Enhanced avidity maturation of antibody to human immunodeficiency virus envelope: DNA vaccination with gp120-C3d fusion proteins

Fusion of the molecular adjuvant C3d to cleavage-independent native-like HIV-1 Env trimers improves the elicited antibody response

An effective HIV vaccine likely requires the elicitation of neutralizing antibodies (NAbs) against multiple HIV-1 clades. The recently developed cleavage-independent native flexibly linked (NFL) envelope (Env) trimers exhibit well-ordered conformation and elicit autologous tier 2 NAbs in multiple animal models. Here, we investigated whether the fusion of molecular adjuvant C3d to the Env trimers can improve B- cell germinal center (GC) formation and antibody responses. To generate Env-C3d trimers, we performed a glycine-serine- based (G4S) flexible peptide linker screening and identified a linker range that allowed native folding. A 30-60- amino- acid- long linker facilitates Env-to-C3d association and achieves the secretion of well-ordered trimers and the structural integrity and functional integrity of Env and C3d. The fusion of C3d did not dramatically affect the antigenicity of the Env trimers and enhanced the ability of the Env trimers to engage and activate B cells in vitro. In mice, the fusion of C3d enhanced germinal center formation, the magnitude of Env-specific binding antibodies, and the avidity of the antibodies in the presence of an adjuvant. The Sigma Adjuvant System (SAS) did not affect the trimer integrity in vitro but contributed to altered immunogenicity in vivo, resulting in increased tier 1 neutralization, likely by increased exposure of variable region 3 (V3). Taken together, the results indicate that the fusion of the molecular adjuvant, C3d, to the Env trimers improves antibody responses and could be useful for Env-based vaccines against HIV.

Profiling the Quality and Quantity of Naturally Induced Antibody Responses Against Pfs230 and Pfs48/45 Among Non-Febrile Children Living in Southern Ghana: A Longitudinal Study

A clear understanding of the properties of naturally induced antibody responses against transmission-blocking vaccine candidates can accelerate the understanding of the development of transmission-blocking immunity. This study characterized the naturally induced IgG responses against two leading transmission-blocking vaccine antigens, Pfs230 and Pfs48/45, in non-febrile children living in Simiw, Ghana. Consecutive sampling was used to recruit 84 non-febrile children aged from 6 to 12 years old into the 6-month (November 2017 until May 2018) longitudinal study. Venous blood (1 ml) was collected once every 2 months and used to determine hemoglobin levels, P. falciparum prevalence using microscopy and polymerase chain reaction, and the levels and relative avidity of IgG responses against Pfs230 and Pfs48/45 using indirect ELISA. IgG levels against Pfs230 and Pfs48/45 decreased from the start (November) to the middle (January) and end (March) of the dry season respectively, then they began to increase. Participants, especially older children (10-12 years old) with active infections generally had lower antibody levels against both antigens. The relative avidities of IgG against both antigens followed the trend of IgG levels until the middle of the dry season, after which the relative avidities of both antigens correlated inversely with the antibody levels. In conclusion, although IgG antibody levels against both Pfs48/45 and Pfs230 began to increase by the early rainy season, they were inversely correlated to their respective relative avidities.

Modular complement assemblies for mitigating inflammatory conditions

Complement protein C3dg, a key linkage between innate and adaptive immunity, is capable of stimulating both humoral and cell-mediated immune responses, leading to considerable interest in its use as a molecular adjuvant. However, the potential of C3dg as an adjuvant is limited without ways of controllably assembling multiple copies of it into vaccine platforms. Here, we report a strategy to assemble C3dg into supramolecular nanofibers with excellent compositional control, using β-tail fusion tags. These assemblies were investigated as therapeutic active immunotherapies, which may offer advantages over existing biologics, particularly toward chronic inflammatory diseases. Supramolecular assemblies based on the Q11 peptide system containing β-tail-tagged C3dg, B cell epitopes from TNF, and the universal T cell epitope PADRE raised strong antibody responses against both TNF and C3dg, and prophylactic immunization with these materials significantly improved protection in a lethal TNF-mediated inflammation model. Additionally, in a murine model of psoriasis induced by imiquimod, the C3dg-adjuvanted nanofiber vaccine performed as well as anti-TNF monoclonal antibodies. Nanofibers containing only β-tail-C3dg and lacking the TNF B cell epitope also showed improvements in both models, suggesting that supramolecular C3dg, by itself, played an important therapeutic role. We observed that immunization with β-tail-C3dg caused the expansion of an autoreactive C3dg-specific T cell population, which may act to dampen the immune response, preventing excessive inflammation. These findings indicate that molecular assemblies displaying C3dg warrant further development as active immunotherapies.

Immunogenicity of HIV-1-Based Virus-Like Particles with Increased Incorporation and Stability of Membrane-Bound Env

An optimal prophylactic vaccine to prevent human immunodeficiency virus (HIV-1) transmission should elicit protective antibody responses against the HIV-1 envelope glycoprotein (Env). Replication-incompetent HIV-1 virus-like particles (VLPs) offer the opportunity to present virion-associated Env with a native-like structure during vaccination that closely resembles that encountered on infectious virus. Here, we optimized the incorporation of Env into previously designed mature-form VLPs (mVLPs) and assessed their immunogenicity in mice. The incorporation of Env into mVLPs was increased by replacing the Env transmembrane and cytoplasmic tail domains with those of influenza haemagglutinin (HA-TMCT). Furthermore, Env was stabilized on the VLP surface by introducing an interchain disulfide and proline substitution (SOSIP) mutations typically employed to stabilize soluble Env trimers. The resulting mVLPs efficiently presented neutralizing antibody epitopes while minimizing exposure of non-neutralizing antibody sites. Vaccination of mice with mVLPs elicited a broader range of Env-specific antibody isotypes than Env presented on immature VLPs or extracellular vesicles. The mVLPs bearing HA-TMCT-modified Env consistently induced anti-Env antibody responses that mediated modest neutralization activity. These mVLPs are potentially useful immunogens for eliciting neutralizing antibody responses that target native Env epitopes on infectious HIV-1 virions.

B cell targeting by molecular adjuvants for enhanced immunogenicity

INTRODUCTION

Adjuvants are critical components of vaccines to improve the quality and durability of immune responses. Molecular adjuvants are a specific subclass of adjuvants where ligands of known immune-modulatory receptors are directly fused to an antigen. Co-stimulation of the B cell receptor (BCR) and immune-modulatory receptors through this strategy can augment downstream signaling to improve antibody titers and/or potency, and survival in challenge models.

AREAS COVERED

C3d has been the most extensively studied molecular adjuvant and shown to improve immune responses to a number of antigens. Similarly, tumor necrosis superfamily ligands, such as BAFF and APRIL, as well as CD40, CD180, and immune complex ligands can also improve humoral immunity as molecular adjuvants.

EXPERT OPINION

However, no single strategy has emerged that improves immune outcomes in all contexts. Thus, systematic exploration of molecular adjuvants that target B cell receptors will be required to realize their full potential as next-generation vaccine technologies.

Conjugation of an scFab domain to the oligomeric HIV envelope protein for use in immune targeting

A promising strategy for the enhancement of vaccine-mediated immune responses is by directly targeting protein antigens to immune cells. Targeting of antigens to the dendritic cell (DC) molecule Clec9A has been shown to enhance antibody affinity and titers for model antigens, and influenza and enterovirus antigens, and may be advantageous for immunogens that otherwise fail to elicit antibodies with sufficient titers and breadth for broad protection, such as the envelope protein (Env) of HIV. Previously employed targeting strategies often utilize receptor-specific antibodies, however it is impractical to conjugate a bivalent IgG antibody to oligomeric antigens, including HIV Env trimers. Here we designed single chain variable fragment (scFv) and single chain Fab (scFab) constructs of a Clec9A-targeting antibody, expressed as genetically fused conjugates with the soluble ectodomain of Env, gp140. This conjugation did not affect the presentation of Env neutralising antibody epitopes. The scFab moiety was shown to be more stable than scFv, and in the context of gp140 fusions, was able to mediate better binding to recombinant and cell surface-expressed Clec9A, although the level of binding to cell-surface Clec9A was lower than that of the anti-Clec9A IgG. However, binding to Clec9A on the surface of DCs was not detected. Mouse immunization experiments suggested that the Clec9A-binding activity of the scFab-gp140 conjugate was insufficient to enhance Env-specific antibody responses. This is an important first proof of principle study demonstrating the conjugation of a scFab to an oligomeric protein antigen, and that an scFab displays better antigen binding than the corresponding scFv. Future developments of this technique that increase the scFab affinity will provide a valuable means to target oligomeric proteins to cell surface antigens of interest, improving vaccine-generated immune responses.

