Restoration of lymphoid organ integrity through the interaction of lymphoid tissue-inducer cells with stroma of the T cell zone.

The generation of lymphoid microenvironments in early life depends on the interaction of lymphoid tissue-inducer cells with stromal lymphoid tissue-organizer cells. Whether this cellular interface stays operational in adult secondary lymphoid organs has remained elusive. We show here that during acute infection with lymphocytic choriomeningitis virus, antiviral cytotoxic T cells destroyed infected T cell zone stromal cells, which led to profound disruption of secondary lymphoid organ integrity. Furthermore, the ability of the host to respond to secondary antigens was lost. Restoration of the lymphoid microanatomy was dependent on the proliferative accumulation of lymphoid tissue-inducer cells in secondary lymphoid organs during the acute phase of infection and lymphotoxin alpha(1)beta(2) signaling. Thus, crosstalk between lymphoid tissue-inducer cells and stromal cells is reactivated in adults to maintain secondary lymphoid organ integrity and thereby contributes to the preservation of immunocompetence.

Phenotypic plasticity in salmonid embryos : characterizing pathogen-induced stress effects on heritable variation in traits and reaction norms

Les pressions écologiques peuvent varier tant en nature qu'en intensité dans le temps et l'espace. C'est pourquoi, un phénotype unique ne peut pas forcément conférer la meilleure valeur sélective. La plasticité phénotypique peut être un moyen de s'accommoder de cette situation, en augmentant globalement la tolérance aux changements environnementaux. Comme pour tout trait de caractère, une variation génétique doit persister pour qu'évoluent les traits plastiques dans une population donnée. Cependant, les pressions extérieures peuvent affecter l'héritabilité, et la direction de ces changements peut dépendre du caractère en question, de l'espèce mais aussi du type de stress. Dans la présente thèse, nous avons cherché à élucider les effets des pressions pathogéniques sur les phénotypes et la génétique quantitative de plusieurs traits plastiques chez les embryons de deux salmonidés, la palée (Coregonus palaea), et la truite de rivière (Salmo trutta). Les salmonidés se prêtent à de telles études du fait de leur extraordinaire variabilité morphologique, comportementale et des traits d'histoire de vie. Par ailleurs, avec le déclin des salmonidés dans le monde, il est important de savoir combien la variabilité génétique persiste dans les normes de réaction afin d'aider à prédire leur capacité à répondre aux changements de leur milieu. Nous avons observé qu'une augmentation de la croissance des communautés microbiennes symbiotiques entraînait une mortalité accrue et une éclosion précoce chez la palée, et dévoilait la variance génétique additive pour ces deux caractères (Chapitres 1-2). Bien qu'aucune variation génétique n'ait été trouvée pour les normes de réaction, nous avons observé une variabilité de la plasticité d'éclosion. Néanmoins, on a trouvé que les temps d'éclosion étaient corrélés entre les environnements, ce qui pourrait limiter l'évolution de la norme de réaction. Le temps d'éclosion des embryons est lié à la taille des géniteurs mâles, ce qui indique des effets pléiotropiques. Dans le Chapitre 3, nous avons montré qu'une interaction triple entre la souche bactérienne {Pseudomonas fluorescens}, l'état de dévelopement de l'hôte ainsi que ses gènes ont une influence sur la mortalité, le temps d'éclosion et la taille des alevins de la palée. Nous avons démontré qu'une variation génétique subsistait généralement dans les normes de réaction des temps d'éclosion, mais rarement pour la taille des alevins, et jamais pour la mortalité. Dans le même temps, nous avons exhibé que des corrélations entre environnements dépendaient des caractères phénotypiques, mais contrairement au Chapitre 2, nous n'avons pas trouvé de preuve de corrélations transgénérationnelles. Le Chapitre 4 complète le chapitre précédent, en se plaçant du point de vue moléculaire, et décrit comment le traitement d'embryons avec P. fluorescens s'est traduit par une régulation négative d'expression du CMH-I indépendemment de la souche bactérienne. Nous avons non seulement trouvé une variation génétique des caractères phénotypiques moyens, mais aussi de la plasticité. Les deux derniers chapitres traitent de l'investigation, chez la truite de rivière, des différences spécifiques entre populations pour des normes de réaction induites par les pathogènes. Dans le Chapitre 5, nous avons illustré que le métissage entre des populations génétiquement distinctes n'affectait en rien la hauteur ou la forme des normes de réaction d'un trait précoce d'histoire de vie suite au traitement pathogénique. De surcroît, en dépit de l'éclosion tardive et de la réduction de la taille des alevins, le traitement n'a pas modifié la variation héritable des traits de caractère. D'autre part, dans le Chapitre 6, nous avons démontré que le traitement d'embryons avec des stimuli contenus dans l'eau de conspécifiques infectés a entraîné des réponses propre à chaque population en terme de temps d'éclosion ; néanmoins, nous avons observé peu de variabilité génétique des normes de réaction pour ce temps d'éclosion au sein des populations. - Ecological stressors can vary in type and intensity over space and time, and as such, a single phenotype may not confer the highest fitness. Phenotypic plasticity can act as a means to accommodate this situation, increasing overall tolerance to environmental change. As with any trait, for plastic traits to evolve in a population, genetic variation must persist. However, environmental stress can alter trait heritability, and the direction of this shift can be trait, species, and stressor-dependent. In this thesis, we sought to understand the effects of pathogen stressors on the phenotypes and genetic architecture of several plastic traits in the embryos of two salmonids, the whitefish (Coregonus palaea), and the brown trout (Salmo trutta). Salmonids lend themselves to such studies because their extraordinary variability in morphological, behavioral, and life-history traits. Also, with declines in salmonids worldwide, knowing how much genetic variability persists in reaction norms may help predict their ability to respond to environmental change. We found that increasing growth of symbiotic microbial communities increased mortality and induced hatching in whitefish, and released additive genetic variance for both traits (Chapters 1-2). While no genetic variation was found for survival reaction norms, we did find variability in hatching plasticity. Nevertheless, hatching time was correlated across environments, which could constrain evolution of the reaction norm. Hatching time in the induced environment was also correlated to sire size, indicating pleiotropic effects. In Chapter 3 we report that a three-way interaction between bacterial strain (Pseudomonas fluorescens), host developmental stage, and host genetics impacted mortality, hatching time, and hatchling size in whitefish. We also showed that genetic variation generally persisted in hatching age reaction norms, but rarely for hatchling length, and never for mortality. At the same time, we demonstrated that cross-environmental correlations were trait-dependent, and unlike Chapter 2, we found no evidence of cross-generational correlations. Chapter 4 expands on the previous chapter, moving to the molecular level, and describes how treatment of embryos with P. fluorescens resulted in strain-independent downregulation of MHC class I. Genetic variation was evident not only in trait means, but also in plasticity. In the last two chapters, we investigated population level differences in pathogen- induced reaction norms in brown trout. In Chapter 5, we found that interbreeding between genetically distinct populations did not affect the elevation or shapes of the reaction norms of early life-history traits after pathogen challenge. Moreover, despite delaying hatching and reducing larval length, treatment produced no discernable shifts in heritable variation in traits. On the other hand, in Chapter 6, we found that treatment of embryos with water-borne cues from infected conspecifics elicited population-specific responses in terms of hatching time; however, we found little evidence of genetic variability in hatching reaction norms within populations. We have made considerable progress in understanding how pathogen stressors affect various early life-history traits in salmonid embryos. We have demonstrated that the effect of a particular stressor on heritable variation in these traits can vary according to the trait and species under consideration, in addition to the developmental stage of the host. Moreover, we found evidence of genetic variability in some, but not all reaction norms in whitefish and brown trout.

