Academic literature on the topic 'Stage-specific vaccine candidates'

The Duffy binding protein (DBP) is a vital ligand forPlasmodium vivaxblood-stage merozoite invasion, making the molecule an attractive vaccine candidate against vivax malaria. Similar to other blood-stage vaccine candidates, DBP allelic variation eliciting a strain-specific immunity may be a major challenge for development of a broadly effective vaccine against vivax malaria. To understand whether conserved epitopes can be the target of neutralizing anti-DBP inhibition, we generated a set of monoclonal antibodies to DBP and functionally analyzed their reactivity to a panel of allelic variants. Quantitative analysis by enzyme-linked immunosorbent assay (ELISA) determined that some monoclonal antibodies reacted strongly with epitopes conserved on all DBP variants tested, while reactivity of others was allele specific. Qualitative analysis characterized by anti-DBP functional inhibition using anin vitroerythrocyte binding inhibition assay indicated that there was no consistent correlation between the endpoint titers and functional inhibition. Some monoclonal antibodies were broadly inhibitory while inhibition of others varied significantly by target allele. These data demonstrate a potential for vaccine-elicited immunization to target conserved epitopes but optimization of DBP epitope target specificity and immunogenicity may be necessary for protection against diverseP. vivaxstrains.

The use of dengue virus (DENV) vaccines has been hindered by the complexities of antibody dependent enhancement (ADE). Current late-stage vaccine candidates utilize attenuated and chimeric DENVs that produce particles of varying maturities. Antibodies that are elicited by preferentially exposed epitopes on immature virions have been linked to increased ADE. We aimed to further understand the humoral immunity promoted by DENV particles of varying maturities in an AG129 mouse model using a chimeric insect specific vaccine candidate, bDENV-2. We immunized mice with mature, partially mature, and immature bDENV-2 and found that immunization with partially mature bDENV-2 produced more robust and cross-neutralizing immune responses than immunization with immature or mature bDENV-2. Upon challenge with mouse adapted DENV-2 (D220), we observed 80% protection for mature bDENV-2 vaccinated mice and 100% for immature and partially mature vaccinated mice, suggesting that protection to homotypic challenge is not dependent on maturation. Finally, we found reduced in vitro ADE at subneutralising serum concentrations for mice immunized with mature bDENV-2. These results suggest that both immature and mature DENV particles play a role in homotypic protection; however, the increased risk of in vitro ADE from immature particles indicates potential safety benefits from mature DENV-based vaccines.

Hepatitis E virus (HEV) infection causes sporadic outbreaks of acute hepatitis worldwide. HEV was previously considered to be restricted to resource-limited countries with poor sanitary conditions, but increasing evidence implies that HEV is also a public health problem in developed countries and regions. Fortunately, several vaccine candidates based on virus-like particles (VLPs) have progressed into the clinical development stage, and one of them has been approved in China. This review provides an overview of the current HEV vaccine pipeline and future development with the emphasis on defining the critical quality attributes for the well-characterized vaccines. The presence of clinically relevant epitopes on the VLP surface is critical for eliciting functional antibodies against HEV infection, which is the key to the mechanism of action of the prophylactic vaccines against viral infections. Therefore, the epitope-specific immunochemical assays based on monoclonal antibodies (mAbs) for HEV vaccine antigen are critical methods in the toolbox for epitope characterization and for in vitro potency assessment. Moreover, serological evaluation methods after immunization are also discussed as biomarkers for clinical performance. The vaccine efficacy surrogate assays are critical in the preclinical and clinical stages of VLP-based vaccine development.

Malaria vaccines that disrupt the Plasmodium life cycle in mosquitoes and reduce parasite transmission in endemic areas are termed transmission-blocking vaccines (TBVs). Despite decades of research, there are only a few Plasmodium falciparum antigens that indisputably and reproducibly demonstrate transmission-blocking immunity. So far, only two TBV candidates have advanced to phase 1/2 clinical testing with limited success. By applying an unbiased transcriptomics-based approach, we have identified Pf77 and male development gene 1 (PfMDV-1) as two P. falciparum TBV antigens that, upon immunization, induced antibodies that caused reductions in oocyst counts in Anopheles mosquito midguts in a standard membrane feeding assay. In-depth studies were performed to characterize the genetic diversity of, stage-specific expression by, and natural immunity to these two molecules to evaluate their suitability as TBV candidates. Pf77 and PfMDV-1 display limited antigenic polymorphism, are pan-developmentally expressed within the parasite, and induce naturally occurring antibodies in Ghanaian adults, which raises the prospect of natural boosting of vaccine-induced immune response in endemic regions. Together, these biological properties suggest that Pf77 and PfMDV-1 may warrant further investigation as TBV candidates.

