The Role of Interleukin-6 in the Febrile Response

Abstract: Everyone who has been exposed to influenza or a bacterial infection knows how it feels to be sick. Apart from not being willing to participate in social activities, losing your appetite and experiencing pain, you have also most likely suffered from increased body temperature which defines fever, one of the most prominent signs of an acute ongoing infection. Invading the body, the infectious microorganisms are combated by the activated innate and adaptive immune systems, and the impaired balance is thus restored. While fever is an event that is controlled by the central nervous system, it has for long been debated how the inflammatory signals generated in the periphery communicate with the brain that is protected by the bloodbrain barrier which prevents large molecules such as cytokines from entering into the brain parenchyma.Previous studies from our group have provided evidence in support of the existence of a pathway across the blood-brain barrier by demonstrating that proinflammatory cytokine interleukin-1 transfers the inflammatory message to the brain through binding to its receptors situated in the brain vessels. This will subsequently trigger the production of the prostaglandin E2 (PGE2) that enters the brain and exerts its effect by binding to the receptors located on the thermoregulatory neurons. Interleukin-6 (IL-6) is another cytokine essential for fever signaling; however, the mechanism has not yet been identified. The research on which this thesis is based aimed at elucidating the role of IL-6 in inflammatory induced fever.In paper I, we demonstrated that mice incapable of producing inflammatory PGE2 still responded with an intact cytokine production in the brain upon peripheral lipopolysaccharide (LPS) stimulation. Thus, although the mice had induced expression of inflammatory cytokines in the brain, this was not sufficient for a fever response without simultaneous production of PGE2. The relationship between IL-6 and PGE2, both essential for fever, was further investigated in paper II, focusing on clarifying the mechanism by which IL-6 controls fever. We demonstrated that mice deficient in IL-6 did not respond with fever upon peripheral LPS administration despite an intact expression of PGE2 in the brain. In contrast, upon intracerebroventricular administration of PGE2 into the brain, a dose-dependent fever response was monitored in IL-6 deficient mice. Thus, we suggest that IL-6 exerts its effect neither up- nor downstream from PGE2, and propose instead that IL-6 may act alongside the PGE2 and regulate the process that deals with the transport of and binding of PGE2 onto its receptors. To further investigate the elusive role of IL-6 in fever, we performed a microarray analysis to identify the genes that were differentially expressed in the brain of LPS-challenged IL-6 deficient mice compared to wild type mice (paper III). We demonstrated that mice lacking IL-6 displayed two-times lower expression of lipocalin-2 in the hypothalamus. IL-6 and lipocalin-2 were directly related to each other since peripherally administrated IL-6 induced the expression of lipocalin-2 in cells associated with the brain vessels. Lipocalin-2 induced by LPS was expressed by brain endothelial cells and partly co-localized with cyclooxygenase-2, one of the enzymes essential for inflammatory PGE2 production in the endothelial cells. We also demonstrated that lipocalin-2 in a sex-dependent and ambient temperature-specific manner may be implicated in thermogenesis. We have thus identified a new factor in the IL-6 regulated fever pathway, but the pathway is still not understood. On important question that remained to be answered was in which  compartment IL-6 was needed for the signaling. This question was studied further in paper IV, where we investigated the role of hematopoietically produced IL-6 in fever by constructing chimeric mice. We concluded that IL-6 produced by cells of non-hematopoietic origin is critical for the LPSinduced fever while hematopoietically produced IL-6 plays only a minor role in contributing to fever.