Red cell receptors as access points for malaria infection

Targeting PfProhibitin 2-Hu-Hsp70A1A complex as a unique approach towards malaria vaccine development

Malaria parasite invasion to host erythrocytes is mediated by multiple interactions between merozoite ligands and erythrocyte receptors that contribute toward the development of disease pathology. Here, we report a novel antigen Plasmodium prohibitin "PfPHB2" and identify its cognate partner "Hsp70A1A" in host erythrocyte that plays a crucial role in mediating host-parasite interaction during merozoite invasion. Using small interfering RNA (siRNA)- and glucosamine-6-phosphate riboswitch (glmS) ribozyme-mediated approach, we show that loss of Hsp70A1A in red blood cells (RBCs) or PfPHB2 in infected red blood cells (iRBCs), respectively, inhibit PfPHB2-Hsp70A1A interaction leading to invasion inhibition. Antibodies targeting PfPHB2 and monoclonal antibody therapeutics against Hsp70A1A efficiently block parasite invasion. Recombinant PfPHB2 binds to RBCs which is inhibited by anti-PfPHB2 antibody and monoclonal antibody against Hsp70A1A. The validation of PfPHB2 to serve as antigen is further supported by detection of anti-PfPHB2 antibody in patient sera. Overall, this study proposes PfPHB2 as vaccine candidate and highlights the use of monoclonal antibody therapeutics for future malaria treatment.

Structure-guided design of VAR2CSA-based immunogens and a cocktail strategy for a placental malaria vaccine

Placental accumulation of Plasmodium falciparum infected erythrocytes results in maternal anemia, low birth weight, and pregnancy loss. The parasite protein VAR2CSA facilitates the accumulation of infected erythrocytes in the placenta through interaction with the host receptor chondroitin sulfate A (CSA). Antibodies that prevent the VAR2CSA-CSA interaction correlate with protection from placental malaria, and VAR2CSA is a high-priority placental malaria vaccine antigen. Here, structure-guided design leveraging the full-length structures of VAR2CSA produced a stable immunogen that retains the critical conserved functional elements of VAR2CSA. The design expressed with a six-fold greater yield than the full-length protein and elicited antibodies that prevent adhesion of infected erythrocytes to CSA. The reduced size and adaptability of the designed immunogen enable efficient production of multiple variants of VAR2CSA for use in a cocktail vaccination strategy to increase the breadth of protection. These designs form strong foundations for the development of potent broadly protective placental malaria vaccines.

P. falciparum Invasion and Erythrocyte Aging

Plasmodium parasites need to find red blood cells (RBCs) that, on the one hand, expose receptors for the pathogen ligands and, on the other hand, maintain the right geometry to facilitate merozoite attachment and entry into the red blood cell. Both characteristics change with the maturation of erythrocytes. Some Plasmodia prefer younger vs. older erythrocytes. How does the life evolution of the RBC affect the invasion of the parasite? What happens when the RBC ages? In this review, we present what is known up until now.

Proteome Analysis for Inflammation Related to Acute and Convalescent Infection

Infectious diseases are a significant burden in global healthcare. Pathogens engage with different host defense mechanisms. However, it is currently unknown if there are disease-specific immune signatures and/or if different pathogens elicit common immune-associated molecular entities to common therapeutic interventions. We studied patients enrolled through the Human Immunology Project Consortium (HIPC), which focuses on immune responses to various infections. Blood samples were collected and analyzed from patients during infection and follow-up time points at the convalescent stage. The study included samples from patients with Lyme disease (LD), tuberculosis (TB), malaria (MLA), dengue virus (DENV), and West Nile virus (WNV), as well as kidney transplant patients with cytomegalovirus (CMV) and polyomavirus (BKV) infections. Using an antibody-based assay, we quantified ~ 350 cell surface markers, cytokines, and chemokines involved in inflammation and immunity. Unique protein signatures were identified specific to the acute phase of infection irrespective of the pathogen type, with significant changes during convalescence. In addition, tumor necrosis factor receptor superfamily member 6 (TNR6), C-C Motif Chemokine Receptor 7 (CCR7), and C-C motif chemokine ligand-1 (CCL1) were increased in the acute and convalescent phases across all viral, bacterial, and protozoan compared to blood from healthy donors. Furthermore, despite the differences between pathogens, proteins were enriched in common biological pathways such as cell surface receptor signaling pathway and response to external stimulus. In conclusion, we demonstrated that irrespective of the pathogen type, there are common immunoregulatory and proinflammatory signals.

