Beat A. Imhof

Cell adhesion in the immune system

Cell adhesion molecules have an important role to play in many facets of the immune system. At a recent meeting their role in leukocyte migration, inflammation, cancer metastasis and lymphocyte development was discussed.

To stick or not to stick: the new leukocyte homing paradigm

The immune system is formed by leukocytes. They are passively transported through the body by the vascular system, but their entrance into tissues requires a coordinated series of events, namely activation of leukocyte integrins, adhesion to the vascular endothelium, and migration. There are four steps in this process, which begin with the rolling of leukocytes along the vascular endothelium, followed by signaling which activates leukocyte integrins, thus leading to tight adhesion to the endothelium and finally transmigration. Substantial progress has been made recently in elucidating the molecular events that induce rolling and signaling, partly as a result of the study of double-knockout mice that are deficient for genes encoding two selectins.

Vanin-1, a Novel GPI-Linked Perivascular Molecule Involved in Thymus Homing

Migration of hematopoietic precursor cells to the thymus is shown to depend upon a novel molecule called Vanin-1 expressed by perivascular thymic stromal cells. An anti-Vanin-1 antibody blocks the binding of pro-T cells to thymic sections in vitro, the in vivo accumulation of bone marrow cells around cortical thymic vessels, and long-term thymic regeneration. Thus, it interferes with the entry, and not the differentiation, of hematopoietic precursor cells. The Vanin-1 gene codes for a GPI-anchored 70 kDa protein that shows homology only with human biotinidase. Transfection of thymic stromal cells with the Vanin-1 cDNA enhances thymocyte adhesion in vitro. These data suggest that Vanin-1 regulates late adhesion steps of thymus homing under physiological, noninflammatory conditions.

Suppression of mouse melanoma metastasis by EA-1, a monoclonal antibody specific for alpha 6 integrins

The rat monoclonal antibody (mAb) termed EA-1 was originally selected for its capacity to block the adhesion of T lymphocyte progenitors to mouse thymic endothelium. Here we show that the mAb EA-1 recognizes the alpha 6 chain of alpha 6 beta 1 and alpha 6 beta 4 integrins. Both molecules are present at a high level on the luminal and basolateral side of vascular endothelium and alpha 6 beta 1 integrin is expressed on the highly metastatic cell lines B16/129 (melanoma) and KLN-205 (carcinoma). These lung specific tumors bind preferentially to lung frozen sections, and EA-1 blocked this interaction in vitro. Moreover, mAb EA-1 inhibited experimental metastasis to the lung of B16/129 cells injected intravenously. Metastasis in vivo was blocked when the antibody was injected into mice before or simultaneously with the melanoma cells, as well as when melanoma cells were precoated with EA-1 before injection. We suggest that alpha 6 integrins play a dual role in the metastatic process, mediating the adhesion of tumor cells to the luminal surface of the endothelium and the adhesion to laminin in the subendothelial extracellular matrix during extravasation. Despite the fact that alpha 6 integrins are laminin receptors, EA-1 did not interfere with melanoma cell binding to laminin fragments. Our antibody EA-1 may therefore recognize a binding domain on alpha 6 integrins of a novel ligand involved in cell-cell interaction.

Alpha 6 integrin distribution in human embryonic and adult tissues.

Alpha 6 integrin is an adhesion molecule that connects cells with extracellular matrix molecules of the laminin family. The laminin interaction seems to be essential for cell differentiation during embryogenesis and for the subsequent maintenance of tissue integrity in the adult. Alpha 6 integrin can also interact with laminin-independent cellular ligands and in this way plays a role in homing of leucocytes. Furthermore, in cancer biology α6 integrin has an important role in metastasis and as a possible new prognostic factor; exact knowledge of α6 integrin distribution in normal human tissues is therefore a crucial element. By immuno-histochemical methods we have screened α6 integrin expression of representative human tissues from the adult and the embryonic organism. All tested epithelia were α6 integrin positive, except for the endocrine cells of the pancreas and the adrenal glands. Heterogeneous staining was found on non-epithelial tissues. Strong staining was evident in peripheral nerves (Schwann cells), germ and Sertoli cells, endothelia, and smooth muscle cells of the myometrium. Weak staining was found in nerve cells of the stratum granulosum, the microglia, Kupffers cells and stromal cells of the ovary. All fibroblasts, striated muscle cells and astrocytes were negative. The tissue distribution of α6 integrin and the semi-quantitative estimation of their expression level should provide a better understanding of α6 integrin function under normal and phathological conditions, in particular in tumour progression.

