Allergens displayed on virus-like particles are highly immunogenic but fail to activate human mast cells.

BACKGROUND: The goal of allergen-specific immunotherapy is the induction of protective immune responses in the absence of anaphylactic reactions. We have previously shown that Fel d 1, the major cat allergen, displayed in a repetitive fashion on virus-like particles (VLPs) may fulfill these criteria. Specifically, Fel d 1 on VLPs induced strongly increased protective IgG responses compared to free allergen in mice while anaphylactic reactions were essentially abolished. Here we extend these findings to human mast cells and offer a mechanistic explanation for the reduced anaphylactic activity. METHODS: We differentiated human mast cells in vitro from blood-derived stem cell progenitors and sensitized the cells with a monoclonal Fel d 1-specific IgE. We compared the capability of Fel d 1 to induce mast cell activation in its free form versus displayed on VLPs and we performed allergen binding studies by surface plasmon resonance as well as flow cytometry. RESULTS: We show that free Fel d 1 induces degranulation of IgE-sensitized mast cells whereas Fel d 1 displayed on VLPs fails to induce mast cell activation. We demonstrate that this inability to activate mast cells is based on a biophysical as well as a biochemical mechanism. Firstly, Fel d 1 on VLPs showed a strongly impaired ability to bind to surface-bound IgE. Secondly, despite residual binding, repetitively displayed allergen on VLPs failed to cause mast cell activation. CONCLUSION: These findings indicate that repetitively displaying allergens on VLPs increases their immunogenicity while reducing their potential to cause anaphylactic reactions by essentially eliminating IgE-mediated activation of mast cells.

Allergen-specific immunotherapy: is it vaccination against toxins after all?

IgE-mediated allergies, in particular allergic rhinoconjunctivitis and asthma, have reached epidemic proportions, affecting about one-third of the population in developed countries. The most effective treatment for allergies is specific immunotherapy (SIT), which involves the injection of increasing doses of an allergen extract to allergic individuals. The current form of SIT was first introduced in 1911 and recently celebrated its 100th birthday for the treatment of hay fever. The concept of this therapy at the time was straightforward, as it was believed that pollen contained toxins against which the patient could be vaccinated. However, the understanding became blurred with the discovery that IgE antibodies were the effector molecules of the allergic response. Subsequent research focused on the idea that SIT should induce tolerance keeping the IgE antibodies at bay. In this review, we will discuss the various hypotheses for the mechanism of SIT and we will put forward the concept that allergens may be viewed as 'protoxins' which need to be activated by IgE antibodies. Within this framework, protoxin-neutralizing antibodies are the key effector molecules while a shift to Th1 or Treg cells mainly contributes to the efficacy of SIT by helping B cells to produce neutralizing IgG antibodies.

IgG-mediated down-regulation of IgE bound to mast cells: a potential novel mechanism of allergen-specific desensitization.

BACKGROUND: Allergen-specific IgGs are known to inhibit IgE-mediated mast cell degranulation by two mechanisms, allergen-neutralization and engagement of the inhibitory FcγRIIB recruiting the phosphatase SHIP-1. Here we unravel an additional mechanism of IgG-mediated mast cell desensitization in mice: down-regulation of allergen-specific IgE. METHODS: Mast cells were loaded in vitro and in vivo with monoclonal IgE antibodies specific for Fel d1 and exposed to immune complexes consisting of Fel d1-specific IgG antibodies recognizing different epitopes. Down regulation of IgE was followed by flow cytometry. RESULTS: Mast cells loaded with 2 different IgE antibodies efficiently internalized the IgE antibodies if exposed to recombinant Feld d1. In contrast, no down-regulation occurred if mast cells were loaded with IgE antibodies exhibiting a single specificity before stimulation with recombinant Fel d1 [corrected]. Interestingly, however, IgEs of a single specificity were rapidly down-regulated in vitro and in vivo in the presence of Fel d1-specific monoclonal IgGs recognizing another epitope on Fel d1. Despite FceRI-internalization, little calcium flux or mast cell degranulation occurred. FcγRIIB played a dual role in the process since it enhanced IgE internalization and prevented cellular activation as documented by the inhibited calcium flux and mast cell degranulation. Similar observations were made in the presence of low concentrations of IgEs recognizing several epitopes on Fel d1. CONCLUSION: We demonstrate here that Fel d1-specific IgG antibodies interact with FcγRIIB which (i) promotes IgE internalization; and (ii) inhibits mast cell activation. These results broaden our understanding of allergen-specific desensitization and may provide a mechanism for long-term desensitization of mast cells by selective removal of long-lived IgE antibodies on mast cells.

