Inclusion Criteria:

• * Be ≥6 and \<18 years of age, at the time of signing the informed consent and informed assent (if applicable).
• * Have a history of pulmonary alveolar proteinosis, based on examination of a lung biopsy, bronchoalveolar lavage cytology, or a high-resolution computed tomogram of the chest.
• * Have a positive serum anti-GM-CSF autoantibody test result confirming aPAP.
• * Have a hemoglobin (Hb)-adjusted diffusing capacity of the lung for carbon monoxide (DLCO) ≤70% predicted at Screening.

Exclusion Criteria:

• * Have a diagnosis of hereditary (congenital) or secondary PAP, or a metabolic disorder of surfactant production.
• * Have undergone treatment with Lung Lavage (WLL) within 1 month of Baseline