Biology of germinal centers in lymphoid tissue

Repetitive mRNA vaccination is required to improve the quality of broad-spectrum anti-SARS-CoV-2 antibodies in the absence of CXCL13

Since the initial spread of severe acute respiratory syndrome coronavirus 2 infection, several viral variants have emerged and represent a major challenge for immune control, particularly in the context of vaccination. We evaluated the quantity, quality, and persistence of immunoglobulin G (IgG) and IgA in individuals who received two or three doses of messenger RNA (mRNA) vaccines, compared with previously infected vaccinated individuals. We show that three doses of mRNA vaccine were required to match the humoral responses of preinfected vaccinees. Given the importance of antibody-dependent cell-mediated immunity against viral infections, we also measured the capacity of IgG to recognize spike variants expressed on the cell surface and found that cross-reactivity was also strongly improved by repeated vaccination. Last, we report low levels of CXCL13, a surrogate marker of germinal center activation and formation, in vaccinees both after two and three doses compared with preinfected individuals, providing a potential explanation for the short duration and low quality of Ig induced.

Low quantity and quality of anti-spike humoral response is linked to CD4 T-cell apoptosis in COVID-19 patients

In addition to an inflammatory reaction, Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2)-infected patients present lymphopenia, which we recently reported as being related to abnormal programmed cell death. As an efficient humoral response requires CD4 T-cell help, we hypothesized that the propensity of CD4 T cells to die may impact the quantity and quality of the humoral response in acutely infected individuals. In addition to specific immunoglobulins (Ig)A, IgM, and IgG against SARS-CoV-2 nucleocapsid (N), membrane (M), and spike (S1) proteins, we assessed the quality of IgG response by measuring the avidity index. Because the S protein represents the main target for neutralization and antibody-dependent cellular cytotoxicity responses, we also analyzed anti-S-specific IgG using S-transfected cells (S-Flow). Our results demonstrated that most COVID-19 patients have a predominant IgA anti-N humoral response during the early phase of infection. This specific humoral response preceded the anti-S1 in time and magnitude. The avidity index of anti-S1 IgG was low in acutely infected individuals compared to convalescent patients. We showed that the percentage of apoptotic CD4 T cells is inversely correlated with the levels of specific IgG antibodies. These lower levels were also correlated positively with plasma levels of CXCL10, a marker of disease severity, and soluble Fas ligand that contributes to T-cell death. Finally, we found lower S-Flow responses in patients with higher CD4 T-cell apoptosis. Altogether, these results demonstrate that individuals with high levels of CD4 T-cell apoptosis and CXCL10 have a poor ability to build an efficient anti-S response. Consequently, preventing CD4 T-cell death might be a strategy for improving humoral response during the acute phase, thereby reducing COVID-19 pathogenicity.

TIR-Domain-Containing Adapter-Inducing Interferon-β (TRIF)-Dependent Antiviral Responses Protect Mice against Ross River Virus Disease

Ross River virus (RRV) is the major mosquito-borne virus in the South Pacific region. RRV infections are characterized by arthritic symptoms, which can last from several weeks to months. Type I interferon (IFN), the primary antiviral innate immune response, is able to modulate adaptive immune responses. The relationship between the protective role of type I IFN and the induction of signaling proteins that drive RRV disease pathogenesis remains poorly understood. In the present study, the role of TIR-domain-containing adapter-inducing interferon-β (TRIF), an essential signaling adaptor protein downstream of Toll-like receptor (TLR) 3, a key single-stranded RNA (ssRNA)-sensing receptor, was investigated. We found that TRIF^-/- mice were highly susceptible to RRV infection, with severe disease, high viremia, and a low type I IFN response early during disease development, which suggests the TLR3-TRIF axis may engage early in response to RRV infection. The number and the activation level of CD4⁺ T cells, CD8⁺ T cells, and NK cells were reduced in TRIF^-/- mice compared to those in infected wild-type (WT) mice. In addition, the number of germinal center B cells was lower in TRIF^-/- mice than WT mice following RRV infection, with lower titers of IgG antibodies detected in infected TRIF^-/- mice compared to WT. Interestingly, the requirement for TRIF to promote immunoglobulin class switch recombination was at the level of the local immune microenvironment rather than B cells themselves. The slower resolution of RRV disease in TRIF^-/- mice was associated with persistence of the RRV genome in muscle tissue and a continuing IFN response. IMPORTANCE RRV has been prevalent in the South Pacific region for decades and causes substantial economic and social costs. Though RRV is geographically restricted, a number of other alphaviruses have spread globally due to expansion of the mosquito vectors and increased international travel. Since over 30 species of mosquitoes have been implicated as potent vectors for RRV dissemination, RRV has the potential to further expand its distribution. In the pathogenesis of RRV disease, it is still not clear how innate immune responses synergize with adaptive immune responses. Type I IFN is crucial for bridging innate to adaptive immune responses to viral invasion. Hence, key signaling proteins in type I IFN induction pathways, which are important for type I IFN modulation, may also play critical roles in viral pathogenesis. This study provides insight into the role of TRIF in RRV disease development.

Review article: targeting the B cell activation system in autoimmune hepatitis

BACKGROUND

The B cell activation system, consisting of B cell activating factor and a proliferation-inducing ligand, may have pathogenic effects in autoimmune hepatitis.

AIMS

To describe the biological actions of the B cell activation system, indicate its possible role in autoimmune diseases, and evaluate its prospects as a therapeutic target in autoimmune hepatitis METHODS: English abstracts were identified in PubMed by multiple search terms. Full length articles were selected for review, and secondary and tertiary bibliographies were developed.

RESULTS

The B cell activating factor is crucial for the maturation and survival of B cells, and it can co-stimulate T cell activation, proliferation, and survival. It can also modulate the immune response by inducing interleukin 10 production by regulatory B cells. A proliferation-inducing ligand modulates and diversifies the antibody response by inducing class-switch recombination in B cells. It can also increase the proliferation, survival, and antigen activation of T cells. These immune stimulatory actions can be modulated by inducing proliferation of regulatory T cells. The B cell activation system has been implicated in diverse autoimmune diseases, and therapeutic blockade is a management strategy now being evaluated in autoimmune hepatitis.

CONCLUSIONS

The B cell activation system has profound effects on B and T cell function in autoimmune diseases. Blockade therapy is being actively evaluated in autoimmune hepatitis. Clarification of the critical pathogenic components of the B cell activation system will improve the targeting, efficacy, and safety of blockade therapy in this disease.