Axial spondylometaphyseal dysplasia: Additional reports.

Axial spondylometaphyseal dysplasia (SMD) (OMIM 602271) is an uncommon skeletal dysplasia characterized by metaphyseal changes of truncal-juxtatruncal bones, including the proximal femora, and retinal abnormalities. The disorder has not attracted much attention since initially reported; however, it has been included in the nosology of genetic skeletal disorders [Warman et al. (2011); Am J Med Genet Part A 155A:943-968] in part because of a recent publication of two additional cases [Isidor et al. (2010); Am J Med Genet Part A 152A:1550-1554]. We report here on the clinical and radiological manifestations in seven affected individuals from five families (three sporadic cases and two familial cases). Based on our observations and Isidor's report, the clinical and radiological hallmarks of axial SMD can be defined: The main clinical findings are postnatal growth failure, rhizomelic short stature in early childhood evolving into short trunk in late childhood, and thoracic hypoplasia that may cause mild to moderate respiratory problems in the neonatal period and later susceptibility to airway infection. Impaired visual acuity comes to medical attention in early life and function rapidly deteriorates. Retinal changes are diagnosed as retinitis pigmentosa or pigmentary retinal degeneration on fundoscopic examination and cone-rod dystrophy on electroretinogram. The radiological hallmarks include short ribs with flared, cupped anterior ends, mild spondylar dysplasia, lacy iliac crests, and metaphyseal irregularities essentially confined to the proximal femora. Equally affected sibling pairs of opposite gender and parental consanguinity are strongly suggestive of autosomal recessive inheritance. © 2011 Wiley-Liss, Inc.

Increased male offspring's risk of metabolic-neuroendocrine dysfunction and overweight after fructose-rich diet intake by the lactating mother.

