Quantitative analysis of molecules which distinguish functional compartments within germinal centers

CNTools: A computational toolbox for cellular neighborhood analysis from multiplexed images

Recent studies show that cellular neighborhoods play an important role in evolving biological events such as cancer and diabetes. Therefore, it is critical to accurately and efficiently identify cellular neighborhoods from spatially-resolved single-cell transcriptomic data or single-cell resolution tissue imaging data. In this work, we develop CNTools, a computational toolbox for end-to-end cellular neighborhood analysis on annotated cell images, comprising both the identification and analysis steps. It includes state-of-the-art cellular neighborhood identification methods and post-identification smoothing techniques, with our newly proposed Cellular Neighbor Embedding (CNE) method and Naive Smoothing technique, as well as several established downstream analysis approaches. We applied CNTools on three real-world CODEX datasets and evaluated identification methods with smoothing techniques quantitatively and qualitatively. It shows that CNE with Naive Smoothing overall outperformed other methods and revealed more convincing biological insights. We also provided suggestions on how to choose proper identification methods and smoothing techniques according to input data.

T cell help induces Myc transcriptional bursts in germinal center B cells during positive selection

Antibody affinity maturation occurs in secondary lymphoid organs within germinal centers (GCs). At these sites, B cells mutate their antibody-encoding genes in the dark zone, followed by preferential selection of the high-affinity variants in the light zone by T cells. The strength of the T cell-derived selection signals is proportional to the B cell receptor affinity and to the magnitude of subsequent Myc expression. However, because the lifetime of Myc mRNA and its corresponding protein is very short, it remains unclear how T cells induce sustained Myc levels in positively selected B cells. Here, by direct visualization of mRNA and active transcription sites in situ, we found that an increase in transcriptional bursts promotes Myc expression during B cell positive selection in GCs. Elevated T cell help signals predominantly enhance the percentage of cells expressing Myc in GCs as opposed to augmenting the quantity of Myc transcripts per individual cell. Visualization of transcription start sites in situ revealed that T cell help promotes an increase in the frequency of transcriptional bursts at the Myc locus in GC B cells located primarily in the LZ apical rim. Thus, the rise in Myc, which governs positive selection of B cells in GCs, reflects an integration of transcriptional activity over time rather than an accumulation of transcripts at a specific time point.

A Shiga Toxin B-Subunit-Based Lectibody Boosts T Cell Cytotoxicity towards Gb3-Positive Cancer Cells

Aberrant glycosylation plays a crucial role in tumour progression and invasiveness. Tumour-associated carbohydrate antigens (TACAs) represent a valuable set of targets for immunotherapeutic approaches. The poor immunogenicity of glycan structures, however, requires a more effective and well-directed way of targeting TACAs on the surface of cancer cells than antibodies. The glycosphingolipid globotriaosylceramide (Gb3) is a well-established TACA present in a multitude of cancer types. Its overexpression has been linked to metastasis, invasiveness, and multidrug resistance. In the present study, we propose to use a dimeric fragment of the Shiga toxin B-subunit (StxB) to selectively target Gb3-positive cancer cells in a StxB-scFv UCHT1 lectibody. The lectibody, comprised of a lectin and the UCHT1 antibody fragment, was produced in E. coli and purified via Ni-NTA affinity chromatography. Specificity of the lectibody towards Gb3-positive cancer cell lines and specificity towards the CD3 receptor on T cells, was assessed using flow cytometry. We evaluated the efficacy of the lectibody in redirecting T cell cytotoxicity towards Gb3-overexpressing cancer cells in luciferase-based cytotoxicity in vitro assays. The StxB-scFv UCHT1 lectibody has proven specific for Gb3 and could induce the killing of up to 80% of Gb3-overexpressing cancer cells in haemorrhagic and solid tumours. The lectibody developed in this study, therefore, highlights the potential that lectibodies and lectins in general have for usage in immunotherapeutic approaches to boost the efficacy of established cancer treatments.

Tissue schematics map the specialization of immune tissue motifs and their appropriation by tumors

A schematic of a biological system, i.e., a representation of its pieces, how they are combined, and what they do, would facilitate understanding its essential organization and alteration in pathogenesis or evolution. We present a computational approach for constructing tissue schematics (TSs) from high-parameter imaging data and a biological model for interpreting them. TSs map the spatial assembly of cellular neighborhoods into tissue motifs, whose modular composition, we propose, enables the generation of complex outputs. We developed our approach in human lymphoid tissue (HLT), identifying the follicular outer zone as a potential relay between neighboring zones and a core lymphoid assembly with modifications characteristic of each HLT type. Applying the TS approach to the tumor microenvironment in human colorectal cancer identified a higher-order motif, whose mutated assembly was negatively associated with patient survival. TSs may therefore elucidate how immune architectures can be specialized and become vulnerable to reprogramming by tumors.

Landscape of T Follicular Helper Cell Dynamics in Human Germinal Centers

T follicular helper (Tfh) cells play a very important role in mounting a humoral response. Studies conducted in mouse models have revealed with good kinetic and spatial resolution the dynamics of these cells in germinal centers (GC) and their cross-talk with B cells upon an immune response. However, whether a similar migratory behavior is performed by human Tfh cells is unclear, as technology to track them in situ has been lacking. In this study, we combined traditional immunohistochemistry and real-time fluorescent imaging approaches on fresh human adenoid slices to provide static and dynamic information on Tfh cells. Our data indicate that GC light zones are composed of two distinct areas in terms of Tfh cell distribution and migration. In the outer GC light zones, Tfh cells migrate actively and with a high ability to form dynamic clusters showing intense and rapid reorganization. In these outer regions, Tfh cells demonstrate multiple interactions between each other. Conversely, in central regions of GC light zones, Tfh cells are much more static, forming long-lasting conjugates. These findings reveal for the first time, to our knowledge, the dynamic behavior whereby Tfh cells migrate in human GC and highlight the heterogeneity of GC for Tfh cell motility.

