Review: Immune-mediated necrotizing myopathies - a heterogeneous group of diseases with specific myopathological features

Immune-mediated necrotizing myopathy, associated with antibodies to signal recognition particle, together with lupus nephritis: case presentation and management

A male patient with limb weakness, myalgia and edema was subsequently found to have an immune-mediated necrotizing myopathy (IMNM) on biopsy. Targeted myopathic antibody analysis revealed antibodies to signal recognition particle (SRP). Anti-SRP-associated necrotizing myopathy was diagnosed. This case was complicated by the concurrent development of class III lupus nephritis. We discuss an interesting case progression and development as well as the management of these difficult to treat conditions.

Necrotizing myopathy as a paraneoplastic syndrome associated with renal cell carcinoma

We report a 49-year-old patient with necrotizing myopathy and a right renal mass. After laparoscopic radical nephrectomy, a remission of myopathy was seen. Pathologic evaluation of the nephrectomy specimen revealed a clear cell renal cell carcinoma. Relapse of myopathy 6 months postoperatively coincided with the diagnosis of the appearance of liver metastatic disease. After initiation of treatment with an mTOR-inhibitor, myopathy became less active requiring smaller amounts of corticosteroids with a complete remission of myopathy after 3 months of systemic treatment for metastatic renal cell cancer.

Cytokines in immune-mediated inflammatory myopathies: cellular sources, multiple actions and therapeutic implications

The idiopathic inflammatory myopathies are a heterogeneous group of disorders characterised by diffuse muscle weakness and inflammation. A common immunopathogenic mechanism is the cytokine-driven infiltration of immune cells into the muscle tissue. Recent studies have further dissected the inflammatory cell types and associated cytokines involved in the immune-mediated myopathies and other chronic inflammatory and autoimmune disorders. In this review we outline the current knowledge of cytokine expression profiles and cellular sources in the major forms of inflammatory myopathy and detail the known mechanistic functions of these cytokines in the context of inflammatory myositis. Furthermore, we discuss how the application of this knowledge may lead to new therapeutic strategies for the treatment of the inflammatory myopathies, in particular for cases resistant to conventional forms of therapy.

Clinical and genetic associations of autoantibodies to 3-hydroxy-3-methyl-glutaryl-coenzyme a reductase in patients with immune-mediated myositis and necrotizing myopathy

INTRODUCTION

Inhibition of 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMGCR) with statins may trigger idiopathic inflammatory myositis (IIM) or immune-mediated necrotizing myopathy (IMNM). Anti-HMGCR antibodies have been detected in patients with IIM/IMNM. We aimed to determine the associations of anti-HMGCR in IIM/IMNM.

METHODS

Anti-HMGCR antibodies were detected by ELISA in sera from patients with IIM/IMNM.

RESULTS

Anti-HMGCR antibodies were detected in 19 of 207 patients with IIM/IMNM, and there was a trend toward an association with male gender (P = 0.079). Anti-HMGCR antibodies were associated strongly with statin exposure (OR = 39, P = 0.0001) and HLA-DRB1*11 (OR = 50, P < 0.0001). The highest risk for development of anti-HMGCR antibodies was among HLA-DR11 carriers exposed to statins. Univariate analysis showed a strong association of anti-HMGCR antibodies with diabetes mellitus (P = 0.008), which was not confirmed by multiple regression. Among anti-HMGCR(+) patients there was a trend toward increased malignancy (P = 0.15).

CONCLUSIONS

Anti-HMGCR antibodies are seen in all subtypes of IIM and IMNM and are associated strongly with statin use and HLA-DR11. Muscle Nerve 52: 196-203, 2015.

