Date of Degree
PhD (Doctor of Philosophy)
Allen, Lee-Ann H.
First Committee Member
Barker, Jason H.
Second Committee Member
Horswill, Alexander R.
Third Committee Member
Jones, Bradley D.
Fourth Committee Member
McCarter, Linda L.
Francisella tularensis is a Gram-negative, facultative intracellular bacterium and the etiologic agent of the zoonosis tularemia. Inhalation of as few as 10 organisms can cause a severe pneumonic disease with a mortality rate reported to be around 30-60% in untreated cases. Due to its highly infectious nature, high mortality rate, and ease of aerosolization, the Centers for Disease Control considers F. tularensis a Tier 1 select agent and potential bioweapon. This organism is capable of producing a severe infection as it gains entry into a number of different host cell types and modulates numerous key innate immune responses. It is noteworthy that neutrophils contribute to tissue destruction and disease severity, as exemplified by studies demonstrating that blocking neutrophil recruitment into infected tissues leads to reduced bacterial load and an overall increase in host survival. Therefore, we hypothesized that neutrophil function is dysregulated in the context of tularemia, a result of modification of neutrophil antimicrobial mechanisms by F. tularensis.
Previously, we demonstrated that F. tularensis prolongs human neutrophil lifespan by interfering with the intrinsic, extrinsic, and phagocytosis-induced apoptotic pathways. How this prolongation occurs is incompletely defined; however, our published data suggest secreted or extracellular factors function in this process. The major aims of the studies outline in this thesis were aimed at investigating the effectors produced by F. tularensis that function in the inhibition of human neutrophil apoptosis and the survival signaling within these immune cells that may lead to their prolonged lifespan. Moreover, we examined the related, less virulent strain, F. novicida, to determine the extent to which this organism shares the ability to modulate neutrophil apoptosis like F. tularensis.
Herein, we report that F. tularensis activates multiple survival signaling pathways in neutrophils. In addition, we provide insight into the properties of the extracellular, antiapoptotic factors produced by F. tularensis, and furthermore, describe the identification of Francisella-derived lipoproteins as functional antiapoptotic effectors acting specifically via TLR2/1. Lastly, we report the novel finding that F. novicida manipulates apoptosis and extends neutrophil lifespan by utilizing a similar, yet distinct mechanism as F. tularensis.
The immune system protects the body from infection and consists of a number of different cells work together to detect and kill microbes before they can establish an infection and cause disease. Neutrophils, the most common white blood cell in the bloodstream, recognize and eat infectious microbes and kill them with a number of toxic, antimicrobial compounds. Following destruction of invading microbes, neutrophils die and are subsequently removed by another type of immune cell, macrophages. This process helps keep all of the toxic neutrophil particles contained to prevent host tissue damage and helps the body recover from infection.
Some bacteria prevent neutrophils from doing their job of killing microbes and dying. Then, bacteria survive and multiply inside the neutrophil, and eventually these cells burst and release all of the bacteria and toxic cell material. This progression of infection can lead to damage to host tissues and can worsen infection and disease severity. One type of bacteria, Francisella tularensis, can cause a severe, life-threatening illness known as tularemia. One of the ways it causes severe disease is by preventing crucial neutrophil antimicrobial mechanisms, described above. Francisella avoids killing by neutrophils and prevents these cells from dying so it can replicate within them and hide from recognition by the host immune system.
The overall aim of this research is to determine how Francisella prevents neutrophil cell death by examining the signaling within the neutrophil that conveys the message to live longer, as well as the molecules that Francisella uses to activate this signaling. We demonstrate that this organism triggers multiple survival signaling pathways in neutrophils; moreover, we report that specific proteins found in the outer membrane of Francisella can prolong neutrophil lifespan. There is, however, at least one other survival factor produced by Francisella. Our data provides insight into the mechanism by which this bacterium manipulates the functions of an important immune cell resulting in more severe disease.
xvii, 169 pages
Includes bibliographical references (pages 145-169).
Copyright © 2017 Lauren Cornick Kinkead
Kinkead, Lauren Cornick. "Mechanisms of human neutrophil apoptosis inhibition by Francisella." PhD (Doctor of Philosophy) thesis, University of Iowa, 2017.