Serum soluble interleukin‐6 receptor in MRL/lpr mice is elevated with age and mediates the interleukin‐6 signal

The characteristics of soluble interleukin‐6 receptor (sIL‐6R) in murine sera were examined. To investigate a relationship between serum sIL‐6R level and autoimmune diseases, quantitative analysis of serum sIL‐6R in MRL/lpr mice was performed by an enzyme‐linked immunosorbent assay. The serum sIL‐6R level in MRL/lpr mice of both sexes was below the detection limit (< 1.0 ng/ml) at 8 weeks of age, but it increased in accordance with age and reached 42 ± 9.3 ng/ml in female and 31 ± 13 ng/ml in male mice at 30 weeks of age. In MRL/+ mice, although an age‐associated increase in serum sIL‐6R level was observed, it was much less extensive than that in MRL/lpr mice. Elevated serum sIL‐6R level at the age of 30 weeks was observed in female and male (NZB × NZW)F1 mice (32 ± 10 ng/ml and 17 ± 5.0 ng/ml, respectively), and male BXSB/Mpj Yaa mice (42 ± 18 ng/ml), suggesting that elevated serum sIL‐6R in aged mice is one of the characteristics of autoimmune‐prone mice. Quantitative analysis of serum IL‐6 in MRL/lpr revealed that the serum sIL‐6R level correlated well with the serum IL‐6 level. We also showed that sIL‐6R in the sera from MRL/lpr mice could mediate the IL‐6 functions through the IL‐6 signal‐transducing receptor component gpl30, suggesting that elevated production of sIL‐6R may partly contribute to development of autoimmune disease in MRL/lpr mice.