Cutaneous Dengue Virus Inoculation Triggers Strong B Cell Reactions but Contrastingly Poor T Cell Responses

Dengue is a global health problem without current specific treatment nor safe vaccines available. While severe dengue is related to pre-existing non-neutralizing dengue virus (DENV) antibodies, the role of T cells in protection or pathology is unclear. Using cutaneous DENV infection in immunocompetent mice we previously showed the generation of PNA+ germinal centers (GCs), now we assessed the activation and proliferation of B and T cells in draining lymph nodes (DLNs). We found a drastic remodelling of DLN compartments from 7 to 14 days post-infection (dpi) with greatly enlarged B cell follicles, occupying almost half of the DLN area compared to ~24% in naïve conditions. Enormous clusters of proliferating (Ki-67+) cells inside B follicles were found 14 dpi, representing ~33% of B cells in DLNs but only ~2% in non-infected mice. Inside GCs, we noticed an important recruitment of tingle body macrophages removing apoptotic cells. In contrast, the percentage of paracortex area and total T cells decreased by 14–16 dpi, compared to controls. Scattered randomly distributed Ki-67+ T cells were found, similar to non-infected mice. CD69 expression by CD4+ and CD8+ T cells was minor, while it was remarkable in B cells, representing 1764.7% of change from basal levels 3 dpi. The apparent lack of T cell responses cannot be attributed to apoptosis since no significant differences were observed compared to non-infected mice. This study shows massive B cell activation and proliferation in DLNs upon DENV infection. In contrast, we found very poor, almost absent CD4+ and CD8+ T cell responses.

Plasma cells, plasmablasts, and AID+/CD30+ B lymphoblasts inside and outside germinal centres: details of the basal light zone and the outer zone in human palatine tonsils

Plasma cells (PCs) in human palatine tonsils are predominantly located in the germinal centres (GCs), in the subepithelial space and near the deep connective tissue septa surrounding each crypt. We analysed the location, phenotype, and proliferation of GC PCs by immunohistology comparing them to PCs in the other two locations. Most PCs in GCs were strongly positive for CD38, CD138, CD27, IRF4, and intracellular (ic) IgG. They often accumulated in the basal light zone, but could also be found scattered in the entire light zone. In addition, rows of PCs occurred at the surface of the GC bordering the mantle zone, i.e., in the outer zone, and at the surface of the dark zone. The latter cells were often continuous with PCs in the extrafollicular area. The vast majority of GC PCs were negative for Ki-67. Only a few Ki-67⁺ plasmablasts, predominantly icIgG⁺ or icIgM⁺, were found inside GCs. In certain GCs PCs accumulated around capillaries and the adjacent perikarya of follicular dendritic cells (FDCs). Newly formed PCs might migrate from the basal to the superficial part of the light zone and then back to the dark zone surface to leave the GC. This guarantees an even distribution of secreted Ig for exchange with immune complexes on FDCs. The surface of the dark zone may also be an exit site for Ki-67⁺CD30⁺ B lymphoblasts, which seed perifollicular and extrafollicular sites. We speculate that these cells tend to downmodulate CD20 and activation-induced deaminase and further up-regulate CD30 when developing into pre-plasmablasts.

Germinal center B cell initiation, GC maturation, and the coevolution of its stromal cell niches

Throughout the developing GC response, B cell survival and fate choices made at the single cell level are dependent on signals received largely through interactions with other cells, often with cognate T cells. The type of signals that a given B cell can encounter is dictated by its location within tissue microarchitecture. The focus of this review is on the initiation and evolution of the GC response at the earliest time points. Here, we review the key factors influencing the progression of GC B cell differentiation that are both stage and context dependent. Finally, we describe the coevolution of niches within and surrounding the GC that influence the outcome of the GC response.

G-protein coupled receptors and ligands that organize humoral immune responses

B-cell responses are dynamic processes that depend on multiple types of interactions. Rare antigen-specific B cells must encounter antigen and specialized systems are needed-unique to each lymphoid tissue type-to ensure this happens efficiently. Lymphoid tissue barrier cells act to ensure that pathogens, while being permitted entry for B-cell recognition, are blocked from replication or dissemination. T follicular helper (Tfh) cells often need to be primed by dendritic cells before supporting B-cell responses. For most responses, antigen-specific helper T cells and B cells need to interact, first to initiate clonal expansion and the plasmablast response, and later to support the germinal center (GC) response. Newly formed plasma cells need to travel to supportive niches. GC B cells must become confined to the follicle center, organize into dark and light zones, and interact with Tfh cells. Memory B cells need to be positioned for rapid responses following reinfection. Each of these events requires the actions of multiple G-protein coupled receptors (GPCRs) and their ligands, including chemokines and lipid mediators. This review will focus on the guidance cue code underlying B-cell immunity, with an emphasis on findings from our laboratory and on newer advances in related areas. We will discuss our recent identification of geranylgeranyl-glutathione as a ligand for P2RY8. Our goal is to provide the reader with a focused knowledge about the GPCRs guiding B-cell responses and how they might be therapeutic targets, while also providing examples of how multiple types of GPCRs can cooperate or act iteratively to control cell behavior.

Nonredundant Roles of IL-21 and IL-4 in the Phased Initiation of Germinal Center B Cells and Subsequent Self-Renewal Transitions

We examined the unique contributions of the cytokines IL-21 and IL-4 on germinal center (GC) B cell initiation and subsequent maturation in a murine model system. Similar to other reports, we found T follicular helper cell expression of IL-21 begins prior to T follicular helper cell migration into the B cell follicle and precedes that of IL-4. Consistent with this timing, IL-21 signaling has a greater influence on the perifollicular pre-GC B cell transition to the intrafollicular stage. Notably, Bcl6^hi B cells can form in the combined absence of IL-21R- and STAT6-derived signals; however, these nascent GC B cells cease to proliferate and are more prone to apoptosis. When B cells lack either IL-21R or STAT6, aberrant GCs form atypical centroblasts and centrocytes that differ in their phenotypic maturation and costimulatory molecule expression. Thus, IL-4 and IL-21 play nonredundant roles in the phased progression of GC B cell development that can initiate in the combined absence of these cytokine signals.

Sphingolipids role in the regulation of inflammatory response: From leukocyte biology to bacterial infection

Sphingolipids (SLs) are amphiphilic molecules mainly associated with the external leaflet of eukaryotic plasma membrane, and are structural membrane components with key signaling properties. Since the beginning of the last century, a large number of papers described the involvement of these molecules in several aspects of cell physiology and pathology. Several lines of evidence support the critical role of SLs in inflammatory diseases, by acting as anti- or pro-inflammatory mediators. They are involved in control of leukocyte activation and migration, and are recognized as essential players in host response to pathogenic infection. We propose here a critical overview of current knowledge on involvement of different classes of SLs in inflammation, focusing on the role of simple and complex SLs in pathogen-mediated inflammatory response.

