Establishing two systems to study MAIT cell responses during bacterial infection

Abstract

Mucosal-associated invariant T (MAIT) cells are a population of innate-like T cells, which are abundant in humans and also present in many mammals, including mice [1]. MAIT cells express semi-invariant T cell receptors (TCR) are activated by recognizing microbial vitamin B2 (riboflavin) metabolites presented on non-classical major histocompatibility complex (MHC) class I related protein 1 (MR1) [1-6]. Upon activation, MAIT cells can either directly kill infected cells or secrete IFN, IL-17, TNF and other functional molecules that enhance the anti-microbial activity of other immune cells [7, 8]. These characteristics suggest an important role played by MAIT cells in anti-bacterial immunity in mammals.

Bacterial infections are major problems in clinical settings. While MAIT cells have shown antibacterial properties, clinically relevant in vivo animal models have not been extensively developed for further exploration of the mechanisms of MAIT cell protection. Using three clinically common bacteria; Klebsiella pneumoniae, Staphylococcus aureus, and Escherichia coli, this study established bacterial peritonitis models in mice for MAIT cell research. The three bacteria were all able to produce antigens that stimulate MAIT cells in vitro. For mouse infection models, the optimal dose of K. pneumoniae for inducing productive but nonlethal infection was not established in this study. S. aureus and E. coli peritonitis incurred mild and robust MAIT cell responses in vivo, respectively. Further experiments with E. coli peritonitis showed that MAIT cells accumulated in the intraperitoneal cavity, the spleen and liver, which contributed to bacteria control. The finding suggests possible MAIT cell-based treatments in clinical conditions.

While in vitro studies have shown that MAIT cells can be stimulated by various types of cells upon infection, there have been few investigations into how MAIT cells are activated in vivo. In the second project presented in this thesis, the contributions of antigen presenting cells (APCs) in initiating activation in MAIT cells was studied with Francisella tularensis infection. We found that APCs were burdened with bacteria in the early phase of infection. With an ex vivo cellular co-culture assay was specifically optimized for assessing MAIT cell activation induced by the F. tularensis burdened APCs, we found that alveolar macrophages contributed to the best activation of MAIT cells in an MR1 dependent manner. Other cell subsets: monocytes, neutrophils and dendritic cells were also capable of inducing a mild activation.

In summary, the present study helped optimize two novel systems for MAIT cell research. With the ex vivo cellular co-culture system, this thesis contributes to understanding how APCs activate MAIT cells in vivo upon bacterial infection. With the in vivo peritonitis models, this study introduces clinically relevant bacteria to the current MAIT cell research in mice, and ultimately to inform and complement clinical research. It is hoped that the insights gained from this study could be of assistance to current research methods and to achieve a better understanding of MAIT cell responses during bacterial infection.