E. C. M. Hoefsmit

Ultrastructure of the rat thymus: The micro-environment of T-lymphocyte maturation

SummaryThe ultrastructure of the micro-environment of the fully functional rat thymus was studied. The thymus consists of two discrete compartments, viz., an epithelial and a mesenchymal compartment. Thymus fibroblasts/fibrocytes, mast cells and granulocytes, are restricted to the mesenchymal compartment. The thymocyte maturation process seems to occur in the epithelial compartment in a network of reticular epithelial cells. The cortex is finely meshed and filled with proliferating thymocytes and some scattered macrophages. Moreover, in the medulla vacuolated epithelial cells form part of a loosely meshed reticulum which is filled with thymocytes and interdigitating cells (IDCs). IDCs frequently contain Birbeck granules and appear to be phagocytic. Together with macrophages, they probably enter the thymus, predominantly in the cortico-medullary region, and cross the separating wall between the two compartments. Some functional aspects of the non-lymphoid cells and in particular the IDCs, which form the micro-environment of the thymus, are discussed with respect to T-cell development.

Lymph node macrophages and reticulum cells in the immune response

SummaryThe morphology and kinetics of macrophages and reticulum cells of rat lymph nodes have been studied in relation to the immune response to a second exposure to antigen. During the first 24 h after stimulation monocyte-like exudate macrophages, including some scattered interdigitating cells (IDC), contain granules similar to those present in epidermal Langerhans cells and lymph-borne veiled cells. In this induction phase these macrophages migrate from the marginal sinus into the paracortex and during the migration they gradually transform into IDC. In the proliferation phase the paracortex is mainly populated by transitional macrophages and there are almost no typical IDC present between the lymphoblasts. In the memory phase the relative number of IDC again rapidly increases. During this period in the paracortex there are often typical IDC which contain partially digested necrotic lymphocytes, thus resembling tingible body macrophages (TBM) of the germinal centre in this respect.It is suggested that the newly arrived macrophages induce the lymphoblast reaction, while mature IDC may have an inhibitory function in the memory phase of the immune response. In this phase the phagocytic potential of IDC is clearly shown.

Depletion of macrophages and disappearance of postcapillary high endothelial venules in lymph nodes deprived of afferent lymphatic vessels.

SummaryThe afferent lymphatic vessels of rat popliteal lymph nodes were interrupted, and the histological alterations in the lymph nodes occurring 1 to 14 weeks after operation were studied. One week after operation the number of macrophages was considerably reduced and continued to decrease during the subsequent time periods studied. A 6 weeks most macrophages had disappeared. Simultaneously the immunological activity diminished and had completely disappeared 8 weeks after operation. Three weeks after operation the endothelial cells of the postcapillary high endothelial venules had flattened, and the number of immigrating lymphocytes was greatly reduced. Subsequently the lymph nodes became depleted of both macrophages and lymphocytes, leaving only the reticuloendothelial framework.

Interdigitating cells and macrophages in the acute involuting rat thymus

SummaryPhagocytic activity and population development of medullary interdigitating cells (IDC) and cortical macrophages have been studied in the rat thymus after irradiation-induced thymocyte necrosis. IDC clearly demonstrate phagocytic activity, particularly in the 16h stage after irradiation. At this stage the number of necrotic thymocytes is maximal and the total number of phagocytic cells is insufficient to phagocytize all necrotic material. IDC increase in number slightly and are predominantly phagocytizing in the corticomedullary region (CMR), where they appear to develop from macrophage-like cells.The results indicate that medullary IDC can be phagocytic but have a different developmental pathway than cortical macrophages. Cortical macrophages greatly increase in number and acquire the appearance of tingible body macrophages by phagocytizing many necrotic thymocytes. They seem to develop from monocytes that normally enter the thymus at the CMR. During the acute involution macrophages probably also enter the cortex via the connective tissue capsule. It is suggested that thymus medullary IDC probably belong to the mononuclear phagocytes, as do the cortical macrophages. In the specific medullary environment IDC gradually develop their characteristic ultrastructure for an apparently other than phagocytic function. The similarities between IDC and epidermal Langerhans cells are discussed.

