Soluble Interleukin‐6 Receptor Is Released from Receptor‐bearing Cell Linesin vitro

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Abstract
Soluble interleukin‐6 receptor (sIL‐6R) was found to be spontaneously released from human myeloma cell line U266 cells into culture supernatant, and was quantitatively measured with a fluorescence sandwich enzyme‐linked immunosorbent assay employing antibodies specific to IL‐6R. The supernatant IL‐6R was generated only from IL‐6R‐positive cell lines; myeloma cell lines RPMI8226 and RPMI1788, and myelomonocytic cell lines U937, THP‐1, and HL‐60. In contrast, it was not released from the IL‐6R‐negative cells; T cell line MoIt‐4 and Burkitt lympboma cell line Raji. SDS‐PAGE analysis of the soluble IL‐6R from U266 cells suggested a molecular weight of approximately 50‐55 kDa, 25–30 kDa smaller than the mature cell surface receptor. These results suggest that the generation of soluble IL‐6R may be a maker of myeloma cells and myelomonocytic cells.