How to assess the binding strength of antibodies elicited by vaccination against HIV and other viruses

Vaccines that protect against viral infections generally induce neutralizing antibodies. When vaccines are evaluated, the need arises to assess the affinity maturation of the antibody responses. Binding titers of polyclonal sera depend not only on the affinities of the constituent antibodies but also on their individual concentrations, which are difficult to ascertain. Therefore an assay based on chaotrope disruption of antibody-antigen complexes was designed for measuring binding strength. This assay works well with many viral antigens but gives differential results depending on the conformational dependence of epitopes on complex antigens such as the envelope glycoprotein of HIV-1. Kinetic binding assays might offer alternatives, since they can measure average off-rate constants for polyclonal antibodies in a serum. Here, potentials and fallacies of these techniques are discussed.

What Do Chaotrope-Based Avidity Assays for Antibodies to HIV-1 Envelope Glycoproteins Measure?

When HIV-1 vaccine candidates that include soluble envelope glycoproteins (Env) are tested in humans and other species, the resulting antibody responses to Env are sifted for correlates of protection or risk. One frequently used assay measures the reduction in antibody binding to Env antigens by an added chaotrope (such as thiocyanate). Based on that assay, an avidity index was devised for assessing the affinity maturation of antibodies of unknown concentration in polyclonal sera. Since a high avidity index was linked to protection in animal models of HIV-1 infection, it has become a criterion for evaluating antibody responses to vaccine candidates. But what does the assay measure and what does an avidity index mean? Here, we have used a panel of monoclonal antibodies to well-defined epitopes on Env (gp120, gp41, and SOSIP.664 trimers) to explore how the chaotrope acts. We conclude that the chaotrope sensitivity of antibody binding to Env depends on several properties of the epitopes (continuity versus tertiary- and quaternary-structural dependence) and that the avidity index has no simple relationship to antibody affinity for functional Env spikes on virions. We show that the binding of broadly neutralizing antibodies against quaternary-structural epitopes is particularly sensitive to chaotrope treatment, whereas antibody binding to epitopes in variable loops and to nonneutralization epitopes in gp41 is generally resistant. As a result of such biases, the avidity index may at best be a mere surrogate for undefined antibody or other immune responses that correlate weakly with protection.

IMPORTANCE

An effective HIV-1 vaccine is an important goal. Such a vaccine will probably need to induce antibodies that neutralize typically transmitted variants of HIV-1, preventing them from infecting target cells. Vaccine candidates have so far failed to induce such antibody responses, although some do protect weakly against infection in animals and, possibly, humans. In the search for responses associated with protection, an avidity assay based on chemical disruption is often used to measure the strength of antibody binding. We have analyzed this assay mechanistically and found that the epitope specificity of an antibody has a greater influence on the outcome than does its affinity. As a result, the avidity assay is biased toward the detection of some antibody specificities while disfavoring others. We conclude that the assay may yield merely indirect correlations with weak protection, specifically when Env vaccination has failed to induce broad neutralizing responses.

DNA vaccine molecular adjuvants SP-D-BAFF and SP-D-APRIL enhance anti-gp120 immune response and increase HIV-1 neutralizing antibody titers

Broadly neutralizing antibodies (bNAbs) specific for conserved epitopes on the HIV-1 envelope (Env) are believed to be essential for protection against multiple HIV-1 clades. However, vaccines capable of stimulating the production of bNAbs remain a major challenge. Given that polyreactivity and autoreactivity are considered important characteristics of anti-HIV bNAbs, we designed an HIV vaccine incorporating the molecular adjuvants BAFF (B cell activating factor) and APRIL (a proliferation-inducing ligand) with the potential to facilitate the maturation of polyreactive and autoreactive B cells as well as to enhance the affinity and/or avidity of Env-specific antibodies. We designed recombinant DNA plasmids encoding soluble multitrimers of BAFF and APRIL using surfactant protein D as a scaffold, and we vaccinated mice with these molecular adjuvants using DNA and DNA-protein vaccination strategies. We found that immunization of mice with a DNA vaccine encoding BAFF or APRIL multitrimers, together with interleukin 12 (IL-12) and membrane-bound HIV-1 Env gp140, induced neutralizing antibodies against tier 1 and tier 2 (vaccine strain) viruses. The APRIL-containing vaccine was particularly effective at generating tier 2 neutralizing antibodies following a protein boost. These BAFF and APRIL effects coincided with an enhanced germinal center (GC) reaction, increased anti-gp120 antibody-secreting cells, and increased anti-gp120 functional avidity. Notably, BAFF and APRIL did not cause indiscriminate B cell expansion or an increase in total IgG. We propose that BAFF and APRIL multitrimers are promising molecular adjuvants for vaccines designed to induce bNAbs against HIV-1.

IMPORTANCE

Recent identification of antibodies that neutralize most HIV-1 strains has revived hopes and efforts to create novel vaccines that can effectively stimulate HIV-1 neutralizing antibodies. However, the multiple immune evasion properties of HIV have hampered these efforts. These include the instability of the gp120 trimer, the inaccessibility of the conserved sequences, highly variable protein sequences, and the loss of HIV-1-specific antibody-producing cells during development. We have shown previously that tumor necrosis factor (TNF) superfamily ligands, including BAFF and APRIL, can be multitrimerized using the lung protein SP-D (surfactant protein D), enhancing immune responses. Here we show that DNA or DNA-protein vaccines encoding BAFF or APRIL multitrimers, IL-12p70, and membrane-bound HIV-1 Env gp140 induced tier 1 and tier 2 neutralizing antibodies in a mouse model. BAFF and APRIL enhanced the immune reaction, improved antibody binding, and increased the numbers of anti-HIV-1 antibody-secreting cells. Adaptation of this vaccine design may prove useful in designing preventive HIV-1 vaccines for humans.

Protective immunity against enteral stages of Trichinella spiralis elicited in mice by live attenuated Salmonella vaccine that secretes a 30-mer parasite epitope fused to the molecular adjuvant C3d-P28

The development of a veterinary vaccine against T. spiralis infection is an alternative strategy to control trichinellosis. In an effort to develop an efficient vaccine, BALB/c mice were immunized with attenuated Salmonella enterica serovar Typhimurium SL3261 that expresses a 30-mer peptide (Ag30) derived from the gp43 of T. spiralis muscle larvae fused to three copies of the molecular adjuvant P28 (Ag30-P283) and it was either displayed on the surface or secreted by recombinant Salmonella strains. Salmonella strain secreting Ag30-P283, reduced the adult worm burden 92.8% following challenge with T. spiralis muscle larvae compared to 42% achieved by recombinant Salmonella displaying Ag30-P283 on the surface. The protection induced by secreted Ag30-P283 was associated with a mixed Th1/Th2 with predominance of Th2 phenotype, which was characterized by the production of IgG1, intestinal IgA antibodies and IL-5 secretion. This finding could provide an efficient platform technology for the design of novel vaccination strategies.

DNA Immunization for HIV Vaccine Development

DNA vaccination has been studied in the last 20 years for HIV vaccine research. Significant experience has been accumulated in vector design, antigen optimization, delivery approaches and the use of DNA immunization as part of a prime-boost HIV vaccination strategy. Key historical data and future outlook are presented. With better understanding on the potential of DNA immunization and recent progress in HIV vaccine research, it is anticipated that DNA immunization will play a more significant role in the future of HIV vaccine development.