The enhanced immunopharmacology of VIB4920, a novel Tn3 fusion protein and CD40L antagonist, and assessment of its safety profile in cynomolgus monkeys

BACKGROUND AND PURPOSE

Inhibition of the T- and B-cell interaction through the CD40/CD40 ligand (L) axis is a favourable approach for inflammatory disease treatment. Clinical studies of anti-CD40L molecules in autoimmune diseases have met challenges because of thromboembolic events and adverse haemostasis. VIB4920 (formerly MEDI4920) is a novel CD40L antagonist and Tn3 fusion protein designed to prevent adverse haemostasis and immunopharmacology. We evaluated the pharmacokinetics, activity and toxicity of VIB4920 in monkeys.

EXPERIMENTAL APPROACH

Cynomolgus monkeys received i.v. or s.c. 5-300 mg·kg^-1 VIB4920 or vehicle, once weekly for 1 month (Studies 1 and 2) or 28 weeks (Study 3). VIB4920 exposure and bioavailability were determined using pharmacokinetic analyses, and immune cell population changes via flow cytometry. Pharmacological activity was evaluated by measuring the animals' capacity to elicit an immune response to keyhole limpet haemocyanin (KLH) and tetanus toxoid (TT).

KEY RESULTS

VIB4920 demonstrated linear pharmacokinetics at multiple doses. Lymphocyte, monocyte, cytotoxic T-cell and NK cell counts were not significantly different between treatment groups. B-cell counts reduced dose-dependently and the T-cell dependent antibody response to KLH was suppressed by VIB4920 dose-dependently. The recall response to TT was similar across treatment groups. No thromboembolic events or symptoms of immune system dysfunctionality were observed.

CONCLUSIONS AND IMPLICATIONS

VIB4920 demonstrated an acceptable safety profile in monkeys. VIB4920 showed favourable pharmacokinetics, dose-dependent inhibition of a neoantigen-specific immune response and no adverse effects on immune function following long-term use. Our data support the use of VIB4920 in clinical trials.

PTEN-Regulated AID Transcription in Germinal Center B Cells Is Essential for the Class-Switch Recombination and IgG Antibody Responses

Class-switch recombination (CSR) and somatic hypermutation (SHM) occur during the differentiation of germinal center B cells (GCBs). Activation-induced cytidine deaminase (AID) is responsible for both CSR and SHM in GCBs. Here, we show that ablation of PTEN through the Cγ1-Cre mediated recombination significantly influences the CSR and SHM responses. The GCs fail to produce the IgG1 B cells, the high affinity antibodies and nearly lost the dark zone (DZ) in Pten^fl/flCγ1^Cre/+ mice after immunization, suggesting the impaired GC structure. Further mechanistic investigations show that LPS- and interleukin-4 stimulation induced the transcription of Cγ1 in IgM-BCR expressing B cells, which efficiently disrupts PTEN transcription, results in the hyperphosphorylated AKT and FoxO1 and in turn the suppression of AID transcription. Additionally, the reduced transcription of PTEN and AID is also validated by investigating the IgM-BCR expressing GCBs from Pten^fl/flCγ1^Cre/+ mice upon immunization. In conclusion, PTEN regulated AID transcription in GCBs is essential for the CSR and IgG antibody responses.

Toll-like receptor 3 adjuvant in combination with virus-like particles elicit a humoral response against HIV

Human Immunodeficiency Virus (HIV) Virus-Like Particles (VLPs) composed of HIV_IIIB Gag and HIV_BaL gp120/gp41 envelope are a pseudovirion vaccine capable of presenting antigens in their native conformations. To enhance the immunogenicity of the HIV Env antigen, VLPs were coupled to VesiVax Conjugatable Adjuvant Lipid Vesicles (CALV) containing one of four toll-like-receptor (TLR) ligands, each activating a receptor with distinct cellular localization and downstream pathways. C57BL/6 mice were vaccinated by intranasal prime followed by two sub-cheek boosts and their sera immunoglobulin and neutralizing potency were measured over a duration of 3months after vaccination. PBS control, VLPs alone, CALV+VLPs, and VLPs complexed with CALV and ligands for TLR2 (PAM3CAG), TLR3 (dsRNA), TLR4 (MPLA), or TLR7/8 (resiquimod) were evaluated based on antibody titer, IgG1 and IgG2c class switching, germinal center formation, T follicular cells and potency of neutralizing antibodies. Consistently, the TLR3 ligand dsRNA complexed to CALV and in combination with VLPs (CALV(dsRNA)+VLPs) induced the strongest response. CALV(dsRNA)+VLPs induced the highest titers against the recombinant vaccine antigens clade B Bal gp120 and pr55 Gag. Additionally, CALV(dsRNA)+VLPs induced cross-clade antibodies, represented by high titers of antibody to clade c 96ZM651 gp120. CALV(dsRNA)+VLPs induced predominantly IgG2c over IgG1, a response associated with T helper type 1 (Th1)-like cytokines. In turn, CALV(dsRNA)+VLP immunized mice generated the most potent neutralizing antibodies against HIV strain MN.3. Finally, at time of sacrifice, a significant increase in germinal center B cells and T follicular cells was detected in mice which received CALV(dsRNA)+VLPs compared to PBS. Our results indicate that CALV(dsRNA) is a superior adjuvant for HIV VLPs in generating a Th1-like immunoglobulin profile, while prolonging lymph node germinal centers, T follicular cells and generating neutralizing antibodies to a highly sensitive tier 1A variant of HIV.

