Lipopolysaccharide induces human interleukin‐1 receptor antagonist and interleukin‐1 production in the same cell

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Abstract
A new member of the interleukin‐1 (IL‐1) family has recently been described. Human IL‐1 receptor antagonist (IL‐1ra) is structurally related to IL‐1α and IL‐1β but binds to IL‐1 receptors on various target cells without demonstrable agonist activity. Understanding the mechanisms of regulation of IL‐1ra production may clarify the biology of this unique cytokine as well as elucidate its possible role as a natural anti‐inflammatory protein. The effects of lipopolysaccharide (LPS) on IL‐1α, IL‐1β and IL‐1ra production was studied at a single‐cell level by use of cytokine‐specific antibodies and indirect immunofluorescence technique. The peak synthesis of IL‐1ra and IL‐1α/β occurred in peripheral blood monocytes obtained from healthy blood donors within 4 and 6 h of cell stimulation, respectively. By double‐staining procedure all IL‐1ra‐positive cells were also IL‐1α and/or β positive. Thus, endotoxin induced simultaneous synthesis of the IL‐1 gene family in the same cells. Only monocytes contributed to the production of IL‐1α, β and IL‐1ra during the 96 h of cell culture. The maximum number of IL‐1ra‐producing monocytes was 48 ± 16% as compared to peak production of IL‐1α and β which occurred in 75 ± 9% and 80 ± 12% (p < 0.001), respectively, of all peripheral blood monocytes. The incidence of IL‐1α and β‐containing cells was not only significantly higher but also occurred for a longer time period, 72 h as compared to 24 h for IL‐1ra localized in the Golgi organelle. However, IL‐1ra‐containing cells with a diffuse cytoplasmic appearance were also evident (20%–30%) at a later stage, 12 to 72 h after stimulation. Blocking IL‐1 surface receptors by addition of exogenous recombinant IL‐1β before stimulation could not inhibit the diffuse cytosolic localization. This indicates that the „late”︁ staining pattern did not reflect IL‐1ra being secreted and internalized after binding to extracellular receptors. Thus, perhaps IL‐1ra modulates IL‐1 effector mechanisms by receptor interactions both inside and outside the cell.