Shoji Uehara

A Role for CCR9 in T Lymphocyte Development and Migration

CCR9 mediates chemotaxis in response to CCL25/thymus-expressed chemokine and is selectively expressed on T cells in the thymus and small intestine. To investigate the role of CCR9 in T cell development, the CCR9 gene was disrupted by homologous recombination. B cell development, thymic αβ-T cell development, and thymocyte selection appeared unimpaired in adult CCR9-deficient (CCR9−/−) mice. However, competitive transplantation experiments revealed that bone marrow from CCR9−/− mice was less efficient at repopulating the thymus of lethally irradiated Rag-1−/− mice than bone marrow from littermate CCR9+/+ mice. CCR9−/− mice had increased numbers of peripheral γδ-T cells but reduced numbers of γδTCR+ and CD8αβ+αβTCR+ intraepithelial lymphocytes in the small intestine. Thus, CCR9 plays an important, although not indispensable, role in regulating the development and/or migration of both αβ− and γδ− T lymphocytes.

Characterization of CCR9 Expression and CCL25/Thymus-Expressed Chemokine Responsiveness During T Cell Development: CD3highCD69+ Thymocytes and γδTCR+ Thymocytes Preferentially Respond to CCL25

CCR9 mediates chemotaxis of thymocytes in response to CCL25/thymus-expressed chemokine, and its mRNA is selectively expressed in thymus and small intestine, the two known sites of T lymphopoiesis. To examine the expression of CCR9 during lymphocyte development, we generated polyclonal Ab that recognizes murine CCR9. CCR9 was expressed on the majority of immature CD4+CD8+ (double-positive) thymocytes, but not on immature CD4−CD8− (double-negative) thymocytes. CCR9 was down-regulated during the transition of double-positive thymocytes to the CD4+ or CD8+ (single-positive) stage, and only a minor subset of CD8+ lymph node T cells expressed CCR9. All CCR9+ thymocyte subsets migrated in response to CCL25; however, CD69+ thymocytes demonstrated enhanced CCL25-induced migration compared with CD69− thymocytes. Ab-mediated TCR stimulation also enhanced CCL25 responsiveness, indicating that CCL25-induced thymocyte migration is augmented by TCR signaling. Approximately one-half of all γδTCR+ thymocytes and peripheral γδTCR+ T cells expressed CCR9 on their surface, and these cells migrated in response to CCL25. These findings suggest that CCR9 may play an important role in the development and trafficking of both αβTCR+ and γδTCR+ T cells.

Premature Expression of Chemokine Receptor CCR9 Impairs T Cell Development

During thymocyte development, CCR9 is expressed on late CD4−CD8− (double-negative (DN)) and CD4+CD8+ (double-positive) cells, but is subsequently down-regulated as cells transition to the mature CD4+ or CD8+ (single-positive (SP)) stage. This pattern of expression has led to speculation that CCR9 may regulate thymocyte trafficking and/or export. In this study, we generated transgenic mice in which CCR9 surface expression was maintained throughout T cell development. Significantly, forced expression of CCR9 on mature SP thymocytes did not inhibit their export from the thymus, indicating that CCR9 down-regulation is not essential for thymocyte emigration. CCR9 was also expressed prematurely on immature DN thymocytes in CCR9 transgenic mice. Early expression of CCR9 resulted in a partial block of development at the DN stage and a marked reduction in the numbers of double-positive and SP thymocytes. Moreover, in CCR9-transgenic mice, CD25high DN cells were scattered throughout the cortex rather than confined to the subcapsular region of the thymus. Together, these results suggest that regulated expression of CCR9 is critical for normal development of immature thymocytes, but that down-regulation of CCR9 is not a prerequisite for thymocyte emigration.