Muscle biopsy features of idiopathic inflammatory myopathies and differential diagnosis

The gold standard to characterize idiopathic inflammatory myopathies is the morphological, immunohistochemical and immunopathological analysis of muscle biopsy. Mononuclear cell infiltrates and muscle fiber necrosis are commonly shared histopathological features. Inflammatory cells that surround, invade and destroy healthy muscle fibers expressing MHC class I antigen are the typical pathological finding of polymyositis. Perifascicular atrophy and microangiopathy strongly support a diagnosis of dermatomyositis. Randomly distributed necrotic muscle fibers without mononuclear cell infiltrates represent the histopathological hallmark of immune-mediated necrotizing myopathy; meanwhile, endomysial inflammation and muscle fiber degeneration are the two main pathological features in sporadic inclusion body myositis. A correct differential diagnosis requires immunopathological analysis of the muscle biopsy and has important clinical implications for therapeutic approach. In particular, unnecessary, potentially harmful, immune-suppressive therapy should be avoided alike in dystrophic myopathies with secondary inflammation.

7-Tesla magnetic resonance imaging precisely and noninvasively reflects inflammation and remodeling of the skeletal muscle in a mouse model of antisynthetase syndrome

Inflammatory myopathies comprise heterogeneous disorders. Their etiopathogenesis is poorly understood, because of the paucity of informative experimental models and of approaches for the noninvasive study of inflamed tissues. Magnetic resonance imaging (MRI) provides information about the state of the skeletal muscle that reflects various facets of inflammation and remodeling. This technique has been scarcely used in experimental models of inflammatory myopathies. We characterized the performance of MRI in a well-established mouse model of myositis and the antisynthetase syndrome, based on the immunization of wild-type mice with the amino-terminal fragment of histidyl-tRNA synthetase (HisRS). Over an eight-week period following myositis induction, MRI enabled precise identification of pathological events taking place in muscle tissue. Areas of edema and of active inflammation identified by histopathology paralleled muscle modifications detected noninvasively by MRI. Muscles changes were chronologically associated with the establishment of autoimmunity, as reflected by the development of anti-HisRS antibodies in the blood of immunized mice. MR imaging easily appreciated muscle damage and remodeling even if actual disruption of myofiber integrity (as assessed by serum concentrations of creatinine phosphokinase) was limited. Thus, MR imaging represents an informative and noninvasive analytical tool for studying in vivo immune-mediated muscle involvement.

Diagnosis, pathogenesis and treatment of myositis: recent advances

Dermatomyositis (DM), polymyositis (PM), necrotizing myopathy (NM) and inclusion body myositis (IBM) are four distinct subtypes of idiopathic inflammatory myopathies - in short myositis. Recent studies have shed some light on the unique pathogenesis of each entity. Some of the clinical features are distinct, but muscle biopsy is indispensable for making a reliable diagnosis. The use of magnetic resonance imaging of skeletal muscles and detection of myositis-specific autoantibodies have become useful additions to our diagnostic repertoire. Only few controlled trials are available to substantiate current treatment approaches for myositis and hopes are high that novel modalities will become available within the next few years. In this review we provide an up-to-date overview of the pathogenesis and diagnostic approach of myositis. We aim to present a guide towards therapeutic and general management.

[Necrotizing myopathies: From genetic to acquired forms]

Necrotizing myopathies (NM) are defined by histological features. Muscle biopsy demonstrates marked muscle necrosis with regeneration, with little or an absence of inflammatory infiltrate. Histological pattern of NM is unspecific and can be encountered in diverse conditions as acquired myopathies and muscular dystrophies. Among acquired forms of NM, necrotizing autoimmune myopathy (NAM) is a recently recognized sub-group of the idiopathic inflammatory myopathies. Classically, patients present with a subacute severe proximal myopathy, associated with a markedly elevated creatine kinase level, usually greater than 10 times the upper limit of normal. Nevertheless, the clinical presentation can be misleading, with chronic course mimicking muscular dystrophy. Different forms of NAM can be distinguished with various underlying inciting conditions, including autoantibodies to the SRP, autoantibodies to the HMG-CoA reductase, association to connective tissue disease or underlying malignancy. Other associated conditions need yet to be identified. To confirm a diagnosis of NAM, other causes of NM should be excluded as toxic myopathies, muscular dystrophies and other inflammatory myopathies with a misleading histological pattern. NAM is a rare condition but is probably underdiagnosed. Both clinicoserologic and pathologic data must be taken into account to improve this diagnosis. We propose guidelines for diagnosis of NAM according to clinical course, to be used in clinical practice.