Plasmodium falciparum exploits CD44 as a coreceptor for erythrocyte invasion

The malaria parasite Plasmodium falciparum invades and replicates asexually within human erythrocytes. CD44 expressed on erythrocytes was previously identified as an important host factor for P falciparum infection through a forward genetic screen, but little is known about its regulation or function in these cells, nor how it may be used by the parasite. We found that CD44 can be efficiently deleted from primary human hematopoietic stem cells using CRISPR/Cas9 genome editing, and that the efficiency of ex vivo erythropoiesis to enucleated cultured red blood cells (cRBCs) is not affected by lack of CD44. However, the rate of P falciparum invasion was reduced in CD44-null cRBCs relative to isogenic wild-type control cells, validating CD44 as an important host factor for this parasite. We identified 2 P falciparum invasion ligands as binding partners for CD44, erythrocyte binding antigen 175 (EBA-175) and EBA-140 and demonstrated that their ability to bind to human erythrocytes relies primarily on their canonical receptors, glycophorin A and glycophorin C, respectively. We further show that EBA-175 induces phosphorylation of erythrocyte cytoskeletal proteins in a CD44-dependent manner. Our findings support a model in which P falciparum exploits CD44 as a coreceptor during invasion of human erythrocytes, stimulating CD44-dependent phosphorylation of host cytoskeletal proteins that alter host cell deformability and facilitate parasite entry.

Sialylated Glycan Bindings from SARS-CoV-2 Spike Protein to Blood and Endothelial Cells Govern the Severe Morbidities of COVID-19

Consistent with well-established biochemical properties of coronaviruses, sialylated glycan attachments between SARS-CoV-2 spike protein (SP) and host cells are key to the virus's pathology. SARS-CoV-2 SP attaches to and aggregates red blood cells (RBCs), as shown in many pre-clinical and clinical studies, causing pulmonary and extrapulmonary microthrombi and hypoxia in severe COVID-19 patients. SARS-CoV-2 SP attachments to the heavily sialylated surfaces of platelets (which, like RBCs, have no ACE2) and endothelial cells (having minimal ACE2) compound this vascular damage. Notably, experimentally induced RBC aggregation in vivo causes the same key morbidities as for severe COVID-19, including microvascular occlusion, blood clots, hypoxia and myocarditis. Key risk factors for COVID-19 morbidity, including older age, diabetes and obesity, are all characterized by markedly increased propensity to RBC clumping. For mammalian species, the degree of clinical susceptibility to COVID-19 correlates to RBC aggregability with p = 0.033. Notably, of the five human betacoronaviruses, the two common cold strains express an enzyme that releases glycan attachments, while the deadly SARS, SARS-CoV-2 and MERS do not, although viral loads for COVID-19 and the two common cold infections are similar. These biochemical insights also explain the previously puzzling clinical efficacy of certain generics against COVID-19 and may support the development of future therapeutic strategies for COVID-19 and long COVID patients.

Designing an effective malaria vaccine targeting Plasmodium vivax Duffy-binding protein

Malaria caused by the Plasmodium vivax parasite is a major global health burden. Immunity against blood-stage infection reduces parasitemia and disease severity. Duffy-binding protein (DBP) is the primary parasite protein responsible for the invasion of red blood cells and it is a leading subunit vaccine candidate. An effective vaccine, however, is still lacking despite decades of interest in DBP as a vaccine candidate. This review discusses the reasons for targeting DBP, the challenges associated with developing a vaccine, and modern structural vaccinology methods that could be used to create an effective DBP vaccine. Next-generation DBP vaccines have the potential to elicit a broadly protective immune response and provide durable and potent protection from P. vivax malaria.

