The heterogeneity of the reticulum of rat peripheral lymphoid organs identified by monoclonal antibodies

Expression of mucosal addressin cell adhesion molecule-1 on the reticular framework between white pulp and the marginal zone in the human spleen

The antigenic heterogeneity of the reticular framework of the white pulp and marginal zone is well documented in the human adult spleen. Immunostaining of α-smooth muscle actin characterizes the heterogeneity of the reticular framework of the white pulp and marginal zone. In the human spleen, the blood cells flow in an open circulation. T and B lymphocytes flow out from the arterial terminal, and migrate in the reticular framework. Homing of lymphocytes to lymphoid tissues is regulated by selective interactions between cell surface homing receptors and tissue vascular addressins at sites of lymphocyte recruitment from the blood. In the present study, mucosal addressin cell adhesion molecule-1 was selectively expressed on α-smooth muscle actin-positive reticular framework. The reticular framework may function in lymphocyte homing and segregation into the periarteriolar lymphoid sheath, lymph follicle and marginal zone.

The actin-binding protein caldesmon is in spleen and lymph nodes predominately expressed by smooth-muscle cells, reticular cells, and follicular dendritic cells

Reticular cells and follicular dendritic cells (FDCs) build up a framework that underlies the compartmentalization of spleens and lymph nodes. Subpopulations of reticular cells express the smooth-muscle isoform of actin, indicative of a specialized contractile apparatus. We have investigated the distribution of the actin-binding protein caldesmon in spleen and lymph nodes of mice and rats. Caldesmon modulates contraction and regulates cell motility. Alternative splicing of transcripts from a single gene results in high-molecular-mass isoforms (h-caldesmon) that are predominately expressed by smooth-muscle cells (SMCs), and low-molecular-mass isoforms (l-caldesmon) that are thought to be widely distributed in non-muscle tissues, but the distribution of caldesmon in spleen and lymph nodes has not been reported. We have performed Western blot analysis and immunohistochemistry using four different antibodies against caldesmon, among these a newly developed polyclonal antibody directed against recombinant mouse caldesmon. Western blot analysis showed the preponderance of l-caldesmon in spleen and lymph nodes. Our results from immunohistochemistry demonstrate caldesmon in SMCs, as expected, but also in reticular cells and FDCs, and suggest that the isoform highly expressed by reticular cells is l-caldesmon. In spleen of SCID mice, caldesmon was expressed by reticular cells in the absence of lymphocytes.

Reactive and proliferative changes of splenic reticulum cells of rats investigated with special staining methods and immunohistochemistry

In the rat spleen, reactive and proliferative changes of the reticulum cells are rare events and seem to occur almost exclusively in the red pulp. The normal structure of the splenic reticulum cell and fiber lattice and examples of spontaneous and induced pathological alterations were investigated by immunohistochemistry (smooth muscle-actin, vimentin, S100 and proliferating cell nuclear antigen) and special stains for extracellular fibers (silver impregnation, azan). In response to congestion, systemic tumor growth or treatment with a hematotoxic compound, the scaffold cells increased either their contractile properties or their production of extracellular fibers. Primary focal hyperplasias of stromal cells which had developed without obvious cause were characterized by vanishing of sinuses, increased fiber content, increased expression of sm-actin or foci of lipomatosis. The borders of focal hyperplasias were indistinct and they did not infiltrate the white pulp compartments. Neoplasms of the stromal reticulum cells resembled soft tissue tumors in other organs. Specific tumor entities as described in other species have so far not been observed in the rat.

Normal B cell homeostasis requires B cell activation factor production by radiation-resistant cells

The cellular source of B cell activation factor (BAFF) required for peripheral B cell survival/maturation is unknown. To determine the nature of BAFF-producing cells we established and analyzed reciprocal bone marrow (BM) chimeras with wild-type (WT) and BAFF-deficient mice. The results revealed that BAFF production by radiation-resistant stromal cells is completely sufficient to provide a necessary signal for B cell survival/maturation, as BAFF-/- BM cells transferred into lethally irradiated WT mice gave rise to normal numbers of follicular (FO) and marginal zone (MZ) B cell subpopulations. On the other hand, transfer of WT BM into BAFF-/- lethally irradiated mice resulted only in minimal reconstitution of mature FO B cells and no restoration of MZ B cells. Thus, in the absence of BAFF+/+ stromal cells, BAFF production by BM-derived cells, presumably by macrophages, dendritic cells, and/or neutrophils, was not at all sufficient to support normal B cell homeostasis. Interestingly, immunization of both types of chimeras stimulated high levels of antigen-specific antibody secretion, indicating that either stromal cell- or hematopoietic cell-derived BAFF is sufficient for B cell antibody responses.