DNMT1 is predictive of survival and associated with Ki-67 expression in R-CHOP-treated diffuse large B-cell lymphomas

DNMT1 is a target of approved anti-cancer drugs including decitabine. However, the prognostic value of DNMT1 protein expression in R-CHOP-treated diffuse large B-cell lymphomas (DLBCLs) remains unexplored. Here we showed that DNMT1 was expressed in the majority of DLBCL cases (n = 209/230, 90.9%) with higher expression in germinal centre B-cell-like (GCB)-DLBCL subtype. Low and negative DNMT1 expression (20% cut-off, n = 33/230, 14.3%) was predictive of worse overall survival (OS; p < 0.001) and progression-free survival (PFS; p < 0.001). Nonetheless, of the 209 DNMT1 positive patients, 33% and 42% did not achieve 5-year OS and PFS, respectively, indicating that DNMT1 positive patients showed considerably heterogeneous outcomes. Moreover, DNMT1 was frequently expressed in mitotic cells and significantly correlated with Ki-67 or BCL6 expression (r = 0.60 or 0.44, respectively; p < 0.001). We demonstrate that DNMT1 is predictive of DLBCL patients' survival, and suggest that DNMT1 could be a DLBCL therapeutic target due to its significant association with Ki-67.

Implications of Intravital Imaging of Murine Germinal Centers on the Control of B Cell Selection and Division

Intravital imaging of antibody optimization in germinal center (GC) reactions has set a new dimension in the understanding of the humoral immune response during the last decade. The inclusion of spatio-temporal cellular dynamics in the research on GCs required analysis using the agent-based mathematical models. In this study, we integrate the available intravital imaging data from various research groups and incorporate these into a quantitative mathematical model of GC reactions and antibody affinity maturation. Interestingly, the integration of data concerning the spatial organization of GCs and B cell motility allows to draw conclusions on the strength of the selection pressure and the control of B cell division by T follicular helper cells.

The Transcription Factor AP4 Mediates Resolution of Chronic Viral Infection through Amplification of Germinal Center B Cell Responses

B cells diversify and affinity mature their antigen receptor repertoire in germinal centers (GCs). GC B cells receive help signals during transient interaction with T cells, yet it remains unknown how these transient T-B interactions in the light zone sustain the subsequent proliferative program of selected B cells that occurs in the anatomically distant dark zone. Here, we show that the transcription factor AP4 was required for sustained GC B cell proliferation and subsequent establishment of a diverse and protective antibody repertoire. AP4 was induced by c-MYC during the T-B interactions, was maintained by T-cell-derived interleukin-21 (IL-21), and promoted repeated rounds of divisions of selected GC B cells. B-cell-specific deletion of AP4 resulted in reduced GC sizes and reduced somatic hypermutation coupled with a failure to control chronic viral infection. These results indicate that AP4 integrates T-cell-mediated selection and sustained expansion of GC B cells for humoral immunity.

Germinal Center B-Cell-Associated Nuclear Protein (GANP) Involved in RNA Metabolism for B Cell Maturation

Germinal center B-cell-associated nuclear protein (GANP) is upregulated in germinal center B cells against T-cell-dependent antigens in mice and humans. In mice, GANP depletion in B cells impairs antibody affinity maturation. Conversely, its transgenic overexpression augments the generation of high-affinity antigen-specific B cells. GANP associates with AID in the cytoplasm, shepherds AID into the nucleus, and augments its access to the rearranged immunoglobulin (Ig) variable (V) region of the genome in B cells, thereby precipitating the somatic hypermutation of V region genes. GANP is also upregulated in human CD4(+) T cells and is associated with APOBEC3G (A3G). GANP interacts with A3G and escorts it to the virion cores to potentiate its antiretroviral activity by inactivating HIV-1 genomic cDNA. Thus, GANP is characterized as a cofactor associated with AID/APOBEC cytidine deaminase family molecules in generating diversity of the IgV region of the genome and genetic alterations of exogenously introduced viral targets. GANP, encoded by human chromosome 21, as well as its mouse equivalent on chromosome 10, contains a region homologous to Saccharomyces Sac3 that was characterized as a component of the transcription/export 2 (TREX-2) complex and was predicted to be involved in RNA export and metabolism in mammalian cells. The metabolism of RNA during its maturation, from the transcription site at the chromosome within the nucleus to the cytoplasmic translation apparatus, needs to be elaborated with regard to acquired and innate immunity. In this review, we summarize the current knowledge on GANP as a component of TREX-2 in mammalian cells.

Pharmacological targeting of valosin containing protein (VCP) induces DNA damage and selectively kills canine lymphoma cells

Background

Valosin containing protein (VCP) is a critical mediator of protein homeostasis and may represent a valuable therapeutic target for several forms of cancer. Overexpression of VCP occurs in many cancers, and often in a manner correlating with malignancy and poor outcome. Here, we analyzed VCP expression in canine lymphoma and assessed its potential as a therapeutic target for this disease.

Methods

VCP expression in canine lymphomas was evaluated by immunoblotting and immunohistochemistry. The canine lymphoma cell lines CLBL-1, 17–71 and CL-1 were treated with the VCP inhibitor Eeyarestatin 1 (EER-1) at varying concentrations and times and were assessed for viability by trypan blue exclusion, apoptosis by TUNEL and caspase activity assays, and proliferation by propidium iodide incorporation and FACS. The mechanism of EER-1 action was determined by immunoblotting and immunofluorescence analyses of Lys48 ubiquitin and markers of ER stress (DDIT3), autophagy (SQSTM1, MAP1LC3A) and DNA damage (γH2AFX). TRP53/ATM-dependent signaling pathway activity was assessed by immunoblotting for TRP53 and phospho-TRP53 and real-time RT-PCR measurement of Cdkn1a mRNA.

Results

VCP expression levels in canine B cell lymphomas were found to increase with grade. EER-1 treatment killed canine lymphoma cells preferentially over control peripheral blood mononuclear cells. EER-1 treatment of CLBL-1 cells was found to both induce apoptosis and cell cycle arrest in G1. Unexpectedly, EER-1 did not appear to act either by inducing ER stress or inhibiting the aggresome-autophagy pathway. Rather, a rapid and dramatic increase in γH2AFX expression was noted, indicating that EER-1 may act by promoting DNA damage accumulation. Increased TRP53 phosphorylation and Cdkn1a mRNA levels indicated an activation of the TRP53/ATM DNA damage response pathway in response to EER-1, likely contributing to the induction of apoptosis and cell cycle arrest.

Conclusions

These results correlate VCP expression with malignancy in canine B cell lymphoma. The selective activity of EER-1 against lymphoma cells suggests that VCP will represent a clinically useful therapeutic target for the treatment of lymphoma. We further suggest a mechanism of EER-1 action centered on the DNA repair response that may be of central importance for the design and characterization of VCP inhibitory compounds for therapeutic use.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1489-1) contains supplementary material, which is available to authorized users.

