An analysis of B cell selection mechanisms in germinal centers.

Affinity maturation of antibodies during immune responses is achieved by multiple rounds of somatic hypermutation and subsequent preferential selection of those B cells that express B cell receptors with improved binding characteristics for the antigen. The mechanism underlying B cell selection has not yet been defined. By employing an agent-based model, we show that for physiologically reasonable parameter values affinity maturation can be driven by competition for neither binding sites nor antigen--even in the presence of competing secreted antibodies. Within the tested mechanisms, only clonal competition for T cell help or a refractory time for the interaction of centrocytes with follicular dendritic cells is found to enable affinity maturation while generating the experimentally observed germinal centre characteristics and tolerating large variations in the initial antigen density.     

Clonal proliferation of B lymphocytes in the germinal centers of human reactive lymph nodes: possibility of overdiagnosis of B cell clonal proliferation.

Clonal expansion of the germinal center B cells of human reactive lymph nodes was analyzed. By micromanipulation, 28 germinal centers were microdissected from three nonneoplastic lymph nodes that had been fixed with formalin. Immunoglobulin heavy chain variable (V) region gene rearrangement was examined by seminested polymerase chain reaction (PCR) using two sets of primers (FR2-J and FR3A-J). An oligoclonal development (one to five clones) was found in each germinal center. Depending on the primer used, four or five (16%) of the germinal centers showed a single rearrangement band. The average number of B-cell clones in each germinal center was approximately 2.5. Next, the authors analyzed 50 endoscopic biopsy specimens from 6 patients with non-mucosa-associated lymphoid tissue (MALT) type gastric lymphoma, 25 patients with chronic gastritis, and 19 patients with nonspecific colitis. In addition to the samples from the 6 patients with malignant lymphoma, 8 of 44 biopsy samples (18.2%) from patients diagnosed as having chronic gastritis or nonspecific colitis showed one or two amplified bands. These results indicate that PCR analysis of immunoglobulin heavy chain V region gene rearrangement in small biopsy specimens could be misleading, causing overdiagnosis of reactive lymphoid tissue as B-cell clonal proliferation.     

Follicular dendritic cells restrict interleukin-4 availability in germinal centers and foster memory B cell generation

1. Download : Download high-res image (126KB)
2. Download : Download full-size image

     

The role of germinal centers for antiviral B cell responses

Germinal centers (GCs) are crucially involved in T cell-dependent B cell responses. B cells rapidly proliferate within GCs and their Ig variable region genes undergo hypermutation. Cognate T helper cells and antigen presented in native form on follicular dendritic cells (FDCs) select B cells expressing high-affinity Igs, leading to affinity maturation and the generation of memory B cells. In addition to these well-established functions of GCs, this article presents evidence that they also play a crucial role for the maintenance of specific memory Ig titers and for the prevention of viral antibody escape mutants.     

Subpopulations of B lymphocytes in germinal centers, II. A germinal center B cell subpopulation expresses sIgD and CD23

To understand B cell development in germinal centers, it is important to delineate the expression of surface antigens among germinal center cells. Because it is unclear whether germinal center cells express common antigens such as sIgD and CD23, we studied their expression among tonsillar lymphocytes with flow cytometry, immunohistochemistry, and in vitro stimulation. Upon studying a large number of tonsils with flow cytometry, we found that occasional tonsils have a very large number of sIgD+ cells among their PNA+ cells. Furthermore, the occasional tonsils with a large number of sIgD+ and PNA+ cells also have many CD23+ cells among their PNA+ cells. Tonsil sections stained immunohistochemically revealed germinal centers containing sIgD+ cells. In addition, PNA- and sIgD+ cells can be induced to express PNA binding sites in vitro without losing the expression of sIgD. Taking these findings together, we conclude that a subpopulation of germinal center B cells coexpresses sIgD and CD23.     

