SUMMARY: Enterococcus faecalis and Enterococcus faecium are human pathobionts that exhibit a dual lifestyle as commensal and pathogenic bacteria. The pathogenic lifestyle is associated with specific conditions involving host susceptibility and intestinal overgrowth or the use of a medical device. Although the virulence of E. faecium appears to benefit from its antimicrobial resistance, E. faecalis is recognized for its higher pathogenic potential. E. faecalis has long been considered a predominantly extracellular pathogen; it adheres to and is taken up by a wide range of mammalian cells, albeit with less efficiency than classical intracellular enteropathogens. Carbohydrate structures, rather than proteinaceous moieties, are likely to be primarily involved in the adhesion of E. faecalis to epithelial cells. Consistently, few adhesins have been implicated in the adhesion of E. faecalis to epithelial cells. On the host side, very little is known about cognate receptors, except for the role of glycosaminoglycans during macrophage infection. Several lines of evidence indicate that E. faecalis internalization may involve a zipper-like mechanism as well as a macropinocytosis pathway. Conversely, E. faecalis can use several strategies to prevent engulfment in phagocytes. However, the bacterial and host mechanisms underlying cell infection by E. faecalis are still in their infancy. The most recent striking finding is the existence of an intracellular lifestyle where E. faecalis can replicate within a variety of host cells. In this review, we summarize and discuss the current knowledge of E. faecalis –host cell interactions and argue on the need for further mechanistic studies to prevent or reduce infections.

Interactions between the immune system and the microbiome play a crucial role on the human health. These interactions start in the prenatal period and are critical for the maturation of the immune system in newborns and infants. Several factors influence the composition of the infant’s microbiota and subsequently the development of the immune system. They include maternal infection, antibiotic treatment, environmental exposure, mode of delivery, breastfeeding, and food introduction. In this review, we focus on the ontogeny of the immune system and its association to microbial colonization from conception to food diversification. In this context, we give an overview of the mother–fetus interactions during pregnancy, the impact of the time of birth and the mode of delivery, the neonate gastrointestinal colonization and the role of breastfeeding, weaning, and food diversification. We further review the impact of the vaccination on the infant’s microbiota and the reciprocal case. Finally, we discuss several potential therapeutic interventions that might help to improve the newborn and infant’s health and their responses to vaccination. Throughout the review, we underline the main scientific questions that are left to be answered and how the non-human primate model could help enlighten the path.

Enterococcus faecalis is a bacterial species present at a subdominant level in the human gut microbiota. This commensal turns into an opportunistic pathogen under specific conditions involving dysbiosis and host immune deficiency. E. faecalis is one of the rare pathobionts identified to date as contributing to liver damage in alcoholic liver disease. We have previously observed that E. faecalis is internalized in hepatocytes. Here, the survival and fate of E. faecalis was examined in hepatocytes, the main epithelial cell type in the liver. Although referred to as an extracellular pathogen, we demonstrate that E. faecalis is able to survive and divide in hepatocytes, and form intracellular clusters in two distinct hepatocyte cell lines, in primary mouse hepatocytes, as well as in vivo. This novel process extends to kidney cells. Unraveling the intracellular lifestyle of E. faecalis, our findings contribute to the understanding of pathobiont-driven diseases.

Étude de l’interaction de Enterococcus faecalis avec les cellules de l’hôte: Enterococcus faecalis est une bactérie commensale du microbiote intestinal humain. Inoffensive chez l'homme sain, E. faecalis est aussi un pathogène opportuniste. En conditions de dysbiose post-antibiotique, E. faecalis peut devenir une espèce dominante, traverser la barrière intestinale avant de disséminer. E. faecalis se classe désormais comme la troisième cause d'infections nosocomiales dans le monde. La pathogénie de E. faecalis est un processus multifactoriel dont les mécanismes cellulaires de son interaction avec l'hôte sont encore mal compris. À l'aide de modèles cellulaires d'infection et de modèles in vivo, nous avons entrepris de caractériser le rôle du facteur de virulence ElrA pendant l'infection cellulaire.Notre objectif était également de déterminer si FHL2, un partenaire eucaryote de ElrA, était impliqué dans l'infection par E. faecalis et de determiner l'impact de l'interaction ElrA-FHL2. Nous avons démontré que ElrA agit comme une cape d'invisibilité permettant à E. faecalis de ne pas être détecté par des macrophages. Nous avons également montré que FHL2 est impliqué dans la défense de l’hôte contre l'infection par E. faecalis, mais ce rôle implique partiellement ElrA. Parallèlement, nous avons montré pour la première fois que E. faecalis est capable de se multiplier dans les hepatocytes. En conclusion, ce travail apporte de nouvelles perspectives sur les interactions de E. faecalis avec son hôte.

