Modeling the Impact of a Highly Potent Plasmodium falciparum Transmission-Blocking Monoclonal Antibody in Areas of Seasonal Malaria Transmission.

Transmission-blocking interventions can play an important role in combating malaria worldwide. Recently, a highly potent Plasmodium falciparum transmission-blocking monoclonal antibody (TB31F) was demonstrated to be safe and efficacious in malaria-naive volunteers. Here we predict the potential public health impact of large-scale implementation of TB31F alongside existing interventions. We developed a pharmaco-epidemiological model, tailored to 2 settings of differing transmission intensity with already established insecticide-treated nets and seasonal malaria chemoprevention interventions. Community-wide annual administration (at 80% coverage) of TB31F over a 3-year period was predicted to reduce clinical incidence by 54% (381 cases averted per 1000 people per year) in a high-transmission seasonal setting, and 74% (157 cases averted per 1000 people per year) in a low-transmission seasonal setting. Targeting school-aged children gave the largest reduction in terms of cases averted per dose. An annual administration of the transmission-blocking monoclonal antibody TB31F may be an effective intervention against malaria in seasonal malaria settings.

Mass campaigns combining antimalarial drugs and anti-infective vaccines as seasonal interventions for malaria control, elimination and prevention of resurgence: a modelling study.

BACKGROUND: The only licensed malaria vaccine, RTS,S/AS01, has been developed for morbidity-control in young children. The potential impact on transmission of deploying such anti-infective vaccines to wider age ranges, possibly with co-administration of antimalarial treatment, is unknown. Combinations of existing malaria interventions is becoming increasingly important as evidence mounts that progress on reducing malaria incidence is stalling and threatened by resistance. METHODS: Malaria transmission and intervention dynamics were simulated using OpenMalaria, an individual-based simulation model of malaria transmission, by considering a seasonal transmission setting and by varying epidemiological and setting parameters such as transmission intensity, case management, intervention types and intervention coverages. Chemopreventive drugs and anti-infective vaccine efficacy profiles were based on previous studies in which model parameters were fitted to clinical trial data. These intervention properties were used to evaluate the potential of seasonal mass applications of preventative anti-infective malaria vaccines, alone or in combination with chemoprevention, to reduce malaria transmission, prevent resurgence, and/or reach transmission interruption. RESULTS: Deploying a vaccine to all ages on its own is a less effective intervention strategy compared to chemoprevention alone. However, vaccines combined with drugs are likely to achieve dramatic prevalence reductions and in few settings, transmission interruption. The combined mass intervention will result in lower prevalence following the intervention compared to chemoprevention alone and will increase chances of interruption of transmission resulting from a synergistic effect between both interventions. The combination of vaccine and drug increases the time before transmission resurges after mass interventions cease compared to mass treatment alone. Deploying vaccines and drugs together requires fewer rounds of mass intervention and fewer years of intervention to achieve the same public health impact as chemoprevention alone. CONCLUSIONS: Through simulations we identified a previously unidentified value of deploying vaccines with drugs, namely the greatest benefit will be in preventing and delaying transmission resurgence for longer periods than with other human targeted interventions. This is suggesting a potential role for deploying vaccines alongside drugs in transmission foci as part of surveillance-response strategies.

Safety and efficacy of novel malaria vaccine regimens of RTS,S/AS01B alone, or with concomitant ChAd63-MVA-vectored vaccines expressing ME-TRAP.

