Isolation of functionally active murine follicular dendritic cells

NGFR regulates stromal cell activation in germinal centers

Nerve growth factor receptor (NGFR) is expressed by follicular dendritic cells (FDCs). However, the role of NGFR in the humoral response is not well defined. Here, we study the effect of Ngfr loss on lymph node organization and function, demonstrating that Ngfr depletion leads to spontaneous germinal center (GC) formation and an expansion of the GC B cell compartment. In accordance with this effect, stromal cells are altered in Ngfr-/- mice with a higher frequency of FDCs, characterized by CD21/35, MAdCAM-1, and VCAM-1 overexpression. GCs are located ectopically in Ngfr-/- mice, with lost polarization together with impaired high-affinity antibody production and an increase in circulating autoantibodies. We observe higher levels of autoantibodies in Bcl2 Tg/Ngfr-/- mice, concomitant with a higher incidence of autoimmunity and lower overall survival. Our work shows that NGFR is involved in maintaining GC structure and function, participating in GC activation, antibody production, and immune tolerance.

The role of lymphoid tissue SPARC in the pathogenesis and response to treatment of multiple myeloma

Background

Despite the significant progress in the treatment of multiple myeloma (MM), the disease remains untreatable and its cure is still an unmet clinical need. Neoplastic transformation in MM is initiated in the germinal centers (GCs) of secondary lymphoid tissue (SLT) where B cells experience extensive somatic hypermutation induced by follicular dendritic cells (FDCs) and T-cell signals.

Objective

We reason that secreted protein acidic and rich in cysteine (SPARC), a common stromal motif expressed by FDCs at the origin (SLTs) and the destination (BM) of MM, plays a role in the pathogenesis of MM, and, here, we sought to investigate this role.

Methods

There were 107 BM biopsies from 57 MM patients (taken at different time points) together with 13 control specimens assessed for SPARC gene and protein expression and compared with tonsillar tissues. In addition, regulation of myeloma-promoting genes by SPARC-secreting FDCs was assessed in in vitro GC reactions (GCRs).

Results

SPARC gene expression was confirmed in both human primary (BM) and secondary (tonsils) lymphoid tissues, and the expression was significantly higher in the BM. Sparc was detectable in the BM and tonsillar lysates, co-localized with the FDC markers in both tissues, and stimulation of FDCs in vitro induced significantly higher levels of SPARC expression than unstimulated controls. In addition, SPARC inversely correlated with BM PC infiltration, ISS staging, and ECOG performance of the MM patients, and in vitro addition of FDCs to lymphocytes inhibited the expression of several oncogenes associated with malignant transformation of PCs.

Conclusion

FDC-SPARC inhibits several myelomagenic gene expression and inversely correlates with PC infiltration and MM progression. Therapeutic induction of SPARC expression through combinations of the current MM drugs, repositioning of non-MM drugs, or novel drug discovery could pave the way to better control MM in clinically severe and drug-resistant patients.

Follicular dendritic cell differentiation is associated with distinct synovial pathotype signatures in rheumatoid arthritis

Follicular dendritic cells (FDCs) fundamentally contribute to the formation of synovial ectopic lymphoid-like structures in rheumatoid arthritis (RA) which is associated with poor clinical prognosis. Despite this critical role, regulation of FDC development in the RA synovium and its correlation with synovial pathotype differentiation remained largely unknown. Here, we demonstrate that CNA.42⁺ FDCs distinctively express the pericyte/fibroblast-associated markers PDGFR-β, NG2, and Thy-1 in the synovial perivascular space but not in established follicles. In addition, synovial RNA-Seq analysis revealed that expression of the perivascular FDC markers was strongly correlated with PDGF-BB and fibroid synovitis, whereas TNF-α/LT-β was significantly associated with lymphoid synovitis and expression of CR1, CR2, and FcγRIIB characteristic of mature FDCs in lymphoid follicles. Moreover, PDGF-BB induced CNA.42⁺ FDC differentiation and CXCL13 secretion from NG2⁺ synovial pericytes, and together with TNF-α/LT-β conversely regulated early and late FDC differentiation genes in unsorted RA synovial fibroblasts (RASF) and this was confirmed in flow sorted stromal cell subsets. Furthermore, RASF TNF-αR expression was upregulated by TNF-α/LT-β and PDGF-BB; and TNF-α/LT-β-activated RASF retained ICs and induced B cell activation in in vitro germinal center reactions typical of FDCs. Additionally, FDCs trapped peptidyl citrulline, and strongly correlated with IL-6 expression, and plasma cell, B cell, and T cell infiltration of the RA synovium. Moreover, synovial FDCs were significantly associated with RA disease activity and radiographic features of tissue damage. To the best of our knowledge, this is the first report describing the reciprocal interaction between PDGF-BB and TNF-α/LT-β in synovial FDC development and evolution of RA histological pathotypes. Selective targeting of this interplay could inhibit FDC differentiation and potentially ameliorate RA in clinically severe and drug-resistant patients.