DENV-2 subunit proteins fused to CR2 receptor-binding domain (P28)-induces specific and neutralizing antibodies to the Dengue virus in mice

Domain III (DIII) of the dengue virus (DENV) envelope (E) protein induces strong neutralizing type-specific antibodies. In addition, a region near the fusion loop in domain II (DII) induces the production of cross-reactive antibodies with neutralizing potential. Thus, this study aimed to generate DENV-2 recombinant fusion proteins (i.e., rEII*EIII and rEII*EIII/NS1*) either alone or fused to 3 copies of P28, the minimum CR2-binding domain of the complement protein C3d. The 4 recombinant proteins were generated in a Drosophila melanogaster Schneider 2 (S2) cell system. The expression and secretion of the recombinant proteins were confirmed in vitro using immunofluorescence (IF) and western blot (WB) analyses. Human dengue immune serum samples recognized recombinant proteins. The immunogenicity of the 4 proteins in BALB/c mice was analyzed using ELISA, and the results revealed that the induced specific antibody response was higher in the groups of mice immunized with the P28 fusion proteins. Interestingly, although the 4 recombinant proteins were able to elicit high levels of neutralizing antibodies in BALB/c mice; no adjuvant effect was observed in terms of neutralizing antibodies in the groups immunized with proteins containing P28. Thus, ELISA and PRNT50 assays may evaluate different epitopes and responses, where ELISA showed a wider response that did not always correlate with neutralization. Furthermore, the elicited antibodies were able to recognize the immobilized E glycoprotein of DENV. All mice vaccinated with the DENV-2 recombinant proteins showed induction of higher levels of IgG1 antibodies than of IgG2a antibodies.

The MSHA strain of Pseudomonas aeruginosa activated TLR pathway and enhanced HIV-1 DNA vaccine immunoreactivity

The mannose-sensitive hemagglutination pilus strain of Pseudomonas aeruginosa (PA-MSHA) has been shown to trigger naïve immune responses through the activation of monocytes, macrophages, natural killer cells (NK cells) and antigen presenting cells (APCs). Based on the hypothesis that PA-MSHA activates natural immunity through the Toll-like receptor (TLR) pathway, we scanned several critical TLR pathway molecules in mouse splenocytes using high-throughput real-time QRT-PCR and co-stimulatory molecule in bone marrow-derived dendritic cells (BMDCs) following in vitro stimulation by PA-MSHA. PA-MSHA enabled activation of the TLR pathway mediated by NF-κB and JNK signaling in splenocytes, and the co-stimulatory molecule CD86 was up-regulated in BMDCs. We then assessed the adjuvant effect of PA-MSHA for HIV-1 DNA vaccines. In comparison to DNA inoculation alone, co-inoculation with low dosage of PA-MSHA enhanced specific immunoreactivity against HIV-1 Env in both cellular and humoral responses, and promoted antibody avidity maturation. However, high doses of adjuvant resulted in an immunosuppressive effect; a two- or three-inoculation regimen yielded low antibody responses and the two-inoculation regimen exhibited only a slight cellular immunity response. To our knowledge, this is the first report demonstrating the utility of PA-MSHA as an adjuvant to a DNA vaccine. Further research is needed to investigate the exact mechanisms through which PA-MSHA achieves its adjuvant effects on innate immune responses, especially on dendritic cells.

Heterologous prime-boost-boost immunisation of Chinese cynomolgus macaques using DNA and recombinant poxvirus vectors expressing HIV-1 virus-like particles

Background

There is renewed interest in the development of poxvirus vector-based HIV vaccines due to the protective effect observed with repeated recombinant canarypox priming with gp120 boosting in the recent Thai placebo-controlled trial. This study sought to investigate whether a heterologous prime-boost-boost vaccine regimen in Chinese cynomolgus macaques with a DNA vaccine and recombinant poxviral vectors expressing HIV virus-like particles bearing envelopes derived from the most prevalent clades circulating in sub-Saharan Africa, focused the antibody response to shared neutralising epitopes.

Methods

Three Chinese cynomolgus macaques were immunised via intramuscular injections using a regimen composed of a prime with two DNA vaccines expressing clade A Env/clade B Gag followed by boosting with recombinant fowlpox virus expressing HIV-1 clade D Gag, Env and cholera toxin B subunit followed by the final boost with recombinant modified vaccinia virus Ankara expressing HIV-1 clade C Env, Gag and human complement protein C3d. We measured the macaque serum antibody responses by ELISA, enumerated T cell responses by IFN-γ ELISpot and assessed seroneutralisation of HIV-1 using the TZM-bl β-galactosidase assay with primary isolates of HIV-1.

Results

This study shows that large and complex synthetic DNA sequences can be successfully cloned in a single step into two poxvirus vectors: MVA and FPV and the recombinant poxviruses could be grown to high titres. The vaccine candidates showed appropriate expression of recombinant proteins with the formation of authentic HIV virus-like particles seen on transmission electron microscopy. In addition the b12 epitope was shown to be held in common by the vaccine candidates using confocal immunofluorescent microscopy. The vaccine candidates were safely administered to Chinese cynomolgus macaques which elicited modest T cell responses at the end of the study but only one out of the three macaques elicited an HIV-specific antibody response. However, the antibodies did not neutralise primary isolates of HIV-1 or the V3-sensitive isolate SF162 using the TZM-bl β-galactosidase assay.

Conclusions

MVA and FP9 are ideal replication-deficient viral vectors for HIV-1 vaccines due to their excellent safety profile for use in humans. This study shows this novel prime-boost-boost regimen was poorly immunogenic in Chinese cynomolgus macaques.

Low concentrations of anti-Aβ antibodies generated in Tg2576 mice by DNA epitope vaccine fused with 3C3d molecular adjuvant do not affect AD pathology

It has been demonstrated that an active vaccination strategy with protein- or DNA-based epitope vaccines composed of the immunodominant self B cell epitope of amyloid-β₄₂ (Aβ₄₂) and a non-self T helper (Th) cell epitope is an immunotherapeutic approach to preventing or treating Alzheimer's disease (AD). As a DNA-based epitope vaccine, we used a plasmid encoding three copies of Aβ(1-11) and Th cell epitope, PADRE (p3Aβ(1-11)-PADRE). We have previously reported that three copies of component of complement C3d (3C3d) acts as a molecular adjuvant significantly enhancing immune responses in wild-type mice of the H2(b) haplotype immunized with p3Aβ(1-11)-PADRE. Here, we tested the efficacy of p3Aβ(1-11)-PADRE and the same vaccine fused with 3C3d (p3Aβ(1-11)-PADRE-3C3d) in a transgenic (Tg) mouse model of AD (Tg2576) of the H2(bxs) immune haplotype. The overall responses to both vaccines were very weak in Tg2576 mice despite the fact that the 3C3d molecular adjuvant significantly enhanced the anti-Aβ response to 3Aβ(1-11)-PADRE. Importantly, generation of low antibody responses was associated with the strain of amyloid precursor protein Tg mice rather than with a molecular adjuvant, as a p3Aβ(1-11)-PADRE-3C3d vaccine induced significantly higher antibody production in another AD mouse model, 3xTg-AD of the H2(b) haplotype. Finally, this study demonstrated that low concentrations of antibodies generated by both DNA vaccines were not sufficient for the reduction of Aβ pathology in the brains of vaccinated Tg2576 animals, confirming previous reports from preclinical studies and the AN-1792 clinical trials, which concluded that the concentration of anti-Aβ antibodies may be essential for the reduction of AD pathology.

