Inhibition of parasite invasion by monoclonal antibody against epidermal growth factor-like domain of Plasmodium vivax merozoite surface protein 1 paralog

Prevalence and Characteristics of Plasmodium vivax Gametocytes in Duffy-Positive and Duffy-Negative Populations across Ethiopia

Plasmodium parasites replicate asexually in human hosts. The proportion of infections that carries gametocytes is a proxy for human-to-mosquito transmissibility. It is unclear which proportion of Plasmodium vivax infections in Duffy-negative populations carries gametocytes. We determined the prevalence and characteristics of P. vivax gametocytes in Duffy-positive and -negative populations across broad regions of Ethiopia. Finger-prick blood samples were collected for microscopic and molecular screening of Plasmodium parasites and Duffy status of individuals. Molecular screening of Plasmodium species and Duffy blood group genotyping was done using SYBR green and the Taqman quantitative polymerase chain reaction method. Of the 447 febrile patients who were shown to be P. vivax smear positive, 414 (92.6%) were confirmed by molecular screening as P. vivax and 16 (3.9%) of them were from Duffy-negative individuals. Of these, 5 of 16 (31.3%) Duffy-negative P. vivax-infected samples were detected with gametocytes. Of the 398 Duffy-positive P. vivax-infected samples, 150 (37.7%) were detected with gametocytes, slightly greater than that in Duffy-negative samples. This study highlights the presence of P. vivax gametocytes in Duffy-negative infections, suggestive of human-to-mosquito transmissibility. Although P. vivax infections in Duffy-negative individuals were commonly associated with low parasitemia, some of these infections were shown to have relatively high parasitemia and may represent a prominent erythrocyte invasion capability of P. vivax, and hidden reservoirs that can contribute to transmission. A better understanding of P. vivax transmission biology and gametocyte function particularly in Duffy-negative populations would aid future treatment and management of P. vivax malaria in Africa.

The Plasmodium vivax MSP1P-19 is involved in binding of reticulocytes through interactions with the membrane proteins band3 and CD71

The parasite Plasmodium vivax preferentially invades human reticulocytes. Its merozoite surface protein 1 paralog (PvMSP1P), particularly the 19-kDa C-terminal region (PvMSP1P-19), has been shown to bind to reticulocytes, and this binding can be inhibited by antisera obtained by PvMSP1P-19 immunization. The molecular mechanism of interactions between PvMSP1P-19 and reticulocytes during P. vivax invasion, however, remains unclear. In this study, we analyzed the ability of MSP1P-19 to bind to different concentrations of reticulocytes and confirmed its reticulocyte preference. LC-MS analysis was used to identify two potential reticulocyte receptors, band3 and CD71, that interact with MSP1P-19. Both PvMSP1P-19 and its sister taxon Plasmodium cynomolgi MSP1P-19 were found to bind to the extracellular loop (loop 5) of band3, where the interaction of MSP1P-19 with band3 was chymotrypsin sensitive. Antibodies against band3-P5, CD71, and MSP1P-19 reduced the binding activity of PvMSP1P-19 and Plasmodium cynomolgi MSP1P-19 to reticulocytes, while MSP1P-19 proteins inhibited Plasmodium falciparum invasion in vitro in a concentration-dependent manner. To sum up, identification and characterization of the reticulocyte receptor is important for understanding the binding of reticulocytes by MSP1P-19.

Characterization of merozoite-specific thrombospondin-related anonymous protein (MTRAP) in Plasmodium vivax and P. knowlesi parasites

