Biomarkers for Diagnostic, Prognostic and of Response to Treatment in Adult Langerhans Cell Histiocytosis

Adult Langerhans histiocytosis (LCH) is a rare disease of unknown etiology, characterized by the activation of the MAPK (Mitogen-activated protein kinases) pathway, driven by various somatic mutations in the specific lesions of involved organs/tissues. LCH is currently classified as myeloid neoplasia with an inflammatory component. In patients with active systemic LCH, MAPK mutations may also be identified in plasma free cell DNA in patients. In contrast, circulating MAPK mutations seem more rarely detected in patients with LCH limited to a single organ/tissue (single system disease), but this has not been accurately assessed in a large series of patients. The clinical presentation of LCH is very diverse, the prognosis variable, and the evolution marked by the occurrence of flares of the disease. A definitive diagnosis of LCH warrants histological confirmation obtained by a biopsy of an involved organ. In case of Pulmonary Langerhans cell histiocytosis (PLCH), a presumptive diagnosis is often acceptable when lung-computed tomography (CT) shows a nodulo-cystic pattern after excluding alternative diagnoses. In contrast, in case of purely cystic lung CT pattern, PLCH may be difficult to differentiate from other diffuse cystic lung diseases (mainly lymphangioléiomyomatose (LAM) and BHD (Birt-Hogg-Dubé syndrom), and eventually other rare disorders). Advanced PLCH may even be misdiagnosed as pulmonary emphysema that also occurs in smokers. In these situations, confirmation of PLCH warrants lung tissue, obtained most often by surgical lung biopsy that comprises significant morbidity or is not feasible in patients with altered lung function. Thus, the identification of specific blood biomarkers of cystic PLCH would be very useful. On another hand, personalized management of adult patients with LCH is limited given the absence of predictive factors for prognosis or response to treatment. The aim of this prospective study is to describe precisely the clinical phenotype at