Elisabeth C. M. Hoefsmit

Mannose Receptor Mediated Antigen Uptake and Presentation in Human Dendritic Cells

In an immature state, dendritic cells (DC) can capture antigen via at least two mechanisms. First, DC use macropinocytosis for continuous uptake of large amounts of soluble antigens. Second, they express high levels of mannose receptor that can mediate internalization of glycosylated ligands. We found that dendritic cells can present mannosylated antigen 100-1000 fold more efficiently than non-mannosylated antigen. Immunocytochemistry as well as subcellular fractionation demonstrated that the mannose receptor and MHC class II molecules were located in distinct subcellular compartments. These results demonstrate that the mannose receptor endows DC with a high capacity to present glycosylated antigens at very low concentrations.

Effect of Bacillus Calmette-Guérin Inoculation on Numbers of Dendritic Cells in Bronchoalveolar Lavages of Rats

Dendritic cells (DC) are present in all lymphoid tissues and are widely distributed in the airway epithelium and lung parenchyma. In this study DC were morphologically and cytochemically identified in normal rat bronchoalveolar lavage (BAL), although in very low percentages. Furthermore, the total population as well as different Percoll density fractions demonstrated poor antigen-presenting capacity and even suppressed antigen-specific stimulation by rat splenic DC. In contrast, when an inflammatory response was induced by intratracheal inoculation of Bacillus Calmette-Guérin (BCG), an increase of Ia-positive cells, containing high percentages of monocytes and DC (MDC) was found. In BAL, DC increased about 25 times within 48 h after BCG inoculation. These BCG-induced BAL cells as well as the different density fractions showed a high antigen-presenting capacity at low concentrations. However, at higher concentrations they were suppressive, except for the highest density fraction which lacked alveolar macrophages (AM). These results indicate that the increased numbers of Ia-positive MDC during an inflammatory reaction are very likely responsible for antigen presentation in vitro. In contrast, AM suppress the antigen-specific T cell proliferation in a concentration dependent manner.

Localization of Class II Molecules in Storage Vesicles, Endosomes and Lysosomes in Human Dendritic Cells

Class II molecules are a prerequisite for antigen presentation. We studied whether class II molecules can be found in the endocytic and/or lysosomal route of dendritic cells (DC), which are very potent antigen-presenting cells. Therefore first immunolabelling for HLA-DR alpha chain was applied on ultrathin cryosections of cells of which plasma membrane HLA-DR/DQ molecules were labelled in suspension, followed by incubation with the endocytic marker BSA-gold. Second, immunolabelling for HLA-DR alpha chains was applied on ultrathin cryosections of cells on which enzyme cytochemistry for acid phosphatase (APh) was performed to see whether the class II positive vesicles belong to the lysosomal compartment. Third, this immunolabelling was applied on cryosections of cells pretreated with the protein synthesis inhibitor cycloheximide (CHX) to see whether the class II positive vesicles are derived from biosynthesis. We found limited uptake of BSA-gold into endosomes and lysosomes, some of which also contained endocytozed HLA-DR/DQ. APh and HLA-DR were observed in the same vesicles but also vesicles containing either HLA-DR or APh were found. However, many class II positive vesicles were found, which were apparently not accessible to exogenous molecules. Moreover, the amount of class II positive vesicles decreased after treatment of the cells with CHX, suggesting that these vesicles form part of the biosynthetic route. These results imply that there is a cluster of class II positive vesicles, probably a storage compartment, that has connections with the lysosomal system. The concentration of lysosomes and class II positive vesicles in the juxtanuclear area of DC is probably of crucial importance in the processing of antigens.

Development of Exudate-Resident Macrophages, on the Basis of the Pattern of Peroxidatic Activity in Vivo and in Vitro

The diaminobenzidine (DAB) technique developed by Graham and Karnovsky (1966) has been widely used in the study of the localization of peroxidatic activity in various types of cell, including macrophages. On the basis of the intercellular distribution of peroxidatic activity, two types of macrophage have been distinguished, viz. resident macrophages and exudate macrophages. There are discrepancies between the findings of different authors as well as differences between the animal species used, as described below.

