A 73-year-old woman received an orthotopic liver transplant for hepatitis C virus (HCV) cirrhosis. Initial immunosuppression was tacrolimus, azathioprine, and prednisolone. A liver biopsy 18 months later showed mild-grade, moderate-stage recurrent HCV-related liver disease prompting replacement of tacrolimus with sirolimus, which is antifibrotic. Over the next 2 months, the serum sirolimus concentration was 6.1 to 13.1 ng/mL (normal range, 5 to 15 ng/mL). Two weeks after starting sirolimus, a nonproductive cough, dyspnea, and malaise developed. On examination, she was febrile (38.5°C), had a respiratory rate of 16 breaths/min, and had saturations of 97% breathing room air. She had fine end-inspiratory crepitations audible at the left lung base. Her chest radiograph is shown in Figure 1
, top, A. The WBC count was 6.4 × 109/L, and C-reactive protein was 15 mg/L (normal range, 0 to 6 mg/L). Peripheral blood cytomegalovirus polymerase chain reaction, blood cultures, and atypical respiratory serology were negative. Pleural fluid analysis revealed a benign, sterile transudate. Antinuclear and antineutrophil cytoplasmic antibodies and rheumatoid factor were negative. She was treated with 5 days of IV meropenem without improvement. Thoracic CT is shown in Figure 1, center, B.