Follicular dendritic cells

Follicular dendritic cells (FDCs) are unique accessory immune cells that contribute to the regulation of humoral immunity. They are multitasker cells essential for the organization and maintenance of the lymphoid architecture, induction of germinal center reaction, production of B memory cells, and protection from autoimmune disorders. They perform their activities through both antigen-driven and chemical signaling to B cells. FDCs play a crucial role in the physiological regulation of the immune response. Dis-regulation of this immune response results when FDCs retain antigens for years. This provides a constant antigenic stimulation for B cells resulting in the development of immune disorders. Antigen trapped on FDCs is resistant to therapeutic intervention causing chronicity and recurrences. Beyond their physiological immunoregulatory functions, FDCs are involved in the pathogenesis of several immune-related disorders including HIV/AIDS, prion diseases, chronic inflammatory, and autoimmune disorders. FDCs have also been recently implicated in rare neoplasms of lymphoid and hematopoietic tissues. Understanding FDC biology is essential for better control of humoral immunity and opens the gate for therapeutic management of FDC-mediated immune disorders. Thus, the biology of FDCs has become a hot research area in the last couple of decades. In this review, we aim to provide a comprehensive overview of FDCs and their role in physiological and pathological conditions.

The CTA1-DD adjuvant strongly potentiates follicular dendritic cell function and germinal center formation, which results in improved neonatal immunization

Vaccination of neonates and young infants is hampered by the relative immaturity of their immune systems and the lack of safe and efficacious vaccine adjuvants. Immaturity of the follicular dendritic cells (FDCs), in particular, appears to play a critical role for the inability to stimulate immune responses. Using the CD21mT/mG mouse model we found that at 7 days of life, FDCs exhibited a mature phenotype only in the Peyer´s patches (PP), but our unique adjuvant, CTA1-DD, effectively matured FDCs also in peripheral lymph nodes following systemic, as well as mucosal immunizations. This was a direct effect of complement receptor 2-binding to the FDC and a CTA1-enzyme-dependent enhancing effect on gene transcription, among which CR2, IL-6, ICAM-1, IL-1β, and CXCL13 encoding genes were upregulated. This way we achieved FDC maturation, increased germinal center B-cell- and Tfh responses, and enhanced specific antibody levels close to adult magnitudes. Oral priming immunization of neonates against influenza infection with CTA1-3M2e-DD effectively promoted anti-M2e-immunity and significantly reduced morbidity against a live virus challenge infection. To the best of our knowledge, this is the first study to demonstrate direct effects of an adjuvant on FDC gene transcriptional functions and the subsequent enhancement of neonatal immune responses.

Immunophenotyping of Sheep Paraffin-Embedded Peripheral Lymph Nodes

Sheep are not only a major livestock species globally, they are also an important large animal model for biomedical research and have contributed to our understanding of the ontogeny and architecture of the mammalian immune system. In this study, we applied immunohistochemistry and multicolor immunofluorescence in fixed and paraffin-embedded lymph nodes to phenotype the key populations of antigen presenting cells, lymphocytes, and stromal cells that orchestrate the host adaptive immune response. We used an extensive panel of antibodies directed against markers associated with dendritic cells (MHC class II, CD83, and CD208), macrophages (CD11b, CD163, and CD169), stromal cells (CNA.42, S100, and CD83), and lymphocytes (CD3, Pax5, CD4, CD8). Using different methods of tissue fixation and antigen retrieval, we provide a detailed immunophenotyping of sheep lymph nodes including the identification of potential subpopulations of antigen presenting cells and stromal cells. By characterizing cells expressing combinations of these markers in the context of their morphology and location within the lymph node architecture, we provide valuable new tools to investigate the structure, activation, and regulation of the sheep immune system in health and disease.

Isolation and Characterization of Mouse and Human Follicular Dendritic Cells

Follicular dendritic cells (FDCs) reside in the B cell follicles of secondary and tertiary lymphoid tissues where they play key roles in the development and maintenance of lymphoid tissue architecture and function. FDCs trap native antigens for extended periods of time in the form of immune complexes which critcally regulate germinal center reactions in health and disease. Here, we describe how to isolate and characterize FDCs from murine and human lymphoid tissues.

