T cells in myositis

Pretreatment mortality risk prediction model in patients with polymyositis/dermatomyositis-associated interstitial lung disease

OBJECTIVES

Risk prediction for patients with polymyositis/dermatomyositis-associated interstitial lung disease (PM/DM-ILD) is challenging due to heterogeneity in the disease course. We aimed to develop a mortality risk prediction model for PM/DM-ILD.

METHODS

This prognostic study analysed patients with PM/DM-ILD admitted to Nanjing Drum Hospital from 2016 to 2021. The primary outcome was mortality within 1 year. We used a least absolute shrinkage and selection operator (LASSO) logistic regression model to identify predictive laboratory indicators. These indicators were used to create a laboratory risk score, and we developed a mortality risk prediction model by incorporating clinical factors. The evaluation of model performance encompassed discrimination, calibration, clinical utility and practical application for risk prediction and prognosis.

RESULTS

Overall, 418 patients with PM/DM-ILD were enrolled and randomly divided into development (n=282) and validation (n=136) cohorts. LASSO logistic regression identified four optimal features in the development cohort, forming a laboratory risk score: C reactive protein, lactate dehydrogenase, CD3+CD4+ T cell counts and PO2/FiO2. The final prediction model integrated age, arthralgia, anti-melanoma differentiation-associated gene 5 antibody status, high-resolution CT pattern and the laboratory risk score. The prediction model exhibited robust discrimination (area under the receiver operating characteristic: 0.869, 95% CI 0.811 to 0.910), excellent calibration and valuable clinical utility. Patients were categorised into three risk groups with distinct mortality rates. The internal validation, sensitivity analyses and comparative assessments against previous models further confirmed the robustness of the prediction model.

CONCLUSIONS

We developed and validated an evidence-based mortality risk prediction model with simple, readily accessible clinical variables in patients with PM/DM-ILD, which may inform clinical decision-making.

Single-cell profiling of muscle-infiltrating T cells in idiopathic inflammatory myopathies

Idiopathic inflammatory myopathies (IIM) are rare autoimmune systemic diseases characterized by muscle weakness and the presence of muscle-infiltrating T cells. IIM represent a clinical challenge due to heterogeneity of symptoms and variability of response to immunosuppressive treatment. Here, we performed in-depth single-cell sequencing on muscle-infiltrating T cells and peripheral blood memory T cells in six patients with recently diagnosed IIM. We identified tissue resident memory T-cell (TRM ) signatures including the expression of HOBIT, XCL1 and CXCR6 in the muscle biopsies of all patients with IIM. Clonally expanded T-cell clones were mainly found among cytotoxic and TRM implying their role in the disease pathogenesis. Finally, identical expanded T-cell clones persisting at follow-up in the muscle tissue of two patients suggest their involvement in disease chronicity. Our study reveals a muscle tissue resident memory T-cell signature in patients with IIM and a transcriptomic map to identify novel therapeutic targets in IIM.

Immune cell profiles of idiopathic inflammatory myopathy patients expressed anti-aminoacyl tRNA synthetase or anti-melanoma differentiation-associated gene 5 autoantibodies

BACKGROUND

Patients with idiopathic inflammatory myopathy (IIM) often express a different type of myositis-specific autoantibodies (MSAs), each associated with different clinical symptoms. Understanding the immunopathogenesis of various IIM subgroups can help improve the diagnosis and prognosis of IIM patients with different MSAs. However, the immune cell profiles of these IIM patients with anti-aminoacyl tRNA synthetase (ARS) or anti-melanoma differentiation-associated gene 5 (MDA5) autoantibodies remain unclear. We focused on the immune cell profiles of IIM patients with anti-ARS or anti-MDA5 autoantibodies.

