Pulmonary alveolar proteinosis: from classification to therapy

Pulmonary alveolar proteinosis (PAP) is a rare respiratory syndrome characterised by the accumulation of surfactant lipoproteins within the alveoli. According to various pathogenetic mechanisms and aetiologies, PAP is classified as primary, secondary or congenital. Primary PAP is led by a granulocytemacrophage colony-stimulating factor (GM-CSF) signalling disruption; the autoimmune form is driven by the presence of anti GM-CSF autoantibodies and represents 90% of all the PAP cases; and the hereditary form is the result of mutations in genes encoding GM-CSF receptor. Secondary PAP is associated with various diseases causing a reduction in function and/or number of alveolar macrophages. Congenital PAP emerges as a consequence of corrupted surfactant production, due to mutations in surfactant proteins or lipid transporter, or mutations affecting lung development. The clinical manifestations are various, ranging from insidious onset to acute or progressive respiratory failure, including premature death within the first days of life in neonates with congenital surfactant production disorders. The diagnostic workup includes clinical and radiological assessment (respiratory function test, high-resolution chest computed tomography), laboratory tests (anti-GM-CSF autoantibodies dosage, GM-CSF serum level and GM-CSF signalling test), and genetic tests. Whole-lung lavage is the current gold standard of care of PAP; however, the therapeutic approach depends on the pathogenic form and disease severity, including GM-CSF augmentation strategies in autoimmune PAP and other promising new treatments.