some signalling capablenesss and besides might be involved in leptin conveyance through the blood-brain barrier and possibly in other unknown maps [ 29 ] .
The cytokine construction of leptin and recent grounds has indicated that it has a pleiotropic nature [ 30 ] . Probably the chief function of leptin is to modulate organic structure weight through the suppression of nutrient consumption and to increase energy ingestion by increased thermogenesis. Leptin appears to be portion of the complex web that coordinates immune responses to assorted stimulations. It balances the organic structure ‘s energy position and therefore adjusts the immune response to an appropriate degree. Immune response is an energy-demanding procedure, and damage of this procedure during famishment may salvage energy necessary for critical organic structure maps. Such interaction between energy homeostasis and the immune system appears to be bi-directional [ 31 ] .
a. Leptin and the immune system
The pleiotropic function for leptin in mammalian physiology is clearly shown by the complex syndrome exhibited by leptin-deficient ob/ob mice and leptin receptor-deficient db/db mice. Those mice are non merely corpulent, but besides have abnormalcies in generative map, endocrine degrees, lesion fix, bone construction, and immune map [ 24, 32-36 ] . In add-on, the ob/ob and db/db mice suffer from thymic wasting and hold reduced Numberss of go arounding lymph cells [ 37-39 ] . Impaired T cell unsusceptibility in these mice indicates towards a direct consequence of leptin on T lymph cells, which might be due to the fact that CD4+ and CD8+ T cells express functional leptin receptor [ 40, 41 ] . Leptin concentrations lowered by famishment appear to correlate with impaired immune responses in mice [ 42 ] . Since disposal of leptin in ob/ob but non db/db mice normalised the immune disfunctions, a direct function for leptin on immune system has been suggested [ 33, 43 ] .
Several writers have reviewed the recent findings about leptin and its relationship with immune system and autoimmune diseases [ 44-54 ] . The effects of leptin on adaptative immune responses have been more extensively investigated compared to innate immunity.In vitro surveies have shown that leptin enhances proliferation of go arounding blood T lymphocytes in a dose-dependent mode [ 40, 41 ] . Addition of physiological concentrations of leptin to a Mixed Lymphocytes Reaction ( MLR ) induces a dose-dependent addition the proliferation of CD4+ T cell [ 25 ] . Sing that inborn lack of leptin increases the frequence of infection and related mortality [ 66 ] , it was hypothesized that a low concentration of serum leptin might lend to increased susceptibleness to infection by cut downing T assistant cell priming and by impacting thymic map [ 25, 33 ] . Leptin appears to impact the T assistant ( Th ) subsets, switching the balance towards the T assistant one ( Th1 ) subtype by exciting production of the Th1 proinflammatory cytokines such as, IL-2, interferon gamma ( IFN-I? ) , Tumour Necrosis Factor alpha ( TNF-I± ) , and IL-18, and decreases production of the Th2 cytokines: IL-4, IL-5 and IL-10 [ 40, 41 ] . These effects are non seen on T lymph cells of db/db mice, back uping the construct that this consequence is straight mediated through the leptin receptor, expressed on the T lymph cells.
Leptin besides exerts some effects on the immune cells. Peritoneal macrophages from ob/ob mice display a lower phagocytic activity, compared to macrophages from normal mice, and when leptin was administred, the phagocytic activity was restored [ 55 ] . Furthermore, the production of granulocyte macrophage settlement exciting factor ( GM-CSF ) , G-CSF [ 56 ] and the proinflammatory cytokines such as, TNF-I± , IL-6 and IL-12 [ 55 ] , by murine macrophages is enhanced after intervention with leptin. It was besides shown that leptin induces production of TNF-I± , IL-6 and IFN-I? from resting human peripheral blood mononuclear cells ( PBMCs ) and enhances release of these cytokines from stimulated PBMCs [ 57 ] . In human neutrophils, leptin seems to intercede its effects through an indirect mechanism, likely affecting the release of TNF-I± from monocytes [ 58 ] . This protein acts as a chemo-attractant for lymph cells and monocytes [ 59 ] , enrolling activated T cells to the site of redness [ 59, 60 ] . Furthermore, in ob/ob mice, the figure of intraepithelial lymph cells is reduced and these cells show a reduced IFN-I? secernment, while the lamina propria mononucleate cells of these mice show increased programmed cell death [ 62 ] .
