Air-liquid_interface_Caco-2_culture_with_vasointestinal_peptide_ mimicks_ gut_ mucosal_ barrier_ function_ in_ permeability_ and_bacterial infection

The intestinal mucosal barrier is a dynamic system that allows nutrient uptake, stimulates healthy microbe-host interactions, and prevents invasion by pathogens. The mucosa consists of epithelial cells connected by cellular junctions that regulate the passage of nutrients covered by a mucus layer that plays an important role in host-microbiome interactions. Mimicking the intestinal mucosa for in vitro assays, in particular the generation of a viscous mucus layer, has proven to be challenging. Here, we present a novel in vitro intestinal culture model that produces a robust mucus layer and is based on the widely used intestinal Caco-2 cell line. We investigated the effects of air-liquid interface (ALI) culturing compared to liquid-liquid interface (LLI) on Caco-2 cells grown under low-glucose conditions in Transwell plates. In addition, we determined the impact of vasointestinal peptide (VIP) on mucus secretion, epithelial barrier properties and microbe-mucus interactions. A combination of ALI culturing with VIP addition led to formation of a robust mucus layer on the apical surface of the Caco-2 confluent layer in which the intestinal secreted mucin MUC2 was a major component. RNAseq analysis demonstrated upregulation of unique gene clusters in response to ALI and VIP conditions, but the ALI-VIP combination treatment resulted in a significant upregulation of multiple mucin genes and proteins including MUC2, MUC13 and MUC17. Expression of tight junction proteins was significantly altered in the ALI-VIP condition leading to increased permeability to small molecules, more closely reflecting an intestinal epithelium permissive for nutrient uptake. In infection experiments with commensal Lactobacillus plantarum, pathogenic Salmonella enterica serovar Enteritidis, or enterotoxigenic Escherichia coli (ETEC), the ALI-VIP mucus layer separated the bacteria from the underlying epithelium. In conclusion, ALI-VIP culture of Caco-2 cells provides an attractive in vitro model to study the function of the intestinal mucosal barrier and pathogenic and commensal microbe-host interactions.