Anti-PL-12 and Interstitial Lung Disease as the First Signs of Antisynthetase Syndrome in Patients With Previously Diagnosed Systemic Lupus Erythematosus.

Antisynthetase syndrome (ASTS) is an infrequent autoimmune condition marked by chronic inflammation and the presence of autoantibodies targeting aminoacyl-tRNA synthetases. Clinically, it often manifests through a constellation of features, including inflammatory arthritis, myositis, Raynaud's phenomenon, hyperkeratotic skin changes (mechanic's hands), and interstitial lung disease (ILD). Among these, ILD is the most significant in terms of prognosis, as it contributes to elevated rates of morbidity and mortality, surpassing those seen in other idiopathic inflammatory myopathies. We report the clinical course of a 56-year-old woman with a prior diagnosis of systemic lupus erythematosus (SLE) established in 2019, based on the 2017 European League Against Rheumatism (EULAR)/American College of Rheumatology (ACR) classification criteria. She was admitted to the Institute for Pulmonary Diseases of Vojvodina (IPBV) in Sremska Kamenica, Serbia, due to respiratory symptoms suggestive of ILD. Although ILD is an uncommon manifestation in SLE, the patient was assessed for the potential coexistence of other connective tissue diseases, such as systemic sclerosis (SSc) and inflammatory myopathies. Laboratory testing demonstrated a strong antinuclear antibody (ANA) reactivity on Hep-2 cells and low anti-double-stranded DNA (anti-dsDNA) levels (<10 IU/ml). Extended myositis panel testing revealed marked positivity for anti-PL-12 and anti-Ro-52 antibodies. Given the presence of pre-existing ILD and clinical signs of arthritis and proximal muscle weakness, alongside the identification of anti-tRNA synthetases antibodies, a diagnosis of ASTS was established. Immunosuppressive treatment was intensified by increasing the glucocorticoid (GC) dosage and introducing intravenous cyclophosphamide (CYC, 1 g). The patient demonstrated a favorable clinical and biochemical response to this regimen.