T follicular helper cells have distinct modes of migration and molecular signatures in naive and memory immune responses

B helper follicular T (Tfh) cells are critical for long-term humoral immunity. However, it remains unclear how these cells are recruited and contribute to secondary immune responses. Here we show that primary Tfh cells segregate into follicular mantle (FM) and germinal center (GC) subpopulations that display distinct gene expression signatures. Restriction of the primary Tfh cell subpopulation in the GC was mediated by downregulation of chemotactic receptor EBI2. Following collapse of the GC, memory T cells persisted in the outer follicle where they scanned CD169(+) subcapsular sinus macrophages. Reactivation and intrafollicular expansion of these follicular memory T cells in the subcapsular region was followed by their extrafollicular dissemination via the lymphatic flow. These data suggest that Tfh cells integrate their antigen-experience history to focus T cell help within the GC during primary responses but act rapidly to provide systemic T cell help after re-exposure to the antigen.

CD9 may contribute to the survival of human germinal center B cells by facilitating the interaction with follicular dendritic cells

The germinal center (GC) is a dynamic microenvironment where antigen (Ag)-activated B cells rapidly expand and differentiate, generating plasma cells (PC) that produce high-affinity antibodies. Precise regulation of survival and proliferation of Ag-activated B cells within the GC is crucial for humoral immune responses. The follicular dendritic cells (FDC) are the specialized stromal cells in the GC that prevent apoptosis of GC-B cells. Recently, we reported that human GC-B cells consist of CD9+ and CD9- populations and that it is the CD9+ cells that are committed to the PC lineage. In this study, we investigated the functional role of CD9 on GC-B cells. Tonsillar tissue section staining revealed that in vivo CD9+ GC-B cells localized in the light zone FDC area. Consistent this, in vitro CD9+ GC-B cells survived better than CD9- GC-B cells in the presence of HK cells, an FDC line, in a cell-cell contact-dependent manner. The frozen tonsillar tissue section binding assay showed that CD9+ GC-B cells bound to the GC area of tonsillar tissues significantly more than the CD9- GC-B cells did and that the binding was significantly inhibited by neutralizing anti-integrin β1 antibody. Furthermore, CD9+ cells bound to soluble VCAM-1 more than CD9- cells did, resulting in activation and stabilization of the active epitope of integrin β1. All together, our data suggest that CD9 on GC-B cells contributes to survival by strengthening their binding to FDC through the VLA4/VCAM-1 axis.

Tracking plasma cell differentiation and survival

Plasma cells play a crucial role for the humoral immune response as they represent the body's factories for antibody production. The differentiation from a B cell into a plasma cell is controlled by a complex transcriptional network and happens within secondary lymphoid organs. Based on their lifetime, two types of antibody secreting cells can be distinguished: Short-lived plasma cells are located in extrafollicular sites of secondary lymphoid organs such as lymph node medullary cords and the splenic red pulp. A fraction of plasmablasts migrate from secondary lymphoid organs to the bone marrow where they can become long-lived plasma cells. Bone marrow plasma cells reside in special microanatomical environments termed survival niches, which provide factors promoting their longevity. Reticular stromal cells producing the chemokine CXCL12, which is known to attract plasmablasts to the bone marrow but also to promote plasma cell survival, play a crucial role in the maintenance of these niches. In addition, hematopoietic cells are contributing to the niches by providing other soluble survival factors. Here, we review the current knowledge on the factors involved in plasma cell differentiation, their localization and migration. We also give an overview on what is known regarding the maintenance of long lived plasma cells in survival niches of the bone marrow.

Reduced T cell-dependent humoral immune response in microsomal prostaglandin E synthase-1 null mice is mediated by nonhematopoietic cells

Microsomal PGE synthase-1 (mPGES-1) is an inducible enzyme that specifically catalyzes the conversion of PGH2 to PGE2. We showed that mPGES-1 null mice had a significantly reduced incidence and severity of collagen-induced arthritis compared with wild-type (WT) mice associated with a marked reduction in Abs to type II collagen. In this study, we further elucidated the role of mPGES-1 in the humoral immune response. Basal levels of serum IgM and IgG were significantly reduced in mPGES-1 null mice. Compared with WT mice, mPGES-1 null mice exhibited a significant reduction of hapten-specific serum Abs in response to immunization with the T cell-dependent (TD) Ag DNP-keyhole limpet hemocyanin. Immunization with the T cell-independent type 1 Ag trinitrophenyl-LPS or the T cell-independent type 2 Ag DNP-Ficoll revealed minimal differences between strains. Germinal center formation in the spleen of mPGES-1 null and WT mice were similar after immunization with DNP-keyhole limpet hemocyanin. To determine whether the effect of mPGES-1 and PGE2 was localized to hematopoietic or nonhematopoietic cells, we generated bone marrow chimeras. We demonstrated that mPGES-1 deficiency in nonhematopoietic cells was the critical factor for reduced TD Ab production. We conclude that mPGES-1 and PGE2-dependent phenotypic changes of nonhematopoietic/mesenchymal stromal cells play a key role in TD humoral immune responses in vivo. These findings may have relevance to the pathogenesis of rheumatoid arthritis and other autoimmune inflammatory diseases associated with autoantibody formation.

The proto-oncogene MYC is required for selection in the germinal center and cyclic reentry

After antigenic challenge, B cells enter the dark zone (DZ) of germinal centers (GCs) to proliferate and hypermutate their immunoglobulin genes. Mutants with greater affinity for the antigen are positively selected in the light zone (LZ) to either differentiate into plasma and memory cells or reenter the DZ. The molecular circuits that govern positive selection in the GC are not known. We show here that the GC reaction required biphasic regulation of expression of the cell-cycle regulator c-Myc that involved its transient induction during early GC commitment, its repression by Bcl-6 in DZ B cells and its reinduction in B cells selected for reentry into the DZ. Inhibition of c-Myc in vivo led to GC collapse, which indicated an essential role for c-Myc in GCs. Our results have implications for the mechanism of GC selection and the role of c-Myc in lymphomagenesis.

Identification of human germinal center light and dark zone cells and their relationship to human B-cell lymphomas

Germinal centers (GCs) are sites of B-cell clonal expansion, hypermutation, and selection. GCs are polarized into dark (DZ) and light zones (LZ), a distinction that is of key importance to GC selection. However, the difference between the B cells in each of these zones in humans remains unclear. We show that, as in mice, CXCR4 and CD83 can be used to distinguish human LZ and DZ cells. Using these markers, we show that LZ and DZ cells in mice and humans differ only in the expression of characteristic "activation" and "proliferation" programs, suggesting that these populations represent alternating states of a single-cell type rather than distinct differentiation stages. In addition, LZ/DZ transcriptional profiling shows that, with the exception of "molecular" Burkitt lymphomas, nearly all human B-cell malignancies closely resemble LZ cells, which has important implications for our understanding of the molecular programs of lymphomagenesis.

