Phase I Clinical Trial of a Recombinant Blood Stage Vaccine Candidate for Plasmodium falciparum Malaria Based on MSP1 and EBA175

Plasmodium falciparum merozoite surface protein 1 as asexual blood stage malaria vaccine candidate

INTRODUCTION

Malaria represents a public health challenge in tropical and subtropical regions, and currently deployed control strategies are likely insufficient to drive elimination of malaria. Development and improvement of malaria vaccines might be key to reduce disease burden. Vaccines targeting asexual blood stages of the parasite have shown limited efficacy when studied in human trials conducted over the past decades.

AREAS COVERED

Vaccine candidates based on the merozoite surface protein 1 (MSP1) were initially envisioned as one of the most promising approaches to provide immune protection against asexual blood-stage malaria. Successful immunization studies in monkey involved the use of the full-length MSP1 (MSP1FL) as vaccine construct. Vaccines using MSP1FL for immunization have the potential benefit of including numerous conserved B-cell and T-cell epitopes. This could result in improved parasite strain-transcending, protective immunity in the field. We review outcomes of clinical trials that utilized a variety of MSP1 constructs and formulations, including MSP1FL, either alone or in combination with other antigens, in both animal models and humans.

EXPERT OPINION

Novel approaches to analyze breadth and magnitude of effector functions of MSP1-targeting antibodies in volunteers undergoing experimental vaccination and controlled human malaria infection will help to define correlates of protective immunity.

Reporting quality of CONSORT flow diagrams in published early phase dose-finding clinical trial reports: Improvement is needed

BACKGROUND

This project aims to: (1) assess the completeness of information in flow diagrams of published early phase dose-finding (EPDF) trials based on CONSORT recommendations, and if additional features on dose (de-)escalation were presented; (2) propose new flow diagrams presenting how doses were (de-)escalated throughout the trial.

METHODS

Flow diagrams were extracted from a random sample of 259 EPDF trials, published from 2011 to 2020 indexed in PubMed. Diagrams were scored out of 15 following CONSORT recommendations with an additional score for presence of (de-)escalation. New templates were proposed for features that were deficient and presented to 39 methodologists and 11 clinical trialists in October and December 2022.

RESULTS

98 (38%) papers included a flow diagram. Flow diagrams were most deficient in the reporting of reasons for lost to follow up (2%) and reasons for not receiving allocated intervention (14%). Few (39%) presented sequential dose-decision stages. Of voting methodologists, 33/38 (87%) agreed or strongly agreed that for participants recruited in cohorts, presenting the (de-)escalation steps in the flow diagram is a useful feature, also expressed by the trial investigators. Most workshop attendees (35/39, 90%) preferred a larger dose to be displayed higher up within the flow diagram than a smaller dose.

CONCLUSION

Most published trials do not provide a flow diagram, and for those that do, essential information is often omitted. EPDF flow diagrams capturing information on participant flow in the trial's journey, encapsulated within one figure, are highly recommended to promote transparency and interpretability of trial results.

Neutralizing and interfering human antibodies define the structural and mechanistic basis for antigenic diversion

Defining mechanisms of pathogen immune evasion and neutralization are critical to develop potent vaccines and therapies. Merozoite Surface Protein 1 (MSP-1) is a malaria vaccine antigen and antibodies to MSP-1 are associated with protection from disease. However, MSP-1-based vaccines performed poorly in clinical trials in part due to a limited understanding of the protective antibody response to MSP-1 and of immune evasion by antigenic diversion. Antigenic diversion was identified as a mechanism wherein parasite neutralization by a MSP-1-specific rodent antibody was disrupted by MSP-1-specific non-inhibitory blocking/interfering antibodies. Here, we investigated a panel of MSP-1-specific naturally acquired human monoclonal antibodies (hmAbs). Structures of multiple hmAbs with diverse neutralizing potential in complex with MSP-1 revealed the epitope of a potent strain-transcending hmAb. This neutralizing epitope overlaps with the epitopes of high-affinity non-neutralizing hmAbs. Strikingly, the non-neutralizing hmAbs outcompete the neutralizing hmAb enabling parasite survival. These findings demonstrate the structural and mechanistic basis for a generalizable pathogen immune evasion mechanism through neutralizing and interfering human antibodies elicited by antigenic diversion, and provides insights required to develop potent and durable malaria interventions.

