The role and regulation of MHC-II expression on intestinal macrophages
Guillaume, Joel; Pabst, Oliver (Thesis advisor); Zenke, Martin (Thesis advisor); Blank, Lars M. (Thesis advisor)
Aachen : RWTH Aachen University (2020, 2021)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2020
Intestinal macrophages play a vital role in controlling the balance of immunity and tolerance. Much is known about their ability to regulate both tolerogenic and inflammatory T cell responses by the production of a variety of cytokines, both in the small intestine (SI) and colon. Traditionally, macrophages have been considered antigen-presenting cells (APCs), based on their expression of the antigen presenting molecule MHC-II. However, little is known about the capacity of intestinal macrophages to present antigen and stimulate CD4 T cell responses locally. Furthermore, it is unclear which molecular mechanisms and environmental factors regulate the high expression of MHC-II on the surface of intestinal macrophages. In this study, we demonstrate that MHC-II expression on macrophages depends both on their ontogeny and tissue of residence. Monocyte-derived tissue macrophages almost exclusively had high levels of surface MHC-II, regardless of the tissue of residence. Interestingly, amongst different prenatally derived tissue macrophages, both SI and colonic Tim4+ macrophages in mice were the highest MHC-II-expressing populations. In the SI, all macrophages were uniformly high in their MHC-II expression. However, in the colon, a substantial population of prenatally derived Tim4+ macrophages expressed low levels of surface MHC-II. Furthermore, MHC-II expression on mouse SI and colonic macrophages exhibited different kinetics during neonatal development and in particular around weaning. Since colonic macrophages are in close contact with the gut microbiota, we hypothesised that this interaction may play a major role in regulating their MHC-II expression. In accordance with this, we observed lower frequencies of MHC-IIhigh and an increase in MHC-IIlow macrophages in the colon, but not in the small intestine of germ-free (GF) or antibiotics-treated (ABX) mice. Notably, in both cases, the MHC-IIlow colonic macrophages were Tim4 positive, suggesting that the microbiota specifically regulates MHC-II expression on prenatally-derived resident colonic macrophages. The microbiota of mice that were treated with ABX showed a significant reduction in crucial producers of short chain fatty acids (SCFAs) and supplementation of ABX-treated mice with SCFAs partially ameliorated the effect. Interestingly, we also observed a significant decrease in MHC-IIhigh Tim4+ macrophages in the colon of MyD88-/- mice, indicating that direct recognition of the microbial PAMPs regulates expression of MHC-II on prenatally-derived colonic macrophages. Taken together, our data suggest that MHC-II expression on resident colonic, but not monocyte-derived or SI macrophages, is, at least in part, regulated by the microbiota. We next addressed the functional relevance of macrophage MHC-II expression. Intestinal macrophages were capable of presenting soluble protein and peptide antigens to both naïve and activated CD4 T cells in vitro, although at a lower efficiency compared to dendritic cells (DCs). To study the role of MHC-II on intestinal macrophages in stimulating local CD4 T cell responses in vivo, we established a novel bone marrow chimeric model, which resulted in a macrophage-specific MHC-II knockout, while minimally affecting other cell populations. Notably, in these mice, we observed decreased frequencies and numbers of resident effector SI CD4 T cells. Moreover, we observed decreased proliferation amongst endogenous resident CD4 T cells, but also newly-arrived CD4 T cells in the SI, but not the colon. Our data therefore suggest a role for MHC-II expression on SI macrophages in stimulating local effector CD4 T cell expansion. These data can contribute to a better understanding of the role of local antigen presentation by macrophages in stimulating T cell responses in the intestine and the role of antigen presentation by intestinal macrophages in regulating local immune responses, such as in food allergy or inflammatory bowel disease (IBD).