Only a small proportion of splenic B cells in adults are short-lived virgin cells

A cohort of newly produced virgin B cells was followed from the marrow to the spleen of non-immunized clean rats, which showed minimal antigen-driven proliferation of B cells in their spleens. The progenitors of this cohort of virgin cells were labeled in vivo over 12 h with the thymidine analogue 5-bromo-2′-deoxyuridine (BrdUrd) and their non proliferating progeny left the marrow 2-3 days later. This coincided with the arrival of labeled B cells in the red pulp and T zones of the spleen. These appear to be short-lived as few remained a week after the label was given. The short-lived newly produced virgin B cells can only comprise a minority of splenic B cells, for it is shown that only 20% of splenic B cells are found in the red pulp and T zone. It is calculated that newly produced virgin B cells are likely to make up between 5% and 10% of splenic B cells. In the marginal zones and follicular mantle respective medians of 3.3% and 1.8% were already labeled at 1 day from the start of the BrdUrd pulse. The appearance of these cells seems likely to result from antigen-driven B cell proliferation outside the marrow, for labeled virgin B cells have not started to leave the marrow at this stage. During day 2 and 3 the proportion of labeled follicular mantle B cells rose to 3.4%, which might in part reflect the recruitment of newly produced virgin B cells to the pool of recirculating follicular B cells. After day 3 in the follicles and day 1 in the marginal zones the proportion of labeled cells did not vary significantly through day 7. This appears to confirm the comparative longevity of the cells in these zones, which contain 80% of the non-proliferating splenic B cells of adult rats.     

Distribution and identity of the earliest proliferating progeny of colony-forming cells in regenerating murine spleen and bone marrow

A study was made of the sites of development and the types of cells found in very early hemopietic colomes in the mouse spleen. Two, 3, and 4 days after transplantation, the proliferating descendants of transplanted bone-marrow cells were identified on radioautographs of spleen sections and on spleen and bone-marrow smears of supralethally irradiated recipient mice which were injected with ³H-TdR at 12, 6, and 0.5 hours before sacrifice. Surprisingly the spleens of nontransplanted, irradiated mice contained proliferating medium and large lymphocytes in the white pulp which increased in numbers during the observation period. The early descendants of transplanted cells that lodged in the spleen could be clearly distinguished from the labeled indigenous cells because they formed discrete nodules or colonies beneath the splenic capsule or in the vicinity of venules and trabeculae of the red pulp. These cells were identifiable on day 2 as transitional cells or unknown hemopoietic blasts and on day 4 included early erythroid cells and small lymphocytes. There was evidence for the traffic of ³H-TdR-labeled cells through the splenic sinusoids.     

Directions of Migration of Bone Marrow Mononuclears after Intracoronary Transventricular Injection

Directions of migration of mononuclear bone marrow cells after intracoronary transventricular injection procedure developed by us were experimentally studied. After nonselective injection of cells into the right and left coronary arteries in rats, the labeled cells were detected only in the damaged zone of the myocardium. Localization of transplanted mononuclears in the scar attests to their homing into the damaged zone. Numerous cells were found in the red pulp of the spleen and solitary cells were detected in the liver and lungs. In the heart, the labeled transplanted cells were detected only in the scar zone at all terms of the study; they were not incorporated into the vascular walls, but were surrounded by thick bundles of collagen fibers and probably underwent differentiation into fibroblasts. No data on possible differentiation of the transplanted cells into vascular cells or cardiomyocytes were obtained.     

Human spleen microanatomy: why mice do not suffice

The microanatomical structure of the spleen has been primarily described in mice and rats. This leads to terminological problems with respect to humans and their species‐specific splenic microstructure. In mice, rats and humans the spleen consists of the white pulp embedded in the red pulp. In the white pulp, T and B lymphocytes form accumulations, the periarteriolar lymphatic sheaths and the follicles, located around intermediate‐sized arterial vessels, the central arteries. The red pulp is a reticular connective tissue containing all types of blood cells. The spleen of mice and rats exhibits an additional well‐delineated B‐cell compartment, the marginal zone, between white and red pulp. This area is, however, absent in human spleen. Human splenic secondary follicles comprise three zones: a germinal centre, a mantle zone and a superficial zone. In humans, arterioles and sheathed capillaries in the red pulp are surrounded by lymphocytes, especially by B cells. Human sheathed capillaries are related to the splenic ellipsoids of most other vertebrates. Such vessels are lacking in rats or mice, which form an evolutionary exception. Capillary sheaths are composed of endothelial cells, pericytes, special stromal sheath cells, macrophages and B lymphocytes. Human spleens most probably host a totally open circulation system, as connections from capillaries to sinuses were not found in the red pulp. Three stromal cell types of different phenotype and location occur in the human white pulp. Splenic white and red pulp structure is reviewed in rats, mice and humans to encourage further investigations on lymphocyte recirculation through the spleen.     