Assessment of clinical efficacy of CYT003-QbG10 in patients with allergic rhinoconjunctivitis: a phase IIb study.

BACKGROUND: Allergic symptoms are generally caused by exposure to substances to which people have become sensitized. Associated with this is an 'unbalanced' Th1/Th2 immune response with T cell responses skewed towards the production of Th2 cytokines, IL-4, 5, and 13 and high levels of IgE antibodies. Current immune modulating therapies require the use of allergens, carrying the risk to induce potentially severe allergic reactions. OBJECTIVE: Goal of the present study was to assess the safety and efficacy of an allergen-free immune modulator in patients suffering from perennial allergy. METHODS: In order to be protected from immediate degradation upon injection, a toll-like receptor 9 (TLR9) agonist was packaged into virus-like particles. These nanoparticles loaded with TLR9 ligands (CYT003-QbG10) were injected six times, at weekly intervals, into patients with house dust mite allergy in an attempt to ameliorate allergic symptoms by modifying the immune response towards allergens. Two different doses were compared against placebo in this double-blind, randomized phase IIb study (n=299). Public trial registry: http://clinicaltrials.gov (NCT00800332). RESULTS: The treatment was safe and generally well tolerated. Rhinoconjunctivitis symptoms were significantly lower in patients treated with high dose of CYT003-QbG10 as compared with placebo (scores 0.31 vs. 0.52, P=0.04) based on a standardized average combined symptom and medication score. Furthermore, patients in the high dose group reported a significantly better quality of life score post-treatment than patients on placebo (scores 0.71 vs. 1.21, P=0.02). The conjunctival provocation test revealed a median 10-fold increase in allergen tolerance in the high dose group while in the placebo group it remained unchanged. CONCLUSION AND CLINICAL RELEVANCE: Treatment with high-dose CYT003-QbG10 improved disease symptoms and reduced medication use in allergic individuals thus providing first evidence for a new potential immunotherapeutic.

T-cell independent IgM and enduring protective IgG antibodies induced by chimeric measles viruses.

B-cell activation depends on the intensity of B-cell receptor cross-linking. Studies of haptenated antigens and vesicular stomatitis virus (VSV) have demonstrated a correlation between antigen repetitiveness and the degree to which B-cell activation is independent of T cells. Here, we compare neutralizing antibody responses to inactivated VSV with those to two inactivated human pathogenic viruses: highly cytopathic poliovirus (PV) and poorly cytopathic measles virus (MV). The rigidly structured PV efficiently induced neutralizing IgM antibodies independent of T cells. In contrast, neutralizing antibodies to the pleomorphic MV were dependent on helper T cells. To test whether this resulted from the differences in virus structure or the capacity of MV to induce cell fusion and/or immunosuppression, we analyzed antibody responses to chimeric MV expressing VSV glycoprotein instead of MV fusion protein and hemagglutinin. IgM antibodies were independent of T cells; in addition, we found IgG responses dependent on T-cell help that were enduring and protective against lethal VSV infection. Because chimeric MV viruses look like MV ultrastructurally, we conclude that not only structural differences in the envelope but also the ability of MV to induce immunosuppression may limit its capacity to directly activate B cells. These findings are relevant for our understanding of B-cell activation by two prototypic human pathogenic viruses and for the design of new recombinant vaccines.

CD2 sets quantitative thresholds in T cell activation.

It has been proposed that CD2, which is highly expressed on T cells, serves to enhance T cell-antigen presenting cell (APC) adhesion and costimulate T cell activation. Here we analyzed the role of CD2 using CD2-deficient mice crossed with transgenic mice expressing a T cell receptor specific for lymphocytic choriomeningitis virus (LCMV)-derived peptide p33. We found that absence of CD2 on T cells shifted the p33-specific dose-response curve in vitro by a factor of 3-10. In comparison, stimulation of T cells in the absence of lymphocyte function-associated antigen (LFA)-1-intercellular adhesion molecule (ICAM)-1 interaction shifted the dose-response curve by a factor of 10, whereas absence of both CD2-CD48 and LFA-1-ICAM-1 interactions shifted the response by a factor of approximately 100. This indicates that CD2 and LFA-1 facilitate T cell activation additively. T cell activation at low antigen density was blocked at its very first steps, as T cell APC conjugate formation, TCR triggering, and Ca(2+) fluxes were affected by the absence of CD2. In vivo, LCMV-specific, CD2-deficient T cells proliferated normally upon infection with live virus but responded in a reduced fashion upon cross-priming. Thus, CD2 sets quantitative thresholds and fine-tunes T cell activation both in vitro and in vivo.