A theory of germinal center B cell selection, division, and exit

High-affinity antibodies are generated in germinal centers in a process involving mutation and selection of B cells. Information processing in germinal center reactions has been investigated in a number of recent experiments. These have revealed cell migration patterns, asymmetric cell divisions, and cell-cell interaction characteristics, used here to develop a theory of germinal center B cell selection, division, and exit (the LEDA model). According to this model, B cells selected by T follicular helper cells on the basis of successful antigen processing always return to the dark zone for asymmetric division, and acquired antigen is inherited by one daughter cell only. Antigen-retaining B cells differentiate to plasma cells and leave the germinal center through the dark zone. This theory has implications for the functioning of germinal centers because compared to previous models, high-affinity antibodies appear one day earlier and the amount of derived plasma cells is considerably larger.

Recent progress in histochemistry and cell biology

Studies published in Histochemistry and Cell Biology in the year 2011 represent once more a manifest of established and newly sophisticated techniques being exploited to put tissue- and cell type-specific molecules into a functional context. The review is therefore the Histochemistry and Cell Biology's yearly intention to provide interested readers appropriate summaries of investigations touching the areas of tissue biology, developmental biology, the biology of the immune system, stem cell research, the biology of subcellular compartments, in order to put the message of such studies into natural scientific-/human- and also pathological-relevant correlations.

Human follicular dendritic cells promote the APC capability of B cells by enhancing CD86 expression levels

Follicular dendritic cells (FDCs) are an essential cellular component of the germinal center (GC) and are believed to exert regulatory effects on the various stages of GC reactions. According to our previous reports, human FDCs express prostacyclin synthase, and prostacyclin analogues augment adhesion and co-stimulatory molecules on the surface of activated B cells. These findings prompted us to investigate whether FDCs would contribute to the antigen-presenting capability of B cells by using the well-established FDC-like cells, HK cells, and tonsillar B cells. Our results show that HK cells significantly enhance the expression levels of CD54, CD80, and CD86 on the surface of activated B cells. The enhancing effect of HK cells on CD86 is impeded by indomethacin and an EP4 antagonist, implying that a certain prostaglandin is mediating the up-regulation. Prostacyclin indeed recapitulates the enhancing effect on CD86, which is inhibited by EP4 as well as IP antagonists. B cells co-cultured with HK cells exhibit an augmented APC activity, which is inhibited by CD86 neutralization. These results reveal another unrecognized function of human FDC.

Cellular choreography in the germinal center: new visions from in vivo imaging

Germinal centers (GC) are large aggregates of proliferating B lymphocytes within follicles of lymphoid tissue that form during adaptive immune responses. GCs are the source of long-lived B cells that form the basis for pathogen-specific lifelong B cell immunity. The complex architecture of these structures includes subdomains that differ significantly in their stromal cell and T lymphocyte subset composition. In part due to their structural complexity and potential to generate some lymphomas, much interest and many theories about GC dynamics have emerged. Here, we review recent research employing in vivo imaging that has begun to untangle some of the mysteries.

Enzymatic study of tonsil tissue alkaline and acid phosphatase in children with recurrent tonsillitis and tonsil hypertrophy

OBJECTIVE

Indications for tonsillectomy in recurrent tonsillitis are defined according to the number of episodes of acute bacterial infections in a year. However, little is known about the tonsil immune competence status in patients presenting with recurrent tonsillitis with either hypertrophied or atrophied tonsils, or in patients presenting with obstructive sleep apnoea. In this study we examined the tonsil immune status in children with 3-5 acute recurrent infections a year and in children with obstructive sleep apnoea by comparing the activity of tonsil and adenoid tissue nonspecific alkaline and acid phosphatase.

METHODS

Specific activity of tonsil and adenoid tissue nonspecific alkaline and acid phosphatase was investigated in children who underwent tonsillectomy and adenoidectomy for recurrent infection (72 children) and for obstructive sleep apnoea (10 children). Tissue enzyme activities were measured using p-nitrophenylphosphate as a substrate. Tissue samples were examined by the haematoxylin-eosin histological technique. Statistical analyses were performed using SPSS v. 16 software.

RESULTS

The tissue nonspecific alkaline phosphatase activity was similar in hypertrophied tonsils in the recurrent infection group and in the obstructive sleep apnoea group (3.437+/-1.226 and 3.978+/-0.762 U/mg of protein, respectively). The enzyme activity in both hypertrophied tonsil groups was significantly higher as compared to atrophied tonsils in the recurrent tonsillitis group, p=0.021 and p=0.006, respectively. The enzyme activity was significantly higher in the adenoids compared to the tonsils from all three groups. Contrary to this, no significant differences were noticed for tonsil and adenoid acid phosphatase activities among the groups.

CONCLUSION

Similar acid phosphatase activity in all three groups implies that all three groups have preserved antigen presenting cell activity. In patients with hypertrophied tonsils similar tissue nonspecific alkaline phosphatase activity suggests preserved B cell tonsil immune activity, regardless of the pathology. Patients with atrophied tonsils had significantly lower alkaline phosphatase activity, indicating relative tonsil B cell immune deficiency. Thus, different immunological status in patients presenting with hypertrophied vs. atrophied tonsils could point to a different underlying pathophysiologic mechanism of the disease.

Lymphocyte-rich classical Hodgkin's lymphoma: distinctive tumor and microenvironment markers

The existence, diagnostic features, and the biological and clinical relevance of lymphocyte-rich classical Hodgkin's lymphoma remain controversial. A comparative marker analysis of lymphocyte-rich classical Hodgkin's lymphoma, nodular lymphocyte-predominance Hodgkin's lymphoma, and of other subtypes of classical Hodgkin's lymphoma was carried out. Markers were selected focusing on B-cell lineage and transcription program (OCT.1, OCT.2, BOB.1, BCL6, PAX-5, GCET1, KLHL6, and BLIMP1), the NF-kappaB signaling pathway (REL-B, C-REL, TRAF-1, p-50, and MUM-1) and the T-cell microenvironment (CD3, CD57, PD-1, CXCL-13, and CD10, BCL-6, CD23). Lymphocyte-rich classical Hodgkin's lymphoma cases displayed features intermediate between those of classical Hodgkin's lymphoma and nodular lymphocyte-predominance Hodgkin's lymphoma. The expression of B-cell transcription factors such as OCT.1, OCT.2, BOB.1, and BCL6 was more frequent in lymphocyte-rich classical Hodgkin's lymphoma than in classical Hodgkin's lymphoma. A follicular T-cell microenvironment was also identified in 50% of lymphocyte-rich classical Hodgkin's lymphoma cases. NF-kB markers were expressed at frequencies comparable with those observed in classical Hodgkin's lymphoma. The neoplastic cell immunophenotype and microenvironment in lymphocyte-rich classical Hodgkin's lymphoma closely mimic that which are observed in the outer zone of the germinal center, where B-cell blasts with germinal-center markers co-express CD30 and the B-cell transcription program, surrounded by follicular T-cell rosettes. Lymphocyte-rich classical Hodgkin's lymphoma seems to be characterized by a stronger expression of the B-cell transcription program by the neoplastic cells and by a follicular T-cell background, occupying an intermediate position between classical Hodgkin's lymphoma and nodular lymphocyte-predominance Hodgkin's lymphoma.

