Abstract
Long-term intravenous (IV) epoprostenol is a key therapy for pulmonary arterial hypertension (PAH); however, rare immune-mediated conditions occurring during therapy remain incompletely characterised. We report a 34-year-old man with idiopathic PAH who developed low-grade fever and bilateral pulmonary infiltrates after 11 years of IV epoprostenol. Despite empirical antibiotics, his condition persisted, and a lip biopsy confirmed IgG4-related disease (IgG4-RD). Systemic corticosteroids were initiated, and the patient was transitioned from IV epoprostenol to a combination of IV and inhaled treprostinil; IgG4-RD has remained in remission to date, with normalisation of serum IgG4. This case highlights a practical, minimally invasive diagnostic approach for suspected IgG4-RD in severe PAH and demonstrates the feasibility of prostacyclin modification to sustain PAH control while reducing immunosuppression.