Surface Of Follicular Dendritic Cells Research Articles

Cellular distribution of the prion protein in palatine tonsils of mule deer (Odocoileus hemionus) and Rocky Mountain elk (Cervus elaphus nelsoni).

Chronic wasting disease (CWD) is a transmissible spongiform encephalopathy (TSE) that affects members of the Cervidae family, including deer (Odocoileus spp.), elk (Cervus Canadensis spp.), and moose (Alces alces spp.). While CWD is a neurodegenerative disease, lymphoid accumulation of the abnormal isoform of the prion protein (PrPSc) is detectable early in the course of infection. It has been shown that a large portion of the PrPSc lymphoid accumulation in infected mule deer takes place on the surface of follicular dendritic cells (FDCs). In mice, FDC expression of PrPC has been shown to be essential for PrPSc accumulation. FDCs have been shown to normally express high levels of PrPC in mice and humans but this has not been examined in natural hosts for CWD. We used double immunofluorescent labeling and confocal microscopy to determine the PrPC expression characteristics of B and T lymphocytes as well as FDCs in palatine tonsils of CWD-negative mule deer and elk. We detected substantial PrPC colocalization with all cellular phenotypic markers used in this study, not just with FDC phenotypic markers.

The B-Cell Follicle in HIV Infection: Barrier to a Cure.

The majority of HIV replication occurs in secondary lymphoid organs (SLOs) such as the spleen, lymph nodes, and gut-associated lymphoid tissue. Within SLOs, HIV RNA+ cells are concentrated in the B-cell follicle during chronic untreated infection, and emerging data suggest that they are a major source of replication in treated disease as well. The concentration of HIV RNA+ cells in the B-cell follicle is mediated by several factors. Follicular CD4+ T-cell subsets including T-follicular helper cells and T-follicular regulatory cells are significantly more permissive to HIV than extrafollicular subsets. The B cell follicle also contains a large reservoir of extracellular HIV virions, which accumulate on the surface of follicular dendritic cells (FDCs) in germinal centers. FDC-bound HIV virions remain infectious even in the presence of neutralizing antibodies and can persist for months or even years. Moreover, the B-cell follicle is semi-immune privileged from CTL control. Frequencies of HIV- and SIV-specific CTL are lower in B-cell follicles compared to extrafollicular regions as the majority of CTL do not express the follicular homing receptor CXCR5. Additionally, CTL in the B-cell follicle may be less functional than extrafollicular CTL as many exhibit the recently described CD8 T follicular regulatory phenotype. Other factors may also contribute to the follicular concentration of HIV RNA+ cells. Notably, the contribution of NK cells and γδ T cells to control and/or persistence of HIV RNA+ cells in secondary lymphoid tissue remains poorly characterized. As HIV research moves increasingly toward the development of cure strategies, a greater understanding of the barriers to control of HIV infection in B-cell follicles is critical. Although no strategy has as of yet proven to be effective, a range of novel therapies to address these barriers are currently being investigated including genetically engineered CTL or chimeric antigen receptor T cells that express the follicular homing molecule CXCR5, treatment with IL-15 or an IL-15 superagonist, use of bispecific antibodies to harness the killing power of the follicular CD8+ T cell population, and disruption of the follicle through treatments such as rituximab.

Unique properties of GC B cells permit optimal BCR affinity discrimination

Abstract Upon encountering pathogens, B cells within lymphoid organs enter specialized micro-environments of germinal centers (GCs). In the dark zones (DZ) of GCs, B cells proliferate and undergo somatic hypermutation, and subsequently migrate to the light zone (LZ) of the GC where selection of B cells that express the highest affinity B cell receptors (BCRs) occurs. Affinity selection is driven by the ability of BCRs to signal and extract antigen from the surfaces of follicular dendritic cells. Here we provide evidence that human tonsil LZ GC B cells are inherently better able to undergo affinity selection as compared to naïve tonsil B cells. LZ GC B cells engaged membranes that contained F(ab’)2 anti-light chain as a surrogate antigen through highly dynamic F-actin-containing pod-like structures that concentrated BCRs at their contact points. Using DNA-based mechanical force nano-sensors we observed robust pulling forces localized to the contact sites of the pods with the membrane. In contrast naïve B cells formed flat stable contacts with the membranes and showed only defuse weak pulling forces. Using antikappa antibodies of low (KD=3.9×10−7) versus high (KD=2.4×10−9) affinities as surrogate antigens, we show that LZ GC B cells were able to discriminate between these and responded only to the high affinity antigen. In contrast, naïve B cells responded equally to both antigens. Both LZ GC B cells and naive B cells were able to extract and internalize membrane-bound antigen, however, only in LZ GC B cells was the subsequent trafficking of the antigen to acidic intracellular compartments for processing highly dependent on the amount of antigen internalized. Thus, LZ GC B cells appear to be intrinsically programmed for optimal affinity selection.