A Novel Prime and Boost Regimen of HIV Virus-Like Particles with TLR4 Adjuvant MPLA Induces Th1 Oriented Immune Responses against HIV

HIV virus-like particles (VLPs) present the HIV envelope protein in its native conformation, providing an ideal vaccine antigen. To enhance the immunogenicity of the VLP vaccine, we sought to improve upon two components; the route of administration and the additional adjuvant. Using HIV VLPs, we evaluated sub-cheek as a novel route of vaccine administration when combined with other conventional routes of immunization. Of five combinations of distinct prime and boost sequences, which included sub-cheek, intranasal, and intradermal routes of administration, intranasal prime and sub-cheek boost (IN+SC) resulted in the highest HIV-specific IgG titers among the groups tested. Using the IN+SC regimen we tested the adjuvant VesiVax Conjugatable Adjuvant Lipid Vesicles (CALV) + monophosphoryl lipid A (MPLA) at MPLA concentrations of 0, 7.5, 12.5, and 25 μg/dose in combination with our VLPs. Mice that received 12.5 or 25 μg/dose MPLA had the highest concentrations of Env-specific IgG2c (20.7 and 18.4 μg/ml respectively), which represents a Th1 type of immune response in C57BL/6 mice. This was in sharp contrast to mice which received 0 or 7.5 μg MPLA adjuvant (6.05 and 5.68 μg/ml of IgG2c respectively). In contrast to IgG2c, MPLA had minor effects on Env-specific IgG1; therefore, 12.5 and 25 μg/dose of MPLA induced the optimal IgG1/IgG2c ratio of 1.3. Additionally, the percentage of germinal center B cells increased significantly from 15.4% in the control group to 31.9% in the CALV + 25 μg MPLA group. These mice also had significantly more IL-2 and less IL-4 Env-specific CD8⁺ T cells than controls, correlating with an increased percentage of Env-specific central memory CD4⁺ and CD8⁺ T cells. Our study shows the strong potential of IN+SC as an efficacious route of administration and the effectiveness of VLPs combined with MPLA adjuvant to induce Env specific Th1-oriented HIV-specific immune responses.

Malignancies in HIV/AIDS: from epidemiology to therapeutic challenges

The incidence of AIDS-defining cancers (ADCs) - Kaposi sarcoma, primary central nervous system lymphoma, non-Hodgkin lymphoma, and cervical cancer - although on the decline since shortly after the introduction of HAART, has continued to be greater even in treated HIV-infected persons than in the general population. Although the survival of newly infected people living with HIV/AIDS now rivals that of the general population, morbidity and mortality associated with non-AIDS-defining cancers (NADCs) such as lung, liver, anal, and melanoma are significant and also continue to rise. Increasing age (i.e. longevity) is the greatest risk factor for NADCs, but longevity alone is not sufficient to fully explain these trends in cancer epidemiology. In this review, we briefly review the epidemiology and etiology of cancers seen in HIV/AIDS, and in this context, discuss preclinical research and broad treatment considerations. Investigation of these considerations provides insight into why malignancies continue to be a major problem in the current era of HIV/AIDS care.

Gpr97 is essential for the follicular versus marginal zone B-lymphocyte fate decision

Gpr97 is an orphan adhesion GPCR and is highly conserved among species. Up to now, its physiological function remains largely unknown. Here, we show that Gpr97 deficiency results in an extensive reduction in B220⁺ lymphocytes in mice. More intensive analyses reveal an expanded marginal zone but a decreased follicular B-cell population in Gpr97^−/−spleen, which displays disorganized architecture characterized by diffuse, irregular B-cell areas and the absence of discrete perifollicular marginal and mantle zones. In vivo functional studies reveal that the mutant mice could generate antibody responses to T cell-dependent and independent antigens, albeit enhanced response to the former and weakened response to the latter. By screening for the molecular events involved in the observed phenotypes, we found that lambda 5 expression is downregulated and its upstream inhibitor Aiolos is increased in the spleen of mutant mice, accompanied by significantly enhanced phosphorylation and nuclear translocation of cAMP response element-binding protein. Interestingly, increased constitutive Nf-κb p50/p65 expression and activity were observed in Gpr97^−/− spleen, implicating a crucial role of Gpr97 in regulating Nf-κb activity. These findings uncover a novel biological function of Gpr97 in regulating B-cell development, implying Gpr97 as a potential therapeutic target for treatment of immunological disorders.

An HIV-1 envelope glycoprotein trimer with an embedded IL-21 domain activates human B cells

Broadly neutralizing antibodies (bNAbs) that target the HIV-1 envelope glycoproteins (Env) can prevent virus acquisition, but several Env properties limit its ability to induce an antibody response that is of sufficient quantity and quality. The immunogenicity of Env can be increased by fusion to co-stimulatory molecules and here we describe novel soluble Env trimers with embedded interleukin-4 (IL-4) or interleukin-21 (IL-21) domains, designed to activate B cells that recognize Env. In particular, the chimeric Env_IL-21 molecule activated B cells efficiently and induced the differentiation of antibody secreting plasmablast-like cells. We studied whether we could increase the activity of the embedded IL-21 by designing a chimeric IL-21/IL-4 (ChimIL-21/4) molecule and by introducing amino acid substitutions in the receptor binding domain of IL-21 that were predicted to enhance its binding. In addition, we incorporated IL-21 into a cleavable Env trimer and found that insertion of IL-21 did not impair Env cleavage, while Env cleavage did not impair IL-21 activity. These studies should guide the further design of chimeric proteins and Env_IL-21 may prove useful in improving antibody responses against HIV-1.

Lung involvement in connective tissue diseases: a comprehensive review and a focus on rheumatoid arthritis

The lungs are frequently involved in Connective Tissue Diseases (CTDs). Interstitial lung disease (ILD) is one of the most common pleuropulmonary manifestations that affects prognosis significantly. In practice, rheumatologists and other physicians tend to underestimate the impact of CTD-ILDs and diagnose respiratory impairment when it has reached an irreversible fibrotic stage. Early investigation, through clinical evidence, imaging and - in certain cases - lung biopsy, is therefore warranted in order to detect a possible ILD at a reversible initial inflammatory stage. In this review, we focus on lung injury during CTDs, with particular attention to ILDs, and examine their prevalence, clinical manifestations and histological patterns, as well as therapeutic approaches and known complications till date. Although several therapeutic agents have been approved, the best treatment is still not certain and additional trials are required, which demand more knowledge of pulmonary involvement in CTDs. Our central aim is therefore to document the impact that lung damage has on CTDs. We will mainly focus on Rheumatoid Arthritis (RA), which - unlike other rheumatic disorders - resembles Idiopathic Pulmonary Fibrosis (IPF) in numerous aspects.

Follicular dendritic cells in health and disease

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

Bronchus-associated lymphoid tissue (BALT) structure and function

Bronchus-associated lymphoid tissue (BALT) is a constitutive mucosal lymphoid tissue adjacent to major airways in some mammalian species, including rats and rabbits, but not humans or mice. A related tissue, inducible BALT (iBALT), is an ectopic lymphoid tissue that is formed upon inflammation or infection in both mice and humans and can be found throughout the lung. Both BALT and iBALT acquire antigens from the airways and initiate local immune responses and maintain memory cells in the lungs. Here, we discuss the development and function of BALT and iBALT in the context of pulmonary immunity to infectious agents, tumors, and allergens as well as autoimmunity and inflammatory diseases of the lung.