RESULTS

The peripheral blood from IIM patients with anti-MDA5 autoantibody (MDA5 + group, n = 24) or one of the anti-ARS autoantibodies (ARS + group, n = 40) autoantibodies, and healthy controls (HC group, n = 60) were collected and examined. We found that IIM patients had a lower CD3 T cell population compared to the HC group. IIM patients showed a significantly lower TN cell population and a higher TEMRA cell population. Higher Th17 and Treg cell populations were found in these IIM patients than in the HC group. In these IIM patients, the MDA5 + group exhibited the higher percentages of Th17 and Treg cells than the ARS + group. It is noteworthy that the percentage of Th1 cells in the survival subgroup was higher than in the death subgroup in IIM patients with ARS + or MDA5 + . Furthermore, in the MDA5 + group, the percentage of Treg cells was higher in the survival subgroup compared to the death subgroup.

CONCLUSIONS

Our study demonstrated that elevated Th1 may be a good prognostic indicator in IIM patients with ARS + or MDA5 + . Elevated Treg may also help predict a good prognosis in MDA5 + IIM patients. However, more large-scale studies and clinical samples are needed to verify the significance of Th1 and Treg cell subsets in clinical outcomes for these IIM patients with ARS + or MDA5 + . These data may help design a therapeutic approach that specifically targets the pathogenic immune molecular responsible for autoimmune attacks in IIM.

Antigen-driven T cell-macrophage interactions mediate the interface between innate and adaptive immunity in histidyl-tRNA synthetase-induced myositis

Introduction

Previous work in humans has demonstrated that both innate and adaptive immune signaling pathways contribute to the pathogenesis of idiopathic inflammatory myopathy (IIM), a systemic autoimmune disease targeting muscle as well as extra-muscular organs. To better define interactive signaling networks in IIM, we characterized the cellular phenotype and transcriptomic profiles of muscle-infiltrating cells in our established murine model of histidyl-tRNA synthetase (HRS)-induced myositis.

Methods

Myositis was induced in wild type (WT) and various congenic/mutant strains of C57BL/6 mice through intramuscular immunization with recombinant HRS. Histopathological, immunohistochemical, flow cytometric, and transcriptomic assessments were used to characterize the functional relationship between muscle-infiltrating cell populations in these strains lacking different components of innate and/or adaptive immune signaling.

Results

RAG1 KO mice developed markedly reduced muscle inflammation relative to WT mice, demonstrating a key requirement for T cells in driving HRS-induced myositis. While the reduction of mononuclear cell infiltrates in CD4-Cre.MyD88fl/fl conditional knockout mice and OT-II TCR transgenic mice highlighted roles for both innate and TCR-mediated/adaptive immune signaling in T cells, diminished inflammation in Lyz2-Cre.MyD88fl/fl conditional knockout mice underscored the importance of macrophage/myeloid cell populations in supporting T cell infiltration. Single cell RNA sequencing-based clustering of muscle-infiltrating subpopulations and associated pathway analyses showed that perturbations of T cell signaling/function alter the distribution and phenotype of macrophages, fibroblasts, and other non-lymphoid cell populations contributing to HRS-induced myositis.

Discussion

Overall, HRS-induced myositis reflects the complex interplay between multiple cell types that collectively drive a TH1-predominant, pro-inflammatory tissue phenotype requiring antigen-mediated activation of both MyD88- and TCR-dependent T cell signaling pathways.

Highly multiplexed imaging mass cytometry identifies similarities between antisynthetase syndrome and dermatomyositis skin lesions

OBJECTIVE

Antisynthetase syndrome (AS) and Dermatomyositis (DM) are autoimmune disorders that overlap clinically. Given the presence of DM skin lesions in AS patients, there is debate about whether AS is distinct or a subclassification of DM. Recently studies identified differences in type I interferon (IFN) between AS and DM muscle and finger eruptions. The aim of this study is to elucidate cutaneous disease pathogenic similarities and differences on a single cell level.

METHODS

Five AS and seven DM patients were recruited from a prospectively collected database of well-characterized DM patients. AS patients were clinically confirmed with anti-synthetase syndrome by the Connors and Solomon et al. criteria and aminoacyl-transfer ribonucleic acid synthetase antibodies. Immunophenotyping conducted using immunofluorescence (IF) and imaging mass cytometry (IMC).

