REGULATION OF INTERLEUKIN-6 (IL-6)

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Interleukin-6 (IL-6) is a multifunctional cytokine. It stimulates activation and differentiation of B and T lymphocytes, induces thrombopoiesis and fever, regulates the acute phase response of inflammation, and is a growth factor for some tumors. Dysregulation of the production or turnover of IL-6 leads to abnormal levels of the cytokine and has pathologic consequences such as those observed in patients with multiple myeloma and rheumatoid arthritis. In most cases, the causes for abnormal IL-6 levels in vivo are not known. The goal of this work is to identify mechanisms of dysregulation of interleukin-6 during chronic inflammation. We have established an experimental model that involves injection of the mineral oil pristane (2,6,10,14- tetramethyl- pentadecane) into the peritoneal cavities of inbred mice. The treatment induces a chronic peritonitis that is accompanied by dramatically elevated levels of intraperitoneal IL-6 that persist for months after injection of the oil. This condition is associated with development of plasma cell tumors in some strains of mice. Two approaches are being followed to elucidate the mechanisms responsible for the elevation of IL-6. The first is to pursue our earlier observation that the increase in IL-6 can be inhibited by co-administration of the cyclooxygenase inhibitor indomethacin to pristane-treated mice. This result suggested that prostaglandins might be responsible for stimulating IL-6 production during the inflammatory response. In vitro experiments in which peritoneal macrophages were isolated and treated with prostaglandin E2 now support this hypothesis, and the mechanism appears to be mediated by adenylate cyclase. Several different types of agents that elevate intracellular cAMP were tested and all acted to stimulate IL-6 secretion. The results suggest that indomethacin lowers IL-6 levels after pristane treatment by inhibiting cyclooxygenase activity and that cAMP serves as a second messenger for macrophage IL-6 production. Future studies will be directed toward establishing a direct link between abnormal prostaglandin and IL-6 production in vivo. Our second approach to identifying the pathways responsible for dysregulation of IL-6 is to determine whether genes that regulate macrophage IL-6 production can be identified. The first step has been to treat different inbred mouse strains with pristane and measure the levels of intraperitoneal IL-6 that develop. Significant heritable differences have been observed. Most notably, BALB/c mice develop the highest IL-6 levels (~ 350 pg/ml on average) while C3H mice (HeJ and HeN) incur only marginal levels of induction that are below or near the lower assay limit of 15 pg/ml. Progeny of the first generation cross between these two strains of mice show only low levels of IL-6 in response to pristane. Experiments are underway to determine whether the trait resides in a single gene. In addition, expression of the cyclooxygenase genes is being compared in inflammatory macrophages from BALB/c and C3H mice.