An adverse endogenous environment during early life predisposes the organism to develop metabolic disorders. We evaluated the impact of intake of an iso-caloric fructose rich diet (FRD) by lactating mothers (LM) on several metabolic functions of their male offspring. On postnatal d 1, ad libitum eating, lactating Sprague-Dawley rats received either 10% F (wt/vol; FRD-LM) or tap water (controls, CTR-LM) to drink throughout lactation. Weaned male offspring were fed ad libitum a normal diet, and body weight (BW) and food intake were registered until experimentation (60 d of age). Basal circulating levels of metabolic markers were evaluated. Both iv glucose tolerance and hypothalamic leptin sensitivity tests were performed. The hypothalamus was dissected for isolation of total RNA and Western blot analysis. Retroperitoneal (RP) adipose tissue was dissected and either kept frozen for gene analysis or digested to isolate adipocytes or for histological studies. FRD rats showed increased BW and decreased hypothalamic sensitivity to exogenous leptin, enhanced food intake (between 49-60 d), and decreased hypothalamic expression of several anorexigenic signals. FRD rats developed increased insulin and leptin peripheral levels and decreased adiponectinemia; although FRD rats normally tolerated glucose excess, it was associated with enhanced insulin secretion. FRD RP adipocytes were enlarged and spontaneously released high leptin, although they were less sensitive to insulin-induced leptin release. Accordingly, RP fat leptin gene expression was high in FRD rats. Excessive fructose consumption by lactating mothers resulted in deep neuroendocrine-metabolic disorders of their male offspring, probably enhancing the susceptibility to develop overweight/obesity during adult life.

Dental service utilization by Europeans aged 50 plus.

OBJECTIVES: To describe variations in the utilization of dental services by persons aged 50+ from 14 European countries and to identify the extent to which such variations are attributable to differences in oral health need and in accessibility of dental care. METHODS: We use data from the Survey of Health, Ageing, and Retirement in Europe (SHARE Waves 2 and 3) and estimate a series of multivariate logistic regression models to analyze variations in dental service utilization (overall dental attendance, preventive treatment and/or operative treatment, dental attendance in early life years) RESULTS: Overall dental attendance and incidence of solely preventive treatment are comparatively high in the Netherlands, Sweden, Denmark, Germany, and Switzerland. In contrast, overall dental attendance is relatively low in Spain, Italy, France, Greece, Poland, and Ireland. Moreover, a high incidence of solely operative treatment is observed in Austria, Italy, and France, whereas in the Netherlands, Sweden, Denmark, Switzerland, and Ireland, the incidence of solely operative treatment is comparably low. By and large, these variations persist even when controlling for cross-country differences in oral health need and in accessibility of dental care. CONCLUSIONS: In comparison with other European regions, there is a tendency toward more frequent and preventive dental treatment of the elderly populations residing in Scandinavia and Western Europe. Such utilization patterns appear only partially attributable to differences in need for and accessibility of dental care.

Consequences of gestational stress on GABAergic modulation of respiratory activity in developing newborn pups.

The GABAergic system modulates respiratory activity and undergoes substantial changes during early life. Because this maturation process is sensitive to stress, we tested the hypothesis that gestational stress (GS) alters development of GABAergic modulation of respiratory control in rat pups. The respiratory responses to the selective GABAA receptor agonist muscimol were compared between pups born to dams subjected to GS (bright light and predator odor; 20 min/day from G9 to G19) or maintained under standard (control) conditions. Respiratory activity was measured on 1 and 4 days old pups of both sexes using in vivo (whole body plethysmography) and in vitro (isolated brainstem-spinal cord preparation) approaches. In intact pups, muscimol injection (0.75 mg/kg; i.p.) depressed minute ventilation; this response was less in GS pups, and at P4, muscimol augmented minute ventilation in GS females. Bath application of muscimol (0.01-0.5 μM) onto brainstem preparations decreased inspiratory (C4) burst frequency and amplitude in a dose-dependent manner; the responsiveness decreased with age. However, GS had limited effects on these results. We conclude that the results obtained in vivo are consistent with our hypothesis and show that GS delays maturation of GABAergic modulation of respiratory activity. The differences in the results observed between experimental approaches (in vivo versus in vitro) indicate that the effect of prenatal stress on maturation of GABAergic modulation of respiratory control mainly affects the peripheral/metabolic components of the respiratory control system.

Environmental and T cell-intrinsic factors limit the expansion of neonatal follicular T helper cells but may be circumvented by specific adjuvants.

Follicular Th (T(FH)) cells have emerged as a new Th subset providing help to B cells and supporting their differentiation into long-lived plasma cells or memory B cells. Their differentiation had not yet been investigated following neonatal immunization, which elicits delayed and limited germinal center (GC) responses. We demonstrate that neonatal immunization induces CXCR5(high)PD-1(high) CD4(+) T(FH) cells that exhibit T(FH) features (including Batf, Bcl6, c-Maf, ICOS, and IL-21 expression) and are able to migrate into the GCs. However, neonatal T(FH) cells fail to expand and to acquire a full-blown GC T(FH) phenotype, as reflected by a higher ratio of GC T(FH)/non-GC CD4(+) T cells in immunized adults than neonates (3.8 × 10(-3) versus 2.2 × 10(-3), p = 0.01). Following the adoptive transfer of naive adult OT-II CD4(+) T cells, OT-II T(FH) cells expand in the vaccine-draining lymph nodes of immunized adult but not infant recipients, whereas naive 2-wk-old CD4(+) OT-II cells failed to expand in adult hosts, reflecting the influence of both environmental and T cell-intrinsic factors. Postponing immunization to later in life increases the number of T(FH) cells in a stepwise manner, in direct correlation with the numbers of GC B cells and plasma cells elicited. Remarkably, adjuvantation with CpG oligonucleotides markedly increased GC T(FH) and GC B cell neonatal responses, up to adult levels. To our knowledge, this is the first demonstration that the T(FH) cell development limits early life GC responses and that adjuvants/delivery systems supporting T(FH) differentiation may restore adultlike early life GC B cell responses.

