Recently, Eberl et al. identified CP cells as the adult counterparts of fetal lymphoid tissue inducer (LTi) cells that are among the first haematopoetic cells to colonize developing lymphoid tissue such as Peyer’s patch anlagen [8,9]. By tracking the retinoic acid-related orphan receptor gamma T (RORγt), the authors found that this receptor is basically expressed by all buy AG-014699 lin- c-kit+ lamina propria lymphocytes (LPL). RORγt-deficient mice have an impaired thymic lymphopoiesis and, strikingly, have no CP and no isolated lymphoid follicles. The presence of regular numbers of γδTCR-positive IEL suggests that these cells are not the progeny of CP cells. The authors conclude that CP are more likely to serve as organizers

of inducible tertiary lymphoid tissue inside the gut. In this report, we show that lin- c-kit+ lymphocytes see more are not restricted to CP inside the gut. Even though this cell population expresses a broad variety of chemokine receptors, the expression of CCR6 identifies specifically those cells located within CP whereas diffusely distributed LTi cells express the chemokine receptor CXCR3, suggesting that CCR6 is a marker for CP cells. In addition, we show that CCR6 positive aggregates are also found within the human lamina propria, suggesting that these organized structures might have a role for inflammatory responses inside the human gut. The gene targeting strategy employed

to generate CCR6 enhanced green fluorescent protein (EGFP) knock-in mice has been described previously [10]. The homozygous CCR6-deficient mice used for these studies were back-crossed eight times to C57BL/6. CCR6 knock-out mice and heterozygous CCR6-deficient mice shared the same background. When genotyping of the individual offspring was required, a three-primer polymerase chain reaction (PCR) method was used. Comparisons of CCR6-deficient and knock-out mice were

made using heterozygous and homozygous knock-out mice that were typically littermates between 6 and 8 weeks of age. All experiments including mice were approved by the Institutional Animal Care and Use Committee (authorization no. 9·93·2·10·36·07·081). Experiments using human tissue were approved by the local ethical committee Sitaxentan (authorization no. 2007-206-f-S). Lamina propria lymphocytes were prepared by a standard method with minor modifications. Briefly, the small intestine was removed from euthanized mice, followed by identification and resection of Peyer’s patches. The remaining small intestinal tissue specimens were opened longitudinally and cut into 0·5-cm pieces, washed four times with cold Ca2+/Mg2+ (CMF) solution [Ca2+ and Mg2+-free Hanks's balanced salt solution (HBSS), 10 mM HEPES, 25 mM NaHCO3, 2% (v/v) fetal bovine serum (FBS), pH 7·2]. The intestinal tissue specimens were transferred into 30 ml of CMF/FBS/ethylenediamine tetraacetic acid (EDTA) solution [Ca2+ and Mg2+-free HBSS, 15 mM HEPES, 5 mM EDTA, 100 µg/ml gentamycin, 10% (v/v) FBS, pH 7·2].