Angioimmunoblastic T-cell lymphoma with hyperplastic germinal centres: a neoplasia with origin in the outer zone of the germinal centre? Clinicopathological and immunohistochemical study of 10 cases with follicular T-cell markers

Angioimmunoblastic T-cell lymphoma is an aggressive peripheral T-cell lymphoma whose natural history is not fully understood. Up to 17% of cases can present histologically with hyperplastic germinal centres (pattern I). The accurate recognition of Angioimmunoblastic T-cell lymphoma with pattern I remains a challenge and therefore the aim of this study is to phenotypically and morphologically characterize this variant with the use of the follicular helper T-cell (T(FH)) markers PD1, CXCL-13 and ICOS. Out of the 88 Angioimmunoblastic T-cell lymphoma cases reviewed, 10 showed hyperplastic follicles. Molecular probe methods for the detection of T-cell and B-cell clonality, as well as in-situ hybridization probes for EBV RNA expression, were carried out to leave no question as to the establishment of the diagnosis in each case. Of the 10 cases, all (100%) showed strong positive PD1 staining in perifollicular areas and in neoplastic cells surrounding small veins. CXCL13 and ICOS showed a similar staining pattern. By contrast, CD10 was found to only weakly label the neoplastic T cells, with only 5-10% of the target cell population staining for this marker. EBV was found in 9/10 cases. Clinically, 8/9 cases presented with stage IIIB/IVB and in 2/10 cases consecutive biopsies showed 'progression' from pattern I to classical Angioimmunoblastic T-cell lymphoma. In conclusion we have shown that the T(FH) cells markers PD1, CXCL13 and ICOS are useful adjuncts in the diagnosis of Angioimmunoblastic T-cell lymphoma with hyperplastic germinal centres. PD1 also highlighted the presence of neoplastic cells in the outer zone of lymphoid follicles, suggesting that Angioimmunoblastic T-cell lymphoma (pattern I) may originate from T(FH) cells in this region, in accordance with previous immunological studies. As the majority of cases in our series presented clinically with advanced stage disease, progression from pattern I to classical Angioimmunoblastic T-cell lymphoma may represent histological evolution rather than clinical progression.

The strand bias paradox of somatic hypermutation at immunoglobulin loci

Somatic hypermutation has two phases: phase 1 affects cytosine-guanine (C/G) pairs and is triggered by the deamination of cytosine residues in DNA to uracil; phase 2 affects mostly adenine-thymine (A/T) pairs and is induced by the detection of uracil lesions in DNA. It is not known how, at V(D)J genes in mice, hypermutations accumulate at A/T pairs with strand bias without perturbing the strand unbiased accumulation of hypermutations at C/G pairs. Additionally, it is not known why, in contrast, at switch regions in mice, both C/G-targeted and A/T-targeted hypermutations accumulate in a strand unbiased manner. To explain the strand bias paradox, we propose that phase 1 and phase 2 hypermutations are generated at different stages of the cell cycle.

The Bak protein expression in germinal centers of hypertrophied adenoids in children

The aim of this study was to evaluate pro-apoptotic Bak expression in the germinal centers of adenoid in children on the assumption of the potential usefulness of Bak as adenoid function marker. The study involved 95 children undergoing adenoidectomy; divided into three age groups: aged up to 5 years (25 children), 5-10 years (54 children) and over 10 years (16 children). The analyzed material was adenoids removed on the ground of hypertrophy. Immunohistochemical analyses were carried out using goat polyclonal Bak antibodies (DAKO) directed against human Bak protein. The presence of Bak positive lymphocytes within germinal centers and Bak immunostaining were scored. The immunohistochemical staining showed the Bak positive lymphocytes mainly within the germinal centers of the lymphoid follicles. The Bak reactivity was also present in hyperplastic lymphoid tissue within the subepithelial B lymphocytes. We have not found statistically significant correlation between Bak expression and clinical status and change in Bak expression level according to age. The apoptotic presence within the germinal centers are the manifestation of which is Bak expression and its lack in the mantle zone, what we confirmed in our former study by describing Bcl-2 expression, seems to be a proper B cells maturation marker within lymphoid follicles. Our finding shows that these processes are not influenced by age and supports our thesis that adenoid involution is rather the effect of changes in the number of lymphoid follicles that changes in them.

Germinal-center organization and cellular dynamics

Germinal centers (GCs) are important sites of antibody affinity maturation. In the classical model, the GC dark zone contains large centroblasts that are rapidly proliferating and undergoing somatic hypermutation of their antibody variable-region genes. Centroblasts give rise to smaller nonproliferating centrocytes in the light zone that compete for binding antigen on follicular dendritic cells. Recently, the approach of real-time imaging of GCs by two-photon microscopy of intact lymph nodes has provided new insights into GC dynamics that both support and challenge fundamental aspects of this model. Here we review recent and older findings on cell migration, proliferation, and interaction dynamics in the GC and discuss a model in which dark- and light-zone cells are morphologically similar, proliferation occurs in both zones, and GC B cells compete for T cell help as well as antigen.

Definition of germinal-center B cell migration in vivo reveals predominant intrazonal circulation patterns

Proliferation, mutation, and selection in the germinal center (GC) are thought to occur in distinct microanatomical compartments-the dark zone (DZ) and the light zone (LZ). Thus, affinity maturation has been posited to require frequent trafficking between zones. Here we report the use of multiphoton in vivo microscopy to determine migration patterns of GC B cells. Analysis of time-resolved images revealed unexpected patterns of movement as well as GC B cell morphology. Though frequent movement between the DZ and LZ was anticipated, few cells were observed to cross the interface between the two compartments. Moreover, cell-track trajectories indicated that cell movement in this region is predominantly parallel to the interface, suggesting that B cells circulate within individual LZ and DZ compartments. The results suggest a revision to our views of B cell circulation within GCs and the functional relationship of its two major compartments.

