Helper T-cell-regulated B-cell immunity

Helper T Cells are Hyperactive and Contribute to the Dysregulation of Antibody Production in Patients with Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a systemic autoimmune disease, mediated by a complex interaction between B cells and various subsets of T cells. Dysfunction of helper T (Th) and regulatory T (Treg) cells may contribute to the breakdown of self-tolerance and the progression of autoimmune disease. In this study, we investigated the activity of Th and Treg cells on the differentiation of autologous B cells in vitro using cell cultures from the peripheral blood of healthy controls (HCs) and RA patients. The expressions of programmed death 1 (PD-1) and IL-21 were monitored as activation markers for Th cells. Unstimulated Th cells from RA patients showed remarkably higher PD-1 expression than HC samples. Stimulation of Th cells from RA patients with Staphylococcus enterotoxin B (SEB) in the presence of B cells significantly induced their PD-1 and IL-21 expression at a considerably higher level in RA compared to HCs, and Treg cells did not affect IL-21 production. When monitoring B-cell differentiation, a significantly higher frequency of plasma cells was observed, even in unstimulated samples of RA patients compared to HCs. In the SEB-stimulated co-cultures of the RA samples, plasma cell frequency and IgG production were considerably higher than in HCs and were not significantly affected by Tregs. These findings demonstrate that Th cells are constitutively active in RA, and their hyperactivity upon interaction with diseased B cells may lead to uncontrolled antibody production.

A cyclophosphamide-induced immunosuppression Swiss Albino mouse model unveils a potential role for cow urine distillate as a feed additive

BACKGROUND

Traditional and natural immunomodulators are increasingly used as supplements in animal feeds and as interventions in the prevention and treatment of disease in animals.

OBJECTIVE

The aim of this study was to examine the immunomodulatory characteristics of distilled cow urine in vivo using two mouse models, a normal mouse model and an immunosuppressive mouse model.

METHODOLOGY

We divided 144 Swiss Albino mice weighing between 15 g and 30 g, aged between two and three months, into two groups of 72 mice each. In the first group, we subdivided the animals into six subgroups of 12 each. In this group paramerters such as, body weight, organ weights of liver and kidney, haemagglutination titre, Jerne plaque-forming assay, and bone marrow cellularity were measured. We divided the second group into six subgroups for the assessment of delayed-type hypersensitivity (DTH).

RESULTS

As compared to normal control mice, immunocompetent and immunosuppressed mice (given cow urine distillate) had significant increases in body weight, spleen weight, liver weight, total leucocyte count, lymphocyte count, serum protein, and globulin contents. In the treatment groups, the titre of antibodies, the number of antibody- producing cells, the cellularity of bone marrow, and foot pad thickness also increased. In the treatment group, both humoral and cellular immunity were altered compared to the control group, suggesting cow urine distillate to be a potential animal feed ingredient for immunoregulation.

CONCLUSION

This study was able to demonstrate the experimental validity of natural compounds as immunomodulators that can be used in feed supplements for animals. Various compounds could be tested for immunomodulatory effects using this technique in experimental animals.

Mouse Liver B Cells Phagocytose Streptococcus pneumoniae and Initiate Immune Responses against Their Antigens

Recent studies have revealed that mammalian B cells ingest particulate Ags, such as bacteria, although little is known about the effect of this function on acquired immunity. We investigated the role of bacterium-phagocytosing B cells in acquired host immune responses. Cultured mouse liver B cells substantially phagocytosed serum-opsonized Streptococcus pneumoniae and produced IgM. On adoptive transfer of liver B cells that phagocytose S. pneumoniae labeled with pHrodo Red succinimidyl ester, recipient mice showed elevated plasma levels of IgG specific for bacterial Ags. In particular, the levels of IgG2a and IgG2b specific for pneumococcal surface protein A, as well as IgG3 for pneumococcal polysaccharide, were markedly increased compared with total IgG specific for each Ag. When phagocytic liver B cells were cultured with spleen CD4⁺ T cells obtained from mice primed with heat-killed S. pneumoniae 7 d before, they induced IL-2 production and proliferation of the CD4⁺ T cells, along with Th1 cytokine production. However, they induced neither the CD4⁺ T cell production of IL-21, a suggested marker promoting B cell proliferation and differentiation, nor the expression of genes important for somatic hypermutation or isotype switching; such responses were particularly evident when splenic B cells merely capturing S. pneumoniae without processing them were cultured with spleen CD4⁺ T cells. These findings suggest that phagocytic liver B cells may be involved in acquired immune responses by presenting derivative peptides to CD4⁺ T cells without their own somatic hypermutation or isotype switching.

