Macrophage foam cells are the major culprits in atherosclerotic lesions, having a prominent role in both lesion initiation and progression. With atherosclerosis being the main factor underlying cardiovascular complications, there is a long-standing interest on finding ways to reverse lipid buildup in plaques. Studies have shown that promoting reverse cholesterol transport (RCT) from macrophage foam cells is anti-atherogenic because it alleviates the cholesterol burden of the plaques. The goal of this thesis was to gain insight into the mechanisms that govern cholesterol efflux from macrophage foam cells. The first part of this study looked at the ability of different oxysterols to promote cholesterol efflux in unloaded as compared to lipid-loaded macrophages, and our major finding here is that epoxycholesterol decreases efflux in lipid-loaded macrophages. It appears that epoxycholesterol does so by impairing the release cholesterol from its cellular storage site, the lipid droplet (LD), where it accumulates in the form of cholesteryl esters (CE). These results highlighted the importance of cholesterol release from LDs for efflux; indeed, this process is increasingly being recognized as the rate-limiting step for RCT in vivo. Subsequent experiments aimed at elucidating the mechanisms that govern LD CE hydrolysis in macrophage foam cells lead to the discovery of a novel pathway involved in cholesterol efflux. Macrophage CE hydrolysis is classically defined as being entirely dependent on neutral CE hydrolases. In the second part of this study, we demonstrate that in addition to the canonical CE hydrolases, which mediate neutral lipid hydrolysis, lysosomal acid lipase (LAL) also participates in the hydrolysis of cytoplasmic CE. Autophagy is specifically triggered in macrophages by atherogenic lipoproteins and delivers LD CE to LAL in lysosomes, thus generating free cholesterol for efflux. This autophagy-mediated cholesterol efflux is a process that is primarily dependant on the ABCA1 transporter and, importantly, is important for whole-body RCT. Overall, the studies presented in this thesis support that macrophage LD CE hydrolysis is rate-limiting for cholesterol efflux and shed light on the mechanisms of cholesterol mobilization for efflux in macrophage foam cells.

Cell death plays a vital role throughout the immune response, from the onset of inflammation to the elimination of primed T cells. Understanding the regulation of cell death within immune cells is of vital importance to understanding the immune system and developing therapies against various immune-disorders. In this thesis I have investigated the regulation of cell death and its functional role in of the innate and adaptive arms of the immune system. The mechanisms that govern expansion and contraction of antigen stimulated CD8+ T cells are not well understood. In the first section of this thesis, I show that caspase-3 becomes activated in proliferating CD8+ proliferation, yet this does not result in cell death. I used both in vivo and in vitro models to demonstrate that caspase-3 activation is specifically driven by antigen presentation and not inflammation, and that it likely plays a role in promoting T cell proliferation. Next, I present novel data regarding the regulation of a newly identified form of programmed cell death via necrosis, known as necroptosis. I show that the cellular inhibitor of apoptosis (cIAP) proteins act to limit activation of key necroptosis proteins in macrophage cells. Furthermore, I show that necroptosis can be exploited by intracellular bacterial pathogens to escape removal by the immune system. I also demonstrate that necroptosis is highly intertwined with the pathway of inflammation, and the autocrine production of type-I interferon constitutes a vital positive feedback loop in the induction of inflammatory cell death. In the final section of my thesis work, I delve into the specific regulation of Rip1 kinase and demonstrate that in addition to previously demonstrated regulation by caspase-8, cathepsins are also able to cleave Rip1 kinase and limit necroptosis. This thesis presents a wide variety of novel data regarding the regulation of cell death within immune cells. In total, the results reveal a picture of two divergent forms of programmed cell death, apoptosis and necroptosis. Through improving the understanding of the cross-regulation of these two key cell death pathways this work aims to improve the understanding of the immune function.