Leptin besides seems to be a regulator of natural slayer ( NK ) cells development and activation. The db/db mice show decreased Numberss of NK cells in the liver, lien, lung and peripheral blood, and in normal mice leptin disposal increases the basal or induced lysis of splenocytes, but non in db/db mice [ 61 ] .
b. Leptin and Autoimmunity
Leptin, as mentioned before, plays an of import function in CD4+ T cell mediated immune responses, advancing a proinflammatory Th1 response. The Th1 advancing effects of leptin have been linked to an enhanced susceptibleness to develop by experimentation induced autoimmune disease including Experimental Autoimmune Encephalomyelitis ( EAE ) , type 1 diabetes melitus ( T1D ) , and Antigen-Induced Arthritis ( AIA ) [ 67 ] . Roll uping groundss suggest that leptin besides plays a polar function in the development of CD4+ T cell mediated autoimmune diseases including Crohn ‘s disease, Rheumatoid Arthritis ( RA ) , MS and type I diabetes melitus. ob/ob mice are immune to the initiation of several experimental theoretical accounts of inflammatory and autoimmune diseases, such as experimental arthritis [ 63 ] , T cell-mediated hepatitis [ 64 ] and ague and chronic enteric redness [ 65 ] .
In experimental mouse theoretical account systems for human inflammatory intestine disease ( Crohn ‘s disease with ague and chronic inflammatory bowel disease ) , leptin-deficient ob/ob mice showed a 72 % decrease of inflammatory bowel disease badness and a pronounced lessening of proinflammatory cytokines ( IFN-I? , TNF-I± , IL-1I? , IL-18 and IL-6 ) in colon cell civilization supernatants, compared to wild type mice. Administration of leptin in ob/ob mice extinguish this opposition against by experimentation induced inflammatory bowel disease [ 62 ] . In this theoretical account, Clostridium difficile toxin A caused terrible inflammatory bowel disease but ob/ob every bit good as db/db mice were partly protected against the toxin A induced enteric secernment and redness. In this instance besides leptin disposal in ob/ob, but non in db/db mice reversed this consequence [ 65 ] .
Chronic idiopathic thrombocytopenic peliosis ( ITP ) is an organ-specific autoimmune disease characterized by the production of antibodies against antigens on the membranes of thrombocytes, ensuing in enhanced devastation of the thrombocytes by macrophages. Leptin enhances in vitro secernment of IgG anti-platelet antibodies by splenocytes and PBMCs from patients with chronic ITP. After depletion of CD4+ T cells, leptin lost this map. Further surveies showed that leptin could increase platelet reactive T cells. These findings suggest that leptin may be involved in the pathogenesis of chronic ITP and might offer a possible mark for the intervention of this disease [ 68 ] .
There are informations proposing a function for leptin in the development of Rheumatoid Arthritis ( RA ) . Injection of methylated bovine serum albumen ( BSA ) in the articulatio genuss of mice consequences in the development of antigen-induced arthritis. Whereas, ob/ob and db/db mice develop less terrible arthritis as compared to wild type mice, with decreased IL-1I? and TNF-I± in the articulatio genus synovial fluid and decreased serum degrees of anti-methylated BSA antibodies. Furthermore, a reduced antigen-specific T cell proliferative response, with a lower IFN-I? and a higher IL-10 secernment, typical for a displacement towards an anti-inflammatory Th2 phenotype is besides reported [ 63 ] . Reducing leptin degrees in RA patients by fasting better the clinical marks of the disease [ 69 ] .
In the Non-Obese Diabetic ( NOD ) mouse, an carnal theoretical account for type 1 diabetes ( an autoimmune disease, in which the pancreatic I?-cells are destroyed by inflammatory procedures ) , an increased serum degree of leptin precedes the diabetes in susceptible females, while injection of leptin accelerates the autoimmune devastation of the pancreatic I?-cells and increases the IFN-I? production in peripheral T cells. These effects indicate that leptin promotes the development of type 1 diabetes through Th1 responses [ 70 ] . It has been found that natural leptin receptor mutations of the NOD/LtJ strain of mice ( named NOD/LtJ-db5J ) show reduced susceptibleness to T1D [ 71 ] .
Womans are more prone to get autoimmune diseases [ 73 ] , this sexual preponderance could be justified by higher norm leptin concentrations in adult females [ 53 ] .
c. Leptin and multiple induration
MS is an autoimmune neurologic upsets which affects immature grownups and, although its cause is non to the full understood, it appears to affect a complex interplay of familial, environmental and immunologic factors [ 2 ] . Disease get downing normally occurs in immature grownups, is more common in adult females [ 3 ] . Multiple induration is an illustration of an autoimmune disease that many cytokines and chemokines can act upon its patterned advance and badness.