HIV/AIDS患者淋巴结生发中心及滤泡树突状细胞分布的变化

淋巴结是成熟T细胞和B细胞在抗原刺激下发生免疫应答的部位。淋巴结皮质淋巴滤泡中的生发中心，是由抗原刺激机体而出现的抗原依存性组织。滤泡树突状细胞（follicular dendritic cell，FDC）存在于生发中心，与T、B淋巴细胞紧密接触，能够捕捉极微量的抗原并长期保持微量抗原活性，维持长达数年的记忆性免疫反应。     

Cellular composition of germinal centers in lymph nodes after HIV-1 infection: evidence for an inadequate support of germinal center B lymphocytes by follicular dendritic cells.

Germinal centers in lymph nodes with follicular hyperplasia from 15 patients with HIV-1 infection were analyzed by qualitative and quantitative electron microscopical methods and compared with control follicular hyperplasia (FH). Using a pattern recognition method, two main clusters were recognized within the germinal centers of HIV and FH lymph nodes on the basis of the relative frequencies of small centroblast and centrocytes. All FH lymph nodes and 6 HIV-1 lymph nodes (HIV-Clu-1) were placed in cluster 1; 9 HIV-1 lymph nodes (HIV-Clu-2) formed cluster 2. Germinal centers in the HIV-Clu-2 lymph nodes were characterized by a cell composition of predominantly lymphoid blasts and decreased numbers of centrocytes, but without altered numbers of mitotic figures. The frequency distribution of ultrastructurally distinct FDC subtypes differed between these clusters. In HIV-Clu-2 the frequencies of FDC types with an undifferentiated and regressive morphology occurred at a higher frequency, whereas FDC types with a highly differentiated morphology had a lower frequency. We conclude that 9 out of 15 lymph nodes with HIV-1 associated follicular hyperplasia show changes in FDC morphology indicative of a less differentiated functional stage of FDC. The changes in FDC morphology are closely associated with changes in the germinal center B-cell population resulting in an inverted blast to the centrocyte ratio.     

Follicular dendritic cell networks of primary follicles and germinal centers: phenotype and function

Follicular dendritic cells (FDCs) were identified decades ago by their ability to retain immune complexes and more recent findings indicate that they are a source of B cell attractants and trophic factors. New imaging studies have shown that B cells closely associate with their dendritic processes during migration. Here we will review the properties of these specialized follicular stromal cells and provide an update on the requirements for their maturation into phenotypically distinct cells within germinal center light and dark zones. We will then discuss current understanding of how they help support the B cell immune response.     

Classic follicular dendritic reticulum cell tumor of the lymph node developing in a patient with a previous inflammatory pseudotumor-like proliferation

Inflammatory pseudotumor (IPT) and follicular dendritic reticulum cell tumor (FDRCT) are rare entities of the lymph node characterized by spindle-cell proliferation. We report a case of a 31-year-old woman, who was admitted for biopsy of a lymph node in the left submandibular area. The microscopic examination revealed a proliferation of spindle cells, partially replacing the normal lymph node architecture, suggestive of an IPT. The preserved peripheral portion showed follicular hyperplasia with Castleman-like appearance. Six years later she presented with a new enlargement in the same submandibular area. The nodule was removed, and a diagnosis of a classic FDRCT of the lymph node was made. The present case is remarkable, and clinicopathological data show that IPT-like proliferations could be in some case an early presentation of FDRCT.     

CD137 is expressed by follicular dendritic cells and costimulates B lymphocyte activation in germinal centers

CD137, a member of the TNF receptor family, and its ligand are expressed on T lymphocytes and antigen-presenting cells (APC), respectively. During interaction with APC, T lymphocytes receive a potent, costimulatory signal through CD137. Reverse signaling has been demonstrated for the CD137 ligand, which causes activation in monocytes. Here we show that B lymphocytes also receive costimulatory signals through the CD137 ligand. Immobilized CD137 augmented proliferation of preactivated B lymphocytes up to fivefold and immunoglobulin synthesis, up to threefold. CD137 had no effect on resting cells. Further, we show that CD137 is expressed in vivo by follicular dendritic cells (FDC) in germinal centers. Germinal centers form during humoral immune responses and are essential for B lymphocyte affinity maturation. These data imply that, similar to the CD40 receptor/ligand system, which mediates T lymphocyte help to B lymphocytes after the first antigen encounter, the CD137 receptor/ligand system may mediate costimulation of B lymphocytes by FDC during affinity maturation.     