The Plasmodium falciparum Merozoite Surface Protein 1 (MSP-1) is a prime vaccine candidate for malaria. Here, the authors structurally and functionally characterise a panel of naturally acquired MSP-1 specific antibodies to identify one with potent broadly neutralising activity and better understand immune evasion mechanisms.

Phagocytosis of Plasmodium falciparum ring-stage parasites predicts protection against malaria

Ring-infected erythrocytes are the predominant asexual stage in the peripheral circulation but are rarely investigated in the context of acquired immunity against Plasmodium falciparum malaria. Here we compare antibody-dependent phagocytosis of ring-infected parasite cultures in samples from a controlled human malaria infection (CHMI) study (NCT02739763). Protected volunteers did not develop clinical symptoms, maintained parasitaemia below a predefined threshold of 500 parasites/μl and were not treated until the end of the study. Antibody-dependent phagocytosis of both ring-infected and uninfected erythrocytes from parasite cultures was strongly correlated with protection. A surface proteomic analysis revealed the presence of merozoite proteins including erythrocyte binding antigen-175 and -140 on ring-infected and uninfected erythrocytes, providing an additional antibody-mediated protective mechanism for their activity beyond invasion-inhibition. Competition phagocytosis assays support the hypothesis that merozoite antigens are the key mediators of this functional activity. Targeting ring-stage parasites may contribute to the control of parasitaemia and prevention of clinical malaria.

Adjuvants Differentially Modulate the Immunogenicity of Lassa Virus Glycoprotein Subunits in Mice

Lassa Fever (LF) is an acute viral hemorrhagic fever caused by Lassa virus (LASV) that is primarily transmitted through contact with wild rodents in West Africa. Although several advanced vaccine candidates are progressing through clinical trials, some effective vaccines are virally vectored and thus require a stringent cold-chain, making distribution to rural and resource-poor areas difficult. Recombinant subunit vaccines are advantageous in this aspect as they can be thermostabilized and deployed with minimal storage and transportation requirements. However, antigen dose and adjuvant formulation must be carefully selected to ensure both the appropriate humoral and cell-mediated immune responses are elicited. In this study, we examine the immunogenicity of a two-step immunoaffinity-purified recombinant LASV glycoprotein (GP) with five clinical- and preclinical-grade adjuvants. Swiss Webster mice immunized intramuscularly with 2 or 3 doses of each vaccine formulation showed complete seroconversion and maximal GP-specific antibody response after two immunizations. Formulations with GPI-0100, LiteVax, Montanide™ ISA 51, and Montanide™ ISA 720 induced both IgG1 and IgG2 antibodies suggesting a balanced Th1/Th2 response, whereas formulation of LASV GP with Alhydrogel elicited a IgG1-dominant response. Splenocytes secreting both Th1 and Th2 cytokines i.e., IFN-γ, TNF-α, IL-2, IL-4 and IL-5, were observed from mice receiving both antigen doses formulated with ISA 720, LiteVax and GPI-0100. However, robust, multifunctional T-cells were only detected in mice receiving a higher dose of LASV GP formulated with GPI-0100. Our results emphasize the importance of careful adjuvant selection and lay the immunological basis for a recombinant subunit protein LF vaccine formulation.

Optimal control analysis of hepatocytic-erythrocytic dynamics of Plasmodium falciparum malaria

This paper presents an in-host malaria model subject to anti-malarial drug treatment and malaria vaccine antigens combinations. Pontryagin's Maximum Principle is applied to establish optimal control strategies against infected erythrocytes, infected hepatocytes and malaria parasites. Results from numerical simulation reveal that a combination of pre-erythrocytic vaccine antigen, blood schizontocide and gametocytocide drugs would offer the best strategy to eradicate clinical P. falciparum malaria. Sensitivity analysis, further reveal that the efficacy of blood schizontocides and blood stage vaccines are crucial in the control of clinical malaria infection. Futhermore, we found that an effective blood schizontocide should be used alongside efficacious blood stage vaccine for rapid eradication of infective malaria parasites. The authors hope that the results of this study will help accelerate malaria elimination efforts by combining malaria vaccines and anti-malarial drugs against the deadly P. falciparum malaria.