Plasmodium vivax, the most widespread human malaria parasite, and P. knowlesi, an emerging Plasmodium that infects humans, are the phylogenetically closest malarial species that infect humans, which may induce cross-species reactivity across most co-endemic areas in Southeast Asia. The thrombospondin-related anonymous protein (TRAP) family is indispensable for motility and host cell invasion in the growth and development of Plasmodium parasites. The merozoite-specific TRAP (MTRAP), expressed in blood-stage merozoites, is supposed to be essential for human erythrocyte invasion. We aimed to characterize MTRAPs in blood-stage P. vivax and P. knowlesi parasites and ascertain their cross-species immunoreactivity. Recombinant P. vivax and P. knowlesi MTRAPs of full-length ectodomains were expressed in a mammalian expression system. The MTRAP-specific immunoglobulin G, obtained from immune animals, was used in an immunofluorescence assay for subcellular localization and invasion inhibitory activity in blood-stage parasites was determined. The cross-species humoral immune responses were analyzed in the sera of patients with P. vivax or P. knowlesi infections. The MTRAPs of P. vivax (PvMTRAP) and P. knowlesi (PkMTRAP) were localized on the rhoptry body of merozoites in blood-stage parasites. Both anti-PvMTRAP and anti-PkMTRAP antibodies inhibited erythrocyte invasion of blood-stage P. knowlesi parasites. The humoral immune response to PvMTRAP showed high immunogenicity, longevity, and cross-species immunoreactivity with P. knowlesi. MTRAPs are promising candidates for development of vaccines and therapeutics against vivax and knowlesi malaria.

Prevalence and characteristics of Plasmodium vivax Gametocytes in Duffy-positive and Duffy-negative populations across Ethiopia

Plasmodium parasites replicate asexually in the human host. The proportion of infections that carries gametocytes is a proxy for human-to-mosquito transmissibility. It is unclear what proportion of P. vivax infections in Duffy-negatives carries gametocytes. This study aims to determine the prevalence of P. vivax in Duffy-negatives across broad regions of Ethiopia and characterize parasite stages. Finger-prick blood samples were collected for microscopic and molecular screening of Plasmodium parasites and Duffy status of individuals. Molecular screening of plasmodium species and Duffy blood group genotyping was done using SYBR green and Taqman qPCR method. Among the total 447 samples, 414 (92.6%) were P. vivax confirmed and, 16 (3.9%) of them were from Duffy-negatives. Of these, 5/16 (31.3%) Duffy-negative P. vivax-infected samples were detected with gametocytes. Of the 398 Duffy-positive P. vivax-infected samples, 150 (37.7%) were detected with gametocytes, slightly higher than that in Duffy-negatives. This study highlights the presence of P. vivax gametocytes in Duffy-negative infections, suggestive of human-to-mosquito transmissibility. Although P. vivax infections in Duffy-negatives are commonly associated with low parasitemia, some of these infections were shown with relatively high parasitemia and may represent better erythrocyte invasion capability of P. vivax and hidden reservoirs that can contribute to transmission. A better understanding of P. vivax transmission biology and gametocyte function particularly in Duffy-negative populations would aid future treatment and management of vivax malaria in Africa.

Merozoite surface protein 1 paralog is involved in the human erythrocyte invasion of a zoonotic malaria, Plasmodium knowlesi

The zoonotic malaria parasite Plasmodium knowlesi is an important public health concern in Southeast Asia. Invasion of host erythrocytes is essential for parasite growth, and thus, understanding the repertoire of parasite proteins that enable this process is vital for identifying vaccine candidates and how some species are able to cause zoonotic infection. Merozoite surface protein 1 (MSP1) is found in all malaria parasite species and is perhaps the most well-studied as a potential vaccine candidate. While MSP1 is encoded by a single gene in P. falciparum, all other human infective species (P. vivax, P. knowlesi, P. ovale, and P. malariae) additionally encode a divergent paralogue known as MSP1P, and little is known about its role or potential functional redundancy with MSP1. We, therefore, studied the function of P. knowlesi merozoite surface protein 1 paralog (PkMSP1P), using both recombinant protein and CRISPR-Cas9 genome editing. The recombinant 19-kDa C-terminus of PkMSP1P (PkMSP1P-19) was shown to bind specifically to human reticulocytes. However, immunoblotting data suggested that PkMSP1P-19-induced antibodies can recognize PkMSP1-19 and vice versa, confounding our ability to separate the properties of these two proteins. Targeted disruption of the pkmsp1p gene profoundly impacts parasite growth, demonstrating for the first time that PkMSP1P is important in in vitro growth of P. knowlesi and likely plays a distinct role from PkMSP1. Importantly, the MSP1P KO also enabled functional characterization of the PkMSP1P-19 antibodies, revealing clear immune cross-reactivity between the two paralogues, highlighting the vital importance of genetic studies in contextualizing recombinant protein studies.