Among naive precursor cell subpopulations only progenitors of memory B cells originate germinal centers.

Immunization leads to the generation of both antibody-forming cells (AFC) and memory B cells which are thought to arise in germinal centers within lymphoid follicles. The findings that the precursors to memory B cells reside in the J11Dlo subpopulation of the spleens in non-immune mice and that this subpopulation is distinct from conventional AFC precursors, including CD5+ B cells, suggest that the precursors of germinal centers might also reside in the J11Dlo subpopulation. To test this hypothesis, SCID mice were repopulated with CD4+ carrier-primed T cells and T-depleted J11Dlo, J11Dhi or CD5+ B cells and immunized with a hapten-carrier conjugate. Only the J11Dlo population was enriched for cells that produced germinal centers. Thus, the subpopulation of precursors that generates memory B cells also originates germinal centers.     

B cell receptor signaling in germinal centers prolongs survival and primes B cells for selection

1. Download : Download high-res image (117KB)
2. Download : Download full-size image

     

Epithelioid germinal centers in overwhelming childhood infections. The aftermath of nonspecific destruction of follicular B cells by natural killer cells

The destruction of proliferating lymphoid cells within germinal centers with subsequent replacement by histiocytoid cells has been described in infants and children dying of viral and bacterial infections. The etiology and significance of "epithelioid germinal centers" (EGCs) are unknown. The cells implicated in forming EGCs have included histiocytes and dendritic reticulum cells. We have studied four children at autopsy who died at ages ranging from 10 months to 7 years. Three contracted fatal infections, one with fulminant meningococcemia, one with bacterial sepsis, and one with viral hepatitis. The fourth child contracted viral pneumonitis and died of acetaminophen toxicity. Epithelioid germinal centers were found in numerous lymphoid organs (spleen, lymph nodes, and Peyer's patches) in all four cases. Avidin-biotin complex immunohistochemical analysis performed on formalin-fixed splenic tissue from the first three cases and snap-frozen splenic tissue from the second case revealed an absence of B cells in the follicular centers. The mantle zones surrounding follicles were thin but intact. The histiocytoid cells expanding the germinal centers were positive for S100 and R4/23 (dendritic reticulum cells) and negative for numerous histiocyte markers (alpha 1-antitrypsin, alpha 1-antichymotrypsin, and lysozyme). Increased numbers of killer cells (Leu-7) were present within the affected germinal centers in the three cases in which material was available for immunohistochemical studies. Overwhelming infections in these patients seem to result in anomalous natural killer cell activation resulting in localized nonselective destruction of follicular centers similar to anomalous natural killer cell activity reported to occur in fatal infectious mononucleosis. This may lead to an acquired immunodeficiency that precludes long-term survival in affected patients.     

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.     

Follicular dendritic cell and ICCOSOMES in germinal center reactions.

The development and destination of immune complex coated bodies (ICCOSOMES) was investigated at the ultrastructural level in the germinal centers (GC) of murine lymph nodes after secondary-immunization with horseradish peroxidase. Immediately after a booster injection, large amounts of immune complex (IC) were trapped on the surface of the cytoplasmic extensions of follicular dendritic cells (FDC). Cytoplasmic extensions, consisting mainly of FDC processes with associated IC, formed complex lamellar labyrinth-like structures (LBS). We describe here morphological changes of LBS. Lamellar or stationary LBS before the injection sequentially changed into distorted, filiform, glomerated and transitional LBS. Around day 1, IC-coated cytoplasmic extensions developed into glomerated LBS (G-LBS) which were invaginated into their own cytoplasmic ends, which swelled up to several microns in diameter. Around day 2, tufts of the glomerated LBS were unlaced and dispersed as (ICCOSOMES) in the interstitium. Some ICCOSOMES were endocytosed by germinal center cells (GCCs) and transported to the Golgi apparatus and perinuclear space. These ICCOSOME-bearing GCCs were B220-positive and some produced specific antibody (spAb) in the perinuclear space indicating they are in the B cell lineage. ICCOSOME-ingesting GCCs are thought to be able to process the antigen and present it to T cells. This may be important in the development of plasma cells and memory cells which developed from the GC B cells.     

