Adrian M. Duijvestijn

The role of nasopharyngeal lymphoid tissue

Nasal-associated lymphoid tissue (NALT), which comprises paired lymphoid organs in the nasopharynx of rodents, is the principal mucosal lymphoid tissue of the respiratory tract. As described in this review, NALT bears certain similarities to the Peyers patches of the intestine but the two differ remarkably in morphology, lymphoid migration patterns and the binding properties of their high endothelial venules (HEV).

Mechanisms and regulation of lymphocyte migration

Lymphocyte traffic seems to be an essential requirement for an adequate immune response both in lymphoid tissues and local inflammatory sites. In this review, Adrian Duijvestijn and Alf Hamann discuss how selective migration of lymphocytes is directed by lymphocyte-endothelial interactions and what mechanisms may control this.

The endothelium of the high endothelial venule: a specialized endothelium with unique properties.

High endothelial venules (HEV) in lymphoid organs are specialized to facilitate the passage of lymphocytes into lymphoid parenchyma. This is accomplished by a ligand-receptor system on the endothelium and lymphocytes, which differs between lymph nodes and Peyers patch. Experiments discussed in this paper show that other non-HEV-derived endothelial cells can acquire the characteristics of HEV and that HEV may differentiate under influences from their local micro-environments.

DIFFERENTIAL KINETICS OF VARIOUS SUBSETS OF THYMIC BONE MARROW-DERIVED STROMAL CELLS IN RAT CHIMERAS

Identification of those cells within the thymic stroma which are responsible for tolerance induction remains controversial. Evidence derived from studies of bone marrow chimeras or thymus transplants attributed this function to cells of haematopoietic origin, usually class II positive medullary dendritic cells (DC). Recent data suggest, however, that a stromal element located in the thymus cortex might be involved in negative selection. To further explore this issue we used immunohistology and immunocytology with a combination of allotype and cell type specific monoclonal antibodies (MoAb) to study the turnover of thymic stromal cells of haematopoietic origin to different rat models of allogeneic and congenic bone marrow (BM) radiation chimeras. Use of CFU‐GM cultured BM inoculum for congenic recipients allowed us to distinguish between direct homing of donor mycloid cells and the delayed migration of the donor stem cell progeny after the post‐irradiation recovery of the recipient. Our data indicate a heterogeneity in the turnover rate of thymic mobile stromal cells. While DC and a subset of macrophages located m the cortex as well as m the medulla (EDI+). within 4 weeks were virtually all of donor type, cortical macrophages detected by ED2 MoAbs were still incompletely replaced after a period as long as 20 weeks. Slow turnover, location and variable class II expression may imply a role for thymic cortical macrophages in (self‐) tolerance induction.

Functional Capacities of High Endothelial Venules Appear not to be Controlled by Recirculating Lymphocytes

The influence of recirculating lymphocytes on the function and morphology of high endothelial venules (HEV) has been studied. Mice were depleted of lymphocytes by lethal (1200 cGy) total body irradiation; subsequently, the HEV in mesenteric and cervical lymph nodes were studied up to 7 days after irradiation for: 1) capacity to bind lymphocytes by using the in vitro HEV-binding assay, 2) for morphological aspects such as ultrastructure and endothelial height, 3) for presence of RNA (pyroninophylia) and MECA-325 expression. Although, commencing 3 days after irradiation, lymphocyte depletion was intense and no extravasation of lymphocytes was observed; HEV were capable of binding lymphocytes at normal levels. Also the ratio of B/T cell binding to HEV was comparable to normal. MECA-325 expression, pyroninophilia, and ultrastructure of high endothelial cells were not affected by lymphocyte depletion. However, the average height of endothelial cells, which is a measurement related to cell volume, declined during lymphocyte depletion, stabilizing at about 70% of normal levels from day 4. After intravenous injection of viable lymph node cells, endothelial cell height rapidly increased within a few hours in conjunction with lymphocyte extravasation and homing into the nodes. Restoration of endothelial cell height was not observed after infusion of thymocytes, lethally irradiated lymph node cells or supernatants rich in cytokines. We conclude that recirculating lymphocytes in blood and lymphoid tissues are not involved in controlling high endothelial cell activity including the specific function in lymphocyte extravasation. However, recirculating/extravasating lymphocytes contribute to the development of endothelial cell height. The significance of non-lymphoid (radioresistant) cells in the control of characteristic high endothelial function is suggested.

