Cyclin D1 is Expressed in Neoplastic Cells of Langerhans Cell Histiocytosis but Not Reactive Langerhans Cell Proliferations

Langerhans cell histiocytosis (LCH) is characterized by frequent activating mutations involving the mitogen-activated protein kinase (MAPK) pathway. Therefore, downstream markers of MAPK pathway activation such as cyclin D1 may be useful as novel diagnostic markers of neoplasia in LCH. The goal of this study was to investigate cyclin D1 expression in LCH and reactive Langerhans cell accumulations using immunohistochemistry on archival tissue. All LCH cases tested (39/39) showed cyclin D1 expression in CD1a/Langerin cells. Most cases (22/39; 56%) showed strong cyclin D1 expression in the majority (≥50%) of lesional cells. Only a few cases (6/39; 15%) showed cyclin D1 expression in a small subset (