Improved protocol for the isolation of naïve follicular dendritic cells

Follicular dendritic cells (FDCs) in lymphoid organs play an important role in the humoral immune response. However, because the isolation of FDCs is difficult due to their very small population size and fragility under mechanical and chemical stresses, the genetic and biochemical characteristics of FDCs remain unclear. Previously, we identified FDCs as ICAM-1⁺ cells in the CD45^- non-hematopoietic cell fraction from naïve mouse spleen after cell separation by means of digestion with a combination of enzymes. In the present study, using a new combination of enzymes, we found that FDCs are highly enriched in the CD45^-ICAM-1⁺CD21/35⁺ cell fraction. CD45^-ICAM-1⁺CD21/35⁺ cells in the mouse spleen retained an antigen administered in vivo for more than 7 days. Moreover, CD45^-ICAM-1⁺CD21/35⁺ cells isolated from the spleen of mice administered with a cognate antigen enhanced the survival and proliferation of antigen-specific B cells in vitro. Our improved protocol for the isolation of naïve FDCs will be useful for the analysis of FDCs in vitro and in vivo.

Modeling the efficiency of a magnetic needle for collecting magnetic cells

As new magnetic nanoparticle-based technologies are developed and new target cells are identified, there is a critical need to understand the features important for magnetic isolation of specific cells in fluids, an increasingly important tool in disease research and diagnosis. To investigate magnetic cell collection, cell-sized spherical microparticles, coated with superparamagnetic nanoparticles, were suspended in (1) glycerine-water solutions, chosen to approximate the range of viscosities of bone marrow, and (2) water in which 3, 5, 10 and 100% of the total suspended microspheres are coated with magnetic nanoparticles, to model collection of rare magnetic nanoparticle-coated cells from a mixture of cells in a fluid. The magnetic microspheres were collected on a magnetic needle, and we demonstrate that the collection efficiency versus time can be modeled using a simple, heuristically-derived function, with three physically-significant parameters. The function enables experimentally-obtained collection efficiencies to be scaled to extract the effective drag of the suspending medium. The results of this analysis demonstrate that the effective drag scales linearly with fluid viscosity, as expected. Surprisingly, increasing the number of non-magnetic microspheres in the suspending fluid results increases the collection of magnetic microspheres, corresponding to a decrease in the effective drag of the medium.

Cochlin produced by follicular dendritic cells promotes antibacterial innate immunity

Cochlin, an extracellular matrix protein, shares homologies with the Factor C, a serine protease found in horseshoe crabs, which is critical for antibacterial responses. Mutations in the COCH gene are responsible for human DFNA9 syndrome, a disorder characterized by neurodegeneration of the inner ear that leads to hearing loss and vestibular impairments. The physiological function of cochlin, however, is unknown. Here, we report that cochlin is specifically expressed by follicular dendritic cells and selectively localized in the fine extracellular network of conduits in the spleen and lymph nodes. During inflammation, cochlin was cleaved by aggrecanases and secreted into blood circulation. In models of lung infection with Pseudomonas aeruginosa and Staphylococcus aureus, Coch(-/-) mice show reduced survival linked to defects in local cytokine production, recruitment of immune effector cells, and bacterial clearance. By producing cochlin, FDCs thus contribute to the innate immune response in defense against bacteria.

Follicular dendritic cells in health and disease

Follicular dendritic cells (FDCs) are unique immune cells that contribute to the regulation of humoral immune responses. These cells are located in the B-cell follicles of secondary lymphoid tissues where they trap and retain antigens (Ags) in the form of highly immunogenic immune complexes (ICs) consisting of Ag plus specific antibody (Ab) and/or complement proteins. FDCs multimerize Ags and present them polyvalently to B-cells in periodically arranged arrays that extensively crosslink the B-cell receptors for Ag (BCRs). FDC-FcγRIIB mediates IC periodicity, and FDC-Ag presentation combined with other soluble and membrane bound signals contributed by FDCs, like FDC-BAFF, -IL-6, and -C4bBP, are essential for the induction of the germinal center (GC) reaction, the maintenance of serological memory, and the remarkable ability of FDC-Ags to induce specific Ab responses in the absence of cognate T-cell help. On the other hand, FDCs play a negative role in several disease conditions including chronic inflammatory diseases, autoimmune diseases, HIV/AIDS, prion diseases, and follicular lymphomas. Compared to other accessory immune cells, FDCs have received little attention, and their functions have not been fully elucidated. This review gives an overview of FDC structure, and recapitulates our current knowledge on the immunoregulatory functions of FDCs in health and disease. A better understanding of FDCs should permit better regulation of Ab responses to suit the therapeutic manipulation of regulated and dysregulated immune responses.