<p>Essential and diverse biological processes such as cell division, morphogenesis and migration are regulated by a family of molecular switches called Rho GTPases. These proteins cycle between active, GTP-bound states and inactive, GDP-bound state and this cycle is regulated by families of proteins called Rho GEFs and GAPs. GAPs are proteins that stimulate the intrinsic GTPase activity of Rho-family proteins, potentiating the active to inactive transition. GAPs target specific spatiotemporal pools of GTPases by responding to cellular cues and utilizing protein-protein interactions. By dissecting these interactions and pathways, we can infer and then decipher the biological functions of these GAPs.</p><p>This work focuses on the characterization of a novel Rho-family GAP called srGAP2. In this study, we identify that srGAP2 is a Rac-specific GAP that binds a Formin-family member, Formin-like 1 (FMNL1). FMNL1 is activated by Rac and polymerizes, bundles and severs actin filaments. srGAP2 specifically inhibits the actin severing of active FMNL1, and the assembly of an srGAP2-FMNL1 complex is regulated by Rac. Work on FMNL1 shows that it plays important roles in regulating phagocytosis and adhesion in macrophages. To learn more about srGAP2 and its role in regulating FMNL1, we studied macrophages isolated from an srGAP2 KO mouse we have recently generated. This has proven quite fruitful: loss of srGAP2 decreases the ability for macrophages to invade through extracellular matrix but increases phagocytosis. These results suggest that these two processes might be coordinated in vivo by srGAP2 and that srGAP2 might be a critical regulator of the innate immune system.</p> / Dissertation

Elements that are foreign to the human body, such as bacteria, viruses and fungi, are recognised by cells of the innate immune system. Through a process termed phagocytosis, microorganisms are bound, internalised and destroyed. In this thesis, we focus upon how host cells respond to IgG-opsonised targets, studying both the initial stages of Fc-receptor (FcR) ligation and the later stages of phagocytic cup formation. We provide evidence that after clustering of the receptors, the mobility of diacylated probes such as those found in Src-family of kinases, was reduced. This immobilisation was found to be insensitive to cholesterol depletion, arguing against a role for conventional ‘lipid rafts’ in the initiation of receptor signalling. Furthermore, decreased mobility was only partially dependent upon the presence of actin which could provide a physical restriction. Importantly, inhibiting Src-family kinase activity, completely abrogated immobilisation. These results are highly suggestive of a previously unrecognised mechanism for the initation of FcR signalling. At later stages, receptor-derived signalling leads to the formation of an actin-rich phagocytic ‘cup’. We found that even before a large particle was fully internalised, actin cleared from the base of the phagocytic cup. This clearance was necessary for the internalisation of large particles, as chemically stabilising actin prior to clearance, abrogated internalisation. Actin clearance was shown to be the indirect result of the localised disappearance of phosphatidylinositol 4,5-bisphosphate and the dephosphorylation of tyrosine-phosphorylated proteins. Strikingly, phosphatidylinositol 3-kinase activity was required for both the protein dephosphorylation and for the phosphatidylinositol 4,5-bisphosphate hydrolysis that was responsible for actin disassembly. We propose that actin disassembly is required to recycle actin to the advancing pseudopods, in order to complete phagocytosis. For many microorganisms, internalisation through phagocytosis means certain death. Obligate intracellular bacteria, such as Salmonella enterica serovar Typhimurium however, can readily survive inside host cells. This is achieved through modulation of the host-cell signalling pathways that normally lead to microbial destruction. In S. Typhimurium, a needle-like complex, delivers small protein effectors that aid in the survival of the bacterium. We studied one such effector: SigD, that had been suggested to have phosphatidylinositol phosphatase activity. Indeed, we showed that when the cDNA for SigD was exogenously expressed in mammalian cells, phosphatidylinositol 4,5-bisphosphate was depleted and phosphatidylinositol 5-phosphate was formed. We characterised the physiological effects of this 4-phosphatase activity and furthermore, describe the use of SigD as a research tool for modulating host cell phospholipids.