A highly effective vaccine would be a valuable weapon in the drive toward malaria elimination. No such vaccine currently exists, and only a handful of the hundreds of potential candidates in the parasite genome have been evaluated. In this study, we systematically evaluated 29 antigens likely to be involved in erythrocyte invasion, an essential developmental stage during which the malaria parasite is vulnerable to antibody-mediated inhibition. Testing antigens alone and in combination identified several strain-transcending targets that had synergistic combinatorial effects in vitro, while studies in an endemic population revealed that combinations of the same antigens were associated with protection from febrile malaria. Video microscopy established that the most effective combinations targeted multiple discrete stages of invasion, suggesting a mechanistic explanation for synergy. Overall, this study both identifies specific antigen combinations for high-priority clinical testing and establishes a generalizable approach that is more likely to produce effective vaccines.

ABSTRACT Lymphatic filariasis is a major tropical disease caused by the mosquito-borne nematodes Brugia and Wuchereria. About 120 million people are infected and at risk of lymphatic pathology such as acute lymphangitis and elephantiasis. Vaccines against filariasis must generate immunity to the infective mosquito-derived third-stage larva (L3) without accentuating immunopathogenic responses to lymphatic-dwelling adult parasites. We have identified two highly expressed genes, designated abundant larval transcript-1 and -2 (alt-1 and alt-2), from each of which mRNAs account for >1% of L3 cDNAs. ALT-1 and ALT-2 share 79% amino acid identity across 125 residues, including a putative signal sequence and a prominent acidic tract. Expression ofalt-1 and alt-2 is initiated midway through development in the mosquito, peaking in the infective larva and declining sharply following entry into the host. Humans exposed toBrugia malayi show a high frequency of immunoglobulin G1 (IgG1) and IgG3 antibodies to ALT-1 and -2, distinguishing them from adult-stage antigens, which are targeted by the IgG4 isotype. Immunization of susceptible rodents (jirds) with ALT-1 elicited a 76% reduction in parasite survival, the highest reported for a single antigen from any filarial parasite. ALT-1 and the closely related ALT-2 are therefore strong candidates for a future vaccine against human filariasis.

Eimeria tenella (E. tenella) is a highly pathogenic and prevalent species of Eimeria that infects chickens, and it causes a considerable disease burden worldwide. The secreted proteins and surface antigens of E. tenella at the sporozoite stage play an essential role in the host–parasite interaction, which involves attachment and invasion, and these interactions are considered vaccine candidates based on the strategy of cutting off the invasion pathway to interrupt infection. We selected two highly expressed surface antigens (SAGs; Et-SAG13 and Et-SAG) and two highly expressed secreted antigens (rhoptry kinases Eten5-A, Et-ROPK-Eten5-A and dense granule 12, Et-GRA12) at the sporozoite stage. Et-ROPK-Eten5-A and Et-GRA12 were two unexplored proteins. Et-ROPK-Eten5-A was an E. tenella-specific rhoptry (ROP) protein and distributed in the apical pole of sporozoites and merozoites. Et-GRA12 was scattered in granular form at the sporozoite stage. To evaluate the potential of rEt-ROPK-Eten5-A, rEt-GRA12, rEt-SAG13 and rEt-SAG proteins as a coccidiosis vaccine, the protective efficacy was examined based on survival rate, lesion score, body weight gain, relative body weight gain and oocyst output. The survival rate was significantly improved in rEt-ROPK-Eten5-A (100%) and rEt-GRA12 (100%) immune chickens compared to the challenged control group (40%). The average body weight gains of rEt-ROPK-Eten5-A, rEt-GRA12, rEt-SAG13 and rEt-SAG immunized chickens were significantly higher than those of unimmunized chickens. The mean lesion score and oocyst output of the rEt-ROPK-Eten5-A immunized chickens were significantly reduced compared to unimmunized challenged chickens. These results suggest that the rEt-ROPK-Eten5-A protein effectively triggered protection against E. tenella in chickens and provides a useful foundation for future work developing anticoccidial vaccines.

