Lymphocyte homing to secondary lymphoid tissue is defined by a multistep sequence of interactions between lymphocytes and endothelial cells in high endothelial venules (HEVs). After initial selectin-mediated tethering and rolling, firm adhesion of lymphocytes requires rapid upregulation of lymphocyte integrin adhesiveness. This step is mediated in part by the HEV-derived chemokine SLC (secondary lymphoid-tissue chemokine, or CCL21) that binds to the CC chemokine receptor (CCR)7 on lymphocytes. However, the CC chemokine ELC (Epstein-Barr virus–induced molecule 1 ligand chemokine, or CCL19) shares the same receptor, and ELC transcripts have been observed in the T cell areas of lymphoid organs. Here, we show that perivascular ELC is transcytosed to the luminal surfaces of HEVs and enables efficient T cell homing to lymph nodes. In situ hybridization on sections of human tonsil showed no ELC mRNA in HEVs, but immunostaining revealed ELC protein in cytoplasmic vesicles of HEV cells. Furthermore, ELC injected into the footpads of mice entered the draining lymph nodes and was presented by HEVs. Finally, intracutaneous injections of ELC in mice lacking functionally relevant ELC and SLC (plt/plt mice) restored T cell trafficking to draining lymph nodes as efficiently as SLC. We conclude that perivascular ELC is transcytosed to the luminal surfaces of HEVs and participates in CCR7-mediated triggering of lymphocyte arrest.

IntroductionRheumatoid arthritis (RA) and ulcerative colitis (UC) are chronic inflammatory diseases associated with de novo formation of irregular T-and B-cell aggregates in the synovium 1 and large bowel mucosa, 2 respectively. Such local development of lymphoid tissue is thought to contribute to the pathology of chronic inflammation. 3,4 Lymphoid tissue organization is orchestrated by a subset of the chemokine family, termed homeostatic or lymphoid chemokines because of their constitutive expression in secondary lymphoid tissues. 5 One of these lymphoid chemokines, CXCL13, also called B cell-attracting chemokine 1 (BCA-1) 6 or B-lymphocyte chemoattractant (BLC), 7 and its receptor CXCR5, also called Burkitt lymphoma receptor-1 (BLR1), are required for normal development of secondary lymphoid organs in mice. 8,9 CXCL13 and CXCR5 have, in addition, been detected in various chronic human inflammatory diseases where lymphoid neogenesis occurs, such as Helicobacter pylori gastritis, 10 RA, 1,11 Sjögren syndrome, 12-14 and UC 15 as well as in gastric 10 and primary central nervous system lymphoma. 16 Follicular dendritic cells (FDCs) are generally believed to be the main source of CXCL13 in normal 6,7,17 as well as inflamed lymphoid tissue. 1,10,11 In addition, vascular expression of the CXCL13 protein has been reported, 1,15,16,18,19 and CXCL13 and CXCR5 have been shown to play an important role in B-cell homing to murine Peyer patches. 20 However, no expression analysis of CXCL13 mRNA in human lymphoid tissue 10,15,16,18 has directly shown that CXCL13 is actually produced by FDCs or endothelial cells. Importantly, available data do not exclude the possibility that murine FDCs in fact bind CXCL13 rather than produce it. 7,17 Also of note, CXCL13 protein is undetectable in human FDC-like cells stimulated in vitro. 21 We have shown earlier that another lymphoid chemokine, CCL19, is present and functions in endothelial cells that are not its source. 22 In our recent human study of normal and aberrant gutassociated lymphoid tissue, we found that CXCL13 was mainly associated with extracellular fibrils and only minimally with cells displaying the traditional FDC phenotype. 15 This prompted us to explore more extensively the source of this chemokine in human lymphoid neogenesis represented by RA synovium and UC large bowel mucosa. Furthermore, we investigated the potential mechanistic role monocyte-derived cells could play in CXCL13 secretion. Our data documented for the first time that human monocytes/ macrophages are a potent inducible source of CXCL13 and, in fact, appear to be the main producers of this chemokine in inflammatory lesions where lymphoid neogenesis occurs. This suggested a role of recently extravasated macrophages in the formation of such ectopic follicles. Patients, materials, and methods PatientsClinicopathologic data and patient treatment schedules are provided in Table 1. Cryosections (8 m) were cut serially from various human tissue For personal use only. on May 10, 2018. by guest www.bloodjournal.o...

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Naive T cells are usually excluded from nonlymphoid tissues. Only when such tertiary tissues are subjected to chronic inflammation, such as in some (but not all) autoimmune diseases, are naive T cells recruited to these sites. We show that the CCR7 ligand CC chemokine ligand (CCL)21 is sufficient for attracting naive T cells into tertiary organs. We performed intravital microscopy of cremaster muscle venules in T-GFP mice, in which naive T cells express green fluorescent protein (GFP). GFP+ cells underwent selectin-dependent rolling, but no firm adherence (sticking). Superfusion with CCL21, but not CXC chemokine ligand 12, induced integrin-dependent sticking of GFP+ cells. Moreover, CCL21 rapidly elicited accumulation of naive T cells into sterile s.c. air pouches. Interestingly, a second CCR7 ligand, CCL19, triggered T cell sticking in cremaster muscle venules, but failed to induce extravasation in air pouches. Immunohistochemistry studies implicate ectopic expression of CCL21 as a mechanism for naive T cell traffic in human autoimmune diseases. Most blood vessels in tissue samples from patients with rheumatoid arthritis (85 ± 10%) and ulcerative colitis (66 ± 1%) expressed CCL21, and many perivascular CD45RA+ naive T cells were found in these tissues, but not in psoriasis, where CCL21+ vessels were rare (17 ± 1%). These results identify endothelial CCL21 expression as an important determinant for naive T cell migration to tertiary tissues, and suggest the CCL21/CCR7 pathway as a therapeutic target in diseases that are associated with naive T cell recruitment.