Targeting Oxidative Stress and Aberrant Critical Period Plasticity in the Developmental Trajectory to Schizophrenia.

Schizophrenia is a neurodevelopmental disorder reflecting a convergence of genetic risk and early life stress. The slow progression to first psychotic episode represents both a window of vulnerability as well as opportunity for therapeutic intervention. Here, we consider recent neurobiological insight into the cellular and molecular components of developmental critical periods and their vulnerability to redox dysregulation. In particular, the consistent loss of parvalbumin-positive interneuron (PVI) function and their surrounding perineuronal nets (PNNs) as well as myelination in patient brains is consistent with a delayed or extended period of circuit instability. This linkage to critical period triggers (PVI) and brakes (PNN, myelin) implicates mistimed trajectories of brain development in mental illness. Strategically introduced antioxidant treatment or later reinforcement of molecular brakes may then offer a novel prophylactic psychiatry.

Microbiome influences on allergy in mice and humans.

The microbiota plays a pivotal role in the development and calibration of host immunity. Over many millennia, finely balanced interactions between the microbiota and host tissue compartments have evolved, imparting metabolic advantages and protection against pathogens, while restricting deleterious immune responses against innocuous antigens. Perturbations in host-microbiota crosstalk at critical developmental windows in early life may underlie allergy and chronic inflammation. Although the microbiota's of the gut and skin have been extensively characterized, the lung microbiota has also, in recent years, received considerable attention. This ever-expanding field is pushing the boundaries of pulmonary research, with potential implications for novel strategies in the treatment and prevention of chronic lung diseases. In this article, we provide a summary of the development of the microbiota in early life, and describe the evidence from human and murine studies of how microbial dysbiosis in early life can alter the trajectory of immune development and provide the setting for allergic disorders in later life.

Selective serotonin reuptake inhibitor antidepressant use in first trimester pregnancy and risk of specific congenital anomalies: a European register-based study.

Evidence of an association between early pregnancy exposure to selective serotonin reuptake inhibitors (SSRI) and congenital heart defects (CHD) has contributed to recommendations to weigh benefits and risks carefully. The objective of this study was to determine the specificity of association between first trimester exposure to SSRIs and specific CHD and other congenital anomalies (CA) associated with SSRI exposure in the literature (signals). A population-based case-malformed control study was conducted in 12 EUROCAT CA registries covering 2.1 million births 1995-2009 including livebirths, fetal deaths from 20 weeks gestation and terminations of pregnancy for fetal anomaly. Babies/fetuses with specific CHD (n = 12,876) and non-CHD signal CA (n = 13,024), were compared with malformed controls whose diagnosed CA have not been associated with SSRI in the literature (n = 17,083). SSRI exposure in first trimester pregnancy was associated with CHD overall (OR adjusted for registry 1.41, 95 % CI 1.07-1.86, fluoxetine adjOR 1.43 95 % CI 0.85-2.40, paroxetine adjOR 1.53, 95 % CI 0.91-2.58) and with severe CHD (adjOR 1.56, 95 % CI 1.02-2.39), particularly Tetralogy of Fallot (adjOR 3.16, 95 % CI 1.52-6.58) and Ebstein's anomaly (adjOR 8.23, 95 % CI 2.92-23.16). Significant associations with SSRI exposure were also found for ano-rectal atresia/stenosis (adjOR 2.46, 95 % CI 1.06-5.68), gastroschisis (adjOR 2.42, 95 % CI 1.10-5.29), renal dysplasia (adjOR 3.01, 95 % CI 1.61-5.61), and clubfoot (adjOR 2.41, 95 % CI 1.59-3.65). These data support a teratogenic effect of SSRIs specific to certain anomalies, but cannot exclude confounding by indication or associated factors.

The biological embedding of social differences in ageing trajectories.

The occurrence of adult disease is related to lifetime experiences and, at least in part, to early life events. It is now well established that socioeconomic circumstances across the lifetime are major determinants of adult health and disease, and the current economic crisis is amplifying susceptibility to disease and unhealthy ageing in disadvantaged subgroups of the population. In adulthood, the gap between social groups is extensive in terms of mortality, functional performances and cognitive capacity. Since the occurrence of adult disease is related to lifetime experiences, including early life exposures, late-life preventive efforts may be of limited efficacy, particularly in disadvantaged subgroups. We now have the analytical tools to understand mechanisms that underlie life-long susceptibility to unhealthy ageing, and new knowledge can lead to better and more effective mechanisms to prevent diseases and reduce health inequalities. In this perspective, we first discuss the impact of recent changes in the understanding of chronic disease aetiology on our interpretation of the influence of life-course socioeconomic status (SES) on health and ageing. We then propose a model for integrating the exposome concept (the myriad of exposures derived from exogenous and endogenous sources) into the analysis of life-course socioeconomic differentials in ageing.