Lymphocyte circulation in the spleen: Marginal zone bridging channels and their possible role in cell traffic

Histological evidence is presented for distinct, anatomically determined pathways in the spleen for cells in transit between the white pulp and the red pulp prior to entering the draining veins. In rats and mice these appear as narrow channels of lymphocytes which run between both the periarteriolar lymphatic sheath and the red pulp sinuses, and the peripheral white pulp and the red pulp sinuses, crossing the marginal zone in association with fine argentophilic fibres. These marginal zone bridging channels were found to contain labelled T or B cells 4 and 8 hours after injection which suggested that transit was occurring in the direction from white pulp to red pulp rather than the reverse.

Additional histological evidence is given to suggest that, after antigenic stimulation, germinal centre dissociation occurs by release of the germinal centre cells towards the periarteriolar lymphatic sheath before they are shed into the red pulp through marginal zone bridges occurring in the periarteriolar region.

The data are incorporated into a scheme of unidirectional lymphoid cell flow through the spleen. This proposes that the spleen is composed of many functionally discrete units in which the anatomical matrix, reflected by the reticulin fibre pattern, plays a major role. It further implies that the periarteriolar region of the spleen is not totally thymus dependent.

     

Proliferation of mononuclear phagocytes (Kupffer cells) and endothelial cells in regenerating rat liver. A light and electron microscopic cytochemical study.

The proliferation of littoral cells in regenerating rat liver after partial hepatectomy has been investigated using endogenous peroxidase and uptake of large (0.8-mu) latex particles as markers of Kupffer cells. Female rats were subjected to 2/3 partial hepatectomy and sacrificed at intervals up to 11 days. Prior to sacrifice, animals were injected with latex and their livers were fixed by perfusion and were processed for cytochemical localization of peroxidase. The sinusoidal cells exhibited a marked regenerative response after partial hepatectomy. Peroxidase activity persisted in endoplasmic reticulum of Kupffer cells during mitosis. Furthermore, latex particles were exclusively localized in such peroxidase-positive cells, thus confirming their identity as Kupffer cells. Quantitative counts revealed that the peak mitotic activity of Kupffer cells occurred at 48 hours, whereas that of endothelial cells was at 96 hours after partial hepatectomy. Our findings indicate that Kupffer cells are capable of dividing locally in the liver; no morphologic evidence of transformation of endothelial cells or monocytes to Kupffer cells was found. The significance of these observations concerning the origin of Kupffer cells is discussed, and it is concluded that in the model of liver regeneration after partial hepatectomy the Kupffer cells are formed predominantly by local cell division.     

The regenerating liver: A site of erythropoiesis in the adult long-evans rat

Erythropoiesis, which is primarily hepatic in the rat during fetal and early neonatal life, shifts almost entirely to the bone marrow in the neonatal-adolescent stage of development. In the adult, extramedullary erythropoiesis has been demonstrated in the liver and spleen under certain pathological conditions when bone marrow red cell production is insufficient. In the present study, erythropoietic foci have been found in young-adult rat liver regenerating 24–72 hr after subtotal hepatectomy. This erythropoiesis is both extravascular and sinusoidal, with some erythroblastic islands noted. The centrolobular hepatic area contains the highest concentration of erythroblasts. Peripheral blood reticulocytosis coincides with the appearance of these cells and this is considered as an indicator of effective erythropoiesis. Liver regenerating after partial hepatectomy produces significant quantities of erythropoietin (Ep) in response to hypoxia. Subtotal hepatectomy may confer upon the adult liver the ability to revert to a fetal-like condition both in its ability to produce Ep and to function as a hematopoietic inductive microenvironment for erythropoiesis.     

Morphologic and cell kinetic investigations of the spleen after repeated in situ freezing of liver and kidney.