We assessed a combination multi-stage malaria vaccine schedule in which RTS,S/AS01B was given concomitantly with viral vectors expressing multiple-epitope thrombospondin-related adhesion protein (ME-TRAP) in a 0-month, 1-month, and 2-month schedule. RTS,S/AS01B was given as either three full doses or with a fractional (1/5th) third dose. Efficacy was assessed by controlled human malaria infection (CHMI). Safety and immunogenicity of the vaccine regimen was also assessed. Forty-one malaria-naive adults received RTS,S/AS01B at 0, 4 and 8 weeks, either alone (Groups 1 and 2) or with ChAd63 ME-TRAP at week 0, and modified vaccinia Ankara (MVA) ME-TRAP at weeks 4 and 8 (Groups 3 and 4). Groups 2 and 4 received a fractional (1/5th) dose of RTS,S/AS01B at week 8. CHMI was delivered by mosquito bite 11 weeks after first vaccination. Vaccine efficacy was 6/8 (75%), 8/9 (88.9%), 6/10 (60%), and 5/9 (55.6%) of subjects in Groups 1, 2, 3, and 4, respectively. Immunological analysis indicated significant reductions in anti-circumsporozoite protein antibodies and TRAP-specific T cells at CHMI in the combination vaccine groups. This reduced immunogenicity was only observed after concomitant administration of the third dose of RTS,S/AS01B with the second dose of MVA ME-TRAP. The second dose of the MVA vector with a four-week interval caused significantly higher anti-vector immunity than the first and may have been the cause of immunological interference. Co-administration of ChAd63/MVA ME-TRAP with RTS,S/AS01B led to reduced immunogenicity and efficacy, indicating the need for evaluation of alternative schedules or immunization sites in attempts to generate optimal efficacy.

The frontline of controlled human malaria infections: A report from the controlled human infection models Workshop in Leiden University Medical Centre 5 May 2016.

Controlled Human Malaria Infection (CHMI) is the most practiced controlled human infection model nowadays and there is an exponential increase in implementation of the model worldwide. During the Controlled Human Infection Models Workshop in Leiden, one day was dedicated to the discussion of the advances made and gaps in Controlled Human Malaria Infection (CHMI) trials. Factors contributing to this impressive expansion in the number of CHMI trials have been related to the ability to perform CHMI using injectable cryopreserved sporozoites (a product from Sanaria Inc. - PfSPZ Challenge), the development of a transmission blocking CHMI model and the need to test more vaccine candidates particularly in the field of whole-sporozoite vaccine development. However, with an increasing number of CHMI trials being undertaken, in an ever-growing number of trial sites, heterogeneity in trial design may compromise universal interpretation of results and require an ongoing dialogue on the need and feasibility of standardization. At the workshop, CHMI investigators convened to share their experiences in CHMI trials and discuss the possibilities for future trials.

Assessment of Pfs25 expressed from multiple soluble expression platforms for use as transmission-blocking vaccine candidates.

BACKGROUND: Transmission-blocking vaccines (TBVs) have become a focus of strategies to control and eventually eliminate malaria as they target the entry of sexual stage into the Anopheles stephensi mosquito thereby preventing transmission, an essential component of the parasite life cycle. Such vaccines are envisioned as complements to vaccines that target human infection, such as RTS,S as well as drug treatment, and vector control strategies. A number of conserved proteins, including Pfs25, have been identified as promising TBV targets in research or early stage development. Pfs25 is a 25 kDa protein of Plasmodium falciparum expressed on the surface of zygotes and ookinetes. Its complex tertiary structure, including numerous cysteines, has led to difficulties in the expression of a recombinant protein that is homogeneous, with proper conformation, and free of glycosylation, a phenomenon not found in native parasite machinery. METHODS: While the expression and purification of Pfs25 in various systems, has been previously independently reported, here a parallel analysis of Pfs25 is presented to inform on the biochemical features of Pfs25 and their impact on functionality. Three scalable expression systems were used to express, purify, and evaluate Pfs25 both in vitro and in vivo, including the ability of each protein to produce functional antibodies through the standard membrane feeding assay. RESULTS: Through numerous attempts, soluble, monomeric Pfs25 derived from Escherichia coli was not achieved, while Pichia pastoris presented Pfs25 as an inhomogeneous product with glycosylation. In comparison, baculovirus produced a pure, monomeric protein free of glycosylation. The glycosylation present for Pichia produced Pfs25, showed no notable decrease in the ability to elicit transmission reducing antibodies in functional evaluation, while a reduced and alkylated Pfs25 (derived from plant and used as a control) was found to have significantly decreased transmission reducing activity, emphasizing the importance of ensuring correct disulfide stabilized conformation during vaccine design and production. CONCLUSIONS: In this study, the biochemical features of Pfs25, produced from different expression systems, are described along with their impact on the ability of the protein to elicit functional antibodies. Pfs25 expressed using baculovirus and Pichia showed promise as candidates for vaccine development.

