Human Malaria Infection Research Articles

Plasmodium falciparum specific helicase 2 is a dual, bipolar helicase and is crucial for parasite growth

Human malaria infection is a major challenge across the globe and is responsible for millions of deaths annually. Rapidly emerging drug resistant strains against the new class of anti-malarial drugs are major threat to control the disease burden worldwide. Helicases are present in every organism and have important role in various nucleic acid metabolic processes. Previously we have reported the presence of three parasite specific helicases (PSH) in Plasmodium falciparum 3D7 strain. Here we present the detailed biochemical characterization of PfPSH2. PfPSH2 is DNA and RNA stimulated ATPase and is able to unwind partially duplex DNA and RNA substrates. It can translocate in both 3′ to 5′ and 5′ to 3′ directions. PfPSH2 is expressed in all the stages of intraerythrocytic development and it is localized in cytoplasm in P. falciparum 3D7 strain. The dsRNA mediated inhibition study suggests that PfPSH2 is important for the growth and survival of the parasite. This study presents the detailed characterization of PfPSH2 and lays the foundation for future development of PfPSH2 as drug target.

Antibody Responses to Antigenic Targets of Recent Exposure Are Associated With Low-Density Parasitemia in Controlled Human Plasmodium falciparum Infections.

The majority of malaria infections in low transmission settings remain undetectable by conventional diagnostics. A powerful model to identify antibody responses that allow accurate detection of recent exposure to low-density infections is controlled human malaria infection (CHMI) studies in which healthy volunteers are infected with the Plasmodium parasite. We aimed to evaluate antibody responses in malaria-naïve volunteers exposed to a single CHMI using a custom-made protein microarray. All participants developed a blood-stage infection with peak parasite densities up to 100 parasites/μl in the majority of participants (50/54), while the remaining four participants had peak densities between 100 and 200 parasites/μl. There was a strong correlation between parasite density and antibody responses associated with the most reactive blood-stage targets 1 month after CHMI (Etramp 5, GLURP-R2, MSP4 and MSP1-19; Spearman’s ρ = 0.82, p < 0.001). Most volunteers developed antibodies against a potential marker of recent exposure: Etramp 5 (37/45, 82%). Our findings justify validation in endemic populations to define a minimum set of antigens needed to detect exposure to natural low-density infections.

Controlled Human Malaria Infection (CHMI) Studies: Over 100 Years of Experience with Parasite Injections.

Human experimentation by deliberate infection with malarial parasites seems unethical yet has a long history in infectious disease research. After rigorous screening, volunteers are inoculated with Plasmodium sporozoites or blood stages and monitored under strict clinical supervision until they are treated with a licensed malaria drug and the infection is completely resolved. Historically, experimental Plasmodium challenge infections were applied to confirm that Anopheles mosquitoes were the malaria vector and to treat neurosyphilis in Treponema pallidum-infected patients. The lifesaving treatment with reliable parasite inoculation, monitoring, and drug cure was awarded with a Nobel Prize in 1927 and paved the way for human trials for clinical tests of candidate drugs and vaccines. Importantly, controlled human malaria infection (CHMI) studies are indispensable to bridge the major gap between phase I safety and phase II field trials. Here, we describe the biological basis, historical experiences, applications, and ethical considerations for CHMI studies. Acceleration of antimalarial drug and vaccine development remains a priority in medical research and critically depends on capacity building for CHMI studies.

Exploring Ethical Concerns About Human Challenge Studies: A Qualitative Study of Controlled Human Malaria Infection Study Participants' Motivations and Attitudes.

Controlled human malaria infection (CHMI) studies deliberately infect healthy participants with malaria to test interventions faster and more efficiently. Some argue the study design and high payments offered raise ethical concerns about participants' understanding of risks and undue inducement. We conducted baseline and exit interviews with 16 CHMI study participants to explore these concerns. Participants described themes including decision-making tension with friends and family, mixed motivations for participating, low study risks but high burdens, fair compensation, sacrificing values, deceiving researchers, and perceived benefits. Our findings do not support concerns that high payments limit understanding of study risks, but suggest participants may lack appreciation of study burdens, withhold information or engage in deception, and experience conflict with others regarding study participation.