Elements that are foreign to the human body, such as bacteria, viruses and fungi, are recognised by cells of the innate immune system. Through a process termed phagocytosis, microorganisms are bound, internalised and destroyed. In this thesis, we focus upon how host cells respond to IgG-opsonised targets, studying both the initial stages of Fc-receptor (FcR) ligation and the later stages of phagocytic cup formation. We provide evidence that after clustering of the receptors, the mobility of diacylated probes such as those found in Src-family of kinases, was reduced. This immobilisation was found to be insensitive to cholesterol depletion, arguing against a role for conventional ‘lipid rafts’ in the initiation of receptor signalling. Furthermore, decreased mobility was only partially dependent upon the presence of actin which could provide a physical restriction. Importantly, inhibiting Src-family kinase activity, completely abrogated immobilisation. These results are highly suggestive of a previously unrecognised mechanism for the initation of FcR signalling. At later stages, receptor-derived signalling leads to the formation of an actin-rich phagocytic ‘cup’. We found that even before a large particle was fully internalised, actin cleared from the base of the phagocytic cup. This clearance was necessary for the internalisation of large particles, as chemically stabilising actin prior to clearance, abrogated internalisation. Actin clearance was shown to be the indirect result of the localised disappearance of phosphatidylinositol 4,5-bisphosphate and the dephosphorylation of tyrosine-phosphorylated proteins. Strikingly, phosphatidylinositol 3-kinase activity was required for both the protein dephosphorylation and for the phosphatidylinositol 4,5-bisphosphate hydrolysis that was responsible for actin disassembly. We propose that actin disassembly is required to recycle actin to the advancing pseudopods, in order to complete phagocytosis. For many microorganisms, internalisation through phagocytosis means certain death. Obligate intracellular bacteria, such as Salmonella enterica serovar Typhimurium however, can readily survive inside host cells. This is achieved through modulation of the host-cell signalling pathways that normally lead to microbial destruction. In S. Typhimurium, a needle-like complex, delivers small protein effectors that aid in the survival of the bacterium. We studied one such effector: SigD, that had been suggested to have phosphatidylinositol phosphatase activity. Indeed, we showed that when the cDNA for SigD was exogenously expressed in mammalian cells, phosphatidylinositol 4,5-bisphosphate was depleted and phosphatidylinositol 5-phosphate was formed. We characterised the physiological effects of this 4-phosphatase activity and furthermore, describe the use of SigD as a research tool for modulating host cell phospholipids.

The proteoglycan CD44 is a widely expressed cell surface receptor for the extracellular matrix glycosaminoglycan hyaluronan, and is involved in processes ranging from metastasis to wound healing. In the immune system, leukocyte activation induces hyaluronan binding through changes in CD44 post-translational modification, but these changes have not been well characterized. Here I identify chondroitin sulfate addition to CD44 as a negative regulator of hyaluronan binding. Chondroitin sulfate addition was analyzed by sulfate incorporation and Western blotting and determined to occur at serine 180 in human CD44 using site-directed mutagenesis. Mutation of serine 180 increased hyaluronan binding by both a CD44-immunoglobulin fusion protein expressed in HEK293 cells, and full-length CD44 expressed in murine L fibroblast cells. In bone marrow-derived macrophages, hyaluronan binding induced by the inflammatory cytokines tumor necrosis factor-α and interferon-γ corresponded with reduced chondroitin sulfate addition to CD44. Retroviral infection of CD44⁻/⁻ macrophages with mouse CD44 containing a mutation at serine 183, equivalent to serine 180 in human CD44, resulted in hyaluronan binding that was constitutively high and no longer enhanced by stimulation. These results demonstrate that hyaluronan binding by CD44 is regulated by chondroitin sulfate addition in macrophages. A functional consequence of altered chondroitin sulfate addition and increased hyaluronan binding was observed in Jurkat T cells, which became more susceptible to activation-induced cell death when transfected with mutant CD44. The extent of cell death was dependent upon both the hyaluronan binding ability of CD44 and the size of hyaluronan itself, with high molecular mass hyaluronan having a greater effect than intermediate or low molecular mass hyaluronan. The addition of hyaluronan to pre-activated Jurkat T cells induced rapid cell death independently of Fas and caspase activation, identifying a unique Fas-independent mechanism for inducing cell death in activated cells. Results were comparable in splenic T cells, where high hyaluronan binding correlated with increased phosphatidylserine exposure, and hyaluronan-dependent cell death occurred in a population of restimulated cells in the absence of Fas-dependent cell death. Together these results reveal a novel mechanism for regulating hyaluronan binding and demonstrate that altered chondroitin sulfate addition can affect CD44 function.