Kinetics of Rat Thymic Dendritic Cells in Bone Marrow-Reconstituted Radiation Chimeras

The thymus meets unique microenvironmental conditions essential for pre-T cell maturation and generation of the complete T cell repertoire. During intrathymic development thymocytes acquire qualities essential for adequate immune functioning of T cells, such as tolerance for self, antigen receptors, and restriction to class I or II MHC antigens. With respects to restriction of T helper cells to class II (Ia) antigens of the MHC, it is more or less generally accepted that class II positive cells in the thymus control this aspect in T cell ‘education’. Potential candidates for regulating restriction of T helper cells to class II molecules are thymic reticular epithelial cells and bone marrow-derived thymic dendritic cells (DC), both known to be Ia+. Attending the question which cell type indeed is controlling class II restriction in the thymus, one can study T helper cell restriction to Ia antigens in allogeneic or semi-allogeneic bone marrow-reconstituted radiation chimeras (BMRRC) or thymus-grafted animals. Results obtained from these studies are conflicting, suggesting on one hand, that class II restriction of T cells is dictated by thymic epithelial cells (1), or on the other hand, that DC (antigen presenting cells) in the thymus are involved in this (2,3). Of crucial importance in such restriction studies is to know at what experimental stage the thymus can be considered class II-chimeric with respects to epithelial cell versus DC, viz. how long does it take in BMRRC for the recipient thymic DC to be replaced by allogeneic or semi-allogeneic donor type DC?

Changes in Lymphocyte Binding and Expression of his 22 of High Endothelial Venules in Rat Lymph Nodes after Occlusion of Afferent Lymph Flow

Recirculating lymphocytes migrate from the blood into lymph nodes through the walls of the high endothelial venules (HEV). HEV are lined by large plump so-called high endothelial cells (HE cells) to which lymphocytes specifically adhere. This specific interaction can be studied in vitro by overlaying lymphocyte suspensions on cryostat sections of lymph nodes (1-3).

Vascular Thymustransplantation in Rats: A New Method to Study Thymocyte Kinetics

Although many studies have been performed to unravel the cellular kinetics of thymic immigration, intrathymic differentation and thymic emigration, there is still not a generally accepted view on this issue (for review see Scollay (1,2)). An important reason for the contradictory results is the lack of an experimental model mimicking the normal physiological situation of the thymus.

Regulation of Functional and Morphological Aspects of High Endothelium in Mouse

Lymphocyte migration and recirculation between lymphoid and also nonlymphoid tissues is essential for effective immunological surveillance. The entrance of blood-borne lymphocytes into peripheral lymphoid organs, such as lymph nodes and Peyer’s patches, occurs at particular vascular sites, so called post-capillary high endothelial venules (HEV;1,2). The endothelium of these HEV is unique in it’s capacity to mediate lymphocyte extravasation, and its characteristic cuboidal or ‘high’ cellular appearance. From recent investigations, it has become clear that subtile mechanisms are controlling a balanced and selective immigration of lymphocytes and lymphocyte subsets via HEV into mucosal and non-mucosal lymphoid tissues (3,4). Major contributions to studying lymphocyte extravasation and homing via interaction with high endothelium have been the production of monoclonal antibodies (MABs) against lymphocyte ‘homingreceptors’ for HEV (5,6,7), and the development of an elegant in vitro assays for lymphocyte-binding to HEV in frozen sections by Stamper and Woodruff (8). With these tools the nature and many aspects of the regulation of homing receptors on lymphocytes and lymphocyte subsets have been unraveled. However, little is known of the nature and regulation mechanisms of the specific sets of ligands on high endothelium involved in lymphocyte binding, nor about why this endothelium develops it’s characteristic high appearance and what mechanisms regulate this process. In this paper we will address mechanisms controlling specific functional and morphological qualities of high endothelium. We summarize some previous results obtained with HEV-specific MAB MECA-325, e.g. expression of the MECA-325 antigen in chronic inflammation and induction of the antigen in cultured endothelial cells by lymphokines (9,10). In addition, an in vivo model is described in which severe depletion of recirculating lymphocytes by irradiation affects endothelial height but not its lymphocyte-binding capacity or MECA-325 antigen expression. By injecting recirculating lymphocytes the characteristic high endothelial cell appearance can be quickly restored.

Kinetics of Thymocyte Regeneration in Adult Adriamycin Treated Rats: Evidence for a Bone Marrow Derived Prothymocyte Precursor

The thymus is the central organ for T lymphocyte maturation in which prothymocytes undergo rearrangement of T cell receptor genes, acquire function associated antigens and become “educated ” to respond to foreign antigens in a self MHC class I or class II restricted manner.