Homing of antibody secreting cells

As activated B cells differentiate into plasma cells they complete a final series of migration steps that take them to locations where they can efficiently carry out their effector function, secreting immunoglobulin (Ig) M or IgG into circulation or releasing dimeric IgA adjacent to the epithelium for transcytosis. Recent experiments have established a key role for chemokines in directing antibody secreting cell (ASC) movement within the secondary lymphoid organs where they are generated, as well as in guiding the cells to the bone marrow or mucosal surfaces. This review discusses the chemokines involved in directing ASC movements, particularly focusing on the role of CXCR4 and CXCL12/SDF1. The function of CCR9 and CCR10 in IgA ASC homing and contributions made by integrins and lectins are also discussed.

Lectin histochemistry of the rat lymph node: visualisation of stroma, blood vessels, sinuses, and macrophages. A contribution to the concept of an immune accessory role of sinus-lining endothelia

The lectin Chelidonium majus agglutinin (CMA) was previously shown to visualise endothelia of all blood vessels and those lining sinuses of red pulp, stromal reticular meshwok (RM) and dendritic cells of lymphatic follicles in white pulp of the spleen in rats. The aim of the present study was the analysis of CMA and some other lectins in labelling RM, vascular structures and macrophages in lymph nodes of rats. It appeared that CMA stained the entire RM, dendritic cells, lining cells of sinuses and all types of blood vessels. Sinus-lining cells of lymph nodes were labelled with CMA and mannose-, GalNac-, and sialic acid-specific lectins. Moreover, lymph node macrophages were labelled above all by mannose specific lectins. The broad lectin-binding pattern of sinuses--not observed in rat spleen- and CMA-reactivity of both sinus-lining and dendritic cells corroborates the hypothesis that lymph node sinus-lining endothelia are precursors or a special type of immune accessory cells.

Differential galactosylation of neuronal and haematopoietic signal regulatory protein-(α) determines its cellular binding-specificity

Signal regulatory protein-(α) (SIRP(α)) is a member of the Ig superfamily selectively expressed by neuronal and myeloid cells. The molecule mediates functional interactions with CD47/integrin-associated protein. Here we provide evidence for the tissue-specific glycosylation of neuronal and haematopoietic SIRP(α). We demonstrate a major difference in the galactosylation of N-linked glycans isolated from neuronal (i.e. brain-derived) SIRP(α) as compared to myeloid (i.e. spleen-derived) SIRP(α), with neuronal SIRP(α) almost completely lacking galactose. (β)4-galactosyltransferase assays demonstrated that this is most likely due to a low galactosylation capacity of the brain. In order to investigate the role of galactosylation of SIRP(α) in cellular interactions, soluble recombinant SIRP(α) glycoforms containing galactose (SIRP(α)-Fc) or lacking galactose (SIRP(α)((Δ)Gal)-Fc) were produced. Binding studies demonstrated superior binding of SIRP(α)((Δ)Gal)-Fc to cerebellar neurons and isolated lymphocytes. In contrast, SIRP(α)-Fc bound relatively strong to macrophages. These data show that the galactosylation of SIRP(α) determines its cellular binding specificity.

Lectin histochemistry of the spleen: a new lectin visualizes the stromal architecture of white pulp and the sinuses of red pulp