Forest malaria and prospects for anti-malarial chemoprophylaxis among forest goers: findings from a qualitative study in Thailand

BACKGROUND

Across the Greater Mekong Subregion, malaria remains a dangerous infectious disease, particularly for people who visit forested areas where residual transmission continues. Because vector control measures offer incomplete protection to forest goers, chemoprophylaxis has been suggested as a potential supplementary measure for malaria prevention and control. To implement prophylaxis effectively, additional information is needed to understand forest goers' activities and their willingness to use malaria prevention measures, including prophylaxis, and how it could be delivered in communities. Drawing on in-depth interviews with forest goers and stakeholders, this article examines the potential acceptability and implementation challenges of malaria prophylaxis for forest goers in northeast Thailand.

METHODS

In-depth interviews were conducted with forest goers (n = 11) and stakeholders (n = 16) including healthcare workers, community leaders, and policymakers. Interviews were audio-recorded, transcribed and coded using NVivo, employing an inductive and deductive approach, for thematic analysis.

RESULTS

Forest goers were well aware of their (elevated) malaria risk and reported seeking care for malaria from local health care providers. Forest goers and community members have a close relationship with the forest but are not a homogenous group: their place and time-at-risk varied according to their activities and length of stay in the forest. Among stakeholders, the choice and cost of anti-malarial prophylactic regimen-its efficacy, length and complexity, number of tablets, potential side effects, and long-term impact on users-were key considerations for its feasibility. They also expressed concern about adherence to the preventive therapy and potential difficulty treating malaria patients with the same regimen. Prophylaxis was considered a low priority in areas with perceived accessible health system and approaching malaria elimination.

CONCLUSIONS

In the context of multi-drug resistance, there are several considerations for implementing malaria prophylaxis: the need to target forest goers who are at-risk with a clear period of exposure, to ensure continued use of vector control measures and adherence to prophylactic anti-malarials, and to adopt an evidence-based approach to determine an appropriate regimen. Beyond addressing current intervention challenges and managing malaria incidence in low-transmission setting, it is crucial to keep malaria services available and accessible at the village level especially in areas home to highly mobile populations.

Combating Human Viral Diseases: Will Plant-Based Vaccines Be the Answer?

Molecular pharming or the technology of application of plants and plant cell culture to manufacture high-value recombinant proteins has progressed a long way over the last three decades. Whether generated in transgenic plants by stable expression or in plant virus-based transient expression systems, biopharmaceuticals have been produced to combat several human viral diseases that have impacted the world in pandemic proportions. Plants have been variously employed in expressing a host of viral antigens as well as monoclonal antibodies. Many of these biopharmaceuticals have shown great promise in animal models and several of them have performed successfully in clinical trials. The current review elaborates the strategies and successes achieved in generating plant-derived vaccines to target several virus-induced health concerns including highly communicable infectious viral diseases. Importantly, plant-made biopharmaceuticals against hepatitis B virus (HBV), hepatitis C virus (HCV), the cancer-causing virus human papillomavirus (HPV), human immunodeficiency virus (HIV), influenza virus, zika virus, and the emerging respiratory virus, severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) have been discussed. The use of plant virus-derived nanoparticles (VNPs) and virus-like particles (VLPs) in generating plant-based vaccines are extensively addressed. The review closes with a critical look at the caveats of plant-based molecular pharming and future prospects towards further advancements in this technology. The use of biopharmed viral vaccines in human medicine and as part of emergency response vaccines and therapeutics in humans looks promising for the near future.

Prefoldin subunit 6 of Plasmodium falciparum binds merozoite surface protein-1 (MSP-1)

Malaria is a human disease caused by eukaryotic protozoan parasites of the Plasmodium genus. Plasmodium falciparum (Pf) causes the most lethal form of human malaria and is responsible for widespread mortality worldwide. Prefoldin is a heterohexameric molecular complex that binds and delivers unfolded proteins to chaperonin for correct folding. The prefoldin PFD6 is predicted to interact with merozoite surface protein‐1 (MSP‐1), a protein well known to play a pivotal role in erythrocyte binding and invasion by Plasmodium merozoites. We previously found that the P. falciparum (Pf) genome contains six prefoldin genes and a prefoldin‐like gene whose molecular functions are unidentified. Here, we analyzed the expression of PfPFD‐6 during the asexual blood stages of the parasite and investigated its interacting partners. PfPFD‐6 was found to be significantly expressed at the trophozoite and schizont stages. Pull‐down assays suggest PfPFD‐6 interacts with MSP‐1. In silico analysis suggested critical residues involved in the PfPFD‐6‐MSP‐1 interaction. Our data suggest PfPFD‐6 may play a role in stabilizing or trafficking MSP‐1.