Resident tissue macrophages are an integral component of many tissues and are important in development, homeostasis and repair. Macrophages are present at sites of both pathologic bone deposition and loss, and can produce osteo-active factors. These observations link macrophages to bone disease, however their contribution to bone dynamics is poorly understood. The molecular and cellular mechanisms driving osteoblast differentiation, matrix deposition and mineralization in vivo are incompletely understood and this deficiency is translated to limited ability to clinically manipulate bone formation. The emerging understanding of the bi-directional interactions between the osseus and immune systems (osteoimmunology) provides a novel avenue to identify mechanisms involved in the regulation of bone formation. In this study, the presence and distribution of macrophages on bone surfaces was systematically analysed and their functional contribution to the bone microenvironment was investigated. Using immunohistochemistry a discrete population of mature resident tissue macrophages was demonstrated throughout resting murine osteal tissues, termed OsteoMacs. Utilising MacGreen mice (csf1r promoter drives eGFP transgene expression in macrophages and other myeloid cells), it was demonstrated that OsteoMacs were intercalated amongst other bone lining cells in both the endosteum and periosteum. OsteoMacs were TRAPneg in situ and had limited osteoclastogenic ability in vitro therefore they are unlikely to serve as the immediate physiologic osteoclast precursors in vivo. Microarray gene expression profiling demonstrated that macrophage gene expression was regulated in response to a characteristic feature of the bone microenvironment, elevated extracellular calcium. Quantitative PCR validated upregulation of sphingosine kinase 1, interleukin 1 receptor antagonise, progressive ankylosis, vascular endothelial growth factor c and dipepetidase 2 mRNA in response to elevated extracellular calcium, suggesting the potential roles of these genes in this unique niche. GNF Symatlas microarray and quantitative PCR demonstrated the expression of macrophage-restricted genes throughout a 21-day primary osteoblast differentiation time course, suggesting co-isolation of OsteoMacs with primary osteoblasts. Flow cytometry analysis confirmed that over all 15.9% of the digested primary calvarial cell preparations were OsteoMacs. Immunocytochemistry demonstrated that OsteoMacs persisted and expanded in standard 21-day osteoblast differentiation assays. Contrary to previous studies, we demonstrated it was the OsteoMacs, and not osteoblasts, within calvarial preparations that selectively detected patho-physiological concentrations of the bacterial product lipopolysaccharide (LPS). A protocol was developed to deplete OsteoMacs from calvarial digests to determine if their presence within these cultures facilitates osteoblast differentiation or function. OsteoMac removal did not affect expression of the early osteoblast differentiation marker genes collagen type I or alkaline phosphatase. However, OsteoMac removal significantly decreased gene expression of the osteoblast mineralisation marker osteocalcin and mineralisation function, assessed by von Kossa staining. Microarray gene expression profiling demonstrated that osteoblast enrichment had a broad impact on transcription within the culture, identifying both candidate OsteoMac marker genes as well as osteoblast expressed genes that are regulated by OsteoMacs. Potential OsteoMac-enriched candidate genes insulin-like growth factor a, dipepetidase 2, glycoprotein NMB, and macrophage expressed gene 1 as well as osteoblast-specific genes bone sialoprotein and thrombospondin 1 were selected based on their potential involvement in osteoblast function. In a transwell co-culture system of enriched osteoblasts and macrophages, it was demonstrated that macrophages were required for osteoblast mineralisation in response to the physiologic remodelling stimulus, elevated extracellular calcium. A blocking soluble receptor strategy provided evidence that this is mediated in a BMP-2 and -4 independent manner. To investigate the relevance of OsteoMacs to bone formation in vivo, immunohistochemistry staining for the mature tissue macrophage marker F4/80 was performed in long bone sections from rapidly growing mice. OsteoMacs were closely associated with areas of bone formation in situ, forming a distinctive canopy structure over mature cuboidal osteoblasts (collagen type I+, osteocalcin+) on endosteal cortical surfaces. Using adapted histomorphometic analysis, we determined that 77 ± 2.1% (n = 7) of the endosteal mature osteoblast surface was covered by the F4/80+ OsteoMac canopy. This observation suggested that OsteoMacs are optimally located to regulate osteoblast function in vivo. In summary, we have demonstrated that OsteoMacs are an integral component of bone lining tissues and play a novel role in bone dynamics through regulating osteoblast function. These observations implicate OsteoMacs, in addition to osteoclasts and osteoblasts, as principal participants in bone dynamics. Further delineation of OsteoMac functions is likely to provide new avenues for treating bone disease and assisting bone repair.