Nanocarrier based active targeting strategies against erythrocytic stage of malaria

The Global Technical Strategy for Malaria 2016-2030 aims to achieve a 90% reduction in malaria cases, and strategic planning and execution are crucial for accomplishing this target. This review aims to understand the complex interaction between erythrocytic receptors and parasites and to use this knowledge to actively target the erythrocytic stage of malaria. The review provides insight into the malaria life cycle, which involves various receptors such as glycophorin A, B, C, and D (GPA/B/C/D), complement receptor 1, basigin, semaphorin 7a, Band 3/ GPA, Kx, and heparan sulfate proteoglycan for parasite cellular binding and ingress in the erythrocytic and exo-erythrocytic stages. Synthetic peptides mimicking P. falciparum receptor binding ligands, human serum albumin, chondroitin sulfate, synthetic polymers, and lipids have been utilized as ligands and decorated onto nanocarriers for specific targeting to parasite-infected erythrocytes. The need of the hour for treatment and prophylaxis against malaria is a broadened horizon that includes multiple targeting strategies against the entry, proliferation, and transmission stages of the parasite. Platform technologies with established pre-clinical safety and efficacy should be translated into clinical evaluation and formulation scale-up. Future development should be directed towards nanovaccines as proactive tools against malaria infection.

A human antibody epitope map of the malaria vaccine antigen Pfs25

Pfs25 is a leading antigen for a malaria transmission-blocking vaccine and shows moderate transmission-blocking activity and induction of rapidly decreasing antibody titers in clinical trials. A comprehensive definition of all transmission-reducing epitopes of Pfs25 will inform structure-guided design to enhance Pfs25-based vaccines, leading to potent transmission-blocking activity. Here, we compiled a detailed human antibody epitope map comprising epitope binning data and structures of multiple human monoclonal antibodies, including three new crystal structures of Pfs25 in complex with transmission-reducing antibodies from Malian volunteers immunized with Pfs25 conjugated to EPA and adjuvanted with AS01. These structures revealed additional epitopes in Pfs25 capable of reducing transmission and expanded this characterization to malaria-exposed humans. This work informs immunogen design to focus the antibody response to transmission-reducing epitopes of Pfs25, enabling development of more potent transmission-blocking vaccines for malaria.

Genetic variants of TLR4, including the novel variant, rs5030719, and related genes are associated with susceptibility to clinical malaria in African children

BACKGROUND

Malaria is a deadly disease caused by Plasmodium spp. Several blood phenotypes have been associated with malarial resistance, which suggests a genetic component to immune protection.

METHODS

One hundred and eighty-seven single nucleotide polymorphisms (SNPs) in 37 candidate genes were genotyped and investigated for associations with clinical malaria in a longitudinal cohort of 349 infants from Manhiça, Mozambique, in a randomized controlled clinical trial (RCT) (AgeMal, NCT00231452). Malaria candidate genes were selected according to involvement in known malarial haemoglobinopathies, immune, and pathogenesis pathways.

RESULTS

Statistically significant evidence was found for the association of TLR4 and related genes with the incidence of clinical malaria (p = 0.0005). These additional genes include ABO, CAT, CD14, CD36, CR1, G6PD, GCLM, HP, IFNG, IFNGR1, IL13, IL1A, IL1B, IL4R, IL4, IL6, IL13, MBL, MNSOD, and TLR2. Of specific interest, the previously identified TLR4 SNP rs4986790 and the novel finding of TRL4 SNP rs5030719 were associated with primary cases of clinical malaria.

CONCLUSIONS

These findings highlight a potential central role of TLR4 in clinical malarial pathogenesis. This supports the current literature and suggests that further research into the role of TLR4, as well as associated genes, in clinical malaria may provide insight into treatment and drug development.