The subcompartmentalization of the white pulp in the spleen is the result of interactions of specific resident stromal cells and migrating subtypes of lymphocytes. Because carbohydrate residues of cell membranes and extracellular matrices are involved in cell-cell and cell-matrix interactions, they were investigated in rat spleen by a broad panel of lectins. Splenic macrophages, which were also demonstrated by Perls' Prussian blue reaction, were labeled selectively by most mannose-specific lectins and gave the characteristic distribution patterns in all splenic (sub)compartments. One recently isolated lectin, Chelidonium majus agglutinin (CMA), visualized predominantly central arterioles, the reticular meshwork (RM) in the periarteriolar lymphatic sheaths (PALS), the circumferential reticulum cells limiting PALS and follicles, and some follicular dendritic cells (FDCs) in white pulp. The endothelial cells of venous sinuses in red pulp were also labeled by CMA and, if frozen sections were used, CMA also labeled the macrophages of the red pulp. Compared to CMA, the monoclonal antibody CD11, which can be used only in frozen sections, stained almost solely the fibrous (extracellular) component of the RM. Because CMA stains the reticulum cells in particular, it is better suited to visualize the stromal architecture of splenic white pulp than the monoclonal antibody. Because CMA can be applied to paraffin-embedded material, it is a particularly useful tool to study the splenic stromal architecture in archival material.

Expression of the gene encoding the chemorepellent semaphorin III is induced in the fibroblast component of neural scar tissue formed following injuries of adult but not neonatal CNS

This study evaluates the expression of the chemorepellent semaphorin III (D)/collapsin-1 (sema III) following lesions to the rat CNS. Scar tissue, formed after penetrating injuries to the lateral olfactory tract (LOT), cortex, perforant pathway, and spinal cord, contained numerous spindle-shaped cells expressing high levels of sema III mRNA. The properties of these cells were investigated in detail in the lesioned LOT. Most sema III mRNA-positive cells were located in the core of the scar and expressed proteins characteristic for fibroblast-like cells. Neuropilin-1, a sema III receptor, was expressed in injured neurons with projections to the lesion site, in a subpopulation of scar-associated cells and in blood vessels around the scar. In contrast to lesions made in the mature CNS, LOT transection in neonates did not induce sema III mRNA expression within cells in the lesion and was followed by vigorous axonal regeneration. The concomitant expression of sema III and its receptor neuropilin-1 in the scar suggests that sema III/neuropilin-1-mediated mechanisms are involved in CNS scar formation. The expression of the secreted chemorepellent sema III following CNS injury provides the first evidence that chemorepulsive semaphorins may contribute to the inhibitory effects exerted by scars on the outgrowth of injured CNS neurites. The vigorous regrowth of injured axons in the absence of sema III following early neonatal lesions is consistent with this notion. The inactivation of sema III in scar tissue by either antibody perturbation or by genetic or pharmacological intervention could be a powerful means to promote long-distance regeneration in the adult CNS.

Signal-Regulatory Protein Is Selectively Expressed by Myeloid and Neuronal Cells

Signal-regulatory proteins (SIRP) are transmembrane glycoproteins with three extracellular Ig-like domains, closely related to Ag receptors Ig, TCR, and MHC, and a cytoplasmic domain with two immunoreceptor with tyrosine-based inhibition motifs that can interact with src homology 2 domain-containing phosphatases. SIRP have previously been shown to inhibit signaling through receptor tyrosine kinases, but their physiologic function is unknown. Here we demonstrate by expression cloning that the mAbs ED9, ED17, and MRC-OX41 recognize rat SIRP. In addition, we show for the first time that rat SIRP is selectively expressed by myeloid cells (macrophages, monocytes, granulocytes, dendritic cells) and neurons. Moreover, SIRP ligation induces nitric oxide production by macrophages. This implicates SIRP as a putative recognition/signaling receptor in both immune and nervous systems.

Immunohistochemical and structural characteristics of the reticular framework of the white pulp and marginal zone in the human spleen

BACKGROUND

The reticular framework of the white pulp (WP) and marginal zone (MZ) consists of reticulum cells and reticulin fibers. The antigenic heterogeneity of the reticular framework is well documented in the mouse and rat spleen. The aim of the present study is to characterize the reticular framework of the WP and MZ of the human spleen.

METHODS

Nine surgically resected human spleens were investigated. Five of the nine spleens were perfused. Formalin-fixed materials were embedded in paraffin and serial sections prepared for hematoxylin-eosin, silver staining, and immunohistochemical examination. Electron and immuno-electron microscopy were also applied. Using confocal laser scanning microscopy, the reticular framework was analyzed three-dimensionally.

