Merozoite Surface Protein 1 from Plasmodium falciparum Is a Major Target of Opsonizing Antibodies in Individuals with Acquired Immunity against Malaria

Full-length MSP1 is a major target of protective immunity after controlled human malaria infection

The merozoite surface protein 1 (MSP1) is the most abundant protein on the surface of the invasive merozoite stages of Plasmodium falciparum and has long been considered a key target of protective immunity. We used samples from a single controlled human malaria challenge study to test whether the full-length version of MSP1 (MSP1FL) induced antibodies that mediated Fc-IgG functional activity in five independent assays. We found that anti-MSP1FL antibodies induced complement fixation via C1q, monocyte-mediated phagocytosis, neutrophil respiratory burst, and natural killer cell degranulation as well as IFNγ production. Activity in each of these assays was strongly associated with protection. The breadth of MSP1-specific Fc-mediated effector functions was more strongly associated with protection than the individual measures and closely mirrored what we have previously reported using the same assays against merozoites. Our findings suggest that MSP1FL is an important target of functional antibodies that contribute to a protective immune response against malaria.

IgG Subclass Switch in Volunteers Repeatedly Immunized with the Full-Length Plasmodium falciparum Merozoite Surface Protein 1 (MSP1)

Vaccines are highly effective tools against infectious diseases and are also considered necessary in the fight against malaria. Vaccine-induced immunity is frequently mediated by antibodies. We have recently conducted a first-in-human clinical trial featuring SumayaVac-1, a malaria vaccine based on the recombinant, full-length merozoite surface protein 1 (MSP1FL) formulated with GLA-SE as an adjuvant. Vaccination with MSP1FL was safe and elicited sustainable IgG antibody titers that exceeded those observed in semi-immune populations from Africa. Moreover, IgG antibodies stimulated various Fc-mediated effector mechanisms associated with protection against malaria. However, these functionalities gradually waned. Here, we show that the initial two doses of SumayaVac-1 primarily induced the cytophilic subclasses IgG1 and IgG3. Unexpectedly, a shift in the IgG subclass composition occurred following the third and fourth vaccinations. Specifically, there was a progressive transition to IgG4 antibodies, which displayed a reduced capacity to engage in Fc-mediated effector functions and also exhibited increased avidity. In summary, our analysis of antibody responses to MSP1FL vaccination unveils a temporal shift towards noninflammatory IgG4 antibodies. These findings underscore the importance of considering the impact of IgG subclass composition on vaccine-induced immunity, particularly concerning Fc-mediated effector functions. This knowledge is pivotal in guiding the design of optimal vaccination strategies against malaria, informing decision making for future endeavors in this critical field.

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

INTRODUCTION

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

AREAS COVERED

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

EXPERT OPINION

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

Accumulation of Neutrophil Phagocytic Antibody Features Tracks With Naturally Acquired Immunity Against Malaria in Children

BACKGROUND

Studies have demonstrated the protective role of antibodies against malaria. Young children are known to be particularly vulnerable to malaria, pointing to the evolution of naturally acquired clinical immunity over time. However, whether changes in antibody functionality track with the acquisition of naturally acquired malaria immunity remains incompletely understood.

METHODS

Using systems serology, we characterized sporozoite- and merozoite-specific antibody profiles of uninfected Malian children before the malaria season who differed in their ability to control parasitemia and fever following Plasmodium falciparum (Pf) infection. We then assessed the contributions of individual traits to overall clinical outcomes, focusing on the immunodominant sporozoite CSP and merozoite AMA1 and MSP1 antigens.

RESULTS

Humoral immunity evolved with age, with an expansion of both magnitude and functional quality, particularly within blood-stage phagocytic antibody activity. Moreover, concerning clinical outcomes postinfection, protected children had higher antibody-dependent neutrophil activity along with higher levels of MSP1-specific IgG3 and IgA and CSP-specific IgG3 and IgG4 prior to the malaria season.

CONCLUSIONS

These data point to the natural evolution of functional humoral immunity to Pf with age and highlight particular antibody Fc-effector profiles associated with the control of malaria in children, providing clues for the design of next-generation vaccines or therapeutics.

