We developed a monoclonal antibody with neutralizing activity against the rabbit IL-1 receptor antagonist (IL-1ra) and a sandwich ELISA for rabbit IL-1ra. Using this ELISA, we measured IL-1ra contents in inflammatory exudate cells and culture supernatants of LPS-stimulated macrophages. The production of IL-1ra occurred rapidly after the onset of inflammation in vivo and after stimulation of macrophages in vitro and was sustained for a long period. The time interval for detection of IL-1 activity overlapped with the above mentioned period of IL-1ra production. In the presence of this neutralizing monoclonal IgG, the effect of IL-1ra in these biological specimens was nil and heretofore unrecognized IL-1 activity in the samples was measurable; the rate of this increment in activity ranging between 1.5- and 4.8-fold. We conclude that underestimations of IL-1 activity have been made and previously reported results on IL-1 activities in many biological materials have to be re-examined.
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