Reclassification of Seronegative Rheumatoid Arthritis as Anti-PL-12 Antisynthetase Syndrome with Interstitial Lung Disease and Joint Involvement–Case Report

<b>Background and Clinical Significance:</b> Antisynthetase syndrome (ASyS) is a rare autoimmune entity defined by the presence of anti-aminoacyl-t ribonucleic acid (RNA) synthetase autoantibodies and classically associated with a triad of interstitial lung disease (ILD), inflammatory myopathy, and arthritis. Additional clinical features may include Raynaud's phenomenon and "mechanic's hands". Among antisynthetase antibodies, anti-PL-12 is notably associated with predominant or isolated ILD and may occur in the absence of clinically evident myositis, thereby complicating timely diagnosis. <b>Case Presentation:</b> We are presenting a 45-year-old non-smoking female patient with a prior diagnosis of seronegative rheumatoid arthritis (RA) who developed progressive dyspnea, dry cough, and sicca symptoms. High-resolution computed tomography revealed a nonspecific interstitial pneumonia (NSIP) pattern. Despite normal creatine kinase and lactate dehydrogenase levels, serological work-up revealed positive anti-PL-12 and anti-Ro52 antibodies, supporting a diagnosis of antisynthetase syndrome without myositis, fulfilling the diagnostic criteria for ASyS per Connors and Solomon. Treatment with corticosteroids and cyclophosphamide induced clinical and functional respiratory improvement, while azathioprine was initiated for maintenance. <b>Conclusions:</b> This case underscores the clinical heterogeneity of antisynthetase syndrome and highlights the diagnostic challenge posed by anti-PL-12-associated ILD in the absence of myositis. Importantly, it demonstrates that in patients with pre-existing rheumatologic diagnoses, the emergence of atypical pulmonary manifestations warrants repeat serologic evaluation to assess ASyS and other autoimmune conditions. Early diagnosis and immunosuppressive treatment are essential to optimize outcomes.