Plasmodium vivax vaccine: What is the best way to go?

Malaria is one of the most devastating human infectious diseases caused by Plasmodium spp. parasites. A search for an effective and safe vaccine is the main challenge for its eradication. Plasmodium vivax is the second most prevalent Plasmodium species and the most geographically distributed parasite and has been neglected for decades. This has a massive gap in knowledge and consequently in the development of vaccines. The most significant difficulties in obtaining a vaccine against P. vivax are the high genetic diversity and the extremely complex life cycle. Due to its complexity, studies have evaluated P. vivax antigens from different stages as potential targets for an effective vaccine. Therefore, the main vaccine candidates are grouped into preerythrocytic stage vaccines, blood-stage vaccines, and transmission-blocking vaccines. This review aims to support future investigations by presenting the main findings of vivax malaria vaccines to date. There are only a few P. vivax vaccines in clinical trials, and thus far, the best protective efficacy was a vaccine formulated with synthetic peptide from a circumsporozoite protein and Montanide ISA-51 as an adjuvant with 54.5% efficacy in a phase IIa study. In addition, the majority of P. vivax antigen candidates are polymorphic, induce strain-specific and heterogeneous immunity and provide only partial protection. Nevertheless, immunization with recombinant proteins and multiantigen vaccines have shown promising results and have emerged as excellent strategies. However, more studies are necessary to assess the ideal vaccine combination and test it in clinical trials. Developing a safe and effective vaccine against vivax malaria is essential for controlling and eliminating the disease. Therefore, it is necessary to determine what is already known to propose and identify new candidates.

Host-parasite interactions during Plasmodium infection: Implications for immunotherapies

Malaria is a global infectious disease that remains a leading cause of morbidity and mortality in the developing world. Multiple environmental and host and parasite factors govern the clinical outcomes of malaria. The host immune response against the Plasmodium parasite is heterogenous and stage-specific both in the human host and mosquito vector. The Plasmodium parasite virulence is predominantly associated with its ability to evade the host's immune response. Despite the availability of drug-based therapies, Plasmodium parasites can acquire drug resistance due to high antigenic variations and allelic polymorphisms. The lack of licensed vaccines against Plasmodium infection necessitates the development of effective, safe and successful therapeutics. To design an effective vaccine, it is important to study the immune evasion strategies and stage-specific Plasmodium proteins, which are targets of the host immune response. This review provides an overview of the host immune defense mechanisms and parasite immune evasion strategies during Plasmodium infection. Furthermore, we also summarize and discuss the current progress in various anti-malarial vaccine approaches, along with antibody-based therapy involving monoclonal antibodies, and research advancements in host-directed therapy, which can together open new avenues for developing novel immunotherapies against malaria infection and transmission.

Monoclonal Antibodies for Protozoan Infections: A Future Reality or a Utopic Idea?

Following the SARS-CoV-2 pandemic, several clinical trials have been approved for the investigation of the possible use of mAbs, supporting the potential of this technology as a therapeutic approach for infectious diseases. The first monoclonal antibody (mAb), Muromonab CD3, was introduced for the prevention of kidney transplant rejection more than 30 years ago; since then more than 100 mAbs have been approved for therapeutic purposes. Nonetheless, only four mAbs are currently employed for infectious diseases: Palivizumab, for the prevention of respiratory syncytial virus (RSV) infections, Raxibacumab and Obiltoxaximab, for the prophylaxis and treatment against anthrax toxin and Bezlotoxumab, for the prevention of Clostridium difficile recurrence. Protozoan infections are often neglected diseases for which effective and safe chemotherapies are generally missing. In this context, drug resistance and drug toxicity are two crucial problems. The recent advances in bioinformatics, parasite genomics, and biochemistry methodologies are contributing to better understand parasite biology, which is essential to guide the development of new therapies. In this review, we present the efforts that are being made in the evaluation of mAbs for the prevention or treatment of leishmaniasis, Chagas disease, malaria, and toxoplasmosis. Particular emphasis will be placed on the potential strengths and weaknesses of biological treatments in the control of these protozoan diseases that are still affecting hundreds of thousands of people worldwide.