Controlled Human Malaria Infection in Semi-Immune Kenyan Adults (CHMI-SIKA): a study protocol to investigate in vivo Plasmodium falciparum malaria parasite growth in the context of pre-existing immunity

Malaria remains a major public health burden despite approval for implementation of a partially effective pre-erythrocytic malaria vaccine. There is an urgent need to accelerate development of a more effective multi-stage vaccine. Adults in malaria endemic areas may have substantial immunity provided by responses to the blood stages of malaria parasites, but field trials conducted on several blood-stage vaccines have not shown high levels of efficacy. We will use controlled human malaria infection (CHMI) studies with malaria-exposed volunteers to identify correlations between immune responses and parasite growth rates in vivo. Immune responses more strongly associated with control of parasite growth should be prioritized to accelerate malaria vaccine development. We aim to recruit up to 200 healthy adult volunteers from areas of differing malaria transmission in Kenya, and after confirming their health status through clinical examination and routine haematology and biochemistry, we will comprehensively characterize immunity to malaria using >100 blood-stage antigens. We will administer 3,200 aseptic, purified, cryopreserved Plasmodium falciparum sporozoites (PfSPZ Challenge) by direct venous inoculation. Serial quantitative polymerase chain reaction to measure parasite growth rate in vivo will be undertaken. Clinical and laboratory monitoring will be undertaken to ensure volunteer safety. In addition, we will also explore the perceptions and experiences of volunteers and other stakeholders in participating in a malaria volunteer infection study. Serum, plasma, peripheral blood mononuclear cells and extracted DNA will be stored to allow a comprehensive assessment of adaptive and innate host immunity. We will use CHMI in semi-immune adult volunteers to relate parasite growth outcomes with antibody responses and other markers of host immunity. Registration: ClinicalTrials.gov identifier NCT02739763.

Antiplasmodial Activity of Ethanolic Leaf Extract of Eucalyptus citriodora in Swiss Albino Mice Infected with Plasmodium berghei NK 65

Malaria is a life-threatening disease and emergence of malaria parasite resistance to antimalarial drugs, has necessitated the need for discovery and development of an alternative to malaria medicine. This study assessed the ethanolic leaf extract of Eucalyptus citriodora for the presence of bioactive components qualitatively and efficacy of the extract against the malaria parasite. Standard methods were used to determine the bioactive components of the leaf extract. Twenty (20) albino mice of body weight between 18-25 g were randomised into 5 groups of four mice each for acute toxicity test, while twenty-four (24) mice were randomised into six groups of four mice each (group 1, 2, 3, 4, 5 and 6) for antiplasmodial activity. All the groups were infected with P. berghei, except group 3 (normal control). Group 4, 5 and 6 were treated with 0.2 mL of 200, 400 and 800 mg/kg body weight of extract respectively. Group 2 (positive control) were treated with 0.2 mL of 5 mg/kg body weight of chloroquine. Group 1 (negative control) were administered with 0.2 mL of normal saline, while group 3 (normal control) were administered with 0.2 mL of normal saline for four consecutive days. Phytochemical screening revealed the presence of alkaloids, saponins, tannins, anthraquinone, flavonoids and cardiac glycosides and the extract was found safe and nontoxic. The antiplasmodial investigation revealed a decrease in percentage parasitaemia level in mice of group 4, 5 and 6 (extract treated group) compared with mice of group 1 (infected and not treated). The parasitaemia reduction was higher in group 6 (800 mg/kg). This significant decrease (P&lt;0.05) in percentage parasitaemia level in the study was dose and time-dependent. The study revealed the potency of E. citriodora leaf extract as a future herbal candidate for the treatment of human malaria infection.

Ethical considerations in Controlled Human Malaria Infection studies in low resource settings: Experiences and perceptions of study participants in a malaria Challenge study in Kenya.

Background: The range and amount of volunteer infection studies, known as Controlled Human Infection Model (CHMI) studies, in Low-Middle Income Countries (LMICs) is increasing with rapid technological advancement, world-class laboratory facilities and increasing capacity development initiatives. However, the ethical issues these studies present in LMICs have not been empirically studied. We present findings of a descriptive social science study nested within a malaria volunteer infection study, on-going at the time of writing, at the KEMRI-Wellcome Trust Research Programme (KWTRP) on the Kenyan Coast. Methods: The study included non-participant observations, five group discussions with more than half of the CHMI study participants, two in-depth interviews with study team members, and an exit questionnaire administered to the participants. Results: Participants understood the key elements of the study, including that they would be deliberately infected with malaria parasites and may get malaria as a result, there would be regular blood draws, and they would spend up to 24 days in a residence facility away from their homes. The greatest motivation for participation was the monetary compensation of 20 USD per overnight stay given as a lump-sum at the end of their residency stay. Also appreciated were the health screening tests prior to enrolment and the positive relations with the study team. Concerns raised included the amount and regularity of blood draws experienced, and concerns that this type of research may feed into on-going rumours about research generally. Conclusion: With the increasing range and number of CHMI studies being conducted in LMICs, current ethical guidance are inadequate. This study highlights some of the ethical issues that could emerge in these settings, emphasizing the heavy responsibility placed on research review and regulatory systems, researchers and funders, as well as the importance of carefully tailored community engagement and consent processes.