A quantitative assessment of primary and secondary immune responses in cattle using a B cell ELISPOT assay

The aim of the study was to build a comprehensive picture of the appearance in the blood stream of Ag-specific plasma cells and memory B cells in the bovine model. For this purpose, we have developed a method allowing the detection and quantification of both cell types within individual calves immunised with ovalbumin. During the primary response, we detected a burst of ovalbumin-specific plasma cells at days 6 and 7 post-immunisation, which was followed by the production of specific Ab, whereas a gradual increase of memory B cells was only detected from day 15. As expected, a boost immunisation performed 7 weeks later induced a quicker and stronger secondary response. Indeed, a burst of plasma cells was detected in the blood at days 3 and 4, which was followed by a strong increase in Ab titres. Furthermore, a burst of memory B cells, and not a gradual increase, was detected at days 5 and 6 post-boost immunisation. Importantly, we showed a strong correlation between the anti-ovalbumin-specific IgG titres detected 5 months after secondary immunisation and the plasma cell numbers detected in the blood at the peak response after secondary immunisation. The detection and quantification of plasma cells following an immunisation/vaccination strategy could constitute a very effective means for predicting the magnitude and longevity of an Ab response.

The Germinal centre-derived lymphomas seen through their cellular microenvironment

The human lymph node is a complex tissue resulting from the microenvironmental organisation of different cell populations linked by topographical and/or functional relationships. Germinal centres (GCs) of lymphoid follicles contain a meshwork of follicular dendritic cells in addition to B-cells and some CD4(+) T cells. Moreover, there is a sharp demarcation around the whole follicle centre, which is highlighted by fibroblastic reticulum cells. On the whole, GC exerts a role in B cell physiology and malignancy. In GC-derived lymphomas, gene expression profiling studies have raised the possibility that survival of the affected patients may be associated with signatures preferentially expressed in non-malignant T cells and macrophages and/or dendritic cells. Immunohistological analyses in lymphoma biopsy samples have confirmed that the biological behaviour and tumour progression may be influenced by the tumour microenvironment. This review will examine GC-derived lymphomas, including follicular lymphomas, Hodgkin lymphomas and angioimmunoblastic T-cell lymphoma, through their integrated cellular microenvironment, highlighting those findings which may serve as a useful surrogate marker for tumour diagnosis or tumour progression, together with key molecules involved in tumour development.

Scrapie-specific pathology of sheep lymphoid tissues

Transmissible spongiform encephalopathies (TSEs) or prion diseases often result in accumulation of disease-associated PrP (PrP^d) in the lymphoreticular system (LRS), specifically in association with follicular dendritic cells (FDCs) and tingible body macrophages (TBMs) of secondary follicles. We studied the effects of sheep scrapie on lymphoid tissue in tonsils and lymph nodes by light and electron microscopy. FDCs of sheep were grouped according to morphology as immature, mature or regressing. Scrapie was associated with FDC dendrite hypertrophy and electron dense deposit or vesicles. PrP^d was located using immunogold labelling at the plasmalemma of FDC dendrites and, infrequently, mature B cells. Abnormal electron dense deposits surrounding FDC dendrites were identified as immunoglobulins suggesting that excess immune complexes are retained and are indicative of an FDC dysfunction. Within scrapie-affected lymph nodes, macrophages outside the follicle and a proportion of germinal centre TBMs accumulated PrP^d within endosomes and lysosomes. In addition, TBMs showed PrP^d in association with the cell membrane, non-coated pits and vesicles, and also with discrete, large and random endoplasmic reticulum networks, which co-localised with ubiquitin. These observations suggest that PrP^d is internalised via the caveolin-mediated pathway, and causes an abnormal disease-related alteration in endoplasmic reticulum structure. In contrast to current dogma, this study shows that sheep scrapie is associated with cytopathology of germinal centres, which we attribute to abnormal antigen complex trapping by FDCs and abnormal endocytic events in TBMs. The nature of the sub-cellular changes in FDCs and TBMs differs from those of scrapie infected neurones and glial cells suggesting that different PrP^d/cell membrane interactions occur in different cell types.

Fibrinogen is localized on dark zone follicular dendritic cells in vivo and enhances the proliferation and survival of a centroblastic cell line in vitro

Follicular dendritic cells (FDC) in the germinal centers (GC) of secondary lymphoid organs increase the survival and proliferation of antigen-stimulated B cells and are pivotal for the affinity maturation of an antibody response and for maintenance of B cell immunological memory. The dark zone (DZ) and the light zone (LZ) constitute distinct areas of the GC containing different subtypes of FDC as identified by their morphology and phenotype. Until now, most available FDC-specific reagents identify LZ FDC, and there are no reagents recognizing DZ FDC specifically. Here, we report a new mAb, D46, which stains FDC specifically in the DZ of bovine and ovine GC within the secondary follicles. We identify its ligand as bovine fibrinogen, and using commercially available anti-human fibrinogen antibodies, show that this inflammatory protein is also present on DZ FDC of human GC within palatine tonsils. In vitro, the addition of exogenous fibrinogen stimulates the proliferation and survival of BCR-stimulated L3055 cells, which constitute a clonal population of centroblastic cells and retain important features of normal GC B cells. Together, our results suggest that fibrinogen localized on DZ FDC could support the extensive proliferation and survival of GC B cells within the DZ in vivo.

Natural Humoral Immune Competence and Survival in Layers

The relation between survival and levels of humoral components of innate (and specific) immune competence of laying hens was investigated in a population of 1,063 laying hens from 12 purebred layer lines. Natural immune competence of the chickens was studied by measuring levels of natural antibodies (NAb) binding to keyhole limpet hemocyanin (KLH) or lipopolysaccharide (LPS), respectively, and hemolytic (classical and alternative) complement activity at 20, 40, and 65 wk of age. In addition, levels of antibodies binding a Newcastle disease vaccine strain as a measure of specific immunity were investigated at 20 wk of age. A distinction could be made between lines showing high or low immune competence with respect to NAb, complement activity, and specific antibodies. Within lines, significant correlations were found for each of the innate parameters among the 3 ages. The innate and specific parameters were, however, not correlated with each other. Based on the limited data set, it was not possible to draw conclusions on line differences for innate or specific immune competence in relation to survival. However, regardless of line, low levels of NAb binding to KLH or high levels of NAb binding to LPS were detected in chickens that did not survive the laying period. The major difference between the responses of NAb binding to KLH or LPS was that the chickens probably did not encounter KLH, which suggests a reflection of the capacity to respond, whereas the chickens most probably did encounter LPS, which suggests a reflection of the active status of the innate humoral immune system. In conclusion, we propose that levels (KLH) and activation (LPS) of components of natural antibodies are indicative for the probability that chickens survive a laying period.

