Germinal-center cell proliferation in response to T-independent antigens: a stathmokinetic, morphometric and immunohistochemical study in vivo

Intrinsic B cell TLR-BCR linked coengagement induces class-switched, hypermutated, neutralizing antibody responses in absence of T cells

Maturation of antibody responses entails somatic hypermutation (SHM), class-switch DNA recombination (CSR), plasma cell differentiation, and generation of memory B cells, and it is thought to require T cell help. We showed that B cell Toll-like receptor 4 (TLR4)-B cell receptor (BCR) (receptor for antigen) coengagement by 4-hydroxy-3-nitrophenyl acetyl (NP)-lipopolysaccharide (LPS) (Escherichia coli lipid A polysaccharide O-antigen) or TLR5-BCR coengagement by Salmonella flagellin induces mature antibody responses to NP and flagellin in Tcrβ-/-Tcrδ-/- and NSG/B mice. TLR-BCR coengagement required linkage of TLR and BCR ligands, "linked coengagement." This induced B cell CSR/SHM, germinal center-like differentiation, clonal expansion, intraconal diversification, plasma cell differentiation, and an anamnestic antibody response. In Tcrβ-/-Tcrδ-/- mice, linked coengagement of TLR4-BCR by LPS or TLR5-BCR by flagellin induced protective antibodies against E. coli or Salmonella Typhimurium. Our findings unveiled a critical role of B cell TLRs in inducing neutralizing antibody responses, including those to microbial pathogens, without T cell help.

Characterization of Thymus-dependent and Thymus-independent Immunoglobulin Isotype Responses in Mice Using Enzyme-linked Immunosorbent Assay

Antibodies, also termed as immunoglobulins (Ig), secreted by differentiated B lymphocytes, plasmablasts/plasma cells, in humoral immunity provide a formidable defense against invading pathogens via diverse mechanisms. One major goal of vaccination is to induce protective antigen-specific antibodies to prevent life-threatening infections. Both thymus-dependent (TD) and thymus-independent (TI) antigens can elicit robust antigen-specific IgM responses and can also induce the production of isotype-switched antibodies (IgG, IgA and IgE) as well as the generation of memory B cells with the help provided by antigen presenting cells (APCs). Here, we describe a protocol to characterize TD and TI Ig isotype responses in mice using enzyme-linked immunosorbent assay (ELISA). In this protocol, TD and TI Ig responses are elicited in mice by intraperitoneal (i.p.) immunization with hapten-conjugated model antigens TNP-KLH (in alum) and TNP-polysaccharide (in PBS), respectively. To induce TD memory response, a booster immunization of TNP-KLH in alum is given at 3 weeks after the first immunization with the same antigen/adjuvant. Mouse sera are harvested at different time points before and after immunization. Total serum Ig levels and TNP-specific antibodies are subsequently quantified using Ig isotype-specific Sandwich and indirect ELISA, respectively. In order to correctly quantify the serum concentration of each Ig isotype, the samples need to be appropriately diluted to fit within the linear range of the standard curves. Using this protocol, we have consistently obtained reliable results with high specificity and sensitivity. When used in combination with other complementary methods such as flow cytometry, in vitro culture of splenic B cells and immunohistochemical staining (IHC), this protocol will allow researchers to gain a comprehensive understanding of antibody responses in a given experimental setting.

TLR4 signaling augments B lymphocyte migration and overcomes the restriction that limits access to germinal center dark zones

B lymphocyte–intrinsic Toll-like receptor (TLR) signals amplify humoral immunity and can exacerbate autoimmune diseases. We identify a new mechanism by which TLR signals may contribute to autoimmunity and chronic inflammation. We show that TLR4 signaling enhances B lymphocyte trafficking into lymph nodes (LNs), induces B lymphocyte clustering and interactions within LN follicles, leads to sustained in vivo B cell proliferation, overcomes the restriction that limits the access of nonantigen-activated B cells to germinal center dark zones, and enhances the generation of memory and plasma cells. Intravital microscopy and in vivo tracking studies of B cells transferred to recipient mice revealed that TLR4-activated, but not nonstimulated, B cells accumulated within the dark zones of preexisting germinal centers even when transferred with antigen-specific B cells. The TLR4-activated cells persist much better than nonstimulated cells, expanding both within the memory and plasma cell compartments. TLR-mediated activation of B cells may help to feed and stabilize the spontaneous and ectopic germinal centers that are so commonly found in autoimmune individuals and that accompany chronic inflammation.

AID constrains germinal center size by rendering B cells susceptible to apoptosis

The germinal center (GC) is a transient lymphoid tissue microenvironment that fosters T cell–dependent humoral immunity. Within the GC, the B cell–specific enzyme, activation-induced cytidine deaminase (AID), mutates the immunoglobulin locus, thereby altering binding affinity for antigen. In the absence of AID, larger GC structures are observed in both humans and mice, but the reason for this phenomenon is unclear. Because significant apoptosis occurs within the GC niche to cull cells that have acquired nonproductive mutations, we have examined whether a defect in apoptosis could account for the larger GC structures in the absence of AID. In this report, we reveal significantly reduced death of B cells in AID−/− mice as well as in B cells derived from AID−/− bone marrow in mixed bone marrow chimeric mice. Furthermore, AID-expressing B cells show decreased proliferation and survival compared with AID−/− B cells, indicating an AID-mediated effect on cellular viability. The GC is an etiologic site for B-cell autoimmunity and lymphomagenesis, both of which have been linked to aberrant AID activity. We report a link between AID-induced DNA damage and B-cell apoptosis that has implications for the development of B-cell disorders.