Multifunctional IgG/IgM antibodies and cellular cytotoxicity are elicited by the full-length MSP1 SumayaVac-1 malaria vaccine

Radical control of malaria likely requires a vaccine that targets both the asymptomatic liver stages and the disease-causing blood stages of the human malaria parasite Plasmodium falciparum. While substantial progress has been made towards liver stage vaccines, the development of a blood stage vaccine is lagging behind. We have recently conducted a first-in-human clinical trial to evaluate the safety and immunogenicity of the recombinant, full-length merozoite surface protein 1 (MSP1FL) formulated with GLA-SE as adjuvant. Here, we show that the vaccine, termed SumayaVac-1, elicited both a humoral and cellular immune response as well as a recall T cell memory. The induced IgG and IgM antibodies were able to stimulate various Fc-mediated effector mechanisms associated with protection against malaria, including phagocytosis, release of reactive oxygen species, production of IFN-γ as well as complement activation and fixation. The multifunctional activity of the humoral immune response remained for at least 6 months after vaccination and was comparable to that of naturally acquired anti-MSP1 antibodies from semi-immune adults from Kenya. We further present evidence of SumayaVac-1 eliciting a recallable cellular cytotoxicity by IFN-γ producing CD8+ T cells. Our study revitalizes MSP1FL as a relevant blood stage vaccine candidate and warrants further evaluation of SumayaVac-1 in a phase II efficacy trial.

Mouse Models for Unravelling Immunology of Blood Stage Malaria

Malaria comprises a spectrum of disease syndromes and the immune system is a major participant in malarial disease. This is particularly true in relation to the immune responses elicited against blood stages of Plasmodium-parasites that are responsible for the pathogenesis of infection. Mouse models of malaria are commonly used to dissect the immune mechanisms underlying disease. While no single mouse model of Plasmodium infection completely recapitulates all the features of malaria in humans, collectively the existing models are invaluable for defining the events that lead to the immunopathogenesis of malaria. Here we review the different mouse models of Plasmodium infection that are available, and highlight some of the main contributions these models have made with regards to identifying immune mechanisms of parasite control and the immunopathogenesis of malaria.

Targeting the Plasmodium falciparum proteome and organelles for potential antimalarial drug candidates

There is an unmet need to unearth alternative treatment options for malaria, wherein this quest is more pressing in recent times due to high morbidity and mortality data arising mostly from the endemic countries coupled with partial diversion of attention from the disease in view of the SARS-Cov-2 pandemic. Available therapeutic options for malaria have been severely threatened with the emergence of resistance to almost all the antimalarial drugs by the Plasmodium falciparum parasite in humans, which is a worrying situation. Artemisinin combination therapies (ACT) that have so far been the mainstay of malaria have encountered resistance by malaria parasite in South East Asia, which is regarded as a notorious ground zero for the emergence of resistance to antimalarial drugs. This review analyzes a few key druggable targets for the parasite and the potential of specific inhibitors to mitigate the emerging antimalarial drug resistance problem by providing a concise assessment of the essential proteins of the malaria parasite that could serve as targets. Moreover, this work provides a summary of the advances made in malaria parasite biology and the potential to leverage these findings for antimalarial drug production.

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.

Structure of the merozoite surface protein 1 from Plasmodium falciparum

The merozoite surface protein 1 (MSP-1) is the most abundant protein on the surface of the erythrocyte-invading Plasmodium merozoite, the causative agent of malaria. MSP-1 is essential for merozoite formation, entry into and escape from erythrocytes, and is a promising vaccine candidate. Here, we present monomeric and dimeric structures of full-length MSP-1. MSP-1 adopts an unusual fold with a large central cavity. Its fold includes several coiled-coils and shows structural homology to proteins associated with membrane and cytoskeleton interactions. MSP-1 formed dimers through these domains in a concentration-dependent manner. Dimerization is affected by the presence of the erythrocyte cytoskeleton protein spectrin, which may compete for the dimerization interface. Our work provides structural insights into the possible mode of interaction of MSP-1 with erythrocytes and establishes a framework for future investigations into the role of MSP-1 in Plasmodium infection and immunity.

Recent advances in immunotherapies against infectious diseases

Immunotherapies are disease management strategies that target or manipulate components of the immune system. Infectious diseases pose a significant threat to human health as evidenced by countries continuing to grapple with several emerging and re-emerging diseases, the most recent global health threat being the SARS-CoV2 pandemic. As such, various immunotherapeutic approaches are increasingly being investigated as alternative therapies for infectious diseases, resulting in significant advances towards the uncovering of pathogen-host immunity interactions. Novel and innovative therapeutic strategies are necessary to overcome the challenges typically faced by existing infectious disease prevention and control methods such as lack of adequate efficacy, drug toxicity, and the emergence of drug resistance. As evidenced by recent developments and success of pharmaceuticals such as monoclonal antibodies (mAbs), immunotherapies already show abundant promise to overcome such limitations while also advancing the frontiers of medicine. In this review, we summarize some of the most notable inroads made to combat infectious disease, over mainly the last 5 years, through the use of immunotherapies such as vaccines, mAb-based therapies, T-cell-based therapies, manipulation of cytokine levels, and checkpoint inhibition. While its most general applications are founded in cancer treatment, advances made towards the curative treatment of human immunodeficiency virus, tuberculosis, malaria, zika virus and, most recently COVID-19, reinforce the role of immunotherapeutic strategies in the broader field of disease control. Ultimately, the comprehensive specificity, safety, and cost of immunotherapeutics will impact its widespread implementation.