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

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

Serum haemolytic complement activities in 11 different MHC (B) typed chicken lines

To study the relation between serum complement levels and the chicken MHC (B) complex, complement haemolytic activity was measured in sera from hens from seven pure-bred B-typed White and one Brown Leghorn lines, and three ISA-Warren lines that had been divergently selected for antibody responses to sheep red blood cells (SRBC). Significant differences occurred in the serum haemolytic complement activities, both belonging to the classic (CPW) and the alternative (APW) pathways, among the 11 different haplotyped chicken lines. Hens with high CPW and high APW titres predominantly displayed the B2 or B21 haplotypes. Chickens with low CPW and APW were found in B14 and B15 haplotypes. Haplotype B14 appears to be different in complement levels when present into the pure-bred lines or into the ISA-Warren line selected for low antibody responses to SRBC. Otherwise, the presence of B21 in ISA-Warren line selected for high antibody responses to SRBC does not differ with the B21 in the inbred lines (except in the NL-line for CPW values). In general the haplotypes B2 and B21 are found in chicken lines with enhanced disease resistance, and the B15 haplotype has been connected with enhanced disease susceptibility. Our results suggest that levels of haemolytic complement activity, either from the classical or from the alternative pathways, may underlie part of the immunocompetence ascribed to the MHC (B) complex in chickens.

Autoimmune glomerulonephritis with spontaneous formation of splenic germinal centers in mice lacking the estrogen receptor alpha gene

In mice, ovariectomy accelerates the progression of the end-stage renal disease glomerulosclerosis. In women, the incidence of this disease increases after menopause, and estrogen alters its progression. Polymorphisms in the human estrogen receptor alpha (ERalpha) gene have been suggested to constitute a genetic predisposition for lupus nephritis. Here we show that by 1 year of age, mice lacking ERalpha (ERalpha(-/-)) but not those lacking ERbeta (ERbeta(-/-)) exhibit immune complex-type glomerulonephritis, proteinuria, and destruction of tubular cells with severe infiltration of B lymphocytes in the kidney and the presence of anti-DNA antibodies in serum. No gender difference occurred in the incidence or severity of these symptoms. However, in female but not in male ERalpha(-/-) mice there were elevated serum levels of IgG3. Other prominent features of these mice were (i) spontaneous formation of germinal centers in the spleen in the absence of antigen challenge and (ii) infiltration of plasma cells in the kidney and plasmacytosis in the spleen. Immunohistochemistry indicated a selective expression of ERalpha protein in the germinal centers but not in the follicular mantle zone of murine spleens and human tonsils. Our results indicate that ERalpha has indispensable functions in the kidney and in germinal centers, and that defective ERalpha signaling results in glomerulonephritis.

In vivo expression of interleukin-8, and regulated on activation, normal, T-cell expressed, and secreted, by human germinal centre B lymphocytes

T-cell homing within germinal centres (GCs) is required for humoral B-cell responses. However, the mechanisms implicated in the recruitment of T cells into the GC are not completely understood. Here we show, by immunohistology, and Northern and Western blots, that in vivo human GC B lymphocytes can express CxC and CC chemokines. Moreover, B-cell subset-specific experiments reveal that interleukin (IL)-8 and regulated on activation, normal, T-cell expressed, and secreted (RANTES) are predominantly expressed by GC centroblast and centrocytes, suggesting that chemokine expression is essential at stages in which B-lymphocytes engage in active antigen-dependent interactions with T lymphocytes. In keeping with this hypothesis, we show that the T cells recruited into the GC correlatively express the receptors for IL-8 and RANTES. We propose that chemokine expression is a key B-cell function that facilitates T-lymphocyte recruitment into the GCs and supports cognate B-cell : T-cell encounters. Moreover, our data are consistent with the impaired homing of T cells to secondary lymphoid organs in mice that are either deficient in CC and CxC chemokines or their receptors.

Life and death within germinal centres: a double-edged sword

Within germinal centres, B lymphocytes are destined to die by apoptosis via Fas signalling, unless they are positively rescued by antigen and by signals initiated by CD40-CD154 interactions. Thus, while the germinal centre microenvironment can become a virtual graveyard for most B lymphocytes that fail to bind antigen with high affinity, it concomitantly provides the necessary stimuli for the survival of cells that successfully accomplish affinity maturation. Such dichotomy in the physiology of germinal centre reaction that results in survival of the functional B-cell repertoire and the elimination of abnormal cells, dictates the fate towards B-cell homeostasis or disease. Consequently, the death and survival-signalling arms within germinal centres predominantly reside on the timely and controlled expression of Fas and its ligand (FasL), and CD40 and CD154, respectively. In keeping with this notion, lymphoproliferation or deficient immunity are documented landmarks of inactivation of either the Fas/FasL or CD40/CD154 signalling pathways. The present review considers two different scenarios in the control of B-cell survival and death within germinal centres. The first is an idealistic scenario, in which a discriminatory and co-ordinate signalling initiated by the CD40/CD154 and Fas/FasL pairs, respectively, leads the rescue of the functional B-cell repertoire and the elimination of the abnormal phenotype. The second is a gloomy scenario in which both the lack and the hyperexpression of either receptor/ligand pairs, are seen as equally deleterious.

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.

