The ontogeny and function of intestinal dendritic cells

Detta är en avhandling från Mucosal Immunology, EMV, Lund University

Sammanfattning: The intestinal mucosa presents a huge surface area to the external environment and the intestinal immune system is continually exposed to a wide array of foreign antigens derived from resident intestinal microbiota as well as dietary constituents, including food proteins. In order to maintain intestinal homeostasis, the intestinal immune system relies on the ability to generate tolerogenic responses to innocuous antigen while maintaining the ability to develop effective immune responses to intesinal pathogens. Dysregulated intestinal immune responses are thought to contribute to the development of inflammatory bowel diseases (IBD) and food allergies. Conventional dendritic cells (cDCs) are key regulators of adaptive immune responses. cDCs are found throughout the intestinal lamina propria and in gut associated lymphoid tissues (GALT) of mice and humans. Intestinal cDCs migrate constitutively to mesenteric lymph nodes (MLNs) where they present luminal derived antigen to T cells. Intestinal derived cDCs are thought to play an major role in the initiation and regulation of intestinal adaptive immune responses. The aim of the work described in this thesis was to study the development and function of intestinal cDC subsets.

At the start of the thesis work, CD103+ DCs had been identified in the intestinal lamina propria (LP) that were found to have an enhanced capacity to induce gut-homing receptors and generate FoxP3+ Tregs in vitro. Separate studies had identified a major subset of CX3CR1+ mononuclear phagocytes which, by virtue of their capacity to sample antigen from the intestinal lumen, were thought to play key roles in the initiation of intestinal adaptive immune responses. In paper I, we assess the relationship between CD103+ DCs and CX3CR1+ mononuclear phagocytes and establish that they represent distinct populations with separate ontogenies and diverse functional properties. By mulitphoton confocal microscopy of intestinal draining lymphatics, and flow cytrometric analysis of lymph, we show that only CD103+ cDCs migrate to MLNs to participate in the initiation of adaptive immune responses at this site. In contrast CX3CR1+ represent a tissue resident macrophage population that may influence immune responses locally in the gut.

Phenotypic analysis of CD103+ small intestinal cDC revealed that CD103+ cDCs can be divided into two distinct subsets based on their expression of CD11b. In paper II, we assess the ontogeny and in vivo functions of intestinal CD103+CD11b- and CD103+CD11b+ intestinal cDCs. cDC subset development depends on distinct transcription factors (TFs) and while the development of CD103+CD11b- cDC depends on the TFs Id2, IRF8 and BatF3, the TF requirements for CD103+CD11b+ were not known. Using a mouse model that selectively depletes the TF IRF4 selectively in CD11c+ cells, we show that, similar to splenic/lymph node CD11b+ cDCs, intestinal CD103+CD11b+ cDCs depend on IRF4 for their development. Further, we demonstrate that IRF4-dependent cDCs play a key role in the generation and maintenance of the intestinal interleukin 17 (IL-17)-secreting helper T (Th17) cell compartment. Mechanistically, in vitro and in vivo experiments suggest that the ability of CD103+CD11b+ intestinal cDCs to induce Th17 differentiation is linked to their capacity to produce IL-6.

In contrast, mice lacking IRF4-depending cDCs had normal number of intestinal regulatory T cells (Tregs). In paper III, we address the role of IRF4-dependent cDCS in the regualation of CD4+ T cell responses to orally administered innocuous antigen in vivo, with regards to induced Treg (iTreg) differentiation, and the ability to develop oral tolerance to soluble innocuous antigen. Moreover, we addressed the role of IRF4-dependent cDCs in the induction of gut-homing receptors on CD4+ T cells in vivo. Suprisingly, we find that IRF4-dependent cDCs are critical for the induction of CD4+ T cell responses to intraperitoneally, but not orally administered antigen. Moreover, we show that IRF4-dependent cDCS are not required for the development of iTregs or the establishment of oral toleranc. In contrast, they appear to play key roles in regulating gut-homing receptor expression and IL-10 production by CD4+ T cells in the MLNs.

Collectively, the studies included in this thesis help to broaden our understanding of the ontogeny and in vivo functions of intestinal cDC populations.