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Monocytes, macrophages, and dendritic cells are a family of cells collectively referred to as the ‘mononuclear phagocyte system’ (MPS) that mediates and regulates inflammation. Cells of the MPS scavenge dead cells, toxic molecules, and kill or wall off infectious agents. They are a critical mediator of the inflammatory response, which is a powerful mechanism to control proliferation of microorganisms, but that also –unfortunately- damages host tissues and contribute in the long term to many features of ageing and to cardiovascular and neurological diseases that decrease quality of life and shorten our lifespan. However, deficient scavenging of dead cells and bacteria is also associated with inflammation and autoimmune diseases such as lupus, and may impair the defence against pathogens.

Members of the Geissmann lab Development and Functions of Mononuclear Phagocytes focus their efforts on understanding the molecular and cellular basis for the functional heterogeneity of the mononuclear phagocyte system in vivo. We have described the common precursor for macrophage, monocytes and dendritic cells (Fogg et al., Science 2006; Auffray et al., JEM 2009), discovered specialized population of monocytes (Geissmann et al., Immunity 2003, Auffray et al., Science 2007, Auffray et al., Annu. Rev. Immunol. 2009), and investigated the pathophysiology of diseases of this cellular system, incliding Langerhans cell histiocytosis (Senechal et al., Plos Meddicine 2007). To study the development and functions of phagocytes in vivo, in animal models and in man, we use fate mapping strategies and intravital imaging in mouse models in vivo, high-throughput or multiplex analysis of gene and protein expression ex-vivo from human cells purified by flow cytometry, and we develop new models for the study of the genetic control of phagocytes development and functions, such as the fruit fly Drosphila melanogaster. We plan to build on hypotheses and results generated using mouse and drosophila models to identify candidate genes responsible for human inflammatory diseases, to model and test (ex vivo) the functions of human monocytes and their roles in diseases, and -in collaboration with clinicians- to develop prospective cohorts to test biomarkers, diagnostic tools and therapeutic strategies.

In the next years we have 3 specific objectives:

1- Investigate the development and functions of monocytes and dendritic cells, and their roles in inflammatory diseases.
1.1- Characterize the functions of ‘patrolling’ monocytes in mouse and human In this part of our work, we aim i) to characterize the molecular mechanisms that control the adherence to endothelium and the crawling behavior of monocytes that ‘patrol’ the blood vessels in mice, ii) to understand their functions in steady state, during vascular inflammation, and in models of atherosclerosis, lupus nephritis, and arthritis. iii) to characterize their homologs in human, and their roles in diseases. These experiments will involve techniques and protocols for intravital imaging in normal and mutants mice that have set up in the laboratory, and new methods that we are developing. We are using new reporter mice to follow the inflammatory response in vivo, (such as the production of iNOs) in blood vessels, skin, joints and kidney. We will try to to use in vivo FRET to monitor integrin activation in vivo and in real time, in collaboration with the Randall division of molecular biophysics.

1.2- Investigate the development and functions of ‘resident’ networks of macrophages and Dcs such as langerhans cells and microglia, and their potential roles in inflammation. We are studying the cellular origin and the mechanisms that control the establishments and maintainance of these networks of ‘resident’ cell in skin and brain, i) whether they are renewed from bone marrow or ‘niche’ precursors, or if mature cells divide and self renew, ii) whether similar or separate mechanisms control their accumulation and activation during inflammation, and iii) their role in skin and brain inflammation.

2- Drosophila melanogaster as a model for the cellular immunology of phagocytes, and a tool for genome wide quantitative screens for the control of cytokine production by phagocytes. In close collaboration with Marc Dionne’s Lab in the CMCBI we aim to combine Drosophila melanogaster well-known potential for high-throughput genetic screening, its more recently discovered strengths for reverse genetics and intravital imaging experiments, and the cellular and molecular similarities that exist between the fly immune cells (hemocytes) and vertebrate monocytes/macrophages, to establish Dm as a model to investigate the cellular immunology of phagocytes. We are developing a genome wide RNAi screen for genes that control the production of inflammatory cytokines by hemocytes during wounding and infection. We will focus on fly cytokines homologous to TNFa, IL1 and IL6. This screen will use a cutting edge imaging technology in order to investigate gene expression quantitatively, by individual hemocytes, in living animals, its variation over time, the number of cells involved, and associated phenotypes (including recruitment/migration of cytokine producing cells). We anticipate that this screen will identify new genes and pathways involved in the control of inflammation that we will investigate in mouse and human. This work will also provide us with new tools and a better knowledge of the cellular immunology of Drosophila .

3- Improving diagnosis, prognosis, and treatment of histiocytoses by identifying genetic mutations associated with these diseases. The “histiocytoses” represent a group of diseases that include Langerhans cell histiocytosis (LCH), juvenile xanthogranuloma (JXG), Rosai-Dorfman disease (RD), Multicentric reticulohistiocytosis (MR). These conditions features inflammatory granuloma rich in macrophages / dendritic cells in soft tissues, mainly the skin, but also bones, joints, lung, liver, and lymphoid organs. Although clonality of lesional macrophages has been proposed, little is currently known about the underlying molecular pathology. This can lead to delay or errors in diagnosis and difficulties in determining optimal treatments and predicting prognosis. To address these deficiencies, in collaboration with a national clinical network in France (Dr Jean Donadieu) we have begun to collect patient/family details and samples. Based on our preliminary results, we postulate that histiocytosis lesions may result from an inappropriate response to infectious or mechanic stimuli in individuals with genetically susceptibility, including germline mutations. Our project cover: i) The establishment of a network of patients with histiocytoses in the UK, in collaboration with Prof John McGrath, at the St John’s Institute of Dermatology at the Guy’s and St Thomas’ NHS Foundation Trust, ii) the identification of genetic polymorphisms associated with histiocytoses using génome wide association studies, linkage studies, and candidate gene analysis, and assessment of their relevance to diagnostic and clinical parameters/disease characteristics, and iii) to test the biological relevance of genetic polymorphisms/mutations to the pathophysiology of the diseases and their possible link to drug targets, e.g. by lentiviral transduction of relevant mutant molecules in myeloid cells.