RESULTS

The reticulin fibers of the framework were immunostained for type IV collagen in the WP and MZ. The WP was three-dimensionally delimited by the alpha-smooth muscle actin (alpha-SMA)-positive reticulum cells. In the WP, the distribution of alpha-SMA-positive reticulum cells formed the reticular framework of the periarteriolar lymphoid sheath (PALS). They also ensheathed the reticulin fibers. Interdigitating cells (IDCs) were scattered throughout the framework. A few IDCs attached to the framework. In the lymph follicle (LF), reticulum cells were not alpha-SMA-positive. The mesh of follicular dendritic cells (FDCs) was found in the germinal center. In places, the reticulin fibers were involved in the mesh of the FDCs and covered by the cytoplasm of FDCs. In the MZ, alpha-SMA-positive reticulum cells were arranged in a mesh pattern and ensheathed the fine reticulin fibers.

CONCLUSION

The reticular framework of the PALS, LF, and MZ is specialized into heterogeneous components in the human spleen. The heterogeneity of the framework may induce the segregation of T and B lymphocytes.

Monoclonal antibodies against mouse splenic stromal cells

A panel of rat monoclonal antibodies directed against mouse splenic stromal cells were isolated. These monoclonal antibodies were immunohistochemically divided into four groups which reacted with non-lymphoid cells of the murine spleen; (I) in the white pulp, (II) at the marginal zone, (III) in the red pulp, and (IV) on the endothelium of splenic blood vessels. These monoclonal antibodies were studied immunohistochemically in lymphoid organs by means of light and electron microscopy. Monoclonal antibodies SS-4 (group I) reacted with fibroblastic reticulum cells that were distributed only in the white pulp of the spleen and in the follicular areas of lymph nodes. The SS-4 staining cell, in clustered splenic stromal cells, formed colonies which included a small number of Thy-1 positive lymphocytes. Therefore, we concluded that SS-4 staining stromal cells comprise the lymphoid compartment. In contrast, monoclonal antibodies SS-1, SS-3 and SS-5 (group II) reacted with dendritic shaped cells in the marginal zone of the spleen. Examination of splenic extramedullary hematopoiesis in mice rescued by bone marrow transplantation after lethal irradiation revealed that SS-3 and SS-5 reacted with dendritic shaped stromal cells in clonal nodules of engrafted marrow in the red pulp. SS-3 and SS-5 staining cells could not be observed in physiologic hematopoiesis of non-transplanted mice. It was suggested that SS-3 and SS-5 staining stromal cells are involved in primitive hematopoiesis. Monoclonal antibodies SS-2, SS-6 and SS-7 (group III) mainly reacted with dendritic cells and macrophages in the red pulp. Monoclonal antibodies SS-B and SS-9 (group IV) reacted with endothelial cells of blood vessels and sinuses. These findings of heterogeneity in mouse splenic stromal cells are further evidence that specific micro-environments are composed by specialized stromal cells.

Omental milky spots in the local immune response in the peritoneal cavity of rats

BACKGROUND

Milky spots have been described as reactive structures, their classification varying from inflamed or haematopoietic tissue to lymphoid organs. In this study we investigated the reactivity of the milky spots in the omentum of rats upon induction of a chronic immune response in the peritoneal cavity.

METHODS

At different time points after intraperitoneal administration of Bacillus Calmette-Guérin (BCG), a peritoneal lavage was made, and the omentum and the draining parathymic lymph nodes were taken out. The cellular composition of these tissues was examined on the light microscopic level, using a panel of monoclonal antibodies, and also by electron microscopy.

RESULTS

During the first 4 months after administering BCG, the number and size of the milky spots increased enormously. Separate macrophage, T, and B cell areas were formed, but interdigitating cells and follicular dendritic cells were not observed. The number of cells in the peritoneal cavity also increased, and the cellular composition showed a strong similarity with that of the milky spots. Especially during the onset of the experiment, most bacteria were observed in the macrophages in the milky spots rather than in the draining lymph nodes. A cellular immune response was observed in the parathymic lymph nodes but not in the milky spots.

CONCLUSIONS

Milky spots, either unstimulated or stimulated, should be classified as perivascular infiltrates. They play a role in the initial clearance of bacteria from the peritoneal cavity. Although the large increase in cell number is predominantly caused by immigration of cells, the results do support the role of milky spots as a site for local proliferation and maturation of especially macrophages and also B cells. The obtained data, however, do not support the earlier made assumption that milky spots function as a secondary lymphoid organ in the peritoneal cavity.