The effect of immunization schedule with the malaria vaccine candidate RTS,S/AS01E on protective efficacy and anti-circumsporozoite protein antibody avidity in African infants.

BACKGROUND: The malaria vaccine RTS,S induces antibodies against the Plasmodium falciparum circumsporozoite protein (CSP) and the concentration of Immunoglobulin G (IgG) against the repeat region of CSP following vaccination is associated with protection from P. falciparum malaria. So far, only the quantity of anti-CSP IgG has been measured and used to predict vaccination success, although quality (measured as avidity) of the antigen-antibody interaction shall be important since only a few sporozoites circulate for a short time after an infectious mosquito bite, likely requiring fast and strong binding. METHODS: Quantity and avidity of anti-CSP IgG in African infants who received RTS,S/AS01E in a 0-1-2-month or a 0-1-7-month schedule in a phase 2 clinical trial were measured by enzyme-linked immunosorbent assay. Antibody avidity was defined as the proportion of IgG able to bind in the presence of a chaotropic agent (avidity index). The effect of CSP-specific IgG concentration and avidity on protective efficacy was modelled using Cox proportional hazards. RESULTS: After the third dose, quantity and avidity were similar between the two vaccination schedules. IgG avidity after the last vaccine injection was not associated with protection, whereas the change in avidity following second and third RTS,S/AS01E injection was associated with a 54% risk reduction of getting malaria (hazard ratio: 0.46; 95% confidence interval (CI): 0.22-0.99) in those participants with a change in avidity above the median. The change in anti-CSP IgG concentration following second and third injection was associated with a 77% risk reduction of getting malaria (hazard ratio: 0.23, 95% CI: 0.11-0.51). CONCLUSIONS: Change in IgG response between vaccine doses merits further evaluation as a surrogate marker for RTS,S efficacy. TRIAL REGISTRATION: ClinicalTrials.gov Identifier NCT00436007 .

Safety and comparability of controlled human Plasmodium falciparum infection by mosquito bite in malaria-naïve subjects at a new facility for sporozoite challenge.

BACKGROUND: Controlled human malaria infection (CHMI) studies which recapitulate mosquito-borne infection are a critical tool to identify protective vaccine and drug candidates for advancement to field trials. In partnership with the Walter Reed Army Institute of Research, the CHMI model was established at the Seattle Biomedical Research Institute's Malaria Clinical Trials Center (MCTC). Activities and reagents at both centers were aligned to ensure comparability and continued safety of the model. To demonstrate successful implementation, CHMI was performed in six healthy malaria-naïve volunteers. METHODS: All volunteers received NF54 strain Plasmodium falciparum by the bite of five infected Anopheles stephensi mosquitoes under controlled conditions and were monitored for signs and symptoms of malaria and for parasitemia by peripheral blood smear. Subjects were treated upon diagnosis with chloroquine by directly observed therapy. Immunological (T cell and antibody) and molecular diagnostic (real-time quantitative reverse transcriptase polymerase chain reaction [qRT-PCR]) assessments were also performed. RESULTS: All six volunteers developed patent parasitemia and clinical malaria. No serious adverse events occurred during the study period or for six months post-infection. The mean prepatent period was 11.2 days (range 9-14 days), and geometric mean parasitemia upon diagnosis was 10.8 parasites/µL (range 2-69) by microscopy. qRT-PCR detected parasites an average of 3.7 days (range 2-4 days) earlier than blood smears. All volunteers developed antibodies to the blood-stage antigen merozoite surface protein 1 (MSP-1), which persisted up to six months. Humoral and cellular responses to pre-erythrocytic antigens circumsporozoite protein (CSP) and liver-stage antigen 1 (LSA-1) were limited. CONCLUSION: The CHMI model was safe, well tolerated and characterized by consistent prepatent periods, pre-symptomatic diagnosis in 3/6 subjects and adverse event profiles as reported at established centers. The MCTC can now evaluate candidates in the increasingly diverse vaccine and drug pipeline using the CHMI model. TRIAL REGISTRATION: ClinicalTrials.gov NCT01058226.

