Pathophysiological Roles Of Interleukin 21 And Its Receptor In Polymyositis And Dermatomyositis

BackgroundIdiopathic inflammatory myopathies(IIMs)are a heterogeneous group of skelet-al muscle diseases,which usually characterized by chronic or subacute progressive muscle weakness and skeletal muscle fiber necrosis with or without inflammatory cell infiltrates.According to distinct clinical and pathological features,IIMs can be classi-fied into the following subtypes:polymyositis(PM),dermatomyositis(DM),sporadic inclusion body myositis and immune mediated necrotizing myopathy etc.These sub-types usually have common clinical features:patients with IIMs usually have proxim-al muscle weakness during the early stage of the disease except the patients with spo-radic inclusion body myositis,distal muscles are affected early in patients with spo-radic inclusion body myositis;the neck extensor or pharyngeal muscle can be affected in all subtypes,which then result in head drop or dysphagia;in advanced cases,respi-ratory muscle can also be affected which can result in dyspnea;however,the ocular muscle can be spared in all the disease subtypes.Moreover,extramuscular manifes-tions may occur in IIMs,such as fever,arthralgia,Raynaud’s phenomenon,etc.,heart involvements usually include cardiac arrhythmia and heart failure,interstitial lung disease usually occurs when the lung is affected.Polymyositis and dermatomyositis are two important subtypes during the clinical course.Dermatomyositis can occur in both children and adults;in these patients,skin rashes can occur accompanying or preceding the muscle weakness,the rashes are photosensitive and can be aggravated by ultraviolet.Dilated capillary loops at the base of patients’ fingernails and the irregular thickened and cracked cuticles in palm and fingertips(mechanic’s hands)are also characteristic manifestations of dermato-myositis.Patients with juvenile dermatomyositis usually have cutaneous or gastroin-testinal ulcer,calcinosis can occur in the subcutaneous tissues and muscles,some-t:imes lipodystrophy can also occur.The incidence of accompanying cancer increases along with the duration of patients with dermatomyositis.Muscle biopsy of patients with dermatomyositis can find the necrosis and regeneration of muscle fibers and in-flammatory cell infiltrates,the inflammatory cells mainly locate in the perivascular and perifascicular area,reduced density of capillaries and dilated capillary lumen can also be found,typical perifascicular atrophy often occurs.Unlike dermatomyositis,polymyositis usually occurs in adults and rarely in children,patients with polymyosi-tis usually have symmetrical and progressive proximal muscle weakness with no skin involvement.Muscle biopsy usually finds that the inflammatory cells surround and invade healthy appearing muscle fibers besides the necrosis and regeneration of mus-cle fibers,the inflammatory cells can locate in the endomysium but rarely in the peri-vascualr area.Major histocompatibility complex(MHC)-I,which does not express under normal condition can widely express on the membrane of the muscle fibers.Up to now,the immunopathogenesis underline polymyositis and dermatomyosi-tis is still not well known.During the occurrence of dermatomyositis,antibodies against vascular endothelial cell bind with antigen and activate complement 3,then C5b9 membrane attack complex forms and deposits on the endomysial capillary,sub-sequently the endothelial cells swell,vacuolize and then induce the necrosis of capil-laries,ischemic damage can occur in the supplied muscle fibers;the density of capil-lary reduce and compensatory dilation of the residual capillaries’ lumen occurs,hy-poperfusion and ischemic necrosis of muscle fibers usually occur in the peripheral of fascicles and induce the formation of perifascicular atrophy.The pathogenic process of polymyositis is mediated by CD8+T lymphocytes;activated CD8+T lymphocytes can surround and invade healthy appearing muscle fibers and then secret granzyme and perforin directly to the surface of muscle fibers to induce its necrosis.In patients with polymyositis,the CD8/MHC-I complex formed by CD8+T lymphocytes invad-ing muscle fibers that aberrantly express MHC-I is its characteristic change,as it does not occur in other myopathies like some muscular dystrophy accompanied with in-flammation.Cytokines are small molecule proteins,they play an important role in the patho-genic process of polymyositis and dermatomyositis after secretion.The source of cy-tokines in myositis muscle tissues is various,the infiltrated inflammatory cells in muscle tissues are the major source of cytokines,secreted cytokines can then regulate the function of inflammatory cells and maintain the inflammatory process;muscle fibres itself can also secret cytokines and participate in the inflammation,moreover,the vascular endothelial cells can secret cytokines to facilitate peripheral activated in-flammatory cells to migrate to the affected muscle tissues.In patients with polymyo-sitis and dermatomyositis,cytokines can activate innate immunity,recruit inflamma-tory cells,activate macrophages,facilitate the differentiation and activation of T cells,and regulate the differentiation and function of B cells.Interleukin(IL)21 was first discovered in the year 2000,which belongs to type 1 cytokine family;type 1 cytokine family also includes IL-2,IL-4,IL-7,IL-9 and IL-15,IL-21 shares similar structure with these cytokines.Among different subtypes of CD4+T lymphocytes,T follicular helper(Tfh)cells and T helper(Th)17 cells are the two major sources of IL-21;moreover,natural killer T(NKT)cells are also the other major source of IL-21.IL-21 can exert its biological effect through binding