Isolation of follicular dendritic cells from human tonsils and adenoids. I. Procedure and morphological characterization

Isolation of functionally active murine follicular dendritic cells

Biochemical, genetic, and immunological studies of follicular dendritic cells (FDCs) have been hampered by difficulty in obtaining adequate numbers of purified cells in a functional state. To address this obstacle, we enriched FDCs by irradiating mice to destroy most lymphocytes, excised the lymph nodes, and gently digested the nodes with an enzyme cocktail to form single cell suspensions. The FDCs in suspension were selected using the specific mAb FDC-M1 with magnetic cell separation technology. We were able to get nearly a million viable lymph node FDCs per mouse at about 90% purity. When examined under light and transmission electron microscopy, the cytological features were characteristic of FDCs. Furthermore, the cells were able to trap and retain immune complexes and were positive for important phenotypic markers including FDC-M1, CD21/35, CD32, CD40, and CD54. Moreover, the purified FDCs exhibited classical FDC accessory activities including: the ability to co-stimulate B cell proliferation, augment antibody responses induced by mitogens or antigens, maintain B cell viability for weeks, and protect B lymphocytes from anti-FAS induced apoptosis. In short, this combination of methods made it possible to obtain a substantial number of highly enriched functional murine FDCs.

New histochemical and ultrastructural observations on normal bovine tonsils

Samples of normal bovine palatine tonsils were examined by light and electron microscopy. Like human tonsils, they were composed of crypts, subepithelial areas, follicles, and T-dependent zones, but their well-developed capsule subdivided the lymphoid tissue by connective septa. B cells formed the major lymphoid component. The follicles and T-dependent zones had morphological and histochemical features typical of peripheral lymph organs. Follicular dendritic cells were isolated and shown to be similar to human follicular dendritic cells.

Functional effects of TNF and lymphotoxin alpha1beta2 on FDC-like cells

Recent studies suggest that tumor necrosis factor (TNF) family members such as TNFalpha and lymphotoxin alphabeta (LTalpha1beta2) are important in the development of follicular dendritic cells (FDCs) and maintenance of FDC function. In this study we used FDC-like cells (FDC-LC) cultured from normal human tonsil and investigated the effects of TNF and LTalpha1beta2 on expression of adhesion molecules and the production of cytokines and chemokines. TNF and LTalpha1beta2 both increased the expression of VCAM-1 and ICAM-1 on FDC-LC. In addition, IL-4 with LTalpha1beta2 synergistically increased the expression of VCAM-1, but not ICAM-1. Cytokine IL-6 and IL-15 mRNAs were induced following stimulation with TNF and LTalpha1beta2. These two cytokines were present in FDC-LC supernatants by ELISA and increased following TNF and LTalpha1beta2 stimulation. We also examined FDC-LC for chemokines, which affect B cells, including IL-8, SDF-1, MIP3beta/ELC, and BCA-1/BLC. SDF-1 mRNA and protein were expressed by FDC-LC, and following stimulation with TNF and LTalpha1beta2, decreases in both were observed. Therefore, TNF and LTalpha1beta2, which are produced by activated B cells, increased the expression of adhesion molecules and cytokines from FDC-LC, potentially providing key signals to support germinal center B cell survival and differentiation.

Human follicular dendritic cells remain uninfected and capture human immunodeficiency virus type 1 through CD54-CD11a interaction

It has been reported that human immunodeficiency virus type 1 (HIV-1) bound to follicular dendritic cells (FDCs) remains highly infectious to CD4(+) T cells even when it forms immune complexes with neutralizing antibody (HIV-1/IC). To elucidate the role of FDCs in HIV-1 transmission to CD4(+) T cells in lymph nodes, we have isolated and purified FDCs from human tonsils and examined whether the HIV-1/IC trapped on their surface is infectious to CD4(+) T cells. To our surprise, not the HIV-1/IC but the antibody-free HIV-1 on FDCs could be transmitted to CD4(+) T cells. Furthermore, in contrast to previous studies showing that FDCs are productively infected with HIV-1, the present study clearly demonstrated that FDCs were not the target cells for HIV-1 infection. FDCs could capture the viral particles on their surface; however, the binding of HIV-1 to FDCs was strongly inhibited by the presence of anti-CD54 (ICAM-1) monoclonal antibody (MAb) and anti-CD11a (LFA-1) MAb, suggesting that the adhesion molecules play an important role in the interaction between HIV-1 and FDCs.

Morphology, function and pathology of follicular dendritic cells

The precise ultrastructural morphology and functions in reactive conditions of lymphoid follicles (LF) and dendritic cells, including follicular dendritic cells (FDC) are reviewed; as well as the pathognomonic role of FDC in some disease conditions and finally, the cellular origin of FDC. In reactive conditions, FDC in each of the five follicular zones have distinct ultrastructural features, reflecting the different three-dimensional structures and functions of these zones. The FDC framework may be supported by some characteristic factors, including desmosome-like junctions between FDC and the expression of fibronectin and laminin receptors and caldesmon on FDC. FDC, especially in the light zone, express various cytokine receptors, but produce only one cytokine, TGF-beta. The outer zone may not only be a cellular pathway in the LF, but may also provide a site for germinal center B cell proliferation, and the FDC-lymphocyte cluster is not the site of germinal center B cell division. In patients with auto-immune diseases, such as Hashimoto's thyroiditis and rheumatoid arthritis, FDC may be in a hyperfunctional state, whereas those in patients with immunosuppressive disorders, such as Kimura's disease and AIDS, may be in a dysfunctional state. FDC may be derived from fibroblastic reticulum cells in lymphatic tissues rather than in bone marrow cells. The data discussed in this review provide fascinating insight into the roles of FDC, which are intimately related to the migration, proliferation, cell selection and differentiation of B cells in secondary LF.