Immunodominance in CD4 T-cell responses: implications for immune responses to influenza virus and for vaccine design

CD4 T cells play a primary role in regulating immune responses to pathogenic organisms and to vaccines. Antigen-specific CD4 T cells provide cognate help to B cells, a requisite event for immunoglobulin switch and affinity maturation of B cells that produce neutralizing antibodies and also provide help to cytotoxic CD8 T cells, critical for their expansion and persistence as memory cells. Finally, CD4 T cells may participate directly in pathogen clearance via cell-mediated cytotoxicity or through production of cytokines. Understanding the role of CD4 T-cell immunity to viruses and other pathogens, as well as evaluation of the efficacy of vaccines, requires insight into the specificity of CD4 T cells. This review focuses on the events within antigen-presenting cells that focus CD4 T cells toward a limited number of peptide antigens within the pathogen or vaccine. The molecular events are discussed in light of the special challenges that the influenza virus poses, owing to the high degree of genetic variability, unpredictable pathogenicity and the repeated encounters that human populations face with this highly infectious pathogenic organism.

The pivotal position of the actin cytoskeleton in the initiation and regulation of B cell receptor activation

The actin cytoskeleton is a dynamic cellular network known for its function in cell morphology and motility. Recent studies using high resolution and real time imaging techniques have revealed that actin plays a critical role in signal transduction, primarily by modulating the dynamics and organization of membrane-associated receptors and signaling molecules. This review summarizes what we have learned so far about a regulatory niche of the actin cytoskeleton in the signal transduction of the B cell receptor (BCR). The activation of the BCR is initiated and regulated by a close coordination between the dynamics of surface BCRs and the cortical actin network. The actin cytoskeleton is involved in regulating the signaling threshold of the BCR to antigenic stimulation, the kinetics and amplification of BCR signaling activities, and the timing and kinetics of signaling downregulation. Actin exerts its regulatory function by controlling the kinetics, magnitude, subcellular location, and nature of BCR clustering and BCR signaling complex formation at every stage of signaling. The cortical actin network is remodeled by initial detachment from the plasma membrane, disassembly and subsequent reassembly into new actin structures in response to antigenic stimulation. Signaling responsive actin regulators translate BCR stimulatory and inhibitory signals into a series of actin remodeling events, which enhance signaling activation and down-regulation by modulating the lateral mobility and spatial organization of surface BCR. The mechanistic understanding of actin-mediated signaling regulation in B cells will help us explore B cell-specific manipulations of the actin cytoskeleton as treatments for B cell-mediated autoimmunity and B cell cancer. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

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.

Extrafollicular B cell activation by marginal zone dendritic cells drives T cell-dependent antibody responses

DCIR2⁺ DCs can initiate extrafollicular B cell responses to T cell–dependent antigen.

Dendritic cells (DCs) are best known for their ability to activate naive T cells, and emerging evidence suggests that distinct DC subsets induce specialized T cell responses. However, little is known concerning the role of DC subsets in the initiation of B cell responses. We report that antigen (Ag) delivery to DC-inhibitory receptor 2 (DCIR2) found on marginal zone (MZ)–associated CD8α⁻ DCs in mice leads to robust class-switched antibody (Ab) responses to a T cell–dependent (TD) Ag. DCIR2⁺ DCs induced rapid up-regulation of multiple B cell activation markers and changes in chemokine receptor expression, resulting in accumulation of Ag-specific B cells within extrafollicular splenic bridging channels as early as 24 h after immunization. Ag-specific B cells primed by DCIR2⁺ DCs were remarkably efficient at driving naive CD4 T cell proliferation, yet DCIR2-induced responses failed to form germinal centers or undergo affinity maturation of serum Ab unless toll-like receptor (TLR) 7 or TLR9 agonists were included at the time of immunization. These results demonstrate DCIR2⁺ DCs have a unique capacity to initiate extrafollicular B cell responses to TD Ag, and thus define a novel division of labor among splenic DC subsets for B cell activation during humoral immune responses.

