Activated CCR9+ OT-1 cells selectively localize to the small intestine. (a) CCR9 expression on CD8αβ+ lymphocytes isolated from tissue effector sites. Cells were stained with anti-CCR9 antibody (open) or anti-CCR9 antibody that had been preincubated with the CCR9-immunizing peptide (shaded), and analyzed by flow cytometry. Results are representative stainings from six mice for small-intestinal and colonic IELs and three mice for small-intestinal LPLs. Skin CD8αβ+ lymphocytes were obtained from the ears of mice with 2,4-dinitro-1-fluorobenzene–induced DTH and are from two stainings using ears pooled from a total of 40 mice. Numbers represent percentage of CD8αβ+ lymphocytes that are CCR9+ with background staining removed. (b) OT-1 cells (Ly5.2+) were injected into C57BL/6J-Ly5.1 mice. Three days after intraperitoneal administration of OVA, lymphocytes were isolated from the small intestine, and the expression of CCR9 on OT-1 lymphocytes was determined by flow cytometry. Results are from five (IEL) and one (LPL) separate stainings using a total of eleven and three mice respectively. Numbers represent the percentage of OT-1 cells expressing CCR9 with background staining removed. (c) CCL25 is selectively expressed in the murine small intestine (SI). Quantitative analysis of CCL25 mRNA expression was determined by real-time RT-PCR.