The myeloma cell antigen syndecan-1 is lost by apoptotic myeloma cells

Syndecan-1 is a cell membrane proteoglycan that binds extracellular matrix components and various growth factors, It is ex-pressed only on malignant plasma cells in bone marrow samples from patients with multiple myeloma (MM). Several reports have suggested that syndecan-1 was present only on a part of the myeloma cells, By using either IL-6-dependent myeloma cell lines or primary myeloma cells stained by annexin V, rye report here that syndecan-1 was rapidly lost by myeloma cells undergoing apoptosis. In the same experimental conditions, expression of other cell membrane antigens such as CD38, HLA class-I or CD49d on apoptotic myeloma cells was not affected. In addition, we show that syndecan-1 loss was independent of activation of the gp130 IL-6 transducer. Dexamethasone induced a strong apoptosis of myeloma cells associated with the loss of syndecan-1. Finally by using freshly-explanted tumoural samples, we show that syndecan-1 rapidly disappeared from myeloma cells in association with induction of apoptosis. In conclusion we showed that syndecan-1 is a marker for viable myeloma cells which is rapidly lost by apoptotic cells. These results emphasize the usefulness of anti-syndecan-1 antibodies to purge tumoural cells from haemopoietic grafts or to purify these cells for further manipulations for immune or gene therapies.