Differential nuclear localization of p50, p52, and RelB proteins in human accessory cells of the immune response in situ

The Rel/NF-kappa B proteins, p50, p52, p65, c-Rel, and RelB, constitute a family of transcription factors involved in the positive regulation of a variety of genes during the immune response. Recently, it has been shown that RelB knockout mice have no dendritic cells (DC). An overexpression of p50 has been described in follicular dendritic cells (FDC). A constitutive NF-kappa B activity has been reported in mature macrophages. This led to the hypothesis that some of the Rel/NF-kappa B proteins were key nuclear factors in functions of accessory cells of the immune response. Therefore, we investigated in situ the nuclear localization of Rel/NF-kappa B proteins in accessory cells of the immune system by immunohistochemistry and double labeling by immunofluorescence from five normal human tonsils and five lymph nodes with follicular hyperplasia. Nuclear p65 and c-Rel proteins were found in all cell types including lymphocytes. In germinal centers GC, p50, p52, and RelB were found in the nuclei of FDC only and were not detected in the nuclei of CD68+ cells. In T cell areas, p50, p52, and RelB were found in the nuclei of HLA-DR+ cells with an antigen-presenting cell (APC) morphology. p52 and RelB were detected in the nuclei in both CD1a+ and CD68+ cells from the T cell area, whereas p50 was found only in CD68- and CD1a- cells. Cells with nuclear p50 were negative for the CD38, CD20 and CD2 markers. These results show that, physiologically, high levels of nuclear of p50, p52 and RelB are restricted to accessory cells of the immune system, which include FDC in GC, and DC and macrophages in the T cell zone, that specialized scavenger macrophages from GC do not have detectable levels of p52 and RelB, whereas macrophages from the T cell area, known to present the antigen to T cells, do have both nuclear p52 and RelB, and that in the T cell zone, p52 and RelB are located in nuclei of both CD1a+, CD68+ or both, cells APC, whereas p50 is restricted to CD1a- and CD68- APC. The different patterns of p50, p52 and RelB protein nuclear localization may provide insight into their different roles during the immune response in vivo.

Isolation of alkaline phosphatase-positive gingival fibroblasts from patients with chronic inflammatory periodontal disease

We have reported recently that increased expression of membrane alkaline phosphatase (ALP) activity is a phenotypical characteristic of gingival fibroblasts located in chronic inflammatory periodontal lesions. To understand the cellular properties of these cells, we isolated ALP-positive gingival fibroblasts from patients with adult periodontitis and evaluated their proliferative potential. Using an enzymatic digestion procedure, we prepared gingival cell suspensions containing ALP-positive fibroblasts without affecting their ALP activities. These cell suspensions were then subjected to 1 g sedimentation, followed by allowing cells to adhere to substrata. Using this procedure, 71.9% of isolated cells were ALP-positive. Dissociation of ALP-positive fibroblasts and contamination by non-fibroblastic cells were examined by cytochemical and immunocytochemical analyses. The proliferative capacity of ALP-positive fibroblasts in culture was assessed by monitoring the proportion of ALP-positive cells after repeated subculture passages and by labelling DNA-synthesizing cells with bromodeoxyuridine (BrdU). The proportion of ALP-positive fibroblasts decreased during cell culture passages without an apparent change in the ALP-positive phenotype. The percentage of BrdU-positive cells was significantly lower among ALP-positive than among ALP-negative fibroblasts. These results indicate that ALP-positive fibroblasts in chronic inflammatory periodontal lesions have low growth potential. We suggest that their reduced capacity to grow in vitro reflects a more differentiated state induced under inflammatory conditions in vivo.

Chemotaxis-promoting and adhesion properties of human tonsillar follicular dendritic cell clusters

Lymph follicles are globular and compact due to aggregation of lymphoid cells on follicular dendritic cells (FDC). To probe the mechanisms underlying this accumulation of cells, we analyse here the role played by FDCs in attracting and binding cells. FDCs prepared from human tonsils by mild separation techniques appeared in the form of clusters (FDC clusters), where, via cytoplasmic extensions, they enveloped lymphoid cells. Using Boyden's chambers, we demonstrated that these FDC clusters produced one or more chemoattractants capable of inducing chemotaxis of lymphoid cells. Supernatants of FDC cluster cultures also exerted a chemotaxis-promoting effect. FDC clusters induced true chemotaxis, not merely chemokinesis due to cell activation. They secreted a substance or substances that stuck to the substrate (a cellulose filter) and thus induced haptotaxis. B as well as T cells were attracted, but B cells apparently required the presence of T cells to respond fully to the chemoattractant(s). Subtypes of B cells (IgD+ and IgD-) and T cells (CD4+, CD8+, CD57+ AND CD57-) were tested and all were attracted. Since purified lymphoid cells did not induce these phenomena, FDCs were suspected to do so. FDCs have been shown to establish contact with lymphoid cells. Here we have determined that CD4+ T cells adhere in greater number to FDC clusters than do CD8+ T cells. We thus propose that FDCs specifically contribute to construction of lymph follicles by attracting and determining their cell composition.

