A sensitive, IL-2-independent, assay for IL-1.

The thymocyte costimulator (LAF) assay, the standard biological test used for IL-1 titration, has a low sensitivity and lacks specificity since it can be potentiated by the IL-2 which is frequently present in IL-1-containing biological fluids. We describe here a new IL-1 titration method which takes advantage of the capacity of a thymoma line, EL4-6.1, to differentiate and express IL-2 receptors upon stimulation by IL-1 in the presence of a suboptimal dose of phorbol diester. Membrane IL-2R measurement on this indicator cell line permits the detection of 1-2 X 10(-4) ng/ml IL-1, compared to 5 X 10(-2) ng/ml in the LAF assay. In addition, rIL-2 up to 250 U/ml has no effect on IL-1 measurement by this assay, which also exhibits a 100-fold lower sensitivity to inhibitory effects of prostaglandin, compared to the LAF assay. Finally, tumor necrosis factor alpha only exerts a weak costimulation effect at very high doses. A flow cytometry technique and an ELISA are described for IL-2 receptor detection. Due to its high sensitivity and specificity, this novel assay should now permit reliable IL-1 titration in biological fluids such as IL-2-rich lymphocyte culture supernatants.