Background/Purpose . Myositis-specific auto-antibodies (Ab) include those directed against aminoacyl-tRNA synthetases (ARS), signal recognition particle (SRP) and nuclear helicase Mi-2. Anti-SRP Ab are among the most abundant and best characterized myositis-specific Ab. Patients diagnosed with myositis associated with anti-SRP Ab are mainly associated with aggressive disease and poor prognosis and are refractory to glucocorticoids / To determine the clinical features, serological features and long-term prognosis among patients with anti-SRP Ab.

Methods . We retrospectively analyzed 8 patients with positive anti-SRP Ab, detected between 2000 and 2013 in our hospital (Tertiary, referral: 850,000 inhabitants). Medical records of patients with polymyositis (PM) or dermatomyositis according to the Bohan and Peter criteria, and patients with anti-cytoplasmic Ab, were reviewed regardless of diagnosis. Past and current status were assessed from hospital records. A post-questionnaire, personal interview and electromyography (EMG) test were performed in patients without myositis. Anti-nuclear Ab and anti-tissue Ab (AMA, LC, LKM) were tested at diagnosis by indirect immunofluorescence on Hep-2000 cells (Immunoconcepts®) and in-house tissue, respectively. Myositis-specific Ab: anti-ARS (Jo-1, PL7, PL12) and anti-SRP were tested by immunoblot (Orgentec®). In addition, levels of creatine kinase (CK) were measured by turbidimetry (Roche®)

Results . Patients with anti-SRP Ab were 6 Caucasians and 2 north-African women, mean age 63 years; three (37.5%) had PM (1 associated with systemic sclerosis). At diagnosis, muscle weakness was severe in 2 patients (defined as < 3 of manual muscle strength testing), associated with dysphagia and respiratory muscle involvement. Season of onset of muscle weakness: 2 in spring and 1 in winter. All 3 patients were treated with glucocorticoids; 2 required immunosuppressive agents (methotrexate, azathioprine, rituximab or intravenous immunoglobulin). Histological study of biceps brachii showed histological changes consistent with necrotising myopathy with scarce inflammatory cells. Five (62.5%) patients showed no features of myositis after a follow-up of 6 months to 3.5 years; 4 had a normal EMG and normal CK levels. Their diagnoses were: rheumatoid arthritis, Sjögren´s syndrome, autoimmune hepatitis, and primary biliary cirrhosis (pure and associated with systemic sclerosis). There were no deaths or history of malignancy among the 8 anti-SRP patients. 

Race/Age (years)	Diagnosis	Treatment	EMG	CK U/L                       (maximum)	Other Ab
North african/36	Autoimmune hepatitis	Azathioprine	N/A	40	Ro52, La
White/65	Primary biliary cirrhosis, sarcoidosis	Ursobilane	Normal	42	M2
White/69	Primary biliary cirrhosis, systemic sclerosis	Ursobilane	Normal	72	M2, Cep B
White/83	Rheumatoid arthritis	Azathioprine, prednisone	Normal	37	Anti-CCP antibodies , ANA Homogenous Pattern 1/640
White/74	Sjogren´s Syndrome	No	Normal	41	Ro 52, Ro 60, La
North african/46	PM	Methotrexate, azathioprine, prednisone, Ig, Rituximab	polymyositis	3000	Ro 52, La
White/80	PM	Methotrexate, prednisone	polymyositis	2339	No
White/58	PM, systemic sclerosis	Prednisone	polymyositis	1100	Ro52, SP100

N/A  not available.

Conclusion . Although anti-SRP remains a specific Ab for PM, it is occasionally detected in patients with other rheumatic diseases and autoimmune hepatitis in the absence of myositis.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/are-anti-srp-auto-antibodies-specific-for-myositis/