An AMA1/MSP119 Adjuvanted Malaria Transplastomic Plant-Based Vaccine Induces Immune Responses in Test Animals

Malaria is a tropical human disease, caused by protozoan parasites, wherein a significant number of the world's population is at risk. Annually, more than 219 million new cases are reported. Although there are prevention treatments, there are no highly and widely effective licensed anti-malarial vaccines available for use. Opportunities for utilization of plant-based vaccines as novel platforms for developing safe, reliable, and affordable treatments offer promise for developing such a vaccine against malaria. In this study, a Malchloroplast candidate vaccine was designed, composed of segments of AMA1 and MSP1 proteins, two epitopes of Plasmodium falciparum, along with a GK1 peptide from Taenia solium as adjuvant, and this was expressed in tobacco chloroplasts. Transplastomic tobacco lines were generated using biolistic transformation, and these were confirmed to carry the synthetic gene construct. Expression of the synthetic GK1 peptide was confirmed using RT-PCR and Western blots. Furthermore, the GK1 peptide was detected by HPLC at levels of up to 6 µg g-1 dry weight of tobacco leaf tissue. The plant-derived Malchloroplast candidate vaccine was subsequently tested in BALB/c female mice following subcutaneous administration, and was found to elicit specific humoral responses. Furthermore, components of this candidate vaccine were recognized by antibodies in Plasmodium falciparum malaria patients and were immunogenic in test mice. Thus, this study provided a 'proof of concept' for a promising plant-based candidate subunit vaccine against malaria.

Antibody Combinations Targeting the Essential Antigens CyRPA, RH5, and MSP-119 Potently Neutralize Plasmodium falciparum Clinical Isolates From India and Africa

BACKGROUND

Targeting multiple key antigens that mediate distinct Plasmodium falciparum erythrocyte invasion pathways is an attractive approach for the development of blood-stage malaria vaccines. However, the challenge is to identify antigen cocktails that elicit potent strain-transcending parasite-neutralizing antibodies efficacious at low immunoglobulin G concentrations feasible to achieve through vaccination. Previous reports have screened inhibitory antibodies primarily against well adapted laboratory parasite clones. However, validation of the parasite-neutralizing efficacy against clinical isolates with minimal in vitro cultivation is equally significant to better ascertain their prospective in vivo potency.

METHODS

We evaluated the parasite-neutralizing activity of different antibodies individually and in combinations against laboratory adapted clones and clinical isolates. Clinical isolates were collected from Central India and Mozambique, Africa, and characterized for their invasion properties and genetic diversity of invasion ligands.

RESULTS

In our portfolio, we evaluated 25 triple antibody combinations and identified the MSP-Fu+CyRPA+RH5 antibody combination to elicit maximal parasite neutralization against P. falciparum clinical isolates with variable properties that underwent minimal in vitro cultivation.

CONCLUSIONS

The MSP-Fu+CyRPA+RH5 combination exhibited highly robust parasite neutralization against P. falciparum clones and clinical isolates, thus substantiating them as promising candidate antigens and establishing a proof of principle for the development of a combinatorial P. falciparum blood-stage malaria vaccine.

Functional Characterization of Plasmodium falciparum Surface-Related Antigen as a Potential Blood-Stage Vaccine Target

Plasmodium falciparum erythrocyte invasion is a multistep process that involves a spectrum of interactions that are not well characterized. We have characterized a 113-kDa immunogenic protein, PF3D7_1431400 (PF14_0293), that possesses coiled-coil structures. The protein is localized on the surfaces of both merozoites and gametocytes, hence the name Plasmodium falciparum surface-related antigen (PfSRA). The processed 32-kDa fragment of PfSRA binds normal human erythrocytes with different sensitivities to enzyme treatments. Temporal imaging from initial attachment to internalization of viable merozoites revealed that a fragment of PfSRA, along with PfMSP1_19, is internalized after invasion. Moreover, parasite growth inhibition assays showed that PfSRA P1 antibodies potently inhibited erythrocyte invasion of both sialic acid–dependent and –independent parasite strains. Also, immunoepidemiological studies show that malaria-infected populations have naturally acquired antibodies against PfSRA. Overall, the results demonstrate that PfSRA has the structural and functional characteristics of a very promising target for vaccine development.