Enhancement of immunohistochemical staining of scrapie proteins and immune cells within lymph nodes of early scrapie-infected sheep

Transmissible spongiform encephalopathies (TSE) are a group of fatal neurodegenerative diseases that affect animals as well as humans. The oldest of these diseases is Scrapie seen in sheep. Scrapie is caused by an altered form (PrP(sc)), capable of inducing "self-replication" of the normal host prion protein (PrP(c)). There is currently no universal standard for antigen retrieval when using immunohistochemistry to simultaneously stain the PrP(c) protein and other cellular markers. The use of formalin-fixed tissue creates a challenge by concealing the antigenic sites where an antibody would bind, and lengthy antigen retrieval methods must be applied in order to facilitate staining. Further complicating sheep tissue immunohistochemistry is a significant lack of commercial antibodies to sheep cell markers available in research. Here we developed a novel immunohistochemical technique using trypsin, formic acid, and hydrated autoclaving using citraconic anhydride buffer to increase sensitivity of staining for scrapie proteins and immune cell subsets. This allowed us to stain and identify cells within lymphoid tissue associated with early lymphoid pathogenesis in scrapie.

Antigen presenting cells in a non-mammalian model system, the chicken

The chicken has a different repertoire of tissues, cells and genes of the immune response compared to mammals, yet generally survives infection with viral, bacterial, protozoal and fungal pathogens, and also worms and ectoparasites, just like mammals. Poultry are also probably the most heavily vaccinated group of farmed animals. Antigen presentation to the adaptive immune response therefore obviously normally occurs efficiently in birds. Although comparatively much is known about macrophages and B cells in the chicken, there is as yet little work on the other, professional, antigen-presenting cells, the dendritic cells (DC). Birds also have at least two other sets of phagocytic cells, heterophils and thrombocytes, which may also have the ability to present antigen. Here we review the current state of knowledge about antigen presenting cells in the chicken, concentrating mainly on recent advances in our knowledge of DC.

Stromal cell-immune cell interactions

Interaction between different types of hematopoietic cells is essential for proper functioning of the immune system. For instance, the cytokines produced by antigen-presenting dendritic cells will determine the type of T cell response that is induced. However, hematopoietic cells are also strongly influenced by the surrounding nonhematopoietic cells. The cells that form these microenvironments are collectively called stromal cells. Here, we focus on the stromal cells present within secondary lymphoid organs and discuss their importance for various aspects of the immune system.

Oxidative stress induces protein and DNA radical formation in follicular dendritic cells of the germinal center and modulates its cell death patterns in late sepsis

Profound depletion of follicular dendritic cells (FDCs) is a hallmark of sepsis-like syndrome, but the exact causes of the ensuing cell death are unknown. The cell death-driven depletion contributes to immunoparalysis and is responsible for most of the morbidity and mortality in sepsis. Here we have utilized immuno-spin trapping, a method for detection of free radical formation, to detect oxidative stress-induced protein and DNA radical adducts in FDCs isolated from the spleens of septic mice and from human tonsil-derived HK cells, a subtype of germinal center FDCs, to study their role in FDC depletion. At 24h post-lipopolysaccharide administration, protein radical formation and oxidation were significantly elevated in vivo and in HK cells as shown by ELISA and confocal microscopy. The xanthine oxidase inhibitor allopurinol and the iron chelator desferrioxamine significantly decreased the formation of protein radicals, suggesting the role of xanthine oxidase and Fenton-like chemistry in radical formation. Protein and DNA radical formation correlated mostly with apoptotic features at 24h and necrotic morphology of all the cell types studied at 48h with concomitant inhibition of caspase-3. The cytotoxicity of FDCs resulted in decreased CD45R/CD138-positive plasma cell numbers, indicating a possible defect in B cell differentiation. In one such mechanism, radical formation initiated by xanthine oxidase formed protein and DNA radicals, which may lead to cell death of germinal center FDCs.