Textbook Germinal Centers?

Models for the development and function of germinal centers (GCs) have been so widely discussed in the original literature that they now appear in immunology textbooks. Unfortunately, many of the tenets of these models have not yet been subjected to adequate experimental scrutiny. Indeed, recent studies have called several of their principal assumptions into question. In addition, the term germinal center has been applied to a diverse assortment of focal processes of B cell proliferation and differentiation. This variability might be explained by alterations in the progression of a single textbook GC process. Alternatively, distinct developmental pathways may create unique classes of GCs with specialized functions.

Fibrates and medroxyprogesterone acetate induce apoptosis of primary Burkitt's lymphoma cells and cell lines: potential for applying old drugs to a new disease

Current therapies for Burkitt's lymphoma (BL) utilise combined cytotoxic chemotherapy, but these treatments are not always available in areas where the disease is endemic and are also markedly less successful in AIDS-related BL. Therefore, additional therapies are urgently required. We demonstrate here that combined fibrates and MPA exert powerful, antiproliferative actions against well-characterised Daudi, Raji and L3055 BL cell lines and primary BL cells. Detailed studies in L3055 demonstrated that this activity was mediated by induced apoptosis and confirmed by observations that overexpression of the antiapoptotic genes bcl-2 or bcl-x(L) conferred significant protection against the drugs. Importantly, since fibrates and MPA are inexpensive and stable with minimal-associated toxicities, we suggest that these drugs should be considered as adjuncts to currently available treatments for BL in endemic and AIDS-related disease.

Cutting edge: germinal centers can be induced in the absence of T cells

Immunization of mice containing mutations that inactivate the TCR Cbeta and Cdelta genes with the T cell-independent (TI) type 2 Ag (4-hydroxy-3-nitrophenyl)acetyl-Ficoll induces clusters of peanut agglutinin-binding B cells in the spleen. These clusters are histologically indistinguishable from germinal centers (GCs) typical of T cell-dependent immune responses. They are located in follicles, and contain mature follicular dendritic cells, immune complex deposits, and B cells that display the phenotypic qualities of conventional GC B cells. However, the kinetics of this TI GC response differ from T cell-dependent GC responses in being rapidly induced and of short duration. Moreover, the Ab V genes expressed in TI GCs have not undergone somatic hypermutation. Therefore, T cells may be required for B cell differentiation processes associated with the intermediate and latter stages of the GC reaction, but they are dispensable for the induction and initial development of this response.

Germinal centers without T cells

Germinal centers are critical for affinity maturation of antibody (Ab) responses. This process allows the production of high-efficiency neutralizing Ab that protects against virus infection and bacterial exotoxins. In germinal centers, responding B cells selectively mutate the genes that encode their receptors for antigen. This process can change Ab affinity and specificity. The mutated cells that produce high-affinity Ab are selected to become Ab-forming or memory B cells, whereas cells that have lost affinity or acquired autoreactivity are eliminated. Normally, T cells are critical for germinal center formation and subsequent B cell selection. Both processes involve engagement of CD40 on B cells by T cells. This report describes how high-affinity B cells can be induced to form large germinal centers in response to (4-hydroxy-3-nitrophenyl) acetyl (NP)-Ficoll in the absence of T cells or signaling through CD40 or CD28. This requires extensive cross-linking of the B cell receptors, and a frequency of antigen-specific B cells of at least 1 in 1,000. These germinal centers abort dramatically at the time when mutated high-affinity B cells are normally selected by T cells. Thus, there is a fail-safe mechanism against autoreactivity, even in the event of thymus-independent germinal center formation.

Role of T Cells and Germinal Center Formation in the Generation of Immune Responses to the Thymus-Independent Carbohydrate Dextran B512

Immunization with the thymus-independent (TI) Ag native dextran (DX) B512 induces germinal center (GC) formation in the spleen. However, despite this GC formation, the anti-DX response is poor, and no affinity maturation can be observed. Using cholera toxin (CT) as an adjuvant, splenic as well as humoral responses to DX are improved. In this study, we investigated immune responses against DX in mice lacking TNF receptor I and in athymic mice. The adjuvant effect of CT on these responses was also evaluated. Mice lacking the TNF receptor I allowed us to investigate the role of follicular dendritic cell networks and GC formation in the spleen for the generation of Ab responses to DX, whereas we could investigate the role of T cells in GC development to TI Ags using athymic mice. We found that the humoral immune response to TI DX B512 was not dependent upon T cells or the presence of GCs, although GC development occurred after DX immunization. However, T cells were required for this GC formation, since athymic mice could not develop GCs after immunization with DX. We also show that even if CT is able to directly activate B cells when administered as an adjuvant, the major effect may require T cell participation; this is also the case for TI Ags. In contrast, CT adjuvancy is independent of GC formation.

Germinal centre cell kinetics

The germinal centre is a fundamental component of the humoral immune response, representing a unique microenvironment where antigen-activated B lymphocytes undergo clonal expansion, mutate their immunoglobulin, and are subject to a stringent selection process based on their antigen affinity. This review highlights recent advances in the understanding of the cell kinetic process of activation, proliferation, differentiation, and death of germinal centre cells, which are beginning to provide important insights into the regulation of this highly complex reaction. Their definition may have considerable pathological import given the involvement of the germinal centre in non-Hodgkin's lymphomas and recent evidence suggesting that abnormal germinal centre reactions may be involved in the pathogenesis of Hodgkin's disease and some autoimmune diseases.