Efficacy of recombinant circumsporozoite protein vaccine regimens against experimental Plasmodium falciparum malaria

A systematic review on malaria and dengue vaccines for the effective management of these mosquito borne diseases: Improving public health

Insect vector-borne diseases (VBDs) pose significant global health challenges, particularly in tropical and subtropical regions. The WHO has launched the "Global Vector Control Response (GVCR) 2017-2030" to address these diseases, emphasizing a comprehensive approach to vector control. This systematic review investigates the potential of malaria and dengue vaccines in controlling mosquito-borne VBDs, aiming to alleviate disease burdens and enhance public health. Following PRISMA 2020 guidelines, the review incorporated 39 new studies out of 934 identified records. It encompasses various studies assessing malaria and dengue vaccines, emphasizing the significance of vaccination as a preventive measure. The findings indicate variations in vaccine efficacy, duration of protection, and safety considerations for each disease, influencing public health strategies. The review underscores the urgent need for vaccines to combat the increasing burden of VBDs like malaria and dengue, advocating for ongoing research and investment in vaccine development.

Multistage protective anti-CelTOS monoclonal antibodies with cross-species sterile protection against malaria

CelTOS is a malaria vaccine antigen that is conserved in Plasmodium and other apicomplexan parasites and plays a role in cell-traversal. The structural basis and mechanisms of CelTOS-induced protective immunity to parasites are unknown. Here, CelTOS-specific monoclonal antibodies (mAbs) 7g7 and 4h12 demonstrated multistage activity, protecting against liver infection and preventing parasite transmission to mosquitoes. Both mAbs demonstrated cross-species activity with sterile protection against in vivo challenge with transgenic parasites containing either P. falciparum or P. vivax CelTOS, and with transmission reducing activity against P. falciparum. The mAbs prevented CelTOS-mediated pore formation providing insight into the protective mechanisms. X-ray crystallography and mutant-library epitope mapping revealed two distinct broadly conserved neutralizing epitopes. 7g7 bound to a parallel dimer of CelTOS, while 4h12 bound to a novel antiparallel dimer architecture. These findings inform the design of antibody therapies and vaccines and raise the prospect of a single intervention to simultaneously combat P. falciparum and P. vivax malaria.

A phase IIa, randomized, double-blind, safety, immunogenicity and efficacy trial of Plasmodium falciparum vaccine antigens merozoite surface protein 1 and RTS,S formulated with AS02 adjuvant in healthy, malaria-naïve adults

BACKGROUND

To improve the efficacy of Plasmodium falciparum malaria vaccine RTS,S/AS02, we conducted a study in 2001 in healthy, malaria-naïve adults administered RTS,S/AS02 in combination with FMP1, a recombinant merozoite surface-protein-1, C-terminal 42kD fragment.

METHODS

A double-blind Phase I/IIa study randomized N = 60 subjects 1:1:1:1 to one of four groups, N = 15/group, to evaluate safety, immunogenicity, and efficacy of intra-deltoid half-doses of RTS,S/AS02 and FMP1/AS02 administered in the contralateral (RTS,S + FMP1-separate) or same (RTS,S + FMP1-same) sites, or FMP1/AS02 alone (FMP1-alone), or RTS,S/AS02 alone (RTS,S-alone) on a 0-, 1-, 3-month schedule. Subjects receiving three doses of vaccine and non-immunized controls (N = 11) were infected with homologous P. falciparum 3D7 sporozoites by Controlled Human Malaria Infection (CHMI).

RESULTS

Subjects in all vaccination groups experienced mostly mild or moderate local and general adverse events that resolved within eight days. Anti-circumsporozoite antibody levels were lower when FMP1 and RTS,S were co-administered at the same site (35.0 µg/mL: 95 % CI 20.3-63), versus separate arms (57.4 µg/mL: 95 % CI 32.3-102) or RTS,S alone (62.0 µg/mL: 95 % CI: 37.8-101.8). RTS,S-specific lymphoproliferative responses and ex vivo ELISpot CSP-specific interferon-gamma (IFN-γ) responses were indistinguishable among groups receiving RTS,S/AS02. There was no difference in antibody to FMP1 among groups receiving FMP1/AS02. After CHMI, groups immunized with a RTS,S-containing regimen had ∼ 30 % sterile protection against parasitemia, and equivalent delays in time-to-parasitemia. The FMP1/AS02 alone group showed no sterile immunity or delay in parasitemia.

CONCLUSION

Co-administration of RTS,S and FMP1/AS02 reduced anti-RTS,S antibody, but did not affect tolerability, cellular immunity, or efficacy in a stringent CHMI model. Absence of efficacy or delay of patency in the sporozoite challenge model in the FMP1/AS02 group did not rule out efficacy of FMP1/AS02 in an endemic population. However, a Phase IIb trial of FMP1/AS02 in children in malaria-endemic Kenya did not demonstrate efficacy against natural infection.

CLINICALTRIALS

gov identifier: NCT01556945.

A replication competent Plasmodium falciparum parasite completely attenuated by dual gene deletion

Vaccination with infectious Plasmodium falciparum (Pf) sporozoites (SPZ) administered with antimalarial drugs (PfSPZ-CVac), confers superior sterilizing protection against infection when compared to vaccination with replication-deficient, radiation-attenuated PfSPZ. However, the requirement for drug administration constitutes a major limitation for PfSPZ-CVac. To obviate this limitation, we generated late liver stage-arresting replication competent (LARC) parasites by deletion of the Mei2 and LINUP genes (mei2-/linup- or LARC2). We show that Plasmodium yoelii (Py) LARC2 sporozoites did not cause breakthrough blood stage infections and engendered durable sterilizing immunity against various infectious sporozoite challenges in diverse strains of mice. We next genetically engineered a PfLARC2 parasite strain that was devoid of extraneous DNA and produced cryopreserved PfSPZ-LARC2. PfSPZ-LARC2 liver stages replicated robustly in liver-humanized mice but displayed severe defects in late liver stage differentiation and did not form liver stage merozoites. This resulted in complete abrogation of parasite transition to viable blood stage infection. Therefore, PfSPZ-LARC2 is the next-generation vaccine strain expected to unite the safety profile of radiation-attenuated PfSPZ with the superior protective efficacy of PfSPZ-CVac.

Malaria Vaccines: Progress to Date

Malaria is a mosquito-borne disease caused by protozoan parasites of the genus Plasmodium. Despite significant declines in malaria-attributable morbidity and mortality over the last two decades, it remains a major public health burden in many countries. This underscores the critical need for improved strategies to prevent, treat and control malaria if we are to ultimately progress towards the eradication of this disease. Ideally, this will include the development and deployment of a highly effective malaria vaccine that is able to induce long-lasting protective immunity. There are many malaria vaccine candidates in development, with more than a dozen of these in clinical development. RTS,S/AS01 (also known as Mosquirix) is the most advanced malaria vaccine and was shown to have modest efficacy against clinical malaria in phase III trials in 5- to 17-month-old infants. Following pilot implementation trials, the World Health Organisation has recommended it for use in Africa in young children who are most at risk of infection with P. falciparum, the deadliest of the human malaria parasites. It is well recognised that more effective malaria vaccines are needed. In this review, we discuss malaria vaccine candidates that have progressed into clinical evaluation and highlight the most advanced candidates: Sanaria's irradiated sporozoite vaccine (PfSPZ Vaccine), the chemoattenuated sporozoite vaccine (PfSPZ-CVac), RTS,S/AS01 and the novel malaria vaccine candidate, R21, which displayed promising, high-level efficacy in a recent small phase IIb trial in Africa.

Advancements and Challenges in Developing Malaria Vaccines: Targeting Multiple Stages of the Parasite Life Cycle

Malaria, caused by Plasmodium species, remains a major global health concern, causing millions of deaths annually. While the introduction of the RTS,S vaccine has shown promise, there is a pressing need for more effective vaccines due to the emergence of drug-resistant parasites and insecticide-resistant vectors. However, the complex life cycle and genetic diversity of the parasite, technical obstacles, limited funding, and the impact of the 2019 pandemic have hindered progress in malaria vaccine development. This review focuses on advancements in malaria vaccine development, particularly the ongoing clinical trials targeting antigens from different stages of the Plasmodium life cycle. Additionally, we discuss the rationale, strategies, and challenges associated with vaccine design, aiming to enhance the immune response and protective efficacy of vaccine candidates. A cost-effective and multistage vaccine could hold the key to controlling and eradicating malaria.