Comparative analyses of thymocyte and thymic low-density adherent cell functions

Thymocytes which have developed in the C3H thymus showed depressed proliferative responses to stimulation with anti-CD3 antibody as compared with those which have developed in the thymus of other strains of mice (i.e. AKR). The present study was conducted to analyze immunological functions of the thymic stromal cell population (low-density adherent cells, LDAC) in the C3H mice using allogeneic bone marrow (BM) chimeras established by BM transplantation in the reciprocal combination of AKR and C3H mice as donor or recipient. The thymic LDAC from C3H mice or the [AKR(donor)-->C3H(recipient)] chimeras contained a high proportion of Mac-1+ cells as compared to AKR mice or the [C3H-->AKR] chimeras. The proportion of Mac-1+ cells paralleled the IL-1- and PGE2-secreting ability of the LDAC cultured either in the presence or absence of LPS and also paralleled the antigen-presenting cell functions of the LDAC. Furthermore, after anti-CD3 stimulation the PGE2 inhibited more profoundly proliferative responses of [AKR-->C3H] or normal C3H thymocytes than those of the [C3H-->AKR] chimera or normal AKR thymocytes. A PGE2 inhibitor, indomethacin, reversed the depressed responses of the thymocytes which had developed in the C3H thymus. These findings suggest that the lower responsiveness of thymocytes from [AKR-->C3H] chimeras to anti-CD3 stimulation may be attributable to large amounts of PGE2 secreted by LDAC and/or to increased sensitivity of thymocytes themselves to PGE2.

An immortalized cell line with features of human follicular dendritic cells. Antigen and cytokine expression analysis

Follicular dendritic cells (FDC) are specialized cells residing primarily within lymphoid follicles. They bind immunocomplexes and play an important role in the presentation of antigen to follicular B cells. Isolation of FDC for in vitro studies, however, is difficult because they constitute only about 1% of the cells in lymphoid tissue and form tight clusters entrapping lymphocytes within their dendritic processes. The monoclonal antibody (mAb) Ki-M4, which is highly restricted in its binding to FDC, is used to identify these cells. In order to establish a new immortalized cell line with features of FDC, we applied a modified procedure to isolate and enrich FDC from human tonsils and fused them with the myeloma cell line SP2/0-Ag14. The new hybrid cell line, designated FDC-H1, is of both mouse lymphoid and human FDC origin. FDC-H1 was found to have unlimited growth potential and to consistently express the Ki-M4 antigen and other surface antigens of human FDC. Semiquantitative reverse-transcribed polymerase chain reaction (RT-PCR) of enriched FDC and FDC-H1 revealed the same highly restricted cytokine/mRNA profile for both, with detectable levels of interleukin (IL)-1 alpha and surface CD23 and a lack of mRNA for IL-1 beta, IL-2, IL-3, IL-4, IL-7, IL-9, IL-10, interferon-gamma, tumor necrosis factor-alpha, transforming growth factor-beta and granulocyte/macrophage-colony-stimulating factor. Additionally a weak but constant IL-6 mRNA expression was found in the cell line FDC-H1 by RT-PCR. In situ hybridization experiments in tonsils revealed IL-6 transcripts in cells with a staining pattern characteristic of a dendritic cell only in a few germinal centers. To our knowledge, FDC-H1 is the first cell line that constantly expresses surface antigens and a cytokine profile characteristic of FDC. It is, therefore, well suited for studying the biology of FDC and the functional relationship between FDC and normal or neoplastic lymphatic cells.

Follicular dendritic cells in non-Hodgkin's lymphomas

Follicular dendritic cells (FDC) are restricted to the B-cell regions of secondary lymphoid tissue and to non-Hodgkin's lymphomas derived from the follicular center or the mantle zone. With their cytoplasmic ramifications they form a dense network which contains the B-lymphocytes. In situ, FDC are only detectable at the ultrastructural level or when stained with anti FDC-reagents. On the surface of their dendritic extensions they express transferrin receptors (CD71), the B-cell epitope CD20, class II antigens, the myelomonocytic molecule CD14, the glycoprotein gp50 (CD40), and several receptors for components of the complement system (CD11b, CD21, CD35). Subsequent to an antigen challenge, FDC trap and retain immune-complexes for a long period of time. In vitro FDC and neoplastic lymphocytes spontaneously form small cellular aggregates. This adhesion is mediated by the LFA-1-alpha/beta = ICAM-1, the VLA-4 = VCAM-1, and the ICAM-1 = C3bi- receptor ligand pathways on B-cells and on FDC, respectively. The loss of LFA-1- alpha/beta and ICAM-1 molecules may enable neoplastic lymphocytes to detach from FDC. The monoclonal B-cells now invade new compartments. In vitro, FDC have the capacity to activate resting B-cells and to save them from dying by apoptosis. Signals involved in this activation include cell-surface immunoglobulin and CD40. Immunocytochemistry and autoradiography with single cell suspensions of neoplastic B cells suggest that FDC also provide signals leading to the continued stimulation of lymphoma lymphocytes. During the early stage of HIV infection lymph nodes show an immense follicular hyperplasia, with a massive increase of the dendritic network of FDC. In the later stage of the disease, the continuous involution of the germinal centers is associated with a progressive destruction of FDC.

Optimizing follicular dendritic cell isolation by discontinuous gradient centrifugation and use of the magnetic cell sorter (MACS)

Follicular dendritic cells (FDC) contribute minimally to the total cell population of lymphatic tissue. In order to obtain higher numbers of viable FDC with only a small fraction of contaminating cells the following procedure was developed. Subsequent to the usual mechanical and enzymatical digestion of human tonsils, single cells were layered on top of a discontinuous bovine albumin gradient and centrifuged at 8500 x g. The suspension collected from the 1.052-1.030 interphase contained an average of 10.5% FDC. Next, the preparation was subjected to a new step involving separation of FDC previously treated with biotin-labelled KiM4 monoclonal antibody, raised against FDC, and attached via biotin-streptavidin bonding to streptavidin-conjugated paramagnetic beads. Purification on a magnetic cell sorter (MACS) yielded 3.3-10.1 x 10(6) cells with an average FDC content of 78.4%. The viability and morphology of the resulting FDC population was examined using trypan blue staining or electron microscopy. This technique will permit in vitro studies and long term cultures with FDC isolated from human lymphatic tissue.