Dense display of HIV-1 envelope spikes on the lambda phage scaffold does not result in the generation of improved antibody responses to HIV-1 Env

The generation of strong, virus-neutralizing antibody responses to the HIV-1 envelope spike (Env) is a major goal in HIV-1 vaccine research. To try to enhance the Env-specific response, we displayed oligomeric gp140 on a virus-like scaffold provided by the lambda phage capsid. To do this, an in vitro complementation system was used to "decorate" phage particles with glycosylated, mammalian cell-derived envelope oligomers. We compared the immune response to lambda phage particles displaying HIV-1 Env to that elicited by soluble oligomeric gp140 in rabbits. Env-binding antibody titers were higher in animals that received oligomeric gp140 as compared to Env decorated phage particles, as were virus neutralizing antibody responses. The Env decorated phage particles were, however, able to efficiently boost a protein-primed humoral response to levels equivalent to those elicited by high-dose adjuvanted Env oligomers. These results show that display of HIV-1 envelope spikes on the bacteriophage lambda capsid does not result in an improved, Env-specific humoral immune response.

Construction and immunogenicity of DNA vaccines encoding fusion protein of murine complement C3d-p28 and GP5 gene of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) has recently caused catastrophic losses in swine industry worldwide. The commercial vaccines only provide a limited protection against PRRSV infection. At present, DNA vaccine is the focus on the new vaccines. The gene fragment (p28) coding for the molecular adjuvants complement protein C3d (mC3d) from BALB/c mouse was cloned and expressed as a fusion protein for its application in the vaccine study of mice. Three potential vaccines construct units were engineered to contain two, four and six copies of mC3d-p28 coding gene linked to the GP5 gene of PRRSV and one vaccine expressing GP5 alone (pcDNA3.1-GP5) was constructed. Subsequently, the vaccines' abilities to elicit the humoral and cellular immune responses were investigated in mice. These results showed that significantly enhanced GP5-specific ELISA antibody, GP5-specific neutralizing antibody, IFN-γ level, and IL-4 level, could be induced in mice immunized with DNA construct units encoding the pcDNA3.1-C3d-p28.n-GP5 than those received DNA vaccine expressing GP5 alone (pcDNA3.1-GP5). Analysis of the immunogenicity of different repeats of mC3d-p28 revealed that mC3d-p28 had an enhancing effect on the immunogenicity of antigens, and that six or more repeats of mC3d-p28 may be necessary for efficient enhancement of antigen specific immune responses. This approach may provide a new strategy for the development of efficient vaccines against the PRRSV for pigs in the future.

Contribution of Chinese Pekin duck complement component C3d-P29 repeats to enhancement of Th2-biased immune responses against NDV F gene induced by DNA immunization

BACKGROUND AND AIM

C3d, a split product of C3, interacts with its receptor (CR2 or CD21) on B cells and follicular dendritic cells (FDCs) and is crucial for induction and maintenance of a normal humoral immune response. This fragment of complement protein C3 (C3d) has also been shown to enhance B cell responses when complexed with antigen and C3d fusion increased Th2-biased immune response by inducing IL-4 production.

MATERIALS AND METHODS

The gene fragment coding for Chinese Pekin duck (Anas platyrhynchos) C3d gene (duC3d) was cloned and expressed as a component of fusion proteins destined for use in in vitro experiments. Two, four and six copies of CR2-binding domain duC3d-P29 were fused, respectively, to truncated Newcastle disease virus (NDV) F gene encoding soluble glycoprotein F in pcDNA3.1.All recombinant proteins were analyzed by SDS-PAGE and Western immunoblot. BALB/c mice were, respectively, immunized with recombinant plasmids, blank vector, and inactivated vaccine.

RESULTS

The result of immunogenicity detections of The IL-4 level for F-C3d-P29.6 DNA immunization approached that for the inactivated vaccine. Compared to C3d-P29.6, C3d-P29.4 enhanced F DNA immunogenicity to a lesser extent. Furthermore, C3d-P29.n fusion increased Th2-biased immune response by inducing IL-4 production.

CONCLUSION

We demonstrated that C3d-P29 could enhance immunogenicity by directing Th1-biased to a balanced and more effective Th1/Th2 response. The expression of the duck C3d fusion proteins in this study which was the first reported, and the detections of the cytokine level for F-C3d-P29.n in DNA immunization using the BALB/c mice as the model animal, will provide the basis for immunization trials in chicken or other poultry, studies of receptor binding and cell activation of animal lymphocytes, and investigations of new types of vaccine, including genetic recombinant and DNA vaccines for the future against relevant pathogens.

Vaccination with DNA plasmids expressing Gn coupled to C3d or alphavirus replicons expressing gn protects mice against Rift Valley fever virus

Background

Rift Valley fever (RVF) is an arthropod-borne viral zoonosis. Rift Valley fever virus (RVFV) is an important biological threat with the potential to spread to new susceptible areas. In addition, it is a potential biowarfare agent.

Methodology/Principal Findings

We developed two potential vaccines, DNA plasmids and alphavirus replicons, expressing the Gn glycoprotein of RVFV alone or fused to three copies of complement protein, C3d. Each vaccine was administered to mice in an all DNA, all replicon, or a DNA prime/replicon boost strategy and both the humoral and cellular responses were assessed. DNA plasmids expressing Gn-C3d and alphavirus replicons expressing Gn elicited high titer neutralizing antibodies that were similar to titers elicited by the live-attenuated MP12 virus. Mice vaccinated with an inactivated form of MP12 did elicit high titer antibodies, but these antibodies were unable to neutralize RVFV infection. However, only vaccine strategies incorporating alphavirus replicons elicited cellular responses to Gn. Both vaccines strategies completely prevented weight loss and morbidity and protected against lethal RVFV challenge. Passive transfer of antisera from vaccinated mice into naïve mice showed that both DNA plasmids expressing Gn-C3d and alphavirus replicons expressing Gn elicited antibodies that protected mice as well as sera from mice immunized with MP12.

Conclusion/Significance

These results show that both DNA plasmids expressing Gn-C3d and alphavirus replicons expressing Gn administered alone or in a DNA prime/replicon boost strategy are effective RVFV vaccines. These vaccine strategies provide safer alternatives to using live-attenuated RVFV vaccines for human use.

Rift Valley fever virus (RVFV) is an arthropod-borne phlebovirus associated with abortion storms, neonatal mortality in livestock and hemorrhagic fever or fatal encephalitis in a proportion of infected humans. Requirement of multiple booster immunizations to maintain the level of protective immunity with the inactivated vaccines and the ability of live-attenuated vaccines to cause detrimental side-effects are major limitations preventing the widespread use of current vaccines. In this paper, we describe the use of DNA and alphavirus replicon based vaccination approaches to elicit a protective immune response against RVFV. While both vaccines elicited high titer antibodies, DNA vaccination elicited high titer neutralizing antibodies, whereas the replicon vaccine elicited cellular immune responses. Both strategies alone or in combination elicited immune response that completely protected against not only mortality, but also illness. Even though the delivery vectors elicited some protection on their own, they did not prevent severe morbidity. These promising vaccines provide an alternative RVFV vaccine for livestock and humans.

Enhancement of anti-DIII antibodies by the C3d derivative P28 results in lower viral titers and augments protection in mice

Antibodies generated against West Nile virus (WNV) during infection are essential for controlling dissemination. Recent studies have demonstrated that epitopes in all three domains of the flavivirus envelope protein (E) are targets for neutralizing antibodies, with determinants in domain III (DIII) eliciting antibodies with strong inhibitory properties. In order to increase the magnitude and quality of the antibody response against the WNV E protein, DNA vaccines with derivatives of the WNV E gene (full length E, truncated E, or DIII region, some in the context of the pre-membrane [prM] gene) were conjugated to the molecular adjuvant P28. The P28 region of the complement protein C3d is the minimum CR2-binding domain necessary for the adjuvant activity of C3d. Delivery of DNA-based vaccines by gene gun and intramuscular routes stimulated production of IgG antibodies against the WNV DIII region of the E protein. With the exception of the vaccine expressing prM/E given intramuscularly, only mice that received DNA vaccines by gene gun produced protective neutralizing antibody titers (FRNT80 titer >1/40). Correspondingly, mice vaccinated by the gene gun route were protected to a greater level from lethal WNV challenge. In general, mice vaccinated with P28-adjuvated vaccines produced higher IgG titers than mice vaccinated with non-adjuvanted vaccines.