Influence of disulfide-stabilized structure on the specificity of helper T-cell and antibody responses to HIV envelope glycoprotein gp120

CD4(+) helper T cells specific for human immunodeficiency virus type 1 (HIV-1) are associated with control of viremia. Nevertheless, vaccines have had limited effectiveness thus far, in part because sequence variability and other structural features of the HIV envelope glycoprotein deflect the immune response. Previous studies indicated that CD4(+) T-cell epitope dominance is controlled by antigen three-dimensional structure through its influence on antigen processing and presentation. In this work, three disulfide bonds in the outer domain of gp120 were individually deleted in order to destabilize the local three-dimensional structure and enhance the presentation of nearby weakly immunogenic epitopes. However, upon immunization of groups of BALB/c mice, the CD4(+) T-cell response was broadly reduced for all three variants, and distinct epitope profiles emerged. For one variant, antibody titers were sharply increased, and the antibody exhibited significant CD4-blocking activity.

Uncoupling CD21 and CD19 of the B-cell coreceptor

Complement receptors (CRs) CD21 and CD35 form a coreceptor with CD19 and CD81 on murine B cells that when coligated with the B-cell receptor lowers the threshold of activation by several orders of magnitude. This intrinsic signaling role is thought to explain the impaired humoral immunity of mice bearing deficiency in CRs. However, CRs have additional roles on B cells independent of CD19, such as transport of C3-coated immune complexes and regulation of C4 and C3 convertase. To test whether association of CR with CD19 is necessary for their intrinsic activation-enhancing role, knockin mice expressing mutant receptors, Cr2(Delta/Deltagfp), that bind C3 ligands but do not signal through CD19 were constructed. We found that uncoupling of CR and CD19 significantly diminishes survival of germinal center B cells and secondary antibody titers. However, B memory is less impaired relative to mice bearing a complete deficiency in CRs on B cells. These findings confirm the importance of interaction of CR and CD19 for coreceptor activity in humoral immunity but identify a role for CR in B-cell memory independent of CD19.

Antigen structure influences helper T-cell epitope dominance in the human immune response to HIV envelope glycoprotein gp120

The development of an effective vaccine against HIV/AIDS has been hampered, in part, by a poor understanding of the rules governing helper T-cell epitope immunodominance. Studies in mice have shown that antigen structure modulates epitope immunodominance by affecting the processing and subsequent presentation of helper T-cell epitopes. Previous epitope mapping studies showed that the immunodominant helper T-cell epitopes in mice immunized with gp120 were found flanking flexible loops of the protein. In this report, we show that helper T-cell epitopes against gp120 in humans infected with HIV are also found flanking flexible loops. Immunodominant epitopes were found to be located primarily in the outer domain, an average of 12 residues C-terminal to flexible loops. In the less immunogenic inner domain, epitopes were found an average of five residues N-terminal to conserved regions of the protein, once again placing the epitopes C-terminal to flexible loops. These results show that antigen structure plays a significant role in the shaping of the helper T-cell response against HIV gp120 in humans. This relationship between antigen structure and helper T-cell epitope immunodominance may prove to be useful in the development of rationally designed vaccines against pathogens such as HIV.

Mapping and restriction of a dominant viral CD4+ T cell core epitope by both MHC class I and MHC class II

Virus-specific CD4(+) T cells contribute to effective virus control through a multiplicity of mechanisms including direct effector functions as well as "help" for B cell and CD8(+) T cell responses. Here, we have used the lymphocytic choriomeningitis virus (LCMV) system to assess the minimal constraints of a dominant antiviral CD4(+) T cell response. We report that the core epitope derived from the LCMV glycoprotein (GP) is 11 amino acids in length and provides optimal recognition by epitope-specific CD4(+) T cells. Surprisingly, this epitope is also recognized by LCMV-specific CD8(+) T cells and thus constitutes a unique viral determinant with dual MHC class I- and II-restriction.