Immediate and delayed effects of subchronic Paraquat exposure during an early differentiation stage in 3D-rat brain cell cultures.

Xenobiotic exposure is a risk factor in the etiology of neurodegenerative disease. It was recently hypothesized that restricted exposure during brain development could predispose for a neurodegenerative disease later in life. As neuroinflammation contributes to progressive neurodegeneration, it is suspected that neurodevelopmental xenobiotic exposure could elicit a neuroinflammatory process, which over time may assume a detrimental character. We investigated the neurotoxic effects of paraquat (PQ) in three-dimensional whole rat brain cell cultures, exposed during an early differentiation stage, comparing immediate effects-directly post exposure-with long-term effects, 20 days after interrupted PQ-administration. Adverse effects and neuroinflammatory responses were assessed by measuring changes in gene- and protein-expression as well as by determining cell morphology changes. Differentiating neural cultures were highly susceptible to PQ and showed neuronal damage and strong astrogliosis. After the 20-day washout period, neurons partially recovered, whereas astrogliosis persisted, and was accompanied by microglial activation of a neurodegenerative phenotype. Our data shows that immediate and long-term effects of subchronic PQ-exposure differ. Also, PQ-exposure during this window of extensive neuronal differentiation led to a delayed microglial activation, of a character that could promote further pro-inflammatory signals that enable prolonged inflammation, thereby fueling further neurodegeneration.

Integrating human indoor air pollutant exposure within Life Cycle Impact Assessment.

Neglecting health effects from indoor pollutant emissions and exposure, as currently done in Life Cycle Assessment (LCA), may result in product or process optimizations at the expense of workers' or consumers' health. To close this gap, methods for considering indoor exposure to chemicals are needed to complement the methods for outdoor human exposure assessment already in use. This paper summarizes the work of an international expert group on the integration of human indoor and outdoor exposure in LCA, within the UNEP/ SETAC Life Cycle Initiative. A new methodological framework is proposed for a general procedure to include human-health effects from indoor exposure in LCA. Exposure models from occupational hygiene and household indoor air quality studies and practices are critically reviewed and recommendations are provided on the appropriateness of various model alternatives in the context of LCA. A single-compartment box model is recommended for use as a default in LCA, enabling one to screen occupational and household exposures consistent with the existing models to assess outdoor emission in a multimedia environment. An initial set of model parameter values was collected. The comparison between indoor and outdoor human exposure per unit of emission shows that for many pollutants, intake per unit of indoor emission may be several orders of magnitude higher than for outdoor emissions. It is concluded that indoor exposure should be routinely addressed within LCA.

Early Cretaceous life, climate and anoxia

Early Cretaceous life and the environment were strongly influenced by the accelerated break up of Pangaea, which was associated with the formation of a multitude of rift basins, intensified spreading, and important volcanic activity on land and in the sea. These processes likely interacted with greenhouse conditions, and Early Cretaceous climate oscillated between "normal" greenhouse, predominantly arid conditions, and intensified greenhouse, predominantly humid conditions. Arid conditions were important during the latest Jurassic and early Berriasian, the late Barremian, and partly also during the late Aptian. Humid conditions were particularly intense and widespread during shorter episodes of environmental change (EECs): the Valanginian Weissert, the latest Hauterivian Faraoni, the latest Barremian earliest Aptian Taxy, the early Aptian Selli, the early late Aptian Fallot and the late Aptian-early Albian Paquier episodes. Arid conditions were associated with evaporation, low biogeochemical weathering rates, low nutrient fluxes, and partly stratified oceans, leading to oxygen depletion and enhanced preservation of laminated, organic-rich mud (LOM). Humid conditions enabled elevated biogeochemical weathering rates and nutrient fluxes, important runoff and the buildup of freshwater lids in proximal basins, intensified oceanic and atmospheric circulation, widespread upwelling and phosphogenesis, important primary productivity and enhanced preservation of LOM in expanded oxygen-minimum zones. The transition of arid to humid climates may have been associated with the net transfer of water to the continent owing to the infill of dried-out groundwater reservoirs in internally drained inland basins. This resulted in shorter-term sea-level fall, which was followed by sea-level rise. These sea-level changes and the influx of freshwater into the ocean may have influenced oxygen-isotope signatures. Climate change preceding and during the Early Cretaceous EECs may have been rapid, but in general, the EECs had a "pre"-history, during which the stage was set for environmental change. Negative feedback on the climate through increased marine LOM preservation was unlikely, because of the low overall organic-carbon accumulation rates during these episodes. Life and climate co-evolved during the Early Cretaceous. Arid conditions may have affected continental life, such as across the Tithonian/Berriasian boundary. Humid conditions and the corresponding tendency to develop dys- to anaerobic conditions in deeper ocean waters led to phases of accelerated extinction in oceans, but may have led to more luxuriant vegetation cover on continents, such as during the Valanginian, to the benefit of herbivores. During Early Cretaceous EECs, reef systems and carbonate platforms in general were particularly vulnerable. They were the first to disappear and the last to recover, often only after several million years. (C) 2011 Elsevier Ltd. All rights reserved.

