Failure of T cell homing, reduced CD4/CD8αα intraepithelial lymphocytes, and inflammation in the gut of vitamin D receptor KO mice

Specific pathogen- free IL- 10 KO mice failed to develop inflammatory bowel disease (IBD), whereas IL- 10/vitamin D receptor (VDR) double KO mice developed fulminating IBD. WT CD4 T cells inhibited experimental IBD, while VDR KO CD4 T cells failed to suppress IBD. VDR KO mice had normal numbers and functions of regulatory T cells. The percentages of IL- 17- and IFN-gamma-secreting T cells in the gut of mice reconstituted with WT and VDR KO CD4 T cells were also not different. Instead, there were twice as many CD8 alpha alpha intraepithelial lymphocytes (IEL) in mice that were reconstituted with WT CD4 T cells than in mice reconstituted with VDR KO CD4 T cells. Furthermore, VDR KO mice had reduced numbers of CD8 alpha alpha IEL, absent CD4/CD8 alpha alpha populations, and as a result low IL- 10 production in the IEL. The lack of CD8 alpha alpha IEL was due in part to decreased CCR9 expression on T cells that resulted in the failure of the VDR KO T cells to home to the small intestine. We conclude that the VDR mediates T cell homing to the gut and as a result the VDR KO mouse has reduced numbers of CD8 alpha alpha IEL with low levels of IL- 10 leading to increased inflammatory response to the normally harmless commensal flora.