RESULTS

IF revealed type I IFN upregulation in AS and DM compared to HC using MxA and IFNβ expression (p<0.05). IMC showed similar macrophages, T cells, B cells, and dendritic cells in AS and DM with no differences in counts (p>0.05), but an increase in myeloid dendritic cell percentage in DM (p<0.05). Key type I IFN, cytokine, and JAK-STAT pathways were similarly expressed in AS and DM (p>0.05). At a single cell level, pSTING+ macrophages in AS expressed increased TNFα, IL17, and IFNβ (p<0.001).

CONCLUSION

IMC is a powerful tool that identifies a role for the type I IFN system in DM-like skin lesions of AS and DM with some differences at a cellular level, but overall significant overlap exists supporting similar therapeutic decision making.

Cytokeratin 19 fragment is associated with severity and poor prognosis of interstitial lung disease in anti-MDA5 antibody-positive dermatomyositis

OBJECTIVES

In the present study, we aimed to assess the clinical significance of cytokeratin 19 fragment (CYFRA21-1) in patients with anti-melanoma differentiation-associated gene 5 (MDA5) antibody-positive DM-interstitial lung disease (MDA5-DM-ILD).

METHODS

A total of 73 MDA5-DM-ILD patients were retrospectively analysed in this work. Their clinical characteristics, including clinical manifestations, laboratory findings, peripheral blood lymphocyte subsets and lung function, were compared between patients with acute/subacute interstitial pneumonia (A/SIP) and chronic interstitial pneumonia (CIP). The level of serum CYFRA21-1 was also compared between the above-mentioned two groups of patients, and its association with the clinical features and mortality of MDA5-DM-ILD was also evaluated.

RESULTS

Of the 73 MDA5-DM-ILD patients, 26 patients exhibited the A/SIP pattern. The level of serum CYFRA21-1 was higher in MDA5-DM patients with A/SIP compared with the CIP group (P = 0.009). Lower oxygenation index (OI), CD3+CD4+ T cell counts and percentage of CD3+CD4+ cells were also observed in MDA5-DM patients with A/SIP compared with the CIP group. Higher serum CYFRA21-1, lower OI, and lower zone consolidation were associated with a higher risk of A/SIP in MDA5-DM-ILD. In addition, 38 decedents with MDA5-DM-ILD exhibited a greater level of CYFRA21-1 compared with 35 survivors (P < 0.001). Furthermore, it was a prognostic factor and also associated with a higher mortality rate (log-rank test, P < 0.001).

CONCLUSIONS

CYFRA21-1 could be a useful serum indicator associated with occurrence of A/SIP in MDA5-DM-ILD. Moreover, it was associated with a poor survival in MDA5-DM-ILD patients.

Begelomab for severe refractory dermatomyositis: A case report

RATIONALE

Severe refractory idiopathic inflammatory myopathy (IIM) represents a challenge for the clinician. The lack of efficacy of available tools reflects our incomplete insight into the molecular events sustaining the inflammatory tissue damage in these patients. We present the first case of refractory IIM treated with anti-dipeptidyl peptidase-4 (DPP-4)/cluster of differentiation 26 (CD26) monoclonal antibody.

PATIENT CONCERNS

A 55-year old man presented with proximal muscle weakness, diffuse erythematous skin lesions which rapidly evolved into ulcerations, dysphagia and dysphonia.

DIAGNOSIS

Increased serum creatine kinase levels and histological findings at muscle and skin biopsies were compatible with the diagnosis of dermatomyositis (DM). Several lines of treatment failed to control the disease including steroids, mycophenolate mofetil, tacrolimus, intravenous immunoglobulins and rituximab. Despite therapy, the patient also had recurrent intestinal vasculitis causing bowel perforation. Concurrently, DPP-4/CD26 expression in the patient's skin and skeletal muscle was observed.

