Current understanding and management of pulmonary Langerhans cell histiocytosis.

Pulmonary Langerhans cell histiocytosis (PLCH) is a diffuse lung disease that usually affects young adult smokers. PLCH affects different lung compartments; bronchiolar, interstitial and pulmonary vascular dysfunction may coexist to varying extents, resulting in diverse phenotypes. Analyses of PLCH tissues have identified activating mutations of specific mitogen-activated protein kinases (BRAFV600E and others). The current consensus is that PLCH represents a myeloid neoplasm with inflammatory properties: the myeloid tumour cells exhibit surface CD1a expression and up to 50% of the cells harbour activating BRAF or other MAPK mutations. PLCH may be associated with multisystem disease. The detection of disease outside of the thorax is facilitated by whole body positron emission tomography. The natural history of PLCH is unpredictable. In some patients, disease may remit or stabilise following smoking cessation. Others develop progressive lung disease, often associated with evidence of airflow limitation and pulmonary vascular dysfunction. Due to the inability to accurately predict the natural history, it is important that all patients undergo longitudinal follow-up at least twice a year for the first few years following diagnosis. The treatment of PLCH is challenging and should be individualised. While there is no general consensus regarding the role of immunosuppression or chemotherapy in management, selected patients may experience improvement in lung function with therapy. Determination of BRAFV600E or other mutations may assist with the development of an individualised approach to therapy. Patients with progressive disease should be referred to specialised centres and considered for a trial of pharmacotherapy or evaluated for transplantation.