Naturally Acquired Humoral Immunity Against Plasmodium falciparum Malaria

Malaria remains a significant contributor to the global burden of disease, with around 40% of the world's population at risk of Plasmodium infections. The development of an effective vaccine against the malaria parasite would mark a breakthrough in the fight to eradicate the disease. Over time, natural infection elicits a robust immune response against the blood stage of the parasite, providing protection against malaria. In recent years, we have gained valuable insight into the mechanisms by which IgG acts to prevent pathology and inhibit parasite replication, as well as the potential role of immunoglobulin M (IgM) in these processes. Here, we discuss recent advances in our understanding of the mechanisms, acquisition, and maintenance of naturally acquired immunity, and the relevance of these discoveries for the development of a potential vaccine against the blood stage of Plasmodium falciparum.

Breadth of Functional Antibodies Is Associated With Plasmodium falciparum Merozoite Phagocytosis and Protection Against Febrile Malaria

BACKGROUND

The specific targets of functional antibodies against Plasmodium falciparum merozoites remain largely unexplored and, more importantly, their relevance to naturally acquired immunity in longitudinal cohort studies (LCSs) is yet to be tested.

METHODS

Functionality of immunoglobulin G (IgG) antibodies against 24 merozoite antigens was determined at the baseline of an LCS in Ghana using a bead-based opsonic phagocytosis assay (BPA). Antigen-specific IgG3 subclass antibodies were quantified in the same samples by the Luminex multiplex system.

RESULTS

A wide range of BPA activity was observed across the different antigens. High BPA responses of nMSP3K1, GLURP-R2, MSP23D7, MSP119k, and PfRh2-2030 coupled beads were significantly associated with a higher probability of children not experiencing febrile malaria. Children with high breadth of functional antibodies against these antigens together with cMSP33D7 had a significantly reduced risk of febrile malaria (adjusted hazard ratio, 0.36 [95% confidence interval, .18-.72]; P = .004). Five of the 6 BPA activities significantly (likelihood ratio rest, P ≤ .05) contributed to the protective immunity observed with the IgG3 antibodies.

CONCLUSIONS

The development of BPA allowed profiling of functional antibodies in an LCS. Identification of targets of opsonic phagocytosis may have implications in the development of a subunit malaria vaccine.

Immunization with full-length Plasmodium falciparum merozoite surface protein 1 is safe and elicits functional cytophilic antibodies in a randomized first-in-human trial

A vaccine remains a priority in the global fight against malaria. Here, we report on a single-center, randomized, double-blind, placebo and adjuvant-controlled, dose escalation phase 1a safety and immunogenicity clinical trial of full-length Plasmodium falciparum merozoite surface protein 1 (MSP1) in combination with GLA-SE adjuvant. Thirty-two healthy volunteers were vaccinated at least three times with MSP1 plus adjuvant, adjuvant alone, or placebo (24:4:4) to evaluate the safety and immunogenicity. MSP1 was safe, well tolerated and immunogenic, with all vaccinees sero-converting independent of the dose. The MSP1-specific IgG and IgM titers persisted above levels found in malaria semi-immune humans for at least 6 months after the last immunization. The antibodies were variant- and strain-transcending and stimulated respiratory activity in granulocytes. Furthermore, full-length MSP1 induced memory T-cells. Our findings encourage challenge studies as the next step to evaluate the efficacy of full-length MSP1 as a vaccine candidate against falciparum malaria (EudraCT 2016-002463-33).

[Development of malaria vaccines-state of the art]

Weltweit leben 3,1 Mrd. Menschen in Gebieten, in denen Malaria endemisch ist (Tropen, Subtropen). Jährlich erkranken etwa 200 Mio. Menschen, ca. 500.000 sterben daran. Betroffen sind vor allem Kinder. Um die Malaria zu kontrollieren und schlussendlich jegliche Neuinfektion zu verhindern, ist die Entwicklung wirksamer Impfstoffe von großer Bedeutung. In diesem Beitrag werden zunächst Hintergrundinformationen zur Geschichte der Impfstoffentwicklung, zur Malariaerkrankung und zu den Möglichkeiten der Therapie und Ausbreitungskontrolle gegeben. Der Hauptteil widmet sich dem aktuellen Forschungsstand zu Impfstoffen gegen den Erreger Plasmodium falciparum, gefolgt von einer ausführlichen Diskussion.