CD45 links the B cell receptor with cell survival and is required for the persistence of germinal centers

To segregate the many contributions that B cell receptor (BCR)-mediated signals make to immune responses, we have analyzed here B cells deficient in the 'pan-leukocyte' marker CD45. BCR ligation of Cd45-/- B cells failed to activate phosphatidylinositol-3-OH kinase, NF-kappaB, Erk1 or Erk2 kinases or to upregulate cell survival proteins and instead induced apoptosis. Immunization of Cd45-/- B cell chimeras induced germinal centers and antigen-specific immunoglobulin G1 antibody-forming cells early, but both cellular compartments decreased by day 14. Proliferation of Cd45-/- B cells induced by CD40 ligand in vitro was impaired as a result of abrogation by BCR ligation of the upregulation of prosurvival proteins. In contrast, enforced expression of the antiapoptotic factor Bcl-xL prevented the collapse of Cd45-/- B cell germinal centers. These results show mechanistic differences in B cell survival during germinal center initiation and propagation; CD40 signaling is sufficient for the former, whereas the latter requires signaling from the BCR.     

The dendritic reticulum cell pattern in B cell lymphomas of the small cleaved, mixed, and large cell types: an immunohistochemical study of 48 cases

An immunohistochemical study was designed to study the dendritic reticulum cell (DRC) patterns in 48 cases of B cell non-Hodgkin's lymphomas of the small cleaved, mixed, and large cell types, both follicular (20 cases) and diffuse (28 cases), in order to evaluate the possible influence of DRCs on homing and the differentiation of neoplastic B cells. Three DRC patterns were observed. In the follicular lymphomas, DRCs constituted nodular networks of variable density. In the diffuse lymphomas, DRCs were present either as isolated and scattered cells (17 cases) or constituted irregular meshworks of variable sizes (11 cases). These DRC patterns correlate with B cell immunophenotypes. Like follicular lymphomas, and unlike diffuse lymphomas without DRC networks, diffuse lymphomas with DRC networks constantly expressed the pan B antigens and one marker characteristic of normal germinal center cells, CD21 antigen, the C3d receptor. The finding of organized DRC networks in a significant number of diffuse lymphomas does not substantiate the hypothesis that DRCs may play a role in the homing of neoplastic B cells. The correlations observed between DRC patterns and B cell immunophenotypes suggest that the persistence and/or the development of DRC networks within follicular center cell-type lymphomas are related to the degree of functional differentiation of neoplastic B cells.     

Proliferation of germinal center B lymphocytes in vitro by direct membrane contact with follicular dendritic cells.

In secondary lymphoid organs, follicular dendritic cells (FDC) are located within B cell follicles and germinal centers. Through their cytoplasmic extensions they come into contact with a large number of neighboring lymphocytes. Using an enzyme cocktail to digest human tonsils followed by ultracentrifugation on bovine serum albumin gradients, single cell suspensions were obtained. Immunocytochemistry revealed that 7% of the cells were FDC, 5% T cells, and 5% macrophages. The remaining population were B cells with greater than 95% being of the germinal center phenotype (i.e. CD19-positive, CD39/sIgD negative). After 24 h of culture up to 44% of the lymphocytes were found in clusters centered around FDC. At the start of the culture as well as 24 and 72 h later, between 31 and 55% of the B cells within FDC associated clusters were in late G1 to M phase of the cell cycle. In contrast, less than 10% of the B cells not in contact with FDC (i.e. outside the clusters) were in an activated state. Autoradiography revealed that after three days of incubation the rate of proliferation was 26.2 times higher for the lymphocytes involved in cluster formation as compared to those cells not associated with FDC. Furthermore, the number of viable B cells after a 72 h mitogen-free culture period was determined. By adding FDC to these preparations, 31.9% of the lymphocytes were rescued from dying. These data show that FDC provide a microenvironment which can maintain the viability, activation and proliferation of germinal center B cells in vitro.