Ligand binding properties of two Brugia malayi fatty acid and retinol (FAR) binding proteins and their vaccine efficacies against challenge infection in gerbils

Parasitic nematodes produce an unusual class of fatty acid and retinol (FAR)-binding proteins that may scavenge host fatty acids and retinoids. Two FARs from Brugia malayi (Bm-FAR-1 and Bm-FAR-2) were expressed as recombinant proteins, and their ligand binding, structural characteristics, and immunogenicities examined. Circular dichroism showed that rBm-FAR-1 and rBm-FAR-2 are similarly rich in α-helix structure. Unexpectedly, however, their lipid binding activities were found to be readily differentiated. Both FARs bound retinol and cis-parinaric acid similarly, but, while rBm-FAR-1 induced a dramatic increase in fluorescence emission and blue shift in peak emission by the fluorophore-tagged fatty acid (dansyl-undecanoic acid), rBm-FAR-2 did not. Recombinant forms of the related proteins from Onchocerca volvulus, rOv-FAR-1 and rOv-FAR-2, were found to be similarly distinguishable. This is the first FAR-2 protein from parasitic nematodes that is being characterized. The relative protein abundance of Bm-FAR-1 was higher than Bm-FAR-2 in the lysates of different developmental stages of B. malayi. Both FAR proteins were targets of strong IgG1, IgG3 and IgE antibody in infected individuals and individuals who were classified as endemic normal or putatively immune. In a B. malayi infection model in gerbils, immunization with rBm-FAR-1 and rBm-FAR-2 formulated in a water-in-oil-emulsion (®Montanide-720) or alum elicited high titers of antigen-specific IgG, but only gerbils immunized with rBm-FAR-1 formulated with the former produced a statistically significant reduction in adult worms (68%) following challenge with B. malayi infective larvae. These results suggest that FAR proteins may play important roles in the survival of filarial nematodes in the host, and represent potential candidates for vaccine development against lymphatic filariasis and related filarial infections.

Development of a high-throughput flexible quantitative suspension array assay for IgG against multiple Plasmodium falciparum antigens

Background

Antibody responses to Plasmodium falciparum play a critical role in disease control. Finding reliable IgG biomarkers of protection is complicated by a parasite proteome of over 5000 proteins, some with polymorphisms. Studies of anti-malarial naturally acquired and vaccine immunity would benefit from a standard high-throughput immunoassay to measure multiple antibodies. A multiplex quantitative suspension assay to measure antigen-specific IgGs was developed and its precision (reproducibility and repeatability), dynamic range, limits of detection and quantification, and non-specific binding to different P. falciparum proteins tested. A set of 288 human plasma samples from a malaria-endemic region were analysed twice by two different operators. Another set of samples from 9 malaria-naïve and 10 malaria-exposed individuals were repetitively assayed during 22 consecutive days. Positive controls, negative controls, blanks and microspheres coated with bovine serum albumin were included in all assays.

Results

The multiplex quantitative suspension assay demonstrated low non-specific signal and good estimates of precision and reproducibility between operators. The overall mean of non-specific binding measured in 288 plasma samples was 32.83 to ± 44.81 median fluorescence intensity (MFI). Repeatability was 7.66% ± 15.89 between triplicates for all antigens and samples, being lower in samples from malaria-exposed than malaria-naïve individuals. No evidence of significantly different variance across days in MFI or arbitrary units (AU)/mL was found, assuming homogeneity of variance between days of analysis. Intra-class correlation coefficient between 22 days of analysis was 0.98 (0.97–0.98) for MFI units and 0.9 (0.87–0.93) for AU/mL. Reproducibility between operators for all samples and antigens had an overall adjusted correlation of 0.929 for MFI and 0.836 for AU/mL.