with its corresponding transmembrane receptor complex that is formed by two subunits,one subunit is an α chain termed as interleukin 21 receptor(IL-21 R),and the other subunit is a common y chain shared with the other type 1 cytokine family.The trans-membrane receptor can be expressed by various immune cells like CD4+T lympho-cytes,CD8+T lymphocytes,B cells,natural killer(NK)cells,NKT cells,macrophag-es and dendritic cells etc.,binding of IL-21 with the receptor can control the expres-sion of target genes through a series of signal pathway,and then regulate the activa-tion and function of various immune cells.IL-21 can induce the differentiation of naive CD4+T cells and promote them to produce cytokines;IL-21 also plays an im-portant role in regulating the differentiation,proliferation and function of Th17 and Tfh cells.IL-21 can potently induce the differentiation of CD8+T cells,maintain their survival and enhance the cytotoxic function.The role of IL-21 plays on B cells is complicated,it can usually induce the formation of plasma cells from B cells,howev-er,under certain conditions,IL-21 can also induce the apoptosis of B cells.Moreover,IL-21 can promote the differentiation,proliferation and maturation of NK cells,en-hance its cytotoxic effect,IL-21 can also induce the proliferation of NKT cells,main-tain the survival of cells and enhance its cytotoxic function.Macrophages also can be activated by IL-21 and phagocytosis can be enhanced.Up to now,many studies have proven the important role of IL-21 in the patho-genic process of various autoimmune diseases like rheumatoid arthritis,sjogren syn-drome,systemic lupus erythematosus,type 1 diabetes,multiple sclerosis,inflamma-tory bowel disease and psoriasis etc.;however,the expression and role of IL-21 in polymyositis and dermatomyositis is still not studied.ObjectiveTo evaluate the expression of IL-21 and IL-21 R in inflamed muscle tissues and serums of patients with dermatomyositis and polymyositis,and then preliminary dis-cuss the role that IL-21 and IL-21 R play in the pathogenic process of these two dis-eases.MethodsBiopsied muscle tissues were obtained from 12 patients with polymyositis,11 pa-tients with dermatomyositis and 6 normal controls,serums were obtained from 19 pa-tients with polymyositis,36 patients with dermatomyositis and 20 normal controls.Real time quantitative reverse transcription polymerase chain reaction was used to detect the expression of IL-21 mRNA and IL-21 R mRNA in biopsied muscle tissues of patients with polymyositis,dermatomyositis and normal controls.Immunohisto-chemistry staining was used to evaluate the expression and distribution of IL-21 and IL-21 R protein.Enzyme linked immunosorbent assay(ELISA)was used to detect the level of IL-21 in the serum of patients with polymyositis,dermatomyositis and normal controls.Association analysis was performed between IL-21 mRNA level in muscle tissues and the clinical characteristic or lab test of the patients,also association be-tween serum interleukin 21 level and the clinical characteristic or lab test of the pa-tients was studied.Results1.The level of IL-21 mRNA in muscle tissues of patients with polymyositis or dermatomyositis was higher than that in controls(PM vs.control,p<0.05,DM vs.control,p<0.05).The level of IL-21 mRNA in muscle tissues between patients with polymyositis and dermatomyositis was not different from each other(P>0.05).The IL-21 R mRNA level in inflamed muscle tissues of patients with dermatomyositis or polymyositis was not significantly upregulated compared with that in normal controls,also there are no difference in the muscle tissue IL-21 R mRNA level between patients with dermatomyositis and polymyositis.In patients with polymyositis,the IL-21 mRNA level in muscle tissue did not associate with either manual muscle test(MMT)score or serum creatine kinase level,also in patients with dermatomyositis,there was no association between the muscle IL-21 mRNA level and MMT score or serum crea-tine kinase level.2.Immunohistochemistry staining revealed that in inflamed muscle tissues of pa-tients with polymyositis,IL-21 was mainly expressed by inflammatory cells that sur-rounding and invading the healthy appearing muscle fibers;while in muscle tissues of patients with dermatomyositis,IL-21 was mainly expressed by inflammatory cells surrounding the vessels.The expression of IL-21 R had similar features with IL-21.Both IL-21 and IL-21 R were not expressed by the muscle fibers.Muscle tissues of normal controls did not express either IL-21 or IL-21 R.3.The ELISA result revealed that the serum IL-21 level was also significantly higher in patients with polymyositis or dermatomyositis than in controls,serum IL-21 level is 50.77(44.19,60.62)pg/ml in patients with polymyositis,49.12(45.28,60.07)pg/ml in patients with dermatomyositis and 42.54(38,69,48.85)pg/ml in controls(PM vs.control,P<0.05,DM vs.control,P<0.05).There was no difference between these two disease groups.Serum IL-21 level did not associate with MMT score or se-rum creatine kinase level or other clinical characteristics either in patients with poly-myositis or dermatomyositis.ConclusionExpression of IL-21 is upregulated in both muscle tissues and serums of patients with polymyositis and dermatomyositis,IL-21R protein is also highly expressed in affected muscle tissues of patients.IL-21 may act on inflammatory cells through binding with its receptor and then participate in the pathogenic process of polymyosi-tis and dermatomyositis.However,this mechanism may be particularly important during the initial disease phase in patients with polymyositis and dermatomyositis;as to the muscle fiber degeneration and necrosis,the effect of IL-21 is not obvious.