Towards a system analysis of the early steps of infectious endocarditis pathogenesis

L'endocardite infectieuse (EI) est une maladie potentiellement mortelle qui doit être prévenue dans toute la mesure du possible. Au cours de ces dernières 50 années, les recommandations Américaines et Européennes pour la prophylaxie de PEI proposaient aux patients à risques de prendre un antibiotique, préventif avant de subir une intervention médico-chirurgicale susceptible d'induire une bactériémie transitoire. Cependant, des études épidémiologiques récentes ont montré que la plupart des EI survenaient en dehors de tous actes médico-chirurgicaux, et indépendamment de la prise ou non de prophylaxie antibiotique . L'EI pourrait donc survenir suite à la cumulation de bactériémies spontanées de faibles intensités, associées à des activités de la vie courante telle que le brossage dentaire pour le streptocoques, ou à partir de tissus colonisés ou de cathéters infectés pour les staphylocoques. En conséquence, les recommandations internationales pour la prophylaxie de PEI ont été revues et proposent une diminution drastique de l'utilisation d'antibiotiques. Cependant, le risque d'EI représenté par le cumul de bactériémies de faibles intensités n'a pas été démontré expérimentalement. Nous avons développé un nouveau modèle d'EI expérimentale induite par une inoculation en continu d'une faible quantité de bactéries, simulant le cumul de bactériémies de faibles intensités chez l'homme, et comparé l'infection de Streptococcus gordonii et de Staphylococcus aureus dans ce modèle avec celle du modèle d'IE induite par une bactériémie brève, mais de forte intensité. Nous avons démontré, après injection d'une quantité égale de bactéries, que le nombre de végétations infectées était similaire dans les deux types d'inoculations. Ces résultats expérimentaux ont confirmé l'hypothèse qu'une exposition cumulée à des bactériémies de faibles intensités, en dehors d'une procédure médico-chirurgicale, représentait un risque pour le développement d'une El, comme le suggéraient les études épidémiologiques. En plus, ces résultats ont validé les nouvelles recommandations pour la prophylaxie de l'El, limitant drastiquement l'utilisation d'antibiotiques. Cependant, ces nouvelles recommandations laissent une grande partie (> 90%) de cas potentiels d'EI sans alternatives de préventions, et des nouvelles stratégies prophylactiques doivent être investiguées. Le nouveau modèle d'EI expérimentale représente un modèle réaliste pour étudier des nouvelles mesures prophylactiques potentielles appliquées à des expositions cumulées de bactériémies de faible nombre. Dans un contexte de bactériémies spontanées répétitives, les antibiotiques ne peuvent pas résoudre le problème de la prévention de l'EI. Nous avons donc étudié la une alternative de prévention par l'utilisation d'agents antiplaquettaires. La logique derrière cette approche était basée sur le fait que les plaquettes sont des composants clés dans la formation des végétations cardiaques, et le fait que les bactéries capables d'interagir avec les plaquettes sont plus enclines à induire une El. Les agents antiplaquettaires utilisés ont été l'aspirine (inhibiteur du COX1), la ticlopidine (inhibiteur du P2Y12, le récepteur de l'ADP), et l'eptifibatide et Pabciximab, deux inhibiteurs du GPIIb/IIIa, le récepteur plaquettaire pour le fibrinogène. Les anticoagulants étaient le dabigatran etexilate, inhibant lathrombine et l'acenocumarol, un antagoniste de la vitamine K. L'aspirine, la ticlopidine ou l'eptifibatide seuls n'ont pas permis de prévenir l'infection valvulaire (> 75% animaux infectés). En revanche, la combinaison d'aspirine et de ticlopidine, aussi bien que l'abciximab, ont protégé 45% - 88% des animaux de l'EI par S. gordonii et par S. aureus. L'antithrombotique dabigatran etexilate à protégé 75% des rats contre l'EI par S. aureus, mais pas (< 30% de protection) par S. gordonii. L'acenocoumarol n'a pas eu d'effet sur aucun des deux organismes. En général, ces résultats suggèrent un possible rôle pour les antiplaquettaires et du dabigatran etexilate dans la prophylaxie de l'EI dans un contexte de bactériémies récurrentes de faibles intensités. Cependant, l'effet bénéfique des antiplaquettaires doit être soupesé avec le risque d'hémorragie inhérent à ces molécules, et le fait que les plaquettes jouent un important rôle dans les défenses de l'hôte contre les infections endovasculaires. En plus, le double effet bénéfique du dabigatran etexilate devrait être revu chez les patients porteurs de valves prothétiques, qui ont besoin d'une anticoagulation à vie, et chez lesquels l'EI à S. aureus est associée avec une mortalité de près de 50%. Comme l'approche avec des antiplaquettaires et des antithrombotiques pourrait avoir des limites, une autre stratégie prophylactique pourrait être la vaccination contre des adhésines de surfaces des pathogènes. Chez S. aureus, la protéine de liaison au fibrinogène, ou dumping factor A (ClfA), et la protéine de liaison à la fibronectine (FnbpA) sont des facteurs de virulence nécessaires à