Prenatal Exposure to Bacterial Extracellular Vesicles Influences Fetal Gut Immunity and Immune Programming.

Background: The fetal immune system undergoes pivotal development during gestation, preparing for postnatal antigenic challenges. Bacterial extracellular vesicles (bEVs), bioactive particles shed by bacteria, are emerging as modulators of host immunity. However, their role in shaping fetal intestinal immune development remains largely unexplored. Objectives: This study aimed to investigate the effects of bEV exposure on lymphoid and myeloid populations in the fetal murine gut, focusing on their role in priming intestinal immunity, promoting differentiation, and modulating immune cell phenotypes in both normal and germ-free (GF) environments. Materials and Methods: We used a murine model to evaluate the immune-modulating effects of bEVs during fetal development. bEVs were isolated from bacterial cultures and introduced into the amniotic sac of embryonic day 15.5 (E15.5) fetuses through intra-amniotic injection. Fetal and neonatal mice were either raised under conventional conditions (normal environment, NE) or in germ-free (GF) environments to assess microbiota-dependent effects. Immune profiling of fetal (E17) and postnatal (4 weeks) gut tissues was performed using high-dimensional mass cytometry (CyTOF) in both conventionally housed and germ-free (GF) mice. Clustering and differential expression analyses identified lymphoid and myeloid subpopulations, including progenitors, antigen-presenting cells, and intestinal stem cells (ISCs). secondary immune challenge (LPS or TSST-1) was conducted in postnatal bEV-primed mice to assess immune memory responses. Results: bEV exposure significantly increased the prevalence of CD45- CD24+ CD44+ ISCs, promoting intestinal renewal and defense via differentiation into Paneth and tuft cells. These ISCs exhibited potential antigen-presenting capabilities through MHC expression. CD45+ lymphoid progenitors were upregulated, highlighting their role in early differentiation pathways. Myeloid progenitors, particularly monocyte-dendritic progenitor subsets, showed a bias toward antigen-presenting phenotypes.Germ-free models revealed heightened sensitivity to bEVs, with pronounced activation of progenitors and a reduction in exhaustion markers. Interestingly, macrophage and neutrophil populations displayed dose-dependent modulation, with low bEV concentrations promoting their expansion and higher doses leading to reduced incidence. Our findings suggest that bEVs act as immune priming agents in the fetal gut, promoting progenitor expansion and differentiation while preparing the intestine for postnatal challenges. Differences in responses between NE and GF models emphasize the importance of environmental influences, including microbiota, on bEV-mediated immune modulation. Conclusion: bEVs play a pivotal role in shaping fetal intestinal immunity by priming lymphoid and myeloid progenitors and enhancing ISC function. These results open potential avenues for leveraging bEVs in immunomodulation and vaccine strategies. Future studies should explore the functional responses of bEV-primed cells and their translational relevance in humans.