Conclusions

This high-throughput multiplex immunoassay is simple and highly reproducible. This represents an asset for malaria vaccine studies involving CSP-specific antibodies and selected antigens for sero-epidemiological purposes. Measuring a multiplex antigen panel in a single reaction will help to assess not only vaccine immunogenicity but also potential malaria vaccine effects on naturally acquired immune responses. This will accelerate the identification of immune correlates of protection, down-selection of vaccine formulations, antigen discovery and guide second-generation vaccine design.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2365-7) contains supplementary material, which is available to authorized users.

Protein-protein interaction studies reveal the Plasmodium falciparum merozoite surface protein-1 region involved in a complex formation that binds to human erythrocytes

Plasmodium falciparum merozoite surface protein (PfMSP) 1 has been studied extensively as a vaccine candidate antigen. PfMSP-1 undergoes proteolytic processing into four major products, such as p83, p30, p38, and p42, that are associated in the form of non-covalent complex(s) with other MSPs. To delineate MSP1 regions involved in the interaction with other MSPs, here we expressed recombinant proteins (PfMSP-1₆₅) encompassing part of p38 and p42 regions and PfMSP-1₁₉ PfMSP-1₆₅ interacted strongly with PfMSP-3, PfMSP-6, PfMSP-7, and PfMSP-9, whereas PfMSP-1₁₉ did not interact with any of these proteins. Since MSP-1 complex binds human erythrocytes, we examined the ability of these proteins to bind human erythrocyte. Among the proteins of MSP-1 complex, PfMSP-6 and PfMSP-9 bound to human erythrocytes. Serological studies showed that PfMSP-1₆₅ was frequently recognized by sera from malaria endemic regions, whereas this was not the case for PfMSP-1₁₉ In contrast, antibodies against PfMSP-1₁₉ showed much higher inhibition of merozoite invasion compared with antibodies against the larger PfMSP-1₆₅ fragment. Importantly, anti-PfMSP-1₁₉ antibodies recognized both recombinant proteins, PfMSP-1₁₉ and PfMSP-1₆₅; however, anti-PfMSP-1₆₅ antibody failed to recognize the PfMSP-1₁₉ protein. Taken together, these results demonstrate that PfMSP-1 sequences upstream of the 19 kDa C-terminal region are involved in molecular interactions with other MSPs, and these sequences may probably serve as a smoke screen to evade antibody response to the membrane-bound C-terminal 19 kDa region.

Poly(I:C) adjuvant strongly enhances parasite-inhibitory antibodies and Th1 response against Plasmodium falciparum merozoite surface protein-1 (42-kDa fragment) in BALB/c mice

Malaria vaccine development has been confronted with various challenges such as poor immunogenicity of malaria vaccine candidate antigens, which is considered as the main challenge. However, this problem can be managed using appropriate formulations of antigens and adjuvants. Poly(I:C) is a potent Th1 inducer and a human compatible adjuvant capable of stimulating both B- and T-cell immunity. Plasmodium falciparum merozoite surface protein 1₄₂ (PfMSP-1₄₂) is a promising vaccine candidate for blood stage of malaria that has faced several difficulties in clinical trials, mainly due to improper adjuvants. Therefore, in the current study, poly(I:C), as a potent Th1 inducer adjuvant, was evaluated to improve the immunogenicity of recombinant PfMSP-1₄₂, when compared to CFA/IFA, as reference adjuvant. Poly(I:C) produced high level and titers of anti-PfMSP-1₄₂ IgG antibodies in which was comparable to CFA/IFA adjuvant. In addition, PfMSP-1₄₂ formulated with poly(I:C) elicited a higher ratio of IFN-γ/IL-4 (23.9) and IgG2a/IgG1 (3.77) with more persistent, higher avidity, and titer of IgG2a relative to CFA/IFA, indicating a potent Th1 immune response. Poly(I:C) could also help to induce anti-PfMSP-1₄₂ antibodies with higher growth-inhibitory activity than CFA/IFA. Altogether, the results of the current study demonstrated that poly(I:C) is a potent adjuvant that can be appropriate for being used in PfMSP-1₄₂-based vaccine formulations.

