Re-fusion of Small Mononucleated Hodgkin Cells Leads to Multinucleated Reed-Sternberg Cells

Abstract

The multinucleated Reed-Sternberg (RS) cells are pathognomonic for classical Hodgkin lymphoma (HL) and their presence is essential for diagnosis. However, the development of these giant tumor cells is controversially discussed. It was postulated that RS cells arise from mononucleated Hodgkin cells via endomitosis. Conversely, continuous single cell tracking of HL cell lines by long-term time-lapse microscopy showed that cell fusion is the main route of RS cell formation. In contrast to growth-induced formation of giant Hodgkin cells, fusion of small mononuclear cells followed by size increase gives rise to giant RS cells. Importantly, we nearly exclusively observed fusion of cells originating from the same ancestor, termed re-fusion. In the majority of cases, re-fusion of daughter cells was preceded by an incomplete cytokinesis, visualized by a microtubule bond between the cells. We confirm at the level of individual tracked cells that giant Hodgkin and RS cells have little proliferative capacity, further specifying small mononuclear Hodgkin cells as the proliferative compartment of the HL tumor clone. In addition, sister cells showed a shared propensity for re-fusion, which provides evidence of early RS cell fate commitment. Thus, RS cell generation is neither due to cell fusion of unrelated Hodgkin cells nor to endomitosis, but is mediated by re-fusion of daughter cells that underwent mitosis. This surprising finding indicates the existence of a novel mechanism for the generation of multinuclear RS cells, which might have implications beyond HL, as RS-like cells are frequently observed in several other lymphoproliferative diseases.