Plasmodium cynomolgi Berok Growth Inhibition Assay by Thiol-reactive Probe Based Flow Cytometric Measurement

The relapsing malaria species, Plasmodium vivax, is the most widely distributed and difficult-to-treat cause of human malaria. The merozoites of P. vivax preferentially invade ephemeral human CD71⁺ reticulocytes (nascent reticulocytes), thereby limiting the development of a robust continuous culture in vitro. Fortunately, P. vivax's sister species, P. cynomolgi Berok, can be cultured continuously, providing the ability to screen novel therapeutics drug and vaccine candidates in a reliable and high-throughput manner. Based on well-established growth inhibition activity (GIA) assays against P. falciparum and P. knowlesi, this protocol adopts the current flow cytometry assay methodology and investigates P. vivax inhibitory antibodies using the P. cynomolgi Berok invasion model based on the thiol-reactivity and DNA abundance of viable parasites in macaque erythrocytes. Established GIA assays screen antibodies at either a single concentration or high/low dose concentrations to provide quick insights for prioritizing potential antibodies capable of specifically interrupting parasite ligand and host receptor binding with minimal concentrations. Hence, this protocol expands on the existing GIA assay by using serially diluted antibodies and generating a dose-response curve to better quantify the inhibitory efficacy amongst selected vaccine candidates.

Identification and Characterization of Eimeria tenella Microneme Protein (EtMIC8)

Microneme proteins (MICs) of Eimeria tenella play key roles in motility, migration, attachment, and invasion processes. More than 20 apicomplexan parasite's MICs have been identified, with nine Eimeria MICs being reported. In this study, a novel E. tenella MIC was identified, and its gene structural features, developmental expression levels, localization, role in adhesion and invasion, and immunogenicity were studied. The results showed that the open reading frame was 1,650 bp, encoding 550 amino acids. It contains a signal sequence, a transmembrane region, four low-complexity boxes, and five epidermal growth factor-like domains (EGF). Subcellular localization revealed its distribution on the membrane surface of the parasite. These characteristics are consistent with the common features of MICs and are named EtMIC8. Anti-EtMIC8 antibodies recognized a specific binding of about 100 kDa in E. tenella, which was twice as large as the prokaryotic expression (about 50 kDa), suggesting that MIC8 may exist naturally as a dimer. EtMIC8 was expressed at higher levels in sporozoites (3.08-fold) and merozoites (2.1-fold) than in sporulated oocysts. The attachment assays using a yeast surface display of MIC8 and its different domains showed that the adherence rates of EtMIC8 to host cells were significantly higher than those of the control (3.17-fold), which was the full contribution of EGF, but neither was alone. Anti-EtMIC8 antibodies significantly reduced the invasion rate of sporozoites into host cells compared to those of the control (P < 0.01). Recombinant EtMIC8-EGF peptides could provide moderate protective efficacy (anticoccidial index [ACI]: 169.7), induce humoral responses, and upregulate CD3+CD8+ lymphocyte cells.

