Enterohepatic cooperation in the postnatal establishment of immune homeostasis

• Enterohepatische Kooperation in der postnatalen Etablierung der Immunhomöostase

Schwentker, Annika; Hornef, Mathias (Thesis advisor); Pabst, Oliver (Thesis advisor); Blank, Lars M. (Thesis advisor)

Aachen : RWTH Aachen University (2023)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2023

Abstract

Adaption to extra-uterine life after birth represents a challenging task for the host. At this time of high energy demand, nutrition changes from parental to enteral. This requires efficient mucosal absorption of dietary nutrients and might explain the transiently enhanced permeability of the gut mucosa for macromolecules known to cease during what has been termed gut closure. Moreover, the neonatal immune system must adapt to rapid bacterial colonization, especially of the intestinal mucosa, and sudden exposure to microbial stimuli. Gut-absorbed molecules via the portal vein first pass the liver, an organ shifting from a hematopoietic to a metabolic organ in mice only after birth, before entering the systemic circulation. The mechanisms that facilitate efficient nutrient absorption but prevent an inappropriate immune activation by gut-derived microbial stimuli have so far remained ill-defined and are the focus of my work. I could show that oral administration of macromolecular tracers and immune stimuli such as lipopolysaccharide (LPS) to neonate but not adult mice leads to internalization by enterocytes in the distal small intestine and increased serum levels. Since macromolecular internalization by enterocytes was restricted to the distal small intestinal epithelium, characterized in neonates by the presence of supranuclear vacuoles (SNVs) and expression of the endocytic adaptor Disabled homolog 2 (Dab2), I tested the hypothesis of a Dab2-dependent enterocyte internalization and translocation process. Intestinal epithelial cell specific Dab2 knockout mice exhibited reduced protein and fluorescein isothiocyanate (FITC) dextran internalization by enterocytes. However, the translocation of FITC dextran and the internalization and translocation of LPS was unaffected. In contrast, using polarized intestinal epithelial stem cell organoid cultures on transwell filters, I could identify the contribution of the fatty acid translocase CD36, which is highly expressed in the neonatal small intestinal epithelium, for LPS translocation. Furthermore, I could identify increased expression and secretion of the LPS detoxifying enzyme acyloxyacyl hydrolase (AOAH) by the neonatal intestinal epithelium as a potential compensatory mechanism for the elevated LPS exposure. Flow cytometry and quantitative polymerase chain reaction (qPCR) analysis of liver tissue revealed enlarged hepatic immune cell populations in the neonatal liver and enhanced transcriptional activation in neonate but not adult mice upon oral LPS administration. Hepatic cytokine expression was influenced by oral supplementation of the LPS antitoxin Pep19-2.5, altered myeloid differentiation primary response 88 (MyD88) signaling or feeding. Together, my results highlight the complexity of intestinal macromolecular transport during early life, identify a pathway of LPS translocation and characterize a potential compensatory mechanism to prevent inappropriate immune activation to gut-derived LPS. Furthermore, my work helps to explain the previously described immune stimulation during early life and provides the basis for future studies on the long-term consequences for immune homeostasis.

Institutions

• Department of Biology [160000]
• Chair of Applied Microbiology [161710]
• [525500-2]