In vivo imaging studies shed light on germinal-centre development

Affinity maturation of antibodies during the course of an adaptive immune response requires germinal centre (GC) formation within B-cell follicles. Much of the current understanding of GC function has been derived from histology, but these static views have left unresolved many questions about cell movement in GCs. In this Progress article, we describe how several recent studies using time-resolved multiphoton microscopy to track GC B-cell movement within lymph nodes have shed light on the processes that influence GC B-cell dynamics.

Role of ancillary techniques in diagnosing and subclassifying non-Hodgkin's lymphomas on fine needle aspiration cytology

Non-Hodgkin's lymphomas (NHL) are tumours of the lymphoid cells. During the process of development of lymphoid cells, neoplasia may evolve at any point. Neoplastic cells usually carry the imprint of cell of origin at the stage of origin. Various types of NHL may have similar morphology with wide variation in origin, immunophenotype and other biological features. Different ancillary laboratory techniques may help to overcome the limitations of morphology in this aspect. The commonly used ancillary techniques in lymphomas are immunocytochemistry (IC), flow cytometry, Southern blot (SB) technique, polymerase chain reaction (PCR) and fluorescent in situ hybridization (FISH). In addition, laser scanning cytometry (LSC) and DNA microarray technologies are in the research phase. Various laboratory techniques are used for immunophenotyping, demonstration of monoclonality, identification of chromosomal translocation, assessment of cell kinetics and expression of mRNA in the tumour cells. Flow cytometry helps in rapid immunophenotying of NHL and it has an added advantage over IC in recognizing the co-expression of CD markers. Fine needle aspiration cytology (FNAC) combined with flow immunophenotyping may help us to diagnose and subclassify certain NHLs, such as follicular lymphoma and mantle cell lymphoma, which were previously recognized as pure morphological entities. Loss of morphology is one of the important limitations of flow cytometry. LSC can overcome this limitation by studying morphology along with the immunophenotyping pattern of individual cells. Chromosomal changes in NHL can be identified by SB, PCR and FISH. Molecular diagnosis of NHL helps in diagnosis, subclassification, prognostic assessment and even in planning of therapy. DNA microarray is a relatively newer and promising technology. It gives information about the expression of several thousands of genes in a tumour in a single experiment. In the near future, FNAC combined with ancillary techniques may play a major role in diagnosis, subclassification and management of lymphomas.

Nuclear and cytoplasmic AID in extrafollicular and germinal center B cells

Activation-induced cytidine deaminase (AID) is necessary for immunoglobulin somatic hypermutation (SHM) and class switch recombination (CSR) in T-dependent immune response in germinal centers (GCs). The structural similarity of AID with RNA-editing enzymes and its largely cytoplasmic location have fueled controversial views of its mode of interaction with DNA. We show that AID, a mature B-cell-restricted cytoplasmic antigen, is relocated into the nucleus in 2.5% of CDKN1B(-), CCNB1(-) GC cells. The GC dark zone and the outer zone (OZ), but not the light zone, contain nuclear and cytoplasmic AID(+) blasts. AID(+) cells in the OZ are in contact with T cells and CD23(-) follicular dendritic cells. In addition, AID is expressed in extrafollicular large proliferating B cells, 14% of which have nuclear AID. GC and extrafollicular AID(+) cells express E47 but not the inhibiting BHLH protein Id2. Outside the GC, AID(+) B cells are in contact with T cells and show partial evidence of CD40 plus bcr stimulation-dependent signature (CCL22, JunB, cMYC, CD30) but lack early and late plasma cell markers. The distribution of nuclear AID is consistent with the topography of SHM and CSR inside the GC and in extrafollicular activated B cells.

Follicular dendritic cells catalyze hepatocyte growth factor (HGF) activation in the germinal center microenvironment by secreting the serine protease HGF activator

Ag-specific B cell differentiation, the process that gives rise to plasma cells and memory B cells, involves the formation of germinal centers (GC). Within the GC microenvironment, multiple steps of B cell proliferation, selection, and maturation take place, which are controlled by the BCR in concert with cytokines and contact-dependent signals from follicular dendritic cells (FDCs) and T cells. Signaling by the multifunctional cytokine hepatocyte growth factor (HGF) and its receptor MET has been shown to induce integrin-mediated adhesion of B cells to VCAM-1, which is expressed by FDCs. In the present study we have examined the expression of regulatory components of the HGF/MET pathway, including HGF activator (HGFA), within the secondary lymphoid organ microenvironment. We show that MET is expressed by both centroblasts and plasma cells, and that HGFA is expressed by plasma cells. Because we have shown that HGF is a potent growth and survival factor for malignant plasma cells, HGF may also serve as a survival factor for normal plasma cells. Furthermore, we demonstrate that FDCs are the major source for HGF and its activator within the GC microenvironment. Both HGF and HGFA are expressed by FDCs in the GC dark zone (CD21high/CD23low), but not in the light zone (CD21high/CD23high). These findings suggest that HGF and HGFA provided by dark zone FDCs help to regulate the proliferation, survival, and/or adhesion of MET-positive centroblasts.

CD19 Regulates B Cell Maturation, Proliferation, and Positive Selection in the FDC Zone of Murine Splenic Germinal Centers

Mice with mutations in CD19 Y482/Y513 form germinal centers (GC) but fail to produce high-affinity antibodies. In these mice, GC B cell differentiation, proliferation, and class switching occur but are defective. Altered CD19 signaling results in retention of early GC B cells and reduced proliferation in the follicular dendritic cell (FDC) zone of GC, and causes failure to select for high-affinity mutations. In normal mice, the earliest detectable aggregates of GC B cells are in contact with FDC and IgM+ cells are only found in the FDC zone, further evidence that the FDC zone is the site of initial GC B cell proliferation, differentiation, and class switching. Proliferation in the non-FDC zone and somatic mutation are not dependent on CD19, indicating separate signaling requirements for the two GC compartments, but these CD19-independent GC functions are not sufficient to generate high-affinity antibodies and B cell memory.

Cutting edge: back to "one-way" germinal centers

The present status of germinal center (GC) research is revisited using in silico simulations based on recent lymphocyte motility data in mice. The generally adopted view of several rounds of somatic hypermutations and positive selection is analyzed with special emphasis on the spatial organization of the GC reaction. We claim that the development of dark zones is not necessary for successful GC reactions to develop. We find that a recirculation of positively selected centrocytes to the dark zone is rather unlikely. Instead we propose a scenario that combines a multiple-step mutation and selection concept with a "one-way" GC in the sense of cell migration.