Controlled Human Infections As a Tool to Reduce Uncertainty in Clinical Vaccine Development.

Vaccines can be extremely cost-effective public health measures. Unfortunately the research and development (R&D) of novel vaccines is suffering from rising costs and declining success rates. Because many vaccines target low- and middle income markets (LMIC), output needs to be maintained at a constrained budget. In addition, scientific neglect and political uncertainty around reimbursement decisions make it an unattractive arena for private investors. The vaccine development pipeline for LMIC thus is in need for a different, sustainable, and cost-effective development model. In conventional vaccine development, objectives for every clinical development phase have been predefined. However, given the scarcity of resources, the most efficient clinical development path should identify vaccine candidates with the highest potential impact as soon as possible. We argue for a custom-made question-based development path based on the scientific questions, success probabilities and investments required. One question can be addressed by several studies and one study can provide partial answers to multiple questions. An example of a question-based approach is the implementation of a controlled human malaria infection model (CHMI). Malaria vaccine R&D faces major scientific challenges and has limited resources. Therefore, early preliminary efficacy data needs to be obtained in order to reallocate resources as efficiently as possible and reduce clinical development costs. To meet this demand, novel malaria vaccines are tested for efficacy in so-called CHMI trials in which small groups of healthy volunteers are vaccinated and subsequently infected with malaria. Early evaluation studies of critical questions, such as CHMI, are highly rewarding, since they prevent expenditures on projects that are unlikely to succeed. Each set of estimated probabilities and costs (combined with market value) will have its own optimal priority sequence of questions to address. Algorithms can be designed to determine the optimal order in which questions should be addressed. Experimental infections of healthy volunteers is an example of how a question-based approach to vaccine development can be implemented and has the potential to change the arena of clinical vaccine development.

γδ T Cells in Antimalarial Immunity: New Insights Into Their Diverse Functions in Protection and Tolerance.

Uniquely expressing diverse innate-like and adaptive-like functions, γδ T cells exist as specialized subsets, but are also able to adapt in response to environmental cues. These cells have long been known to rapidly proliferate following primary malaria infection in humans and mice, but exciting new work is shedding light into their diverse functions in protection and following repeated malaria infection. In this review, we examine the current knowledge of functional specialization of γδ T cells in malaria, and the mechanisms dictating recognition of malaria parasites and resulting proliferation. We discuss γδ T cell plasticity, including changing interactions with other immune cells during recurrent infection and potential for immunological memory in response to repeated stimulation. Building on recent insights from human and murine experimental studies and vaccine trials, we propose areas for future research, as well as applications for therapeutic development.

Malaria prevention: from immunological concepts to effective vaccines and protective antibodies.

Development of a malaria vaccine remains a critical priority to decrease clinical disease and mortality and facilitate eradication. Accordingly, RTS,S, a protein-subunit vaccine, has completed phase III clinical trials and confers ~30% protection against clinical infection over 4 years. Whole-attenuated-sporozoite and viral-subunit vaccines induce between 20% and 100% protection against controlled human malaria infection, but there is limited published evidence to date for durable, high-level efficacy (>50%) against natural exposure. Importantly, fundamental scientific advances related to the potency, durability, breadth and location of immune responses will be required for improving vaccine efficacy with these and other vaccine approaches. In this Review, we focus on the current understanding of immunological mechanisms of protection from animal models and human vaccine studies, and on how these data should inform the development of next-generation vaccines. Furthermore, we introduce the concept of using passive immunization with monoclonal antibodies as a new approach to prevent and eliminate malaria.