In silico subtraction approach reveals a close lineage relationship between follicular dendritic cells and BP3(hi) stromal cells isolated from SCID mice

Organization of the stromal compartments in secondary lymphoid tissue is a prerequisite for an efficient immune reaction. In particular, follicular dendritic cells (FDC) are pivotal for the activation and differentiation of B cells. To investigate the development of FDC, FDC together with tightly associated B cells (FDC networks) were micro-dissected from frozen tissue sections and follicular B cells were sorted by FACS. Using an in silico subtraction approach, gene expression of FDC was determined and compared with that of follicular stromal cells micro-dissected from the spleen of SCID mice. Nearly 90% of the FDC genes were expressed in follicular stromal cells of the SCID mouse, providing further evidence that FDC develop from the residual network of reticular cells. Thus, it suggests that rather minor modifications in the gene expression profile are sufficient for differentiation into mature FDC. The analysis of different immune-deficient mouse strains shows that a complex pattern of gene regulation controls the development of residual stromal cells into mature FDC. The in silico subtraction approach provides a molecular framework within which to determine the diverse roles of FDC in support of B cells and to investigate the differentiation of FDC from their mesenchymal precursor cells.

The sensing of environmental stimuli by follicular dendritic cells promotes immunoglobulin A generation in the gut

In the Peyer's patches (PPs), germinal centers (GCs) are chronically induced by bacteria and are the major sites for generation of gut immunoglobulin A (IgA) immune responses. Whether follicular dendritic cells (FDCs) within the GCs directly contribute to the IgA production in PPs is unknown. We showed here that direct stimulation of FDCs by bacterial products and retinoic acid synergistically enhanced the expression of the chemokine CXCL13, the survival factor BAFF, and molecules that facilitate the secretion and activation of the cytokine TGF-beta1. A reduced production of these molecules by PP FDCs associated with deficiencies in the Toll-like receptor pathway or vitamin A resulted in decreased numbers of GC B cells and defective generation of IgA(+) B cells within PP GCs. Our data indicate that PP FDCs are conditioned by environmental stimuli to express key factors for B cell migration, survival, and preferential generation of IgA in gut.

IL-6 produced by immune complex-activated follicular dendritic cells promotes germinal center reactions, IgG responses and somatic hypermutation

Reports that follicular dendritic cells (FDCs) produce IL-6 prompted the hypotheses that immune complexes (ICs) induce FDCs to produce IL-6 and that FDC-IL-6 promotes germinal center (GC) reactions, somatic hypermutation (SHM) and IgG production. FDCs were activated in vitro by addition of ICs and FDC-IL-6 production was determined. Wild-type (WT) and IL-6 knockout (KO) mice, as well as chimeras with WT and IL-6 KO cells, were immunized with (4-hydroxy-3-nitrophenyl)-acetyl (NP)-chicken gamma globulin (CGG) and used to study anti-(4-hydroxy-3-iodo-5-nitrophenyl) acetyl (NIP) responses, GC formation and SHM in the VH186.2 gene segment in Ig-gamma. FDC-IL-6 increased when FDCs encountered ICs. At low immunogen dose, 1 microg NP-CGG per mouse, the IgG anti-NIP response in IL-6 KO mice was low and immunohistochemistry revealed a reduction in both the number and size of GCs. The physiological relevance of FDC-IL-6 was apparent in the chimeric mice where total splenocytes from WT mice were unable to provide the IL-6 needed for normal IgG and GC responses in IL-6 KO animals with IL-6-defective FDCs. Moreover, the rate of mutation decreased from 18 to 8.9 mutations per 1000 bases (P < 0.001) in WT versus IL-6 KO mice. Addition of anti-IL-6 to GC reactions in vitro reduced antibody levels and SHM from 3.5 to 0.65 mutations per 1000 bases (P < 0.02). Thus, the absence of FDC-IL-6 correlated with a reduction in SHM that coincided with the reduction in GCs and specific anti-NIP. This is the first study to document that ICs induce FDC-IL-6 and that FDC-derived IL-6 is physiologically relevant in generating optimal GC reactions, SHM and IgG levels.

T-independent antibody responses to T-dependent antigens: a novel follicular dendritic cell-dependent activity

Follicular dendritic cells (FDCs) periodically arrange membrane-bound immune complexes (ICs) of T-dependent Ags 200-500A apart, and in addition to Ag, they provide B cells with costimulatory signals. This prompted the hypothesis that Ag in FDC-ICs can simultaneously cross-link multiple BCRs and induce T cell-independent (TI) B cell activation. TI responses are characterized by rapid IgM production. OVA-IC-bearing FDCs induced OVA-specific IgM in anti-Thy-1-pretreated nude mice and by purified murine and human B cells in vitro within just 48 h. Moreover, nude mice immunized with OVA-ICs exhibited well-developed GL-7(+) germinal centers with IC-retaining FDC-reticula and Blimp-1(+) plasmablasts within 48 h. In contrast, FDCs with unbound-OVA, which would have free access to BCRs, induced no germinal centers, plasmablasts, or IgM. Engagement of BCRs with rat-anti-mouse IgD (clone 11-26) does not activate B cells even when cross-linked. However, B cells were activated when anti-IgD-ICs, formed with Fc-specific rabbit anti-rat IgG, were loaded on FDCs. B cell activation was indicated by high phosphotyrosine levels in caps and patches, expression of GL-7 and Blimp-1, and B cell proliferation within 48 h after stimulation with IC-bearing FDCs. Moreover, anti-IgD-IC-loaded FDCs induced strong polyclonal IgM responses within 48 h. Blockade of FDC-FcgammaRIIB inhibited the ability of FDC-ICs to induce T-independent IgM responses. Similarly, neutralizing FDC-C4BP or -BAFF, to minimize these FDC-costimulatory signals, also inhibited this FDC-dependent IgM response. This is the first report of FDC-dependent but TI responses to T cell-dependent Ags.