Immunohistochemical characteristics of chicken spleen ellipsoids using newly established monoclonal antibodies

Ellipsoids, the extra-vasculature sites surrounding penicilliary capillaries of the chicken spleen, play critical roles in the immune response and also in the clearance of pathogens or other particles. The meshwork of ellipsoids is formed by fibroblastic reticular cells. To characterize ellipsoidal reticular cells, a series of monoclonal antibodies against the chicken spleen have been developed. Of these antibodies, CSA-1 antibody reacts with fibroblastic reticular cells in ellipsoids and with endothelial cells. The reticular nature of positive cells in ellipsoids is indicated by immuno-electron microscopy, and by double staining with anti-heat-shock protein 47 kDa (hsp47) antibody. The reaction of CSA-1 with reticular cells is limited in ellipsoids; CSA-1 does not react with reticular cells in other lymphoid organs. These findings indicate that ellipsoidal reticular cells share the antigen with endothelial cells. Ontogenic studies reveal that, on embryonic day 18, the development of ellipsoids is completed, penicilliary capillaries become fenestrated, and CSA-1 expression in ellipsoids begins. These findings suggest that CSA-1 is expressed on the cell surface of ellipsoidal reticular cells once they are exposed to blood flow.

Distribution of extracellular matrix components and their receptors in human lymphoid tissue and B-cell non-Hodgkin lymphomas

In this study the distribution patterns of various extracellular matrix components and their receptors (i.e. beta 1 integrins) in B-cell non-Hodgkin lymphomas were examined and compared to those in reactive lymphoid tissue. Neoplastic follicles within follicular lymphomas showed similar patterns to that observed in reactive follicles, which appeared to be strongly associated with the presence of follicular dendritic cells. Diffuse lymphomas of low and intermediate malignancy grade revealed features comparable to those of interfollicular areas of reactive lymphoid tissue, irrespective to which compartment the tumour cells were related. Highly malignant lymphomas, however, displayed unique extracellular matrix configurations, resulting from active matrix degradation by macrophages; this may support rapid tumour growth. Extranodal lymphomas showed virtually the same matrix patterns as their nodal counterparts, suggesting that (malignant) lymphoid cells generate (at least partly) their own specific microenvironment. In reactive lymphoid tissue beta 1 integrins were mainly found on resident cells and except for alpha 4, alpha 5 (and beta 1) the lymphoid cells expressed very little, if any, beta 1 integrins. In comparison, expression of these integrins on lymphoma cells was reduced (follicular lymphomas) or could not be detected at all (diffusely growing lymphomas); this might contribute to the growth pattern and metastatic properties of the tumours.

The functional state of follicular dendritic cells in severe combined immunodeficient (SCID) mice: role of the lymphocytes

In the present study, we have investigated the capacity of follicular dendritic cells (FDC) to trap immune complexes (IC) in the splenic white pulp of severe combined immunodeficient (SCID) mice and the influence of lymphocyte transfer on FDC function. FDC are absent in the splenic white pulp of naive SCID mice as revealed by in vitro IC trapping assay. One week after transfer of syngeneic lymphocytes, functional FDC with complement receptors appeared in the primary follicles coincident with B cell segregation, and IC were trapped on those FDC in a complement-dependent manner. Next, we immunized the reconstituted SCID mouse to see whether another type of FDC could be induced in the secondary follicle. Antigenic stimulation induced FDC with an additional capacity to capture IC via FcR gamma II. As seen in immunocompetent mice, this type of FDC was located only in the light zone of the secondary follicle. The newly generated FDC did not carry H-2 antigen of transferred lymphocytes from F1 mice. In SCID mice, in which normally no functional FDC are detectable, the microenvironments of the splenic white pulp have a capacity to develop and differentiate normally after transfer of lymphocytes. Apparently, the generation of functional IC-trapping FDC causes the induction of complement receptor(s) and Fc receptor on meshwork cells, which requires the presence of the lymphocytes.