Ethical considerations in Controlled Human Malaria Infection studies in low resource settings: Experiences and perceptions of study participants in a malaria Challenge study in Kenya

Background: The range and amount of volunteer infection studies, known as Controlled Human Infection Model (CHMI) studies, in Low-Middle Income Countries (LMICs) is increasing with rapid technological advancement, world-class laboratory facilities and increasing capacity development initiatives. However, the ethical issues these studies present in LMICs have not been empirically studied. We present findings of a descriptive social science study nested within a malaria volunteer infection study, on-going at the time of writing, at the KEMRI-Wellcome Trust Research Programme (KWTRP) on the Kenyan Coast. Methods: The study included non-participant observations, five group discussions with more than half of the CHMI study participants, two in-depth interviews with study team members, and an exit questionnaire administered to the participants. Results: Participants understood the key elements of the study, including that they would be deliberately infected with malaria parasites and may get malaria as a result, there would be regular blood draws, and they would spend up to 24 days in a residence facility away from their homes. The greatest motivation for participation was the monetary compensation of 20 USD per overnight stay given as a lump-sum at the end of their residency stay. Also appreciated were the health screening tests prior to enrolment and the positive relations with the study team. Concerns raised included the amount and regularity of blood draws experienced, and concerns that this type of research may feed into on-going rumours about research generally. Conclusion: With the increasing range and number of CHMI studies being conducted in LMICs, current ethical guidance are inadequate. This study highlights some of the ethical issues that could emerge in these settings, emphasizing the heavy responsibility placed on research review and regulatory systems, researchers and funders, as well as the importance of carefully tailored community engagement and consent processes.

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.

Using Machine Learning to Predict Synergistic Antimalarial Compound Combinations With Novel Structures.

The parasite Plasmodium falciparum is the most lethal species of Plasmodium to cause serious malaria infection in humans, and with resistance developing rapidly novel treatment modalities are currently being sought, one of which being combinations of existing compounds. The discovery of combinations of antimalarial drugs that act synergistically with one another is hence of great importance; however an exhaustive experimental screen of large drug space in a pairwise manner is not an option. In this study we apply our machine learning approach, Combination Synergy Estimation (CoSynE), which can predict novel synergistic drug interactions using only prior experimental combination screening data and knowledge of compound molecular structures, to a dataset of 1,540 antimalarial drug combinations in which 22.2% were synergistic. Cross validation of our model showed that synergistic CoSynE predictions are enriched 2.74 × compared to random selection when both compounds in a predicted combination are known from other combinations among the training data, 2.36 × when only one compound is known from the training data, and 1.5 × for entirely novel combinations. We prospectively validated our model by making predictions for 185 combinations of 23 entirely novel compounds. CoSynE predicted 20 combinations to be synergistic, which was experimentally validated for nine of them (45%), corresponding to an enrichment of 1.70 × compared to random selection from this prospective data set. Such enrichment corresponds to a 41% reduction in experimental effort. Interestingly, we found that pairwise screening of the compounds CoSynE individually predicted to be synergistic would result in an enrichment of 1.36 × compared to random selection, indicating that synergy among compound combinations is not a random event. The nine novel and correctly predicted synergistic compound combinations mainly (where sufficient bioactivity information is available) consist of efflux or transporter inhibitors (such as hydroxyzine), combined with compounds exhibiting antimalarial activity alone (such as sorafenib, apicidin, or dihydroergotamine). However, not all compound synergies could be rationalized easily in this way. Overall, this study highlights the potential for predictive modeling to expedite the discovery of novel drug combinations in fight against antimalarial resistance, while the underlying approach is also generally applicable.

Malaria vaccine candidate based on Duffy-binding protein elicits strain transcending functional antibodies in a Phase I trial

Reticulocyte invasion by Plasmodium vivax requires interaction of the Duffy-binding protein (PvDBP) with host Duffy antigen receptor for chemokines (DARCs). The binding domain of PvDBP maps to a cysteine-rich region referred to as region II (PvDBPII). Blocking this interaction offers a potential path to prevent P. vivax blood-stage growth and P. vivax malaria. This forms the rationale for development of a vaccine based on PvDBPII. Here we report results of a Phase I randomized trial to evaluate the safety and immunogenicity of recombinant PvDBPII formulated with glucopyranosyl lipid adjuvant-stable emulsion (GLA-SE). Thirty-six malaria-naive, healthy Indian male subjects aged 18–45 years were assigned into three cohorts corresponding to doses of 10, 25 and 50 µg of PvDBPII formulated with 5 µg of GLA-SE. Each cohort included nine PvDBPII/GLA-SE vaccinees and three hepatitis B control vaccine recipients. Each subject received the assigned vaccine intramuscularly on days 0, 28 and 56, and was followed up till day 180. No serious AE was reported and PvDBPII/GLA-SE was well-tolerated and safe. Analysis by ELISA showed that all three doses of PvDBPII elicited antigen-specific binding-inhibitory antibodies. The 50 µg dose elicited antibodies against PvDBPII that had the highest binding-inhibitory titres and were most persistent. Importantly, the antibody responses were strain transcending and blocked receptor binding of diverse PvDBP alleles. These results support further clinical development of PvDBPII/GLA-SE to evaluate efficacy against sporozoite or blood-stage challenge in controlled human malaria infection (CHMI) models and against natural P. vivax challenge in malaria endemic areas.