Macrophages are involved in many aspects of both the innate and acquired immunity, and participate in tissue homeostasis, bone remodelling, wound healing, and tissue repair. When their function becomes dysregulated, they contribute to the initiation and progression of inflammatory diseases. Genes expressed in activated macrophages are likely to play an important role in inflammation and/or immunity. One family of genes that is highly expressed in activated macrophages is the Schlafen (Slfn) gene family. Given that very little is known about the function of this family, particularly in macrophages, this study focused on the regulation and function of one family member, namely Slfn-4. The transcriptional regulation of Slfn-4 was characterised in murine macrophages. The expression of Slfn-4 was transiently down regulated during macrophage differentiation and dramatically up regulated in response to activation signals including lipopolysaccharide (LPS) and dsRNA Poly(I:C). A potential association with inflammation was further suggested by the enhanced expression of Slfn-4 in a mouse model of rheumatoid arthritis. Further investigations into transcriptional regulation of Slfn-4 revealed that it belongs to the subset of genes that are type I interferon (IFN)-inducible. This finding is consistent with the predicted transcription factor binding sites in the putative promoter of Slfn-4, and suggests a role for Slfn-4 in the antiviral response. To gain further insight into the function of Slfn-4 in macrophage biology, in vitro over-expression approaches were undertaken and its cellular localisation in macrophages was characterised. In resting and activated macrophages, Slfn-4 exhibited a cytoplasmic and strong perinuclear localisation. Additional studies were carried out to investigate the in vivo biology of Slfn-4 in macrophages. For this purpose, the Csf1r-GAL4VP16/UAS-ECFP (referred to as “MacBlue”) transgenic mouse line was first characterised. Enhanced cyan fluorescent (ECFP) reporter expression in the MacBlue transgenic mice was specifically detected in cells of the mononuclear phagocyte system during embryonic development and adulthood. The MacBlue transgenic mouse line was next used to drive expression of Slfn-4 in cells of the myeloid lineage in vivo, and to examine the phenotype of this line. Specific over-expression of Slfn-4 in cells of the myeloid lineage in vivo altered the percentage of peripheral blood monocytes and caused extramedullary hematopoiesis. In summary, this thesis demonstrated that Slfn-4 expression is dynamically regulated during macrophage differentiation and activation, and the enforced Slfn-4 over-expression in cells of the myeloid lineage perturbs normal monocyte/macrophage development.

