Characterization of CCR9 expression and thymus-expressed chemokine responsiveness of the murine thymus, spleen and mesenteric lymph node.

CC chemokine receptor 9 (CCR9) is a receptor expressed at high levels in immature thymocytes, small intestine trafficking T cells and IgA-producing plasma cells. CCR9 mediates chemotaxis in response to thymus-expressed chemokine (TECK) selectively expressed in the thymus and small intestine. CCR9 expression in different subpopulations of thymus, spleen and mesenteric lymph node (MLN) cells was analyzed by flow cytometry and TECK responsiveness of those lymphoid cells was assessed by a Transwell migration assay. CCR9 surface expression level did not completely correlate with cellular chemotaxis to its cognate ligand TECK. The active chemotaxis to TECK was observed in CD4 single positive thymocytes and CD4(-)B220(hi) splenocyte and MLN cells, which poorly expressed CCR9 on their surface. TECK responsiveness of CCR9-abundant subpopulations in the thymus and MLN was unremarkable except for CD4(+)B220(hi) subset of the MLN, and was evident in the CCR3(+) subsets of the thymus and spleen. Exposure to TECK did not affect CCR9 expression in the thymus, spleen and MLN, except for the CD4(+)CD8(+) thymocyte. CCR9 was exuberantly expressed in the cytoplasm of lymphoid cells. CCR9 may act in concert with CCR3 for in terms of TECK responsiveness. Its cytoplasmic location may allow precise regulation of leukocyte responsiveness to TECK.