Impact of RTS,S/AS02(A) and RTS,S/AS01(B) on genotypes of P. falciparum in adults participating in a malaria vaccine clinical trial.

OBJECTIVE: RTS,S, a candidate vaccine for malaria, is a recombinant protein expressed in yeast containing part of the circumsporozoite protein (CSP) sequence of 3D7 strain of Plasmodium falciparum linked to the hepatitis B surface antigen in a hybrid protein. The RTS,S antigen is formulated with GSK Biologicals' proprietary Adjuvant Systems AS02(A) or AS01(B). A recent trial of the RTS,S/AS02(A) and RTS,S/AS01(B) vaccines evaluated safety, immunogenicity and impact on the development of parasitemia of the two formulations. Parasite isolates from this study were used to determine the molecular impact of RTS,S/AS02(A) and RTS,S/AS01(B) on the multiplicity of infection (MOI) and the csp allelic characteristics of subsequent parasitemias. DESIGN: The distribution of csp sequences and the MOI of the infecting strains were examined at baseline and in break-through infections from vaccinated individuals and from those receiving a non-malarial vaccine. SETTING: The study was conducted in Kombewa District, western Kenya. PARTICIPANTS: Semi-immune adults from the three study arms provided isolates at baseline and during break-through infections. OUTCOME: Parasite isolates used for determining MOI and divergence of csp T cell-epitopes were 191 at baseline and 87 from break-through infections. RESULTS: Grouping recipients of RTS,S/AS01(A) and RTS,S/AS02(B) together, vaccine recipients identified as parasite-positive by microscopy contained significantly fewer parasite genotypes than recipients of the rabies vaccine comparator (median in pooled RTS,S groups: 3 versus 4 in controls, P = 0.0313). When analyzed separately, parasitaemic individuals in the RTS,S/AS01(B) group, but not the RTS,S/AS02(A) group, were found to have significantly fewer genotypes than the comparator group. Two individual amino acids found in the vaccine construct (Q339 in Th2R and D371 in Th3R) were observed to differ in incidence between vaccine and comparator groups but in different directions; parasites harboring Q339 were less common among pooled RTS,S/AS vaccine recipients than among recipients of rabies vaccine, whereas parasites with D371 were more common among the RTS,S/AS groups. CONCLUSIONS: It is concluded that both RTS,S/AS vaccines reduce multiplicity of infection. Our results do not support the hypothesis that RTS,S/AS vaccines elicit preferential effects against pfcsp alleles with sequence similarity to the 3D7 pfcsp sequence employed in the vaccine construct.

Advances in immunology and vaccine discovery report of the United States-European Commission workshop.

