Circulating and ex vivo production of pyrogenic cytokines and interleukin-1 receptor antagonist in 123 patients with fever of unknown origin. The Netherlands Fever of Unknown Origin Study Group

Circulating cytokines as mediators of fever

The febrile response is thought to be mediated by endogenous mediators, generically called "endogenous pyrogens." In the classical model of pathogenesis, induction of fever is mediated by the release of pyrogenic cytokines such as tumor necrosis factor (TNF), interleukin (IL)-1, IL-6, and interferons into the bloodstream in response to exogenous pyrogens. These mediators act at the level of the organum vasculosum of the lamina terminalis in the central nervous system (CNS), inducing synthesis of prostaglandins, which are the central mediators of the coordinated responses leading to fever. However, analysis of recent data suggests that multiple pathways may be involved in the induction of fever by cytokines, such as local cytokine production leading to signaling through vagal fibers, release of cytokine-induced circulating mediators at the tissue level, the use of membrane-bound cytokines as mediators, or the local release of cytokines in the hypothalamus by circulating activated monocytes. In addition, certain bacterial products can stimulate cytokine production directly at the level of hypothalamus, probably by activation of Toll-like receptors. A multipathway mechanism for the induction of fever is therefore suggested.

Interleukin-1beta sensitizes abdominal visceral afferents of cats to ischaemia and histamine

1. Activation of abdominal splanchnic visceral afferents during mesenteric ischaemia induces visceral pain and evokes excitatory cardiovascular responses. Previous studies have shown that interleukin-1beta (IL-1beta) concentration is increased locally in tissues during ischaemia and reperfusion. Local administration of IL-1beta sensitizes somatic afferents to mechanical, thermal and chemical stimulation. Therefore, we hypothesized that IL-1beta stimulates or sensitizes splanchnic visceral afferents to ischaemia and to the action of chemical stimuli such as histamine. 2. The concentration of IL-1beta in mesenteric lymph and portal venous plasma in anaesthetized cats was measured with an enzyme-linked immunosorbent assay before, during and after 10 min of abdominal ischaemia. The level of IL-1beta was significantly increased during ischaemia in lymph, but not in plasma. 3. Discharge activity of single-unit abdominal visceral C fibre afferents was measured from the right thoracic sympathetic chain. Ischaemically sensitive C fibre afferents were identified according to their response to 5-10 min of abdominal ischaemia. 4. Intra-arterial (i.a.) injection of a high dose of IL-1beta (500 ng kg-1), but not of a lower dose (i.e. 15, 50 or 150 ng kg-1), stimulated most (six of seven) abdominal visceral afferents. 5. IL-1beta (15 ng kg-1, i.a.) significantly enhanced the increased activity of 11 of 13 C fibre afferents during 10 min of ischaemia. Conversely, an IL-1 type I receptor antagonist (IL-1ra, 1.5 microg kg-1, i.a.) significantly attenuated the increased activity in six of seven other C fibre afferents during ischaemia. 6. IL-1beta (15 ng kg-1, i.a.) significantly augmented the responses of 13 of 16 ischaemically sensitive abdominal afferents to histamine (5-10 microg kg-1, i.a.). Conversely, IL-1ra (1.5 microg kg-1, i.a.) significantly attenuated the responses of five of six other C fibre afferents to histamine. 7. These data strongly suggest that stimulation of IL-1 type I receptors by IL-1beta produced during brief abdominal ischaemia contributes to activation of visceral afferents during ischaemia, at least in part, by sensitizing the afferent nerve endings to ischaemia. Our data also show that exogenous IL-1beta sensitizes visceral afferents to histamine.

Do only circulating pyrogenic cytokines act as mediators in the febrile response? A hypothesis

The classical model of the pathogenesis of fever suggests that pyrogenic cytokines, produced by leucocytes in the bloodstream in response to exogenous pyrogens, represent the distal mediators of the febrile response. They are recognized at the level of the organum vasculosum of the lamina terminalis in the central nervous system, where they induce synthesis of prostaglandins representing the central mediators of the coordinated responses leading to fever. This classical model is challenged by studies showing inconsistencies between the febrile response and the cytokine pattern, as well as by data demonstrating paradoxical hyperfebrile reactions in knock-out mice lacking cytokines or cytokine receptors. Moreover, no measurable cytokine concentrations are to be found in a variety of specific patients groups with febrile conditions. There are recent data in the literature suggesting that alternative pathways may be involved in the induction of fever, ranging from the use of vagal fibres to transmit the signals leading to fever, to local production of cytokines at the level of the hypothalamus, or the use of membrane-bound cytokines as mediators. A multipathway mechanism for the induction of fever is suggested.

Interleukin-1 receptor antagonist: role in biology

The interleukin-1 receptor antagonist (IL-1Ra) is a member of the IL-1 family that binds to IL-1 receptors but does not induce any intracellular response. Two structural variants of IL-1Ra have previously been described: a 17-kDa form that is secreted from monocytes, macrophages, neutrophils, and other cells (sIL-1Ra) and an 18-kDa form that remains in the cytoplasm of keratinocytes and other epithelial cells, monocytes, and fibroblasts (icIL-1Ra). An additional 16-kDa intracellular isoform of IL-1Ra has recently been described in neutrophils, monocytes, and hepatic cells. Both of the major isoforms of IL-1Ra are transcribed from the same gene through the use of alternative first exons. The two promoters regulating transcription of the secreted and intracellular forms have been cloned, and some of the functional cis-acting DNA regions have been characterized. The production of IL-1Ra is stimulated by many substances including adherent IgG, other cytokines, and bacterial or viral components. The tissue distribution of IL-1Ra in mice indicates that sIL-1Ra is found predominantly in peripheral blood cells, lungs, spleen, and liver, while icIL-1Ra is found in large amounts in skin. Studies in transgenic and knockout mice indicate that IL-1Ra is important in host defense against endotoxin-induced injury. IL-1Ra is produced by hepatic cells with the characteristics of an acute phase protein. Endogenous IL-1Ra is produced in numerous experimental animal models of disease as well as in human autoimmune and chronic inflammatory diseases. The use of neutralizing anti-IL-1Ra antibodies has demonstrated that endogenous IL-1Ra is an important natural antiinflammatory protein in arthritis, colitis, and granulomatous pulmonary disease. Treatment of human diseases with recombinant human IL-1Ra showed an absence of benefit in sepsis syndrome. However, patients with rheumatoid arthritis treated with IL-1Ra for six months exhibited improvements in clinical parameters and in radiographic evidence of joint damage.