Recombinant vaccines of a CD4+ T-cell epitope promote efficient control of Paracoccidioides brasiliensis burden by restraining primary organ infection

Paracoccidioidomycosis (PCM) is an infectious disease endemic to South America, caused by the thermally dimorphic fungi Paracoccidioides. Currently, there is no effective human vaccine that can be used in prophylactic or therapeutic regimes. We tested the hypothesis that the immunogenicity of the immunodominant CD4+ T-cell epitope (P10) of Paracoccidioides brasiliensis gp43 antigen might be significantly enhanced by using a hepatitis B virus-derived particle (VLP) as an antigen carrier. This chimera was administered to mice as a (His)6-purified protein (rPbT) or a replication-deficient human type 5 adenoviral vector (rAdPbT) in an immunoprophylaxis assay. The highly virulent Pb18 yeast strain was used to challenge our vaccine candidates. Fungal challenge evoked robust P10-specific memory CD4+ T cells secreting protective Th-1 cytokines in most groups of immunized mice. Furthermore, the highest level of fungal burden control was achieved when rAdPbT was inoculated in a homologous prime-boost regimen, with 10-fold less CFU recovering than in non-vaccinated mice. Systemic Pb18 spreading was only prevented when rAdPbT was previously inoculated. In summary, we present here VLP/P10 formulations as vaccine candidates against PCM, some of which have demonstrated for the first time their ability to prevent progression of this pernicious fungal disease, which represents a significant social burden in developing countries.

Acquisition, maintenance and adaptation of invasion inhibitory antibodies against Plasmodium falciparum invasion ligands involved in immune evasion

Erythrocyte-binding antigens (EBAs) and P. falciparum reticulocyte-binding homologue proteins (PfRhs) are two important protein families that can vary in expression and utilization by P. falciparum to evade inhibitory antibodies. We evaluated antibodies at repeated time-points among individuals living in an endemic region in Nigeria over almost one year against these vaccine candidates. Antibody levels against EBA140, EBA175, EBA181, PfRh2, PfRh4, and MSP2, were measured by ELISA. We also used parasites with disrupted EBA140, EBA175 and EBA181 genes to show that all these were targets of invasion inhibitory antibodies. However, antigenic targets of inhibitory antibodies were not stable and changed substantially over time in most individuals, independent of age. Antibodies levels measured by ELISA also varied within and between individuals over time and the antibodies against EBA181, PfRh2 and MSP2 declined more rapidly in younger individuals (≤15 years) compared with older (>15). The breadth of high antibody responses over time was more influenced by age than by the frequency of infection. High antibody levels were associated with a more stable invasion inhibitory response, which could indicate that during the long process of formation of immunity, many changes not only in levels but also in functional responses are needed. This is an important finding in understanding natural immunity against malaria, which is essential for making an efficacious vaccine.

Yeast-expressed recombinant As16 protects mice against Ascaris suum infection through induction of a Th2-skewed immune response

Background

Ascariasis remains the most common helminth infection in humans. As an alternative or complementary approach to global deworming, a pan-anthelminthic vaccine is under development targeting Ascaris, hookworm, and Trichuris infections. As16 and As14 have previously been described as two genetically related proteins from Ascaris suum that induced protective immunity in mice when formulated with cholera toxin B subunit (CTB) as an adjuvant, but the exact protective mechanism was not well understood.

Methodology/Principal findings

As16 and As14 were highly expressed as soluble recombinant proteins (rAs16 and rAs14) in Pichia pastoris. The yeast-expressed rAs16 was highly recognized by immune sera from mice infected with A. suum eggs and elicited 99.6% protection against A. suum re-infection. Mice immunized with rAs16 formulated with ISA720 displayed significant larva reduction (36.7%) and stunted larval development against A. suum eggs challenge. The protective immunity was associated with a predominant Th2-type response characterized by high titers of serological IgG1 (IgG1/IgG2a > 2000) and high levels of IL-4 and IL-5 produced by restimulated splenocytes. A similar level of protection was observed in mice immunized with rAs16 formulated with alum (Alhydrogel), known to induce mainly a Th2-type immune response, whereas mice immunized with rAs16 formulated with MPLA or AddaVax, both known to induce a Th1-type biased response, were not significantly protected against A. suum infection. The rAs14 protein was not recognized by A. suum infected mouse sera and mice immunized with rAs14 formulated with ISA720 did not show significant protection against challenge infection, possibly due to the protein’s inaccessibility to the host immune system or a Th1-type response was induced which would counter a protective Th2-type response.

Conclusions/Significance

Yeast-expressed rAs16 formulated with ISA720 or alum induced significant protection in mice against A. suum egg challenge that associates with a Th2-skewed immune response, suggesting that rAS16 could be a feasible vaccine candidate against ascariasis.

