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.

During positive selection, thymocytes transition through a stage during which T cell receptor (TCR) signaling controls CD4 versus CD8 lineage choice and subsequent maturation. Here, we describe a new T cell specific protein, THEMIS, that performs a distinct function during this stage. In Themis -/-mice, thymocyte selection was impaired and the number of transitional CD4 + CD8 int thymocytes as well as CD4 and CD8 single positive thymocytes was decreased. Remarkably, although no overt TCR-proximal signaling deficiencies were detected, Themis -/-CD4 + CD8 int thymocytes exhibited developmental defects consistent with attenuated signaling that were reversible by increased TCR stimulation. These results identify THEMIS as a critical component of the T cell developmental program and suggest that THEMIS functions to sustain and/or integrate signals required for proper lineage commitment and maturation.

Thymus seeding by T-lymphoid progenitor cells is a prerequisite for T-cell development. However, molecules guiding thymus colonization and their roles before and after thymus vascularization are unclear. Here we show that mice doubly deficient for chemokine receptors CCR7 and CCR9 were defective specifically in fetal thymus colonization before, but not after, thymus vascularization. The defective prevascular fetal thymus colonization was followed by selective loss of the first wave of T-cell development generating epidermal V␥3 ؉ ␥␦ T cells. Unexpectedly, CCL21, a CCR7 ligand, was expressed not by Foxn1-dependent thymic primordium but by Gcm2-dependent parathyroid primordium, whereas CCL25, a CCR9 ligand, was predominantly expressed by Foxn1-dependent thymic primordium, revealing the role of the adjacent parathyroid in guiding fetal thymus colonization. These results indicate coordination between Gcm2-dependent parathyroid and Foxn1-dependent thymic primordia in establishing CCL21/CCR7-and CCL25/CCR9-mediated chemokine guidance essential for prevascular fetal thymus colonization. (Blood. 2006;108: 2531-2539)