Antigen targeting to MHC class II-enriched compartments in colonic epithelial cells of Crohn's disease patients

Background & Aims: In Crohn's disease (CD), colonic epithelial cells (CECs) are suggested to stimulate pro-inflammatory CD4+ T cells. The endocytic pathways of lumenal antigens, involved in underlying MHC class II presentation by CECs, remain to be defined. Our aim was to elucidate antigen trafficking and associated MHC class II expression in CECs of CD patients in vivo. Methods: In patients with CD and healthy controls undergoing colonoscopy, ovalbumin (OVA)(150mg/mL in saline; 10mL) was sprayed directly onto inflamed or normal mucosa. Mucosal biopsies were taken before and 5, 10 and 20minutes after OVA exposure. The subcellular localisation of OVA and MHC class II was visualized using fluorescence and cryo-electron microscopy. Results: Transport of OVA to late endocytic compartments of CECs was seen in CD and control patients already 10 minutes after exposure. MHC class II expression in CECs was not detected in healthy mucosa but strongly upregulated in CD patients. The majority of MHC class II molecules in CECs, seen during CD, were present at the basolateral membrane and in vacuolar late endosomes. Internalized OVA was sorted into these MHC class II containing late endosomes within a period of 10 minutes after lumenal administration. Conclusion: We suggest MHC class II-enriched late endosomes, found in CECs of CD patients but absent in controls, to represent class II/peptide loading compartments, MIICs. Our data provide in vivo evidence that lumenal antigens access these MIICs in CECs and strenghten the postulated role of CECs in MHC class II-associated antigen-presentation during CD.