Emperipolesis of lymphoid cells by human follicular dendritic cells in vitro

Isolated follicular dendritic cells (FDCs) showed true and pseudoemperipolesis of fresh tonsillar lymphocytes, even after long-term (50-day) cultivation. Emperipolesis by FDCs was not restricted by allotype specificity, nor was it inhibited by the addition of antibodies against MHC-I & II antigens. Follicular dendritic cells predominantly engulfed B-cells; monocytes and macrophages were not found between FDC cytoplasmic extensions. When highly purified T-cell populations were added to FDC cultures emperipolesis of T-cells occurred, particularly those of the CD4-positive phenotype. Mitoses appeared within 6 h in the emperipolesed lymphocytes and, after an additional 18 h, some lymphocytes exhibited apoptosis.

Cellular interactions in the germinal center: role of adhesion receptors and significance for the pathogenesis of AIDS and malignant lymphoma

The germinal center forms a specialized microenvironment that is thought to play a key role in the induction of antibody synthesis, affinity maturation of B cells, isotype switching, and memory B-cell formation. Moreover, the germinal center may also be involved in the maintenance of T-cell memory. In this paper we focus on the role of adhesion receptors in cellular interactions in the germinal center, and discuss evidence indicating that these molecules play an important role in regulating B-cell activation and differentiation. Furthermore, we discuss two important diseases involving the germinal center, i.e., HIV infection and malignant lymphoma. In HIV infection, destruction of the FDC network may explain the selective loss of memory cells observed in otherwise asymptomatic patients and is likely to represent a major pathway leading to AIDS. In follicular lymphoma, escape from physiological apoptosis in the germinal center by overexpression of Bcl-2 appears be a major pathogenetic pathway.

Isolation of normal human follicular dendritic cells and CD4-independent in vitro infection by human immunodeficiency virus (HIV-1)

Immunohistological and electron microscopy studies of lymph nodes from patients infected with the human immunodeficiency virus 1 (HIV-1) demonstrated that follicular dendritic cells (FDC), the antigen-presenting cells of the B cell system, contain and may produce the virus. To elucidate the mode of infection of FDC with HIV-1 in vitro we developed an improved method for the preparation of single-cell suspensions of viable FDC with high purity (greater than 90% FDC). These isolated FDC were subjected to human T cell leukemia virus IIIB infection, which was monitored after 4 days in culture using the polymerase chain reaction. We were able to demonstrate that normal human FDC are highly susceptible to infection by HIV-1. Inhibition experiments with the monoclonal antibody OKT4a demonstrate that this infection is independent of the CD4 molecule.

Adhesion of human B cells to follicular dendritic cells involves both the lymphocyte function-associated antigen 1/intercellular adhesion molecule 1 and very late antigen 4/vascular cell adhesion molecule 1 pathways

Presentation of antigen in the form of immune complexes to B lymphocytes by follicular dendritic cells (FDC) is considered to be a central step in the generation of memory B cells. During this process, which takes place in the microenvironment of the germinal center, B cells and FDC are in close physical contact. In the present study, we have explored the molecular basis of FDC-B cell interaction by using FDC and B cells derived from human tonsils. We found that FDC express high levels of the adhesion receptors intercellular adhesion molecule 1 (ICAM-1 [CD54]) and vascular cell adhesion molecule 1 (VCAM-1), while the B lymphocytes express lymphocyte function-associated antigen 1 (LFA-1 [CD11a/18]), very late antigen 4 (VLA-4 [CD49d], and CD44. Furthermore, we established that both the LFA-1/ICAM-1 and VLA-4/VCAM-1 adhesion pathways are involved in FDC-B lymphocyte binding, and therefore, these pathways might be essential in affinity selection of B cells and in the formation of B memory cells.

Human follicular dendritic cells: isolation and characteristics in situ and in suspension

Follicular dendritic cells (FDC) in human tonsils, either in situ in follicular germinal centres or isolated from tissue, were characterized by immunohistochemical, enzyme cytochemical and electron microscopical methods. Using polyclonal and monoclonal antibodies, expression of DRC-1, Ki-M4, HLA-DR, CR1, C1q antigens, a macrophage marker, and surface IgG and IgM were found on isolated FDC and on FDC in situ. None of these reagents proved to be specific for FDC, e.g. the FDC-directed antibodies DRC-1 and Ki-M4 labelled B lymphocytes in cytofluorography. Enzyme cytochemical staining revealed activities of non-specific esterase, acid alpha-naphthylacetate esterase and ATPase in germinal centres and in freshly isolated FDC. Immunohistochemistry demonstrated a weak expression of CD4 by a fraction of isolated FDC, which was confirmed by two-colour immuno-staining and immuno-electron microscopy.

Responses of single germinal-center B cells in T-cell-dependent microculture

B cells purified from the germinal centers (GCs) of murine Peyer's patches can be stimulated in a clonal microculture containing helper T cells and dendritic cells to divide and secrete immunoglobulin. Intraclonal isotype switching occurs, and a variety of immunoglobulin isotypes, including IgA, is secreted. Memory cells, which generate clones secreting IgA exclusively, are only rarely identified in the GC B-cell subset. Such memory cells can, however, be readily identified among unfractionated Peyer's patch B cells, and in non-GC subsets of B cells. The results suggest that the GC does not contain IgA memory cells that can be restimulated in vitro to secrete only IgA. When division of GC B cells is prevented by irradiation or aphidicholin treatment, a large subset that secretes IgA as the sole immunoglobulin isotype is seen, and the output of presumably single B cells is large enough to be scored by RIA. Both helper T cells and dendritic cells are required for the phenomenon. The data indicate that commitment to IgA secretion occurs in Peyer's patch GCs and suggest that the prolific cell division known to be supported in GCs may forestall terminal differentiation of preplasmablasts to immunoglobulin secretion.