CD8(+) T cells mediate CD40-independent maturation of dendritic cells in vivo.

Induction of cytotoxic T lymphocyte (CTL) responses against minor histocompatibility antigens is dependent upon the presence of T cell help and requires the interaction of CD40 on dendritic cells (DCs) with CD40 ligand on activated T helper cells (Th). This study demonstrates that CD40 is neither involved in Th-dependent nor Th-independent antiviral CTL responses. Moreover, the data show that DC maturation occurs in vivo after viral infection in the absence of CD40 and Th. This maturation did not require viral infection of DCs but was mediated by peptide-specific CD8(+) T cells. Surprisingly, naive CD8(+) T cells were able to trigger DC maturation within 24 h after activation in vivo and in vitro. Moreover, peptide-activated CD8(+) T cells were able to induce maturation in trans, as DCs that failed to present the relevant antigen in vivo also underwent maturation. Upon isolation, the in vivo-stimulated DCs were able to convert a classically Th-dependent CTL response (anti-HY) into a Th-independent response in vitro. Thus, antiviral CD8(+) T cells are sufficient for the maturation of DCs in the absence of CD40.

Induction of long-lived germinal centers associated with persisting antigen after viral infection.

Vesicular stomatitis virus (VSV) induces an early T cell-independent neutralizing lgM response that is followed by a long-lived, T cell-dependent lgG response. We used the specific amplification factor of several 100x of VSV-virions for immunohistology to analyze the localization of VSV-specific B cells at different time points after immunization. At the peak of the IgM response (day 4), VSV-specific B cells were predominantly present in the red pulp and marginal zone but not in the T area. These B cells were mostly stained in the cytoplasm, characterizing them as antibody secreting cells. By day 6 after immunization, germinal centers (GC) containing surface-stained VSV-specific B cells became detectable and were fully established by day 12. At the same time, large VSV-specific B cell aggregates were present in the red pulp. High numbers of VSV-specific GC associated with persisting antigen were present 1 mo after immunization and later, i.e., considerably longer than has been observed for haptens. Some GC, exhibiting follicular dendritic cells and containing VSV-specific, proliferating B cells were still detectable up to 100 d after immunization. Long-lived GC were also observed after immunization with recombinant VSV-glycoprotein in absence of adjuvants. Thus some anti-virally protective (memory) B cells are cycling and locally proliferate in long-lived GC in association with persisting antigen and therefore seem responsible for long-term maintenance of elevated antibody levels. These observations extend earlier studies with carrier hapten antigens in adjuvant depots or complexed with specific IgG; they are the first to show colocalization of antigen and specific memory B cells and to analyze a protective neutralizing antibody response against an acute viral infection.

TRANCE, a tumor necrosis factor family member critical for CD40 ligand-independent T helper cell activation.

CD40 ligand (CD40L), a tumor necrosis factor (TNF) family member, plays a critical role in antigen-specific T cell responses in vivo. CD40L expressed on activated CD4(+) T cells stimulates antigen-presenting cells such as dendritic cells, resulting in the upregulation of costimulatory molecules and the production of various inflammatory cytokines required for CD4(+) T cell priming in vivo. However, CD40L- or CD40-deficient mice challenged with viruses mount protective CD4(+) T cell responses that produce normal levels of interferon gamma, suggesting a CD40L/CD40-independent mechanism of CD4(+) T cell priming that to date has not been elucidated. Here we show that CD4(+) T cell responses to viral infection were greatly diminished in CD40-deficient mice by administration of a soluble form of TNF-related activation-induced cytokine receptor (TRANCE-R) to inhibit the function of another TNF family member, TRANCE. Thus, the TRANCE/TRANCE-R interaction provides costimulation required for efficient CD4(+) T cell priming during viral infection in the absence of CD40L/CD40. These results also indicate that not even the potent inflammatory microenvironment induced by viral infections is sufficient to elicit efficient CD4(+) T cell priming without proper costimulation provided by the TNF family (CD40L or TRANCE). Moreover, the data suggest that TRANCE/TRANCE-R may be a novel and important target for immune intervention.

Developmental regulation of Lck targeting to the CD8 coreceptor controls signaling in naive and memory T cells.