Veiled Cells Resembling Langerhans Cells

Langerhans cells were first described in skin sections stained with gold chloride (Langerhans 1868) and since that time there has been much speculation concerning their origin and function. At first it was thought that they were nerve cells, since they could be visualized by staining methods used for the. demonstration of nervous tissue, such as osmium iodide (Mishima & Miller-Milinska 1961) and the quinone-imine dyes (Billingham & Medawar 1953). In 1948 it was suggested that Langerhans cells were effete melanocytes (Masson 1948). Birbeck et al. (1961), studying vitiligo, described the ultrastructural features of these cells and since that time their criteria have been adopted for the identification of Langerhans cells. According to their description, Langerhans cells have a clear cytoplasm and a lobulated nucleus, lack desmosomes and tonofilaments and also premelanosomes and melanosomes, but contain disc or racket-shaped granules now generally referred to as Langerhans granules or Birbeck granules. The function of these organelles is unknown. It has been suggested that they are secretory organelles, formed from the Golgi apparatus, and that they transport lipid to the cell surface and release it to the exterior (Riley 1974). Other observations suggest that they are formed by an infolding of the plasma membrane. As a result of this movement, substances present on the surface of the cell are transported into the interior (Wolff 1972).

Cells Containing Birbeck Granules in the Lymph and the Lymph Node

The lymph node is composed of functionally different compartments, each with a characteristic type of macrophage. In the marginal zone the plasmacell reaction is induced, in the germinal centre the memory B-cells are generated and in the paracortex recirculating T-cells are stimulated in thymus dependent humoral responses and in cell mediated responses. In normal lymph nodes, the reticulum and endothelial cells composing the stroma of these areas, are clearly distinguished from the macrophages and the lymphocytes (Hoefsmit 1975). Marginal zone macrophages, tingible body macrophages (TBM) and interdigitating cells (IDC) seem to differ in functional activity only.

Reticulum Cells and Macrophages in the Immune Response

At the second Conference on mononuclear phagocytes the ultra-structural differences between reticulum cells and macrophages in rat lymphoid organs were described (Hoefsmit 1975). The reticulum cells in the lymph node were defined as the cells which are responsible for the formation of the fibers of the intercellular skeleton. These cells constitute the framework of the node together with the cells forming the blood and lymph vessels. Macrophages have the morphological characteristics associated with phagocytic activities such as ruffling of the plasma membrane, a well-developed Golgi apparatus surrounding the cytocenter, and the presence of lysosomes. In the normal lymph node the cortex comprises three compartments with specialized immunological functions (Keuning 1972): the marginal zone, the germinal centers, and the paracortex. We suggested that each compartment has its own reticulum cell and its own type of macrophage. In that scheme the antigen-retaining dendritic cells’, which are only found in germinal centers (Nossal et al. 1968) were classified as a special type of reticulum cell (dendritic reticulum cell, DRC). The interdigitating cells (IDC) were classified as macrophages characteristic of the T-cell micro-environment, i.e., the paracortex.

Properties of Kupffer Cells

It is not clear whether Carl Wilhelm von Kupffer was the first to describe the cell type that now bears his name. Nor is it easy to decide what von Kupffer’s star cells ‘Sternzellen’, correspond to in today’s concepts. An excellent account of the old literature is given by Aterman (1963). Most of this material is now mainly of historical interest and has little influence on current scientific nomenclature. As usual in the biological sciences, the notion Kupffer cell has acquired its meaning not by definition but by common usage and assumed agreement. I think it is in accordance with this common usage to define the Kupffer cell as the phagocyte of the liver sinusoidal wall. From this definition it follows that all discussion on the distinction of Kupffer cells from other cell types and of the histogenesis or kinetics of Kupffer cells must start by taking phagocytosis as the central criterion. It is meaningless to discuss questions like these on the basis of other, secondary morphological, cytochemical, or biochemical criteria, without checking all important points against the decisive criterion phagocytosis.

The Vacuolar Apparatus of Alveolar Macrophages and the Turnover of Surfactant

There are profound differences between alveolar macrophages and those elsewhere in the body. Structurally, a prominent distinguishing characteristic of alveolar macrophages is their generous complement of dense inclusions as compared with other populations of macrophages (Karrer 1960; Leake & Heise 1967; Pratt et al. 1971). There are metabolic differences as well, for alveolar macrophages rely primarily on aerobic respiration during phagocytosis, whereas peritoneal macrophages depend primarily upon glycolysis (Oren et al. 1963). The unique properties of alveolar macrophages may result from their location in the lung, since they alone among the phagocytes are continually exposed to noxious and infectious materials in the air.

Origin and Structure of the Osteoclast

The existence of multinucleated giant cells — as distinguished from megakaryocytes — in bone has been known since the middle of the nineteenth century. Kolliker (1873) thought that these cells might represent the ‘agents of bone resorption’ and gave them the name of ‘osteoclasts’.