Circulating interleukin-6 mediates the febrile response to localised inflammation in rats

Abstract

Interleukin (IL)-6 is an important mediator of the host response to disease and has been proposed, largely based upon circumstantial evidence, as the principal endogenous circulating pyrogen responsible for activating CNS mechanisms in fever during infection and inflammation. In the present investigation, we studied the role of peripheral IL-6 in fever and its relationship with IL-1, itself an important endogenous pyrogen and a potent stimulus of IL-6 production. Injection of lipopolysaccharide (LPS) into a sterile, subcutaneous air pouch (i.po.) in rats evoked an increase in body temperature which peaked at 3 h, and which was abolished in animals pretreated (intraperitoneally) with IL-6 antiserum. The increase in body temperature was accompanied by a significant elevation in concentrations of (immunoreactive) IL-1 and IL-6 at the site of inflammation (pouch), but only IL-6 in the circulation and cerebrospinal fluids. We propose that much of the circulating IL-6 originates at the site of inflammation, since injection of human recombinant (hr)IL-6 (i.po.) was detected (10 min after the injection) in the plasma using an ELISA specific for human IL-6. However, despite the relatively high concentration of IL-6 injected (25 microg kg-1, i.po.), this cytokine had no effect on body temperature when injected alone, but did induce fever when co-injected with a non-pyrogenic dose (when given alone) of IL-1beta, and exacerbated the fever to a pyrogenic dose of IL-1beta. The results from the present study demonstrate that IL-6 is a circulating endogenous pyrogen during LPS-induced fever, which acts in concert with IL-1beta at the local site of inflammation, before entering the circulation. Circulating IL-6 can then activate CNS mechanisms resulting in the development of the febrile response during disease.

Figure 1. Concentrations of immunoreactive interleukin (IL)-1β and IL-6 at the site of inflammation, in the plasma, and in the CSF after intrapouch (i.po.) injection of lipopolysaccharide (LPS)

Kinetics of the appearance of immunoreactive IL-1β (hatched columns) and IL-6 (filled columns) at: the site of injection (air pouch lavage fluid, A), in plasma (B), and in the CSF of rats (C) (n = 5) at various time points after injection of LPS (100 μg kg⁻¹, i.po.). Data are expressed as means ±s.e.m. IL-1β and IL-6 were undetectable in pouch fluid, plasma and CSF of vehicle-injected (saline control) animals (data not shown). *P < 0.05, **P < 0.01, ***P < 0.001 (compared with control animals, Student's t test); horizontal dashed line indicates assay detection limit.

Figure 2. Circulating IL-6 derives from the site of inflammation

Kinetics of immunoreactive human IL-6 in the plasma of rats (n = 5) injected with human recombinant IL-6 (25 μg kg⁻¹, i.po). Data are expressed as means ±s.e.m. Human IL-6 was undetectable in the plasma of vehicle-injected animals (n = 3, data not shown). IL-6 was measured using an immunoassay specific for human IL-6 that did not cross-react with rat IL-6; horizontal dashed line indicates assay detection limit.

Figure 3. Intraperitoneal (i.p.) treatment of rats with sheep anti-IL-6 serum abolishes the febrile response to localised (i.po.) injection of LPS

Injection (at time -4 h) of sheep anti-IL-6 serum (1.8 ml, i.p., □, ▪) or pre-immune serum (NSS, ○, •), followed by injection of LPS (100 μg kg⁻¹, filled symbols) or vehicle (saline, 1 ml kg⁻¹, open symbols) into a subcutaneous air pouch, at time 0 h (arrow). Baseline temperatures of all experimental groups were not significantly different from each other at time 0 h (P > 0.05, ANOVA). The results are presented as time curves (means ±s.e.m.) of core body temperature (°C) for: NSS i.p., vehicle i.po. (○, n = 8); NSS i.p., LPS i.po. (•, n = 9); anti-IL-6 serum i.p., vehicle i.po. (□, n = 5); anti-IL-6 serum i.p., LPS i.po. (▪, n = 10).

Figure 4. IL-6 potentiates IL-1β-evoked fever

Body temperature responses of rats injected (i.po.) with: A, vehicle (○, n = 4), rat recombinant (rr)IL-1β (0.03 μg kg⁻¹, □, n = 4) or rrIL-6 (25 μg kg⁻¹, Δ, n = 4) alone, or in combination (•, n = 5); B, vehicle (○, n = 4), rrIL-1β (0.3 μg kg⁻¹, □, n = 4) or rrIL-6 (25 μg kg⁻¹, Δ, n = 4) alone, or in combination (•, n = 5); and C, vehicle at time 0 h (arrow), followed by vehicle (○, n = 6) or a pyrogenic dose of rrIL-1β (0.3 μg kg⁻¹, •, n = 8) at time 1 h (arrow), and rrIL-6 (25 μg kg⁻¹) at time 0 h (arrow) followed by vehicle (□, n = 6) or a pyrogenic dose of rrIL-1β (0.3 μg kg⁻¹, ▪, n = 8) at time 1 h (arrow).