The question of whether enhanced memory T cell responses are simply due to an increased frequency of specific cells or also to an improved response at the single cell level is widely debated. In this study, we analyzed T cell receptor (TCR) transgenic memory T cells and bona fide memory T cells isolated from virally infected normal mice using the tetramer technology. We found that memory T cells are qualitatively different from naive T cells due to a developmentally regulated rearrangement of the topology of the signaling machinery. In naive cytotoxic T cells, only a few CD8 molecules are associated with Lck and the kinase is homogeneously distributed inside the cell. However, in vivo priming of naive T cells induces the targeting of Lck to the CD8 coreceptor in the cell membrane and the consequent organization of a more efficient TCR signaling machinery in effector and memory cells.

Inducible costimulator protein (ICOS) controls T helper cell subset polarization after virus and parasite infection.

It has been shown that certain pathogens can trigger efficient T cell responses in the absence of CD28, a key costimulatory receptor expressed on resting T cells. Inducible costimulator protein (ICOS) is an inducible costimulator structurally and functionally related to CD28. Here, we show that in the absence of CD28 both T helper cell type 1 (Th1) and Th2 responses were impaired but not abrogated after infection with lymphocytic choriomeningitis virus (LCMV), vesicular stomatitis virus (VSV), and the nematode Nippostrongylus brasiliensis. Inhibition of ICOS in CD28-deficient mice further reduced Th1/Th2 polarization. Blocking of ICOS alone had a limited but significant capacity to downregulate Th subset development. In contrast, cytotoxic T lymphocyte (CTL) responses, which are regulated to a minor and major extent by CD28 after LCMV and VSV infection, respectively, remained unaffected by blocking ICOS. Together, our results demonstrate that ICOS regulates both CD28-dependent and CD28-independent CD4(+) subset (Th1 and Th2) responses but not CTL responses in vivo.

Crucial role of interferon consensus sequence binding protein, but neither of interferon regulatory factor 1 nor of nitric oxide synthesis for protection against murine listeriosis.

Listeria monocytogenes is widely used as a model to study immune responses against intracellular bacteria. It has been shown that neutrophils and macrophages play an important role to restrict bacterial replication in the early phase of primary infection in mice, and that the cytokines interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) are essential for protection. However, the involved signaling pathways and effector mechanisms are still poorly understood. This study investigated mouse strains deficient for the IFN-dependent transcription factors interferon consensus sequence binding protein (ICSBP), interferon regulatory factor (IRF) 1 or 2 for their capacity to eliminate Listeria in vivo and in vitro and for production of inducible reactive nitrogen intermediates (RNI) or reactive oxygen intermediates (ROI) in macrophages. ICSBP-/- and to a lesser degree also IRF2-/- mice were highly susceptible to Listeria infection. This correlated with impaired elimination of Listeria from infected peritoneal macrophage (PEM) cultures stimulated with IFN-gamma in vitro; in addition these cultures showed reduced and delayed oxidative burst upon IFN-gamma stimulation, whereas nitric oxide production was normal. In contrast, mice deficient for IRF1 were not able to produce nitric oxide, but they efficiently controlled Listeria in vivo and in vitro. These results indicate that (a) the ICSBP/IRF2 complex is essential for IFN-gamma-mediated protection against Listeria and that (b) ROI together with additional still unknown effector mechanisms may be responsible for the anti-Listeria activity of macrophages, whereas IRF1-induced RNI are not limiting.

Role of repetitive antigen patterns for induction of antibodies against antibodies.

Antibody responses against antibodies, such as rheumatoid factors, are found in several immunopathological diseases and may play a role in disease pathogenesis. Experience shows that they are usually difficult to induce experimentally. Antibodies specific for immunoglobulin constant regions (anti-allotypic) or for variable regions (anti-idiotypic) have been investigated in animal models; the latter have even been postulated to regulate antibody and T cell responses via network-like interactions. Why and how such anti-antibodies are induced during autoimmune diseases, has remained largely unclear. Because repetitively arranged epitopes in a paracrystalline structure of a viral envelope cross-link B cell receptors efficiently to induce a prompt T-independent IgM response, this study used immune complexes containing viruses or bacteria to evaluate the role of antigen pattern for induction of anti-antibody responses. We present evidence that antibodies bound to strictly ordered, but not to irregularly arranged, antigens dramatically enhance induction of anti-antibodies, already after a single immunization and without using adjuvants. The results indicate a novel link between anti-antibody responses and infectious agents, and suggest a similar role for repetitive self-antigens such as DNA or collagen involved in chronic immunopathological diseases.