Dextran-Conjugated Anti-Immunoglobulin Antibodies: A Powerful Tool for Studying B Cell Receptor-Mediated Signaling

The cross-linking of the BCR in a multivalent fashion represents a more physiologic means, relative to using unconjugated bivalent anti-Ig, for studying the functional consequences of B cell encounter with specific antigens in vivo. This reflects either the repeating epitope nature of polysaccharides, or the display of a given intact protein in multiple copies on the surface of follicular dendritic cells, macrophages and dendritic cells where they are encountered by B cells.

Mechanisms of B cell autoimmunity in SLE

Systemic lupus erythematosus (SLE) is a systemic autoimmune disease that is known to be associated with polyclonal B-cell hyperreactivity. The underlying causes of the diffuse B-cell over-reactivity are unclear, but potential candidates include (a) intrinsic hyper-reactivity leading to polyclonal B-cell activation with disturbed activation thresholds and ineffective negative selection; (b) lack of immunoregulatory functions; (c) secondary effects of an overactive inflammatory environment, such as overactive germinal center and ectopic follicular activity; and/or (d) disturbed cytokine production by non-B immune cells. These mechanisms are not mutually exclusive and may operate to varying extents and at varying times in SLE. Phenotypic and molecular studies as well as the results of recent clinical trials have begun to provide new insights to address these possibilities. Of importance, new information has made it possible to distinguish between the contribution played by abnormalities in central checkpoints that could lead to a pre-immune repertoire enriched in autoreactive B cells, on the one hand, and the possibility that autoimmunity arises in the periphery from somatic hypermutation and abnormal selection during T cell-dependent B-cell responses on the other. There is an intriguing possibility that apoptotic material bound to the surface of follicular dendritic cells positively selects autoreactive B cells that arise from non-autoreactive B-cell precursors as a result of somatic hypermutation and thereby promotes the peripheral emergence of autoimmunity.

The missing link in the affinity maturation chain

Knowledge of the B cell's capacity to recognize native antigen has diluted recognition of the importance of B cell interaction and/or communication with accessory cells to initiate the humoral immune response. In addition, the mechanisms by which antigens gain access to the follicle, and thus to the B cell repertoire, were largely unknown until recently. In the July issue of Nature Immunology, Phan et al.1 extensively characterize the macrophage population located at the follicle junction with the lymph node subcapsular sinus (SCS). The authors show an important role for immune complex relay by SCS macrophages and non-cognate B cells in driving antibody affinity maturation at the germinal centre (GC).

Clearance deficiency and systemic lupus erythematosus (SLE)

Systemic lupus erythematosus (SLE) is a fairly heterogeneous autoimmune disease. Impaired clearance functions for dying cells may explain accumulation of nuclear autoantigens in various tissues of SLE patients. Our data show that in a subgroup of patients with SLE, apoptotic cells accumulated in the germinal centres of the lymph nodes. Apoptotic material was attached to the surfaces of follicular dendritic cells. Furthermore, we found an accumulation of apoptotic cells in the skin of patients with cutaneous lupus after UV exposure. Granulocytes and monocytes in whole blood of SLE patients showed a reduced uptake of albumin- and polyglobin-coated beads. Furthermore, we analysed sera from SLE patients in migration assays and observed that the attraction signals for macrophages were reduced by sera of approximately 25% of the SLE patients. Analyses of high-affinity DNA binding IgG autoantibodies of SLE patients revealed that those antibodies had gained their DNA reactivity in a germinal centre reaction. We suggest a stepwise maturation from a non-anti-DNA reactive B cell to an anti-dsDNA autoreactive B cell. We conclude that impaired clearance in early phases of apoptosis leads to a secondary necrotic status of the cells. Danger signals are released; modified autoantigens are accessible, favouring an autoimmune reaction.