The Case for Human Challenge Trials in COVID-19

The COVID-19 pandemic has necessitated rapid research to aid in the understanding of the disease and the development of novel therapeutics. One option is to conduct controlled human infection trials (CHITs). In this article I examine the history of deliberate human infection and CHITs and their utilization prior to the COVID-19 pandemic, key ethical considerations of CHITs in the COVID-19 setting, an analysis of the World Health Organization's (WHO) Key criteria for the ethical acceptability of COVID-19 human challenge studies, and a review of the two COVID-19 CHITs that have already commenced, their compliance with the WHO criteria and other ethical considerations.

Systematic Review of Safety of RTS,S with AS01 and AS02 Adjuvant Systems Using Data from Randomized Controlled Trials in Infants, Children, and Adults

Background

Emergence of antimalarial drugs and insecticides resistance alarms scientists to develop a safe and effective malaria vaccine. A pre-erythrocytic malaria vaccine called RTS,S has made great strides.

Aim

The review was aimed to assess the safety of the candidate malaria vaccine RTS,S with AS01 and AS02 adjuvants using data from Phase I-III randomized controlled clinical trials (RCTs).

Methods

This systematic review was conducted based on PRISMA 2020. Regardless of time of publication year, all articles related with safety of RTS,S, RCTs published in the English language were included in the study. The last search of databases, and registry was conducted on 30 May, 2022. Pubmed, Google Scholar, Cochrane Library, Wiley Online Library, and Clinical trials.gov were thoroughly searched for accessible RCTs on the safety of RTS,S malaria vaccine. The studies were screened in three steps: duplicate removal, title and abstract screening, and full-text review. The included studies' bias risk was assessed using the Cochrane risk of bias tool for RCTs. This systematic review is registered at Prospero (registration number: CRD42021285888). The qualitative descriptive findings from the included published studies were reported stratified by clinical trial phases.

Findings

A total of thirty-five eligible safety studies were identified. Injection site pain and swelling, febrile convulsion, fever, headache, meningitis, fatigue, gastroenteritis, myalgia, pneumonia, reactogenicity, and anemia were the most commonly reported adverse events. Despite few clinical trials reported serious adverse events, none of them were related to vaccination.

Conclusion

Most of the adverse events observed from RTS,S/AS01 and RTS,S/AS02 malaria vaccines were reported in the control group and shared by other vaccines. Hence, the authors concluded that both RTS,S/AS01 and RTS,S/AS02 malaria vaccines are safe.

Humoral Immune Responses to P. falciparum Circumsporozoite Protein (Pfcsp) Induced by the RTS, S Vaccine - Current Update

Malaria vaccines targeting the circumsporozoite protein (CSP) of the P. falciparum parasite have been overall relatively promising. RTS, S is a pre-erythrocytic recombinant protein-based malaria vaccine that targets CSP. RTS, S effectiveness shows some limited success regardless of its 58% efficacy for severe disease. P. falciparum circumsporozoite protein (Pfcsp) has stood to be the main candidate protein for most pre-erythrocytic stage vaccines. Studies on the structural and biophysical characteristics of antibodies specific to CSP (anti-CSP) are underway to achieve fine specificity with the CSP polymorphic regions. More recent studies have proposed the use of different kinds of monoclonal antibodies, the use of appropriate adjuvants, ideal vaccination dose and frequency, and improved targeting of particular epitopes for the robust production of functional antibodies and high complement-fixing activity as other potential methods for achieving long-lasting RTS, S. This review highlights recent findings regarding humoral immune responses to CSP elicited by RTS, S vaccine.

A comprehensive study of epitopes and immune reactivity among Plasmodium species

Background

Malaria is a life-threatening disease caused by protozoan parasite of genus Plasmodium. Various antigenic proteins of Plasmodium are considered as the major targets for the development of an effective vaccine. The aim of the current study was a comprehensive analysis of the experimentally validated epitopes of Plasmodium obtained from various immunoassays.

Methods

Plasmodium species epitopes were prefetched from Immune Epitope Database (IEDB). Species specific classification of available epitopes was done for both human and murine malaria parasites. Further, these T cell and B cell epitopes along with MHC I/II binders of different Plasmodium species were examined to find out their capability to induce IFN-γ and IL-10 using IFNepitope and IL-10 Pred, respectively.

Results

The species-specific classification of 6874 unique epitopes resulted in the selection of predominant human and murine Plasmodium species. Further, the attempt was made to analyse the immune reactivity of these epitopes for their ability to induce cytokines namely IFN-γ and IL-10. Total, 2775 epitopes were predicted to possess IFN-γ inducing ability, whereas 1275 epitopes were found to be involved in the induction of IL-10.

Conclusions

This study facilitates the assessment of Plasmodium epitopes and associated proteins as a potential approach to design and develop an epitope-based vaccine. Moreover, the results highlight the epitope-based immunization in malaria to induce a protective immune response.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-022-02480-7.

Reproducibility of malaria sporozoite challenge model in humans for evaluating efficacy of vaccines and drugs: a systematic review

BACKGROUND

The development of novel malaria vaccines and antimalarial drugs is limited partly by emerging challenges to conduct field trials in malaria endemic areas, including unknown effects of existing immunity and a reported fall in malaria incidence. As a result, Controlled Human Malaria Infection (CHMI) has become an important approach for accelerated development of malarial vaccines and drugs. We conducted a systematic review of the literature to establish aggregate evidence on the reproducibility of a malaria sporozoite challenge model.

METHODS

A systematic review of research articles published between 1990 and 2018 on efficacy testing of malaria vaccines and drugs using sporozoite challenge and sporozoite infectivity studies was conducted using Pubmed, Scopus, Embase and Cochrane Library, ClinicalTrials.gov and Trialtrove. The inclusion criteria were randomized and non-randomized, controlled or open-label trials using P. falciparum or P. vivax sporozoite challenges. The data were extracted from articles using standardized data extraction forms and descriptive analysis was performed for evidence synthesis. The endpoints considered were infectivity, prepatent period, parasitemia and safety of sporozoite challenge.

RESULTS

Seventy CHMI trials conducted with a total of 2329 adult healthy volunteers were used for analysis. CHMI was induced by bites of mosquitoes infected with P. falciparum or P. vivax in 52 trials and by direct venous inoculation of P. falciparum sporozoites (PfSPZ challenge) in 18 trials. Inoculation with P. falciparum-infected mosquitoes produced 100% infectivity in 40 studies and the mean/median prepatent period assessed by thick blood smear (TBS) microscopy was ≤ 12 days in 24 studies. On the other hand, out of 12 infectivity studies conducted using PfSPZ challenge, 100% infection rate was reproduced in 9 studies with a mean or median prepatent period of 11 to 15.3 days as assessed by TBS and 6.8 to 12.6 days by PCR. The safety profile of P. falciparum and P.vivax CHMI was characterized by consistent features of malaria infection.

CONCLUSION

There is ample evidence on consistency of P. falciparum CHMI models in terms of infectivity and safety endpoints, which supports applicability of CHMI in vaccine and drug development. PfSPZ challenge appears more feasible for African trials based on current evidence of safety and efficacy.

Mechanisms and targets of Fcγ-receptor mediated immunity to malaria sporozoites

A highly protective vaccine will greatly facilitate achieving and sustaining malaria elimination. Understanding mechanisms of antibody-mediated immunity is crucial for developing vaccines with high efficacy. Here, we identify key roles in humoral immunity for Fcγ-receptor (FcγR) interactions and opsonic phagocytosis of sporozoites. We identify a major role for neutrophils in mediating phagocytic clearance of sporozoites in peripheral blood, whereas monocytes contribute a minor role. Antibodies also promote natural killer cell activity. Mechanistically, antibody interactions with FcγRIII appear essential, with FcγRIIa also required for maximum activity. All regions of the circumsporozoite protein are targets of functional antibodies against sporozoites, and N-terminal antibodies have more activity in some assays. Functional antibodies are slowly acquired following natural exposure to malaria, being present among some exposed adults, but uncommon among children. Our findings reveal targets and mechanisms of immunity that could be exploited in vaccine design to maximize efficacy.

Circumsporozoite Surface Protein-based malaria vaccines: a review

Malaria represents a serious public health problem, presenting with high rates of incidence, morbidity and mortality in tropical and subtropical regions of the world. According to the World Health Organization, in 2018 there were 228 million cases and 405 thousand deaths caused by this disease in the world, affecting mainly children and pregnant women in Africa. Despite the programs carried out to control this disease, drug resistance and invertebrate vector resistance to insecticides have generated difficulties. An efficient vaccine against malaria would be a strategy with a high impact on the eradication and control of this disease. Researches aimed at developing vaccines have focused on antigens of high importance for the survival of the parasite such as the Circumsporozoite Surface Protein, involved in the pre-erythrocytic cycle during parasites invasion in hepatocytes. Currently, RTS’S is the most promising vaccine for malaria and was constructed using CSP; its performance was evaluated using two types of adjuvants: AS01 and AS02. The purpose of this review was to provide a bibliographic survey of historical researches that led to the development of RTS’S and its performance analysis over the decade. The search for new adjuvants to be associated with this antigen seems to be a way to obtain higher percentages of protection for a future malaria vaccine.