It has long been known that myelin-reactive Th1 CD41 cells can bring on MS, and Th1 cytokines are elevated in the CNS inflammatory lesions of EAE. In contrast, Th2 cytokines typically associate with recovery from EAE and protection from the disease [ 74 ] . As was mentioned before, leptin is known to advance immune response toward Th1. One of the most convincing groundss showing the important function of lepin for initiation of EAE has been presented by Matarese et al [ 75 ] . They have clearly shown that leptin is required for development of EAE, and therefore, perchance besides for multiple induration. Genetically leptin deficient mice ( ob/ob mice ) are immune to initiation of both active and adoptively transferred EAE. This protection is reversed by leptin disposal and associates with a switch from Th2 to Th1 type responses and IgG1 to IgG2a isotype switch. Similarly, in susceptible wild-type C57BL/6J mice, leptin worsens EAE disease by increasing IFN-I? release and IgG2a production. Rush of serum leptin anticipates the oncoming of clinical manifestations of EAE. [ 76 ] . Leptin cistron written text is induced attendant with the polarisation toward Th1 responses which are frequently involved in T-cell-mediated autoimmune diseases including MS and in situ secernment of leptin near inflammatory T cells and macrophages has been observed in active EAE lesions [ 77 ] .
There have been a figure of surveies look intoing the relationship between leptin and multiple induration in patients. Leptin is raised high 6.5-fold in acute/active MS versus chronic silent MS [ 7 ] . In acute stages of MS, leptin secernment and CSF production of IFN-I? are increased. In this status, increased leptin secernment is present both in the serum and in the CSF of patients with MS and does non match with organic structure mass index ( BMI ) [ 48, 78 ] . The addition of leptin in the CSF is higher than in the serum, proposing possible secondary in situ synthesis of leptin in the CNS and/or an increased conveyance across the blood-brain-barrier following enhanced systemic production [ 79 ] . Reports have shown increased secernment of serum leptin before backslidings in patients with MS during intervention with IFN-I? , and a capacity of leptin to heighten in vitro secernment of TNF-I± , IL-6, and IL-10 from peripheral blood mononucleate cells of patients with MS in acute stage of the disease but non in patients with stable disease [ 80 ] . It has been reported that the secernment of leptin is increased in both serum and cerebrospinal fluid ( CSF ) of naive-to-treatment patients with MS, an facet that positively correlates with the secernment of IFN-I? in the CSF and reciprocally correlatives with the per centum of go arounding Regulatory T cells ( or Treg cells, a subset of lymph cells once known as suppresser T cells, are important for the care of immunological tolerance ) is reduced in patients with MS as compared with healthy matched controls. Importantly, the figure of peripheral Treg cells in patients with MS oppositely correlates with the serum degrees of leptin, proposing a nexus between the figure of Treg cells and leptin secernment [ 78 ] . Autoreactive human medulla basic protein ( hMBP ) -specific T cells from patients with MS produced leptin and upregulated the look of leptin receptor after activation [ 48, 77, 78 ] . Up-regulation of Ob-R in mononucleate cells from relapsing-remitting multiple induration ( RRMS ) patients in backsliding, but non in remittal and controls have been observed. This determination suggests that Ob-R could play a function in the pathogenesis of MS by up-regulating the immune response in the acute stage of the disease [ 87 ] .
d. Leptin hostility
The above presented grounds suggesta an engagement of leptin in CNS redness in the EAE theoretical account and MS. Therefore leptin hostility could offer a new intervention option for MS patients.
It has been shown that barricading of leptin with anti-leptin antibodies or with a soluble mouse leptin receptor Chimera either before or after onset of EAE, improved clinical mark, slowed disease patterned advance, reduced disease backslidings, inhibited proteolipid protein 139-151 ( PLP139-151 ) myelin peptide-specific T cell proliferation, and switched cytokine secernment toward a Th2/regulatory profile [ 81 ] . CD4+ T cells from mice treated with leptin adversaries showed hypo-responsiveness to PLP139-151 peptide, which was indicated by accretion of cyclin-dependent kinase inhibitor p27 ( p27Kip-1 ) . Hyporesponsive province induced by leptin hostility was associated with pronounced addition of extracellular signal-regulated kinase 1/2 ( ERK1/2 ) phosphorylation, corroborating engagement of ERK1/2 in the betterment of EAE [ 82 ] .
