Gut Microbiota-Derived Unconventional T Cell Ligands: Contribution to Host Immune Modulation

Unconventional immune cells in the gut mucosal barrier: regulation by symbiotic microbiota

The mammalian gut is the most densely colonized organ by microbial species, which are in constant contact with the host throughout life. Hosts have developed multifaceted cellular and molecular mechanisms to distinguish and respond to benign and pathogenic bacteria. In addition to relatively well-characterized innate and adaptive immune cells, a growing body of evidence shows additional important players in gut mucosal immunity. Among them, unconventional immune cells, including innate lymphoid cells (ILCs) and unconventional T cells, are essential for maintaining homeostasis. These cells rapidly respond to bacterial signals and bridge the innate immunity and adaptive immunity in the mucosal barrier. Here, we focus on the types and roles of these immune cells in physiological and pathological conditions as prominent mechanisms by which the host immune system communicates with the gut microbiota in health and diseases.

Functional and metagenomic level diversities of human gut symbiont-derived glycolipids

Bioactive metabolites produced by symbiotic microbiota causally impact host health and disease, nonetheless, incomplete functional annotation of genes as well as complexities and dynamic nature of microbiota make understanding species-level contribution in production and actions difficult. Alpha-galactosylceramides produced by Bacteroides fragilis (BfaGC) are one of the first modulators of colonic immune development, but biosynthetic pathways and the significance of the single species in the symbiont community still remained elusive. To address these questions at the microbiota level, we have investigated the lipidomic profiles of prominent gut symbionts and the metagenome-level landscape of responsible gene signatures in the human gut. We first elucidated the chemical diversity of sphingolipid biosynthesis pathways of major bacterial species. In addition to commonly shared ceramide backbone synthases showing two distinct intermediates, alpha-galactosyltransferase (agcT), the necessary and sufficient component for BfaGC production and host colonic type I natural killer T (NKT) cell regulation by B. fragilis, was characterized by forward-genetics based targeted metabolomic screenings. Phylogenetic analysis of agcT in human gut symbionts revealed that only a few ceramide producers have agcT and hence can produce aGCs, on the other hand, structurally conserved homologues of agcT are widely distributed among species lacking ceramides. Among them, alpha-glucosyl-diacylglycerol(aGlcDAG)-producing glycosyltransferases with conserved GT4-GT1 domains are one of the most prominent homologs in gut microbiota, represented by Enterococcus bgsB . Of interest, aGlcDAGs produced by bgsB can antagonize BfaGC-mediated activation of NKT cells, showing the opposite, lipid structure-specific actions to regulate host immune responses. Further metagenomic analysis of multiple human cohorts uncovered that the agcT gene signature is almost exclusively contributed by B. fragilis , regardless of age, geographical and health status, where the bgsB signature is contributed by >100 species, of which abundance of individual microbes is highly variable. Our results collectively showcase the diversities of gut microbiota producing biologically relevant metabolites in multiple layers-biosynthetic pathways, host immunomodulatory functions and microbiome-level landscapes in the host.

Die Kämpfe únd schláchten-the struggles and battles of innate-like effector T lymphocytes with microbes

The large majority of lymphocytes belong to the adaptive immune system, which are made up of B2 B cells and the αβ T cells; these are the effectors in an adaptive immune response. A multitudinous group of lymphoid lineage cells does not fit the conventional lymphocyte paradigm; it is the unconventional lymphocytes. Unconventional lymphocytes-here called innate/innate-like lymphocytes, include those that express rearranged antigen receptor genes and those that do not. Even though the innate/innate-like lymphocytes express rearranged, adaptive antigen-specific receptors, they behave like innate immune cells, which allows them to integrate sensory signals from the innate immune system and relay that umwelt to downstream innate and adaptive effector responses. Here, we review natural killer T cells and mucosal-associated invariant T cells-two prototypic innate-like T lymphocytes, which sense their local environment and relay that umwelt to downstream innate and adaptive effector cells to actuate an appropriate host response that confers immunity to infectious agents.

Natural Killer T and Natural Killer Cell-Based Immunotherapy Strategies Targeting Cancer

Both natural killer T (NKT) and natural killer (NK) cells are innate cytotoxic lymphoid cells that produce inflammatory cytokines and chemokines, and their role in the innate immune response to tumors and microorganisms has been investigated. Especially, emerging evidence has revealed their status and function in the tumor microenvironment (TME) of tumor cells. Some bacteria producing NKT cell ligands have been identified to exert antitumor effects, even in the TME. By contrast, tumor-derived lipids or metabolites may reportedly suppress NKT and NK cells in situ. Since NKT and NK cells recognize stress-inducible molecules or inhibitory molecules on cancer cells, their status or function depends on the balance between inhibitory and activating receptor signals. As a recent strategy in cancer immunotherapy, the mobilization or restoration of endogenous NKT or NK cells by novel vaccines or therapies has become a focus of research. As a new biological evidence, after activation, effector memory-type NKT cells lasted in tumor-bearing models, and NK cell-based immune checkpoint inhibition potentiated the enhancement of NK cell cytotoxicity against cancer cells in preclinical and clinical trials. Furthermore, several new modalities based on the characteristics of NKT and NK cells, including artificial adjuvant vector cells, chimeric antigen receptor-expressing NK or NKT cell therapy, or their combination with immune checkpoint blockade have been developed. This review examines challenges and future directions for improving these therapies.