Organizer-like reticular stromal cell layer common to adult secondary lymphoid organs

Mesenchymal stromal cells are crucial components of secondary lymphoid organs (SLOs). Organogenesis of SLOs involves specialized stromal cells, designated lymphoid tissue organizer (LTo) in the embryonic anlagen; in the adult, several distinct stromal lineages construct elaborate tissue architecture and regulate lymphocyte compartmentalization. The relationship between the LTo and adult stromal cells, however, remains unclear, as does the precise number of stromal cell types that constitute mature SLOs are unclear. From mouse lymph nodes, we established a VCAM-1(+)ICAM-1(+)MAdCAM-1(+) reticular cell line that can produce CXCL13 upon LTbetaR stimulation and support primary B cell adhesion and migration in vitro. A similar stromal population sharing many characteristics with the LTo, designated marginal reticular cells (MRCs), was found in the outer follicular region immediately underneath the subcapsular sinus of lymph nodes. Moreover, MRCs were commonly observed at particular sites in various SLOs even in Rag2(-/-) mice, but were not found in ectopic lymphoid tissues, suggesting that MRCs are a developmentally determined element. These findings lead to a comprehensive view of the stromal composition and architecture of SLOs.

Follicular dendritic cells control engulfment of apoptotic bodies by secreting Mfge8

The secreted phosphatidylserine-binding protein milk fat globule epidermal growth factor 8 (Mfge8) mediates engulfment of apoptotic germinal center B cells by tingible-body macrophages (TBMphis). Impairment of this process can contribute to autoimmunity. We show that Mfge8 is identical to the mouse follicular dendritic cell (FDC) marker FDC-M1. In bone-marrow chimeras between wild-type and Mfge8(-/-) mice, all splenic Mfge8 was derived from FDCs rather than TBMphis. However, Mfge8(-/-) TBMphis acquired and displayed Mfge8 only when embedded in Mfge8(+/+) stroma, or when situated in lymph nodes draining exogenous recombinant Mfge8. These findings indicate a licensing role for FDCs in TBMphi-mediated removal of excess B cells. Lymphotoxin-deficient mice lacked FDCs and splenic Mfge8, and suffer from autoimmunity similar to Mfge8(-/-) mice. Hence, FDCs facilitate TBMphi-mediated corpse removal, and their malfunction may be involved in autoimmunity.

Immune complex-bearing follicular dendritic cells deliver a late antigenic signal that promotes somatic hypermutation

We reasoned that immune complex (IC)-bearing follicular dendritic cells (FDCs) promote somatic hypermutation (SHM). This hypothesis was tested in murine germinal center reactions induced in vitro by coculturing 6-day (4-hydroxy-3-nitrophenyl) acetyl-primed but unmutated lambda+ B cells, chicken gamma-globulin (CGG) memory T cells, FDCs, and ICs (anti-CGG plus NP-CGG). Mutations in primed lambda+ B cells were obtained only when both FDCs and immunogen were present. FDCs alone promoted B cell survival and Ab production but there were no mutations without more immunogen. Moreover, the mutation rate was enhanced when FDCs were activated. Trapped ICs ranged from 200 to 500 A apart on FDC membranes and this correlated with the periodicity known to optimally signal BCRs. FDCs are unique in their ability to retain ICs for months and a second signal mediated by FDC-ICs appeared to be needed a week or more after immunization by immunogen persisting on FDCs. However, the time needed to detect extensive SHM could be reduced to 7 days if ICs were injected together with memory T cells in vivo. In marked contrast, no mutations were apparent after 7 days in vivo if ICs were replaced by free Ag that would not load on FDCs until Ab was produced. The data suggest that specific Ab production leads to the following events: Ab encounters Ag and ICs are formed, ICs are trapped by FDCs, B cells are stimulated by periodically arranged Ag in ICs on FDCs, and this late antigenic signal promotes SHM.