Antibody Feedback Limits the Expansion of B Cell Responses to Malaria Vaccination but Drives Diversification of the Humoral Response

Generating sufficient antibody to block infection is a key challenge for vaccines against malaria. Here, we show that antibody titers to a key target, the repeat region of the Plasmodium falciparum circumsporozoite protein (PfCSP), plateaued after two immunizations in a clinical trial of the radiation-attenuated sporozoite vaccine. To understand the mechanisms limiting vaccine responsiveness, we developed immunoglobulin (Ig)-knockin mice with elevated numbers of PfCSP-binding B cells. We determined that recall responses were inhibited by antibody feedback, potentially via epitope masking of the immunodominant PfCSP repeat region. Importantly, the amount of antibody that prevents boosting is below the amount of antibody required for protection. Finally, while antibody feedback limited responses to the PfCSP repeat region in vaccinated volunteers, potentially protective subdominant responses to PfCSP C-terminal regions expanded with subsequent boosts. These data suggest that antibody feedback drives the diversification of immune responses and that vaccination for malaria will require targeting multiple antigens.

Multi-functional antibodies are induced by the RTS,S malaria vaccine and associated with protection in a phase I/IIa trial

Background

RTS,S is the leading malaria vaccine candidate but only confers partial efficacy against malaria in children. RTS,S is based on the major Plasmodium falciparum sporozoite surface antigen, circumsporozoite protein (CSP). The induction of anti-CSP antibodies is important for protection; however, it is unclear how these protective antibodies function.

Methods

We quantified the induction of functional anti-CSP antibody responses in healthy malaria-naive adults (N = 45) vaccinated with RTS,S/AS01. This included the ability to mediate effector functions via the fragment crystallizable (Fc) region, such as interacting with human complement proteins and Fcγ-receptors (FcγRs) that are expressed on immune cells, which promote various immunological functions.

Results

Our major findings were (1) RTS,S-induced antibodies mediated Fc-dependent effector functions, (2) functional antibodies were generally highest after the second vaccine dose, (3) functional antibodies targeted multiple regions of CSP, (4) participants with higher levels of functional antibodies had a reduced probability of developing parasitemia following homologous challenge (P < .05), and (5) nonprotected subjects had higher levels of anti-CSP IgM.

Conclusions

Our data suggest a role for Fc-dependent antibody effector functions in RTS,S-induced immunity. Enhancing the induction of these functional activities may be a strategy to improve the protective efficacy of RTS,S or other malaria vaccines.

Clinical Trials Registration

NCT00075049

Plasmodium falciparum pre-erythrocytic stage vaccine development

Worldwide strategies between 2010 and 2017 aimed at controlling malarial parasites (mainly Plasmodium falciparum) led to a reduction of just 18% regarding disease incidence rates. Many biologically-derived anti-malarial vaccine candidates have been developed to date; this has involved using many experimental animals, an immense amount of work and the investment of millions of dollars. This review provides an overview of the current state and the main results of clinical trials for sporozoite-targeting vaccines (i.e. the parasite stage infecting the liver) carried out by research groups in areas having variable malaria transmission rates. However, none has led to promising results regarding the effective control of the disease, thereby making it necessary to complement such efforts at finding/introducing new vaccine candidates by adopting a multi-epitope, multi-stage approach, based on minimal subunits of the main sporozoite proteins involved in the invasion of the liver.

Combining antimalarial drugs and vaccine for malaria elimination campaigns: a randomized safety and immunogenicity trial of RTS,S/AS01 administered with dihydroartemisinin, piperaquine, and primaquine in healthy Thai adult volunteers

Introduction: RTS,S/AS01 is currently the most advanced malaria vaccine but provides incomplete, short-term protection. It was developed for use within the expanded program on immunizations (EPI) for African children. Another use could be adding mass RTS,S/AS01 vaccination to the integrated malaria elimination strategy in the Greater Mekong Subregion (GMS), where multidrug-resistant P.falciparum strains have emerged and spread. Prior to evaluating RTS,S/AS01 in large-scale trials we assessed whether the vaccine, administered with and without antimalarial drugs, is safe and immunogenic in Asian populations.

Methods: An open-label, randomized, controlled phase 2 trial was conducted in healthy, adult Thai volunteers. Seven vaccine regimens with and without antimalarial drugs (dihydroartemisinin-piperaquine plus a single low dose primaquine) were assessed. Antibody titres against the PfCSP full-length (NANP) 6, PfCSP anti-C–term, PfCSP full-length (N + C-Terminal) were measured by standard enzyme-linked immunosorbent assays. Liquid chromatography was used to measure piperaquine, primaquine and carboxy-primaquine concentrations.

Results: 193 volunteers were enrolled and 186 study participants completed the 6 months follow-up period. One month after the last vaccination all study participants had seroconverted to the PfCSP (NANP)6, and the PfCSP Full Length (N + C-Terminal). More than 90% had seroconverted to the Pfanti-C-Term CSP. There was no indication that drug concentrations were influenced by vaccine regimens or the antibody levels by the drug regimens. Adverse events were similarly distributed between the seven treatment groups. No serious adverse events attributable to the study interventions were detected.

Conclusion: This study found that RTS,S/AS01 with and without dihydroartemisinin-piperaquine plus a single low dose primaquine was safe and immunogenic in a healthy, adult Asian population.

Vaccination With Sporozoites: Models and Correlates of Protection

Despite continuous efforts, the century-old goal of eradicating malaria still remains. Multiple control interventions need to be in place simultaneously to achieve this goal. In addition to effective control measures, drug therapies and insecticides, vaccines are critical to reduce mortality and morbidity. Hence, there are numerous studies investigating various malaria vaccine candidates. Most of the malaria vaccine candidates are subunit vaccines. However, they have shown limited efficacy in Phase II and III studies. To date, only whole parasite formulations have been shown to induce sterile immunity in human. In this article, we review and discuss the recent developments in vaccination with sporozoites and the mechanisms of protection involved.

Zika Vaccines: Role for Controlled Human Infection

Zika virus (ZIKV), a previously little known arbovirus, caused an unprecedented outbreak in Latin America and the Caribbean throughout 2015 and 2016. The virus has been associated with the congenital Zika syndrome (CZS), which can occur with maternal ZIKV infection during any trimester and can result from asymptomatic infection. There is concern that even low levels of viremia can result in CZS, meaning an effective vaccine will need to induce very high levels of protection. Controlled human infection models (CHIMs), in which subjects are infected with a pathogen of interest, have been used to down-select vaccine candidates and have provided efficacy data in support of vaccine licensure.A ZIKV CHIM could be instrumental in determining which of the many ZIKV vaccine candidates provides the highest degree of protection and should be advanced in clinical development. The development of a ZIKV CHIM is not without challenges. The ZIKV, unlike other flaviviruses, is sexually and mosquito-transmitted, and an increase in the incidence of Guillain-Barré syndrome was reported in some countries during the ZIKV outbreak. These obstacles can be overcome with thoughtful study design to ensure maximal risk mitigation. If successful, a ZIKV CHIM could de-risk and accelerate ZIKV vaccine development.

The Controlled Human Malaria Infection Experience at the University of Maryland

Controlled human malaria infection (CHMI) is a powerful tool to evaluate the efficacy of malaria vaccines and pharmacologics. Investigators at the University of Maryland, Baltimore, Center for Vaccine Development (UMB-CVD) pioneered the technique in the 1970s and continue to advance the frontiers of CHMI research. We reviewed the records of 338 malaria-naive volunteers who underwent CHMI at UMB-CVD with Plasmodium falciparum from 1971 until 2017. These 338 volunteers underwent 387 CHMI events, including 60 via intradermal injection or direct venous inoculation (DVI) of purified, cryopreserved sporozoites. No volunteer suffered an unplanned hospitalization or required intravenous therapy related to CHMI. Median prepatency period was longer in challenges using NF54 (9 days) than in those using 7G8 (8 days), P = 0.0006 by the log-rank test. With dose optimization of DVI, the prepatent period did not differ between DVI and mosquito bite challenge (log-rank test, P = 0.66). Polymerase chain reaction (PCR) detected P. falciparum infection 3 days earlier than thick smears (P < 0.001), and diagnosis by ultrasensitive PCR was associated with less severe symptoms than smear-based diagnosis (39% versus 0%, P = 0.0003). Historical studies with NF54 showed a shorter median prepatency period of 10.3 days than more recent studies (median 11.0 days, P = 0.02) despite significantly lower salivary gland scores in earlier studies, P = 0.0001. The 47-year experience of CHMI at UMB-CVD has led to advancements in sporozoite delivery, diagnostics, and use of heterologous challenge. Additional studies on new challenge strains and genomic data to reflect regional heterogeneity will help advance the use of CHMI as supporting data for vaccine licensure.

