A case of anti‐OJ antibody‐positive polymyositis with marked muscle involvement and interstitial lung disease

An 83‐year‐old man with muscle involvement and interstitial lung disease (ILD) was diagnosed with anti‐OJ antibody (Ab)‐positive polymyositis (PM). He did not have Raynaud's phenomenon, heliotrope rash, Gottron's sign, fever, arthralgia, or mechanic's hand. A literature review of 21 anti‐OJ Ab‐positive patients described that the diagnoses were as follows: 10 with PM, five with dermatomyositis, five with ILD, and 1 with overlap syndrome of PM and rheumatoid arthritis. Myositis was observed in 16, ILD in 19, and skin rash in 5. Physicians need to be aware of the existence of patients with anti‐OJ Ab who have myositis and ILD.


| INTRODUC TI ON
anti-ARS Abs, Targoff proposed a disease entity termed "anti-synthetase syndrome," which is characterized by myositis, interstitial lung disease (ILD), Raynaud's phenomenon, fever, arthritis, and mechanic's hand. 2 Although anti-synthetase syndrome has common clinical manifestations, further observations have distinguished differences in clinical features associated with individual anti-ARS Abs. 3 Therefore, the accumulation of single cases with anti-ARS Abs is clinically important to understand the entire picture of anti-synthetase syndrome. Anti-OJ antibody Ab is an anti-ARS Abs that reacts with isoleucyl-tRNA. 4 Clinical features of anti-OJ Ab currently remain unclear due to the small number of reported cases because immunoprecipitation assay is required to identify it. 5 We report the case of an anti-OJ Ab-positive PM patient with marked muscle involvement and ILD who was successfully treated using oral prednisolone and high-dose intravenous immunoglobulin therapy.

| C A S E REP ORT
An 83-year-old Japanese man was referred to our hospital with a 3 month history of muscle pain and weakness in his trunk and extremities. When he visited our hospital, he had difficulties walking and raising the upper extremities. He did not exhibit coughing or dyspnea on exertion. He also did not have Raynaud's phenomenon, fever, heliotrope rash, Gottron's sign, arthralgia, or mechanic's hand.
Capillaroscopy revealed reduced capillaries, irregularly enlarged capillaries, hemorrhage, and capillary ramifications ( Figure 1A).  Figure 1D). Malignancy was not detected in a detailed examination. Therefore, he was diagnosed with anti-OJ Ab-positive PM with ILD.
He was hospitalized and treated using oral prednisolone (PSL) at 50 mg/day (1 mg/kg/day), followed by high-dose intravenous immunoglobulin therapy because intensive immunosuppressive therapy was not recommended due to his age. Muscle weakness rapidly recovered and muscle enzymes decreased. Ground-glass opacities on chest CT also improved. PSL was tapered, and he was discharged from the hospital while receiving PSL at 25 mg/day.

| DISCUSS ION
Although anti-synthetase syndrome has common clinical manifestations, identification of each anti-ARS Ab is clinically important to predict clinical features, severity, and prognosis. 6  NVC findings and disease activity in patients with DM was reported. 13 Although clinical importance of NVC findings in patients with anti-ARS Abs has not been elucidated, our case showed similar NVC findings that are commonly observed in patients with DM.
Accumulation of cases with anti-ARS Abs is needed to evaluate whether NVC findings are a useful marker for diagnosis and disease activity in patients with anti-ARS Abs.
An ELISA system has been developed to detect any one of five anti-ARS Abs (anti-Jo-1 Ab, anti-PL-7 Ab, anti-PL-12 Ab, anti-EJ Ab, and anti-KS Ab); however, anti-ARS Ab specificity cannot be identified because recombinant antigens are coated on the plate as a mixture. In addition, this ELISA Kit does not identify anti-OJ Ab. 6 A line blot assay also cannot detect anti-OJ Ab. 5 Therefore, immunoprecipitation assay is essential to identify anti-OJ Ab.
When patients have severe myopathy and their IIF staining demonstrates a cytoplasmic pattern with negative ELISA results for anti-ARS Abs, physicians may consider the presence of anti-SRP Ab.
Clinical characteristics of anti-SRP Ab include rapidly progressive proximal muscle weakness, dysphagia, and persistent increases in CK levels. Muscle biopsy can distinguish anti-SRP Ab from anti-OJ Ab because anti-SRP Ab typically suggests necrotizing myopathy exhibiting active muscle fiber degeneration and regeneration, and a marked increase in endomysial connective tissue, but negligible or no inflammatory cell inflammation. 14 Thus, although muscle biopsy is useful to distinguish, additional information of MSAs aids in making an accurate diagnosis.
In conclusion, it is important to steadily accumulate single cases of anti-OJ Ab because its clinical features have yet to be fully clarified.

CO N FLI C T O F I NTE R E S T
The authors declare no conflict of interest.

D ECL A R ATI O N S
Approval of the research protocol: N/A.