Soluble Interleukin‐6 Receptor Is Released from Receptor‐bearing Cell Lines in vitro

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.