The immune system is comprised of specific cell types that communicate and interact via a range of soluble and surface-bound molecules to defend the body against pathogens. Many gaps remain in our understanding of the subcellular trafficking pathways that regulate the diverse functions of the immune system. The central aim of this thesis was to investigate transport through the endocytic pathway, focussing in particular on the unique organelles and functions of this pathway in immune cells. Two subsets of immune cells were of particular interest in this thesis, macrophages and natural killer (NK) cells. These cell types both perform a range of functions that contribute to both innate and adaptive immunity. Another common thread between these cells is that they both perform functions involving specialised endocytic organelles and pathways. Macrophages utilise their endocytic pathways to perform several unique functions; phagocytosis, endocytosis and degradation of foreign proteins for presentation on MHC class II molecules, and signalling of Toll-like receptors from endosomes. Even secretion of cytokines such as tumour necrosis factor alpha (TNFα) by macrophages requires transport through an endosomal compartment, the recycling endosome, as recently discovered in this laboratory (Murray et al., 2005a). NK cells utilise specialised secretory lysosomes to deliver a lethal hit to carefully identified target cells, providing an alternative example of specialised endocytic trafficking in the immune system. Of the many protein families that regulate subcellular trafficking, the SNARE, Rab, Munc and exocyst proteins were focussed on during this thesis. The localisation and function of members of these families in the endocytic pathway were investigated. Novel results in macrophages concerned the role of Vti1b in endocytosis, a process with implications for MHC class II antigen presentation and TLR detection of endocytosed particles. Alteration of Vti1b protein levels in the cells significantly decreased uptake and degradation of endocytic cargo. A role for Rab11 and the recycling endosome in antigen presentation was also studied. MHC class II was detected in recycling endosomes, and overexpression of a mutant Rab11 protein altered the distribution of MHC class II, suggesting a role for Rab11 in subcellular trafficking during antigen presentation. Preliminary results also suggest a role for the exocyst protein Sec15 at the recycling endosome in macrophages, providing a new target for investigation of the regulation of TNFα secretion. The recycling endosome is emerging as a vital transport hub during cytokine secretion, phagocytosis and possibly other cellular functions in macrophages. This project also involved the unique opportunity to examine primary NK cells from patients with a number of genetic immunodeficiencies caused by mutations to trafficking proteins. The autosomal recessive immunodeficiencies Griscelli syndrome type 2 (GS2) and familial haemophagocytic lymphohistiocytosis types 3 (FHL3) and 4 (FHL4) are associated with loss-of-function of Rab27a, Munc13-4 and syntaxin 11 (Stx11), respectively. These diseases involve a loss of cytotoxic function by cytotoxic CD8+ T lymphocytes and NK cells, but the precise molecular role of these proteins in granule release is incompletely understood. In freshly isolated, resting NK cells from healthy subjects, PMA and ionomycin stimulation or conjugation to susceptible target cells induced colocalisation of endogenous Rab27a and Munc13-4 to perforin-containing granules. In Rab27a-deficient cells, which showed defective degranulation and cytotoxicity induced by signals for both natural and antibody-dependent cellular cytotoxicity, Munc13-4 failed to colocalise with perforin upon activation. Unexpectedly, Rab27a and Munc13-4 localisation to lytic granules was selectively induced by different receptor signals, demonstrating specificity for regulation of lytic granule maturation by target cell ligand expression. Recruitment of the SNARE protein Vti1b, which has not previously been associated with NK cell function or secretory lysosome release, to perforin granules was also discovered. Unexpectedly, Stx11 was not localised to perforin granules. These experiments have contributed to our understanding of the precise molecular roles of Munc13-4, Rab27a and Stx11 in NK cell granule release. Overall, this thesis presents novel and important results from studies of subcellular transport through the endocytic pathways of macrophages and NK cells. These results advance our understanding of several immune functions, and a number of human genetic immunodeficiencies. This new knowledge of the role of endocytic organelles and fusion machinery in these processes provides exciting targets for future research.

Amendments to thesis in pocket on back cover. / Copy of author's previously published article in pocket on back cover. / Bibliography: leaves 124-172. / viii, 172, [101] leaves, [22] leaves of plates : ill. (chiefly col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Studies the biological effects and leukaemic potential of h[beta]c mutants using murine models. / Thesis (Ph.D.)--University of Adelaide, Dept. of Medicine, 1999?