Association between ovalocytosis and Plasmodium infection: a systematic review and meta-analysis

Reports of an association between ovalocytosis and protection against Plasmodium infection are inconsistent. Therefore, we aimed to synthesise the overall evidence of the association between ovalocytosis and malaria infection using a meta-analysis approach. The systematic review protocol was registered with PROSPERO (CRD42023393778). A systematic literature search of the MEDLINE, Embase, Scopus, PubMed, Ovid, and ProQuest databases, from inception to 30 December 2022, was performed to retrieve studies documenting the association between ovalocytosis and Plasmodium infection. The quality of the included studies was assessed using the Newcastle-Ottawa Scale. Data synthesis included a narrative synthesis and a meta-analysis to calculate the pooled effect estimate (log odds ratios [ORs]) and 95% confidence intervals (CIs) using the random-effects model. Our database search retrieved 905 articles, 16 of which were included for data synthesis. Qualitative synthesis revealed that over half of the studies showed no association between ovalocytosis and malaria infections or severity. Furthermore, our meta-analysis demonstrated no association between ovalocytosis and Plasmodium infection (P = 0.81, log OR = 0.06, 95% CI - 0.44 to 0.19, I²: 86.20%; 11 studies). In conclusion, the meta-analysis results demonstrated no association between ovalocytosis and Plasmodium infection. Hence, the role of ovalocytosis in relation to protection against Plasmodium infection or disease severity should be further investigated in larger prospective studies.

Current advances in nanodrug delivery systems for malaria prevention and treatment

Malaria is a life-threatening, blood-borne disease with over two hundred million cases throughout the world and is more prevalent in Sub-Saharan Africa than anywhere else in the world. Over the years, several treatment agents have been developed for malaria; however, most of these active pharmaceutical ingredients exhibit poor aqueous solubility and low bioavailability and may result in drug-resistant parasites, thus increasing malaria cases and eventually, deaths. Factors such as these in therapeutics have led to a better appreciation of nanomaterials. The ability of nanomaterials to function as drug carriers with a high loading capacity and targeted drug delivery, good biocompatibility, and low toxicity renders them an appealing alternative to conventional therapy. Nanomaterials such as dendrimers and liposomes have been demonstrated to be capable of enhancing the efficacy of antimalarial drugs. This review discusses the recent development of nanomaterials and their benefits in drug delivery for the potential treatment of malaria.

Plasmodium falciparum exploits CD44 as a co-receptor for erythrocyte invasion

The malaria parasite Plasmodium falciparum invades and replicates asexually within human erythrocytes. CD44 expressed on erythrocytes was previously identified as an important host factor for P. falciparum infection through a forward genetic screen, but little is known about its regulation or function in these cells, nor how it may be utilized by the parasite. We found that CD44 can be efficiently deleted from primary human hematopoietic stem cells using CRISPR/Cas9 genome editing, and that the efficiency of ex-vivo erythropoiesis to enucleated cultured red blood cells (cRBCs) is not impacted by lack of CD44. However, the rate of P. falciparum invasion was substantially reduced in CD44-null cRBCs relative to isogenic wild-type (WT) control cells, validating CD44 as an important host factor for this parasite. We identified two P. falciparum invasion ligands as binding partners for CD44, Erythrocyte Binding Antigen-175 (EBA-175) and EBA-140, and demonstrated that their ability to bind to human erythrocytes relies primarily on their canonical receptors-glycophorin A and glycophorin C, respectively. We further show that EBA-175 induces phosphorylation of erythrocyte cytoskeletal proteins in a CD44-dependent manner. Our findings support a model where P. falciparum exploits CD44 as a co-receptor during invasion of human erythrocytes, stimulating CD44-dependent phosphorylation of host cytoskeletal proteins that alter host cell deformability and facilitate parasite entry.