Malaria ist eine parasitäre Infektionskrankheit, die von Einzellern, sog. Plasmodien, verursacht wird. Es werden 5 humanpathogene Spezies unterschieden, von denen P. falciparum über 99 % der Erkrankungen in Afrika verursacht. Überträger (Vektor) ist die Anophelesmücke. Plasmodium bietet innerhalb seines Lebenszyklus verschiedene Ansatzpunkte für die Wirkung von Impfstoffen. Von den insgesamt ca. 70 Impfstoffkandidaten sind die präerythrozytären Impfstoffe, die in den Leberzyklus des Parasiten eingreifen, aktuell am weitesten entwickelt. Die von der Weltgesundheitsorganisation (WHO) angestrebte Wirksamkeit von mindestens 75 % wurde aber längst nicht erreicht.

Mit RTS,S/AS01 wird derzeit erstmals ein mäßig wirksamer Impfstoff großflächig eingesetzt. Schon jetzt ist offensichtlich, dass die Malaria nur im Zusammenspiel mit anderen Maßnahmen eingedämmt werden kann. Expositionsprophylaxe mit imprägnierten Moskitonetzen, der Einsatz von Insektiziden mit Residualeffekt in Innenräumen (Indoor Residual Spraying), die Vernichtung von Moskitobrutplätzen und schnelle Diagnose und Therapie der Erkrankung sind hier wichtige Elemente ebenso wie eine funktionierende Gesundheitsversorgung, die in den von Armut geprägten Gebieten oft nicht gewährleistet ist.

Computational and experimental analysis of the glycophosphatidylinositol-anchored proteome of the human parasitic nematode Brugia malayi

Further characterization of essential systems in the parasitic filarial nematode Brugia malayi is needed to better understand its biology, its interaction with its hosts, and to identify critical components that can be exploited to develop novel treatments. The production of glycophosphatidylinositol-anchored proteins (GPI-APs) is essential for eukaryotic cellular and physiological function. In addition, GPI-APs perform many important roles for cells. In this study, we characterized the B. malayi GPI-anchored proteome using both computational and experimental approaches. We used bioinformatic strategies to show the presence or absence of B. malayi GPI-AP biosynthetic pathway genes and to compile a putative B. malayi GPI-AP proteome using available prediction programs. We verified these in silico analyses using proteomics to identify GPI-AP candidates prepared from the surface of intact worms and from membrane enriched extracts. Our study represents the first description of the GPI-anchored proteome in B. malayi and lays the groundwork for further exploration of this essential protein modification as a target for novel anthelmintic therapeutic strategies.

Plasmodium falciparum merozoite surface antigen-specific cytophilic IgG and control of malaria infection in a Beninese birth cohort

Background

Substantial evidence indicates that cytophilic IgG responses to Plasmodium falciparum merozoite antigens play a role in protection from malaria. The specific targets mediating immunity remain unclear. Evaluating antibody responses in infants naturally-exposed to malaria will allow to better understand the establishment of anti-malarial immunity and to contribute to a vaccine development by identifying the most appropriate merozoite candidate antigens.

Methods

The study was based on parasitological and clinical active follow-up of infants from birth to 18 months of age conducted in the Tori Bossito area of southern Benin. For 399 infants, plasma levels of cytophilic IgG antibodies with specificity for five asexual stage malaria vaccine candidate antigens were determined by ELISA in infants’ peripheral blood at 6, 9, 12 and 15 months of age. Multivariate mixed logistic model was used to investigate the association between antibody levels and anti-malarial protection in the trimester following the IgG quantification. Moreover, the concentrations of merozoite antigen-specific IgG were compared between a group of infants apparently able to control asymptomatic malaria infection (CAIG) and a group of infants with no control of malaria infection (Control group (NCIG)). Protective effect of antibodies was also assessed after 15 months of malaria exposure with a Cox regression model adjusted on environmental risk.

Results

Cytophilic IgG responses to AMA1, MSP1, MSP2-3D7, MSP2-FC27, MSP3 and GLURP R2 were associated with increasing malarial infection risk in univariate analysis. The multivariate mixed model showed that IgG1 and IgG3 to AMA1 were associated with an increased risk of malarial infection. However infants from CAIG (n = 53) had significantly higher AMA1-, MSP2-FC27-, MSP3-specific IgG1 and AMA1-, MSP1-, MSP2-FC27-, MSP3 and GLURP-R2-specific IgG3 than those from NCIG (n = 183). The latter IgG responses were not associated with protection against clinical malaria in the whole cohort when protective effect is assessed after 15 months of malaria exposition.