Enzymatic study of Waldeyer’s ring lymphoid tissue: Activity of alkaline and acid phosphatase in palatinal tonsils and adenoids in children with recurrent infection of the ring

Activity and kinetic properties of tissue nonspecific alkaline phopsphatase and acid phosphatase were investigated in adenoids and tonsils of 62 children tonsiloadenoidectomised because of recurrent infection using p-nitrophenylphosphate as a substrate. Kinetic properties were calculated by the method of Levenberg-Marquardt. The mean value of tonsil TNAP activity was 3.525 U/mg of protein versus 7.280 U/ mg of protein in adenoids (t = 5.928, df = 60, p < 0.01). ACP activity was also significantly lower in tonsils (10.844 U/mg of protein) than in adenoids (13.059 U/mg of protein) (t = 11.318, df = 60, p < 0.01). There were no influence of age and sex to both enzyme activities. TNAP activity was significantly higher in hypertrophic (4.132 U/mg of protein) than in atrophic (2.531 U/mg of protein) (t = 2.361, df = 20, p < 0.05). Tonsillar TNAP was more effective than adenoid TNAP (t = 11.769, df = 60, p < 0.01). Results suggest the possibility that recurrent infection influences the tonsils more than adenoids and age. Hypertrophy could be an adaptive mechanism of palatinal tonsils during the infection.

Tracking CD40 signaling during germinal center development

Substantial evidence indicates that signaling through the CD40 receptor (CD40) is required for germinal center (GC) and memory B-cell formation. However, it is not fully understood at which stages of B-cell development the CD40 pathway is activated in vivo. To address this question, we induced CD40 signaling in human transformed GC B cells in vitro and identified a CD40 gene expression signature by DNA microarray analysis. This signature was then investigated in the gene expression profiles of normal B cells and found in pre- and post-GC B cells (naive and memory) but, surprisingly, not in GC B cells. This finding was validated in lymphoid tissues by showing that the nuclear factor-kappaB (NF-kappaB) transcription factors, which translocate to the nucleus upon CD40 stimulation, are retained in the cytoplasm in most GC B cells, indicating the absence of CD40 signaling. Nevertheless, a subset of centrocytes and B cells in the subepithelium showed nuclear staining of multiple NF-kappaB subunits, suggesting that a fraction of naive and memory B cells may be subject to CD40 signaling or to other signals that activate NF-kappaB. Together, these results show that GC expansion occurs in the absence of CD40 signaling, which may act only in the initial and final stages of the GC reaction.

Two distinct Gb3/CD77 signaling pathways leading to apoptosis are triggered by anti-Gb3/CD77 mAb and verotoxin-1

Globotriasosylceramide (Gb3), a neutral glycosphingolipid, is the B-cell differentiation antigen CD77 and acts as the receptor for most Shiga toxins, including verotoxin-1 (VT-1). We have shown that both anti-Gb3/CD77 mAb and VT-1 induce apoptosis in Burkitt's lymphoma cells. We compared the apoptotic pathways induced by these two molecules by selecting cell lines sensitive to only one of these inducers or to both. In all these cell lines (including the apoptosis-resistant line), VT-1 was transported to the endoplasmic reticulum and inhibited protein synthesis similarly, suggesting that VT-1-induced apoptosis is dissociated from these processes. VT-1 triggered a caspase- and mitochondria-dependent pathway (rapid activation of caspases 8 and 3 associated with a loss of mitochondrial membrane potential (Deltapsim) and the release of cytochrome c from mitochondria). In contrast, the anti-Gb3/CD77 mAb-induced pathway was caspase-independent and only involved partial depolarization of mitochondria. Antioxidant compounds had only marginal effects on VT-1-induced apoptosis but strongly protected cells from anti-Gb3/CD77 mAb-induced apoptosis. VT-1- and anti-Gb3/CD77 mAb-treated cells displayed very different features on electron microscopy. These results clearly indicate that the binding of different ligands to Gb3/CD77 triggers completely different apoptotic pathways.

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.

Human follicular dendritic cells and fibroblasts share the 3C8 antigen

Although the pivotal role of follicular dendritic cells (FDCs) in humoral immune responses has been demonstrated, little is known of the molecular basis of biological functions and the cellular origin of FDC. We have recently generated a monoclonal antibody (mAb) against FDC by immunizing mice with FDC-like tonsillar stromal cells. The mAb 3C8 does not cross-react with bone marrow-derived blood cells. Partial amino acid sequencing revealed that 3C8 Ag is a novel human protein. In this study, we carried out a detailed analysis of 3C8 immunoreactivity with tonsil sections to examine cellular distribution of 3C8 Ag. 3C8 Ab recognized connective tissue fibroblasts in addition to FDC. Western blot analysis indicated that 3C8 antigen is expressed in various fibroblasts and is specific to human species. Furthermore, there was a correlation between 3C8 expression in several stromal cell lines and their co-stimulatory activity of germinal center B cell proliferation. These findings strongly support the view that FDCs originate from local fibroblasts.

Repertoire shift in the humoral response to phosphocholine-keyhole limpet hemocyanin: VH somatic mutation in germinal center B cells impairs T15 Ig function

Phosphocholine (PC) is a naturally occurring Ag common to many pathogenic microorganisms. Early in the primary response to PC conjugated to keyhole limpet hemocyanin (KLH), T15 Id(+) Abs constitute >90% of the serum Ig in BALB/c mice. During the late primary and memory response to PC-protein, a shift in the repertoire occurs and T15 Id(+) Abs lose dominance. In this study, we use immunohistochemistry and single germinal center microdissection to locate T15 Id(+) cells in the spleen in a primary response to PC-KLH. We demonstrate T15 Id(+) B cells and V(H)1-DFL16.1-JH1 and V kappa 22-J kappa 5 rearrangements in germinal centers early in the immune response; thus loss of T15 dominance is not due to lack of T15 cells within germinal centers. One-hundred thirty one V(H)1 and 57 V kappa 22 rearrangements were cloned and sequenced. Thirty four percent of the V(H)1 clones and 37% of the V kappa 22 clones contained somatic mutations indicating participation in the germinal center response. Six variant T15 H clones were expressed with wild-type T15 L chain in vitro. Two of these Abs were defective in secretion providing the first evidence that mutation occurring in vivo can disrupt Ig assembly and secretion. Of the four secretion-competent Abs, two failed to display binding to PC-protein, while the other two displayed altered carrier recognition. These results indicate that somatic mutation of T15 in vivo can result in the loss of binding and secretion, potentially leading to B cell wastage. The failure of T15 to gain affinity enhancing mutations in the face of these detrimental changes may contribute to repertoire shift.