Malaria systems immunology: Plasmodium vivax induces tolerance during primary infection through dysregulation of neutrophils and dendritic cells

To dissect the transcriptional networks underpinning immune cells responses during primary Plasmodium vivax infection of healthy human adults. We conducted network co-expression analysis of next-generation RNA sequencing data from whole blood from P. vivax and P. falciparum controlled human malaria infection (CHMI) of healthy naïve and malaria-exposed volunteers. Single cell transcription signatures were used to deconvolute the bulk RNA-Seq data into cell-specific signals. Initial exposure to P. vivax induced activation of innate immunity, including efficient antigen presentation and complement activation. However, this effect was accompanied by strong immunosuppression mediated by dendritic cells via the induction of Indoleamine 2,3-Dioxygenase 1(IDO1) and Lymphocyte Activation Gene 3 (LAG3). Additionally, P. vivax induced depletion of neutrophil populations associated with down regulation of 3G-protein coupled receptors, CRXCR1, CXCR2 and CSF3R. Accordingly, in malaria-exposed volunteers the inflammatory response was attenuated, with a decreased class II antigen presentation in dendritic cells. While the immunosuppressive signalling was maintained between plasmodium species, response to P. falciparum was significantly more immunogenic. In silico analyses suggest that primary infection with P. vivax induces potent immunosuppression mediated by dendritic cells, conditioning subsequent anti-malarial immune responses. Targeting immune evasion mechanisms could be an effective alternative for improving vaccine efficacy.

Diurnal variation in expired breath volatiles in malaria-infected and healthy volunteers

We previously showed that thioether levels in the exhaled breath volatiles of volunteers undergoing controlled human malaria infection (CHMI) with P. falciparum increase as infection progresses. In this study, we show that thioethers have diurnal cyclical increasing patterns and their levels are significantly higher in P. falciparum CHMI volunteers compared to those of healthy volunteers. The synchronized cycle and elevation of thioethers were not present in P. vivax-infection, therefore it is likely that the thioethers are associated with unique factors in the pathology of P. falciparum. Moreover, we found that time-of-day of breath collection is important to accurately predict (98%) P. falciparum-infection. Critically, this was achieved when the disease was asymptomatic and parasitemia was below the level detectable by microscopy. Although these findings are encouraging, they show limitations because of the limited and logistically difficult diagnostic window and its utility to P. falciparum malaria only. We looked for new biomarkers in the breath of P. vivax CHMI volunteers and found that a set of terpenes increase significantly over the course of the malaria infection. The accuracy of predicting P. vivax using breath terpenes was up to 91%. Moreover, some of the terpenes were also found in the breath of P. falciparum CHMI volunteers (accuracy up to 93.5%). The results suggest that terpenes might represent better biomarkers than thioethers to predict malaria as they were not subject to malaria pathogens diurnal changes.

Liver Injury in Uncomplicated Malaria is an Overlooked Phenomenon: An Observational Study.

Abstract Background Liver injury is a known feature of severe malaria, but is only incidentally investigated in uncomplicated disease. In such cases, drug-induced hepatotoxicity is often thought to be the primary cause of the observed liver injury, and this can be a major concern in antimalaria drug development. We investigated liver function test (LFT) abnormalities in patients with imported uncomplicated malaria, and in Controlled Human Malaria Infection (CHMI) studies. Methods Clinical and laboratory data from 484 imported malaria cases and 254 CHMI participants were obtained from the Rotterdam Malaria Cohort database, and the Radboud University Medical Center database (between 2001 and 2017), respectively. Routine clinical LFTs, clinical profiles, parasite densities, hematological, and inflammation parameters were assessed in 217 patients with imported falciparum malaria upon admission, and from longitudinal data of 187 CHMI participants. Findings Upon admission, the proportion of patients with imported uncomplicated malaria and elevated liver enzymes was 128/186 (69%). In CHMI, 97/187 (52%) participants showed LFT abnormalities, including mild (64%, >1.0 ≤ 2.5× upper limit of normal (ULN)), moderate (20%, >2.5 ≤ 5.0xULN) or severe (16%, >5.0xULN). LFT abnormalities were primarily ALT/AST elevations and to a lesser extent γGT and ALP. LFT abnormalities peaked shortly after initiation of treatment, regardless of drug regimen, and returned to normal within three to six weeks. Positive associations were found with parasite burden and inflammatory parameters, including cumulative inflammatory cytokine responses and oxidative stress markers (r = 0·65, p = 0·008, and r = −0·63, p = 0·001, respectively). Interpretation This study shows that reversible liver injury is a common feature of uncomplicated falciparum malaria, most likely caused by an enduring pro-inflammatory response post treatment. The recognition of this phenomenon is of clinical relevance for individual patient care as well as clinical development of (new) antimalarial drugs. Fund PATH Malaria Vaccine Initiative (MVI)