Complement-Mediated Activation of the Adaptive Immune Responses: Role of C3d in Linking the Innate and Adaptive Immunity

C3d is the final degradation product of the third component of complement (C3). When conjugated to an antigen, C3d enhances immune responses to the fused antigen. Therefore, this molecule has been used as an adjuvant to enhance the immune responses to various foreign and self-proteins. C3d binds to the complement receptor 2 (CR2) that is located on the surface of follicular dendritic cells (FDC), B cells, and T cells in many species. C3d stimulates antigen presentation by FCD and helps to maintain immunological B cell memory. On B cells, C3d interaction with CR2 will collect molecules such as CD19, TAPA (CD81), and Lew 13. CD19 has a long intracellular tail that triggers a signaling cascade that results in cell activation and proliferation. Furthermore, simultaneous C3d-CR2 ligation and surface immunoglobulin (sIg) by antigen, activates two signaling pathways that cross-talk and synergize to activate B cells. However, C3d can enhance antibody titers in the absence of CR2 binding, indicating CR2-independent mechanism(s) of enhancement of the immune response. This review focuses on the complexity of the C3d-CR2 interaction, the importance of this interaction for the enhancement of the immune response by C3d and its derivatives, as well as the paradoxical enhancement of the immune response in the absence of CR2.

The majority of human immunodeficiency virus type 1 particles present within splenic germinal centres are produced locally

In most stages of human immunodeficiency virus (HIV) infection, cell-free viral particles can be detected in germinal centres (GCs) that are principally retained, in the form of immune complexes, on the surface of follicular dendritic cells (FDCs). The source of this virus remains unknown, although it is agreed that the FDCs themselves are not infected productively. By sequencing HIV viral DNA, genomic RNA and spliced mRNA isolated from individual splenic white pulps, it was shown here that the majority of HIV-1 viral particles are produced locally within the supporting lymphoid structure and do not result from trapping of circulating viruses or immune complexes. These findings underline the exquisite spatial organization of HIV-1 replication in vivo, suggesting a local origin for viruses trapped in splenic GCs.

FDC-B1: a new monoclonal antibody directed against bovine follicular dendritic cells.

Follicular dendritic cells (FDCs) are a unique population of accessory cells located in the light zone of the germinal centres of lymphoid follicles. Their involvement in the generation of humoral immune responses implies a potential role for these cells in many disorders. Indeed, in prion diseases, FDCs seem to be the major sites of extraneuronal cellular prion protein expression and the principal sites of the infectious agent accumulation in lymphoid organs. The identification of FDC is useful for the analysis of their distribution in reactive lymphoid tissue as well as in pathological conditions. The production and characterisation of a new mouse monoclonal antibody directed against bovine follicular dendritic cells (FDC-B1) is reported. The antigen detected by FDC-B1 is expressed exclusively on the surface of FDCs in ruminant lymphoid organs. The antigen has an approximate molecular weight of 28 kDa.

Reassessing the function of immune-complex retention by follicular dendritic cells.

The close association of follicular dendritic cells (FDCs) and germinal-centre B cells has fostered the idea that B-cell recognition of retained antigen that is presented on the surface of FDCs is important for affinity maturation and memory B-cell development. We argue that the retention of immune complexes is not required for germinal-centre development, affinity maturation and memory B-cell maintenance. Instead, it is probable that FDCs support B-cell proliferation and differentiation in a non-specific manner. Other potential roles of immune complexes retained by FDCs are discussed.

Apoptosis and nucleosomes.

Drs Berden and Koutouzov presented evidence that nucleosomes are antigens in lupus pathogenesis and that apoptotic cells are the source of nucleosomes. Berden's group measured persistence of circulating nucleosomes and nucleosome-antibody complexes in autoimmune mice and demonstrated nucleosomal deposition in skin of SLE patients as well as in renal lesions. Koutouzov reported that anti-nucleosomes are among the earliest autoantibodies in MRL+/+ mice, appearing several weeks before anti-DNA antibodies. Treatment of the mice with a pro-apoptotic drug, taxotere, accelerated autoantibody production and development of lesions. Herrmann proposed that persistent immunization results from reduced dead cell clearance and reduced production of immunosuppressive cytokines by defective scavenger macrophages. He also described accumulation of apoptotic cells in germinal follicles in SLE patients and attachment of nuclear antigens that are produced in apoptosis to the surface of follicular dendritic cells. Apoptosis-derived nucleosomes may be important in both the immunizing and effector arms of pathogenesis.

