Pulmonary Langerhans cell histiocytosis (PLCH) is a diffuse cystic lung disease that is strongly associated with exposure to cigarette smoke. Recently, activating pathogenic mutations in the mitogen-activated protein kinase pathway have been described in the dendritic cells in patients with PLCH and have firmly established PLCH to be an inflammatory myeloid neoplasm. Disease course and prognosis in PLCH are highly variable among individual patients, ranging from spontaneous resolution to development of pulmonary hypertension and progression to terminal respiratory failure. A subset of patients with PLCH may have extrapulmonary involvement, typically involving the skeletal system in the form of lytic lesions, skin lesions, or the central nervous system most commonly manifesting in the form of diabetes insipidus. Smoking cessation is the cornerstone of treatment in patients with PLCH and can lead to disease regression or stabilization in a substantial proportion of patients. Further insight into the underlying molecular pathogenesis of PLCH has paved the way for the future development of disease-specific biomarkers and targeted treatment options directed against the central disease-driving mutations.

Pulmonary alveolar microlithiasis (PAM) is a rare, autosomal recessive disorder that is caused by mutations in SCL34A2 that encodes for the type IIb sodium-dependent phosphate cotransporter (Npt2b). The loss of Npt2b transporter function from alveolar epithelial cells results in failure to export inorganic phosphate from the alveolar lining fluid, which then accumulates, binds to calcium, and forms hydroxyapatite microliths. Radiographs and computed tomography of the chest demonstrate hyperdense infiltrates that are often quite dramatic and distinctive, and in many cases, the diagnosis can be made without invasive measures. The most common presenting symptom of PAM is dyspnea on exertion, but the disease is frequently first noted as an incidental finding in asymptomatic patients who have chest films performed for unrelated reasons. Pulmonary fibrosis, pulmonary hypertension, and respiratory failure can develop as the disease progresses, and treatment remains supportive. Lung transplantation is an option for those with end stage disease.

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