| Summary: | Peripheral blood monocytes represent the rapid response component of the mononuclear phagocyte host defence. This thesis has examined the fate of highly purified human monocytes following challenge with a range of Gram positive and Gram negative bacterial pathogens. Exposure of monocytes to high bacterial loads resulted in a rapid loss of cell viability and decreased innate responses, including reduced generation of ROS and decreased rates of phagocytosis. Monocyte cell death following exposure to bacteria in many circumstances was due to apoptosis. Exposure of monocytes to high numbers of Escherichia coli and Klebsiella pneumoniae however, resulted in a death process with evidence of caspase-I activation and extracellular trap formation. Reducing the bacterial load with antibiotics converted the death process to apoptosis. Exposure of the monocytes to Neisseria meningitidis showed a delayed death response compared with other bacterial pathogens investigated. Delayed cell death was associated with preservation of intracellular ATP and maintenance of innate anti-microbial function. Delayed induction of cell death however, resulted in prolonged pro-inflammatory cytokine production. Prolonged exposure of monocytes to Neisseria meningitidis eventually induced apoptosis that occurred via the intrinsic (mitochondrial) pathway and was independent of lysosomal mediated apoptosis. Apoptosis was associated with the alternative splicing of certain proapoptotic Bcl-2 proteins and the down regulation of Mcl-1. I also investigated the utility of using differentiated THP-I cells as a model of monocytes and macrophages. Prolonged exposure of differentiated THP-I cells to Neisseria meningitidis resulted in the induction of apoptosis with, in contrast to primary monocytes, relative preservation ofMcl-I, and up-regulation of the pro-apoptotic Mcl-l isoform, Mcl-I Exon-1. Overall this thesis has shown that monocytes are capable of activating a range of cell death programmes and are more closely related to neutrophils with regard to their susceptibility to cell death and the features of cell death they exhibit. The thesis has also shown that failure to engage a death process following exposure to Neisseria meningitidis results in prolonged cytokine release that may have implications in the pathogenesis of meningococcal disease.
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