The role of teleost neutrophils in the regulation of inflammation

• Author / Creator

  Havixbeck, Jeffrey J

• The inflammatory response is a complex biological process initiated following the recognition of noxious stimuli. It is composed of an elaborate cascade of both pro- and anti-inflammatory mediators, essential for effective defenses against infection, the removal of damaged cells, and the initiation of tissue repair processes. At the core of these responses is phagocytosis, an evolutionarily conserved mechanism critical to immune defense and the maintenance of homeostasis. Phagocytosis is initiated through the recognition of large particles (>1μM) by receptors on the cell surface, leading to actin polymerization, particle internalization, and a diverse array of downstream responses. Neutrophils are essential effector cells that form the first line of defense against invading pathogens. The main objective of my thesis research was to characterize the functional roles of teleost neutrophils, where various mechanisms are involved in the induction of pro-inflammatory responses, the regulation of inflammation, and neutrophil-driven mechanisms of resolution. Similar to mammalian phagocytes, I found that goldfish and lamprey phagocytes contribute to both pro-inflammatory and pro-resolving responses ex vivo. However, I found significant differences with regards to the level of responsiveness to zymosan and apoptotic cells. Interestingly, goldfish phagocytes displayed a reduced sensitivity to apoptotic cells, instead displaying a greater induction of antimicrobial respiratory burst responses during co-stimulation. Moreover, teleost phagocytes remained as central contributors to the resolution phase of inflammation, even though they showcased an improved ability to induce strong antimicrobial inflammatory responses. When I began focusing on neutrophils, I found that the number of circulating neutrophils in teleost fish was far below their mammalian counterparts. Despite this, many other aspects of neutrophil biology and regulation remained the same. Within the hematopoietic tissue, I found that teleost neutrophils exist in a large storage pool, from which they rapidly exit, enter into circulation, and infiltrate the inflammatory site when called upon. It has previously been shown that both mammalian and teleost macrophages exhibit extensive roles in inflammatory control. Interestingly, I demonstrated that neutrophils alter their phenotype throughout the acute inflammatory response, and contribute to both the induction and the resolution of inflammation. Neutrophils isolated during the pro-inflammatory phase displayed elevated ROS responses and released inflammation-associated leukotriene B4. In contrast, neutrophils isolated during pro-resolution displayed low levels of ROS and released lipoxin A4. Notably, eicosanoid release by neutrophils played a role, at least in part, in regulating divergent macrophage responses, including inducing their uptake of apoptotic cells. These observations were then applied to study the teleost inflammatory response following infection with the natural fish pathogens, Aeromonas veronii or Mycobacterium fortuitum. Both bacteria have been associated with significant outbreaks in aquaculture, often leading to mass die offs. I found that similar to the zymosan model, neutrophils rapidly migrated to the site of infection. In addition, both pathogens stimulated the production of ROS in teleost neutrophils. However, neutrophils were only capable of significantly killing A. veronii, unlike M. fortuitum, which required the total population of leukocytes for significant elimination. However, most interestingly, was the capacity of neutrophils to internalize dying macrophages previously incubated with A. veronii, despite remaining unable to internalize other forms of apoptotic cells. Notably, following the uptake of dying macrophages, neutrophils remained viable and increased their production of ROS, remaining pro-inflammatory in nature. Overall, the examination of the teleost inflammatory response mounted against pathogen mimics and natural bacterial infections provides additional insights into the complex mechanisms by which neutrophils operate within an inflammatory site and contribute to the induction and regulation of acute inflammatory responses.

• Subjects / Keywords

  • Neutrophils
  • Infections
  • Inflammation
  • Lipid mediators
  • Immunity

• Graduation date

  Fall 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3CN6ZD1J

• License

  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.