Chlamydiosis, a sexually transmitted infection induced by Chlamydia trachomatis (CT), increases local inflammation (cytokine production, recruitment of neutrophils). Vaginal microbiota (VM) is involved in the regulation of local immune responses and STI acquisition where Lactobacillus spp are associated with protection. In this study, the VM of cynomolgus macaques was enriched with Lactobacillus crispatus after local metronidazole treatment followed by repeated intravaginal inoculations of CT. Metronidazole treatment induced drastic VM modifications (decrease abundance of several anaerobes and increase of natural Lactobacillus spp) . After CT exposure, a non-persisting CT infection and no association between L. crispatus enrichment and a lower CT susceptibility were detected. However, serum specific anti-CT IgG production was higher in treated animals. High concentration of blood cytokines after CT infection was observed in untreated animals, whereas treated animals exhibited an increased production of cervicovaginal cytokines. Blood neutrophils were more mature and activated after CT infection in both groups. These results suggest an efficient local immune response associated with a controlled systemic inflammation (low cytokine production and higher anti-CT IgG titer) upon CT infection in L. crispatus treated animals. This study confirmed that VM composition can be a powerful tool to modulate local immune responses to STIs. Authors summary: Chlamydia trachomatis (CT) is a bacterial pathogen responsible for a highly prevalent sexually transmitted infection (STI), particularly among young women. Often asymptomatic, untreated infections can ascend to the upper genital tract, causing inflammation and, in severe cases, infertility. Environmental factors within the female reproductive tract, including the vaginal microbiota, can influence susceptibility to infection by modulating local immune responses. A non- Lactobacillus spp.-dominated microbiota has been associated with an increased risk of CT acquisition. Using the relevant in vivo model of cynomolgus macaques, which closely resemble humans in terms of immune responses and female reproductive tract anatomy, we investigated the impact of vaginal Lactobacillus crispatus colonization on CT infection. The presence of L. crispatus did not prevent CT acquisition but was associated with increased CT-specific antibody levels. Moreover, systemic inflammation was higher in the non- Lactobacillus -treated group, whereas the opposite trend was observed in the cervicovaginal compartment. This study highlights the critical role of microbiota composition in shaping host immune responses to pathogen infection.

Adipose tissue (AT) dysfunction in obesity involves complex interactions among adipocytes, immune cells, endothelial cells and extracellular matrix components. However, most in vitro studies investigating adipocyte–immune interactions rely on simplified two-dimensional systems that incompletely recapitulate human adipose tissue physiology. Here, we developed a patient-derived three-dimensional (3D) human adiposphere model to investigate adipocyte-T-cell crosstalk under controlled metabolic conditions. Adipospheres were generated from the stromal vascular fraction of human subcutaneous AT obtained from donors with clinically distinct metabolic profile. Following 3D adipogenic maturation under normogenic or obesogenic conditions, adipospheres retained donor-specific adipogenic and metabolic characteristics during long-term culture, as demonstrated by differential expression of adipogenic markers, insulin-responsive AKT signaling and lipolytic activity. Compared with normogenic maturation condition, obesogenic maturation induced marked alterations in adiposphere morphology, cytokine secretion and metabolic function. In a second step, mature adipospheres were co-cultured with activated blood-derived T cells using a Transwell-based paracrine system. T-cells co-culture further modulated adiposphere inflammatory and metabolic- related responses. Transcriptomic analyses revealed coordinated regulation of immune- and metabolism related pathways, indicating extensive transcriptional remodeling during adipocyte–T-cell crosstalk. Overall, this modular 3D adiposphere platform reproduces key features of human adipose tissue immunometabolic interactions while preserving donor-specific responses. The system provides a flexible framework for integrating additional cellular components and may facilitate future mechanistic studies and therapeutic screening approaches targeting obesity-associated metabolic and inflammatory disorders.