Reticulum cells in the ontogeny of nasal-associated lymphoid tissue (NALT) in the rat

This study concerns the ontogeny of reticulum cells (RC) in the nasal-associated lymphoid tissue (NALT) of Wistar and Brown-Norway rats. A panel of monoclonal antibodies (mAb) directed against RC in peripheral lymphoid organs (antibodies ED10-ED15) was used, together with a recently developed antibody ED17, which recognizes macrophages and Langerhans cells. Early in embryogenesis, staining with common connective tissue markers, ED14 and ED15, was found. ED17-positive cells were present before cells positive to ED1, a pan-macrophage marker, or Ia glycoproteins were observed. The first differentiation of reticulum was seen at the day of birth, when ED10 recognized a distinct area in the nasal mucosa. The first T-lymphocytes were found at the same time. Two days after birth, B-cells and ED11-positive cells were present in the NALT area. Fourteen days after birth, T- and B-cell compartments were recognizable. ED10 was found predominantly in the T-cell area and ED11 was mainly confined to the B-cell compartment. We conclude that the development of the NALT is closely accompanied by the phenotypic specialization of the reticulum. This suggests that the reticulum plays an important role in the compartmentalization of NALT tissue and in the retention of lymphocyte subsets within these compartments.

Monoclonal antibody 4C7 recognizes an endothelial basement membrane component that is selectively expressed in capillaries of lymphoid follicles

In order to define compartment-related structures within the extracellular matrix of human lymphoid organs, monoclonal antibodies (MAbs) were generated by immunizing mice with stromal fragments of human tonsils. One MAb (4C7) was selected which recognized an endothelial basal membrane component that is selectively expressed in capillaries of lymphoid follicles. The epitope was also present in follicles within chronically inflamed synovial membrane and in a hyperplastic thymus of a patient with myasthenia gravis. B-cell non-Hodgkin's lymphomas with a follicular growth pattern expressed the antigen in neoplastic follicles, whereas diffuse growing lymphomas lacked the antigen. The restricted distribution pattern suggests involvement of the 4C7-defined antigen in the organization of the follicular compartment within human lymphoid tissue.

Development of antigenic heterogeneity in the splenic meshwork of severe combined immunodeficient (SCID) mice after reconstitution with T and B lymphocytes

Recently, we produced monoclonal antibodies reacting specifically with the reticular meshwork (RM) of lymphoid tissues, and demonstrated that, in the splenic white pulp of normal mouse, the antigenic heterogeneity of RM was associated with the segregation of the T and B lymphocytes. In the present study, we attempted to visualize further the interaction between splenic RM and T and B lymphocytes transferred into severe combined immunodeficient (SCID) mice. The splenic white pulp of naive SCID mice, containing a few T and B cells, showed little tendency for T-B segregation and antigenic diversity of RM. Transfer of spleen or bone marrow cells from normal mice resulted in complete recovery of lymphocyte populations, showing not only a clear segregation of T and B lymphocytes but also a remarkable antigenic diversity of RM. The same results were obtained following the transfer of spleen or bone marrow cells from the nude mouse. Next, we transferred purified T lymphocytes to one group of SCID mice and B cells to another. In mice given T cells, a few B cells were observed in the white pulp; T lymphocytes lodged not only in the inner periarterial lymphatic sheath (PALS) but also in the outer PALS and follicles. In the animals to which B cells were transferred, T cells were few and the homing of B cells occurred only into their proper compartments, such as the outer PALS, follicles and marginal zone, but not in the inner PALS. Thus, B cells can home into their proper compartments of the splenic white pulp independently of T lymphocytes.

The in vivo antibody response in rat gut-associated lymphoid tissue (GALT) after immunization with bacterial polysaccharide antigen

Gut mucosal immune responses to bacterial polysaccharide antigen in rats were investigated in vivo. Rats were immunized with pneumococcal polysaccharide type 3 (PPS-3) via different routes, i.e. in the Peyer's patch (iPP), in the colon (ic), in the peritoneal cavity (ip), and intravenously (iv). The development of specific antibody-forming cells (AFC) and their isotypes in the intestinal mucosa, gut-associated lymphoid tissue (GALT), mesenteric lymph nodes (MLN) and spleen were studied by immunohistochemistry. Furthermore, the serum antibody levels were measured by enzyme-linked immunosorbent assay (ELISA). The results showed that iPP immunization evoked high numbers of anti-PPS-3 AFC of the IgA isotype in the mucosa of the small intestine and in the PP. On the contrary, the ic route did not elicit a mucosal response, though a few AFC were found in the MLN and spleen. Following ip priming, a specific IgA response was found, especially in MLN and spleen, and a low response was detected in the villi. A high response was found in the parathymic lymph nodes (PTLN). Iv immunization gave rise to the development of AFC in the spleen, particularly of the IgM isotype. We failed to induce mucosal responses to PPS-3 antigen in the colon, irrespective of the route of immunization.