Characterization of the population of phagocytic cells in thymic cell suspensions

SummaryRat thymic phagocytic cells were characterized in vitro using various light and electron-microscopical techniques. Thymic cell suspensions were mechanically prepared and enriched for non-lymphoid cells, which were predominantly phagocytic and of three types. Type I showed acid phosphatase (APh) activity in small granules dispersed throughout the cytoplasm and were mostly Ia antigen-positive, although the Ia membrane label varied in intensity and distribution among individual cells. Only a few cells had endogenous peroxidase activity. The type-I cells could not be clearly distinguished morphologically from type-II or -III cells, and most likely comprise precursors of both these cell types. Type-II were large pale cells with many slender cell processes. These cells had APh activity centrally positioned, were strongly positive for Ia on the cell membrane and were negative for endogenous peroxidase. The cytoplasm frequently contained Birbeck granules, which unequivocally classifies these cells as the in vitro equivalent of the interdigitating cells present in the medullary area of the thymus in situ. Type-III cells were rounded with a smooth or ruffled cell membrane and contained vacuoles and many phagolysosomes. They were strongly positive for APh which was present throughout the cytoplasm. About 50% of these cells were positive for endogenous peroxidase in a pattern resembling resident macrophages. The cells were negative for Ia antigens. Type-III cells mostly likely represent the macrophages found in the cortical area of the thymus.

Characterization of dendritic cells, isolated from normal and stimulated lymph nodes of the rat

SummaryNon-lymphoid dendritic cells were isolated from normal and paratyphoid vaccine-stimulated lymph nodes draining the rat skin. They were studied using enzymecytochemical, immunocytochemical and electron-microscopical methods. These cells had an irregular outline and an eccentrically situated nucleus. All showed acid phosphatase activity in a central area and expressed Ia antigen on the plasma membrane. Birbeck granules were exclusively present in dendritic cells isolated from lymph nodes in the induction phase of the immune response. This observation concurs with the presence of Birbeck granules in interdigitating cells in situ during the same period of the immune response. It is concluded that the dendritic cells are the in-vitro equivalents of the non-actively phagocytizing population of interdigitating cells.

Lymph node accessory cells in the immune response

SummaryThe immune response in the rat parathymic lymph node was studied after administration of antigen into the peritoneal cavity. Special attention was paid to the accessory cells,which might induce the response. During the induction phase of the response a heterogeneous population of non-lymphoid mononuclear cells was present in the subcapsular sinus and the cortex of the node. These cells resemble “veiled cells” described in skin draining lymph and interdigitating cells in the paracortex of skin draining lymph nodes, but they do not contain Birbeck granules. It is concluded that the appearance of these granules depends on the site of the exudate provocation and that the presence of the organelles in these accessory cells is not obligatory for lymphocyte stimulation.

The relation between rat lymph node interdigitating cells in situ and dendritic cells in suspension

Interdigitating cells (IDC) are present in the thymus dependent areas of peripheral lymphoid organs such as lymph nodes. They resemble the epidermal Langerhans cells in many aspects: both cell types form the same characteristic interdigitations with the surrounding cells, having the pale cytoplasm and the irregularly shaped, most eccentrically situated nucleus. However, in non-stimulated lymph nodes Birbeck granules, which are the characteristic cell organelles of the Langerhans cells, are usually not present in IDC (1). IDC have a central spot of acid phosphatase (APh) activity and the plasma membrane is strongly Ia positive. These cells may function as accessory cells in antigen presentation to immunocompetent T cells (2).

The Relation Between Thymic Interdigitating Cells and Dendritic Cells

In the thymus histologically and functionally distinct areas can be distinguished. The cortex predominantly comprises closely packed proliferating thymocytes. The medulla is characterized by the presence of immunocompetent T cells intermingled with interdigitating cells (IDC) (1). These IDC sometimes show distinct signs of phagocytic activity suggesting that they belong to the mononuclear phagocytes (2). However, it can not be excluded that they belong to a separate population of dendritic cells (DC) recently isolated from rat thymus (3).*