In vivo analysis of MAIT cell in health and disease – MAIT

MAIT cells (MAITs) are innate-like T cells that recognize a new class of antigen, derivatives of riboflavin (Rib) precursors present in >85% of bacteria and yeasts in the gut. The absence of MAITs leads to increased bacterial load in several experimental infection models and MAIT blood numbers are decreased in several human bacterial diseases. MAIT blood levels and activation status are also modified in several pathologies in which dysbiosis of the gut microbiota have been implicated: inflammatory bowel diseases (IBD), Multiple Sclerosis and type 2 diabetes and obesity. MAITs may sense and help to fight invasive bacteria or be involved in commensal/host symbiotic loops that regulate epithelium or gut microbial composition homeostasis.

MAITs are very abundant in human blood (1-8%) and liver (20-40%), but are rare (< 0.1%) in the lymphoid organs of laboratory mice. This has limited the study of MAIT biology as mouse MAITs could not be isolated or manipulated in the absence of specific marker. We recently generated a B6-congenic mouse strain with increased (20 fold) number of GFP tagged MAITs that enables the study of naturally occurring MAITs. This model has recently been improved to further increase MAIT number in the tissues.

We previously showed that human MAITs exit the thymus as naïve cells that are already engaged in a specific differentiation program as shown by IL-18Ra and CD161 expression. MAITs then expand in the periphery in a process requiring both the commensal flora and B cells. In this project we will study the development of MAIT cells in the periphery. We will characterize where and when the bacterial ligand encounters MR1 on B cells to allow for peripheral MAIT cell activation and expansion. We will define the differentiation program of murine MAIT cells in comparison with NKT and conventional T cells to allow the manipulation of their functions in vivo using the relevant KO strains. All the phenotypic features of MAITs point to a tissue residency program. We will make parabiotic mice and inject blocking antibodies such as anti-LFA1 and –ICAM1 to determine whether MAITs behave like NKTs with a loss of tissue residency and influx to the blood.

The ability for a bacterial strain to activate MAITs strictly correlates with the presence of the Rib biosynthesis pathway. Ablation of any enzyme upstream of the intermediary compound 5-Amino-Ribityl Uracil (5-A-RU) in this pathway abolishes the stimulatory capacity of gram+ and gram- bacteria. However, it is currently unknown how the bacterial unstable 5-A-RU molecule or its derivatives have access to the MR1-expressing cells in vivo. The in vivo availability of the activating MR1-ligand complexes were not studied either. To address these questions, we will quantify the expression of the bacterial genes synthesizing the MAIT ligands in the different gut segments. We will also directly measure the amounts of the ligands in the different intestinal segments. Riboflavin deficient or proficient bacteria as well as synthetic MAIT ligand or new more stable analogs will be introduced systemically or per os in germ-free mice. We will determine the in vivo dynamics of synthetic MAIT ligands and MR1/ligand complexes using medium throughput in vitro bioassays with exquisite sensitivity.
To get better insight into MAIT function in health and disease, using our new mouse model, we will manipulate the functions of MAIT cells in vivo by inactivating the candidates genes that are the more likely to be involved in their function. We will study the mechanisms involved in the protective effect of MAIT cells that we observed in an experimental IBD model. We will assess whether MAITs are involved in the anti-bacteria firewall function of the liver. We will examine their ability to clear a systemic bacterial infection and test whether MAIT properties can be used for therapeutic purpose in oncology by injecting MAIT ligands into tumors or as adjuvant to vaccine against nominal antigen.