Fusion to chicken C3d enhances the immunogenicity of the M2 protein of avian influenza virus

BACKGROUND

Current vaccines to avian influenzae virus (AIV), a highly contagious disease of birds, need to be constantly updated due to the high level of variation in the target antigens. Therefore, a vaccine that could induce broad cross protection against AIV is required. The M2 membrane protein is structurally conserved amongst AIV subtypes but tends in induce a poor immune response, whereas C3d has been shown in many species to enhance immunogenicity. In this study, we investigated the potential of M2-avian C3d fusion proteins to provide effective immunity.

RESULTS

We fused chicken complement C3d to sM2 (M2 protein with the transmembrane region deleted) of AIV and expressed four fusion proteins, GST (Glutathione S-transferase tagged proteins in pGEX expression vector) -C3d-sM2, GST-C3d-L2-sM2, GST-C3d-L1-C3d-sM2 and GST-C3d-L1-C3d-L2-sM2 were used to immunize mice. In addition, Specific pathogen free (SPF) chickens were inoculated with the plasmids pcDNA-sM2, pcDNA-C3d-L1-C3d-L2-sM2, GST-sM2 and GST-C3d-L1-C3d-L2-sM2. The immune response was monitored by an enzyme-linked immunosorbent assay (ELISA) for sM2 antibody, and all the test animals were challenged with A/chicken/Bei Jing/WD9/98 (H9N2) virus. Results revealed that the anti-sM2 antibody in mice and chickens vaccinated with these proteins was higher than the nonfused forms of sM2, the GST-C3d-L1-C3d-L2-sM2 groups have conferred the highest 30% and 20% protection ratio in mice and chickens respectively. In addition, the pcDNA-C3d-L1-C3d-L2-sM2 also enhances the antibody responses to sM2 compared to pcDNA-sM2 in chickens, and acquired 13.3% protection ratio.

CONCLUSION

These results indicated that chicken C3d enhanced the humoral immunity against AIV M2 protein either fused proteins expressed by the prokaryotic system or with the DNA vaccine. Nevertheless, in view of the poor protection ratio for these animals, we speculated that this is not a worthy developing of vaccine in these constructs.

Fusion of C3d molecule with neutralization epitope(s) of hepatitis E virus enhances antibody avidity maturation and neutralizing activity following DNA immunization

Previous studies have identified that a hepatits E virus peptide (HEV-p179), spanning amino acids (aa) 439-617 in the 660-aa protein encoded by open reading frame 2(ORF2) of the Chinese epidemic strain (genotype 4), is the minimal size fragment of conformation-dependent neutralization epitope(s). We report here the successful immunization of mice with DNA vaccines expressing the secreted form of HEV-p179 (fused with a human tissue plasminogen activator (tPA) signal sequence) and the tPA-p179-C3d fusion protein (fused with three tandem copies of the murine complement C3d). Analysis of antibody responses in vaccinated mice revealed that immunizations with tPA-p179-C3d3 DNA vaccine dramatically increased both the level and avidity maturation of antibodies against HEV-p179 compared to p179 and tPA-p179 DNA vaccines. In addition, this increased antibody response correlated with neutralizing titers in a PCR-based cell culture neutralization assay. These results indicate that vaccination with C3d conjugated p179 DNA vaccine enhances antibody responses to HEV, and this approach may be applied to overcome the poor immunogenicity of DNA vaccines to generate HEV neutralizing antibodies.

Application of nanotechnologies for improved immune response against infectious diseases in the developing world

There is an urgent need for new strategies to combat infectious diseases in developing countries. Many pathogens have evolved to elude immunity and this has limited the utility of current therapies. Additionally, the emergence of co-infections and drug resistant pathogens has increased the need for advanced therapeutic and diagnostic strategies. These challenges can be addressed with therapies that boost the quality and magnitude of an immune response in a predictable, designable fashion that can be applied for wide-spread use. Here, we discuss how biomaterials and specifically nanoscale delivery vehicles can be used to modify and improve the immune system response against infectious diseases. Immunotherapy of infectious disease is the enhancement or modulation of the immune system response to more effectively prevent or clear pathogen infection. Nanoscale vehicles are particularly adept at facilitating immunotherapeutic approaches because they can be engineered to have different physical properties, encapsulated agents, and surface ligands. Additionally, nanoscaled point-of-care diagnostics offer new alternatives for portable and sensitive health monitoring that can guide the use of nanoscale immunotherapies. By exploiting the unique tunability of nanoscale biomaterials to activate, shape, and detect immune system effector function, it may be possible in the near future to generate practical strategies for the prevention and treatment of infectious diseases in the developing world.

C3d adjuvant activity is reduced by altering residues involved in the electronegative binding of C3d to CR2

The final degradation product of the complement protein C3, C3d, has been used as a molecular adjuvant to various antigens. Chimera proteins of the antigen and multiple copies of C3d were developed to test the adjuvant effect of this molecule. The main mechanism by which C3d enhances the immune response is interaction with CR2. In vitro studies showed that the avidity of C3d for CR2 is affected by residues located at the interacting surface (e.g. 170N) as well as by residues located in other areas. The role of the latter residues has been proposed to depend on the electrostatic nature of the C3d-CR2 interaction, where the charges of the whole molecules are responsible for their binding. C3d is primarily a negatively charged molecule, while CR2 is a positive one. Previous experiments demonstrated that elimination of a positive charge (K162A) in C3d enhanced its avidity for CR2, while elimination of negative charges or addition positives ones (D163A, N170R, respectively), impaired the avidity for CR2. Despite the extensive in vitro research, the role of these residues in the adjuvant effect of C3d is unclear. To study the role of residues at the interacting and non-interacting surface of C3d on the adjuvanticity, single as well as a double residue substitutions were engineered in the murine C3d (R162A, D163A, N170R and D163A-N170R) gene. Two copies of these mutant molecules were fused to HIV-1 Env(gp120) and the proteins were tested for their avidity to bind CR2 (sCR2). Later, these DNA constructs were tested in mice to determine their adjuvant capability. Mutation at residue 162 (R162A) neither enhanced nor impaired the avidity of Env(gp120)-C3d(2) for sCR2 in vitro. Mutations at residues D163A and N170R, on the other hand, reduced the binding affinity of Env(gp120)-C3d(2) for sCR2. Furthermore, these mutations synergized and abolished the interaction of C3d for CR2. The data correlated with the adjuvant capability of these molecules in the mouse model. In summary, residues that alter the electronegative status of C3d (D163A and N170R) impair the binding of chimera proteins to CR2, reducing the adjuvant activity of this molecule.

Replication-deficient rabies virus-based vaccines are safe and immunogenic in mice and nonhuman primates

Although current postexposure prophylaxis rabies virus (RV) vaccines are effective, approximately 40,000-70,000 rabies-related deaths are reported annually worldwide. The development of effective formulations requiring only 1-2 applications would significantly reduce mortality. We assessed in mice and nonhuman primates the efficacy of replication-deficient RV vaccine vectors that lack either the matrix (M) or phosphoprotein (P) gene. A single dose of M gene-deficient RV induced a more rapid and efficient anti-RV response than did P gene-deficient RV immunization. Furthermore, the M gene-deleted RV vaccine induced 4-fold higher virus-neutralizing antibody (VNA) levels in rhesus macaques than did a commercial vaccine within 10 days after inoculation, and at 180 days after immunization rhesus macaques remained healthy and had higher-avidity antibodies, higher VNA titers, and a more potent antibody response typical of a type 1 T helper response than did animals immunized with a commercial vaccine. The data presented in this article suggest that the M gene-deleted RV vaccine is safe and effective and holds the potential of replacing current pre- and postexposure RV vaccines.