In order to evaluate a possible primary or secondary immunologic response of the spleen after single or repeated in situ freezing of parenchymal organs such as liver and kidney within a four week period, light microscopic and cell kinetic investigations with tritiated thymidine were performed on spleens of non-germfree rats. Sham operations served as controls. The sham operations did not induce any significant morphological or cell kinetic changes in the splenic white pulp. After cryolesions were produced in the liver and kidney, the percentages of activated germinal centers, labeled germinal center cells, and cells in the perifollicular area of the lymphatic mantle and marginal zones increased, with maxima during the first 3 days. The investigations show that the cellular reaction of the spleen starts earlier and is more prominent after repeated in situ freezing than after a single cryolesion. These findings point to an immunologic response of the anamnestic type, and correspond to results after repeated freezing of normal and malignant tissue of the urogenital tract. These cell kinetic results are important in the evaluation of further immunologic studies involving the cryotherapy of malignant tissues.     

Lymphocyte migration in the spleen: the effect of macrophage elimination.

To study the influence of macrophages on the migration and distribution of lymphocytes in the spleen, macrophages were eliminated from the spleen of mice by injection of liposomes in which DMDP was encapsulated. This leads to an elimination of macrophages in both the red pulp and marginal zone of the spleen within 1-2 days. In these animals the distribution of lymphocytes was determined by transfer of either syngeneic fluoresceinated or Ly 5 congeneic cells. It was found that after elimination of the macrophages the number of lymphocytes immigrating into the spleen had decreased, although a comparable mode of compartimentalization was found with an initial localization in the marginal zone and a subsequent distribution into the white pulp. After this elimination spleen macrophage subsets return with different kinetics, and in this way the influence of the red pulp macrophages, the marginal zone macrophages and the marginal metallophilic macrophages on lymphocyte immigration and redistribution could be investigated. A quantitative decrease of immigration was still found when red pulp and marginal metallophilic macrophages had repopulated their compartments, but was only fully restored when the last population to repopulate the spleen after treatment with DMDP-liposomes, the marginal zone macrophages, had returned. Experiments with isolated T and B cells showed that the elimination of macrophages had a profound effect on the localization of B cells in the white pulp, whereas it hardly affected T cells.     

Expression of NCAM in activated portal fibroblasts during regeneration of the rat liver after partial hepatectomy

In the portal tract of the regenerating liver after partial hepatectomy, vascular and bile ductular remodeling takes place in response to the portal hyperdynamic state and parenchymal hyperplasia. In order to reveal phenotypical changes in the portal fibroblasts, we immunohistochemically investigated neural cell adhesion molecules (NCAM) and alpha smooth muscle actin (alphaSMA) expression and the ultrastructural changes in them during liver regeneration. In the control rat liver, portal fibroblasts were negative for both NCAM and alphaSMA. They became positive for both markers two days after partial hepatectomy, increased in staining intensity, reached a maximum at three to four days, then decreased, being still clearly positive at 14 days. Under an electron microscope, portal fibroblasts from the regenerating liver had larger amounts of cytoplasm and rough endoplasmic reticulum than those from the control liver; thus they might be activated. Additionally, periportal hepatic stellate cells in the regenerating liver were activated with alphaSMA, but without NCAM. The present study has demonstrated that portal fibroblasts express NCAM and alphaSMA in the regenerating liver after partial hepatectomy via transformation into myofibroblasts following reconstruction of the portal tracts.     

B-cell production and differentiation in adult rats.

The B-cell development in a group of rats was suppressed for the first 45 days of life by serial administration of rabbit anti-rat IgM and IgD antibody. Total or near total suppression of B lymphopoiesis was achieved. At 45 days, suppression was stopped by injection of IgM and IgD rat paraproteins. The sequence of B-cell and plasma cell development following suppression was assessed by immunohistological analysis of spleen lymph nodes and small intestinal lamina propria. The main findings are listed below. Complete reconstitution of B-cell numbers occurs within 8 days, at which stage germinal centres are also present. B lymphopoiesis in the red pulp of the spleen differs from that reported for bone marrow. Cells develop expressing surface sIgM and sIgM with IgA, but not sIgD. sIgD-positive cells first appear in splenic follicles 2 days after stopping suppression, but their appearance in lymph nodes is delayed until after 3 days. At this stage, sIgD-positive cells become apparent in the splenic red pulp. IgM plasma cells appear from day 4. IgA plasma cells in the gut appear in small numbers at day 6, and gradually increase to normal numbers by day 14. sIgG2c expression in the splenic marginal zone did not approach normal levels, even 2 weeks after suppression was stopped.     