Both anti-leptin and anti-leptin receptor barricading antibodies reduced the proliferative responses of the hMBP-specific T cell lines to antigen stimulation, underlying a possibility of leptin-based intercession on this autocrine cringle to barricade autoreactivity [ 78 ] .
Leptin neutralisation with leptin adversaries could better clinical oncoming, patterned advance, and clinical backslidings of both actively induced and passively transferred EAE. This consequence was associated with pronounced suppression of delayed-type hypersensitivity ( DTH ) reaction against PLP139-151 peptide, CD4+ T cell hyporesponsiveness, and increased IL-4 and IL-10 production against medulla antigens. Foxp3 which is a selective marker for Treg cells, a cellular subpopulation known to be involved in the control of immune tolerance, was besides expressed more on CD4+ T cells in leptin-neutralized mice, proposing the initiation of a regulative phenotype. At the biochemical degree, T cell hyporesponsiveness might be explained by the failure to downmodulate the anergy factor p27Kip-1 and by the addition in the tyrosine phosphorylation degrees of ERK1/2 and STAT6. These happening provide a model for leptin-based intercession in EAE and place molecules with possible curative potency for the disease [ 81 ] . It has been besides shown that leptin neutralisation improves the EAE class by deeply changing intracellular signalling of myelin-reactive T cells and increasing the figure of regulative T cells.
A critical point about leptin is its pleiotropic nature as discussed earlier and any effort to barricade the leptin signalling in vivo should be carefully planned as it may do unwanted effects. The concern in the development of leptin-based curative schemes for autoimmune diseases is that complete leptin/leptin receptor obstruction besides interferes with leptins hypothalamic organic structure weight modulating function. Indeed, intervention of mice with the S120A/T121A leptin mutation which act as leptin adversary induces important weight addition [ 84 ] . The weight addition of S120A/T121A treated mice implies that the mutant works centrally, and therefore is actively transported over the blood-brain-barrier. The BBB can be exploited to divide the peripheral ( i.e. on immune cells ) and cardinal ( i.e. on hypothalamic nerve cells ) activities of leptin.
There are different attacks for planing adversaries. Barricading common of import signal tracts, such as JAK-STAT, may consequences in damaging effects. So far, there is no sanctioned commercially available leptin adversary that can be used for clinical surveies. The recent development ofleptin mutations with counter belongingss and other proteins that block leptin activity opens up new possibilities for their usage in research and, finally, therapy [ 85 ] . A monoclonal antibody against human leptin receptor with counter consequence have been antecedently described [ 86 ] . This antibody inhibits the proinflammatory activity of leptin by its ability to barricade peripheral immune actions of leptin and leptin-induced initiation of TNF-I± by human monocytes, and T cell proliferation. The DNA sequence of this antibody is cloned and different parts ( Fab and ScFv ) are produced with the same barricading consequence as whole antibody. The greatest advantage of recombinant antibody ( rAb ) engineering is that rAb can be manipulated genetically ( eg ; humanized conjugated with other molecules, etc ) and more significantly bring forthing bispecific molecules which bind at the same time to at least two different molecules. Therefore they can barricade a particular molecule ( such as leptin receptor ) on a specific mark tissue.
Adequate nutrition is a requirement for bring forthing appropriate immune responses against pathogens. Adversely, sufficient energy shops may be one of the factors required for long-run, damaging immune reactions, as observed in autoimmune diseases. Therefore, leptin can be considered as a nexus between the immune tolerance, metabolic province, and autoimmunity. Leptin as a cytokine might be responsible for finding the balance between sensitivity to infections and sensitivity to autoimmune diseases, by higher go arounding leptin degrees predisposing to autoimmune diseases, and lower go arounding leptin degrees to infection [ 72 ] . As early leptin research has chiefly been focussed on the consequence of leptin on organic structure weight ordinance, small attending has yet been given to the development of leptin adversaries specifically designed for peripheral effects. Harmonizing to the grounds presented above, leptin receptor adversary could stand for a fresh curative attack for autoimmune diseases, including MS. Identification of a monoclonal antibody against the human leptin receptor which blocks leptin signalling is likely a promising tool for planing a tissue specific leptin adversary [ 86 ] .