ABSTRACT The circumsporozoite protein (CSP) from the surface of sporozoite stage Plasmodium sp. malaria parasites is among the most important of the malaria vaccine candidates. Gene gun injection of genetic vaccines encoding Plasmodium berghei CSP induces a significant protective effect against sporozoite challenge; however, intramuscular injection does not. In the present study we compared the immune responses and protective effects induced by P. berghei CSP genetic vaccines delivered intradermally with a needle or epidermally with a gene gun. Mice were immunized three times at 4-week intervals and challenged by a single infectious mosquito bite. Although 50 times more DNA was administered by needle than by gene gun, the latter method induced significantly greater protection against infection. Intradermal injection of the CSP genetic vaccine induced a strong Th1-type immune response characterized by a dominant CSP-specific immunoglobulin G2a (IgG2a) humoral response and high levels of gamma interferon produced by splenic T cells. Gene gun injection induced a predominantly Th2-type immune response characterized by a high IgG1/IgG2a ratio and significant IgE production. Neither method generated measurable cytotoxic T lymphocyte activity. The results indicate that a gene gun-mediated CS-specific Th2-type response may be best for protecting against malarial sporozoite infection when the route of parasite entry is via mosquito bite.

ABSTRACTCircumsporozoite protein (CSP) ofPlasmodium falciparumis a protective human malaria vaccine candidate. There is an urgent need for models that can rapidly down-select novel CSP-based vaccine candidates. In the present study, the mouse-mosquito transmission cycle of a transgenicPlasmodium bergheimalaria parasite stably expressing a functional full-lengthP. falciparumCSP was optimized to consistently produce infective sporozoites for protection studies. A minimal sporozoite challenge dose was established, and protection was defined as the absence of blood-stage parasites 14 days after intravenous challenge. The specificity of protection was confirmed by vaccinating mice with multiple CSP constructs of differing lengths and compositions. Constructs that induced high NANP repeat-specific antibody titers in enzyme-linked immunosorbent assays were protective, and the degree of protection was dependent on the antigen dose. There was a positive correlation between antibody avidity and protection. The antibodies in the protected mice recognized the native CSP on the parasites and showed sporozoite invasion inhibitory activity. Passive transfer of anti-CSP antibodies into naive mice also induced protection. Thus, we have demonstrated the utility of a mouse efficacy model to down-select human CSP-based vaccine formulations.

The liver flukes Fasciola hepatica and F. gigantica cause infectious disease in ruminants and humans. The geographical range of these two parasite species (temperate and tropical respectively) ensures that infection can occur worldwide. Although anthelmintic treatment is effective against disease, emerging drug resistant strains leads to the development of a vaccine. However, despite several decades of research, there is no commercial vaccine available. The main challenge at present is to produce recombinant proteins in an immunologically active form using recombinant DNA technology. This is an essential step in Fasciola vaccine production. Cysteine proteases are probably the most important facilitators of virulence in flukes and are produced by all stages of the fluke life-cycle. Two classes of cysteine protease are found in the excretory and secretory material of liver flukes- these are cathepsin L and cathepsin B. As such, the major aims of this thesis were to investigate the expression and purification of Fasciola recombinant cysteine proteins, and characterisation by SDS-PAGE and immunoblotting using monoclonal and polyclonal antibodies. These studies demonstrate the production of functionally active cathepsin proteins in S. cerevisiae BJ3505 cells which will lead to vaccine candidate analysis. The second aim of this thesis was to determine the protective efficacy of stage specific target antigens against experimental infection. In addressing this issue, the protective efficacy of single and multivalent recombinant protein vaccinations of adult stage F. hepatica cathepsin L5, immature F. gigantica cathepsin L1g and juvenile F. hepatica cathepsin B were analysed in Sprague Dawley rats against F. hepatica infection. This study demonstrates that juvenile fluke target antigen-cathepsin B induces better immune protection than adult fluke antigen-cathepsin L5. Cocktails of juvenile and adult stage fluke recombinant proteins (cathepsin B and L5) elicited the highest protective immunity against experimental infection and this combination showed not only reduction in fluke recovery and size of flukes, but also marked diminution in the intensity of liver lesions in vaccinated rats. In order to assess the immunogenic property of an early infective stage fluke secreting cysteine protease as a vaccine candidate, DNA vaccination vectors encoding cathepsin B were analysed in BALB/c mice. In this study, the ability of four DNA vaccination strategies such as secretory, chemokine-activating, lymph node targeting vectors encoding cathepsin B were assessed by antibody titre, antibody avidity, western blotting and ELIPSOT assay. The results have further validated the immunoprophylactic potential of a cathepsin B vaccine against F. hepatica. In this study, we have expressed and attained high yields of F. gigantica cathepsin L1g from E. coli BL21, and compared this to a yeast-expressed system. This protease was over-expressed and formed insoluble inclusion bodies that were subsequently solubilised with urea or guanidine hydrochloride. In order to purify the urea-solubilised protein, step-wise urea gradient chromatography was used. For refolding of solubilised protein, a dilution and dialysis procedure was utilised. Proteolytic activity was confirmed by gelatin SDS-PAGE analysis. In conclusion, the determination of the immune potential of recombinant stage specific antigens allows the development of effective vaccines against Fasciola infection.