MUC1 mucin expression in follicular dendritic cells and lymphoepithelial lesions of gastric mucosa-associated lymphoid tissue lymphoma.

Membrane-associated mucin MUC1 is expressed in various adenocarcinoma cells and active T lymphocytes. We tried to find out whether MUC1 is expressed in gastric mucosa-associated lymphoid tissue (MALT) lymphoma lesion. MUC1 was not expressed in infiltrating T lymphocytes; however, MUC1 was found on the cell surface of follicular dendritic cells (FDC) of germinal centers and in the epithelial cytoplasm of lymphoepithelial lesion (LEL) of the lymphoma, which were immunohistochemically detected by monoclonal antibodies DF3 and MY.1E12. MUC1 was also expressed in the FDC of control cases (gastrectomy specimen containing reactive lymphoid follicles, n = 10, MUC1/ DF3, 100%; MUC1/MY.1E12, 40%), and FDC in MALT lymphomas (n = 59) showed lower MUC1 expression rates (MUC1/ DF3, 32%; MUC1/MY.1E12, 0%) than the control (P < 0.001). Lymphoepithelial lesion in the low-grade MALT lymphomas (n = 23) showed a higher MUC1/DF3 expression rate (30%) than those in the high-grade MALT lymphomas (n = 36; 6%; P < 0.05). T lymphocytes in the surface mucosa were more frequent in MALT lymphoma (91.4 +/- 80.6/unit area) than those in the control (20.0 +/- 23.6) (P < 0.001). S100-positive dendritic cells around LEL were more frequent in the low-grade (19.0 +/- 9.4/unit area) than in the high-grade (11.7 +/- 9.7) (P < 0.005). This study demonstrated MUC1 mucin expression on FDC for the first time. Mucosa-associated lymphoid tissue lymphoma, especially low-grade, shows immunologically active state, where FDC MUC1 expression may be suppressed by some factors released from lymphoma cells. Further study to elucidate the pathogenetic role of MUC1 in MALT lymphoma is necessary.

Losing Their Grip

To battle infections, the immune system has to recognize and grab invaders. New work suggests that with age, certain immune cells lose their hold on the molecules that identify trespassers. The study identifies for the first time a molecular defect in these cells that could weaken older immune systems. Elderly people succumb to infections such as influenza more easily than do youngsters, and they don't develop the large numbers of specific antibodies that usually arise from repeated exposure to microbes or from vaccinations. Why age undercuts the immune system isn't clear, but researchers know that people's immune systems gradually lose the ability to generate B cells, which produce antibodies against interlopers. Antibodies bind antigens--bits of bacteria and viruses--and bring them to lymph nodes and the spleen, where follicular dendritic cells (FDCs) grab onto this so-called immune complex. FDCs then shuttle the complex to B cells, which produce a second round of antibodies with the help of T cells. The new batch of antibodies prods other immune cells to destroy the interlopers and provide resistance to future infections. Previously, Andras Szakal, an immunologist at Virginia Commonwealth University in Richmond, and colleagues found that FDCs from old animals lose their antigen-antibody-binding power. The team set out to find molecular changes that underlie the deterioration. The team first compared FDCs from old and young mice. Animals carried similar numbers of FDCs regardless of age, but cells from old animals displayed fewer copies of the cell surface receptor that they use to pick up immune complexes. Then the researchers assessed the ability of FDCs to stimulate antibody production in a culture dish. They combined FDCs from either 12-week-old or 26-week-old mice with B cells and T cells from 12-week-olds. After exposure to a test antigen, young FDCs spurred far greater antibody output than did old FDCs. Furthermore, old B and T cells churned out antibodies as well as their younger counterparts did in the presence of young FDCs. The team is currently investigating how FDCs regulate the amount of the cell surface receptor; boosting quantities of this molecule might bolster flagging immunity. "FDCs might be a bottleneck in the aging immune system," says Szakal. The results suggest that an age-related loss of receptors on the surface of FDCs prevents the cells from shepherding antigens to B and T cells and, as a result, suppresses antibody production. "I was taken aback," says immunologist Dennis Taub of the National Institute on Aging's branch in Baltimore, Maryland. Previous studies have shown that B and T cell function declines with age, but the new work "suggests that the FDCs themselves are responsible for the age-based loss of immune function." Not everyone is convinced. Although FDCs might play a role, the study doesn't clear B and T cells, says immunologist Robin Callard of University College London, because the cell culture studies might not re-create what happens in an intact animal. Still, the elderly might defeat infections if scientists can find a way to make their FDCs maintain a tight hold on transgressors. --R. John Davenport Y. Aydar, P. Balogh, J. G. Tew, A. K. Szakal, Age-related depression of FDC accessory functions and CD21 ligand-mediated repair of co-stimulation. Eur. J. Immunol. 32 , 2817-2826 (2002). [Abstract] [Full Text]