DNA epitope vaccine containing complement component C3d enhances anti-amyloid-beta antibody production and polarizes the immune response towards a Th2 phenotype

We have engineered a DNA epitope vaccine that expresses 3 self-B cell epitopes of Abeta(42) (3Abeta(1-11)), a non-self T helper (Th) cell epitope (PADRE), and 3 copies of C3d (3C3d), a component of complement as a molecular adjuvant, designed to safely reduce CNS Abeta. Immunization of mice with 3Abeta(1-11)-PADRE epitope vaccine alone generated only moderate levels of anti-Abeta antibodies and a pro-inflammatory T helper (Th1 phenotype) cellular immune response. However, the addition of 3C3d to the vaccine construct significantly augmented the anti-Abeta humoral immune response and, importantly, shifted the cellular immune response towards the potentially safer anti-inflammatory Th2 phenotype.

Fusion of C3d with hemagglutinin enhances protective immunity against swine influenza virus

H1N1 and H3N2 are the dominant subtypes causing swine influenza in China and other countries. It is important to develop effective vaccines against both H1N1 and H3N2 subtypes of swine influenza virus (SIV). We examined the effects of a DNA vaccine expressing an influenza HA fused to three copies of murine complement C3d in mice. Plasmids encoding soluble HA (sHA), complete HA (tmHA), or a soluble fused form of HA (sHA-mC3d3) were constructed from the H3N2 subtype of SIV. The immune response was monitored by an enzyme-linked immunosorbent assay (ELISA), hemagglutination inhibition (HI) assays, and virus neutralization tests. Analysis of antibody titers indicated that immunization with HA-mC3d3 resulted in higher titers of anti-HA antibodies and higher antibody affinities, compared with serum from mice immunized with sHA or tmHA. Furthermore, the C3d fusion increased the Th2-biased immune response, by inducing IL-4 production. Splenocytes from mice immunized with sHA-mC3d3 produced about three-fold more IL-4 than did splenocytes from mice immunized with sHA or tmHA. Seven days post-challenge with homologous virus (H3N2), no virus was isolated from the mice immunized with HA-expressing plasmids. However, 10 days post-challenge with heterologous virus (H1N1), only mice immunized with sHA-mC3d3 had no virus or microscopic lesions in the kidneys and cerebrum. In conclusion, C3d enhanced antibody responses to hemagglutinin and protective immunity against SIV of different subtypes.

Differential functional phenotypes of two primary HIV-1 strains resulting from homologous point mutations in the LLP domains of the envelope gp41 intracytoplasmic domain

We previously reported that selected mutations of highly conserved arginine residues within the LLP regions of HIV-1(ME46) gp41 had diverse effects on Env function. In the current study, we sought to test if the observed LLP mutant phenotypes would be similar in HIV-1(89.6). The results of the current studies revealed that the LLP-1 mutations conferred reduced Env incorporation, infectivity, and replication phenotypes in both viruses, while homologous LLP-2 mutations had differential phenotypical effects between the two strains. In particular, several of the 89.6 LLP-2 mutant viruses were replication defective in CEMX174 cells despite having increased levels of Env incorporation, and with both strains, there were differential effects on infectivity. This comparison of homologous point mutations in two different strains of HIV supports the role of LLPs as determinants of Env function, but reveals for the first time the influence of virus strain on LLP mutant phenotypes.

The distinct effects of three tandem repeats of C3d in the immune responses against tumor-associated antigen hCGbeta by DNA immunization

Several examples have shown that C3d3, when fused to a corresponding antigen, had a strong adjuvant effect on certain specific antibody production. In a previous study, we attempted to prove that this was the case of the human chorionic gonadotrophin beta chain (hCGbeta)-induced immunity following DNA vaccination. However, we found that C3d3 when fused to hCGbeta inhibited rather than enhanced the antigen-specific immune response. In the present study, using hCGbeta DNA vaccine preparations, we demonstrated that C3d3 inhibited the antigen-specific humoral antibody response and several other immune responses, such as the hCGbeta specific lymphoproliferation. Such inhibitory effects of C3d3 were not related to the expression level of the target protein, the gender of the test mice, or the vector used. Contrastingly, C3d3 fused with the envelope protein of hepatitis B virus (PreS2/S) used as a control system resulted in the enhancement of both humoral and cell-mediated antigen-specific immune responses against HBV-preS2/S, which was consistent with other groups' adjuvant-effect findings. We further showed that the mechanisms involved in the inhibitory effect of C3d3 might be possible due to impairing the function of antigen presenting B lymphocytes and reducing the expression of transcription factors (T-bet and GATA-3) and cytokine IL-4. Collectively, unlike its usual expected adjuvant function, the fusion of C3d3 with the tumor-associated antigen hCGbeta was found to inhibit both humoral and cell-mediated antigen-specific immune responses. These findings indicate that research concerning tumor immune escapes and vaccine designs require further extensive attention.

A minimum CR2 binding domain of C3d enhances immunity following vaccination

The degradation product of the third (C3) complement component, C3d, links innate and adaptive immunity, and the covalent attachment of C3d to an antigen enhances antigen-specific immune responses. C3d has been hypothesized to enhance immunity by direct interaction with complement receptor 2 (CR2/CD21) on immune cells. However, the domains on C3d important for CR2 binding have been controversial, with various studies reaching contradictory conclusions. In addition, the concept of B-cell activation via CR2 by C3d has been questioned, since mice lacking CR2 still elicit C3d-enhanced immunity following vaccination. Therefore, the goal of this study was to determine if a peptide representing one of the proposed CR2 binding domains of C3d could substitute for the entire protein and enhance antigen-specific immunity. Mice (BALB/c) were vaccinated with the HIV-1 gp120 envelope glycoprotein (Env(gp120)) alone or fused to multiple copies of the murine C3d or a twenty-eight amino-acid peptide (P28) containing a minimum CR2 binding domain. Each immunogen was expressed from DNA plasmid in vivo or injected as purified recombinant protein. The fusion of the P28 peptide to Env(gp120) enhanced both humoral and cell-mediated immune responses with similar efficiency as Env(gp120) conjugated to C3d. The fusion of C3d or P28 to Env(gp120) elicited higher-titer anti-Env specific antibody, enhanced avidity maturation of the elicited antibody, and elicited higher numbers of IFN-gamma and IL-4 secreting cells compared to Env(gp120) immunizations. This CR2-binding domain specific 28 amino acid peptide can substitute for the entire C3d molecule and enhance immunity. These results indicate that the adjuvant properties of C3d are associated with CR2 interaction.

Low doses of antigen coupled to anti-CR2 mAbs induce rapid and enduring IgG immune responses in mice and in cynomolgus monkeys

The complement system and B cell complement receptor 2 (CR2), specific for C component C3dg, play important roles in both the innate and adaptive immune response. We used hapten and protein conjugates of anti-CR2 mAbs as models for C3dg-opsonized antigens and immune complexes to examine the handling of and immune response to these reagents in mice and in non-human primates (NHP). Mice immunized and boosted i.v. with only 100 ng of Alexa 488 rat anti-mouse CR1/2 mAb 7G6 had strong IgG immune responses to the Alexa 488 hapten and to rat IgG, compared to very weak immune responses in mice treated with a comparable isotype control; larger doses of Alexa 488 mAb 7G6 did not increase the immune response. A vaccine constructed by cross-linking anthrax protective antigen to mAb 7G6 proved to be effective at low doses in generating sufficiently high titer serum IgG antibodies to neutralize anthrax lethal toxin in vitro and to protect mice from i.v. challenge with anthrax lethal toxin. When biotinylated HB135, a mouse mAb specific for human CR2, was injected i.v. into NHP, the probe manifested the same initial marginal zone B cell binding and subsequent localization to follicular dendritic cells as we have previously reported for comparable experiments in mice. Moreover, i.v. immunization of NHP with 1 microg/kg of Alexa 488 mAb HB135 promoted an IgG immune response to the Alexa 488 hapten and to mouse IgG. Taken together, these results demonstrate the efficacy of using anti-CR2 mAbs as antigen carriers for i.v. immunization with small amounts of antigens without adjuvant.