Induction and decay of functional complement-fixing antibodies by the RTS,S malaria vaccine in children, and a negative impact of malaria exposure

BACKGROUND

Leading malaria vaccine, RTS,S, is based on the circumsporozoite protein (CSP) of sporozoites. RTS,S confers partial protection against malaria in children, but efficacy wanes relatively quickly after primary immunization. Vaccine efficacy has some association with anti-CSP IgG; however, it is unclear how these antibodies function, and how functional antibodies are induced and maintained over time. Recent studies identified antibody-complement interactions as a potentially important immune mechanism against sporozoites. Here, we investigated whether RTS,S vaccine-induced antibodies could function by interacting with complement.

METHODS

Serum samples were selected from children in a phase IIb trial of RTS,S/AS02_A conducted at two study sites of high and low malaria transmission intensity in Manhiça, Mozambique. Samples following primary immunization and 5-year post-immunization follow-up time points were included. Vaccine-induced antibodies were characterized by isotype, subclass, and epitope specificity, and tested for the ability to fix and activate complement. We additionally developed statistical methods to model the decay and determinants of functional antibodies after vaccination.

RESULTS

RTS,S vaccination induced anti-CSP antibodies that were mostly IgG1, with some IgG3, IgG2, and IgM. Complement-fixing antibodies were effectively induced by vaccination, and targeted the central repeat and C-terminal regions of CSP. Higher levels of complement-fixing antibodies were associated with IgG that equally recognized both the central repeat and C-terminal regions of CSP. Older age and higher malaria exposure were significantly associated with a poorer induction of functional antibodies. There was a marked decay in functional complement-fixing antibodies within months after vaccination, as well as decays in IgG subclasses and IgM. Statistical modeling suggested the decay in complement-fixing antibodies was mostly attributed to the waning of anti-CSP IgG1, and to a lesser extent IgG3.

CONCLUSIONS

We demonstrate for the first time that RTS,S can induce complement-fixing antibodies in young malaria-exposed children. The short-lived nature of functional responses mirrors the declining vaccine efficacy of RTS,S over time. The negative influence of age and malaria exposure on functional antibodies has implications for understanding vaccine efficacy in different settings. These findings provide insights into the mechanisms and longevity of vaccine-induced immunity that will help inform the future development of highly efficacious and long-lasting malaria vaccines.

The Advanced Development Pathway of the RTS,S/AS01 Vaccine

Following successful phase 1 and 2 trials, RTS,S/AS01 was evaluated in a large phase 3 trial in 7 African countries in which 8922 young children and 6537 infants were enrolled and followed for a median of 48 and 38 months, respectively. The vaccine efficacy against uncomplicated malaria in children was 28% without booster and 36% with booster. The vaccine received the approval of the European Medical Agency in 2015, but two WHO expert committees requested more data before the programmatic use of RTS,S/AS01 in African children can be recommended. To provide such data, a very large, cluster randomized trial is currently in progress in three African countries. The integration of RTS,S/AS01 in national childhood vaccination programs will benefit the individual vaccinated child but is unlikely to make an impact on malaria transmission. Mass vaccination campaigns that include all age groups over short periods are more likely to harness the short but high protection afforded by RTS,S/AS01.

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.

Ethical challenges posed by human infection challenge studies in endemic settings

Human infection challenge studies (HCS) involve intentionally infecting research participants with pathogens, often with the ultimate aim of developing new interventions against infectious diseases. Despite ethical concerns about research involving vulnerable populations, there are both scientific and ethical reasons to consider conducting more HCS in low- and middle-income countries where neglected diseases are often endemic. HCS researchers can reduce the risks to participants (and the risks of transmission from participants to others) by controlling multiple factors (eg those related to the laboratory environment, participant selection, the pathogen, and the timing of treatment); but HCS nonetheless raise important ethical issues, some of which may be particularly pertinent to HCS in endemic settings. This article provides background on HCS in general, as well as recent HCS in low- and middle-income countries, and an overview of the ethical issues associated with HCS in endemic settings.

The Complement System Contributes to Functional Antibody-Mediated Responses Induced by Immunization with Plasmodium falciparum Malaria Sporozoites

Long-lasting and sterile homologous protection against malaria can be achieved by the exposure of malaria-naive volunteers under chemoprophylaxis to Plasmodium falciparum-infected mosquitoes (chemoprophylaxis and sporozoite [CPS] immunization). While CPS-induced antibodies neutralize sporozoite infectivity in vitro and in vivo, antibody-mediated effector mechanisms are still poorly understood. Here, we investigated whether complement contributes to CPS-induced preerythrocytic immunity. Sera collected before and after CPS immunization in the presence of active or inactive complement were assessed for the recognition of homologous NF54 and heterologous NF135.C10 sporozoites, complement fixation, sporozoite lysis, and possible subsequent effects on in vitro sporozoite infectivity in human hepatocytes. CPS immunization induced sporozoite-specific IgM (P < 0.0001) and IgG (P = 0.001) antibodies with complement-fixing capacities (P < 0.0001). Sporozoite lysis (P = 0.017), traversal (P < 0.0001), and hepatocyte invasion inhibition (P < 0.0001) by CPS-induced antibodies were strongly enhanced in the presence of active complement. Complement-mediated invasion inhibition in the presence of CPS-induced antibodies negatively correlated with cumulative parasitemia during CPS immunizations (P = 0.013). While IgG antibodies similarly recognized homologous and heterologous sporozoites, IgM binding to heterologous sporozoites was reduced (P = 0.023). Although CPS-induced antibodies did not differ in their abilities to fix complement, lyse sporozoites, or inhibit the traversal of homologous and heterologous sporozoites, heterologous sporozoite invasion was more strongly inhibited in the presence of active complement (P = 0.008). These findings demonstrate that CPS-induced antibodies have complement-fixing activity, thereby significantly further enhancing the functional inhibition of homologous and heterologous sporozoite infectivity in vitro The combined data highlight the importance of complement as an additional immune effector mechanism in preerythrocytic immunity after whole-parasite immunization against Plasmodium falciparum malaria.

Safety and Immunogenicity of Seven Dosing Regimens of the Candidate RTS,S/AS01E Malaria Vaccine Integrated Within an Expanded Program on Immunization Regimen: A Phase II, Single-Center, Open, Controlled Trial in Infants in Malawi

BACKGROUND

In a phase III trial, the RTS,S/AS01 malaria vaccine produced lower anti-circumsporozoite (CS) antibody titers when co-administered with Expanded Programme on Immunization vaccines (0-, 1- and 2-month schedule) at 6 to 12 weeks compared with 5 to 17 months at first vaccination. Alternative infant immunization schedules within the Expanded Programme on Immunization were investigated.

METHODS

This phase II, open, single-site (Blantyre, Malawi) trial was conducted in infants 1 to 7 days of age. Subjects were equally randomized across 7 groups to receive 3 doses of RTS,S/AS01E at time points that included ≤7 days, 6, 10, 14 and 26 weeks, and 9 months. All RTS,S/AS01E groups plus a control group (without RTS,S/AS01E) received Bacillus Calmette-Guérin + oral poliovirus vaccine at ≤7 days, diphtheria, tetanus, whole-cell pertussis, hepatitis B and Haemophilus influenzae type b vaccine + oral poliovirus vaccine at 6, 10, and 14 weeks and measles vaccine at 9 months; one RTS,S/AS01E group and the control additionally received hepatitis B vaccination at ≤7 days. Serum anti-CS antibody geometric mean concentration (GMC; enzyme-linked immunosorbent assay) and safety were assessed up to age 18 months.