HLA-DR expression in B-cell non-Hodgkin's malignant lymphomas: a multiparameter flow cytometry study

Ten cases of reactive follicular hyperplasia and 31 cases of B-cell non-Hodgkin's malignant lymphoma were studied using multiparameter flow cytometry. A bimodal distribution for HLA-DR expression, but not for surface immunoglobulin or B cell-specific antigens CD19 and CD20, was observed commonly in mixed cell type and infrequently in non-mixed cell type B-cell malignant lymphomas. On the basis of HLA-DR distribution alone, 31 cases of B-cell malignant lymphomas of low, intermediate, and high grades could be separated into mixed and non-mixed cell types, with only two misclassifications (P = 0.0001). Exceptionally, one case of malignant lymphoma, follicular and diffuse, mixed-cell type had a unimodal HLA-DR distribution, and one case of malignant lymphoma, diffuse, large noncleaved cell type had a bimodal HLA-DR distribution. In all cases of malignant lymphoma, follicular, mixed-cell type studied, low HLA-DR was correlated with small cells, and high HLA-DR was correlated with large cells. In contrast, HLA-DR expression and cell size were not as directly correlated in cases of malignant lymphoma, diffuse, mixed-cell type. These observations suggest that most, but not all, cases of B-cell malignant lymphomas of the mixed cell type can be separated from other B-cell lymphomas on the basis of HLA-DR distribution.

Isolation and long-term cultivation of human tonsil follicular dendritic cells

Highly purified follicular dendritic cells (FDC) were isolated from human tonsils and cultivated for up to 150 days. The cell separation method employed produced pure aggregates (FDC-clusters) composed of FDC and germinal center lymphoid cells, useful for the analysis of the relationship between these two cell types and of the behavior of FDC in culture. During the first few days of culture, lymphoid cells located between FDC extensions survived better than those which were free or partly covered by FDC. After 6 days, the lymphoid population degenerated and only the FDC survived. The unique antigenic pattern of FDC (positive for HLA-DR. DRC-1, CD14b, CD21, CD23, CD35) disappeared within a few days of culture. Recombinant interferon-gamma exerted a positive effect either on retaining HLA-DR expression or on the reexpression of these antigens by FDC. HLA-ABC antigens were traced until the 10th day and desmosomal junctions until the 14th day. Subsequently, FDC presented peculiar features, including oval and rhomboid shapes, one to ten nuclei, fine amoeboid extensions, stress fibers and a radical dense zone in their cytoplasm. FDC possessed actin, tubulin and vimentin, but neither desmin nor cytokeratin. After 40 days of culture, FDC enlarged and were covered with abundant membrane extensions. Even when kept as long as 150 days in vitro. FDC did not proliferate in any of the culture conditions employed.

Vimentin immunostaining in fibroblastic reticulum cells within human reactive and neoplastic lymphoid follicles

We analyzed the distribution of fibroblastic reticulum cells (FRCs), stationary cells of lymphoid tissues, as visualized by the anti-vimentin (V9) monoclonal antibody in human reactive and neoplastic lymphoid follicles, by using immunoenzymatic and immunofluorescence methods on fixed and paraffin-embedded tissue sections from 37 lymphoid specimens with reactive disorders and 10 specimens with nodular/follicular non-Hodgkin's lymphomas (NHLs). The pattern of distribution of the vimentin-positive (VIM+) FRCs was compared with that of follicular dendritic reticulum cells (DRCs) as visualized by anti-S-100 protein antibody. Elongate VIM+ FRCs intimately attached to reticulum fibers were randomly distributed in the paracortical and interfollicular areas of lymph nodes, whereas they were recognized specifically in the mantle zones of the secondary follicles, mostly in the outer margins. Germinal centers were consistently devoid of VIM+ FRCs. Comparative analysis on serial sections as well as paired immunoperoxidase and double immunofluorescence studies demonstrated that there was a sharp difference between the patterns of intrafollicular distribution of VIM+ FRCs and S-100 protein-positive (S-100+) DRCs without juxtaposition, the FRCs being confined to the mantle zones. In the 10 nodular/follicular NHLs VIM+ FRCs could be observed in the thinned mantles of neoplastic nodules displaying a corona-like pattern that accentuated the boundaries of the nodules. The results of this study support the view that the intrafollicular distribution of VIM+ FRCs is specific for the mantle zone. The different microenvironmental organization within the follicles of VIM+ FRCs and S-100+ DRCs suggests that FRCs or at least VIM+ FRCs are stationary cells strictly related to the mantle zone microenvironment, where they may play a role in supposed sustentacular and immunologic functions similar to that of DRCs in the germinal center microenvironment.

Dendritic reticulum cell-related immunostaining for laminin in follicular and diffuse B-cell lymphomas

We performed a comparative immunohistocytochemical study of the distribution patterns of laminin and follicular dendritic reticulum cells (DRCs) within their follicular microenvironment in both nodular or diffuse B-cell non Hodgkin's lymphomas (NHLs). Twenty nine cases of immunophenotypically diagnosed B-cell NHLs (19 of follicular center cell origin-FCCL- and 10 of the diffuse well differentiated lymphocytic type-WDLL-) and five reactive lymph nodes with follicular hyperplasia were analyzed by immunoperoxidase and immunofluorescence techniques. Serial frozen sections and cytospin preparations were tested either with single antibodies anti laminin and DRC-1, or paired reagents in double labeling immunofluorescence. Our results indicated consistently that within both the reactive germinal centers and the neoplastic nodules of FCCL laminin immunostaining visualized a punctate-granular pattern apart from the linear vascular basement membrane positivity. Double immunofluorescence assay demonstrated that there was a close parallelism between this laminin staining pattern and DRC-1 distribution showing a well developed DRCs meshwork; in the diffuse tumour areas of both FCCL and WDLL, laminin immunoreactivity was found only in those cases in which nests of DRCs were observed. Double immunofluorescence studies performed on cytospin preparations demonstrated that the groups of cells containing DRC-1 positive cells, contained a positivity for laminin, although within the cell the staining for DRC-1 was intense and diffuse, while that for laminin was granular and more sparse. Our results suggested that these laminin and DRC-1 positive reactive sites may be present on the same cells. Since the reduction in number or loss of both DRCs and their related immunostaining for laminin within the microenvironment was consistently associated with a loss of nodularity by lymphoma cells, whereas nodularity in reactive and neoplastic conditions was associated with a rich DRCs meshwork and the related laminin immunostaining, a trapping function of DRCs exercised in the presence of laminin should be considered.

