New Insights Into the Molecular Pathogenesis of Langerhans Cell Histiocytosis

Learning Objectives Explain the pathogenesis of Langerhans cell histiocytosis, with particular regard to recent advances in this field. Better identify underdiagnosed disorders such as Langerhans cell histiocytosis. Cite currently available therapeutic opportunities for patients with Langerhans cell histiocytosis. Langerhans cell histiocytosis (LCH) is a rare proliferative disorder characterized by an accumulation of cells sharing the major phenotypic features of cutaneous Langerhans cells. Given its variable clinical evolution, ranging from self‐limiting lesions to multisystemic forms with a poor prognosis, in the last decades it has been debated whether LCH might not have a neoplastic rather than an inflammatory nature. However, although the fundamental events underlying the pathogenesis of LCH are still elusive, recent advances have strikingly improved our understanding of the disease. In particular, the identification of multiple interplays between LCH cells and their tumor microenvironment, along with the recognition of the lesional cytokine storm as a key determinant of LCH progression, has substantiated new opportunities for devising targeted therapeutic approaches. Strikingly, the detection of the rapidly accelerated fibrosarcoma isoform BV600E gain‐of‐function mutation as a genetic alteration recurring in more than 50% of patients has fueled the paradoxical picture of LCH as a tumor of the antigen‐presenting cells that can evade rejection by the immune system. Thus, new evidence regarding the ontogeny of LCH cells, as well as a better understanding of the putative immune system frustrating strategy in LCH, may help to define the precise pathogenesis. Although the fundamental events underlying the pathogenesis of Langerhans cell histiocytosis (LCH) remain elusive, recent advances have significantly improved our understanding of this disease. In particular, the identification of activating mutations of BRAF in LCH patients, along with the recognition of the critical importance of the lesional microenvironment in tumor progression, have overturned the classical—and perhaps ambiguous—view of the disease, substantiating new opportunities for targeted therapies.