RESULTS

Of the 480 infants enrolled, 391 completed the study. No causally related serious adverse event was reported. A higher frequency of fever within 7 days of RTS,S/AS01E vaccination compared with control was observed. Compared with the standard 6-, 10-, 14-week schedule, anti-CS antibody GMC ratios post-dose 3 were significantly higher in the 10-, 14- and 26-week group only (ratio 1.80; 95% confidence interval, 1.24-2.60); RTS,S/AS01E vaccination at ≤7 days and 10 and 14 weeks produced significantly lower anti-CS GMCs (ratio 0.59; 95% confidence interval, 0.38-0.92).

CONCLUSIONS

Initiation of RTS,S/AS01E vaccination above 6 weeks of age tended to improve anti-CS antibody responses. Neonatal vaccination was well tolerated but produced a comparatively lower immune response.

Adenovirus-prime and baculovirus-boost heterologous immunization achieves sterile protection against malaria sporozoite challenge in a murine model

With the increasing prevalence of artemisinin-resistant malaria parasites, a highly efficacious and durable vaccine for malaria is urgently required. We have developed an experimental virus-vectored vaccine platform based on an envelope-modified baculovirus dual-expression system (emBDES). Here, we show a conceptually new vaccine platform based on an adenovirus-prime/emBDES-boost heterologous immunization regimen expressing the Plasmodium falciparum circumsporozoite protein (PfCSP). A human adenovirus 5-prime/emBDES-boost heterologous immunization regimen consistently achieved higher sterile protection against transgenic P. berghei sporozoites expressing PfCSP after a mosquito-bite challenge than reverse-ordered or homologous immunization. This high protective efficacy was also achieved with a chimpanzee adenovirus 63-prime/emBDES-boost heterologous immunization regimen against an intravenous sporozoite challenge. Thus, we show that the adenovirus-prime/emBDES-boost heterologous immunization regimen confers sterile protection against sporozoite challenge by two individual routes, providing a promising new malaria vaccine platform for future clinical use.

Steady progress toward a malaria vaccine

PURPOSE OF REVIEW

Great progress has been made in reducing malaria morbidity and mortality, yet the parasite continues to cause a startling 200 million infections and 500 000 deaths annually. Malaria vaccine development is pushing new boundaries by steady advancement toward a licensed product.

RECENT FINDINGS

Despite 50 years of research, the complexity of Plasmoidum falciparum confounds all attempts to eradicate the organism. This very complexity has pushed the boundaries of vaccine development to new heights, yet it remains to be seen if an affordable vaccine can provide durable and high-level protection. Novel vaccines such as RTS,S/AS01E are on the edge of licensure, but old techniques have resurged with the ability to deliver vialed, whole organism vaccines. Novel adjuvants, multistage/multiantigen approaches and transmission blocking vaccines all contribute to a multipronged battle plan to conquer malaria.

SUMMARY

Vaccines are the most cost-effective tools to control infectious diseases, yet the complexity of malaria has frustrated all attempts to develop an effective product. This review concentrates on recent advances in malaria vaccine development that lend hope that a vaccine can be produced and malaria eradicated.

Controlled Human Malaria Infection: Applications, Advances, and Challenges

Controlled human malaria infection (CHMI) entails deliberate infection with malaria parasites either by mosquito bite or by direct injection of sporozoites or parasitized erythrocytes. When required, the resulting blood-stage infection is curtailed by the administration of antimalarial drugs. Inducing a malaria infection via inoculation with infected blood was first used as a treatment (malariotherapy) for neurosyphilis in Europe and the United States in the early 1900s. More recently, CHMI has been applied to the fields of malaria vaccine and drug development, where it is used to evaluate products in well-controlled early-phase proof-of-concept clinical studies, thus facilitating progression of only the most promising candidates for further evaluation in areas where malaria is endemic. Controlled infections have also been used to immunize against malaria infection. Historically, CHMI studies have been restricted by the need for access to insectaries housing infected mosquitoes or suitable malaria-infected individuals. Evaluation of vaccine and drug candidates has been constrained in these studies by the availability of a limited number of Plasmodium falciparum isolates. Recent advances have included cryopreservation of sporozoites, the manufacture of well-characterized and genetically distinct cultured malaria cell banks for blood-stage infection, and the availability of Plasmodium vivax-specific reagents. These advances will help to accelerate malaria vaccine and drug development by making the reagents for CHMI more widely accessible and also enabling a more rigorous evaluation with multiple parasite strains and species. Here we discuss the different applications of CHMI, recent advances in the use of CHMI, and ongoing challenges for consideration.

Characterization of T-cell immune responses in clinical trials of the candidate RTS,S malaria vaccine

The candidate malaria vaccine RTS,S has demonstrated 45.7% efficacy over 18 months against all clinical disease in a phase-III field study of African children. RTS,S targets the circumsporozoite protein (CSP), which is expressed on the Plasmodium sporozoite during the pre-erythrocyte stage of its life-cycle; the stage between mosquito bite and liver infection.

Early in the development of RTS,S, it was recognized that CSP-specific cell-mediated immunity (CMI) was required to complement CSP-specific antibody-mediated immunity. In reviewing RTS,S clinical studies, associations between protection and various types of CMI (CSP-specific CD4⁺ T cells and INF-γ ELISPOTs) have been identified, but not consistently. It is plausible that certain CD4⁺ T cells support antibody responses or co-operate with other immune-cell types to potentially elicit protection. However, the identities of vaccine correlates of protection, implicating either CSP-specific antibodies or T cells remain elusive, suggesting that RTS,S clinical trials may benefit from additional immunogenicity analyses that can be informed by the results of controlled human malaria infection studies.

Viral Vector Malaria Vaccines Induce High-Level T Cell and Antibody Responses in West African Children and Infants

Heterologous prime-boosting with viral vectors encoding the pre-erythrocytic antigen thrombospondin-related adhesion protein fused to a multiple epitope string (ME-TRAP) induces CD8⁺ T cell-mediated immunity to malaria sporozoite challenge in European malaria-naive and Kenyan semi-immune adults. This approach has yet to be evaluated in children and infants. We assessed this vaccine strategy among 138 Gambian and Burkinabe children in four cohorts: 2- to 6-year olds in The Gambia, 5- to 17-month-olds in Burkina Faso, and 5- to 12-month-olds and 10-week-olds in The Gambia. We assessed induction of cellular immunity, taking into account the distinctive hematological status of young infants, and characterized the antibody response to vaccination. T cell responses peaked 7 days after boosting with modified vaccinia virus Ankara (MVA), with highest responses in infants aged 10 weeks at priming. Incorporating lymphocyte count into the calculation of T cell responses facilitated a more physiologically relevant comparison of cellular immunity across different age groups. Both CD8⁺ and CD4⁺ T cells secreted cytokines. Induced antibodies were up to 20-fold higher in all groups compared with Gambian and United Kingdom (UK) adults, with comparable or higher avidity. This immunization regimen elicited strong immune responses, particularly in young infants, supporting future evaluation of efficacy in this key target age group for a malaria vaccine.

Efficacy of phase 3 trial of RTS, S/AS01 malaria vaccine: The need for an alternative development plan

Although vaccines would be the ideal tool for control, prevention, elimination, and eradication of many infectious diseases, developing of parasites vaccines such as malaria vaccine is very complex. The most advanced malaria vaccine candidate RTS,S, a pre-erythrocytic vaccine, has been recommended for licensure by EMEA. The results of this phase III trial suggest that this candidate malaria vaccine has relatively little efficacy, and the vaccine apparently will not meet the goal of malaria eradication by itself. Since there are many vaccine candidates in the pipeline ¹ that are being evaluated in vaccine trials, further study on using of alternative parasite targets and vaccination strategies are highly recommended.

Attenuated PfSPZ Vaccine induces strain-transcending T cells and durable protection against heterologous controlled human malaria infection

A live-attenuated malaria vaccine, Plasmodium falciparum sporozoite vaccine (PfSPZ Vaccine), confers sterile protection against controlled human malaria infection (CHMI) with Plasmodium falciparum (Pf) parasites homologous to the vaccine strain up to 14 mo after final vaccination. No injectable malaria vaccine has demonstrated long-term protection against CHMI using Pf parasites heterologous to the vaccine strain. Here, we conducted an open-label trial with PfSPZ Vaccine at a dose of 9.0 × 10⁵ PfSPZ administered i.v. three times at 8-wk intervals to 15 malaria-naive adults. After CHMI with homologous Pf parasites 19 wk after final immunization, nine (64%) of 14 (95% CI, 35-87%) vaccinated volunteers remained without parasitemia compared with none of six nonvaccinated controls (P = 0.012). Of the nine nonparasitemic subjects, six underwent repeat CHMI with heterologous Pf7G8 parasites 33 wk after final immunization. Five (83%) of six (95% CI, 36-99%) remained without parasitemia compared with none of six nonvaccinated controls. PfSPZ-specific T-cell and antibody responses were detected in all vaccine recipients. Cytokine production by T cells from vaccinated subjects after in vitro stimulation with homologous (NF54) or heterologous (7G8) PfSPZ were highly correlated. Interestingly, PfSPZ-specific T-cell responses in the blood peaked after the first immunization and were not enhanced by subsequent immunizations. Collectively, these data suggest durable protection against homologous and heterologous Pf parasites can be achieved with PfSPZ Vaccine. Ongoing studies will determine whether protective efficacy can be enhanced by additional alterations in the vaccine dose and number of immunizations.