l'initiation et l'évolution de PEI. Elles représentent donc des cibles potentielles pour le développement de vaccins contre cette infection. Récemment, des nombreuses publications ont décrit que la bactérie Lactococcus lactis pouvait être utilisée comme vecteur pour la diffusion d'antigènes bactériens in vivo, et que cette approche pourrait être une stratégie de vaccination contre les infections bactériennes. Nous avons exploré l'effet de l'immunisation par des recombinant de L. lactis exprimant le ClfA, la FnbpA, ou le ClfA ensemble avec et une forme tronquée de la FnbpA (Fnbp, comprenant seulement le domaine de liaison à la fibronectine mais sans le domaine A de liaison au fibrinogène [L. lactis ClfA/Fnbp]), dans la prophylaxie de PIE expérimentale à S. aureus. L. lactis ClfA a été utilisés comme agent d'immunisation contre la souche S. aureus Newman (qui a particularité de n'exprimer que le ClfA, mais pas la FnbpA). L. lactis ClfA, L. lactis FnbpA, et L. lactis ClfA/Fnbp, ont été utilisé comme agents d'immunisation contre une souche isolée d'une IE, S. aureus P8 (exprimant ClfA et FnbpA). L'immunisation avec L. lactis ClfA a généré des anticorps anti-ClfA fonctionnels, capables de bloquer la liaison de S. aureus Newman au fibrinogène in vitro et protéger 13/19 (69%) animaux d'une El due à S. aureus Newman (P < 0.05 comparée aux contrôles). L'immunisation avec L. lactis ClfA, L. lactis FnbpA, ou L. lactis ClfA/Fnbp, a généré des anticorps contre chacun de ces antigènes. Cependant, ils n'ont pas permis de bloquer l'adhésion de S. aureus P8 au fibrinogène et à la fibronectine in vitro. De plus, l'immunisation avec L. lactis ClfA ou L. lactis FnbpA s'est avérée inefficace in vivo (< 10% d'animaux protégés d'une El) et l'immunisation avec L. lactis ClfA/Fnbp a fourni une protection limitée de l'EI (8/23 animaux protégés; P < 0.05 comparée aux contrôles) après inoculation avec S. aureus P8. Dans l'ensemble, ces résultats indiquent que L. lactis est un système efficace pour la présentation d'antigènes in vivo et potentiellement utile pour la prévention de PEI à S. aureus. Cependant, le répertoire de protéines de surface de S. aureus capable d'évoquer une panoplie d'anticorps efficace reste à déterminer.. En résumé, notre étude a démontré expérimentalement, pour la première fois, qu'une bactériémie répétée de faible intensité, simulant la bactériémie ayant lieu, par exemple, lors des activités de la vie quotidienne, est induire un taux d'EI expérimentale similaire à celle induite par une bactériémie de haute intensité suite à une intervention médicale. Dans ce contexte, où l'utilisation d'antibiotiques est pas raisonnable, nous avons aussi montré que d'autres mesures prophylactiques, comme l'utilisation d'agents antiplaquettaires ou antithrombotiques, ou la vaccination utilisant L. lactis comme vecteur d'antigènes bactériens, sont des alternatives prometteuses qui méritent d'être étudiées plus avant. Thesis Summary Infective endocarditis (IE) is a life-threatening disease that should be prevented whenever possible. Over the last 50 years, guidelines for IE prophylaxis proposed the use of antibiotics in patients undergoing dental or medico-surgical procedures that might induce high, but transient bacteremia. However, recent epidemiological studies indicate that IE occurs independently of medico-surgical procedures and the fact that patients had taken antibiotic prophylaxis or not, i.e., by cumulative exposure to random low-grade bacteremia, associated with daily activities (e.g. tooth brushing) in the case of oral streptococci, or with a colonized site or infected device in the case of staphylococci. Accordingly, the most recent American and European guidelines for IE prophylaxis were revisited and updated to drastically restrain antibiotic use. Nevertheless, the relative risk of IE represented by such cumulative low-grade bacteremia had never been demonstrated experimentally. We developed a new model of experimental IE due to continuous inoculation of low-grade bacteremia, mimicking repeated low-grade bacteremia in humans, and compared the infectivity of Streptococcus gordonii and Staphylococcus aureus in this model to that in the model producing brief, high-level bacteremia. We demonstrated that, after injection of identical bacterial numbers, the rate of infected vegetations was similar in both types of challenge. These experimental results support the hypothesis that cumulative exposure to low-grade bacteremia, outside the context of procedure-related bacteremia, represents a genuine risk of IE, as suggested by human epidemiological studies. In addition, they validate the newer guidelines for IE prophylaxis, which drastic limit the procedures in which antibiotic prophylaxis is indicated. Nevertheless, these refreshed guidelines leave the vast majority (> 90%) of potential IE cases without alternative propositions of prevention, and novel strategies must be considered to propose effective alternative and "global" measures to prevent IE initiation. The more realistic experimental model of IE induced by low-grade bacteremia provides