Studies on cytokines as mediators of fever and sickness

The presence of endotoxin in animals and humans triggers a sequence of acute phase responses, which include the synthesis and release of pro-inflammatory cytokines from immune cells, followed by the development of various symptoms of sickness including fever and an array of behavioural responses, commonly referred to as sickness behaviours. Most experimental investigations examining the mechanisms mediating fever and sickness behaviour responses have used purified lipopolysaccharide (LPS), the glycolipid pyrogenic moiety of the Gram-negative bacterial membrane, to trigger the innate immune system. Results obtained from studies using specific antagonists to block the action of cytokines synthesized following systemic administration of LPS, have uncovered important roles for pro-inflammatory cytokines, such as interleukin (IL)-1b, IL-6, tumour necrosis factor-alpha (TNF-a) and leptin, in mediating fever. Although it has been shown that administration of pro-inflammatory cytokines can induce sickness behaviour in experimental animals, no clear role has been identified for these cytokines as endogenous mediators of sickness behaviours induced following LPS administration. Using rats as experimental animals and endogenous cytokine antagonism, I therefore investigated whether endogenously released IL-1b, IL-6, TNF-a and leptin are physiologically active not only in the generation of fever, but also in the generation of two specific sickness behaviours, lethargy and anorexia, induced by subcutaneous (s.c.) administration of LPS. Lethargy, anorexia and fever were measured as changes in voluntary wheel-running, food intake and body temperature respectively. I antagonized the biological action of these cytokines in the periphery following s.c. administration of LPS by injecting rats intraperitoneally (i.p.) with specific anti-rat sera to one of the following: TNF-a, IL-1b, IL-6 or leptin. Peripherally-released leptin appeared to be an important mediator of both fever and anorexia, as the presence of leptin antibodies in the circulation abolished both the anorexia and fever induced by s.c. administration of LPS. In contrast though, whereas the presence of IL-6 antibodies in the circulation abolished the LPS-induced fever, suppression of voluntary activity was reversed by the presence of IL-6 antibodies only to the extent of 27%, and appetite also was not returned to normal levels in the presence of IL-6 antibodies. Thus, IL-6 may be an essential component of LPS-induced fever, but an additional factor or factors, possibly working in parallel with IL-6, may be required to mediate the lethargy and anorexia induced by s.c. administration of LPS. Injecting rats i.p. with TNF-a antiserum or IL-1b antiserum had no effect on LPS-induced lethargy and LPS-anorexia, indicating that if these cytokines are working with peripherally-released IL-6 to induce sickness behaviour, it is likely due to their synthesis in the brain. Injecting species-homologous rat IL-1β and IL-6 into the brains of conscious rats, I aimed to identify whether either of these two cytokines can act within the brain to induce lethargy and anorexia in the absence of an infection. Intracerebroventricular (i.c.v.) administration of either IL-1β or IL-6 before the night-time active period decreased voluntary activity in the rats in a dose-dependent fashion, whereas only IL-1β decreased food intake and body mass of the rats. It is possible therefore, that increased levels of IL-1β in the brain may be working in parallel with IL-6 released in the periphery to induce lethargy and anorexia following s.c. administration of LPS. Thus I antagonized the biological action of these cytokines endogenously by administering species-specific antiserum to IL-6 (IL-6AS) i.p., and a caspase-1 inhibitor, which prevents the cleavage of pro-IL-1β to biologically active IL-1β, i.c.v. and monitored the symptoms of sickness induced by LPS until they ceased, so as to determine the cytokine involvement not only in the induction of these responses, but also in the resolution of these responses. Pre-treating rats with either IL-6AS i.p. or a caspase-1 inhibitor i.c.v. attenuated the magnitude and the duration of the anorexia and lethargy induced by LPS administration. LPS-induced fever was completely abolished in rats pretreated i.p. with IL-6AS, while it was only partially attenuated in rats pre-treated i.c.v. with a caspase-1 inhibitor. In conclusion, there appears to be some distinct differences in the cytokine-mechanisms regulating fever and sickness behaviours induced by LPS. Identifying the physiological mechanisms mediating fever and sickness behaviours during illness may provide clinicians with more insight into managing not only the thermal, but also the non-thermal responses to infections, responses which may become detrimental to the host if they continue for a prolonged period. My observation that reducing either IL-6 in the circulation or IL-1β in the brain significantly enhances the resolution of anorexia and lethargy, but does not completely prevent them from occurring, appears to indicate that while individual cytokines are possible targets for therapies aimed at alleviating these sickness responses in patients with bacterial infections, to abolish them multiple cytokines may need to be targeted