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Ethical constraints restrict direct tracking of immune-cell migration throughout the human body in vivo. We, therefore, used deletion of the immunoglobulin M (IgM) heavy-chain constant-gene (Cmu) segment as a marker to provide a dispersal signature of an effector B-cell subset (IgD(+)IgM(-)CD38+) induced selectively in human tonsils. By DNA analysis, the Cmu deletion identified dissemination of such blasts and their plasma-cell progeny to peripheral blood, lymph nodes, and bone marrow, as well as to mucosae and glands of the upper airways. Also the endocervix was often positive, while the small intestine was mainly negative, as could be expected from the identified homing-molecule profile of the marker cells, with relatively low levels of integrin alpha4beta7 and CC chemokine receptor 9 (CCR9). Of further importance for vaccine design, the circulating cells expressed abundantly CD62L (L-selectin) and CCR7, which provided a mechanism for integration of respiratory and systemic immunity. Most mucosal vaccines are at present administered perorally, and our results suggested that the nasal route is no alternative for vaccination against rotavirus or other small-intestinal infections in humans. However, immunization of nasopharynx-associated lymphoid tissue clearly appears preferable to target respiratory pathogens and may to some extent also protect against infections of the female genital tract.

T-cell homing to secondary lymphoid tissues generally depends on chemokine-induced firm adhesion in high endothelial venules (HEVs) and is primarily mediated through the CC chemokine receptor 7 (CCR7) on lymphocytes. The CCR7 ligand designated CCL21 is considered the most important trigger because it appears constitutively expressed by murine HEVs. Surprisingly, when we analyzed human tissues, no CCL21 mRNA could be detected in HEVs. In fact, CCL21 mRNA was only expressed in extravascular T-zone cells and lymphatics, whereas immunostaining revealed CCL21 protein within HEVs. This suggests that T-cell recruitment to human lymphoid tissues depends on the transcytosis of lymphoid chemokines through HEV cells because there is at present no evidence of alternative chemokine production in these cells that could explain the attraction of naive T lymphocytes.

The homeostatic chemokine CXCL13 (also called B cell-attracting chemokine 1 [BCA-1] or B-lymphocyte chemoattractant [BLC]) is constitutively expressed in secondary lymphoid tissue and initiates lymphoid neogenesis when expressed aberrantly in mice. CXCL13 has also been detected in chronic inflammation associated with human lymphoid neogenesis, suggesting a pathogenic role. Follicular dendritic cells (FDCs) are generally considered to be the major source of CXCL13 both in normal and aberrant lymphoid tissue. We show here, instead, that most CXCL13-expressing cells in rheumatoid arthritis and ulcerative colitis are of monocyte/macrophage lineage. They are located in irregular lymphoid aggregates within an FDC network, but also within and near smaller collections of B cells in diseased tissue where no FDCs are detected. Some of these CXCL13-expressing cells are CD14(+), suggesting derivation from recently extravasated monocytes. Interestingly, monocytes from healthy donors stimulated in vitro with lipopolysaccharide secrete CXCL13. This induced production is enhanced after in vitro maturation of the monocytes toward macrophages but markedly decreased after maturation toward dendritic cells. Together, our findings strongly suggest that newly recruited monocytes/macrophages play a role for lymphoid neogenesis in human inflammatory diseases. Circulating monocytes are therefore potential candidates for future targeted therapy of chronic inflammation.

Endothelial cells display remarkable heterogeneity in different organs and vascular beds. Although many studies suggest that tissues "speak" to endothelial cells, endothelial cell diversity remains poorly characterized at the molecular level. Here, we describe a novel strategy to characterize tissue-specific endothelial cell phenotypes and to identify endothelial cell genes that are under the control of the local microenvironment. By comparing post-capillary high endothelial venule endothelial cells (HEVECs), freshly isolated from human tonsils without any cell culture step, with HEVECs cultured for 2 days, we found that HEVECs rapidly lost their specialized characteristics when isolated from the lymphoid tissue microenvironment. Striking changes occurred as early as after 48 hours, with complete loss of the postcapillary venule-specific Duffy antigen receptor for chemokines (DARCs) and the HEV-specific fucosyltransferase Fuc-TVII. DNA microarray analysis identified several other candidate HEV genes that were rapidly down-regulated ex vivo, including type XV collagen, which we characterized as a novel, abundant HEV transcript in situ. Together, our results demonstrate that blood vessel type-specific and tissue-specific characteristics of endothelial cells are under the control of their microenvironment. Therefore, even short-term primary cultures of human endothelial cells may not adequately mimic the differentiated endothelial cell phenotypes existing in vivo.