Isolation of chicken follicular dendritic cells

The aim of the present study was to isolate chicken follicular dendritic cells (FDC). A combination of methods involving panning, iodixanol density gradient centrifugation, and magnetic cell separation technology made it possible to obtain functional FDC from the cecal tonsils from chickens, which had been infected with Eimeria tenella. CD45- dendritic cells were selected using the specific monoclonal antibody against chicken CD45, which is a marker for chicken leukocytes, but is not expressed on chicken FDC. Isolated FDC were characterized morphologically, phenotypically and functionally. The phenotype of the selected cells was consistent with FDC in that they expressed IgG, IgM, complement factors C3 and B, ICAM-1, and VCAM-1, but lacked cell surface markers characteristic of macrophages, T-, and B cells. Transmission electron microscopy confirmed their characteristic dendritic morphology. In addition, the identity of the FDC was further confirmed by their ability to trap chicken immune complexes (ICs) on their surface, whereas they did not trap naive antigen (ovalbumin) or ICs generated with mammalian immunoglobulins. Co-culturing allogeneic or autologous isolated FDC with B cells resulted in enhanced B cell proliferation and immunoglobulin production. The lack of MHC restriction, a functional characteristic feature of FDC, further reinforces the identity of the isolated cells as chicken FDC.

Ultrastructural study of highly enriched follicular dendritic cells reveals their morphology and the periodicity of immune complex binding

Follicular dendritic cells (FDCs) are immune accessory cells found in the follicles of secondary lymphoid organs where they promote B cell maturation in germinal centers (GCs) that develop following antigen exposure. Recently, we published a method for isolating functional murine FDCs in high purity. We reasoned that disruption of FDC reticula in vivo would alter FDC morphology. The present study was undertaken to determine the morphological features of isolated FDCs. FDC-M1 and immune complex (IC) labeling were used to identify FDCs in isolated preparations. Results at the light-microscopic level revealed that isolated FDCs trapped ICs, expressed FDC-M1 and cadherins, but generally appeared non-dendritic. However, at the ultrastructural level, the majority of FDCs exhibited dendrites and typical euchromatic nuclei that appeared as single, bilobed, or double nuclei. Based on morphology, four varieties of FDCs were distinguishable, possibly indicative of differences in maturity. Remarkably, ICs trapped by FDCs showed a distinctive periodic arrangement consistent with that known to induce immune responses by thymus independent-2 (TI-2) antigens that engage and cross-link multiple B cell receptors. The ability of FDCs to trap ICs and then display these T-cell-dependent antigens with repeating periodicity suggests that multiple B cell receptors are cross-linked by antigen on FDCs, thus promoting B cell stimulation and proliferation. Rapid proliferation is characteristic of the GC reaction, and the arrangement of T-dependent antigens in this periodic fashion may help to explain the profuse B cell proliferation in the GC microenvironment.

TLR4 on Follicular Dendritic Cells: An Activation Pathway That Promotes Accessory Activity

Microbial molecular patterns engage TLRs and activate dendritic cells and other accessory cells. Follicular dendritic cells (FDCs) exist in resting and activated states, but are activated in germinal centers, where they provide accessory function. We reasoned that FDCs might express TLRs and that engagement might activate FDCs by up-regulating molecules important for accessory activity. To test this hypothesis, TLR4 expression on FDCs was studied in situ with immunohistochemistry, followed by flow cytometry and RT-PCR analysis. TLR4 was expressed on FDC reticula in situ, and flow cytometry indicated that TLR4 was expressed on surface membranes and TLR4 message was readily apparent in FDCs by RT-PCR. Injecting mice or treating purified FDCs with LPS up-regulated molecules important for accessory activity including, FDC-Fc gammaRIIB, FDC-ICAM-1, and FDC-VCAM-1. Treatment of purified FDCs with LPS also induced intracellular phospho-IkappaB-alpha, indicating NF-kappaB activation, and that correlated with increased Fc gammaRIIB, ICAM-1, and VCAM-1. FDCs in C3H/HeJ mice were not activated with LPS even when mice were reconstituted with C3H/HeN leukocytes, suggesting that engagement of FDC-TLR4 is necessary for activation. Moreover, activated FDCs exhibited increased accessory activity in anti-OVA recall responses in vitro, and the FDC number could be reduced 4-fold if they were activated. In short, we report expression of TLR4 on FDCs for the first time and that engagement of FDC-TLR4 activated NF-kappaB, up-regulated expression of molecules important in FDC accessory function, including Fc gammaRIIB, ICAM-1, and VCAM-1, as well as FDC accessory activity in promoting recall IgG responses.