C3d enhanced DNA vaccination induced humoral immune response to glycoprotein C of pseudorabies virus

Murine C3d were utilized to enhance immunogenicity of pseudorabies virus (PrV) gC DNA vaccination. Three copies of C3d and four copies of CR2-binding domain M28(4) were fused, respectively, to truncated gC gene encoding soluble glycoprotein C (sgC) in pcDNA3.1. BALB/c mice were, respectively, immunized with recombinant plasmids, blank vector, and inactivated vaccine. The antibody ELISA titer for sgC-C3d3 DNA was 49-fold more than that for sgC DNA, and the neutralizing antibody obtained 8-fold rise. Protection of mice from death after lethal PrV (316 LD50) challenge was augmented from 25% to 100%. Furthermore, C3d fusion increased Th2-biased immune response by inducing IL-4 production. The IL-4 level for sgC-C3d3 DNA immunization approached that for the inactivated vaccine. Compared to C3d, M28 enhanced sgC DNA immunogenicity to a lesser extent. In conclusion, we demonstrated that murine C3d fusion significantly enhanced gC DNA immunity by directing Th1-biased to a balanced and more effective Th1/Th2 response.

Complement 3d: from molecular adjuvant to target of immune escape mechanisms

C3d is a fragment of the complement factor C3 and is generated in the course of complement activation. When bound to antigen in single or multiple copies, the B cell receptor and complement receptor 2 become co-crosslinked resulting in decreased or increased B cell responses depending on the valence of the antigen-C3d construct. When antigen-C3d constructs are used for the purpose of generating a protective immune response (vaccines), they may either enhance the expected response or suppress it depending on the nature of the antigen. Various pathogens use C3d to evade the immune system by inhibiting complement activation, invading and homing in host cells or masking immunogenic areas of pathogen proteins. Therefore, future vaccination strategies for infectious diseases and cancer employing C3d as a molecular adjuvant need to be carefully evaluated before choosing a target antigen in order to take advantage of the adjuvant effect of the complement component while avoiding potential vaccine complications associated with immune escape mechanisms.

HIV-1 Envgp140 trimers elicit neutralizing antibodies without efficient induction of conformational antibodies

Currently, no vaccine for human immunodeficiency virus (HIV-1) provides protection from virus infection. One reason for these disappointing results has been the difficulty of current vaccine candidates to elicit high-titer, broadly reactive immunity to a large number of viral proteins. Recently, our laboratory demonstrated that the coupling of C3d to a soluble trimerized HIV-1 envelope (Env(gp140(FT))) elicited higher titers of neutralizing antibodies than monomers of Env(gp120) coupled to C3d [Bower JF, Yang X, Sodroski J, Ross TM. Elicitation of neutralizing antibodies with DNA vaccines expressing soluble stabilized human immunodeficiency virus type 1 envelope glycoprotein trimers conjugated to C3d. J Virol 2004;78(9):4710-9]. To determine if the induction of conformational antibodies correlated with neutralization, mice (BALB/c) were primed (2x) with DNA plasmids expressing monomeric Env(gp120) or trimeric Env(gp140) alone or fused to mC3d(3) at one of two doses (2.0microg or 0.2microg), followed by a boost of recombinant uncleaved, trimeric Env(gp140). Regardless of the priming dose of DNA, all mice had high-titer anti-Env IgG antibodies. Interestingly, Env(gp140) trimers did not elicit higher titers of antibodies that recognized conformational Env epitopes compared to monomers of Env(gp120). Therefore, additional parameters were examined for correlation with neutralization. For neutralization-resistant HIV-1 isolates, ADA and YU-2, neutralization correlated with high-titer, high avidity antibodies, with Env(gp140) eliciting slightly higher neutralization titers than Env(gp120). In contrast, none of the measured parameters correlated with neutralization for the more neutralization-sensitive isolates, MN or 89.6. Therefore, even though soluble, uncleaved Env(gp140) trimers may be marginally more effective at eliciting neutralizing antibodies than Env(gp120), neutralization does not appear to correlate with the elicitation of conformationally dependent antibodies.

Gene conjugation of molecular adjuvant C3d3 to hCGβ increased the anti-hCGβ Th2 and humoral immune response in DNA immunization

Human chorionic gonadotropin (hCG) has been used as an anti-fertility vaccine and as a target for cancer immunotherapy. We have explored the use of three copies of C3d in DNA vaccine as molecular adjuvant to improve the immunogenicity of this hormone in previous work and found that the immune response induced by pcDNA3-hCGbeta-C3d3 has been enhanced 243-fold compared with pcDNA3-hCGbeta following DNA immunization in BALB/c mice. In the present study, a new functionally active DNA vaccine of hCGbeta-C3d3 chimera based on pCMV4 vector has been described. We compared the expression efficiency of pCMV4 and pcDNA3 eukaryotic vectors for hCGbeta and hCGbeta-C3d3 fusion protein and the immune response of mice immunized with pcDNA3-hCGbeta, pCMV4-hCGbeta, pcDNA3-hCGbeta-C3d3 and pCMV4-hCGbeta-C3d3, respectively, at 25, 50 and 100 pmol dose, and further analyzed the levels of Th1 and Th2 cytokines produced by spleen lymphocytes of the immunized mice upon hCG restimulation in vitro. It was found that pCMV4 vector achieved 1.3-1.5-fold higher protein expression and raised 1.1-1.2 (primary) and 1.2-1.3 (booster) logs higher titer of anti-hCGbeta IgG than pcDNA3. Mice vaccinated with 50 pmol of hCGbeta-C3d3-DNAs elicited the highest titer of hCGbeta-specific antibody among the serial doses and the immune response induced by pCMV4-hCGbeta-C3d3 were, respectively, 1.3, 1.3 and 1.2 logs higher than that of pcDNA3-hCGbeta-C3d3 and 2.2, 2.9 and 2.4 logs higher than that of pCMV4-hCGbeta at week 2 following the booster immunization. Moreover, we observed that the production of IL-4 and IL-10 increased in mice vaccinated with hCGbeta-C3d3-DNAs and the ratio of IL-4/IFN-(gamma) showed a Th2 bias of immune response in the mice immunized with hCGbeta-C3d3-DNAs. These findings indicated that gene fusion of C3d3 to hCGbeta, as a means of harnessing the adjuvant potential of the innate immune system, may improve the antigen-specific Th2 humoral immune response of the hCGbeta DNA vaccine and the pCMV4 vector is a more ideal eukaryotic vector for DNA vaccine than pcDNA3.

Role of complement receptor type 2 and endogenous complement in the humoral immune response to conjugates of complement C3d and pneumococcal serotype 14 capsular polysaccharide

Conjugation of the complement fragment C3d to both T-cell-dependent (TD) protein and T-cell-independent type 2 (TI-2) polysaccharide antigens enhances the humoral immune response in mice immunized with either type of antigen. However, the ability of C3d-protein conjugates to enhance the antibody response in mice deficient in complement receptor types 1 and 2 (CR1 and CR2) has raised questions about the role of C3d-CR2 interactions in the adjuvant effect of C3d. In this study, we examined the role of CR2 binding and endogenous complement activation in the antibody response to conjugates of C3d and serotype 14 pneumococcal capsular polysaccharide (PPS14). To block binding of PPS14-C3d conjugates to CR2, mice were immunized with a mixture of vaccine and (CR2)2-immunoglobulin G1 (IgG1). Mice receiving (CR2)2-IgG1 at the time of primary immunization had a marked reduction in the primary anti-PPS14 antibody response but an enhanced secondary anti-PPS14 response, suggesting that C3d-CR2 interactions are required for the primary response but can have negative effects on the memory response. Further, compared with mice receiving PPS14-C3d having a high C3d/PPS14 ratio, mice immunized with PPS14-C3d with low C3d/PPS14 ratios had an enhanced secondary antibody response. Treatment of mice with cobra venom factor to deplete complement had insignificant effects on the antibody response to PPS14-C3d. Experiments with CBA/N xid mice confirmed that PPS14-C3d conjugates retain the characteristics of TI-2 rather than TD antigens. Thus, the adjuvant effect of C3d conjugated to PPS14 requires C3d-CR2 interactions, does not require activation of endogenous complement, and is not mediated by TD carrier effects.

