Participants will receive emapalumab administered intravenously at a dose of 6 mg/kg on Day 1, followed by 3 mg/kg every 3 days for 2 weeks, and then 3 mg/kg twice weekly for an additional 2 weeks. Total treatment duration is 12 weeks.

Emapalumab administered intravenously according to the following dosing regimen: 6 mg/kg on Day 1, followed by 3 mg/kg every 3 days for 2 weeks, and then 3 mg/kg twice weekly for 2 weeks.

Supplemental oxygen requirement assessed as the oxygen flow rate (reported in liters per minute [L/min]) while the participant is at rest and, when feasible, while the participant is ambulating during the 6-Minute Walk Test (6MWT). Values are compared with baseline to assess change over time.

Pulmonary function assessed using forced vital capacity (FVC) measured by standard pulmonary function testing (spirometry). The outcome is expressed as percent change from baseline (%) in FVC.

Pulmonary gas exchange capacity assessed using diffusing capacity of the lungs for carbon monoxide (DLCO) measured by standard pulmonary function testing. The outcome is expressed as percent change from baseline (%) in DLCO

Pulmonary consolidations assessed on chest computed tomography (CT) scans. CT images are reviewed by a radiologist and pulmonologist, and percentage of lung affected by consolidations will be documented. Percentage change of affected lung will be evaluated.

Serum ferritin concentration measured using standard clinical laboratory testing and reported in nanograms per milliliter (ng/mL). Values obtained at baseline and during post-baseline assessments are used to assess change from baseline in ferritin level.