INTERVENTIONS

The patient was treated with begelomab, a murine immunoglobulin G2b monoclonal antibody against DPP-4/CD26.

OUTCOMES

Dysphagia, skin lesions and intestinal vasculitis resolved and the patient experienced a significant improvement of his quality of life.

CONCLUSION

Blockade of DPP-4/CD26, which is expressed on T cells and mediates T cell activation and function, is safe and might be effective in patients with refractory DM.

The Role of Immune Cells in the Pathogenesis of Idiopathic Inflammatory Myopathies

Idiopathic inflammatory myopathies (IIMs) are chronic autoimmune disorders involving multiple organs, such as the muscle, skin, lungs and joints. Although the detailed pathogenesis of IIMs remains unclear, immune mechanisms have long been recognised as of key importance. Immune cells contribute to many inflammatory processes via intercellular interactions and secretion of inflammatory factors, and many studies have demonstrated the participation of a variety of immune cells, such as T cells and B cells, in the development of IIMs. Here, we summarise the current knowledge regarding immune cells in IIM patients and discuss their potential roles in IIM pathogenesis.

Soluble IL-2 Receptor in Dermatomyositis: Its Associations with Skin Ulcers and Disease Activity

Objective

To investigate the role of soluble interleukin-2R (sIL-2R) in idiopathic inflammatory myopathies (IIM).

Methods

Serum sIL-2R levels were measured in 74 dermatomyositis (DM), 16 immune-mediated necrotizing myopathy (IMNM), 24 rheumatoid arthritis (RA), 20 systemic lupus erythematosus (SLE), and 20 healthy controls (HCs) by chemiluminescent immunometric assay. Clinical features and laboratory data were collected from electronic medical record. Disease activity was evaluated by using physician global disease activity and myositis disease activity assessment visual analog scale (MYOACT) on admission. 20 DM patients were followed. Serum sIL-2R levels were analyzed and compared with clinical features, laboratory data, and measures of disease activity.

Results

Serum sIL-2R levels were significantly higher in DM patients than in IMNM patients and HCs (648.8 ± 433.1 U/ml vs. 352.3 ± 126.0 U/ml and 648.8 ± 433.1 U/ml vs. 285.8 ± 101.9 U/ml, respectively; all P < 0.001), while there was no significant difference between IMNM and HCs. There were also no significant differences of sIL-2R levels in DM, SLE, and RA. Importantly, serum sIL-2R levels were significantly higher in treatment-naïve or active DM patients than those that are not (1100.9 ± 550.4 U/ml vs. 615.6 ± 330.4 U/ml, P = 0.006; 808.8 ± 421.6 U/ml vs. 339.8 ± 103.4 U/ml, P < 0.001). DM patients with skin ulcers had significantly higher sIL-2R levels than those without (889.3 ± 509.9 U/ml vs. 640.0 ± 368.7 U/ml, P = 0.023). Cross-sectional analysis in DM showed that sIL-2R levels positively correlated with CK, ESR, CRP, ferritin, physician VAS, and MYOACT scores (rho = 0.278, rho = 0.474, rho = 0.469, rho = 0.454, r = 0.646, and r = 0.600, respectively; all P < 0.05), negatively correlated with T cell counts and MMT8 scores (r = −0.380, P = 0.002; rho = −0.394, P = 0.001). Follow-up study showed that changes in sIL-2R levels after treatment correlated with changes in physician VAS and MYOACT scores (r = 0.823 and r = 0.695, respectively; all P < 0.01).

Conclusion

Serum sIL-2R levels were elevated in DM but not in IMNM. Serum sIL-2R could act as a disease activity marker and be associated with ulcerative skin lesions in DM.

CD4+CXCR4+ T cells as a novel prognostic biomarker in patients with idiopathic inflammatory myopathy-associated interstitial lung disease

BACKGROUD

There is an unmet need for the development of new biomarkers for idiopathic inflammatory myopathy-associated interstitial lung disease (IIM-ILD).

