Progress in understanding the pathogenesis of Langerhans cell histiocytosis: back to Histiocytosis X?

Abstract

Langerhans cell histiocytosis (LCH), the most common histiocytic disorder, is characterized by the accumulation of CD1A(+) /CD207(+) mononuclear phagocytes within granulomatous lesions that can affect nearly all organ systems. Historically, LCH has been presumed to arise from transformed or pathologically activated epidermal dendritic cells called Langerhans cells. However, new evidence supports a model in which LCH occurs as a consequence of a misguided differentiation programme of myeloid dendritic cell precursors. Genetic, molecular and functional data implicate activation of the ERK signalling pathway at critical stages in myeloid differentiation as an essential and universal driver of LCH pathology. Based on these findings, we propose that LCH should be re-defined as an inflammatory myeloid neoplasia. Increased understanding of LCH pathogenesis will provide opportunities to optimize and personalize therapy through improved risk-stratification, targeted therapy and assessment of therapy response based on specific molecular features and origin of the pathological myeloid cells.

Keywords: BRAF V600E; Langerhans cell histiocytosis; extracellular signal-regulated kinase signalling pathway; inflammatory myeloid neoplasia; misguided myeloid differentiation.

Fig 1

Routes to ERK activation in LCH. Model of MAPK pathway activation resulting from serial phosphorylation from cellular receptors through RAS, RAF, MEK and, ultimately, ERK. Estimates of frequency of somatic mutations of BRAF and MAP2K1 are illustrated. (*) indicates genes with individual case reports of somatic mutations. ‘Unknown’ indicates ERK activation by mechanisms that have not yet been defined. While activated ERK has been identified in all lesions studied to date, there remains the possibility (dashed line) that Langerhans cell histiocytosis (LCH) may arise from alternative mechanisms in some cases.

Fig 2

Developmental stage of pathological DC precursor defines extent of disease. (A) Somatic mutation of BRAF or other inciting event in CD34⁺ haematopoietic stem cells or early DC progenitors induces proliferation, maturation and migration of pathological DCs in multiple tissues that results in high-risk multisystem LCH. (B) Somatic mutation of BRAF or other inciting event in a tissue-restricted DC precursor induces proliferation, maturation and tissue-limited migration of pathological DCs that results in low-risk multi-system LCH. (C) Somatic mutation of BRAF or other inciting event in mature DC results in proliferation and maturation of pathological DCs leading to low-risk single lesion LCH. (D) Regardless of cell of origin, the pathological DCs recruit ‘bystander’ immune cells in an inflammatory lesion characteristic of LCH. Cells in white areas indicate cells in circulation; cells in grey areas indicate cells that have migrated to tissue targets. LCH, Langerhans Cell Histiocytosis; DC, dendritic cell.