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For the past 20 years, the microcirculation has been regarded as cornerstone in the development of organ failure in critically ill patients. Eventually, the microcirculation became a therapeutic target. Due to the complexity of the microarchitecture of this functional system, varying across organs, one therapy cannot “fit all”. The alterations observed in the critically injured microcirculation involve: (i) the container defined by the different layers of the vascular wall including the endothelial cells and a protective gel called the glycocalyx spread on the surface, where contact with blood is made, (ii) the contents representing the flowing plasma and the different elements of blood and (iii) the extraluminal surrounding tissue. The microcirculation can be injured in various ways, with different levels of injury to these constitutive elements. Thus, to appropriately resuscitate the injured microcirculation, the choice of the optimal therapy or bundle of therapies should be rationalized with a meticulous analysis of the damages suffered by the microcirculation. The evaluation of the microcirculation should be multivariate. In this thesis, the research was mainly focused on the kidney. The first part is dedicated to the review of the structural and functional mechanisms of the renal microcirculation in both healthy and septic states. The second part tries to identify the respective roles of each of the components of the microcirculation in critical conditions especially the glycocalyx and plasma viscosity. The vascular barrier permeability was investigated in hemorrhagic shock and hemodilution models in rodents. The main findings suggest that a gradation in the level of injury to the vascular barrier permeability exist.The last part of the thesis investigated how current and older therapies can modulate microcirculation in terms of oxygenation, inflammation and microcirculatory flow within the kidney. Among therapies investigated, N-acetylcysteine was efficient at limiting inflammation and increasing oxygenation within the kidney. A new generation of hemoglobin-based oxygen carrier showed some efficacy in murine endotoxemic model. Overall, these different findings coalesce to show the importance of having a multivariate analysis of the microcirculation, as each of the therapies acts on a specific aspect of it. Hopefully, this research helped pave the way for a more personalized medicine for the patients., Au cours des 20 dernières années, la microcirculation a été considérée comme la pierre angulaire du développement de la défaillance d’organe chez les patients critiques. De toute évidence, la microcirculation est devenue une cible thérapeutique. En raison de la complexité de la microarchitecture de ce système fonctionnel, variant d'un organe à l'autre, une thérapie ne peut pas «convenir pour tout». Les altérations observées dans la microcirculation sevèrement endommagée sont de 3 ordres: (i) le contenant défini par les différentes couches de la paroi vasculaire, y compris les cellules endothéliales et un gel protecteur appelé glycocalyx répandu à la surface, où le contact avec le sang est établi, (ii) le contenu représentant le plasma qui coule avec les différents éléments figures du sang et (iii) les tissus extraluminaux environnants. La microcirculation peut être endommagée de diverses manières, avec différents niveaux de dommage à ces éléments constitutifs. Ainsi, pour réanimer de manière appropriée la microcirculation lésée, le choix de la thérapie optimale ou du faisceau de thérapies doit être rationalisé avec une analyse méticuleuse des dommages subis par la microcirculation. Ainsi, l'évaluation de la microcirculation doit être obligatoirement multivariée. Dans cette thèse, la recherche s'est principalement concentrée sur un organe, le rein. La première partie est consacrée à la revue des mécanismes structurels et fonctionnels de la microcirculation rénale en condition physiologique et également septique. La deuxième partie tente d'identifier les rôles respectifs de chacun des composants de la microcirculation dans des conditions critiques notamment le glycocalyx et la viscosité du plasma. La perméabilité de la barrière vasculaire a été étudiée dans les modèles de choc hémorragique et d'hémodilution chez les rongeurs. Les principaux résultats suggèrent qu'il existe une gradation du niveau de lésion de la barrière vasculaire. La dernière partie de la thèse a examiné comment les thérapies actuelles et anciennes peuvent moduler la microcirculation en termes d'oxygénation, d'inflammation et de flux microcirculatoire dans le rein. Parmi les thérapies étudiées, la N-acétylcystéine était efficace pour limiter l'inflammation et augmenter l'oxygénation dans le rein. Une nouvelle génération de transporteur d'oxygène à base d'hémoglobine a montré une certaine efficacité dans le modèle endotoxémique murin. Dans l'ensemble, ces différents résultats se rejoignent pour montrer l'importance d'avoir une analyse multivariée de la microcirculation, car chacune des thérapies agit sur un aspect spécifique de celle-ci. Nous espérons que les résultats de cette recherche ouvrent la voie à une médecine plus personnalisée pour les patients.

Photodynamic therapy (PDT) can be used to cause vascular collapse and blood flow stasis of the irradiated pathological neovascularisation appearing in several diseases such as age-related macular degeneration (AMD) and cancer. We hypothesized that this PDT-related interruption of vessel integrity may lead to an increased transvascular passage of drugs that could be used as a drug delivery pathway. Thus, preceding the occlusion of the pathological vasculature, PDT could be used for instance as a local drug delivery pathway to administrate an anti-angiogenic drug in the case of AMD or chemotherapy in the case of cancer, potentially improving combination therapies. In the case of chorioneovessels (CNV) due to AMD, the recurrence of the exudative AMD component of the weeks/months after PDT, due to the re-opening and/or re-growth of neovessels might be avoided by adding an anti-angiogenic factor such as anti-VEGF or anti-inflammatory drug before, during or shortly after PDT. In the case of cancer, the starvation of tumour cells induced by the PDT occlusion of blood vessels feeding the tumour might be combined with a chemotherapeutic agent for the direct kill of the cancer cells themselves. It has been reported that following the light application in PDT, a physiological cascade of responses on the one hand leads to vascular occlusion but may also induce a vascular permeability enhancement. The aim of this thesis is to find conditions where this increase in leakage due to PDT can be observed, to characterize it and to take advantage of this phenomenon to develop the basis of a novel combination therapy approach. Hence in this thesis, pre-clinical experiments were performed in the vasculature of the chorioallantoic membrane model (CAM) of the chicken embryo and in the dorsal skinfold optical chamber of nude mice observed by intravital microscopy (IVM). In the CAM, no PDT-induced leakage of a fluorescent dye (FITC-dextran) was observed unless an anti-aggregating factor, such as aspirin was added. In the chicken embryo model, delaying the blood clot appears to be an essential process to allow effective potential drug delivery. In the dorsal skinfold of the nude mouse, the inflammatory response after PDT was observed and quantified. This revealed that PDT induces a time dependent acute inflammatory response as shown by increased number of leukocytes "rolling" along the vessel wall after treatment. This was observed over a 2 hour period following PDT. The quantification of the microvascular leakage showed a continuous FITC-dextran leakage from the vasculature treated by PDT to the interstitial space. This local leakage was clearly increased by the inflammatory status of the tissue (observed by quantifying the rolling leukocytes and by histology). This concept has the potential to improve the drug delivery of anti-angiogenic drugs in the eyes of patients treated for AMD and could also be applied to improve the uptake of cytostatic drugs in tumours.