Lipopolysaccharide in concentrations above 40 ng/ml stimulates proliferation of the IL-6-dependent B9 cell line

The B9 assay is known to be a specific and sensitive assay for the estimation of interleukin-6 activity. This assay was found to be compromised by lipopolysaccharide in concentrations > or = 40 ng lipopolysaccharide per ml. The lipopolysaccharide stimulates proliferation of the B9 cell line in a dose-dependent manner both when measuring the proliferation by thymidine incorporation and when using the MTT assay. However the LPS dose-response curve is different compared to the dose-response curve for IL-6. A sample containing 100 ng LPS/ml but no IL-6 would be estimated erroneously to contain 12 pg IL-6. The interference of lipopolysaccharide is totally abolished by the addition of polymyxin B to the samples but the addition has no effect on the IL-6 induced proliferation.