Identification of major histocompatibility complex class II-restricted antigens and epitopes of the Epstein-Barr virus by a novel bacterial expression cloning approach

Epstein-Barr virus (EBV)-specific T cells have been successfully used to treat or prevent EBV-positive lymphoproliferative disease in hematopoietic stem cell transplant recipients, but the antigens recognized by the infused CD4+ T cells have remained unknown. Here, we describe a simple procedure that permits the identification of viral T-helper (TH)-cell antigens and epitopes. This direct antigen identification method is based on the random expression of viral polypeptides fused to chloramphenicol acetyltransferase (CAT) in bacteria, which are subsequently fed to major histocompatibility complex class II+ antigen-presenting cells and probed with antigen-specific T cells. The fusion of antigenic fragments to CAT offers several advantages. First, chloramphenicol treatment allows the selection of bacteria expressing antigen-CAT fusion proteins in frame, which greatly reduces the number of colonies to be screened. Second, antigenic fragments fused to CAT are expressed at high levels, even when derived from proteins that are toxic to bacteria. Third, the uniformly high expression level of antigen-CAT fusion proteins permits the establishment of large and representative pool sizes. Finally, antigen identification does not require knowledge of the restriction element and often leads directly to the identification of the T-cell epitope. Using this approach, the BALF4 and BNRF1 proteins were identified as targets of the EBV-specific T-helper-cell response, demonstrating that lytic cycle antigens are a relevant component of the EBV-specific TH-cell response.

Control of Epstein-Barr virus infection in vitro by T helper cells specific for virion glycoproteins

Epstein-Barr virus (EBV) establishes lifelong persistent infections in humans by latently infecting B cells, with occasional cycles of reactivation, virus production, and reinfection. Protective immunity against EBV is mediated by T cells, but the role of EBV-specific T helper (Th) cells is still poorly defined. Here, we study the Th response to the EBV lytic cycle proteins BLLF1 (gp350/220), BALF4 (gp110), and BZLF1 and show that glycoprotein-specific Th cells recognize EBV-positive cells directly; surprisingly, a much higher percentage of target cells than those expressing lytic cycle proteins were recognized. Antigen is efficiently transferred to bystander B cells by receptor-mediated uptake of released virions, resulting in recognition of target cells incubated with <1 virion/cell. T cell recognition does not require productive infection and occurs early after virus entry before latency is established. Glycoprotein-specific Th cells are cytolytic and inhibit proliferation of lymphoblastoid cell lines (LCL) and the outgrowth of LCL after infection of primary B cells with EBV. These results establish a novel role for glycoprotein-specific Th cells in the control of EBV infection and identify virion proteins as important immune targets. These findings have implications for the treatment of diseases associated with EBV and potentially other coated viruses infecting MHC class II–positive cells.

Characterizing biological products and assessing comparability following manufacturing changes

Changes in production methods of a biological product may necessitate an assessment of comparability to ensure that these manufacturing changes have not affected the safety, identity, purity, or efficacy of the product. Depending on the nature of the protein or the change, this assessment consists of a hierarchy of sequential tests in analytical testing, preclinical animal studies and clinical studies. Differences in analytical test results between pre- and post-change products may require functional testing to establish the biological or clinical significance of the observed difference. An underlying principle of comparability is that under certain conditions, protein products may be considered comparable on the basis of analytical testing results alone. However, the ability to compare biological materials is solely dependent on the tests used, since no single analytical method is able to compare every aspect of protein structure or function. The advantages and disadvantages of any given method depends on the protein property being characterized.

Minimizing the immunogenicity of protein therapeutics

As is cautioned in many package inserts, 'with all therapeutic proteins, there is a potential for immunogenicity'. Immunogenicity problems in humans, which currently can be detected only in clinical trials or after product launch, pose a significant barrier to the development and acceptance of protein drugs. Recent and ongoing research, presented in this review, seeks to address the challenge of protein therapeutic immunogenicity by elucidating the mechanisms underlying immune recognition of protein therapeutics, establishing preclinical methods for assessing immunogenicity and developing strategies for minimizing immune responses.