Cytokines produced in lymph follicles

The events occurring inside lymph follicles during a germinal center reaction are poorly understood. Using B and T lymphoid cell populations prepared from human tonsillar lymph follicles, and enriched or not in macrophages or in follicular dendritic cells, we examined the production of cytokines by these cells in vitro. Interleukin 6 (IL-6) and tumor necrosis factor (TNF) were found in the supernatants of cultures stimulated with phytohemagglutinin or pokeweed mitogen. IL-1 beta was occasionally detected; its secretion apparently depends on the origin of the tonsils, the stimulation, and the cell populations. IFN-gamma and IL-2 were not produced in significant amounts by these lymph follicle cells. IL-4 was only found in very low concentrations in the supernatant of the different cell cultures. The cell populations containing follicular dendritic cells produced more IL-6 and TNF than the others, especially than those composed of only B and T cells.

Isolated human follicular dendritic cells display a unique antigenic phenotype

In the present study, follicular dendritic cells (FDCs) were purified to homogeneity in order to define the lineage and function of these cells. FDCs were identified by their characteristic morphology and by their expression of receptors for the third complement component, the myeloid-restricted antigen CD14, and the FDC antigen DRC-1. Unclustered FDCs displayed a unique antigenic phenotype since they expressed several B- and myeloid lineage-restricted antigens, but lacked T and NK cell antigens as well as the leukocyte common antigen. FDCs expressed adhesion molecules, including most of the VLA proteins, intercellular adhesion molecule 1 (ICAM-1), and CD11b. FDCs could be isolated to homogeneity by their intense staining with anti-CD14 using flow cytometric cell sorting. These highly purified FDCs expressed CD14 and CD21 but lacked CD20. This antigen pattern and characteristic morphology confirmed that these cells were, in fact, homogeneous FDC preparations. Analysis of polymerase chain reaction-amplified cDNA from highly purified FDCs showed no transcripts for IL-6. The isolation of homogeneous FDC populations will be important for the analysis of the functional role of FDCs within the lymphoid follicle.

Heterogeneous immunostaining patterns of follicular dendritic reticulum cells in human lymphoid tissue with selected antibodies reactive with different cell lineages

A comparative immunohistologic study of the cell density and distribution pattern of follicular dendritic reticulum cells (DRCs) within their follicular microenvironments (germinal centers and mantle zones) was performed by immunoperoxidase technique with a selected panel of antibodies either operationally specific for DRCs (DRC-1) or reported as having additional immunoreactivity with DRCs (antibodies to B- and T-cells, leukocytes, monocytes/macrophages, desmosomal components, and S-100 protein). Twenty-five biopsy specimens, including reactive lymph nodes and tonsils as well as normal spleen tissue, were analyzed. Serial frozen sections were tested either with single antibodies or paired monoclonal reagents in double-labeling procedures. Our results consistently indicated that DRCs positive for Leu M3 and BA-2 antibodies were confined to the central portion of germinal centers, whereas DRCs immunoreactive for S-100 protein and desmoplakin 1 and 2 were localized mostly in the central and pericentral portion of germinal centers. All the DRCs extending from the central portion of germinal centers to the mantle zones were labeled with DRC-1 and, unexpectedly, with OKB7 antibodies. Other immunostainings, such as those for HLA-DR antigens, common leukocyte antigen, and Leu 3a, were not contributory in defining topographic differences of DRCs within the follicle. The consistent heterogeneity of the labeling patterns appears to suggest a possible in situ immunophenotypic grouping of DRCs, and the concept of their possible heterogeneity appears to be corroborated.

Formation of lymphoepithelial tissue in the sheep's palatine tonsil

Formation of lymphoepithelial tissue was studied in 1-, 10- and 21-day-old sheep. From each of the animals one of the tonsils was fixed in 4% glutaraldehyde for light- and electron microscopy while the other was frozen in liquid nitrogen for immunohistochemistry. These examinations revealed sequential histological events during the formation of reticular epithelium. (i) Appearance of a distinct epithelial cell indicates the initiation of the reticulation. The electron density of these epithelial cells is much lower than that of the common keratinocytes but the presence of cytoplasmic tonofibrils and desmosomes provides evidence that they are of epithelial origin. They may represent the precursors of M cells. Their appearance may be followed by expression of Ia+ molecules on the surrounding keratinocytes showed by isolated Ia+ areas in the epithelium. (ii) In the mesenchyme underneath the Ia+ epithelial areas, Ia+ dendritic-like cells emerge which immigrate through the basement membrane into the epithelium establishing a provisional dendro-epithelial tissue. In this stage of the reticular epithelium's formation large Ia+ areas are shown by immunostaining, which include the epithelium and mesenchyme. The origin of the Ia+ dendritic-like cells is uncertain but their distribution and dense accumulation underneath the epithelium suggest that they are transformed tonsillar mesenchymal cells. Similar cell transformation of mesenchymal cells takes place in the bursa of Fabricius prior to development of lymphoepithelial tissue. (iii) The M cell precursors together with the Ia+ dendritic-like cells adapt the epithelium to be suitable for receiving a large number of lymphoid cells. Immigration of the lymphoid cells into the epithelium transforms the dendro-epithelial tissue to a real lymphoepithelial one.