Conclusion

In this cohort, merozoite antigen-specific cytophilic IgG levels represent a marker of malaria exposure in infants from 6 to 18 months of age. However, infants with resolution of asymptomatic infection (CAIG) seem to have acquired naturally immunity against P. falciparum. This observation is encouraging in the context of the development of multitarget P. falciparum vaccines.

The impact of early life exposure to Plasmodium falciparum on the development of naturally acquired immunity to malaria in young Malawian children

Background

Antibodies targeting malaria blood-stage antigens are important targets of naturally acquired immunity, and may act as valuable biomarkers of malaria exposure.

Methods

Six-hundred and one young Malawian children from a randomized trial of prenatal nutrient supplementation with iron and folic acid or pre- and postnatal multiple micronutrients or lipid-based nutrient supplements were followed up weekly at home and febrile episodes were investigated for malaria from birth to 18 months of age. Antibodies were measured for 601 children against merozoite surface proteins (MSP1 19kD, MSP2), erythrocyte binding antigen 175 (EBA175), reticulocyte binding protein homologue 2 (Rh2A9), schizont extract and variant surface antigens expressed by Plasmodium falciparum-infected erythrocytes (IE) at 18 months of age. The antibody measurement data was related to concurrent malaria infection and to documented episodes of clinical malaria.

Results

At 18 months of age, antibodies were significantly higher among parasitaemic than aparasitaemic children. Antibody levels against MSP1 19kD, MSP2, schizont extract, and IE variant surface antigens were significantly higher in children who had documented episodes of malaria than in children who did not. Antibody levels did not differ between children with single or multiple malaria episodes before 18 months, nor between children who had malaria before 6 months of age or between 6 and 18 months.

Conclusions

Antibodies to merozoite and IE surface antigens increased following infection in early childhood, but neither age at first infection nor number of malaria episodes substantially affected antibody acquisition. These findings have implications for malaria surveillance during early childhood in the context of elimination.

Trials registration Clinical Trials Registration: NCT01239693 (Date of registration: 11-10-2010). URL: http://www.ilins.org

Neutrophils and Malaria

Neutrophils are abundant in the circulation and are one of the immune system's first lines of defense against infection. There has been substantial work carried out investigating the role of neutrophils in malaria and it is clear that during infection neutrophils are activated and are capable of clearing malaria parasites by a number of mechanisms. This review focuses on neutrophil responses to human malarias, summarizing evidence which helps us understand where neutrophils are, what they are doing, how they interact with parasites as well as their potential role in vaccine mediated immunity. We also outline future research priorities for these, the most abundant of leukocytes.

The Rough Guide to Monocytes in Malaria Infection

While half of the world's population is at risk of malaria, the most vulnerable are still children under five, pregnant women and returning travelers. Anopheles mosquitoes transmit malaria parasites to the human host; but how Plasmodium interact with the innate immune system remains largely unexplored. The most recent advances prove that monocytes are a key component to control parasite burden and to protect host from disease. Monocytes' protective roles include phagocytosis, cytokine production and antigen presentation. However, monocytes can be involved in pathogenesis and drive inflammation and sequestration of infected red blood cells in organs such as the brain, placenta or lungs by secreting cytokines that upregulate expression of endothelial adhesion receptors. Plasmodium DNA, hemozoin or extracellular vesicles can impair the function of monocytes. With time, reinfections with Plasmodium change the relative proportion of monocyte subsets and their physical properties. These changes relate to clinical outcomes and might constitute informative biomarkers of immunity. More importantly, at the molecular level, transcriptional, metabolic or epigenetic changes can “prime” monocytes to alter their responses in future encounters with Plasmodium. This mechanism, known as trained immunity, challenges the traditional view of monocytes as a component of the immune system that lacks memory. Overall, this rough guide serves as an update reviewing the advances made during the past 5 years on understanding the role of monocytes in innate immunity to malaria.

Novel Strategies for Malaria Vaccine Design

The quest for a licensed effective vaccine against malaria remains a global priority. Even though classical vaccine design strategies have been successful for some viral and bacterial pathogens, little success has been achieved for Plasmodium falciparum, which causes the deadliest form of malaria due to its diversity and ability to evade host immune responses. Nevertheless, recent advances in vaccinology through high throughput discovery of immune correlates of protection, lymphocyte repertoire sequencing and structural design of immunogens, provide a comprehensive approach to identifying and designing a highly efficacious vaccine for malaria. In this review, we discuss novel vaccine approaches that can be employed in malaria vaccine design.