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When administered parenterally, endotoxin stimulates the synthesis of IL-1, TNF-alpha, and IL-6. However, this initial injection induces tolerance; a second injection of endotoxin results in lower levels of circulating cytokines. In our study, five healthy male volunteers between the ages of 18 and 30 were injected with Escherichia coli endotoxin. Four subjects received only saline. Immediately before the injection and 3, 6, and 24 h afterward, PBMC were isolated and stimulated in vitro with endotoxin, IL-1, or toxic shock syndrome toxin-1. Inasmuch as CD14+ monocytes are the primary source of the cytokines induced by these stimuli, results are expressed as cytokine production per 10(6) CD14+ cells. Six h after endotoxin injection, endotoxin-stimulated CD14+ cells synthesized 66% less IL-1 beta (p < 0.01), 47% less TNF-alpha (p < 0.001), 56% less IL-6 (p < 0.01), and 49% less IL-8 (p < 0.01) than cells obtained before the injection. This suppression was not specific for endotoxin; IL-1 beta-induced IL-1 alpha and TNF-alpha were reduced by 84% (p = 0.01) and 68% (p < 0.001), respectively. A decrease in cytokine synthesis was also observed using toxic shock syndrome toxin-1 as a stimulus: 57% for IL-1 beta (p = 0.06), 70% for TNF-alpha (p < 0.01), 56% for IL-6 (p < 0.05), and 71% for IL-8 (p = 0.001). When data were expressed as cytokine production per 10(6) PBMC, cells isolated 3 h after endotoxin injection synthesized significantly less stimulus-induced IL-1, TNF-alpha, IL-6, and IL-8 than did PBMC from saline-injected controls. We conclude that endotoxin tolerance is due, in part, to changes in the stimulus-induced cytokine response of circulating CD14+ cells.

Interleukin 1 (IL-1) plays an important role in host defense mechanisms by increasing body temperature, inducing the synthesis of a variety of lymphokines and hepatic acute phase proteins and acting as a chemoattractant for lymphocytes. However, in some microenvironments such as injured tissue or joint spaces, elevated IL-1 levels may contribute to pathologic processes, for example, proliferation and fibrosis of tissue involved in pannus formation as well as degradation of matrix and abnormal tissue architecture. To investigate potential mechanisms that may lead to excessive production of IL-1, we have examined the ability of IL-1 to participate in an amplification event by inducing its own gene expression leading to synthesis of biologically active IL-1. When injected into rabbits, recombinant human IL-1-alpha induced biphasic fevers, and during the second temperature elevation 3 hr later, a circulating pyrogenic material was detected by passive transfer of plasma to other rabbits. Induction of the biphasic fever was not caused by endotoxin contamination of the recombinant IL-1. The 3-hr circulating pyrogen was heat-labile and was not residual injected IL-1-alpha. Chromatographic separation of this plasma and biologic assay suggested that it was new IL-1 of rabbit origin. We next incubated human blood mononuclear cells with recombinant IL-1-alpha and measured the intracellular and extracellular levels of IL-1 by bioassay using the D10.G4.1 murine T cell line. In order to control for the carryover of recombinant IL-1-alpha used to stimulate the mononuclear cells (MNC), we used neutralizing antibodies that were specific for IL-1-alpha or IL-1-beta. The results of these neutralizations showed that recombinant human IL-1-alpha induces the synthesis of IL-1-beta in human MNC in vitro. These results were verified with a radioimmunoassay specific for IL-1-beta. At concentrations of 100 ng/ml, IL-1-alpha induced prostaglandin E2 production in the MNC culture, and this was associated with decreased production of immunoreactive IL-1-beta. Adding indomethacin to the cultures prevented the decreased production of IL-1-beta induced by high concentrations of IL-1-alpha. Using nonadherent MNC, we observed an increase in IL-1-beta as well as IL-1-alpha mRNA after 4 hr of exposure to recombinant IL-1-alpha. These results demonstrate that IL-1-alpha induces biologically active and immunoreactive IL-1-beta from MNC in vitro and that the same concentrations of IL-1-alpha induce gene expression for both forms of IL-1.(ABSTRACT TRUNCATED AT 400 WORDS)