CCR9 and colitis: An increase in CD4+ cells in the colonic lamina propria lymphocyte and intraepithelial lymphocyte populations of mdr1a−/− mice with active colitis has been described [27, 28]; it may well be that such an increase results from the concurrent upregulation of CCL25 in the colon coupled with an increase in circulating CCR9+CD4+ T cells, both documented in our studies.