Endotoxin is a component of gram-negative bacteria that causes hematologic and immunologic changes through its induction of cytokines. Interleukin-1 receptor antagonist (IL-1Ra) is a naturally occurring inhibitor of IL-1 that competes with IL-1 for occupancy of cell-surface receptors but possesses no agonist activity. We investigated the ability of human recombinant IL-1Ra to block the effects of low-dose endotoxin. Fourteen healthy male volunteers between 18 and 30 years old were injected intravenously with 3 ng/kg Escherichia coli endotoxin. Concurrent with the injections, nine volunteers received a 3-hour continuous intravenous infusion of IL-1Ra. The other five subjects were given a 3-hour infusion of saline. Volunteers injected with endotoxin experienced a threefold increase in circulating neutrophils over baseline. This neutrophilia was significantly reduced by 48% in subjects administered endotoxin plus IL-1Ra (P = .0253). Ex vivo mitogen-induced peripheral blood mononuclear cell proliferation decreased by greater than 60% at 3 and 6 hours after endotoxin injection (P = .0053). This endotoxin-induced reduction in mitogen response was reversed in subjects coinjected with IL-1Ra (P = .0253). Endotoxin-induced symptoms, fever, and tachycardia were unaffected by IL-1Ra. IL-1 appears to be an important mediator in endotoxemia because some of its hematologic and immunomodulatory effects can be blocked by IL-1Ra.

Interleukin-1 (IL-1) induces IL-1, tumor necrosis factor alpha (TNF alpha), and IL-6 gene expression and synthesis in a variety of cells. In this study, we investigated the ability of human recombinant IL-1 receptor antagonist (IL-1ra) to inhibit IL-1-induced cytokine production in human peripheral blood mononuclear cells (PBMC) and isolated monocytes. IL-1ra alone at concentrations as high as 1 microgram/mL did not induce IL-1 alpha, IL-1 beta, TNF alpha, or IL-6 synthesis. Suppression of IL-1-induced IL-1, TNF alpha, or IL-6 synthesis was dose-dependent (P less than or equal to .0001). At a twofold molar excess, IL-1ra inhibited IL-1-induced IL-1 or TNF alpha synthesis by 50% (P less than .01); an equimolar concentration of IL- 1ra inhibited synthesis of these two cytokines by over 20% (P less than .05). A 10-fold molar excess of IL-1ra over IL-1 beta reduced IL-1 beta- induced IL-1 alpha by 95% (P = .01) and IL-1 alpha-induced IL-1 beta by 73% (P less than .01). IL-1ra added to PBMC 8 hours after stimulation with IL-1 beta was still able to inhibit IL-1 alpha, TNF alpha, and IL- 6 synthesis (P less than or equal to .01). A similar reduction in IL-1 beta-induced IL-1 alpha was observed when IL-1 beta was removed from the cultures after 8 hours of stimulation (P less than .05), suggesting a prolonged presence of IL-1 or restimulation of IL-1 receptors on monocytes is required for the induction of cytokines. In elutriated monocytes, a 10-fold molar excess of IL-1ra reduced IL-1 beta-induced IL-1 alpha by 82% (P less than .05), TNF alpha by 64% (P = .05), and IL- 6 by 47% (P less than .05). 125I-IL-1 beta was bound to purified monocytes, cross-linked, and immunoprecipitated. Sodium dodecyl sulfate- polyacrylamide gel electrophoresis showed a band at 85 Kd corresponding to the 68-Kd IL-1 receptor type II (IL-1RtII). Excess unlabeled IL-1 beta or IL-1ra blocked the binding of 125I-IL-1 beta to the IL-1RtII. We conclude that IL-1ra inhibits IL-1-induced cytokine synthesis and competes with IL-1 for the IL-1RtII on human monocytes.

Lipopolysaccharide (LPS) stimulates interleukin-1 (IL-1) and tumor necrosis factor alpha (TNF alpha) gene expression and synthesis in human peripheral blood mononuclear cells (PBMC). IL-1 can also induce PBMC to synthesize IL-1 and TNF alpha. In the present study, we used IL- 1 receptor antagonist (IL-1ra) to determine the relative contribution of an IL-1-positive feedback loop to the total amount of LPS-induced cytokine synthesis. Pretreatment of PBMC with human recombinant IL-1ra reduced LPS-induced cytokine synthesis in a dose-dependent manner (P less than .001). Maximal inhibition was 33% for IL-1 alpha (P less than .01), 43% for IL-1 beta (P = .001), and 20% for TNF alpha (P less than .05). We consistently observed IL-1ra suppression of LPS-induced cytokines in PBMC of 38 volunteers. However, this phenomenon was not specific for LPS; 1 microgram/mL IL-1ra inhibited IL-1 beta synthesized in response to human recombinant IL-2 by 44% (P less than .001), toxic shock syndrome toxin-1 by 26% (P less than .05), and phorbol 12- myristate 13-acetate by 76% (P less than .001). IL-1ra added to PBMC 4 or 8 hours after stimulation with LPS still inhibited IL-1 beta synthesis by 44% (P less than .001) or 25% (P = .01), respectively. The steady state messenger RNA levels of IL-1 beta were reduced in PBMC stimulated by LPS in the presence of IL-1ra. In monocytes isolated by elutriation, IL-1ra reduced LPS-induced IL-1 alpha by 16% (P less than .001), IL-1 beta by 14% (P less than .05), and TNF alpha by 24% (P = .01). We conclude that IL-1-induced IL-1 significantly contributes to LPS-induced cytokine synthesis.