The Power of Malaria Vaccine Trials Using Controlled Human Malaria Infection

Controlled human malaria infection (CHMI) in healthy human volunteers is an important and powerful tool in clinical malaria vaccine development. However, power calculations are essential to obtain meaningful estimates of protective efficacy, while minimizing the risk of adverse events. To optimize power calculations for CHMI-based malaria vaccine trials, we developed a novel non-linear statistical model for parasite kinetics as measured by qPCR, using data from mosquito-based CHMI experiments in 57 individuals. We robustly account for important sources of variation between and within individuals using a Bayesian framework. Study power is most dependent on the number of individuals in each treatment arm; inter-individual variation in vaccine efficacy and the number of blood samples taken per day matter relatively little. Due to high inter-individual variation in the number of first-generation parasites, hepatic vaccine trials required significantly more study subjects than erythrocytic vaccine trials. We provide power calculations for hypothetical malaria vaccine trials of various designs and conclude that so far, power calculations have been overly optimistic. We further illustrate how upcoming techniques like needle-injected CHMI may reduce required sample sizes.

Controlled human malaria infection (CHMI) in healthy human volunteers is an important and powerful tool in clinical malaria vaccine development. However, to obtain meaningful estimates of protective efficacy, it is important to include an appropriate minimum number of participants, while minimizing the risks and burden for volunteers. Existing power calculations have limited value due to important influential assumptions. To optimize power calculations for malaria vaccine trials, we developed a non-linear, Bayesian statistical model for parasite kinetics as measured by quantitative real-time polymerase chain reaction, using existing data from mosquito-based CHMI experiments. Using our model, we provide improved, robust power calculations for various hypothetical malaria vaccine trials, taking account of important sources of variation between and within individuals. We conclude that so far, power calculations for malaria vaccine trials have been overly optimistic. We further illustrate how upcoming techniques like needle-injected CHMI may reduce required sample sizes.

Efficacy of Phase 3 Trial of RTS, S/AS01 Malaria Vaccine in infants: a systematic review and meta-analysis

Although vaccines would be the ideal tool for control, prevention, elimination, and eradication of many infectious diseases, developing of parasites vaccines such as malaria vaccine is very complex. The most advanced malaria vaccine candidate is RTS,S, a pre-erythrocytic vaccine for which pivotal phase III trial design is underway. Few recent malaria vaccine review articles have attempted to outline of all clinical trials that have occurred globally and no meta-analysis was performed on efficacy of Phase 3 Trial of RTS, S/AS01 Malaria vaccine up to now in infants. Therefore, a systematic review and meta-analysis was carried out to review new and existing data on efficacy of Phase 3 Trial of RTS, S/AS01 Malaria Vaccine in infants. The electronic databases searched were Pubmed (1965-present) and Web of Science (1970-present) (Search date: May, 2016). After full-text review of the papers evaluating clinical/severe malaria in several well-designed phase III field efficacy trials, 5 were determined to meet the eligibility criteria for inclusion in the systematic review. Four out of the 5 publications dealing with efficacy of Phase 3 Trial of RTS, S/AS01 malaria vaccine were included in the qualitative analysis. Pooled estimate of vaccine efficacy in clinical and severe malaria in children aged 5-17 mo was 29% (95% CL: 19%-46%) and 39% (95% CI 20%-74%), while this estimate vaccine in clinical and severe malaria in children aged 6-12 mo was 19% (95% CI 14%-24%) and 21 (95% CI 19%-37%), respectively. On the other hand, higher VE was seen in both per- protocol and intention-to-treat population in children aged 5-17 than the children aged 6-12 mo. The results of this meta-analysis suggest that this candidate malaria vaccine has relatively little efficacy, and the vaccine apparently will not meet the goal of malaria eradication by itself.

A monoclonal antibody-based immunoassay to measure the antibody response against the repeat region of the circumsporozoite protein of Plasmodium falciparum

Background

The malaria vaccine candidate RTS,S/AS01 (GSK Vaccines) induces high IgG concentration against the circumsporozoite protein (CSP) of Plasmodium falciparum. In human vaccine recipients circulating anti-CSP antibody concentrations are associated with protection against infection but appear not to be the correlate of protection. However, in a humanized mouse model of malaria infection prophylactic administration of a human monoclonal antibody (MAL1C), derived from a RTS,S/AS01-immunized volunteer, directed against the CSP repeat region, conveyed full protection in a dose-dependent manner suggesting that antibodies alone are able to prevent P. falciparum infection when present in sufficiently high concentrations. A competition ELISA was developed to measure the presence of MAL1C-like antibodies in polyclonal sera from RTS,S/AS01 vaccine recipients and study their possible contribution to protection against infection.

Results

MAL1C-like antibodies present in polyclonal vaccine-induced sera were evaluated for their ability to compete with biotinylated monoclonal antibody MAL1C for binding sites on the capture antigen consisting of the recombinant protein encompassing 32 NANP repeats of CSP (R32LR). Serum samples were taken at different time points from participants in two RTS,S/AS01 vaccine studies (NCT01366534 and NCT01857869). Vaccine-induced protection status of the study participants was determined based on the outcome of experimental challenge with infected mosquito bites after vaccination. Optimal conditions were established to reliably detect MAL1C-like antibodies in polyclonal sera. Polyclonal anti-CSP antibodies and MAL1C-like antibody content were measured in 276 serum samples from RTS,S/AS01 vaccine recipients using the standard ELISA and MAL-1C competition ELISA, respectively. A strong correlation was observed between the results from these assays. However, no correlation was found between the results of either assay and protection against infection.

Conclusions

The competition ELISA to measure MAL1C-like antibodies in polyclonal sera from RTS,S/AS01 vaccine recipients was robust and reliable but did not reveal the elusive correlate of protection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1596-8) contains supplementary material, which is available to authorized users.

The March Toward Malaria Vaccines

In 2013 there were an estimated 584,000 deaths and 198 million clinical illnesses due to malaria, the majority in sub-Saharan Africa. Vaccines would be the ideal addition to the existing armamentarium of anti-malaria tools. However, malaria is caused by parasites, and parasites are much more complex in terms of their biology than the viruses and bacteria for which we have vaccines, passing through multiple stages of development in the human host, each stage expressing hundreds of unique antigens. This complexity makes it more difficult to develop a vaccine for parasites than for viruses and bacteria, since an immune response targeting one stage may not offer protection against a later stage, because different antigens are the targets of protective immunity at different stages. Furthermore, depending on the life cycle stage and whether the parasite is extra- or intra-cellular, antibody and/or cellular immune responses provide protection. It is thus not surprising that there is no vaccine on the market for prevention of malaria, or any human parasitic infection. In fact, no vaccine for any disease with this breadth of targets and immune responses exists. In this limited review, we focus on four approaches to malaria vaccines, (1) a recombinant protein with adjuvant vaccine aimed at Plasmodium falciparum (Pf) pre-erythrocytic stages of the parasite cycle (RTS,S/AS01), (2) whole sporozoite vaccines aimed at Pf pre-erythrocytic stages (PfSPZ Vaccine and PfSPZ-CVac), (3) prime boost vaccines that include recombinant DNA, viruses and bacteria, and protein with adjuvant aimed primarily at Pf pre-erythrocytic, but also asexual erythrocytic stages, and (4) recombinant protein with adjuvant vaccines aimed at Pf and Plasmodium vivax sexual erythrocytic and mosquito stages. We recognize that we are not covering all approaches to malaria vaccine development, or most of the critically important work on development of vaccines against P. vivax, the second most important cause of malaria. Progress during the last few years has been significant, and a first generation malaria candidate vaccine, RTS,S/AS01, is under review by the European Medicines Agency (EMA) for its quality, safety and efficacy under article 58, which allows the EMA to give a scientific opinion about products intended exclusively for markets outside of the European Union. However, much work is in progress to optimize malaria vaccines in regard to magnitude and durability of protective efficacy and the financing and practicality of delivery. Thus, we are hopeful that anti-malaria vaccines will soon be important tools in the battle against malaria.