As the 21st century unfolds, infectious diseases remain one of the most significant threats to our economy, our food animal production systems, animal welfare, and most importantly, the lives of people worldwide, regardless of their economic standing. The potential use of biological threat agents for terrorism or biowarfare further undermines the security of our society. Arguably, vaccines represent the single most cost-effective, medically delivered strategy for confronting these challenges. The workshop "Advances in Immunology and Vaccine Discovery" was organized to address these challenges, based on the conviction that the interface between immunology and vaccinology offers the best prospects for major breakthroughs in vaccine discovery and development. Six focus areas were identified by workshop organizers: (1) pathogen immune evasion; (2) innate immunity; (3) mucosal immunity; (4) immunogenetics; (5) comparative immunology; and (6) genomics. These areas provided opportunities to elucidate how protective immunity may relate to the disruption of the molecular mechanisms that underlie host-pathogen interactions. A report generated by workshop organizers and participants provides key recommendations and identifies important research gaps, needs, future steps, and potential strategic US-EU collaborations. The report is available on line through ScienceDirect (URL).

Phase I dose escalation safety and immunogenicity trial of Plasmodium falciparum apical membrane protein (AMA-1) FMP2.1, adjuvanted with AS02A, in malaria-naïve adults at the Walter Reed Army Institute of Research.

We report the first safety and immunogenicity trial of the Plasmodium falciparum vaccine candidate FMP2.1/AS02A, a recombinant E. coli-expressed protein based upon the apical membrane antigen-1 (AMA-1) of the 3D7 clone formulated with the AS02A adjuvant. We conducted an open-label, staggered-start, dose-escalating Phase I trial in 23 malaria-naïve volunteers who received 8, 20 or 40microg of FMP2.1 in a fixed volume of 0.5mL of AS02A on a 0, 1, and 2 month schedule. Nineteen of 23 volunteers received all three scheduled immunizations. The most frequent solicited local and systemic adverse events associated with immunization were injection site pain (68%) and headache (29%). There were no significant laboratory abnormalities or vaccine-related serious adverse events. All volunteers seroconverted after second immunization as determined by ELISA. Immune sera recognized sporozoites and merozoites by immunofluorescence assay (IFA), and exhibited both growth inhibition and processing inhibition activity against homologous (3D7) asexual stage parasites. Post-immunization, peripheral blood mononuculear cells exhibited FMP2.1-specific lymphoproliferation and IFN-gamma and IL-5 ELISPOT assay responses. This is the first PfAMA-1-based vaccine shown to elicit both potent humoral and cellular immunity in humans. Encouraged by the potential of FMP1/AS02A to target host immunity against PfAMA-1 that is known to be expressed by sporozoite, hepatic and erythrocytic stages, we have initiated field trials of FMP2.1/AS02A in an endemic population in the Republic of Mali.

Safety, tolerability, and antibody responses in humans after sequential immunization with a PfCSP DNA vaccine followed by the recombinant protein vaccine RTS,S/AS02A.

Optimal protection against malaria may require induction of high levels of protective antibody and CD8(+) and CD4(+) T cell responses. In humans, malaria DNA vaccines elicit CD8(+) cytotoxic T cells (CTL) and IFNgamma responses as measured by short-term (ex vivo) ELISPOT assays, and recombinant proteins elicit antibodies and excellent T cell responses, but no CD8(+) CTL or CD8(+) IFNgamma-producing cells as measured by ex vivo ELISPOT. Priming with DNA and boosting with recombinant pox virus elicits much better T cell responses than DNA alone, but not antibody responses. In an attempt to elicit antibodies and enhanced T cell responses, we administered RTS,S/AS02A, a partially protective Plasmodium falciparum recombinant circumsporozoite protein (CSP) vaccine in adjuvant, to volunteers previously immunized with a P. falciparum CSP DNA vaccine (VCL-2510) and to naïve volunteers. This vaccine regimen was well tolerated and safe. The volunteers who received RTS,S/AS02A alone had, as expected, antibody and CD4(+) T cell responses, but no CD8(+) T cell responses. Volunteers who received PfCSP DNA followed by RTS,S/AS02A had antibody and CD8(+) and CD4(+) T cell responses (Wang et al., submitted). Sequential immunization with DNA and recombinant protein, also called heterologous prime-boost, led to enhanced immune responses as compared to DNA or recombinant protein alone, suggesting that it might provide enhanced protective immunity.