Chimeric Plasmodium falciparum parasites expressing Plasmodium vivax circumsporozoite protein fail to produce salivary gland sporozoites

BackgroundRodent malaria parasites where the gene encoding circumsporozoite protein (CSP) has been replaced with csp genes from the human malaria parasites, Plasmodium falciparum or Plasmodium vivax, are used as pre-clinical tools to evaluate CSP vaccines in vivo. These chimeric rodent parasites produce sporozoites in Anopheles stephensi mosquitoes that are capable of infecting rodent and human hepatocytes. The availability of chimeric P. falciparum parasites where the pfcsp gene has been replaced by the pvcsp would open up possibilities to test P. vivax CSP vaccines in small scale clinical trials using controlled human malaria infection studies.MethodsUsing CRISPR/Cas9 gene editing two chimeric P. falciparum parasites, were generated, where the pfcsp gene has been replaced by either one of the two major pvcsp alleles, VK210 or VK247. In addition, a P. falciparum parasite line that lacks CSP expression was also generated. These parasite lines have been analysed for sporozoite production in An. stephensi mosquitoes.ResultsThe two chimeric Pf-PvCSP lines exhibit normal asexual and sexual blood stage development in vitro and produce sporozoite-containing oocysts in An. stephensi mosquitoes. Expression of the corresponding PvCSP was confirmed in oocyst-derived Pf-PvCSP sporozoites. However, most oocysts degenerate before sporozoite formation and sporozoites were not found in either the mosquito haemocoel or salivary glands. Unlike the chimeric Pf-PvCSP parasites, oocysts of P. falciparum parasites lacking CSP expression do not produce sporozoites.ConclusionsChimeric P. falciparum parasites expressing P. vivax circumsporozoite protein fail to produce salivary gland sporozoites. Combined, these studies show that while PvCSP can partially complement the function of PfCSP, species-specific features of CSP govern full sporozoite maturation and development in the two human malaria parasites.

Correlates of protection from Controlled Human Malaria Infections (CHMI) in semi-immune Kenyan adults

Correlates of protection from Controlled Human Malaria Infections (CHMI) in semi-immune Kenyan adults

CXCR3+ T Follicular Helper Cells Induced by Co-Administration of RTS,S/AS01B and Viral-Vectored Vaccines Are Associated With Reduced Immunogenicity and Efficacy Against Malaria.

A malaria vaccine strategy targeting multiple lifecycle stages may be required to achieve a high level of efficacy. In two Phase IIa clinical trials, we tested immunogenicity and efficacy of RTS,S/AS01B administered alone, in a staggered regimen with viral-vectored vaccines or co-administered with viral-vectored vaccines. RTS,S/AS01B induces high titers of antibody against sporozoites and viral-vectored vaccines ChAd63 ME-TRAP and MVA ME-TRAP induce potent T cell responses against infected hepatocytes. By combining these two strategies, we aimed to improve efficacy by inducing immune responses targeting multiple parasite antigens. Vaccination with RTS,S/AS01B alone or in a staggered regimen with viral vectors produced strong immune responses and demonstrated high levels of protection against controlled human malaria infection. However, concomitant administration of these vaccines significantly reduced humoral immunogenicity and protective efficacy. Strong Th1-biased cytokine responses induced by MVA ME-TRAP were associated with a skew in circulating T follicular helper cells toward a CXCR3+ phenotype and a reduction in antibody quantity and quality. This study illustrates that while a multistage-targeting vaccine strategy could provide high-level efficacy, the regimen design will require careful optimization.