Role of CCR9 in OT-II cell recruitment to the small intestinal LP. (A) CCR9^–/– and WT OT-II cells are first detected in the small intestinal LP 3 days after intraperitoneal administration of OVA plus LPS. (B) The majority of CCR9^–/– and WT OT-II cells entering the small intestinal LP 3 days after intraperitoneal administration of OVA plus LPS express α₄β₇. Numbers represent mean (SEM) positive cells from 3 separate experiments. (C) The CCR9^–/–/WT OT-II ratio was determined in the MLNs, liver, and small intestinal LP 3 days after intraperitoneal administration of OVA plus LPS. CCR9^–/–/WT OT-II cell ratio was calculated by dividing the percentage of CCR9^–/– OT-II cells with the percentage of WT OT-II cells in each organ. The ratios for liver and LP were normalized to the MLN ratio for each animal. Mean (SEM) CCR9^–/–/WT OT-II ratio from 21 (MLNs), 17 (liver), and 21 (small intestinal LP) mice from 5 (liver) and 6 (LP) experiments, respectively. *P < .001, paired 2-tailed Student t test. NS indicates not significant. (D) CD4⁺ T cells within the small intestinal villous LP of CCR9^–/– and WT OT-II mice as assessed by immunohistochemistry analysis.