Follicular dendritic cells in reactive and neoplastic lymphoid tissues: a reevaluation of staining patterns of CD21, CD23, and CD35 antibodies in paraffin sections after wet heat-induced epitope retrieval

Structural alterations in the meshwork of follicular dendritic cells (FDCs) are frequently found in malignant lymphomas. Formaldehyde fixation and paraffin embedding, however, have long prevented consistent detection of FDCs. Wet heat-induced epitope retrieval in Dako Target Retrieval Solution (TRS) (pH 6.0) enabled the reliable detection of FDCs through CD21, CD23, and CD35 antigens in routinely processed tissues from 11 reactive and 69 neoplastic lymphoproliferations, thus allowing the distribution of the FDCs to be reevaluated. Germinal center FDCs in lymphoid hyperplasias and expanded FDC meshworks in the 8 mantle cell lymphomas, 7 low-grade MALT lymphomas, and 6 low-grade follicular lymphomas were intensely stained with all these markers. In 6 cases of B cell chronic lymphocytic leukemia, tumor cells were CD23+. In four cases of nodular lymphocyte predominance Hodgkin's disease (HD), expanded FDC meshwork's sharply delineating negative tumor cells and their rosetting T cell, were revealed mainly with the CD21 and CD35 antibodies. Follicular dendritic cells were also demonstrated in 11 cases of grade I nodular sclerosing HD, including follicular HD. Striking dendritic cell clusters were revealed with all 3 antibodies in 9 angioimmunoblastic T cell lymphomas. Sparse or no FDC meshworks were detected in the 4 cases of grade II nodular sclerosing HD, 5 follicular lymphomas with high-grade transformation, and 5 T cell-rich B cell lymphomas. CD35 immunostaining showed the most consistent labeling in the four FDC sarcomas studied in the current article. Reproducible demonstration of FDCs in routinely processed paraffin sections with CD21, CD23, and CD35 antibodies, as presented here, provides invaluable pieces of information in the diagnosis of lymphoproliferative disorders.

Upregulation of the Netrin Receptor (DCC) Gene during Activation of B Lymphocytes and Modulation by Interleukins

The DCC (deleted in colon cancer) gene has a brain restricted high expression pattern. It encodes a transmembrane protein of the immunoglobulin superfamily identified as the netrin-1 receptor. It might be a member of the so called "brain-lymphoid" molecules, which control key cell surface events. To test this hypothesis we have assessed the DCC mRNA level in human normal and malignant myeloid and lymphoid cells. A high mRNA content has been observed only in mature B cells at the secreting or presecreting stage. Expression of DCC was also assessed in the anti-CD40 model of immunopoiesis. Activation of purified tonsillar B cells by anti-CD 40 antibody strongly increased the DCC mRNA level and this effect was dramatically enhanced by the association of IL-2 + IL-10, which is a potent and selective in vitro inducer of the B cell memory phenotype. In contrast no effect has been detected after activation of T cells by anti-CD3. These data suggest that the DCC encoded netrin receptor is involved in B cell immunopoiesis.

Changes in human lymphocyte subpopulations in tonsils and regional lymph nodes of human head and neck squamous carcinoma compared to control lymph nodes

BACKGROUND

Lymphoid tissues constitute basic structures where specific immune responses take place. This leads to the development of germinal centres (GCs), migration of cells and the generation of memory cells. Here, we have compared human tumour reactive lymph nodes and tonsils with control lymph nodes.

RESULTS

The study by flow cytometry shows that in control lymph nodes the majority of cells were naive T-lymphocytes (CD45RA+/CD7+). In reactive nodes, although the percentage of CD45RO+ T cells remains constant, there is an increase in the number of B-lymphocytes, and a reduction in naive T cells. The percentage of cells expressing CD69 was similar in reactive nodes and in controls. In both cases, we have found two populations of B cells of either CD69- or CD69dull. Two populations of T cells, which are either negative for CD69 or express it in bright levels (CD69bright), were also found. The analysis of tissue sections by confocal microscopy revealed differences between control, tonsils and tumor reactive lymph nodes. In control lymph nodes, CD19 B cells are surrounded by a unique layer of CD69bright/CD45RO+ T cells. GCs from tonsils and from tumour reactive nodes are mainly constituted by CD19 B cells and have four distinct layers. The central zone is composed of CD69- B cells surrounded by CD69bright/CD45RO+ T cells. The mantle region has basically CD69dull B-lymphocytes and, finally, there is an outer zone with CD69-/CD45RO+ T cells.

CONCLUSIONS

Human secondary lymphoid organs react with an increase in the proportion of B lymphocytes and a decrease in the number of CD45RA+ T cells (naive). In tonsils, this is due to chronic pathogen stimulation, whereas in lymph nodes draining head and neck carcinomas the reaction is prompted by surrounded tumors. During this process, secondary lymphoid organs develop secondary follicles with a special organization of T and B cells in consecutive layers, that are described here by confocal microscopy. This pattern of cellular distribution may suggest a model of cell migration into the secondary lymphoid follicles.

Regulation of CD40 and CD40 ligand by the AT-hook transcription factor AKNA

Proteins containing AT hooks bind A/T-rich DNA through a nine-amino-acid motif and are thought to co-regulate transcription by modifying the architecture of DNA, thereby enhancing the accessibility of promoters to transcription factors. Here we describe AKNA, a human AT-hook protein that directly binds the A/T-rich regulatory elements of the promoters of CD40 and CD40 ligand (CD40L) and coordinately regulates their expression. Consistent with its function, AKNA is a nuclear protein that contains multiple PEST protein-cleavage motifs, which are common in regulatory proteins with high turnover rates. AKNA is mainly expressed by B and T lymphocytes, natural killer cells and dendritic cells. During B-lymphocyte differentiation, AKNA is mainly expressed by germinal centre B lymphocytes, a stage in which receptor and ligand interactions are crucial for B-lymphocyte maturation. Our findings show that an AT-hook molecule can coordinately regulate the expression of a key receptor and its ligand, and point towards a molecular mechanism that explains homotypic cell interactions.

The hepatocyte growth factor/Met pathway in development, tumorigenesis, and B-cell differentiation

This article summarizes the structure, signal transduction and physiologic functions of the HGF/Met pathway, as well as its role in tumor growth, invasion, and metastasis. Moreover, it highlights recent studies indicating a role for the HGF/Met pathway in antigen-specific B-cell development and B-cell neoplasia.

Epigenetic determinants of resistance to etoposide regulation of Bcl-X(L) and Bax by tumor microenvironmental factors

BACKGROUND

Epigenetic factors (i.e., alterations of gene activity not involving mutations), as well as genetic changes in surviving cancer cells, may play an important role in drug resistance following cancer chemotherapy-a common cause of tumor relapse. Bcl-2 family proteins are central to the regulation of apoptotic cell death and modulate drug sensitivity. We investigated how survival signals in the cellular microenvironment affect the expression, protein conformation, and protein-protein interactions of the Bcl-2 family proteins Bax and Bcl-x(L) and how changes in response to microenvironmental signals alter the response of cancer cells to the drug etoposide.

