The Cc Chemokine Thymus-Derived Chemotactic Agent 4 (Tca-4, Secondary Lymphoid Tissue Chemokine, 6ckine, Exodus-2) Triggers Lymphocyte Function–Associated Antigen 1–Mediated Arrest of Rolling T Lymphocytes in Peripheral Lymph Node High Endothelial Venules

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Abstract
T cell homing to peripheral lymph nodes (PLNs) is defined by a multistep sequence of interactions between lymphocytes and endothelial cells in high endothelial venules (HEVs). After initial tethering and rolling via L-selectin, firm adhesion of T cells requires rapid upregulation of lymphocyte function–associated antigen 1 (LFA-1) adhesiveness by a previously unknown pathway that activates a Gαi-linked receptor. Here, we used intravital microscopy of murine PLNs to study the role of thymus-derived chemotactic agent (TCA)-4 (secondary lymphoid tissue chemokine, 6Ckine, Exodus-2) in homing of adoptively transferred T cells from T-GFP mice, a transgenic strain that expresses green fluorescent protein (GFP) selectively in naive T lymphocytes (TGFP cells). TCA-4 was constitutively presented on the luminal surface of HEVs, where it was required for LFA-1 activation on rolling TGFP cells. Desensitization of the TCA-4 receptor, CC chemokine receptor 7 (CCR7), blocked TGFP cell adherence in wild-type HEVs, whereas desensitization to stromal cell–derived factor (SDF)-1α (the ligand for CXC chemokine receptor 4 [CXCR4]) did not affect TGFP cell behavior. TCA-4 protein was not detected on the luminal surface of PLN HEVs in plt/plt mice, which have a congenital defect in T cell homing to PLNs. Accordingly, TGFP cells rolled but did not arrest in plt/plt HEVs. When TCA-4 was injected intracutaneously into plt/plt mice, the chemokine entered afferent lymph vessels and accumulated in draining PLNs. 2 h after intracutaneous injection, luminal presentation of TCA-4 was detectable in a subset of HEVs, and LFA-1–mediated TGFP cell adhesion was restored in these vessels. We conclude that TCA-4 is both required and sufficient for LFA-1 activation on rolling T cells in PLN HEVs. This study also highlights a hitherto undocumented role for chemokines contained in afferent lymph, which may modulate leukocyte recruitment in draining PLNs.

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