Roundworms (Ascaris) infect more than 700 million people living in poverty worldwide and cause malnutrition and physical and mental developmental delays in children. As an alternative or complementary approach to global deworming, a pan-anthelminthic vaccine is under development that targets ascariasis in addition to other human intestinal nematode infections. Towards this goal, two Ascaris suum antigens, As16 and As14, were expressed in Pichia pastoris as recombinant proteins. Mice immunized with rAs16 formulated with ISA720 adjuvant produced significant larva reduction (36.7%) and stunted larval development against A. suum egg challenge. The protection was associated with predominant Th2-type responses characterized by high levels of serological IgG1 (IgG1/IgG2a > 2,000) and Th2 cytokines, IL-4 and IL-5. A similar level of protection was observed in mice immunized with rAs16 formulated with alum that induces mainly a Th2-type immune response, whereas mice immunized with rAs16 formulated with MPLA or AddaVax, both inducing major Th1-type responses, were not significantly protected against A. suum infection. High-yield expression of rAs16 in yeast will allow for large-scale manufacture, and its protective efficacy when formulated with alum suggests its suitability as a vaccine candidate.

Malaria Parasites: The Great Escape

Parasites of the genus Plasmodium have a complex life cycle. They alternate between their final mosquito host and their intermediate hosts. The parasite can be either extra- or intracellular, depending on the stage of development. By modifying their shape, motility, and metabolic requirements, the parasite adapts to the different environments in their different hosts. The parasite has evolved to escape the multiple immune mechanisms in the host that try to block parasite development at the different stages of their development. In this article, we describe the mechanisms reported thus far that allow the Plasmodium parasite to evade innate and adaptive immune responses.

Functional Antibodies and Protection against Blood-stage Malaria

Numerous efforts to understand the functional roles of antibodies demonstrated that they can protect against malaria. However, it is unclear which antibody responses are the best correlates of immunity, and which antibody functions are most important in protection from disease. Understanding the role of antibodies in protection against malaria is crucial for antimalarial vaccine design. In this review, the specific functional properties of naturally acquired and vaccine-induced antibodies that correlate to protection from the blood stages of Plasmodium falciparum malaria are re-examined and the gaps in knowledge related to antibody function in malarial immunity are highlighted.

Progress and prospects for blood-stage malaria vaccines

There have been significant decreases in malaria mortality and morbidity in the last 10-15 years, and the most advanced pre-erythrocytic malaria vaccine, RTS,S, received a positive opinion from European regulators in July 2015. However, no blood-stage vaccine has reached a phase III trial. The first part of this review summarizes the pros and cons of various assays and models that have been and will be used to predict the efficacy of blood-stage vaccines. In the second part, blood-stage vaccine candidates that showed some efficacy in human clinical trials or controlled human malaria infection models are discussed. Then, candidates under clinical investigation are described in the third part, and other novel candidates and strategies are reviewed in the last part.

Merozoite surface proteins in red blood cell invasion, immunity and vaccines against malaria

Malaria accounts for an enormous burden of disease globally, with Plasmodium falciparum accounting for the majority of malaria, and P. vivax being a second important cause, especially in Asia, the Americas and the Pacific. During infection with Plasmodium spp., the merozoite form of the parasite invades red blood cells and replicates inside them. It is during the blood-stage of infection that malaria disease occurs and, therefore, understanding merozoite invasion, host immune responses to merozoite surface antigens, and targeting merozoite surface proteins and invasion ligands by novel vaccines and therapeutics have been important areas of research. Merozoite invasion involves multiple interactions and events, and substantial processing of merozoite surface proteins occurs before, during and after invasion. The merozoite surface is highly complex, presenting a multitude of antigens to the immune system. This complexity has proved challenging to our efforts to understand merozoite invasion and malaria immunity, and to developing merozoite antigens as malaria vaccines. In recent years, there has been major progress in this field, and several merozoite surface proteins show strong potential as malaria vaccines. Our current knowledge on this topic is reviewed, highlighting recent advances and research priorities.

The authors summarize current knowledge of merozoite surface proteins of malaria parasites; their function in invasion, processing of surface proteins before, during and after invasion, their importance as targets of immunity, and the current status of malaria vaccines that target merozoite surface proteins.