METHODS

Peripheral CD4+CXCR4+ T cells, stromal cell-derived factor-1 and Krebs von den Lungen-6 were measured in patients with IIM-ILD (n = 85) and controls. The relation to pulmonary functions, high-resolution CT scores, specific clinical phenotypes and survival was analysed. Cytokine-expression profiling of these CD4+CXCR4+ T cells and their co-culture with pulmonary fibroblasts were conducted.

RESULTS

The peripheral percentages of CD4+CXCR4+ T cells were significantly elevated in IIM-ILD patients, and correlated with high-resolution CT score (r = 0.7136, P < 0.0001) and pulmonary function impairments, such as percentage of forced volume vital capacity (r = -0.4734, P = 0.0005). They were associated with anti-melanoma differentiation-associated gene 5 autoantibodies and the amyopathic DM phenotype. In IIM-ILD, peripheral percentages of CD4+CXCR4+ T cells ⩾30% revealed a 6-month mortality as high as 47%. These CD4+CXCR4+ T cells express high levels of IL-21 and IL-6. In vitro blockade of IL-21 signalling by neutralization of IL-21 or Janus kinase inhibitor could abolished the fibroblast proliferation.

CONCLUSION

Overall, peripheral CD4+CXCR4+ T cells appear to be a potentially valuable novel biomarker associated with the severity and prognosis of IIM-ILD. They promote pulmonary fibroblast proliferation via IL-21, which may herald future targeted treatments for this severe disease.

Chemokines in rheumatic diseases: pathogenic role and therapeutic implications

Chemokines, a family of small secreted chemotactic cytokines, and their G protein-coupled seven transmembrane spanning receptors control the migratory patterns, positioning and cellular interactions of immune cells. The levels of chemokines and their receptors are increased in the blood and within inflamed tissue of patients with rheumatic diseases, such as rheumatoid arthritis, systemic lupus erythematosus, systemic sclerosis, vasculitis or idiopathic inflammatory myopathies. Chemokine ligand-receptor interactions control the recruitment of leukocytes into tissue, which are central to the pathogenesis of these rheumatic diseases. Although the blockade of various chemokines and chemokine receptors has yielded promising results in preclinical animal models of rheumatic diseases, human clinical trials have, in general, been disappointing. However, there have been glimmers of hope from several early-phase clinical trials that suggest that sufficiently blocking the relevant chemokine pathway might in fact have clinical benefits in rheumatic diseases. Hence, the chemokine system remains a promising therapeutic target for rheumatic diseases and requires further study.

Estrogen signaling effects on muscle-specific immune responses through controlling the recruitment and function of macrophages and T cells

Background

Estrogen signaling is indispensable for muscle regeneration, yet the role of estrogen in the development of muscle inflammation, especially in the intramuscular T cell response, and the influence on the intrinsic immuno-behaviors of myofibers remain largely unknown. We investigated this issue using the mice model of cardiotoxin (CTX)-induced myoinjury, with or without estrogen level adjustment.

Methods

CTX injection i.m. (tibialis anterior, TA) was performed for preparing mice myoinjury model. Injection s.c. of 17β-estradiol (E₂) or estrogen receptor antagonist 4-OHT, or ovariectomy (OVX), was used to change estrogen level of animal models in vivo. Serum E₂ level was evaluated by ELISA. Gene levels of estrogen receptor (ERs) and cytokines/chemokines in inflamed muscle were monitored by qPCR. Inflammatory infiltration was observed by immunofluorescence. Macrophage and T cell phenotypes were analyzed by FACS. Immunoblotting was used to assess protein levels of ERs and immunomolecules in C₂C₁₂ myotubes treated with E₂ or 4-OHT, in the presence of IFN-γ.