Complement-mediated muscle cell lysis: a possible mechanism of myonecrosis in anti-SRP associated necrotizing myopathy (ASANM)

The mechanism of necrotizing myopathy associated with antibodies to signal recognition particle (SRP) remains unclear. We investigated the effect of anti-SRP+serum and complement on cell viability in myoblast cultures. Cell viability was only slightly reduced by incubation with anti-SRP+serum compared with control serum. However, the addition of fresh complement resulted in a marked reduction in cell survival. Surface immunostaining for SRP, C3c and C5b-9 was demonstrated in cultures pre-incubated with anti-SRP+serum and complement, and in muscle biopsies from patients with myopathy. These findings provide further support for a complement-dependent antibody-mediated mechanism in anti-SRP associated myopathy.

Immune-mediated myopathy related to anti 3-hydroxy-3-methylglutaryl-coenzyme A reductase antibodies as an emerging cause of necrotizing myopathy induced by statins

Immune-mediated necrotizing myopathy (IMNM) associated with statin use and anti 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) antibody is a new and emerging entity that supports a link between statin use and IMNM and raises the questions of distinct clinical phenotypes and treatment strategy. We describe the clinical and histopathological characteristics of a patient and discuss the spectrum of IMNM and statin-induced myopathies. A 65-year-old man was suffering from proximal muscle weakness and elevated CK levels, following exposure to statin therapy. The symptoms worsened despite discontinuation of the drug. At that point, no myositis-specific or -associated antibodies were detected. Malignancy screening did not reveal abnormalities. Muscle biopsy demonstrated a predominantly necrotizing myopathy with minimal lymphocytic infiltrates, MHC class I expression in necrotic muscle fibers, and complement deposition on scattered non-necrotic muscle fibers. Muscle protein analysis by western blot was normal. The patient did not improve with steroid and methotrexate and required monthly intravenous immunoglobulin (IVIG) therapy. Muscle strength gradually improved, CK levels normalized and IVIG were stopped 1 year later. Screening for anti-HMGCR antibodies, not available at the time of presentation, was highly positive. Identification of anti-HMGCR antibodies in statin-exposed patients with myopathy appears to be helpful both for differential diagnosis and for treatment strategy. In patients who did not improve after discontinuation of the statin treatment, a muscle biopsy should be performed as well as screening for anti-HMGCR antibodies. Patients with this disorder require aggressive immunosuppressive treatment.

Statin-induced necrotizing myositis - a discrete autoimmune entity within the "statin-induced myopathy spectrum"

Statin-induced necrotizing myositis is increasingly being recognised as part of the "statin-induced myopathy spectrum". As in other immune-mediated necrotizing myopathies, statin-induced myositis is characterised by proximal muscle weakness with marked serum creatinine kinase elevations and histological evidence of myonecrosis, with little or no inflammatory cell infiltration. Unlike other necrotizing myopathies, statin-induced myopathy is associated with the presence of autoantibodies directed against 3-hydroxy-3-methylglutaryl- coenzyme A reductase (the enzyme target of statin therapies), and with Human Leukocyte Antigen-DRB1*11. This article summarises the clinical presentation, investigations and management of this rare, but serious complication of statin therapy.

T cells in myositis

T cells of both the CD4 and CD8 lineage are commonly found in affected tissues of patients with idiopathic inflammatory myopathies, but understanding the contribution of these cells to immunopathogenesis remains challenging. Given recent advances in identifying more myositis-associated autoantibodies and their putative targets, we suggest that studies on autoreactive T cells targeting those autoantigens are one way forward. Another (so far, more frequently used) approach comes from studies on effector T cells in the context of myositis. This review summarizes recent advances and current hypotheses in both of these contexts.