CD40-CD40 ligand interactions are critical in T-B cooperation but not for other anti-viral CD4+ T cell functions.

CD40-CD40 ligand (CD40L) interaction is required for the generation of antibody responses to T-dependent antigens as well as for the development of germinal centers and memory B cells. The role of the CD40-CD40L interaction in the induction of antigen-specific. Th cells and in mediating Th cell effector functions other than cognate help for B cells is less well understood. Using CD40- and CD40L-deficient mice together with lymphocytic choriomeningitis virus and vesicular stomatitis virus as viral model antigens, this study corroborates earlier findings that no lg isotype switching of virus-specific antibodies was measurable upon infection of CD40- or CD40L-deficient mice. In contrast, in vivo induction of virus-specific CD4+ T cells measured by proliferation and cytokine secretion of primed virus-specific Th cells in vitro was not crucially dependent on the CD40-CD40L interaction. In addition, virus-specific Th cells primed in a CD40-deficient environment, adoptively transferred into CD40-competent recipients, were able to mediate lg isotype switch. Th-mediated effector functions distinct from and in addition to T-B collaboration were analyzed in CD40- and CD40L-deficient and normal mice: (a) local inflammatory reactions upon LCMV infection mediated by LCMV-specific Th cells were not dependent on a functional CD40-CD40L interaction, (b) cytokine-mediated protection by CD4+ T cells primed by vesicular stomatitis virus against a challenge infection with recombinant vaccinia virus expressing the glycoprotein of vesicular stomatitis virus was found to be equivalent in CD40L-deficient and normal mice. Thus, CD40-CD40L interaction plays a crucial role in T-B interactions for Th-dependent activation of B cells but not, or to a much lesser extent, in T cell activation, antigen-specific Th cell responses in vitro, and for interleukin-mediated Th cell effector functions in vivo.

Self antigens expressed by solid tumors Do not efficiently stimulate naive or activated T cells: implications for immunotherapy.

Induction and maintenance of cytotoxic T lymphocyte (CTL) activity specific for a primary endogenous tumor was investigated in vivo. The simian virus 40 T antigen (Tag) expressed under the control of the rat insulin promoter (RIP) induced pancreatic beta-cell tumors producing insulin, causing progressive hypoglycemia. As an endogenous tumor antigen, the lymphocytic choriomeningitis virus (LCMV) glycoprotein (GP) was introduced also under the control of the RIP. No significant spontaneous CTL activation against GP was observed. However, LCMV infection induced an antitumor CTL response which efficiently reduced the tumor mass, resulting in temporarily normalized blood glucose levels and prolonged survival of double transgenic RIP(GP x Tag2) mice (137 +/- 18 d) as opposed to control RIP-Tag2 mice (88 +/- 8 d). Surprisingly, the tumor-specific CTL response was not sustained despite the facts that the tumor cells continued to express MHC class I and LCMV-GP-specific CTLs were present and not tolerized. Subsequent adoptive transfer of virus activated spleen cells into RIP(GP x Tag2) mice further prolonged survival (168 +/- 11 d), demonstrating continued expression of the LCMV-GP tumor antigen and MHC class I. The data show that the tumor did not spontaneously induce or maintain an activated CTL response, revealing a profound lack of immunogenicity in vivo. Therefore, repetitive immunizations are necessary for prolonged antitumor immunotherapy. In addition, the data suggest that the risk for induction of chronic autoimmune diseases is limited, which may encourage immunotherapy against antigens selectively but not exclusively expressed by the tumor.

Vav regulates peptide-specific apoptosis in thymocytes.

The protooncogene Vav functions as a GDP/GTP exchange factor (GEF) for Rho-like small GTPases involved in cytoskeletal reorganization and cytokine production in T cells. Gene-targeted mice lacking Vav have a severe defect in positive and negative selection of T cell antigen receptor transgenic thymocytes in vivo, and vav-/- thymocytes are completely resistant to peptide-specific and anti-CD3/anti-CD28-mediated apoptosis. Vav acts upstream of mitochondrial pore opening and caspase activation. Biochemically, Vav regulates peptide-specific Ca2+ mobilization and actin polymerization. Peptide-specific cell death was blocked both by cytochalasin D inhibition of actin polymerization and by inhibition of protein kinase C (PKC). Activation of PKC with phorbol ester restored peptide-specific apoptosis in vav-/- thymocytes. Vav was found to bind constitutively to PKC-theta in thymocytes. Our results indicate that peptide-triggered thymocyte apoptosis is mediated via Vav activation, changes in the actin cytoskeleton, and subsequent activation of a PKC isoform.