Results

We monitored the increased serum E₂ level and the upregulated ERβ in regenerated myofibres after myotrauma. The absence of estrogen in vivo resulted in the more severe muscle inflammatory infiltration, involving the recruitment of monocyte/macrophage and CD4⁺ T cells, and the heightened proinflammatory (M1) macrophage. Moreover, estrogen signaling loss led to Treg cells infiltration decrease, Th1 response elevation in inflamed muscle, and the markedly expression upregulation of immunomolecules in IFN-γ-stimulated C₂C₁₂ myotubes in vitro.

Conclusion

Our data suggest that estrogen is a positive intervention factor for muscle inflammatory response, through its effects on controlling intramuscular infiltration and phenotypes of monocytes/macrophages, on affecting accumulation and function of Treg cells, and on suppressing Th1 response in inflamed muscle. Our findings also imply an inhibition effect of estrogen on the intrinsic immune behaviors of muscle cells.

Potential role of autophagy in T‑cell survival in polymyositis and dermatomyositis

Peripheral blood T lymphocytopenia has previously been identified in polymyositis/dermatomyositis (PM/DM) patients. Therefore, the present study aimed to examine the potential role of autophagy in peripheral blood T cell survival in PM/DM patients. Transmission electron microscopy was used to detect the formation of autophagosomes of peripheral blood cluster of differentiation (CD)3+ T cells obtained from 24 patients with PM/DM and 21 healthy controls. Protein and mRNA expression levels of autophagy‑related molecules were examined by western blot analysis and reverse transcription‑quantitative polymerase chain reaction, respectively. The number of peripheral blood CD3+ T cells decreased significantly in PM/DM patients. The median percentage of apoptosis of CD3+ T cells in PM/DM patients was significantly increased compared with healthy controls. Furthermore, the number of autophagosomes and the expression of the autophagy markers microtubule‑associated protein 1A/1B‑light chain 3 (LC3) and Beclin‑1 were significantly reduced in the circulating CD3+ T cells of PM/DM patients compared with those of healthy controls. LC3 and Beclin‑1 protein levels correlated negatively with apoptosis rates in circulating CD3+ T cells in patients with PM/DM. CD3+ T cells from PM/DM patients treated with rapamycin increased autophagy and decreased apoptosis compared with untreated cells (P<0.05). In conclusion, these results suggested that autophagy may serve a potential protective role in the peripheral blood T cells of patients with PM/DM.

The NKG2D-IL-15 signaling pathway contributes to T-cell mediated pathology in inflammatory myopathies

NKG2D is an activating receptor on T cells, which has been implicated in the pathogenesis of autoimmune diseases. T cells are critically involved in idiopathic inflammatory myopathies (IIM) and have been proposed as specific therapeutic targets. However, the mechanisms underlying T cell-mediated progressive muscle destruction in IIM remain to be elucidated. We here determined the involvement of the NKG2D – IL-15 signaling pathway. Primary human myoblasts expressed NKG2D ligands, which were further upregulated upon inflammatory stimuli. In parallel, shedding of the soluble NKG2D ligand MICA (sMICA) decreased upon inflammation potentially diminishing inhibition of NKG2D signaling. Membrane-related expression of IL-15 by myoblasts induced differentiation of naïve CD8⁺ T cells into highly activated, cytotoxic CD8⁺NKG2D^high T cells demonstrating NKG2D-dependent lysis of myoblasts in vitro. CD8⁺NKG2D^high T cell frequencies were increased in the peripheral blood of polymyositis (PM) patients and correlated with serum creatinine kinase concentrations, while serum sMICA levels were not significantly changed. In muscle biopsy specimens from PM patients expression of the NKG2D ligand MICA/B was upregulated, IL-15 was expressed by muscle cells, CD68⁺ macrophages as well as CD4⁺ T cells, and CD8⁺NKG2D⁺ cells were frequently detected within inflammatory infiltrates arguing for a local signaling circuit in the inflammatory muscle milieu. In conclusion, the NKG2D – IL-15 signaling pathway contributes to progressive muscle destruction in IIM potentially opening new therapeutic avenues.