Sequential analysis of hepatic carcinogenesis: a comparative study of the ultrastructure of preneoplastic, malignant, prenatal, postnatal, and regenerating liver.

The objective of this study was to compare the fine structure of presumptive preneoplastic hepatocytes at various times during liver carcinogenesis with that of normal, developing, and regenerating liver and of hepatocellular carcinomas, using transmission and scanning electron microscopy. A new model of liver carcinogenesis was used in which several of the early steps are quite well synchronized. A single initiating dose of diethylnitrosamine induced isolated islands of altered hepatocytes. The cells were characterized by persistence of glycogen despite starvation, increase in smooth endoplasmic reticulum, and hypertrophic nucleoli. Following intense selection of the altered hepatocytes by dietary 2-acetylaminofluorene plus partial hepatectomy, the affected hepatocytes proliferated rapidly to produce basophilic foci. These early hyperplastic lesions revealed stellate-shaped dilated bile canaliculi lined by blebs and abnormally thick elongated microvilli, a decreased number of microvilli on the sinusoidal surface, a marked increase in smooth endoplasmic reticulum, large nucleoli, and bundles of pericanalicular microfilaments. A majority of the proliferating lesions reacquired a normal organizational pattern within several weeks after partial hepatectomy and could not be distinguished from normal liver. A small number continued to grow and become typical persistent hyperplastic nodules. These showed significant widening of intercellular spaces between hepatocytes, elongated microvilli over large regions of the cell surface, many invaginations of the cell membrane, and irregularly shaped bile canaliculi. Sequential changes in focal hyperplastic hepatocytes during carcinogenesis could be distinguished from normal, developing, and regenerating liver. The major differences involved the cell surfaces and cytoplasmic organelles. The findings are compatible with the hypothesis that a carcinogen may act by inducing alterations in a small number of hepatocytes and that hepatocellular carcinomas arise through stepwise evolutional changes in these cells.     

Identification of p46 Shc expressed in the nuclei of hepatocytes with high proliferating activity: Study of regenerating rat liver

The adaptor molecule Shc is a proto-oncogene product, and it is known to be associated with cell proliferation. However, the role of Shc in the proliferation and regeneration of hepatocytes remains unknown. In the present study, we report that p46 Shc is specifically expressed in the nuclei of proliferative (or regenerative) hepatocytes, suggesting that p46 Shc protein plays a role in hepatocellular proliferation. The expression of Shc was analyzed in liver tissue after partial hepatectomy (PH) or sham operation in Wistar rats by using immunohistochemistry and/or Western blot analysis. In addition, the expression of various cell cycle-related proteins, such as Cdk4, cyclin D1, PCNA, and Cdk1 was analyzed in the tissues of regenerating rat liver. Furthermore, the tyrosine phosphorylation of Shc was studied in liver tissue after PH or sham operation by immunoprecipitation using a monoclonal phosphotyrosine antibody. Although the protein levels of p52 Shc were unchanged in liver tissues after PH or sham operation, tyrosine phosphorylation was detected only in the regenerating rat liver after PH. The levels of p46 Shc protein were markedly increased in liver tissues during the liver regenerative process. In contrast, p66 Shc was not detected in the liver tissues after PH or sham operation. Western blotting and immunohistochemistry showed that the main location of p46 Shc was in the nuclei of proliferating hepatocytes after PH. These data suggest that p46 Shc expressed in hepatocellular nuclei may be closely related to the proliferation of hepatocytes. Therefore, it is suggested that p46 Shc expressed in hepatocellular nuclei may be a useful marker for detecting hepatocytes with high proliferative activity.     