Inhibition of a malaria host-pathogen interaction by a computationally designed inhibitor

Malaria is a substantial global health burden with 229 million cases in 2019 and 450,000 deaths annually. Plasmodium vivax is the most widespread malaria-causing parasite putting 2.5 billion people at risk of infection. P. vivax has a dormant liver stage and therefore can exist for long periods undetected. Its blood-stage can cause severe reactions and hospitalization. Few treatment and detection options are available for this pathogen. A unique characteristic of P. vivax is that it depends on the Duffy antigen/receptor for chemokines (DARC) on the surface of host red blood cells for invasion. P. vivax employs the Duffy binding protein (DBP) to bind to DARC. We first de novo designed a three helical bundle scaffolding database which was screened via protease digestions for stability. Protease-resistant scaffolds highlighted thresholds for stability, which we utilized for selecting DARC mimetics that we subsequentially designed through grafting and redesign of these scaffolds. The optimized design small helical protein disrupts the DBP:DARC interaction. The inhibitor blocks the receptor binding site on DBP and thus forms a strong foundation for a therapeutic that will inhibit reticulocyte infection and prevent the pathogenesis of P. vivax malaria.

Generation of red blood cells from stem cells: Achievements, opportunities and perspectives for malaria research

Parasites of the genus Plasmodium that cause malaria survive within humans by invasion of, and proliferation within, the most abundant cell type in the body, the red blood cell. As obligate, intracellular parasites, interactions between parasite and host red blood cell components are crucial to multiple aspects of the blood stage malaria parasite lifecycle. The requirement for, and involvement of, an array of red blood cell proteins in parasite invasion and intracellular development is well established. Nevertheless, detailed mechanistic understanding of host cell protein contributions to these processes are hampered by the genetic intractability of the anucleate red blood cell. The advent of stem cell technology and more specifically development of methods that recapitulate in vitro the process of red blood cell development known as erythropoiesis has enabled the generation of erythroid cell stages previously inaccessible in large numbers for malaria studies. What is more, the capacity for genetic manipulation of nucleated erythroid precursors that can be differentiated to generate modified red blood cells has opened new horizons for malaria research. This review summarises current methodologies that harness in vitro erythroid differentiation of stem cells for generation of cells that are susceptible to malaria parasite invasion; discusses existing and emerging approaches to generate novel red blood cell phenotypes and explores the exciting potential of in vitro derived red blood cells for improved understanding the broad role of host red blood cell proteins in malaria pathogenesis.

CD55 Facilitates Immune Evasion by Borrelia crocidurae, an Agent of Relapsing Fever

Relapsing fever, caused by diverse Borrelia spirochetes, is prevalent in many parts of the world and causes significant morbidity and mortality. To investigate the pathoetiology of relapsing fever, we performed a high-throughput screen of Borrelia-binding host factors using a library of human extracellular and secretory proteins and identified CD55 as a novel host binding partner of Borrelia crocidurae and Borrelia persica, two agents of relapsing fever in Africa and Eurasia. CD55 is present on the surface of erythrocytes, carries the Cromer blood group antigens, and protects cells from complement-mediated lysis. Using flow cytometry, we confirmed that both human and murine CD55 bound to B. crocidurae and B. persica. Given the expression of CD55 on erythrocytes, we investigated the role of CD55 in pathological B. crocidurae-induced erythrocyte aggregation (rosettes), which enables spirochete immune evasion. We showed that rosette formation was partially dependent on host cell CD55 expression. Pharmacologically, soluble recombinant CD55 inhibited erythrocyte rosette formation. Finally, CD55-deficient mice infected with B. crocidurae had a lower pathogen load and elevated proinflammatory cytokine and complement factor C5a levels. In summary, our results indicate that CD55 is a host factor that is manipulated by the causative agents of relapsing fever for immune evasion. IMPORTANCE Borrelia species are causative agents of Lyme disease and relapsing fever infections in humans. B. crocidurae causes one of the most prevalent relapsing fever infections in parts of West Africa. In the endemic regions, B. crocidurae is present in ~17% of the ticks and ~11% of the rodents that serve as reservoirs. In Senegal, ~7% of patients with acute febrile illness were found to be infected with B. crocidurae. There is little information on host-pathogen interactions and how B. crocidurae manipulates host immunity. In this study, we used a high-throughput screen to identify host proteins that interact with relapsing fever-causing Borrelia species. We identified CD55 as one of the host proteins that bind to B. crocidurae and B. persica, the two causes of relapsing fever in Africa and Eurasia. We show that the interaction of B. crocidurae with CD55, present on the surface of erythrocytes, is key to immune evasion and successful infection in vivo. Our study further shows the role of CD55 in complement regulation, regulation of inflammatory cytokine levels, and innate immunity during relapsing fever infection. Overall, this study sheds light on host-pathogen interactions during relapsing fever infection in vivo.