an accurate experimental setting to study new preventive measures applying to cumulative exposure to low bacterial numbers. Since in a context of spontaneous low-grade bacteremia antibiotics are unlikely to solve the problem of IE prevention, we addressed the role of antiplatelet and anticoagulant agents for the prophylaxis of experimental IE induced by S. gordonii and S. aureus. The logic of this approach was based on the fact that platelets are key players in vegetation formation and vegetation enlargement, and on the fact that bacteria capable of interacting with platelets are more prone to induce IE. Antiplatelet agents included the COX1 inhibitor aspirin, the inhibitor of the ADP receptor P2Y12 ticlopidine, and two inhibitors of the platelet fibrinogen receptor GPIIb/IIIa, eptifibatide and abciximab. Anticoagulants included the thrombin inhibitor dabigatran etexilate and the vitamin K antagonist acenocoumarol. Aspirin, ticlopidine or eptifibatide alone failed to prevent aortic infection (> 75% infected animals). In contrast, the combination of aspirin with ticlopidine, as well as abciximab, protected 45% to 88% of animals against IE due to S. gordonii and S. aureus. The antithrombin dabigatran etexilate protected 75% of rats against IE due to S. aureus, but failed (< 30% protection) against S. gordonii. Acenocoumarol had no effect against any bacteria. Overall, these results suggest a possible role for antiplatelet agents and dabigatran etexilate in the prophylaxis of IE in humans in a context of recurrent low- grade bacteremia. However, the potential beneficial effect of antiplatelet agents should be balanced against the risk of bleeding and the fact that platelets play an important role in the host defenses against intravascular infections. In addition, the potential dual benefit of dabigatran etexilate might be revisited in patients with prosthetic valves, who require life-long anticoagulation and in whom S. aureus IE is associated with high mortality rate. Because the antiplatelet and anticoagulant approach might be limited in the context of S. aureus bacteremia, other prophylactic strategies for the prevention of S. aureus IE, like vaccination with anti-adhesion proteins was tested. The S. aureus surface proteins fibrinogen-binding protein clumping-factor A (ClfA) and the fibronectin-binding protein A (FnbpA) are critical virulence factors for the initiation and development of IE. Thus, they represent key targets for vaccine development against this disease. Recently, numerous reports have described that the harmless bacteria Lactococcus lactis can be used as a bacterial vector for the efficient delivery of antigens in vivo, and that this approach is a promising vaccination strategy against bacterial infections. We therefore explored the immunization capacity of non- living recombinant L. lactis ClfA, L. lactis FnbpA, or L. lactis expressing ClfA together with Fnbp (a truncated form of FnbpA with only the fibronectin-binding domain but lacking the fibrinogen-binding domain A [L. lactis ClfA/Fnbp]), to protect against S. aureus experimental IE. L. lactis ClfA was used as immunization agent against the laboratory strain S. aureus Newman (expressing ClfA, but lacking FnbpA). L. lactis ClfA, L. lactis FnbpA, as well as L. lactis ClfA/Fnbp, were used as immunization agents against the endocarditis isolate S. aureus P8 (expressing both ClfA and FnbpA). Immunization with L. lactis ClfA produced anti-ClfA functional antibodies, which were able to block the binding of S. aureus Newman to fibrinogen in vitro and protect 13/19 (69%) animals from IE due to S. aureus Newman (P < 0.05 compared to controls). Immunization with L. lactis ClfA, L. lactis FnbpA or L. lactis ClfA/Fnbp, produced antibodies against each antigen. However, they were not sufficient to block S. aureus P8 binding to fibrinogen and fibronectin in vitro. Moreover, immunization with L. lactis ClfA or L. lactis FnbpA was ineffective (< 10% protected animals) and immunization with L. lactis ClfA/Fnbp conferred limited protection from IE (8/23 protected animals; P < 0.05 compared to controls) after challenge with S. aureus P8. Together, these results indicate that L. lactis is an efficient delivering antigen system potentially useful for preventing S. aureus IE. They also demonstrate that expressing multiple antigens in L. lactis, yet to be elucidated, will be necessary to prevent IE due to clinical S. aureus strains fully equipped with virulence determinants. In summary, our study has demonstrated experimentally, for the first time, the hypothesis that low-grade bacteremia, mimicking bacteremia occurring outside of a clinical intervention, is equally prone to induce experimental IE as high-grade bacteremia following medico-surgical procedures. In this context, where the use of antibiotics for the prophylaxis of IE is limited, we showed that other prophylactic measures, like the use of antiplatelets, anticoagulants, or vaccination employing L. lactis as delivery vector of bacterial antigens, are reasonable alternatives that warrant to be further investigated.