Follicular dendritic cells and the persistence of HIV infectivity: the role of antibodies and Fcgamma receptors.

Large quantities of HIV are found trapped on the surface of follicular dendritic cells (FDCs), and virus persists on these cells until they ultimately die. We recently found that FDCs maintain HIV infectivity for long periods in vivo and in vitro. Because FDCs trap Ags (and virus) in the form of immune complexes and are rich in FcgammaRs, we reasoned that Ab and FcgammaRs may be required for FDC-mediated maintenance of HIV infectivity. To investigate this hypothesis, HIV immune complexes were formed in vitro and incubated for increasing times with or without FDCs, after which the remaining infectious virus was determined by HIV-p24 production in rescue cultures. FDCs maintained HIV infectivity in vitro in a dose-dependent manner but required the presence of specific Ab for this activity regardless of whether laboratory-adapted or primary X4 and R5 isolates were tested. In addition, Abs against either virally or host-encoded proteins on the virion permitted FDC-mediated maintenance of HIV infectivity. We found that the addition of FDCs to HIV immune complexes at the onset of culture gave optimal maintenance of infectivity. Moreover, blocking FDC-FcgammaRs or killing the FDCs dramatically reduced their ability to preserve virus infectivity. Finally, FDCs appeared to decrease the spontaneous release of HIV-1 gp120, suggesting that FDC-virus interactions stabilize the virus particle, thus contributing to the maintenance of infectivity. Therefore, optimal maintenance of HIV infectivity requires both Ab against particle-associated determinants and FDC-FcgammaRs.

Impaired uptake of apoptotic cells into tingible body macrophages in germinal centers of patients with systemic lupus erythematosus

To investigate the fate of apoptotic cells in the germinal centers (GCs) of patients with systemic lupus erythematosus (SLE). Lymph node biopsy specimens obtained from 7 SLE patients with benign follicular hyperplasia, 5 non-SLE patients with benign follicular hyperplasia (non-SLE), 5 patients with malignant follicular lymphoma, and 3 patients with dermatopathic lymphadenitis were stained with monoclonal antibodies against macrophages (CD68) and follicular dendritic cells (CR2/CD21). TUNEL staining and transmission electron microscopy were performed to detect apoptotic cells. Confocal microscopy was used to evaluate the in vivo capacity of tingible body macrophages to remove apoptotic cell material. In a subgroup of patients with SLE, apoptotic cells accumulated in the GCs of the lymph nodes. The number of tingible body macrophages, which usually contained engulfed apoptotic nuclei, was significantly reduced in these patients. In contrast to what was observed in all controls, TUNEL-positive apoptotic material from SLE patients was observed to be directly associated with the surfaces of follicular dendritic cells (FDCs). Our findings suggest that in a sub-group of SLE patients, apoptotic cells are not properly cleared by tingible body macrophages of the GCs. Consequently, nuclear autoantigens bind to FDCs and may thus provide survival signals for autoreactive B cells. This action may override an important control mechanism for B cell development, resulting in the loss of tolerance for nuclear antigens.

Isolation of bovine follicular dendritic cells allows the demonstration of a particular cellular prion protein.

