Expression of CD1 molecules and colony‐stimulating factor 1 receptor in indeterminate cell histiocytosis

Indeterminate cell histiocytosis (ICH) and Langerhans cell histiocytosis (LCH) are rare histiocyte proliferating disorders with unknown etiologies. However, both tumor cells immunophenotypically share some features of Langerhans cells (LC), thereby expressing CD1a. Recent transcriptome analysis revealed that circulating CD1c+ myeloid dendritic cells are the potential precursor of LCH tumor cells. LC express CD1a as well as CD1c, but not CD1b. We discovered that both tumor cells express CD1c, but not CD1b, similar to LC. Moreover, like LC, both tumor cells express colony‐stimulating factor 1 receptor (CSF‐1R). Considering the crucial role of the interleukin (IL)‐34/CSF‐1R axis for the development and survival of LC, CSF‐1R on both tumor cells might facilitate their survival and proliferation in situ. These data provide additional evidence to support the fact that ICH and LCH share immunophenotypical features with LC. In addition, we hypothesized that tumor cells in ICH and LCH survive and proliferate through IL‐34‐mediated CSF‐1R signaling.