Proliferation and differentiation of progenitor cells in the cortex and the subventricular zone in the adult rat after focal cerebral ischemia

Progenitor cells in the subventricular zone of the lateral ventricle and in the dentate gyrus of the hippocampus can proliferate throughout the life of the animal. To examine the proliferation and fate of progenitor cells in the subventricular zone and dentate gyrus after focal cerebral ischemia, we measured the temporal and spatial profiles of proliferation of cells and the phenotypic fate of proliferating cells in ischemic brain in a model of embolic middle cerebral artery occlusion in the adult rat. Proliferating cells were labeled by injection of bromodeoxyuridine (BrdU) in a pulse or a cumulative protocol. To determine the temporal profile of proliferating cells, ischemic rats were injected with BrdU every 4 h for 12 h on the day preceding death. Rats were killed 2-14 days after ischemia. We observed significant increases in numbers of proliferating cells in the ipsilateral cortex and subventricular zone 2-14 days with a peak at 7 days after ischemia compared with the control group. To maximize labeling of proliferating cells, a single daily injection of BrdU was administered over a 14-day period starting the day after ischemia. Rats were killed either 2 h or 28 days after the last injection of BrdU. A significant increase in numbers of BrdU immunoreactive cells in the subventricular zone was coincident with a significant increase in numbers of BrdU immunoreactive cells in the olfactory bulb 14 days after ischemia and numbers of BrdU immunoreactive cells did not significantly increase in the dentate gyrus. However, 28 days after the last labeling, the number of BrdU labeled cells decreased by 90% compared with number at 14 days. Clusters of BrdU labeled cells were present in the cortex distal to the infarction. Numerous cells immunostained for the polysialylated form of the neuronal cell adhesion molecule were detected in the ipsilateral subventricular zone. Only 6% of BrdU labeled cells exhibited glial fibrillary acidic protein immunoreactivity in the cortex and subcortex and no BrdU labeled cells expressed neuronal protein markers (neural nuclear protein and microtubule associated protein-2). From these data we suggest that focal cerebral ischemia induces transient and regional specific increases in cell proliferation in the ipsilateral hemisphere and that proliferating progenitor cells may exist in the adult cortex.     

Filtration of circulating particles by splenic autotransplants

To determine whether filtration in reconstituted perifollicular marginal zones underlies particle sequestration in regenerating spleen transplants, the distribution of IV injected particles of tantalum was examined in autografts freely transplanted into subcutaneous pouches in rats. In two to six weeks old autografts removed one to three hours after particle injection, the reconstituted marginal zones contained practically all of the sequestered tantalum. At 48 hours to-20 weeks following injection, marginal zones were largely free of particles which were distributed extracellularly throughout the red pulp. This sequential pattern of distribution conformed closely to that observed in the intact spleen. The findings indicate that a reconstituted marginal zone allows splenic autotransplants to function as simple mechanical filters. Increased demand for this activity rather than for phagocytosis may be the predominant factor regulating spleen growth.     

Light and electron microscopic observations on the spleen and the splenic leukocytes of the newt triturus cristatus

The newt spleen has an outer region, the red pulp, that consists of reticular fibres, reticular cells, and fixed macrophages forming a framework within which erythropoiesis occurs, and an inner region, the white pulp, consisting of a reticular framework containing predominantly lymphocytes. The fine structure of the following leucocytes, namely lymphoid cells, reticular cells, macrophage, haemocytoblasts, plasma cells and neutrophilic, basophilic and eosinophilic granulocytes is, in general, similar to that of the corresponding mammalian blood cells. However, in contrast, all the nuclei contain large numbers of interchromatin granules and a differentiated region termed the nuclear light-staining zone because of its electron-scattering properties. Furthermore the specific granules of the eosinophilic granulocytes are structureless whereas those of the basophilic granulocyte have crystalline regions. The structure and distribution of the granular, fibrillar, tubular and vesicular components of the cytoplasm of each cell type is described.     

Species variation in the structure and function of the marginal zone—an electron microscope study of cat spleen

The marginal zone in the cat spleen consisted of a characteristic mixture of lymphocytes and other blood cells located mainly between the several layers of circumferential reticulum around white pulp. A region of fine-meshed reticulum between white pulp and red pulp, as present in some species, was absent from the cat spleen. Arterial capillaries to the marginal zone were few. Some were continuations of white pulp capillaries, whereas others were red pulp capillaries that likely were continuations of axial capillaries of periarterial macrophage sheaths (PAMS) (ellipsoids). Blood cells deposited in the marginal zone could reach red pulp by passing through the numerous openings in each layer of circumferential reticulum. Lymphocytes appeared to migrate across the marginal zone both toward and away from white pulp. Macrophages lying on the circumferential reticulum of the marginal zone phagocytized cells but did not ingest Thorotrast, although it coated their surfaces. Because of the scarcity of arterial endings and the lack of a macrophage-charged reticular meshwork, the marginal zone in cat spleen is not a major site of blood clearance and phagocytosis. These functions are better served in PAMS and red pulp.     

