Dominique Dunon

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.

Current concepts in lymphocyte homing and recirculation

Referee: Dr. E.J. Kundel, 154-B VAML, 3801 Miranda Ave, Palo Alto, CA 94304 ALTA The immune system consists of a complex collection of leukocytes and dendritic cells that surveys most tissues in the body for the appearance of foreign antigens. For an efficient immune response, the interaction and co-localization of antigen-presenting cells, co-stimulatory helper cells and effector cells are crucial parameters. Therefore, the migration routes of antigen-presenting cells and potential antigen-specific lymphocytes merge in secondary lymphoid organs in order to increase the likelihood and speed of a lymphocyte finding its cognate antigen. Additionally, antigen-primed effector cells are directed to the tissue where they are most likely to encounter their cognate antigen. This highly organized and efficient antigen encounter is based on a continuous recirculation of antigen-specific lymphocytes between blood, peripheral tissue, and secondary lymphoid organs. Moreover, the efficacy of the immune system is further increased by the ability of different lymphocyte subsets to recirculate only through distinct tissues. The scope of this review is to outline the concept and mechanisms of lymphocyte homing and recirculation and to discuss the significance for the immune defense. Current models in leukocyte homing and recirculation and the underlying molecular functions of implicated cell adhesion molecules, chemokines, and chemokine receptors are discussed.

Renewal of thymocyte progenitors and emigration of thymocytes during avian development

The avian thymus is colonized by three waves of hemopoietic progenitors during embryogenesis. An in vivo thymus reconstitution assay based on intrathymic injection of irradiated chicks showed that cells of para-aortic foci were able to differentiate into T lymphocytes, confirming their putative role in the first wave of thymus colonization. This assay was also used to detect and to characterize T cell progenitors from the bone marrow which are involved in the second and third wave of thymus colonization. In the bone marrow, progenitors that differentiated into T cells were found in a subpopulation that expressed the molecules HEMCAM, c-kit and c128. Engraftment of thymus lobes into thymectomized young chick recipients showed that T cell progenitors are replaced in the thymus by subsequent waves of progenitors after hatching. Finally, analysis of thymocyte differentiation suggested that gamma delta and alpha beta T cells migrate from the thymus to the periphery in alternating waves.

Development of the Avian Immune System

The avian embryo provides several advantages for studies on development of the immune system. These include the existence of a clear demarcation between the B and T cell systems, with each population differentiating in a specialized primary lymphoid organ—T cells in the thymus and B cells in the bursa of Fabricius. In addition, there is an availability of large numbers of embryos at precise stages of development. Because of its importance to the poultry industry, much research on the avian system has used the domestic chicken; this has been helped by the ready availability of different congenic and inbred lines, genetic markers and monoclonal antibodies (mAb)—the essential tools for studying the development of the immune system. The quail–chick chimeras have also proved to be an especially informative model, particularly for studying the emergence of hematopoietic stem cells (HSC) and their migration to the primary lymphoid organs during embryogenesis.

A dileucine motif targets MCAM-l cell adhesion molecule to the basolateral membrane in MDCK cells

Melanoma cell adhesion molecule (MCAM), an adhesion molecule belonging to the Ig superfamily, is an endothelial marker and is expressed in different epithelia. MCAM is expressed as two isoforms differing by their cytoplasmic domain: MCAM‐l and MCAM‐s (long and short). In order to identify the respective role of each MCAM isoform, we analyzed MCAM isoform targeting in polarized epithelial Madin–Darby canine kidney (MDCK) cells using MCAM‐GFP chimeras. Confocal microscopy revealed that MCAM‐s and MCAM‐l were addressed to the apical and basolateral membranes, respectively. Transfection of MCAM‐l mutants established that a single dileucine motif (41‐42) of the cytoplasmic domain was required for MCAM‐l basolateral targeting in MDCK cells. Although double labelling experiments showed that MCAM‐l is not a component of adherens junctions and focal adhesions, its expression on basolateral membranes suggests that MCAM‐l is involved in epithelium insuring.

Endoglin expression level discriminates long-term hematopoietic from short-term clonogenic progenitor cells in the aorta

CD105 is an auxiliary receptor for the transforming growth factor beta superfamily, highly expressed on proliferating endothelial cells and adult hematopoietic stem cells. Because CD105 mRNA expression was reported in the developing aortic region, we further characterized its expression profile in the aorta and examined the hematopoietic potential of CD105+ cells. Aortic endothelial cells, intra-aortic hematopoietic cell clusters and the purified cell fraction enriched in progenitor/hematopoietic stem cell activity expressed CD105. Aortic hematopoietic short-term clonogenic progenitors were highly enriched in the CD105intermediate population whereas more immature long-term progenitors/hematopoietic stem cells are contained within the CD105high population. This places CD105 on the short list of molecules discriminating short-term versus long-term progenitors in the aorta. Furthermore, decreasing transforming growth factor beta signaling increases the number of clonogenic progenitors. This suggests that CD105 expression level defines a hierarchy among aortic hematopoietic cells allowing purification of clonogenic versus more immature hematopoietic progenitors, and that the transforming growth factor beta pathway plays a critical role in this process.

Identification of the pre–T-cell receptor α chain in nonmammalian vertebrates challenges the structure–function of the molecule

In humans and mice, the early development of αβ T cells is controlled by the pre–T-cell receptor α chain (pTα) that is covalently associated with the T-cell receptor β (TCRβ) chain to form the pre–T-cell receptor (pre-TCR) at the thymocyte surface. Pre-TCR functions in a ligand-independent manner through self-oligomerization mediated by pTα. Using in silico and gene synteny-based approaches, we identified the pTα gene (PTCRA) in four sauropsid (three birds and one reptile) genomes. We also identified 25 mammalian PTCRA sequences now covering all mammalian lineages. Gene synteny around PTCRA is remarkably conserved in mammals but differences upstream of PTCRA in sauropsids suggest chromosomal rearrangements. PTCRA organization is highly similar in sauropsids and mammals. However, comparative analyses of the pTα functional domains indicate that sauropsids, monotremes, marsupials, and lagomorphs display a short pTα cytoplasmic tail and lack most residues shown to be critical for human and murine pre-TCR self-oligomerization. Chicken PTCRA transcripts similar to those in mammals were detected in immature double-negative and double-positive thymocytes. These findings give clues about the evolution of this key molecule in amniotes and suggest that the ancestral function of pTα was exclusively to enable expression of the TCRβ chain at the thymocyte surface and to allow binding of pre-TCR to the CD3 complex. Together, our data provide arguments for revisiting the current model of pTα signaling.

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.

Avian HSC emergence, migration, and commitment toward the T cell lineage

To date three sites of emergence of hemopoietin cells have been identified during early avian development: the yolk sac, the intraaortic clusters and recently the allantois. However, the contributions of the hematopoietic stem cell (HSC) populations generated by these different sites to definitive hematopoiesis and their migration routes are not fully unraveled. Experimental embryology as well as the establishment of the genetic cascades involved in HSC emergence help now to draw a better scheme of these processes.

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.