T cell migration during ontogeny and T cell repertoire generation

Before their hematopoietic stem cell origin was recognized, chicken thymocytes were thought to be derived from epithelial precursors in the thymus that could be converted into lymphocytes under the influence of surrounding mesenchymal tissue (Auerbach 1961). Evidence obtained later indicated that embryonic yolk sac contains the precursors of lymphoid cells (Moore and Owen 1967). Yolk sac stem cells at the first and second day of embryonation (E1–2) were then shown to be derived from the embryo itself (Martin et al. 1978). Further studies indicated the presence of pluripotent stem cells in the region of the thoracic aorta as early as E4, a few days before some stem cells can be found in the spleen. These studies, conducted in chick-quail chimeras, indicated an aortic (E4) followed by a paraortic (E6) origin of precursors of thymocytes, B cells and myeloid cells. It was suggested that embryonic stem cells native to the aortic region migrate via the circulation and colonize the spleen, yolk sac, and finally the bone marrow. However, recent studies have shown that the stem cells produced in these different organs during ontogeny may correspond to different populations generated separately since they do not present identical properties (see chapter by Dieterlen).

Pro-T cell homing to the thymus.

Thymus colonization during embryogenesis starts with the accumulation of basophilic cells in the jugular vein, in capillaries and in the mesenchyme surrounding the thymus. In birds the extrinsic origin of these basophilic cells, which are considered to be hematopoietic precursors, was established by the construction of quail-chick chimaeras (Le Douarin 1978; Le Douarin and Jotereau 1980). Using this technique the group of Le Douarin demonstrated that the thymus of birds is colonized mainly in three waves during embryogenesis and the first few days following hatching (Jotereau and Le Douarin 1982; Coltey et al. 1987). In mice, the thymus is colonized by hematopoietic precursors at days 10 and 13 of embryogenesis (Jotereau et al. 1987). A similar process, albeit at a lower level, may occur throughout life (Scollay et al. 1986; Donskoy and Goldschneider 1992). The major cellular events of thymus homing are illustrated in Fig. 1.

Adhesion in leukocyte homing and differentiation

The Dominance of Antigen-Specific Receptors in Antigen-Specific Immune Responses.- I Adhesion Molecules.- Integrins and Their Ligands.- Platelet Endothelial Cell Adhesion Molecule (CD31).- CD44: A Multitude of Isoforms with Diverse Functions.- The Selectins and Their Ligands.- II Regulation of Leukocyte-Endothelial Cell Adhesion.- A Model of Leukocyte Adhesion to Vascular Endothelium.- Regulation of Adhesion Receptor Expression in Endothelial Cells.- Regulation of Leukocyte Recruitment by Proadhesive Cytokines Immobilized on Endothelial Proteoglycan.- III Lymphoid Cell Homing Mechanisms.- Migration of Activated Lymphocytes.- The Peyers Patch Homing Receptor.- Pro-T Cell Homing to the Thymus.- Quantitative Analysis of Lymphocyte Fluxes In Vivo.- Lymphocyte Recirculation and Life Span In Vivo.- IV Leukocyte Homing to Inflamed Tissues.- The Contributions of Integrins to Leukocyte Infiltration in Inflamed Tissues.- Regulation of Adhesion and Adhesion Molecules in Endothelium by Transforming Growth Factor-?.- Transendothelial Migration.- V Adhesion Molecules in Differentiation and Activation of Lymphocytes.- CD44 and Other Cell Interaction Molecules Contributing to B Lymphopoiesis.- CD4, CD8, and CD2 in T Cell Adhesion and Signaling.- Activation and Inactivation of Adhesion Molecules.

Involvement of α6 and αv Integrins in Metastasis

The occurrence of metastasis requires detachment of metastatic tumor cells or the primary tumor. It is known that lost of several adhesion molecules by cels of the primary tumor is involved in this event, for example, E-cadherin in carcinomas (Frixen et al. 1991) and α4 integrin in melanomas (Qian et al. 1994). Migrating metastatic cells enter the blood vasculature or the lymphatics through which they are transported to peripherical organs. In the periphery the cells must adhere in some way to the vascular endothelium before they can extravasate and form a secondary tumor. This chapter describes the adhesion molecules α6 and αv integrins and shows at what point they are involved in the adhesion and migration of metastatic tumor cells.

A Chemotactic Assay Enabling Recovery of Cells after Migration

Publisher Summary This chapter discusses a chemotactic assay enabling recovery of cells after migration. Chemotaxis determines the direction of cell locomotion using a soluble gradient of a chemical substance. If the chemotactic substance is present in an isotropic unchanging concentration, then cell locomotion is random. Alternatively, if the substance is present in an anisotropic or gradient concentration, the locomotion becomes directed along this gradient. Chemotaxis can be measured using a number of techniques based on different principles. Using an under-agarose assay, the net movements of a population resulting from a chemotactic response are measured, but cells must survive for a long period and must also be able to migrate in agarose. In this assay, thin micropore filters are used; the attracted cells pass through the filter and fall into the lower part of the well. Migrated cells can then be counted, phenotyped, and/or submitted to differentiation experiments.