Inclusion Body Myositis

Characterizing local antibody responses in the muscle of inclusion body myositis patients

OBJECTIVE

Sporadic inclusion body myositis (IBM) is the most common adult idiopathic inflammatory myopathy. IBM etiology has been elusive, due to both degenerative and autoimmune disease features found on muscle biopsy, and significant disease heterogeneity. Investigating the role of antibodies in the muscle of IBM patients may improve our understanding of disease pathogenesis.

METHODS

We used an IBM xenograft mouse model in which muscle biopsy tissue from IBM patients (n = 98) and controls (n = 131; including 54 from other types of myopathy) were implanted into immunodeficient mice (NOD-Rag1null-IL2rγnull). We quantified the amount of human IgG, IgA, and IgM in xenografted mouse sera using MesoScale Diagnostics (MSD) assay. We detected donor-derived antibody reactivities targeting autoantigens and infectious agents using Phage ImmunoPrecipitation Sequencing (PhIP-Seq). Finally, we used FR3 AmplifiKation Sequencing (FR3AK-Seq) to sequence the antibody mRNAs from a separate cohort of 146 patient biopsies (14 IBM, 22 healthy controls, 110 other myositis subtypes).

RESULTS

With the MSD assay we found human IgG, IgA, and IgM in a larger percentage of IBM xenografted mice versus controls. Using PhIP-Seq, we found anti-microbial reactivities secreted from IBM muscle are prevalent amongst a healthy control population but autoantigen reactivities in IBM are more unique at the peptide and protein level. Additionally, NT5C1A (IgG/IgA and IgM) and TIF1γ (IgG/A) autoantibodies are secreted from muscle tissues of 4/18 and 10/18 IBM xenograft donors, respectively.

CONCLUSION

Our characterization of antibody responses within the muscle of IBM patients reveals that muscle-infiltrating B cells produce both disease-associated autoantibodies and a broad spectrum of antibodies targeting non-self antigens.

Clinical features and diagnostic tools in idiopathic inflammatory myopathies

Idiopathic inflammatory myopathies (IIMs) are rare autoimmune disorders affecting primarily muscles, but other organs can be involved. This review describes the clinical features, diagnosis and treatment for IIMs, namely polymyositis (PM), dermatomyositis (DM), sporadic inclusion body myositis (sIBM), immune-mediated necrotizing myopathy (IMNM), and myositis associated with antisynthetase syndrome (ASS). The diagnostic approach has been updated recently based on the discovery of circulating autoantibodies, which has enhanced the management of patients. Currently, validated classification criteria for IIMs allow clinical studies with well-defined sets of patients but diagnostic criteria to guide the care of individual patients in routine clinical practice are still missing. This review analyzes the clinical manifestations and laboratory findings of IIMs, discusses the efficiency of modern and standard methods employed in their workup, and delineates optimal practice for clinical care. Α multidisciplinary diagnostic approach that combines clinical, neurologic and rheumatologic examination, evaluation of electrophysiologic and morphologic muscle characteristics, and assessment of autoantibody immunoassays has been determined to be the preferred approach for effective management of patients with suspected IIMs.

Biomarker und Histologie bei idiopathischen inflammatorischen Myopathien

Die idiopathischen inflammatorischen Myopathien (IIM) sind eine Gruppe entzündlicher Muskelerkrankungen für deren Diagnosestellung, Verlaufsbeurteilung, Prognoseabschätzung und Risikostratifizierung Biomarker eine jeweils essentielle Rolle spielen. Biomarker in diesem Kontext können sowohl „herkömmliche“ serologische Marker wie Muskelenzyme oder Autoantikörper, histologische Marker wie entitätsspezifische inflammatorische Muster, aber auch genomische und genetische Marker sein. Der vorliegende Artikel gibt einen Überblick über bewährte und innovative Marker.

Sporadic inclusion body myositis: Diagnostic value of p62 immunostaining

BACKGROUND AND OBJECTIVES

Sporadic inclusion body myositis (sIBM) diagnosis is frequently delayed or confused with another class of disorders, and misdiagnosis is common. Sometimes, we have problems diagnosing an sIBM in the early stages or predicting when a PM is going to become an sIBM. In this sense, we believe that p62 immunostaining could help clinicians.

CASE REPORT

We report the case of a 61-year-old patient with sIBM who six years earlier had been diagnosed with polymyositis (PM). After muscle biopsies analyses, we showed the natural history of sIBM by p62 expression.

RESULTS

When we looked for p62 aggregates retrospectively we could see small dotted p62 aggregates in the muscle fibres of the first muscle biopsy. Six years later, the patient presented with the typical clinical picture of sIBM, also the muscle biopsy was characteristic, with large p62 aggregates.

CONCLUSIONS

Probably p62 immunostaining could help to distinguish PM patients that are going to become sIBM, but to date there has been no systematic study to clarify p62 utility in myositis.

Polymyositis with mitochondrial pathology or atypical form of sporadic inclusion body myositis: case series and review of the literature

Polymyositis with mitochondrial pathology (PM-Mito) is a rare form of idiopathic inflammatory myopathy with no definite diagnostic criteria and similarities to both PM and sporadic inclusion body myositis (s-IBM). The aim of this study is to address the dilemma of whether PM-Mito is a subtype of inflammatory myopathy or represents a disease falling into the spectrum of s-IBM. Herein, we report four female patients diagnosed with PM-Mito, highlighting their rather atypical clinical and histopathological characteristics that seem to indicate a diagnosis away from s-IBM. Muscle weakness was rather proximal and symmetrical and lacked the selective pattern observed in s-IBM. Patients had large-scale deletions in mtDNA, reflecting the mitochondrial component in the pathology of the disease. Conclusively, our study adds to the limited data in the literature on whether PM-Mito is a distinct form of myositis or represents a prodromal stage of s-IBM. Although the latter seems to be supported by a substantial body of evidence, there are, however, important differences, such as the different patterns of muscle weakness, and the good response to treatment observed in some patients. Larger-scale studies are certainly needed to clarify pathogenesis and clinical characteristics of PM-Mito patients, especially in therapeutic and prognostic terms.

Distinct interferon signatures stratify inflammatory and dysimmune myopathies

Objective

The role of interferons (IFN) in the pathophysiology of primary inflammatory and dysimmune myopathies (IDM) is increasingly investigated, notably because specific neutralisation approaches may constitute promising therapeutic tracks. In present work we analysed the muscular expression of specific IFNα/β and IFNγ-stimulated genes in patients with various types of IDM.

Methods

39 patients with IDM with inclusion body myositis (IBM, n=9), dermatomyositis (DM, n=10), necrotising autoimmune myopathies (NAM, n=10) and antisynthetase myositis (ASM, n=10), and 10 controls were included. Quantification of expression levels of IFNγ, ISG15, an IFNα/β-inducible gene and of six IFNγ-inducible genes (GBP2, HLA-DOB, HLA-DPB, CIITA, HLA-DRB and HLA-DMB) was performed on muscle biopsy samples.

Results

DM usually associated with strong type I IFNα/β signature, IBM and ASM with prominent type II IFNγ signature and NAM with neither type I nor type II IFN signature. Immunofluorescence study in ASM and IBM showed myofibre expression of major histocompatibility class 2 (MHC-2) and CIITA, confirming the induction of the IFNγ pathway. Furthermore, MHC-2-positive myofibres were observed in close proximity to CD8+ T cells which produce high levels of IFNγ.

Conclusion

Distinct IFN signatures allow a more distinct segregation of IDMs and myofibre MHC-2 expression is a reliable biomarker of type II IFN signature.