As interaction of cellular prion protein (PrPc) and the infectious agent (PrPres) appears to be a crucial pathogenic step promoted by homology, variation in PrPc isoforms on bovine immune cells may explain the absence of infectivity in most bovine lymph organs. In this study, we examined PrPc expression in bovine lymph organs (tonsils and lymph nodes) and on isolated follicular dendritic cells (FDCs). We used a panel of different monoclonal antibodies (MoAbs) raised against different epitopes of prion protein. Two MoAbs recognise amino acids 79-92 (SAF 34 and SAF 32 Mo-Abs); the 6H4 antibody reacts with a specific peptide comprising the 144-152 amino acids, and the 12F10 MoAb recognises the sequence 142-160. After immunolabelling of frozen sections of lymph organs with 6H4 or 12F10 MoAbs, we detected cellular prion protein in germinal centres. However, using the SAF 34 or SAF 32 antibodies, PrPc was revealed outside the lymphoid tissues. No PrPc was observed in the germinal centres. Therefore, we adapted the method of FDC isolation, making it suitable for the study of PrPc expression on their surface. Using electron microscopy, the presence of PrPc on the surface of FDCs was demonstrated only with 6H4 MoAb. These results suggest that bovine follicular dendritic cells express a particular form of prion protein. Either the N-terminal part of PrPc is cleaved or the accessibility of the specific epitope (79-92) of SAF 34 MoAb is abolished by interaction with other molecules. This particular isoform of PrPc on bovine FDCs might be related to the apparent absence of infectivity in lymph organs in cattle affected by bovine spongiform encephalopathy.

Neuroinvasion by a Creutzfeldt-Jakob disease agent in the absence of B cells and follicular dendritic cells.

With the potential spread of bovine spongiform encephalopathy to people as a variant Creutzfeldt-Jakob disease (CJD), it becomes critical to identify cells in the periphery that carry infection. Initial work with scrapie agents suggested that B cells were central vectors for neuroinvasion. Subsequent studies indicated that B cells played an indirect role by promoting the development of follicular dendritic cells (FDCs) that accumulate abnormal prion protein (PrP). The mechanism for the role of FDCs, however, has not been clear. To further dissect potential B cell functions that contribute to neuroinvasion, we inoculated a CJD agent into mutant mice that (i) lacked B cells, (ii) had B cells unable to secrete Ig, or (iii) could secrete only IgM. Remarkably, all these mice developed disease with practically indistinguishable incubation times. The demonstration that neither immune complexes nor B cells were required for neuroinvasion from the periphery mandates a reanalysis of the accepted view of the essential role of B cells and FDC in these infections. Moreover, immune complexes were not required for the accumulation of pathologic PrP on the surface of FDCs, suggesting that PrP can bind to FDCs autonomously or by means of another factor. Wild-type mice had incubation times approximately 50 days less than all mutant mice at the same peripheral doses, indicating that an intact immune system may increase agent uptake and delivery, but this condition is not essential. Specifically, the evidence to date suggests that IgG may enhance pivotal agent interactions with migratory myeloid cells.

Temporary depletion of complement component C3 or genetic deficiency of C1q significantly delays onset of scrapie

Following peripheral exposure to transmissible spongiform encephalopathies (TSEs), infectivity usually accumulates in lymphoid tissues before neuroinvasion. The host prion protein (PrPc) is critical for TSE agent replication and accumulates as an abnormal, detergent insoluble, relatively proteinase-resistant isoform (PrPSc) in diseased tissues. Early PrPSc accumulation takes place on follicular dendritic cells (FDCs) within germinal centers in lymphoid tissues of patients with variant Creutzfeldt-Jakob disease (vCJD), sheep with natural scrapie or rodents following experimental peripheral infection with scrapie. In mouse scrapie models, the absence of FDCs blocks scrapie replication and PrPSc accumulation in the spleen, and neuroinvasion is significantly impaired. The mechanisms by which the TSE agent initially localizes to lymphoid follicles and interacts with FDCs are unknown. Antigens are trapped and retained on the surface of FDCs through interactions between complement and cellular complement receptors. Here we show that in mice, both temporary depletion of complement component C3 or genetic deficiency of C1q significantly delays the onset of disease following peripheral infection, and reduces the early accumulation of PrPSc in the spleen. Thus, in the early stages of infection, C3 and perhaps C1q contribute to the localization of TSE infectivity in lymphoid tissue and may be therapeutic targets.

Unraveling the Mystery of the Lymphoid Follicle

Unraveling the Mystery of the Lymphoid Follicle