Summary Intestinal intraepithelial lymphocytes (IELs) are located at the critical interface between the intestinal lumen, which is chronically exposed to food and microbes, and the core of the body. Using high-resolution microscopy techniques and intersectional genetic tools, we investigated the nature of IEL responses to luminal microbes. We observe that TCRγδ IELs exhibit unique location and movement patterns in the epithelial compartment that are microbiota-dependent. This behavioral pattern quickly changes upon exposure to different enteric pathogens, resulting in increased inter-epithelial cell (EC) scanning, expression of anti-microbial gene expression and glycolysis. Both γδ IEL dynamic and metabolic changes depend on pathogen sensing by ECs. Direct modulation of glycolysis is sufficient to change γδ IEL behavior and susceptibility to early pathogen invasion. Our results uncover a coordinated EC–IEL response to enteric infections that modulates lymphocyte energy utilization and dynamics and supports maintenance of the intestinal epithelial barrier.

The intestinal microbiome modulates host susceptibility to enteric pathogens, but the specific protective factors and mechanisms of individual bacterial species are not fully characterized. We show that secreted antigen A (SagA) from Enterococcus faecium is sufficient to protect Caenorhabditis elegans against Salmonella pathogenesis by promoting pathogen tolerance. The NlpC/p60 peptidoglycan hydrolase activity of SagA is required and generated muramyl-peptide fragments that are sufficient to protect C. elegans against Salmonella pathogenesis in a tol-1-dependent manner. SagA can also be expressed and secreted in other bacteria and improve the protective activity of probiotics against Salmonella pathogenesis in C. elegans and mice. Our study highlights how protective intestinal bacteria can modify microbial-associated molecular patterns to enhance pathogen tolerance.

CTLA-4, an Ig superfamily molecule with homology to CD28, is one of the most potent negative regulators of T-cell responses. In vivo blockade of CTLA-4 exacerbates autoimmunity, enhances tumor-specific T-cell responses, and may inhibit the induction of T-cell anergy. Clinical trials of CTLA-4–blocking antibodies to augment T-cell responses to malignant melanoma are at an advanced stage; however, little is known about the effects of CTLA-4 blockade on memory CD8+T-cell responses and the formation and maintenance of long-term CD8+T-cell memory. In our studies, we show that during in vivo memory CD8+T-cell responses toListeria monocytogenesinfection, CTLA-4 blockade enhances bacterial clearance and increases memory CD8+T-cell expansion. This is followed by an accumulation of memory cells that are capable of producing the effector cytokines IFN-γ and TNF-α. We also demonstrate that in a vaccination setting, blocking CTLA-4 during CD8+T-cell priming leads to increased expansion and maintenance of antigen-specific memory CD8+T cells without adversely affecting the overall T-cell repertoire. This leads to an increase in memory cell effector function and improved protective immunity against further bacterial challenges. These results indicate that transient blockade of CTLA-4 enhances memory CD8+T-cell responses and support the possible use of CTLA-4–blocking antibodies during vaccination to augment memory formation and maintenance.

Summary The intestinal epithelium forms a barrier between the microbiota and the rest of the body. In addition, beyond acting as a physical barrier, the function of intestinal epithelial cells (IECs) in sensing and responding to microbial signals is increasingly appreciated and likely has numerous implications for the vast network of immune cells within and below the intestinal epithelium. IECs also respond to factors produced by immune cells, and these can regulate IEC barrier function, proliferation and differentiation, as well as influence the composition of the microbiota. The mechanisms involved in IEC–microbe–immune interactions, however, are not fully characterized. In this review, we explore the ability of IECs to direct intestinal homeostasis by orchestrating communication between intestinal microbes and mucosal innate and adaptive immune cells during physiological and inflammatory conditions. We focus primarily on the most recent findings and call attention to the numerous remaining unknowns regarding the complex crosstalk between IECs, the microbiota and intestinal immune cells.