Role for CD21 in the establishment of an extracellular HIV reservoir in lymphoid tissues

Follicular dendritic cells (FDC) represent a major extracellular reservoir for HIV. A better understanding of the mechanisms of virion attachment to FDC may offer new avenues for reducing viral burdens in infected individuals. We used a murine model to investigate the establishment of extracellular HIV reservoirs in lymph nodes (LN). Consistent with findings in human tissues, CD21 was required for trapping of HIV to LN cells, as evidenced by significantly reduced virion binding when mice were pretreated with a C3 ligand-blocking anti-CD21 mAb and absence of virion trapping in CD21 knockout mice. Also consistent with findings in human tissues, the majority of HIV virions were associated with the FDC-enriched fraction of LN cell preparations. Somewhat surprisingly, HIV-specific Abs were not essential for HIV binding to LN cells, indicating that seeding of the FDC reservoir may begin shortly after infection and before the development of HIV-specific Abs. Finally, the virion-displacing potential for anti-CD21 mAbs was investigated. Treatment of mice with anti-CD21 mAbs several days after injection of HIV significantly reduced HIV bound to LN cells. Our findings demonstrate a critical role for CD21 in HIV trapping by LN cells and suggest a new therapeutic avenue for reducing HIV reservoirs.

Follicular dendritic cells stimulated by collagen type I develop dendrites and networks in vitro

Follicular dendritic cells (FDCs) reside in germinal centers in which their dendrites interdigitate and form non-mobile networks. FDC purification requires the use of collagenase and selection columns and leaves FDCs without detectable dendrites when examined by light microscopy. We have reasoned that isolated FDCs might reattach to a collagen matrix, extend their processes, and form immobile networks in vitro. As a test for this, cells were plated on collagen type I, laminin, biglycan, and hyaluronan. After 12 h, 80%-90% of FDCs adhered to all tested matrices but not to plastic. Within 2 weeks, FDCs adhering to type I collagen had spread out and had begun to acquire processes with occasional interconnections. By day 30, most FDCs had fine processes that formed networks through interdigitation with neighboring cells. FDC identity was confirmed by FDC-M1 labeling, immune complex trapping, and retention by FDCs in the networks. Scanning electron microscopy confirmed that groups of FDCs were in networks composed of convolutions and branching dendrites emanating from FDC cell bodies. In vivo, collagen type I was co-localized with FDCs, 5 h after challenge of immune mice with antigen. However, 2 days later, the collagen type I fibers were largely found at the periphery of the active follicles. Flow cytometry established the expression of CD29 and CD44 on FDCs; this may have partly mediated FDC-collagen interactions. Thus, we report, for the first time, that FDCs attach to collagen type I in vitro and regenerate their processes and networks with features in common with networks present in vivo.

Follicular dendritic cell (FDC)-FcgammaRIIB engagement via immune complexes induces the activated FDC phenotype associated with secondary follicle development

Follicular dendritic cell (FDC)-FcgammaRIIB levels are up-regulated 1-3 days after challenge of actively immunized mice with Ag. This kinetics suggested that memory cells are not driving this response, prompting the hypothesis that immune complex (IC)-FDC interactions lead to FDC activation. To test this, mice passively immunized with anti-OVA Ab were OVA challenged to produce IC. After 3 days, levels of IC, FcgammaRIIB, ICAM-1, and VCAM-1 on FDC were analyzed. FDC were also stimulated with IC in vitro, and mRNA for FcgammaRIIB, ICAM-1, and VCAM-1 was quantified by quantitative RT-PCR. IC labeling in passively immunized WT and FcgammaRIIB-/- mice revealed five to six FDC-reticula per LN midsagittal section. In WT mice, these IC-bearing FDC-reticula corresponded with FDC-reticula labeling for FcgammaRIIB, ICAM-1, and VCAM-1. Increases in these molecules on IC-stimulated FDC were confirmed by flow cytometry. In marked contrast, in FcgammaRIIB-/- mice, no increased VCAM-1 or ICAM-1 was seen on IC-bearing FDC-reticula or on purified FDC. Addition of IC in vitro resulted in dramatic increases in mRNA for FcgammaRIIB, ICAM-1 and VCAM-1 in WT FDC, but not in FDC from FcgammaRIIB-/- mice, 2.4G2-pretreated WT FDC, B cells, or macrophages. Thus, although FDC-FcgammaRIIB was not essential for IC trapping, engagement of FDC-FcgammaRIIB with IC initiated an FDC activation pathway.