LFA-1-deficient mice show normal CTL responses to virus but fail to reject immunogenic tumor.

The leukocyte integrin LFA-1 (CD11a/CD18) plays an important role in lymphocyte recirculation and homotypic interactions. Leukocytes from mice lacking CD11a displayed defects in in vitro homotypic aggregation, in proliferation in mixed lymphocyte reactions, and in response to mitogen. Mutant mice mounted normal cytotoxic T cell (CTL) responses against systemic LCMV and VSV infections and showed normal ex vivo CTL function. However, LFA-1-deficient mice did not reject immunogenic tumors grafted into footpads and did not demonstrate priming response against tumor-specific antigen. Thus CD11a deficiency causes a selective defect in induction of peripheral immune responses whereas responses to systemic infection are normal.

Use of A-type CpG oligodeoxynucleotides as an adjuvant in allergen-specific immunotherapy in humans: a phase I/IIa clinical trial.

BACKGROUND: B-type CpG oligodeoxynucleotides (ODN) is currently used in clinical trials because of its prolonged half-life, which is due to its phosphorothioate backbone. A-type CpG ODN is a stronger inducer of IFN but has an unstable phosphodiester backbone that has so far prohibited its clinical use. However, upon association with virus-like particles (VLP) consisting of the bacteriophage Qbeta coat protein, A-type CpG ODN can be stabilized and can become an efficient adjuvant in mice. Therefore, the phase I/IIa study presented represents the first test of A-type CpGs in humans. OBJECTIVE: To test the safety, tolerability and clinical efficacy of QbG10 as an adjuvant for subcutaneous immunotherapy with a house dust mite (HDM) allergen extract in allergic patients. METHODS: A single centre, open-label phase I/IIa study evaluated the safety, tolerability and clinical efficacy of QbG10 as an adjuvant to immunotherapy with a subcutaneous HMD allergen extract in 20 patients suffering from HDM allergy. Twenty-one patients were enrolled between March and July 2005. Individual immunotherapy lasted 10 weeks. Clinical end-points included questionnaires, conjunctival provocation, skin prick tests and the measurement of allergen-specific IgG and IgE. RESULTS: QbG10 was well tolerated. Almost complete tolerance to the allergen was observed in conjunctival provocation testing after treatment with QbG10, and symptoms of rhinitis and allergic asthma were significantly reduced. Within 10 weeks of therapy, patients were nearly symptom-free and this amelioration lasted for at least 38 weeks post-treatment. Following injections of QbG10 and HDM allergen extract, allergen-specific IgG increased, while there was a transient increase in allergen-specific IgE titres. Skin reactivity to HDM was reduced. CONCLUSION: The subcutaneous application of HDM allergen, together with A-type CpG ODN packaged into VLP, was safe. All patients achieved practically complete alleviation of allergy symptoms after 10 weeks of immunotherapy. This promising clinical outcome calls for larger placebo-controlled phase II studies.

Pattern recognition by B cells: the role of antigen repetitiveness versus Toll-like receptors.

Viruses induce excellent antibody responses due to several intrinsic features. Their repetitive, organised structure is optimal for the activation of the B cell receptor (BCR), leading to an increased humoral response and a decreased dependence on T cell help. Viruses also trigger Toll-like receptors (TLRs), which in addition to increasing overall Ig levels, drive the switch to the IgG2a isotype. This isotype is more efficient in viral and bacterial clearance and will activate complement, which in turn lowers the threshold of BCR activation. Exploiting these characteristics in vaccine design may help us to create vaccines which are as safe as a recombinant vaccine yet still as effective as a virus in inducing B cell responses.

The influence of antigen organization on B cell responsiveness.

The influence of antigen epitope density and order on B cell induction and antibody production was assessed with the glycoprotein of vesicular stomatitis virus serotype Indiana [VSV-G (IND)]. VSV-G (IND) can be found in a highly repetitive form the envelope of VSV-IND and in a poorly organized form on the surface of infected cells. In VSV-G (IND) transgenic mice, B cells were unresponsive to the poorly organized VSV-G (IND) present as self antigen but responded promptly to the same antigen presented in the highly organized form. Thus, antigen organization influences B cell tolerance.