In vitro behavior of thymic nurse cell-like complexes from mechanically and enzymatically dissociated frog tadpole thymuses

Cellular complexes, analogous by virtue of their external appearance, size, and number of seemingly internalized thymocytes to thymic nurse cells (TNCs) of endothermic vertebrates, were seen in short-term cultures (6-8 days) of mechanically and enzymatically dissociated thymuses of leopard frog tadpoles. Most TNC-like complexes from mechanically disrupted thymuses were covered with many thymocytes that morphologically resembled the "internalized" thymocytes. With time in culture, most complexes remained spherical and lost their externally adherent and "internalized" thymocytes. Some complexes, however, adhered to the glass substratum by means of macrophage-like cells. After one typically appearing TNC from a mechanically dissociated thymus had released its "internalized" thymocytes and spread completely over the glass substratum, it could be seen to consist actually of 9-10 stromal cells with the appearance of epithelial cells, macrophages, and dendritic cells. TNC-like structures from enzymatically dissociated thymuses had few, if any, attached thymocytes. Although these structures closely resembled murine TNCs initially, they displayed abnormal transformations within a few days of culture. Our observations led us to question the assumption that all TNCs from mechanically as well as enzymatically isolated TNCs from vertebrate thymuses are single cells. Rather, some if not all of the so-called TNC may actually be entities composed of several stromal cell types that enclose thymocytes. We suggest that this configuration seen in vitro may reflect the architecture of the compartmentalized reticular stromal cell meshwork that characterizes the intact thymus.

Influence of immunoglobulin isotypes and lymphoid cell phenotype on the transfer of immune complexes to follicular dendritic cells

Follicular dendritic cells (FDC) are located only inside lymph follicles and are characterized mainly by their capacity to retain high amounts of immune complexes by their Fc or C3b receptors. In this work, we examine the influence of immunoglobulin isotypes and the subset of lymphoid cells (B or T) upon the transfer of immune complexes from lymphocytes to FDC. FDC isolated from mice lymph nodes by enzymatic digestion are able to fix, through Fc receptors, gold-labeled immune complexes presented by lymphoid cells. As demonstrated by electron microscopy, this transfer requires the establishment of close contacts between both cell types. Using different cell selection techniques we show that B lymphoid cells take up immune complexes more efficiently than do T lymphoid cells and transfer a larger number of them to FDC. This transfer mechanism is dependent on the immunoglobulin isotype: immune complexes constituted of IgG2a, IgG2b, and IgG1 isotypes are better transferred to FDC than those constituted of IgG3 and IgM.

A light microscopical study of isolated follicular dendritic cell-clusters in human tonsils

In order to re-examine the cellular structure of isolated FDC-culsters in the germinal centers of human tonsils, a light microscopical analysis was made. Approximately 14 FDC-clusters were recovered from one enucleated germinal center using the enzyme digestion technique. The minimum unit of the FDC-clusters was composed of one FDC and 8 to 9 lymphocytes. Most of the FDC-clusters were representative of the microenvironment of the light zone at the germinal center in situ. Half of the engulfed centrocytes were supposed to be at the Go phase, and the others at the G1 to G2 phase. It is suspected that the helper-T cell has some relationship to the FDC microenvironment, and that the suppressor-T cell does not. Most of the CIgG-containing cells in the germinal centers were considered to have infiltrated into the interspace of the FDC microenvironment.

Lipopolysaccharide suppresses immune complex retention by follicular dendritic cells without cytological alterations

Follicular dendritic cells (FDC) are peculiar cells only located inside lymph follicles and which may be characterized by complex dendritic evaginations retaining high quantities of immune complexes by Fc and C3b receptors. After lipopolysaccharide (LPS) injection in mice the retention of gold-labelled immune complexes was abolished in draining lymph nodes. In order to examine the possibility that the transport of immune complexes to lymph follicles was impaired, we isolated FDC from lymph nodes and incubated them in presence of gold-labelled complexes: no or strongly reduced retention was then observed at the ultrastructural level. This LPS-induced impairment of immune complex fixation by FDC is not due to morphological alteration to the cells but to the inhibition of their Fc and C3b receptors. Further, LPS induces changes in the composition of the lymphocyte population in lymph follicles as higher numbers of blast cells and plasmocytes are observed after treatment.

Retention of immune complexes by murine lymph node or spleen follicular dendritic cells. Role of antibody isotype

Using monoclonal anti-trinitrophenyl (TNP) antibodies complexed to TNP-myoglobin-coated gold particles, we analysed at the ultrastructural level the retention by follicular dendritic cells (FDC) of immune complexes containing various antibody isotypes. Gold-labelled immune complexes were injected subcutaneously or intravenously into naive mice and, after 24 h, germinal centres of draining lymph nodes or spleen were examined by electron microscopy. FDC generally retained complexes containing IgG2a and IgG2b better than those formed with IgG1 or IgG3. IgM was rarely retained. FDC isolated from lymph nodes or spleens were incubated in vitro with gold-labelled complexes in a serum-free medium. IgG2a and IgG2b complexes were also retained in vitro in large quantities by FDC; IgG1 and IgG3 complexes were retained in smaller quantities or in highly variable quantities compared with IgG2; IgM complexes were rarely seen on FDC. There was no difference between FDC isolated from lymph nodes or from spleen with respect to the Ig isotypes required for Fc-mediated retention of immune complexes.