Clinical development of RTS,S/AS malaria vaccine: a systematic review of clinical Phase I-III trials

The first clinical Phase III trial evaluating a malaria vaccine was completed in December 2013 at 11 sites from seven sub-Saharan African countries. This systematic review assesses data of Phase I-III trials including malaria-naive adults and adults, children and infants from malaria endemic settings in sub-Saharan Africa. The main endpoint of this systematic review was an analysis of the consistency of efficacy and immunogenicity data from respective Phase I-III trials. In addition, safety data from a pooled analysis of RTS/AS Phase II trials and RTS,S/AS01 Phase III trial were reviewed. The RTS,S/AS01 malaria vaccine may become available on the market in the coming year. If so, further strategies should address challenges on how to optimize vaccine efficacy and implementation of RTS,S/AS01 vaccine within the framework of established malaria control measures.

Optimizing Intradermal Administration of Cryopreserved Plasmodium falciparum Sporozoites in Controlled Human Malaria Infection

Controlled human malaria infection (CHMI) is a powerful tool to evaluate malaria vaccine and prophylactic drug efficacy. Until recently CHMI was only carried out by the bite of infected mosquitoes. A parenteral method of CHMI would standardize Plasmodium falciparum sporozoite (PfSPZ) administration, eliminate the need for expensive challenge facility infrastructure, and allow for use of many P. falciparum strains. Recently, intradermal (ID) injection of aseptic, purified, cryopreserved PfSPZ was shown to induce P. falciparum malaria; however, 100% infection rates were not achieved by ID injection. To optimize ID PfSPZ dosing so as to achieve 100% infection, 30 adults aged 18-45 years were randomized to one of six groups composed of five volunteers each. The parameters of dose (1 × 10(4) versus 5 × 10(4) PfSPZ total dose per volunteer), number of injections (two versus eight), and aliquot volume per ID injection (10 μL versus 50 μL) were studied. Three groups attained 100% infection: 1 × 10(4) PfSPZ in 50 μL/2 doses, 1 × 10(4) PfSPZ in 10 μL/2 doses, and 5 × 10(4) PfSPZ in 10 μL/8 doses. The group that received 5 × 10(4) PfSPZ total dose in eight 10 μL injections had a 100% infection rate and the shortest prepatent period (mean of 12.7 days), approaching the prepatent period for the current CHMI standard of five infected mosquitoes.

Live attenuated pre-erythrocytic malaria vaccines

Although recent control measures have significantly reduced malaria cases and deaths in many endemic areas, an effective vaccine will be essential to eradicate this parasitic disease. Malaria vaccine strategies developed to date focus on different phases of the parasite's complex life cycle in the human host and mosquito vector, and include both subunit-based and whole-parasite vaccines. This review focuses on the 3 live-attenuated malaria vaccination strategies that have been tested in humans to date, and discusses their progress, challenges and the immune correlates of protection that have been identified.

The march toward malaria vaccines

In 2013 there were an estimated 584,000 deaths and 198 million clinical illnesses due to malaria, the majority in sub-Saharan Africa. Vaccines would be the ideal addition to the existing armamentarium of anti-malaria tools. However, malaria is caused by parasites, and parasites are much more complex in terms of their biology than the viruses and bacteria for which we have vaccines, passing through multiple stages of development in the human host, each stage expressing hundreds of unique antigens. This complexity makes it more difficult to develop a vaccine for parasites than for viruses and bacteria, since an immune response targeting one stage may not offer protection against a later stage, because different antigens are the targets of protective immunity at different stages. Furthermore, depending on the life cycle stage and whether the parasite is extra- or intra-cellular, antibody and/or cellular immune responses provide protection. It is thus not surprising that there is no vaccine on the market for prevention of malaria, or any human parasitic infection. In fact, no vaccine for any disease with this breadth of targets and immune responses exists. In this limited review, we focus on four approaches to malaria vaccines, (1) a recombinant protein with adjuvant vaccine aimed at Plasmodium falciparum (Pf) pre-erythrocytic stages of the parasite cycle (RTS,S/AS01), (2) whole sporozoite vaccines aimed at Pf pre-erythrocytic stages (PfSPZ Vaccine and PfSPZ-CVac), (3) prime boost vaccines that include recombinant DNA, viruses and bacteria, and protein with adjuvant aimed primarily at Pf pre-erythrocytic, but also asexual erythrocytic stages, and (4) recombinant protein with adjuvant vaccines aimed at Pf and Plasmodium vivax sexual erythrocytic and mosquito stages. We recognize that we are not covering all approaches to malaria vaccine development, or most of the critically important work on development of vaccines against P. vivax, the second most important cause of malaria. Progress during the last few years has been significant, and a first generation malaria candidate vaccine, RTS,S/AS01, is under review by the European Medicines Agency (EMA) for its quality, safety and efficacy under article 58, which allows the EMA to give a scientific opinion about products intended exclusively for markets outside of the European Union. However, much work is in progress to optimize malaria vaccines in regard to magnitude and durability of protective efficacy and the financing and practicality of delivery. Thus, we are hopeful that anti-malaria vaccines will soon be important tools in the battle against malaria.

Ad35.CS.01-RTS,S/AS01 Heterologous Prime Boost Vaccine Efficacy against Sporozoite Challenge in Healthy Malaria-Naïve Adults

Methods

In an observer blind, phase 2 trial, 55 adults were randomized to receive one dose of Ad35.CS.01 vaccine followed by two doses of RTS,S/AS01 (ARR-group) or three doses of RTS,S/AS01 (RRR-group) at months 0, 1, 2 followed by controlled human malaria infection.

Results

ARR and RRR vaccine regimens were well tolerated. Efficacy of ARR and RRR groups after controlled human malaria infection was 44% (95% confidence interval 21%-60%) and 52% (25%-70%), respectively. The RRR-group had greater anti-CS specific IgG titers than did the ARR-group. There were higher numbers of CS-specific CD4 T-cells expressing > 2 cytokine/activation markers and more ex vivo IFN-γ enzyme-linked immunospots in the ARR-group than the RRR-group. Protected subjects had higher CS-specific IgG titers than non-protected subjects (geometric mean titer, 120.8 vs 51.8 EU/ml, respectively; P = .001).

Conclusions

An increase in vaccine efficacy of ARR-group over RRR-group was not achieved. Future strategies to improve upon RTS,S-induced protection may need to utilize alternative highly immunogenic prime-boost regimens and/or additional target antigens.

Trial Registration

ClinicalTrials.gov NCT01366534

Liver-Stage Specific Response among Endemic Populations: Diet and Immunity

Developing effective anti-malarial vaccine has been a challenge for long. Various factors including complex life cycle of parasite and lack of knowledge of stage specific critical antigens are some of the reasons. Moreover, inadequate understanding of the immune responses vis-à-vis sterile protection induced naturally by Plasmodia infection has further compounded the problem. It has been shown that people living in endemic areas take years to develop protective immunity to blood stage infection. But hardly anyone believes that immunity to liver-stage infection could be developed. Various experimental model studies using attenuated parasite suggest that liver-stage immunity might exist among endemic populations. This could be induced because of the attenuation of parasite in liver by various compounds present in the diet of endemic populations.

Impact of malaria preexposure on antiparasite cellular and humoral immune responses after controlled human malaria infection

To understand the effect of previous malaria exposure on antiparasite immune responses is important for developing successful immunization strategies. Controlled human malaria infections (CHMIs) using cryopreserved Plasmodium falciparum sporozoites provide a unique opportunity to study differences in acquisition or recall of antimalaria immune responses in individuals from different transmission settings and genetic backgrounds. In this study, we compared antiparasite humoral and cellular immune responses in two cohorts of malaria-naive Dutch volunteers and Tanzanians from an area of low malarial endemicity, who were subjected to the identical CHMI protocol by intradermal injection of P. falciparum sporozoites. Samples from both trials were analyzed in parallel in a single center to ensure direct comparability of immunological outcomes. Within the Tanzanian cohort, we distinguished one group with moderate levels of preexisting antibodies to asexual P. falciparum lysate and another that, based on P. falciparum serology, resembled the malaria-naive Dutch cohort. Positive P. falciparum serology at baseline was associated with a lower parasite density at first detection by quantitative PCR (qPCR) after CHMI than that for Tanzanian volunteers with negative serology. Post-CHMI, both Tanzanian groups showed a stronger increase in anti-P. falciparum antibody titers than Dutch volunteers, indicating similar levels of B-cell memory independent of serology. In contrast to the Dutch, Tanzanians failed to increase P. falciparum-specific in vitro recall gamma interferon (IFN-γ) production after CHMI, and innate IFN-γ responses were lower in P. falciparum lysate-seropositive individuals than in seronegative individuals. In conclusion, positive P. falciparum lysate serology can be used to identify individuals with better parasite control but weaker IFN-γ responses in circulating lymphocytes, which may help to stratify volunteers in future CHMI trials in areas where malaria is endemic.

