Modulation of inflammatory cell apoptosis in infection-associated inflammation

Field: Immunology and Microbiology (Life Sciences)

Neutrophils are a central component of the innate immune system, whose major role \nis to defend the host against invading microorganisms. As such they are integral \nplayers in the process of inflammation, the response of vascular tissues to injury. \n \nThey are frequently the first immune cells recruited from the systemic circulation into \na site of tissue injury or infection where they themselves play a key antimicrobial role. \n \nDirect killing of microbes can be accomplished by phagocytosis, degranulation, \nproduction of reactive oxygen species (ROS) or the release of DNA and antimicrobial \npeptides into the extracellular milieu (NETosis). In addition neutrophils orchestrate \nthe recruitment and activation of other leucocytes, further contributing to host \ndefence. The central importance of neutrophils in immunity is revealed by defects in \neither number or function leading to recurrent life threatening infection. \n \nHowever, as \nthe toxic arsenal of neutrophil constituents lack specificity they can also be damaging \nto surrounding host tissues causing exacerbated inflammation. It is therefore essential \nthat neutrophil function is tightly controlled to allow an appropriate response to be \nmounted against invading pathogens while simultaneously minimising host tissue \ninjury. \n \nTherefore, once the inciting inflammatory insult has been successfully cleared or \ncontrolled it is imperative that these non-tissue resident specialised immune cells are \nrapidly ‘switched off’ or cleared to allow the return to homeostasis. This resolution \nphase of the inflammatory cascade is now recognised as an energy dependent, finely \ncontrolled endogenous process, the beginnings of which are activated at the onset of \ninflammation. One of the main aims of resolution is to ensure efficient clearance of \nleucocytes that are no longer necessary. It is likely that a major clearance route is by \nthe highly regulated and energy dependent processes of neutrophil programmed cell \ndeath (apoptosis) with subsequent uptake and disposal of apoptotic neutrophils by \ntissue macrophages. This process of neutrophil apoptosis renders the neutrophils nonfunctional \nand preserves cell membrane integrity, thus preventing further release of \nhistotoxic neutrophil-derived inflammatory mediators into the extracellular \nenvironment. Furthermore, the recognition, uptake and disposal of apoptotic \nneutrophils cause a dynamic change in the phagocytosing macrophage phenotype with \nalterations in inflammatory mediator production. The fundamental importance of \nneutrophil apoptosis and subsequent efferocytosis in inflammation resolution is \nhighlighted by the pathological consequences of neutrophil necrosis or failed \napoptotic cell clearance, which leads to enhanced tissue injury and autoimmunity. \n \nAcute lung infection (pneumonia) is a common and serious condition affecting both \ndeveloped and developing countries; globally, childhood pneumonia is the leading \ncause of death in children aged less than 5 years and pneumonia is the most common \nfatal infection in the developed world. In over half of patients with community \nacquired pneumonia no causative organism is ever isolated suggesting that although \nthe immune response has successfully controlled infection, continued uncontrolled \nneutrophilic inflammation in the lung continues to cause morbidity and mortality. \n \nIndeed, pneumonia frequently progresses to acute respiratory distress syndrome \n(ARDS), a devastating acute inflammatory condition of the lungs characterized by \ninflammatory cell recruitment and accumulation of protein rich oedema fluid leading \nto impaired lung function. ARDS affects 200,000 critically ill patients in the USA per \nyear, and has a substantial mortality rate of up to 40%. Despite advances in intensive \ncare treatment and antimicrobial therapy mortality from pneumonia has not fallen \nsince the 1950s, and at present there are no specific therapies for infection-related \nlung inflammation or ARDS. Understanding the mechanism behind such \nuncontrolled, persisting inflammation, and the need for novel approaches to target \ninfection related lung injury are therefore both urgent and essential. \n \nThis thesis examines the potential of neutrophil apoptosis-inducing pharmacological \nagents as potential treatments for infection-associated lung inflammation. \n \nThe primary \nagents used include a cyclin-dependent kinase inhibitor as well as plant-derived \npolyphenolic flavones. The ability of these compounds to induce human neutrophil \napoptosis in vitro, the key importance of the intracellular neutrophil survival protein \nMcl-1 in mediating this process, and the effect of targeting Mcl-1 in human \nmacrophages is investigated. In addition, neutrophilic inflammation is modelled in \nzebrafish and mice with both sterile and bacterial-driven models of inflammation. A \nkey role for Mcl-1 is delineated in vivo, with it acting as an endogenous controller of \nthe innate immune response by influencing neutrophil apoptosis, but without effects \non macrophage apoptosis or ability to phagocytose apoptotic cells. Driving neutrophil \napoptosis by down-regulation of Mcl-1 accelerates resolution of inflammation in vivo. \n \nThis therapeutic approach is also found to have indirect anti-bacterial effects in a \nmodel of E. Coli induced pneumonia, in stark contrast to established anti-inflammatory \napproaches which routinely cause immune paresis and life threatening \ninfection. As such, targeting inflammatory cell apoptosis by changes in Mcl-1 offers a \npotential new therapeutic approach for the treatment of infection-associated \ninflammation.