Industrially Compatible Transfusable iPSC-Derived RBCs: Progress, Challenges and Prospective Solutions

Amidst the global shortfalls in blood supply, storage limitations of donor blood and the availability of potential blood substitutes for transfusion applications, society has pivoted towards in vitro generation of red blood cells (RBCs) as a means to solve these issues. Many conventional research studies over the past few decades have found success in differentiating hematopoietic stem and progenitor cells (HSPCs) from cord blood, adult bone marrow and peripheral blood sources. More recently, techniques that involve immortalization of erythroblast sources have also gained traction in tackling this problem. However, the RBCs generated from human induced pluripotent stem cells (hiPSCs) still remain as the most favorable solution due to many of its added advantages. In this review, we focus on the breakthroughs for high-density cultures of hiPSC-derived RBCs, and highlight the major challenges and prospective solutions throughout the whole process of erythropoiesis for hiPSC-derived RBCs. Furthermore, we elaborate on the recent advances and techniques used to achieve cost-effective, high-density cultures of GMP-compliant RBCs, and on their relevant novel applications after downstream processing and purification.

The evolution of barriers to exploitation: Sometimes the Red Queen can take a break

We propose a general barrier theory as an evolutionary framework for understanding coevolutionary effects of conflicts of interest in natural and human systems. It is generalized from the barrier theory of cancer, which describes how cancer develops through the evasion of mechanisms, that block unregulated cellular reproduction and survival. Barriers are naturally evolved or artificially implemented mechanisms for blocking exploitation; restraints are mechanisms that impede but do not block exploitation. When conflicts of interest arise, selection will favor exploiters that are capable of overcoming barriers and restraints. When barriers are in place, they halt, at least temporarily, coevolutionary arms races (the Red Queen can stop running). Barriers occur in a broad spectrum of interactions characterized by conflicts of interest: barriers to cellular survival (apoptosis) and reproduction (cell cycle arrest) may block a virus from replicating its genome through reproduction of its host cell. Vaccines may completely protect against targeted pathogens. A plant may escape herbivory by evolving defensive chemicals that block herbivory. Obligate mutualisms may evolve when barriers to horizontal transmission favor symbionts that increasingly lose mechanisms that contribute to horizontal transmission. Here, we show how the barrier theory applies across a spectrum of natural and social systems.

Zoonotic Transmissions and Host Switches of Malaria Parasites

Malaria is a deadly disease that affects the health of hundreds of millions of people annually. There are five Plasmodium parasite species that can naturally infect humans, including Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, Plasmodium ovale and Plasmodium knowlesi. Some of the parasites can also infect various non-human primates. Parasites mainly infecting monkeys such as Plasmodium cynomolgi (in fact P. knowlesi was considered as a parasite of monkeys for years) can also be transmitted to human hosts. Recently, many new Plasmodium species were discovered in African apes, and it is possible that some of the parasites can be transmitted to humans in the future. Here, we searched PubMed and the internet via Google and selected articles concerning zoonotic transmission and evolution of selected malaria parasite species. We reviewed the current advances in the relevant topics emphasizing on transmissions of malaria parasites between humans and non-human primates. We also briefly discuss the transmissions of some avian malaria parasites between wild birds and domestic fowls. Zoonotic malaria transmissions are widespread, which poses a threat to public health. More studies on parasite species identification in non-human primates, transmission, and evolution are needed to reduce or prevent transmission of malaria parasites from non-human primates to humans.

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.

