Modulatory Effects of Behavioural Fever on the Inflammatory Reactions of Teleost Fish

  • Author / Creator
    Wong, Michael E.
  • Since antiquity, fever has been documented as a physiological response to infection characterized by an increase in body temperature. Though fever is commonly regarded as a deleterious symptom of inflammation and is often suppressed using NSAID drugs, little is known about the effects that fever has on immunity due to the difficulty of experimentally blocking fever in traditional endothermic models. Interestingly, ectothermic animals have been seen to undergo a homologous behavioural fever where they exhibit thermoregulatory behaviours to increase body temperature during infection and has been observed in a vast range of distant species including reptiles, amphibians, fish, insects and crustaceans. When combined with traditional metabolic fever the conservation of febrile responses spans over 800 million years of evolution, illustrating the utility of studying fever from a comparative approach using ectothermic models. Many previous studies have displayed strikingly improved pathogen clearance and survival rates in these febrile animals, but few have dissected the mechanisms by which this is achieved. Previous studies using teleost models have shown that behavioural fever is capable of upregulating pro-inflammatory and antiviral gene expression, but none have assessed the impact of behavioural fever on functional immune responses. To address this gap in knowledge, I used a model of zymosan-induced peritonitis in goldfish to study the effect of behavioural fever on functional inflammatory responses. I found that zymosan injection promoted a behavioural fever consisting of a small increase in temperature preference but drastic decreases in velocity and temperature seeking behaviour, illustrating the use of other ‘sickness behaviours’ to define febrile responses. I found that when compared to fish held at normal 16°C housing temperature, the ability to thermoregulate up to 26°C promoted a rapid early influx of leukocytes, and particularly neutrophils, to the inflammatory site. This early cellular response may play an important role in controlling infections by engaging pathogens early before they can replicate to high levels. In addition, I found that behavioural fever caused the rapid reduction of peritoneal cells to homeostatic levels, but with elevated lymphocyte recruitment two-fold higher than controls. This places fever as an inducer and regulator of inflammation and also a possible regulator of downstream adaptive memory responses. When assessing the antimicrobial responses of inflammatory leukocytes, I found that behavioural fever prevents the high production of reactive oxygen species (ROS), but promotes the induction of inducible nitric oxide responses 24 h earlier than controls. This may play a role in promoting the elimination of pathogens while preventing oxidative tissue damage caused by extracellularly released ROS. In addition, while I found that behavioural fever does not inherently affect leukocytes phagocytic ability, increased ex vivo temperature was able to accelerate target internalization in cells derived from both febrile and non-febrile fish, indicating that febrile body temperature likely potentiates phagocytic responses in vivo. Interestingly, I found that manually increasing temperature did not replicate immune efficiencies seen in febrile fish, indicating that the regulation of temperature by each individual animal is critical to inducing febrile immunomodulatory effects. Using a live Aeromonas veronii furunculation infection model that mimics naturally occurring teleost infections, I found that thermoregulation promotes the rapid clearance of Aeromonas by 7 days (50%) earlier than controls, despite increasing growth rate of A. veronii cultures at higher temperatures, indicating that behavioural thermoregulation must modulate host immunity to overcome increased microbial replication rates. Behavioural thermoregulation promoted early iNOS expression, indicating that similar immune-modulatory mechanics were likely taking place within the live infection model. Behavioural thermoregulation also greatly reduced the pathology of induced furuncles and promoted rapid healing of infected lesions. This was associated with early expression of pro-resolution and healing factors IL-10 and VEG-F, indicating that thermoregulation also actively acts to protect host tissues and potentiate healing responses. Overall, my data shows that behavioural thermoregulation acts as a potent regulator of teleost inflammatory responses that drives the efficient induction of immune antimicrobial responses that also prevents host tissue damage while promoting tissue repair and an effective return to homeostasis. The ability to control body temperature appears to have evolved as a ‘sword and shield’ mechanism critical to the efficient clearance of a highly virulent pathogen but also acts to protect against host tissue damage while promoting wound healing to rapidly reach a state of functional tissue homeostasis.

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  • Degree
    Master of Science
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    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.