[Dermatomyositis-update]

Dermatomyositis is a rare idiopathic inflammatory myopathy that affects adults and children, mostly female. Hallmarks of the disease are myositis with necrosis, regeneration and perifascicular atrophy accompanied by a typical skin rash with heliotrope erythema, Gottron's sign, Gottron's papules and nail fold changes with splinter hemorrhage. Typical skin symptoms may appear 6 months up to 2 years before muscle involvement (amyopathic dermatomyositis). New myositis-specific antibodies may allow clinicoserologic correlations within a heterogeneous clinical spectrum. Autoantibody profiles, subtype of myositis, overlap with other collagen vascular disorders and/or malignancy (paraneoplastic dermatomyositis) as well as age of the patients all have a considerable impact on course and prognosis. Infections, drugs and tumors may trigger activation of T and B cells, plasmacytoid dendritic cells, overproduction of type I interferons and complement-mediated endothelial cell damage resulting in vasculopathy. UV radiation may also trigger dermatomyositis. Oral corticosteroids (1.5-2.0 mg/kg body weight/day) are the mainstay of treatment until improvement of muscle symptoms and/or normalization of muscle enzymes with subsequent slow tapering. Corticosteroids may be given as monotherapy or combined with steroid-sparing immunosuppressive agents' i.e. azathioprine, methotrexate, mycophenolate mofetil or high-dose intravenous immunoglobulins. Prognosis has improved considerably since use of high-dose corticosteroids, from 50 to 90% response rate. New therapies with biologicals (anti-CD20-, anti-TNFalpha-, anti-interferon antibodies) and Janus kinase inhibitors are currently being evaluated.

Skeletal muscle interleukin 15 promotes CD8(+) T-cell function and autoimmune myositis

Background

Interleukin 15 (IL-15) is thought to be abundant in the skeletal muscle under steady state conditions based on RNA expression; however, the IL-15 RNA level may not reflect the protein level due to post-transcriptional regulation. Although exogenous protein treatment and overexpression studies indicated IL-15 functions in the skeletal muscle, how the skeletal muscle cell uses IL-15 remains unclear. In myositis patients, IL-15 protein is up-regulated in the skeletal muscle. Given the supporting role of IL-15 in CD8⁺ T-cell survival and activation and the pathogenic role of cytotoxic CD8⁺ T cells in polymyositis and inclusion-body myositis, we hypothesize that IL-15 produced by the inflamed skeletal muscle promotes myositis via CD8⁺ T cells.

Methods

Expression of IL-15 and IL-15 receptors at the protein level by skeletal muscle cells were examined under steady state and cytokine stimulation conditions. The functions of IL-15 in the skeletal muscle were investigated using Il15 knockout (Il15^−/−) mice. The immune regulatory role of skeletal muscle IL-15 was determined by co-culturing cytokine-stimulated muscle cells and memory-like CD8⁺ T cells in vitro and by inducing autoimmune myositis in skeletal-muscle-specific Il15^−/− mice.

Results

We found that the IL-15 protein was not expressed by skeletal muscle cells under steady state condition but induced by tumor necrosis factor alpha (TNF-α) and interferon gamma (IFN-γ) stimulation and expressed as IL-15/IL-15 receptor alpha (IL-15Rα) complex. Skeletal muscle cells expressed a scanty amount of IL-15 receptor beta (IL-15Rβ) under either conditions and only responded to a high concentration of IL-15 hyperagonist, but not IL-15. Consistently, deficiency of endogenous IL-15 affected neither skeletal muscle growth nor its responses to TNF-α and IFN-γ. On the other hand, the cytokine-stimulated skeletal muscle cells presented antigen and provided IL-15 to promote the effector function of memory-like CD8⁺ T cells. Genetic ablation of Il15 in skeletal muscle cells greatly ameliorated autoimmune myositis in mice.

Conclusions

These findings together indicate that skeletal muscle IL-15 directly regulates immune effector cells but not muscle cells and thus presents a potential therapeutic target for myositis.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-015-0058-2) contains supplementary material, which is available to authorized users.