Transcriptome analysis of Anopheles dirus and Plasmodium vivax at ookinete and oocyst stages

Malaria is transmitted by Plasmodium parasites through the bite of female Anopheles mosquitoes. One of the most important mosquito vectors in the Greater Mekong Subregion is Anopheles dirus. This study reports RNA sequencing (RNA-Seq) transcriptome analysis of An. dirus at 18 hours and 7 days after a P. vivax-infected blood meal, which represent infection at the ookinete and oocyst parasite developmental stages, respectively. Following infection, 582 An. dirus transcripts were modulated. The 2,408 P. vivax transcripts could be classified into ookinete-specific, two-stage, and oocyst-specific groups. Results were validated by quantitative reverse transcription polymerase chain reaction. Gene ontology analysis of the vector and parasite revealed several biological pathways for both, providing a better understanding of Anopheles-Plasmodium interactions at the ookinete and oocyst stages.

Plasmodium falciparum Clag9-Associated PfRhopH Complex Is Involved in Merozoite Binding to Human Erythrocytes

Cytoadherence-linked asexual gene 9 (Clag9), a conserved Plasmodium protein expressed during the asexual blood stages, is involved in the cytoadherence of infected red blood cells (RBCs) to the endothelial lining of blood vessels. Here, we show that Plasmodium falciparum Clag9 (PfClag9) is a component of the PfClag9-RhopH complex that is involved in merozoite binding to human erythrocytes. To characterize PfClag9, we expressed four fragments of PfClag9, encompassing the entire protein. Immunostaining analysis using anti-PfClag9 antibodies showed expression and localization of PfClag9 at the apical end of the merozoites. Mass spectrometric analysis of merozoite extracts after immunoprecipitation using anti-PfClag9 antibody identified P. falciparum rhoptry-associated protein 1 (PfRAP1), PfRAP2, PfRAP3, PfRhopH2, and PfRhopH3 as associated proteins. The identified rhoptry proteins were expressed, and their association with PfClag9 domains was assessed by using protein-protein interaction tools. We further showed that PfClag9 binds human RBCs by interacting with the glycophorin A-band 3 receptor-coreceptor complex. In agreement with its cellular localization, PfClag9 was strongly recognized by antibodies generated during natural infection. Mice immunized with the C-terminal domain of PfClag9 were partially protected against a subsequent challenge infection with Plasmodium berghei, further supporting a biological role of PfClag9 during natural infection. Taken together, these results provide direct evidence for the existence of a PfRhopH-Clag9 complex on the Plasmodium merozoite surface that binds to human RBCs.

Infectivity and Screening of Anti-piperaquine Genes in Mice Infected with Piperaquine-Sensitive and Piperaquine-Resistant Plasmodium berghei

BACKGROUND

Piperaquine (PQ) is one of the major components of artemisinin-based combination therapy for malaria. However, the mechanism of PQ resistance has remained unclear.

METHODS

In this study, we infected mice with PQ-resistant Plasmodium berghei ANKA strain line (PbPQR) or PQ-sensitive P. berghei ANKA strain line (PbPQS) and their survival rates, parasitemia, and spleen sizes were compared. In addition, we constructed genomic DNA subtractive library of spleens from the infected mice, and screened the potential PQ-resistant related genes from genomic DNA of PbPQR line using the representational difference analysis (RDA) method. Clones of the subtractive library were screened by PCR, and related genes were sequenced and analyzed using BLAST software of NCBI.

RESULTS

Compared to PbPQS-infected mice, PbPQR-infected mice survived significantly longer, and had significantly lowered parasitemia rate and significantly increased splenomegaly. Among the total of 502 clones picked, 494 were sequenced and 96 unique PCR fragments were obtained; in which 24 DNA fragments were homologous to chromosomes related to immune function of mice. ORF Finder blasting showed that at the protein level, 26 encoded proteins were homologous to 18 hypothetical PbANKA proteins and 13 encoded proteins were homologous to "ferlin-like protein" family of PbANKA. In addition, there were more immune-related DNA molecules, ubiquitous PbANKA homology at the ORF fragment level, and enriched ferlin-like protein families identified from PbPQR-infected mice than those from PbPQS-infected mice.

CONCLUSION

These findings suggest that PbPQR may induce stronger protective immune response than that of PbPQS in infected mice.