Characterization of antibody responses to purified HIV-1 gp120 glycoproteins fused with the molecular adjuvant C3d

The HIV-1 exterior envelope glycoprotein gp120 binds receptor (CD4) and co-receptors (CCR5/CXCR4) and is a major target for neutralizing antibodies. The two functionally conserved regions of gp120 involved in receptor binding are conformational in nature. It is likely that the elicitation of neutralizing antibodies to these targets will benefit by presentation of these sites to the humoral immune system under physiologic conditions. Initially, we investigated the ability of the molecular adjuvant C3d to enhance antibody responses to variant gp120 glycoproteins in phosphate-buffered saline (PBS). We utilized a gp120 variant glycoprotein deleted of N- and C-terminal sequences (gp120DeltaC1/C5) originally designed to eliminate immunodominant, non-neutralizing epitopes and characterized this protein when fused to two C3d elements (gp120DeltaC1/C5(C3d)(2)). In PBS, the gp120DeltaC1/C5(C3d)(2) proteins are able to elicit gp120 binding antibodies more efficiently than gp120 lacking C3d moieties. We then asked if we could observe C3d-enhanced immunogenicity of gp120 in the presence of the classical oil-in-water adjuvant, Ribi. In the presence of the Ribi, which contains the TLR-4 agonist monophospholipid A (MPL), antibodies elicited by the gp120DeltaC1/C5(C3d)(2) were of higher titer than those elicited by the identical protein in PBS. To determine if the elicited secondary response was due to a synergy between the C3d repeats and the Ribi, we then inoculated gp120DeltaC1/C5 protein in Ribi and observed that similar titers of anti-gp120 antibodies were elicited in comparison to the gp120DeltaC1/C5(C3d)(2) protein also inoculated in Ribi adjuvant. In Ribi, there was a small but consistent increase in gp120-specific antibody titer of a gp120DeltaC1/C5(C3d)(2) prime followed by two gp120DeltaC1/C5 boosts compared to three inoculations of either the gp120DeltaC1/C5 proteins or the gp120DeltaC1/C5(C3d)(2) proteins alone. We conclude that the molecular adjuvant C3d demonstrates utility in conditions where physiologic presentation of native protein structures is desired, but may have less benefit in the context of a relatively potent protein adjuvant such as Ribi.

Enhancement of humoral immunity to the hCG beta protein antigen by fusing a molecular adjuvant C3d3

The vaccine directed against human chorionic gonadotropin (hCG) has previously undergone clinical test demonstrating the feasibility of the approach in preventing pregnancy in women. Some individuals, however, did not response adequately despite employing highly immunogenic bacterial toxoids as carriers. In this study, we investigated the potential of three copies of C3d as a new molecular adjuvant to enhance the immunogenicity of hCG beta protein antigen. The antibody response to the hCG beta-C3d3 fusion protein immunization was compared with those resulting from immunization with the hCG beta alone and the hCG beta plus CFA/IFA either in BALB/c mice or in C(57)BL/6J mice. Our results showed that the fusion of C3d3 to hCG beta protein antigen resulted in a significant elevation of the serum anti-hCG beta antibody level in the two mouse strains and the antibodies were capable of effectively neutralizing the bioactivity of hCG. The immunization with C3d3 as a molecular adjuvant favored Th2 bias of immune response. The immunity-enhancing effect of the C3d3 was 10-fold (initial) and 20-32-fold (booster) greater than CFA/IFA. These findings indicated that fusion of C3d3 to hCG beta, as a means of harnessing the adjuvant potential of the innate immune system, may improve immunogenicity of the hCG beta contraceptive vaccine, which is useful to produce a cost-effective vaccine and for the less-responsive population.

Efficient production of complement (C3d)3 fusion proteins using the baculovirus expression vector system

Proteins fused to activated complement (C) fragments elicit enhanced immunogenicity. This "natural adjuvant" effect may have important implications when considering novel vaccination approaches. Here we describe both the construction of a novel fusion protein, consisting of a well characterized test antigen fused to multiple copies of the activated complement component (C3d)3, as well as an efficient method for its expression and production in insect cells. Using the inherent biological advantages of the baculovirus expression system, as well as applying specific infection and harvesting modifications, we have optimized the efficiency of protein production. Our modifications allow purification of fusion proteins directly from cell supernatant in a single anion exchange chromatographic step. This alleviates the requirement for the inclusion of protein affinity tags. The integrity of the purified recombinant protein was evaluated by SDS PAGE analysis, reactivity with antibodies, as well as in vivo by administration as an immunogen.

Current developments in viral DNA vaccines: shall they solve the unsolved?

This review describes the mechanisms of immune response following DNA vaccination. The efficacy of DNA vaccines in animal models is highlighted, especially in viral diseases against which no widely accepted vaccination is currently available. Emphasis is given to possible therapeutic vaccination in chronic infections due to persisting virus genomes, such as recurrent herpes (HSV-1 and HSV-2), pre-AIDS (HIV-1) and/or chronic hepatitis B (HBV). In these, the problem of introducing foreign viral DNA may not be of crucial importance, since the immunised subject is already a viral DNA (or provirus) carrier. The DNA-based immunisation strategies may overcome several problems of classical viral vaccines. Novel DNA vaccines could induce immunity against multiple viral epitopes including the conservative type common ones, which do not undergo antigenic drifts. Within the immunised host, they mimic the effect of live attenuated viral vaccines when continuously expressing the polypeptide in question. For this reason they directly stimulate the antigen-presenting cells, especially dendritic cells. The antigen encoded by plasmid elicits T helper cell activity (Th1 and Th2 type responses), primes the cytotoxic T cell memory and may induce a satisfactory humoral response. The efficacy of DNA vaccines can be improved by adding plasmids encoding immunomodulatory cytokines and/or their co-receptors.

CD21/CD19 Coreceptor Signaling Promotes B Cell Survival during Primary Immune Responses

The adaptive immune response is tightly regulated to limit responding cells in an Ag-specific manner. On B cells, coreceptors CD21/CD19 modulate the strength of BCR signals, potentially influencing cell fate. The importance of the CD95 pathway was examined in response of B cells to moderate affinity Ag using an adoptive transfer model of lysozyme-specific Ig transgenic (HEL immunoglobulin transgene (MD4) strain) B cells. Although adoptively transferred Cr2+/+ MD4 B cells are activated and persist within splenic follicles of duck egg lysozyme-immunized mice, Cr2-/- MD4 B cells do not. In contrast, Cr2-/- MD4 lpr B cells persist after transfer, suggesting that lack of CD21/CD35 signaling results in CD95-mediated elimination. Cr2 deficiency did not affect CD95 levels, but cellular FLIP (c-FLIP) protein and mRNA levels were reduced 2-fold compared with levels in Cr2+/+ MD4 B cells. In vitro culture with Cr2+/+ MD4 B cells demonstrated that equimolar amounts of rHEL-C3d3 were more effective than hen egg lysozyme alone in up-regulating c-FLIP levels and for protection against CD95-mediated apoptosis. Collectively, this study implies a mechanism for regulating B cell survival in vivo whereby the strength of BCR signaling (including coreceptor) determines c-FLIP levels and protection from CD95-induced death.

Complement-opsonized HIV: the free rider on its way to infection

The complement system (C) is one of the main humoral components of innate immunity. Three major tasks of C against invading pathogens are: (i) lysis of pathogens by the formation of the membrane attack complex (MAC); (ii) opsonization of pathogens with complement fragments to favor phagocytosis; and (iii) attraction of inflammatory cells by chemotaxis. Like other particles, HIV activates C and becomes opsonized. To escape complement-mediated lysis, HIV has adopted various properties, which include the acquisition of HIV-associated molecules (HAMs) belonging to the family of complement regulators, such as CD46, CD55, CD59, and the interaction with humoral regulatory factors like factor H (fH). Opsonized virus may bind to complement receptor positive cells to infect them more efficiently or to remain bound on the surface of such cells. In the latter case HIV can be transmitted to cells susceptible for infection. This review discusses several aspects of C-HIV interactions and provides a model for the dynamics of this process.