Antibodies against EGF-like domains in Ixodes scapularis BM86 orthologs impact tick feeding and survival of Borrelia burgdorferi

Ixodes scapularis ticks transmit multiple pathogens, including Borrelia burgdorferi sensu stricto, and encode many proteins harboring epidermal growth factor (EGF)-like domains. We show that I. scapularis produces multiple orthologs for Bm86, a widely studied tick gut protein considered as a target of an anti-tick vaccine, herein termed as Is86. We show that Is86 antigens feature at least three identifiable regions harboring EGF-like domains (termed as EGF-1, EGF-2, and EGF-3) and are differentially upregulated during B. burgdorferi infection. Although the RNA interference-mediated knockdown of Is86 genes did not show any influences on tick engorgement or B. burgdorferi sensu stricto persistence, the immunization of murine hosts with specific recombinant EGF antigens marginally reduced spirochete loads in the skin, in addition to affecting tick blood meal engorgement and molting. However, given the borderline impact of EGF immunization on tick engorgement and pathogen survival in the vector, it is unlikely that these antigens, at least in their current forms, could be developed as potential vaccines. Further investigations of the biological significance of Is86 (and other tick antigens) would enrich our knowledge of the intricate biology of ticks, including their interactions with resident pathogens, and contribute to the development of anti-tick measures to combat tick-borne illnesses.

The biology of unconventional invasion of Duffy-negative reticulocytes by Plasmodium vivax and its implication in malaria epidemiology and public health

Plasmodium vivax has been largely neglected over the past century, despite a widespread recognition of its burden across region where it is endemic. The parasite invades reticulocytes, employing the interaction between Plasmodium vivax Duffy binding protein (PvDBP) and human Duffy antigen receptor for chemokines (DARC). However, P. vivax has now been observed in Duffy-negative individuals, presenting a potentially serious public health problem as the majority of African populations are Duffy-negative. Invasion of Duffy-negative reticulocytes is suggested to be through duplication of the PvDBP and a novel protein encoded by P. vivax erythrocyte binding protein (EBP) genes. The emergence and spread of specific P. vivax strains with ability to invade Duffy-negative reticulocytes has, therefore, drawn substantial attention and further complicated the epidemiology and public health implication of vivax malaria. Given the right environment and vectorial capacity for transmission coupled with the parasite’s ability to invade Duffy-negative individuals, P. vivax could increase its epidemiological significance in Africa. In this review, authors present accruing knowledge on the paradigm shift in P. vivax invasion of Duffy-negative reticulocytes against the established mechanism of invading only Duffy-positive individuals and offer a perspective on the epidemiological diagnostic and public health implication in Africa.

The enigmatic mechanisms by which Plasmodium vivax infects Duffy-negative individuals

The absence of the Duffy protein at the surface of erythrocytes was considered for decades to confer full protection against Plasmodium vivax as this blood group is the receptor for the key parasite ligand P. vivax Duffy binding protein (PvDBP). However, it is now clear that the parasite is able to break through this protection and induce clinical malaria in Duffy-negative people, although the underlying mechanisms are still not understood. Here, we briefly review the evidence of Duffy-negative infections by P. vivax and summarize the current hypothesis at the basis of this invasion process. We discuss those in the perspective of malaria-elimination challenges, notably in African countries.

The immunology of Plasmodium vivax malaria

Plasmodium vivax infection, the predominant cause of malaria in Asia and Latin America, affects ~14 million individuals annually, with considerable adverse effects on wellbeing and socioeconomic development. A clinical hallmark of Plasmodium infection, the paroxysm, is driven by pyrogenic cytokines produced during the immune response. Here, we review studies on the role of specific immune cell types, cognate innate immune receptors, and inflammatory cytokines on parasite control and disease symptoms. This review also summarizes studies on recurrent infections in individuals living in endemic regions as well as asymptomatic infections, a serious barrier to eliminating this disease. We propose potential mechanisms behind these repeated and subclinical infections, such as poor induction of immunological memory cells and inefficient T effector cells. We address the role of antibody-mediated resistance to P. vivax infection and discuss current progress in vaccine development. Finally, we review immunoregulatory mechanisms, such as inhibitory receptors, T regulatory cells, and the anti-inflammatory cytokine, IL-10, that antagonizes both innate and acquired immune responses, interfering with the development of protective immunity and parasite clearance. These studies provide new insights for the clinical management of symptomatic as well as asymptomatic individuals and the development of an efficacious vaccine for vivax malaria.