Characterization and cloning of a novel glycoprotein expressed by stromal cells in T-dependent areas of peripheral lymphoid tissues

A novel glycoprotein (gp) expressed by stromal cells of peripheral lymphoid tissue has been characterized immunohistochemically, biochemically, and at the molecular level. This molecule, gp38, was identified with a monoclonal antibody (mAb) (clone 8.1.1) previously shown to react with a subpopulation of thymic epithelium. This mAb generated a reticular labeling pattern in medullary and paracortical areas of lymph nodes and in splenic white pulp. At the ultrastructural level, labeling by the 8.1.1 mAb was restricted to fibroblastic reticular stromal cells. Serial sections of lymph node and spleen labeled with anti-CD3, anti-B220, and 8.1.1 mAbs clearly showed that the 8.1.1+ cells were associated with T cell-dependent areas. In severe combined immunodeficiency (SCID) or Nu/Nu mice, splenic white pulp also exhibited reticular labeling with the 8.1.1 mAb in the absence of detectable numbers of T cells, indicating that the appearance of 8.1.1-reactive stromal cells in discrete areas of peripheral lymphoid tissue was T cell independent. The cDNA encoding this stromal cell molecule was cloned by direct expression in COS cells and found to encode a 172 amino acid sequence with the typical features of a type I integral membrane protein. COS cells transfected with the gp38 clone direct the expression of an approximately 38-kD protein that reacts with the 8.1.1 mAb but not with isotype-matched controls. Comparison of the predicted amino acid sequence of 8.1.1 mAb but not with isotype-matched controls. Comparison of the predicted amino acid sequence of 8.1.1 with proteins in the National Biomedical Research Foundation (NBRF) data base showed that gp38 is very closely related to the early response protein OTS-8 obtained from a cDNA library of tumor promoting agent (TPA)-induced murine osteoblastic cell line, MC3T3-E1.

Sialoadhesin on macrophages: its identification as a lymphocyte adhesion molecule

In this study we present evidence that the mouse and rat sialoadhesin (originally named sheep erythrocyte receptor) on macrophages can function as a lymphocyte adhesion molecule. Lymphocytes were shown to bind to the splenic marginal zone, and lymph node subcapsular sinus and medulla in a frozen section assay. Selective depletion experiments showed that binding was mediated by macrophages. Adhesion was blocked by preincubation of the sections with monoclonal antibodies against mouse or rat sialoadhesin. Binding was temperature dependent, divalent cation independent, and involved sialic acid residues on the lymphocyte, as it could be inhibited by prior neuraminidase treatment or addition of the ganglioside GD1a. Binding to sialoadhesin was confirmed using the purified receptor and was observed among T cells, T blasts, B cells, and B blasts. Isolated macrophages or dendritic cells showed little binding. Sialoadhesin provides the first example of a macrophage-restricted lymphocyte adhesion molecule.

Macrophage subpopulations and reticulum cells in rat placenta. An immunohistochemical study

The placenta is a unique mixture of histoincompatible cells derived from mother and fetus. The aim of the present study was to obtain information on the development of macrophage subpopulations and reticulum cells during pregnancy in the placenta. Placentas of Wistar rats were removed at several stages of gestation, and were studied by immunohistochemical techniques applying monoclonal antibodies against macrophage subpopulations, lymphoid cells and reticulum cells. The expression of MHC class-II antigens was also studied. Throughout gestation macrophages were demonstrable in large numbers in the endometrium, in the myometrium and in the metrial gland, which is a compartment developing in the myometrium of pregnant rodents. In the labyrinth, a placenta compartment consisting of fetal cells, macrophages (probably of fetal origin) were already found on day 15. In the spongiotrophoblast and decidua basalis, which are layers of the placenta containing both maternal and fetal cells, only a few macrophages were recognized throughout gestation. The monoclonal antibody ED11, raised against reticulum cells, recognized fiber-like structures lining the blood sinuses of the spongiotrophoblast, in which only maternal blood is circulating. As the antigen recognized by ED11 is believed to play a role in the trapping of immune complexes, the spongiotrophoblast may play a role in the protection of the fetus from circulating immune complexes.