Characteristics and functional aspects of nonlymphoid cells in rat germinal centers, recognized by two monoclonal antibodies ED5 and ED6

In this study two newly developed monoclonal antibodies, ED5 and ED6, are described, which specifically recognize nonlymphoid cells in B cell follicles of spleen and lymph nodes. Enzyme and immunocytochemical techniques demonstrated that ED5 and ED6 stain two different types of reticulum cells. The staining patterns were compared with those of the monoclonal antibody Ox2. ED5+ cells are able to retain immune complexes and are considered to be the follicular dendritic cell; in contrast ED6+ lack this capacity. Peyer's patches contain many ED6+ and Ox2+ cells but are completely devoid of ED5+ cells. After the application of immunomodulatory agents, like X-irradiation, cyclophosphamide and lipopolysaccharide, ED5+ and ED6+ cells remain detectable. The nature of the new type of dendritic reticulum cell in lymphoid follicles, recognized by ED6, is discussed.

Transfer of immune complexes from lymphocytes to follicular dendritic cells

Antigens in the form of immune complexes are retained on the membranes of follicular dendritic cells (FDC) for long periods of time. To examine how immune complexes reach germinal centers, where FDC are located, we injected mice with anti-2,4-dinitrophenyl (DNP) antibodies complexed to DNP-myoglobin-coated gold particles. The distribution of the particles in spleens or draining lymph nodes was then determined with the electron microscope. The vast majority of the particles were cell bound. Shortly after injection they were phagocytized by macrophages or fixed on lymphocytes. The latter were found even in the corona of lymph follicles but not in germinal centers. Already 30 min after injection, FDC in contact with the corona were faintly positive but were negative in the center. FDC precursor cells were occasionally observed but in too small a number to account for the transport of immune complexes to the germinal centers. Twenty-four hours after injection colloidal gold particles were found in phagolysosomes of macrophages or on cytoplasmic extensions of FDC in all parts of the germinal centers. Experiments performed on isolated FDC showed that they are not only able to take up free immune complexes but are also able to adsorb immune complexes from pulsed lymphocytes. These results strengthen the idea that lymphoid cells binding immune complexes by their Fc receptors may transport these complexes inside germinal centers.

Demonstration of S-100 protein distribution in human lymphoid tissues by the avidin-biotin complex immunostaining method

Immunoreactivity for S-100 protein was investigated immunohistochemically in a series of 49 fixed and paraffin-embedded normal, reactive, and neoplastic human lymphoid tissue specimens. The avidin-biotin complex immunoperoxidase method was used, with overnight (12-hour) incubation with a commercially available antiserum to S-100 protein. In addition, cryostat sections were tested with DRC 1 monoclonal antibody to dendritic reticulum cells (DRCs) in three cases and with OKT6 antibody to interdigitating reticulum cells (IRCs) in nine cases. All tissues, including lymph nodes, tonsils, adenoid, spleens, appendices, thymuses, and tissues containing nodular reactive lymphoid infiltrates, demonstrated a consistent immune staining pattern. A striking network composed of dendritic processes that showed finely granular S-100 protein immunoreactivity was observed in most of the follicular germinal centers; a similar dendritic pattern was observed in the follicular centers when the corresponding frozen sections were immunostained with DRC 1. In the extrafollicular areas, the S-100-positive cells topographically and morphologically resembled the IRCs that were demonstrated by OKT6 antibody in the corresponding frozen sections. The results seem to indicate that cells topographically and morphologically similar to IRCs and DRCs in human lymphoid tissues from different sites share immunoreactivity for S-100 protein. The present study confirms the unexpected presence of S-100 protein in dendritic cells of follicular germinal centers by a simple and currently available method.

Retention of immune complexes by Fc receptors on mouse follicular dendritic cells

Follicular dendritic cells (FDC) are located inside lymph follicles and are mainly characterized by their capacity to retain antigens. We investigated this aspect in mice lymph nodes by using bovine serum albumin (BSA) labelled with 5-nm colloidal gold particles and homologous anti-BSA antibodies bound to 20-nm gold particles. Gold-labelled BSA injected alone in non-immunized mice was only rarely found in FDC cytoplasmic interdigitations. Injected in the form of immune complexes, it was retained by FDC. Antigen-free anti-BSA antibodies injected under similar conditions as immune complexes were always found in draining lymph nodes in the same locations as BSA-anti-BSA immune complexes. F(ab')2 from mouse immunoglobulins linked to colloidal gold particles were very rarely found between the FDC extensions, whereas it was intensely phagocytosed by macrophages. Our study permitted precise ultrastructural localization between FDC cytoplasmic extensions or inside macrophages and other cells of the lymph nodes, but it also pointed out that homologous antibodies linked to colloidal gold particles might be retained by FDC in the absence of antigens. These observations, carried out with colloidal gold, were checked by using 125I-labelled anti-BSA antibodies. Complement activation determinations of gold-labelled antibodies or immune complexes showed that antibodies or immune complexes fixed on colloidal gold particles do not activate the complement. This observation enabled us to conclude that Fc receptors play a significant part in the retention of gold-labelled antibodies or immune complexes by FDC of lymph nodes.

5-Nucleotidase activity in isolated follicular dendritic cells

Follicular dendritic cells isolated from mouse lymph nodes were incubated in the presence of AMP to test 5-nucleotidase (5-Nase) activity. Ultrastructural observations showed the presence of 5-Nase on external membranes but also some activity inside the nucleus. 5-Nase was found associated to Fc receptors labelled with homologous immunoglobulins fixed on colloidal gold particles. Lymphocytes and macrophages, found in association with the follicular dendritic cells, were either 5-Nase positive or negative. The hypothetical roles played by 5-Nase in germinal centers are discussed.