Antibody and B cell responses to Plasmodium sporozoites

Antibodies are capable of blocking infection of the liver by Plasmodium sporozoites. Accordingly the induction of anti-sporozoite antibodies is a major aim of various vaccine approaches to malaria. In recent years our knowledge of the specificity and quantities of antibodies required for protection has been greatly expanded by clinical trials of various whole sporozoite and subunit vaccines. Moreover, the development of humanized mouse models and transgenic parasites have also aided our ability to assess the specificity of antibodies and their ability to block infection. Nonetheless, considerable gaps remain in our knowledge – in particular in understanding what antigens are recognized by infection blocking antibodies and in knowing how we can induce robust, long-lived antibody responses. Maintaining high levels of circulating antibodies is likely to be of primary importance, as antibodies must block infection in the short time it takes for sporozoites to reach the liver from the skin. It is clear that a better understanding of the development of protective B cell-mediated immunity will aid the development and refinement of malaria vaccines.

Vaccine Adjuvant Systems containing monophosphoryl lipid A and QS-21 induce strong humoral and cellular immune responses against hepatitis B surface antigen which persist for at least 4 years after vaccination

BACKGROUND

Recombinant hepatitis B surface antigen (HBsAg) was used as a model antigen to evaluate persistence of cellular and humoral immune responses when formulated with three different Adjuvant Systems containing 3-O-desacyl-4'-monophosphoryl lipid A (MPL) and QS-21, in an oil-in-water emulsion (AS02B and AS02V), or with liposomes (AS01B).

METHODS

This is an open, 4-year follow-up of a previous randomised, double-blind study. Healthy subjects aged 18-40 years received three vaccine doses on a month 0, 1, 10 schedule and were initially followed for 18 months. A total of 93 subjects (AS02B: n=30; AS02V: n=28; AS01B: n=35) were enrolled in this follow-up and had an additional blood sample taken at Year 4 (NCT02153320). The primary endpoint was the frequency of HBsAg-specific CD4(+) and CD8(+) T-cells expressing cytokines upon short-term in vitro stimulation of peripheral blood mononuclear cells with HBsAg-derived peptides. Secondary endpoints were anti-HBs antibody titres and frequency of HBsAg-specific memory B-cells.

RESULTS

A strong and persistent specific CD4(+) T-cell response was observed at Year 4 in all groups. HBsAg-specific CD4(+) T-cells expressed mainly CD40L and IL-2, and to a lesser extent TNF-α and IFN-γ. HBsAg-specific CD8(+) T-cells were not detected in any group. A high, persistent HBsAg-specific humoral immune response was observed in all groups, with all subjects seroprotected (antibody titre ≥10mIU/mL) at Year 4. The geometric mean antibody titre at Year 4 was above 100,000mIU/mL in all groups. A strong memory B-cell response was observed post-dose 2, which tended to increase post-dose 3 and persisted at Year 4 in all groups.

CONCLUSION

The MPL/QS-21/HBsAg vaccine formulations induced persistent immune responses up to 4 years after first vaccination. These Adjuvant Systems offer potential for combination with recombinant, synthetic or highly purified subunit vaccines, particularly for vaccination against challenging diseases, or in specific populations, although additional studies are needed.

The hunt for protective correlates of immunity to Plasmodium falciparum malaria

Determining an immunologic correlate of protection against Plasmodium falciparum malaria has been the holy grail of natural infection studies, and sought after as an endpoint for malaria vaccine trials. An in vitro assay that provides an accurate and precise assessment of protective immunity to malaria would make smaller, short-duration studies feasible, rather than the currently powered study designs that use morbidity or mortality as outcomes. Such a biomarker would be especially desirable in situations where malaria control measures that result in decreases in clinical endpoints and putatively waning protective immunity have been implemented. In an article published in BMC Medicine, Osier and colleagues addressed this problem, and demonstrated that antibodies promoting opsonic phagocytosis of merozoites provide a functional link between antigen-specific responses and protection. Understanding the mechanisms conferring protection against malaria not only improves our knowledge of basic human immunology, but promises to help in the design of an effective malaria vaccine.

An unstable Th epitope of P. falciparum fosters central memory T cells and anti-CS antibody responses

Malaria is transmitted by Plasmodium-infected anopheles mosquitoes. Widespread resistance of mosquitoes to insecticides and resistance of parasites to drugs highlight the urgent need for malaria vaccines. The most advanced malaria vaccines target sporozoites, the infective form of the parasite. A major target of the antibody response to sporozoites are the repeat epitopes of the circumsporozoite (CS) protein, which span almost one half of the protein. Antibodies to these repeats can neutralize sporozoite infectivity. Generation of protective antibody responses to the CS protein (anti-CS Ab) requires help by CD4 T cells. A CD4 T cell epitope from the CS protein designated T* was previously identified by screening T cells from volunteers immunized with irradiated P. falciparum sporozoites. The T* sequence spans twenty amino acids that contains multiple T cell epitopes restricted by various HLA alleles. Subunit malaria vaccines including T* are highly immunogenic in rodents, non-human primates and humans. In this study we characterized a highly conserved HLA-DRβ1*04:01 (DR4) restricted T cell epitope (QNT-5) located at the C-terminus of T*. We found that a peptide containing QNT-5 was able to elicit long-term anti-CS Ab responses and prime CD4 T cells in HLA-DR4 transgenic mice despite forming relatively unstable MHC-peptide complexes highly susceptible to HLA-DM editing. We attempted to improve the immunogenicity of QNT-5 by replacing the P1 anchor position with an optimal tyrosine residue. The modified peptide QNT-Y formed stable MHC-peptide complexes highly resistant to HLA-DM editing. Contrary to expectations, a linear peptide containing QNT-Y elicited almost 10-fold lower long-term antibody and IFN-γ responses compared to the linear peptide containing the wild type QNT-5 sequence. Some possibilities regarding why QNT-5 is more effective than QNT-Y in inducing long-term T cell and anti-CS Ab when used as vaccine are discussed.

Sequential Phase 1 and Phase 2 randomized, controlled trials of the safety, immunogenicity and efficacy of combined pre-erythrocytic vaccine antigens RTS,S and TRAP formulated with AS02 Adjuvant System in healthy, malaria naïve adults

In an attempt to improve the efficacy of the candidate malaria vaccine RTS,S/AS02, two studies were conducted in 1999 in healthy volunteers of RTS,S/AS02 in combination with recombinant Plasmodium falciparum thrombospondin-related anonymous protein (TRAP). In a Phase 1 safety and immunogenicity study, volunteers were randomized to receive TRAP/AS02 (N=10), RTS,S/AS02 (N=10), or RTS,S+TRAP/AS02 (N=20) at 0, 1 and 6-months. In a Phase 2 challenge study, subjects were randomized to receive either RTS,S+TRAP/AS02 (N=25) or TRAP/AS02 (N=10) at 0 and 1-month, or to a challenge control group (N=8). In both studies, the combination vaccine had an acceptable safety profile and was acceptably tolerated. Antigen-specific antibodies, lymphoproliferative responses, and IFN-γ production by ELISPOT assay elicited with the combination vaccine were qualitatively similar to those generated by the single component vaccines. However, post-dose 2 anti-CS antibodies in the RTS,S+TRAP/AS02 vaccine recipients were lower than in the RTS,S/AS02 vaccine recipients. After challenge, 10 of 11 RTS,S+TRAP/AS02 vaccinees, 5 of 5 TRAP/AS02 vaccinees, and 8 of 8 infectivity controls developed parasitemia, with median pre-patent periods of 13.0, 11.0, and 12.0 days, respectively. The absence of any prevention or delay of parasitemia by TRAP/AS02 suggests no apparent added value of TRAP/AS02 as a candidate vaccine. The absence of significant protection or delay of parasitemia in the 11 RTS,S+TRAP/AS02 vaccine recipients contrasts with previous 2 dose studies of RTS,S/AS02. The small sample size did not permit identifying statistically significant differences between the study arms. However, we speculate, within the constraints of the challenge study, that the presence of the TRAP antigen may have interfered with the vaccine efficacy previously observed with this regimen of RTS,S/AS02, and that any future TRAP-based vaccines should consider employing alternative vaccine platforms.