The complement system is one of the defence systems used by the host to protect\ud itself against pathogens. It is divided into three complement pathways: the antibodydependent classical pathway, the lectin pathway and the alternative pathway. This latter antibody-independent pathway is auto-activated and leads to the generation and the deposition of C3b molecules on the pathogen’s surface. Properdin, the only\ud positive regulator of this pathway, plays a major role by stabilising the alternative C3\ud convertase that leads to the creation of the amplification loop. Only little was known\ud about the biology of properdin at the beginning of this project, despite the fact that properdin was discovered more than 50 years ago and despite the important role\ud played by this molecule in immunity, as illustrated by the higher susceptibility to\ud severe meningococcal disease encountered by properdin-deficient people. My thesis\ud dealt with three experimental outlines to determine the role of properdin in immunity\ud Properdin was examined from its global expression by various organs to its specific\ud expression by different cell types. I have first shown using molecular biology and\ud bioinformatics tools that properdin was expressed at a relatively high level in different lymphoid organs. I have then examined in more detail the expression of properdin by one of these organs, namely the spleen, using immunofluorescence. Properdin was thus shown to be present only in the white pulp compartment of this organ, where it was organised in clusters of properdin-positive cells possessing long cytoplasmic\ud extensions.\ud Next, I studied the expression of properdin by auxiliary cells. I have first given\ud evidence that properdin was deposited on the surface of platelets and that this level of\ud deposition was related to the activation state of the platelets. I have then shown for the first time that properdin was expressed by two mast cell lines. Microscopic analyses then demonstrated that properdin was present on the mast cell membrane and was present mainly as clusters on membrane extensions similar to vesicles in the process of being released. Further analyses on vesicles released by mast cells confirmed that properdin was enriched in a fraction of vesicles similar in size and in shape to microvesicles.\ud Finally, the role played by properdin during bacterial infection was investigated using\ud a pneumococcal pneumonia model. This study showed that properdin-deficient mice\ud presented a worse level of infection than their wild-type littermates 2 days postinfection. This was associated with an unexpected higher survival rate for properdindeficient animals one week following the challenge. Therefore, in this model, while properdin was seen to be beneficial during the first 48 hours post-infection, by controlling the infection, the absence of properdin led to increased survival following infection. This study thus showed for the first time that properdin could play a bivalent role during infection, the higher inflammatory response engendered by properdin turning from being beneficial to being detrimental to the host over time
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