METHODS

JLP119 human B-lymphoma cells were treated with etoposide (40 microM) and then cultured in the presence of an activating anti-CD40 antibody, vascular cellular adhesion molecule-1 (VCAM-1)-to activate VLA-4 (alpha4beta1) integrin, and interleukin 4. Cell fate was monitored after etoposide treatment with or without these microenvironmental signals. Bcl-x(L) gene transcription and protein levels of Bcl-x(L) and Bax were measured by northern and western blotting, respectively. Nuclear translocation of transcription factor NF-kappaB was monitored by immunofluorescence and inhibited by (E)-capsaicin. Bax conformation and Bax-Bcl-x(L) interactions were monitored by immunofluorescence and immunoprecipitation, respectively.

RESULTS

Microenvironmental survival signals produced statistically significant reductions in etoposide-induced apoptotic cell death, from 84.6% (95% confidence interval [CI] = 76.7%-92.4%) to 21.3% (95% CI = 19.5%-23.0%); P<.001. Activation of surface protein CD40 increased Bcl-x(L) protein levels via an (E)-capsaicin-inhibitable activation of NF-kappaB; i.e. , (E)-capsaicin restored etoposide sensitivity. Interleukin 4 had no effect on Bcl-x(L) protein levels but accelerated the increase in Bcl-x(L) protein associated with CD40 activation. VCAM-1- and interleukin 4-mediated signals diminished conformational changes in Bax protein and prevented the etoposide-induced disruption of constitutive Bax-Bcl-x(L) binding.

CONCLUSIONS

Microenvironmental factors reduce the sensitivity of a B-cell lymphoma to etoposide in vitro by modulating the expression and functions of Bax and Bcl-x(L). This interaction may provide a paradigm for epigenetically induced drug resistance in other tumors.

B Cell Responses to a Peptide Epitope. VIII. Immune Complex-Mediated Regulation of Memory B Cell Generation Within Germinal Centers

Using an in vivo reconstitution assay, we examine here the role of immune complexes in both formation of germinal centers (GC) and processes that occur subsequently within. The presence of Ag, as immune complexes, was found not to constitute a limiting requirement for the initiation of GC formation. No detrimental effect either on numbers or sizes of the resulting GC was observed when Ag-containing immune complexes were omitted during reconstitution. Thus, both recruitment and proliferation of Ag-activated B cells within GC appear not to be limited by Ag concentrations. In contrast, the presence of immune complexes was observed to be obligatory for the generation of Ag-specific memory B cells. This optimally required immune complexes to be constituted by IgG-class Abs with epitope specificities that were homologous to those of the GC B cells. The GC reaction was also found to be characterized by an enhancement of Ab specificity for the homologous epitope. Although some improvement in specificity was noted in recall responses from immune complex-deficient GC, the presence of appropriate immune complexes served to further optimize the outcome. Here again, isotype and epitope-specificity of the Ab constituent in immune complexes proved to be important.

Severe attenuation of the B cell immune response in Msh2-deficient mice

Recently, results obtained from mice with targeted inactivations of postreplication DNA mismatch repair (MMR) genes have been interpreted to demonstrate a direct role for MMR in antibody variable (V) gene hypermutation. Here we show that mice that do not express the MMR factor Msh2 have wide-ranging defects in antigen-driven B cell responses. These include lack of progression of the germinal center (GC) reaction associated with increased intra-GC apoptosis, severely diminished antigen-specific immunoglobulin G responses, and near absence of anamnestic responses. Mice heterozygous for the Msh2 deficiency display an "intermediate" phenotype in these regards, suggesting that normal levels of Msh2 expression are critical for the B cell response. Interpretation of the impact of an MMR deficiency on the mechanism of V gene somatic hypermutation could be easily confounded by these perturbations.

Regulation of adhesion and migration in the germinal center microenvironment

T cell dependent humoral immune responses are initiated by the activation of naive B cells in the T cell areas of the secondary lymphoid tissues. This primary B cell activation leads to migration of germinal center (GC) cell precursors into B cell follicles where they engage follicular dendritic cells (FDC) and T cells, and differentiate into memory B cells or plasma cells. Both B cell homing to the GC and interaction with FDC critically depend on integrin-mediated adhesion. We have recently indentified the c-met-encoded receptor tyrosine kinase and its ligand, the growth and motility factor hepatocyte growth factor/scatter factor (HGF/SF), as a novel paracrine signalling pathway regulating B cell adhesion (van der Voort et al., 1997, J. Exp. Med. 185, 2121-2131). The c-Met protein is expressed on B cells localized in the dark zone of the GC (centroblasts) and is induced by CD40 plus BCR ligation. Stimulation of c-Met with HGF/SF, which is produced at high levels by tonsillar stromal cells and FDC, leads to receptor phosphorylation and to enhanced integrin-mediated adhesion of B cells to both VCAM-1 and fibronectin. Interestingly, these responses to HGF/SF are promoted by heparan-sulfate proteoglycan forms of CD44 (CD44-HS). Like c-Met, CD44-HS is induced on B cells by CD40 ligation. It efficiently binds HGF/SF and strongly promotes signalling through c-Met. We conclude that integrin regulation during antigen specific B cell differentiation involves cross-talk between the HGF/SF-c-Met pathway and CD44-HS.

Identification of functional human splenic memory B cells by expression of CD148 and CD27

Memory B cells isolated from human tonsils are characterized by an activated cell surface phenotype, localization to mucosal epithelium, expression of somatically mutated immunoglobulin (Ig) variable (V) region genes, and a preferential differentiation into plasma cells in vitro. In spleens of both humans and rodents, a subset of memory B cells is believed to reside in the marginal zone of the white pulp. Similar to tonsil-derived memory B cells, splenic marginal zone B cells can be distinguished from naive follicular B cells by a distinct cell surface phenotype and by the presence of somatic mutations in their Ig V region genes. Although differences exist between human naive and memory B cells, no cell surface molecules have been identified that positively identify all memory B cells. In this study, we have examined the expression of the receptor-type protein tyrosine phosphatase CD148 on human B cells. CD148(+) B cells present in human spleen exhibited characteristics typical of memory B cells. These included an activated phenotype, localization to the marginal zone, the expression of somatically mutated Ig V region genes, and the preferential differentiation into plasma cells. In contrast, CD148(-) B cells appeared to be naive B cells due to localization to the mantle zone, the expression of surface antigens typical of unstimulated B cells, and the expression of unmutated Ig V region genes. Interestingly, CD148(+) B cells also coexpressed CD27, whereas CD148(-) B cells were CD27(-). These results identify CD148 and CD27 as markers which positively identify memory B cells present in human spleen. Thus, assessing expression of these molecules may be a convenient way to monitor the development of memory B cell responses in immunocompromised individuals or in vaccine trials.