肝再生对大鼠胎肝细胞脾内移植后增殖的影响

目的:研究肝再生状态对大鼠胎肝细胞脾内移植后增殖影响。方法:分离孕3周SD大鼠胚胎肝细胞,将其移植入70%肝切除肝再生模型大鼠脾内,分别于移植后7 d和30 d应用流式细胞仪检测肝切除大鼠残肝细胞的细胞周期,用图像分析法检测脾内移植胎肝细胞面积密度。结果:移植后7 d,肝切除鼠残肝细胞S和G₂/M期细胞比例都明显少于对照组(P＜0.05),而其脾内移植胎肝细胞面积密度则显著高于对照组(P＜0.05);30 d后,各组间残肝细胞再生状态与移植胎肝细胞的面积密度均无明显差异。 结论:肝再生状态有利于大鼠胎肝细胞脾内移植后的增殖。     

Synthesis of DNA by the spleens of germ-free mice during the primary response to sheep red cells

The mitotic activity of the spleens of non-immunized germ-free (GF) mice was less than in similar specific pathogen-free (SPF) animals. No evidence of germinal center activity was noted in these GF mouse spleens before challenge with sheep erythrocytes (SRC). These centers only began to develop 4 days after immunization and were not fully developed before 8 days. In control SPF mice, the spleens showed very few germinal centers which were small in size, but they showed the same pattern of evolution as in similarly immunized GF mice. The changes in the red pulp, characterized by the development of clusters of nuclei labeled with [³H]thymidine, followed closely the development of plaqueforming cells (PFC). The numbers of direct PFC reached the same peak level in GF and SPF mice on day 4 of the response to SRC, but were not so well sustained in the former animals after this time. Indirect PFC were much lower in the spleens of GF mice than in SPF animals. The pattern and degree of increase of DNA synthesis in the spleen of GF and SPF mice following immunization with SRC differed from and was less than that of mice reared in less clean conditions. Increased DNA synthesis occurred very soon after injection of SRC (6 to 24 h) and the increase was sustained for 4 days without further significant rise and then declined. Autoradiographs of the spleen of immunized GF mice given [³H]thymidine showed that the first increase of labeled nuclei in the white pulp occurred around the central arterioles as early as 6 h after SRC. This was followed by increased labeling in the mantle layer of the white pulp and the characteristic pattern of germinal center labeling developed after 4 days. Increased labeling of nuclei developed in the red pulp as early as in the white pulp, while the subcapsular and trabecular areas showed high labeling indices even in the spleens of non-immunized controls. The ratio of labeling index/mitotic index which is governed by the respective durations of DNA synthesis and mitosis in those cells in division cycle, varied between 10 and 280 in different areas of the spleen. This indicated a vast excess of cells synthesising DNA in relation to the numbers of dividing cells actually present in the spleens of these mice.     

The role of macrophages in the immunoadjuvant action of liposomes: effects of elimination of splenic macrophages on the immune response against intravenously injected liposome-associated albumin antigen.

The primary antibody response to intravenously administered and liposome-associated human serum albumin (HSA) was studied in mice under conditions where no response could be detected against the non-liposome-associated form of the antigen. The positive response against the antigen, entrapped in and/or exposed on the surfaces of liposomes, thus resulted from the adjuvant action of the liposomes. In mice intravenously injected with dichloromethylene diphosphonate (C12MDP) also entrapped in liposomes, all red pulp macrophages, marginal metallophilic macrophages and marginal zone macrophages had disappeared from the spleen 2 days after administration. Twenty-two days after such a treatment red pulp macrophages and marginal metallophilic macrophages had reappeared, but marginal zone macrophages were still absent. In mice injected with liposome-associated HSA at 2 days after treatment with the C12MDP liposomes, anti-HSA responses were severely depressed, but administration of the liposome-associated antigen 22 days after C12MDP liposomes elicited a normal response. These results point to a role of splenic macrophages in the processing of liposome-associated antigens, but marginal zone macrophages, which are located close to the open ends of the white pulp capillaries and thus are the first macrophages to meet the antigens arriving in the marginal zone are not required.