Selective inhibition of immune complex trapping by follicular dendritic cells with monoclonal antibodies against rat C3

The role of complement component C3 in the trapping of immune complexes by follicular dendritic cells (FDC) was studied in the rat, by means of the C3-specific monoclonal antibody ED11. Immunocytochemistry revealed the presence of C3 on FDC, where it co-localized with trapped peroxidase anti-peroxidase complexes. Furthermore, C3 was detected on reticular cells occupying the T cell areas of peripheral lymphoid organs, which are not involved in the handling of immune complexes. The in vivo administration of anti-C3 abolished the trapping of immune complexes in splenic follicles, but was unable to release preexisting complexes from the FDC. Trapping of immune complexes was also prevented by treatment of rats with cobra venom factor (CoVF). While CoVF caused massive depletion of C3 from serum, ED11 treatment had no such effect. The effect of anti-C3 appeared at least in part to be due to an inhibition of complement activation by immune complexes. We also analyzed earlier stages of the trapping process, with respect to their C3 dependence. Upon systemic injection immune complexes are initially observed in the marginal zone. Administration of anti-C3 reduced this localization, indicating a role for C3 in the entry of immune complexes into the spleen. Our results confirm experiments in CoVF-treated animals and extend the evidence for a role of C3 in the follicular trapping process using anti-C3 in vivo. The mechanism of immune complex trapping and the role of complement therein is discussed.

Recognition of self within self: specific lymphocyte positioning and the extracellular matrix

In this article, Maria de Sousa and colleagues address the importance of interactions between lymphocytes and extracellular matrix components to lymphocyte migration and positioning, emphasizing the role of the basement membrane and the complex stromal matrix of the lymphoid organs. They conclude that such interactions are of great importance, both in the physiological functioning of the immune system and in organ transplantation.

The functional activity of high endothelial venules: a role for the subcapsular sinus macrophages in the lymph node

Adherence of lymphocytes to high endothelial venules (HEV3) is the first step in normal lymphocyte emigration and recirculation. At sites of chronic inflammation, venules often become high-walled and may also be a site for leukocytes to leave the bloodstream. The immunologic and inflammatory mediators, responsible for these effects on endothelial cells, may be important for the maintenance and function of HEV in physiological conditions. It is reported here that the morphological and functional aspects of HEV can be studied by organ cultures of lymph nodes (LN). At 24 h of culture, the appearance of the node was still quite normal, whereas the HEV became flat-walled, with a 45-50% reduction in the capacity to bind lymphocytes. This decrease in function of HEV could be reduced when LN were cultured in the presence of lipopolysaccharides (LPS). The effect of LPS on the function of HEV was presumably mediated by macrophages in the subcapsular sinus, because HEV in LN, which were depleted of subcapsular sinus and medullary macrophages previous to culture, could not be stimulated by addition of LPS to the cultures.

Ontogeny of macrophage subpopulations and Ia-positive dendritic cells in pulmonary tissue of the rat

The development of macrophage subpopulations dendritic cells in the rat lung was studied from day 15 of gestation until day 21 after birth by means of immunohistochemical techniques combined with acid phosphatase staining. To characterize these cell populations, monoclonal antibodies raised against rat macrophage subpopulations were used (ED1, ED2, ED7, and ED8) in addition to anti-Ia antibodies. Ia-positive cells with a dendritic morphology were found on day 16 of gestation. During ontogeny, the number of these cells gradually increased. They were always found in mesenchymal lung tissue between the epithelial tubules of future alveoli, and in perivascular or peribronchial areas. ED1-positive macrophages were found on day 17 of gestation, with a distribution different from that of Ia-positive dendritic cells. The distribution of ED1-positive cells changed during ontogeny: before birth, ED1-positive cells were present in mesenchymal areas of lung tissue, whereas after the first week of postnatal life ED1 recognized all free alveolar macrophages. No Ia-expression was found on free alveolar macrophages. This developmental pattern resembles the ontogeny of Ia-positive dendritic cells and ED1-positive macrophages in gut-associated tissue. The comparable development of these cell populations in gut and lung tissue indicates a common ontogeny in the mucosal immune system.