Regulation of anti-DNA B cells in recombination-activating gene-deficient mice

Anti-DNA antibodies are regulated in normal individuals but are found in high concentration in the serum of systemic lupus erythematosus (SLE) patients and the MRL lpr/lpr mouse model of SLE. We previously studied the regulation of anti-double-stranded (ds)DNA and anti-single-stranded (ss)DNA B cells in a nonautoimmune background by generating mice carrying immunoglobulin transgenes coding for anti-DNAs derived from MRL lpr/lpr. Anti-dsDNA B cells undergo receptor editing, but anti-ssDNA B cells seem to be functionally silenced. Here we have investigated how anti-DNA B cells are regulated in recombination- activating gene (RAG)-2-/- mice. In this setting, anti-dsDNA B cells are eliminated by apoptosis in the bone marrow and anti-ssDNA B cells are partially activated.

Age-associated modulation of apoptosis and activation in murine B lymphocytes

We have investigated the influence of age on B-cell responsiveness. The present study showed that the B-cell mitogen, lipopolysaccharide (LPS), similarly stimulated the proliferation of purified B lymphocytes obtained from either young mice (3 months) or old mice (24 months). In contrast, expression of the differentiation marker, alkaline phosphatase (ALP), was about fourfold higher in young mice than in older mice upon stimulation with LPS or with dextran sulfate (DXS) and interleukin-5 (IL-5). The occurrence of apoptosis during aging was then studied: unexpectedly, spontaneous cell death was double in B lymphocytes from young mice compared to older animals. Stimulation with DXS with or without IL-5 rescued B lymphocytes from cell death in young mice but protection decreased with aging, and no longer occurred in 24-month-old mice B cells. Meanwhile, the protective activity conferred by IL-4 was maintained at similar levels throughout aging. However, B cells from old mice were more responsive to apoptosis induction with cycloheximide, dibutyryl cAMP and dexamethasone. Together, the present results indicate an age-associated alteration in apoptosis and activation of B lymphocytes which could contribute to the age-related decline of the immune response.

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.

Multiple hemopoietic defects and lymphoid hyperplasia in mice lacking the transcriptional activation domain of the c-Rel protein

The c-rel protooncogene encodes a member of the Rel/nuclear factor (NF)-kappaB family of transcriptional factors. To assess the role of the transcriptional activation domain of c-Rel in vivo, we generated mice expressing a truncated c-Rel (Deltac-Rel) that lacks the COOH-terminal region, but retains a functional Rel homology domain. Mice with an homozygous mutation in the c-rel region encoding the COOH terminus of c-Rel (c-relDeltaCT/DeltaCT) display marked defects in proliferative and immune functions. c-relDeltaCT/DeltaCT animals present histopathological alterations of hemopoietic tissues, such as an enlarged spleen due to lymphoid hyperplasia, extramedullary hematopoiesis, and bone marrow hypoplasia. In older c-relDeltaCT/DeltaCT mice, lymphoid hyperplasia was also detected in lymph nodes, liver, lung, and stomach. These animals present a more severe phenotype than mice lacking the entire c-Rel protein. Thus, in c-relDeltaCT/DeltaCT mice, the lack of c-Rel activity is less efficiently compensated by other NF-kappaB proteins.

The roles of antibody variable region hypermutation and selection in the development of the memory B-cell compartment

Somatic hypermutation and selection of immunoglobulin (Ig) variable (V)-region genes, working in concert, appear to be essential for memory B-cell development in mammals. There has been substantial progress on the nature of the cis-acting DNA elements that regulate hypermutation. The data obtained suggest that the mechanisms of Ig gene hypermutation and transcription are intimately intertwined. While it has long been appreciated that stringent phenotypic selection forces are imposed on the somatically mutated Ig V regions generated during a T-cell dependent B-cell response, the mechanisms involved in this selection have remained enigmatic. Our studies have questioned the role of foreign antigen deposited on follicular dendritic cells in affinity-based positive selection of V regions, and have shown that this selection takes place in a "clone-autonomous" fashion. In addition, our data strongly suggest that affinity for antigen alone is not the driving force for selection of B-cell clones into the memory compartment. In contrast, we suggest that a combination of positive selection for increased foreign antigen binding, and negative selection of antibody V regions that are autoreactive at the onset of the response, or have acquired autoreactivity via hypermutation, results in the "specificity maturation" of the memory B-cell response.

Selective expression of beta 7 integrin on lymphocytes undergoing apoptosis in lymphoid tissues

It has been previously shown that the beta 7 chain of integrin forms heterodimers with the alpha 4 or alpha E chain, which plays essential roles in lymphocyte homing to mucosal lymphoid tissues. The aim of this study was to re-evaluate the possible role of the beta 7 integrin other than lymphocyte homing. We prepared spleen and lymph node lymphocytes from biopsied specimens from macaque monkeys and examined for the reactivity with a monoclonal antibody specific for the beta 7 chain. As a result, a minor population of the lymphocytes with a smaller size, which were in the early stage of apoptosis, was found to express a higher level of the beta 7 integrin than a majority of the lymphocytes with a normal size. Interestingly, the apoptotic lymphocytes expressed neither alpha 4 nor alpha E chains, suggesting that the beta 7 chain on these cells may be associated with an undefined alpha chain. These findings indicate that in the lymphoid tissues the shrunken lymphocytes undergoing apoptosis selectively express a unique beta 7 integrin.

Bcl-2 obstructs negative selection of autoreactive, hypermutated antibody V regions during memory B cell development

We analyzed the participation of a predominant B cell clonotype in the anti-arsonate immune response of mice in which Bcl-2 expression was enforced in B cells. Many of the antibodies expressed by the arsonate-induced memory compartment of these mice were "dual-reactive," displaying increased affinity acquired via V region somatic hypermutation for both arsonate and the autoantigen DNA. The hypermutated antibodies expressed by the anti-arsonate memory B cell compartment of normal mice have increased affinity for arsonate but lack measurable affinity for DNA. Thus, interference with apoptotic pathways allows developing memory B cells that have acquired autoreactivity to bypass a peripheral tolerance checkpoint. These data demonstrate that both positive and negative selection, working in concert with V gene somatic hypermutation, result in the "specificity maturation" of the antibody response.