Genetic manipulation of cell line derived reticulocytes enables dissection of host malaria invasion requirements

Investigating the role that host erythrocyte proteins play in malaria infection is hampered by the genetic intractability of this anucleate cell. Here we report that reticulocytes derived through in vitro differentiation of an enucleation-competent immortalized erythroblast cell line (BEL-A) support both successful invasion and intracellular development of the malaria parasite Plasmodium falciparum. Using CRISPR-mediated gene knockout and subsequent complementation, we validate an essential role for the erythrocyte receptor basigin in P. falciparum invasion and demonstrate rescue of invasive susceptibility by receptor re-expression. Successful invasion of reticulocytes complemented with a truncated mutant excludes a functional role for the basigin cytoplasmic domain during invasion. Contrastingly, knockout of cyclophilin B, reported to participate in invasion and interact with basigin, did not impact invasive susceptibility of reticulocytes. These data establish the use of reticulocytes derived from immortalized erythroblasts as a powerful model system to explore hypotheses regarding host receptor requirements for P. falciparum invasion.

Complement Receptor 1 availability on red blood cell surface modulates Plasmodium vivax invasion of human reticulocytes

Plasmodium vivax parasites preferentially invade reticulocyte cells in a multistep process that is still poorly understood. In this study, we used ex vivo invasion assays and population genetic analyses to investigate the involvement of complement receptor 1 (CR1) in P. vivax invasion. First, we observed that P. vivax invasion of reticulocytes was consistently reduced when CR1 surface expression was reduced through enzymatic cleavage, in the presence of naturally low-CR1-expressing cells compared with high-CR1-expressing cells, and with the addition of soluble CR1, a known inhibitor of P. falciparum invasion. Immuno-precipitation experiments with P. vivax Reticulocyte Binding Proteins showed no evidence of complex formation. In addition, analysis of CR1 genetic data for worldwide human populations with different exposure to malaria parasites show significantly higher frequency of CR1 alleles associated with low receptor expression on the surface of RBCs and higher linkage disequilibrium in human populations exposed to P. vivax malaria compared with unexposed populations. These results are consistent with a positive selection of low-CR1-expressing alleles in vivax-endemic areas. Collectively, our findings demonstrate that CR1 availability on the surface of RBCs modulates P. vivax invasion. The identification of new molecular interactions is crucial to guiding the rational development of new therapeutic interventions against vivax malaria.

Shed EBA-175 mediates red blood cell clustering that enhances malaria parasite growth and enables immune evasion

Erythrocyte Binding Antigen of 175 kDa (EBA-175) has a well-defined role in binding to glycophorin A (GpA) during Plasmodium falciparum invasion of erythrocytes. However, EBA-175 is shed post invasion and a role for this shed protein has not been defined. We show that EBA-175 shed from parasites promotes clustering of RBCs, and EBA-175-dependent clusters occur in parasite culture. Region II of EBA-175 is sufficient for clustering RBCs in a GpA-dependent manner. These clusters are capable of forming under physiological flow conditions and across a range of concentrations. EBA-175-dependent RBC clustering provides daughter merozoites ready access to uninfected RBCs enhancing parasite growth. Clustering provides a general method to protect the invasion machinery from immune recognition and disruption as exemplified by protection from neutralizing antibodies that target AMA-1 and RH5. These findings provide a mechanistic framework for the role of shed proteins in RBC clustering, immune evasion, and malaria.

Human erythrocyte band 3 is a host receptor for Plasmodium falciparum glutamic acid-rich protein

Malaria remains a major global threat to human health and economic development. Microvascular lesions caused by Plasmodium falciparum-infected human erythrocytes/red blood cells are hallmarks of severe pathogenesis contributing to high mortality, particularly in children from sub-Saharan Africa. In this study, we used a phage display complementary DNA library screening strategy to identify P falciparum glutamic acid-rich protein (PfGARP) as a secreted ligand that recognizes an ectodomain of human erythrocyte anion-exchanger, band 3/AE1, as a host receptor. Domain mapping of PfGARP revealed distinct nonoverlapping repeats encoding the immune response epitopes and core erythrocyte-binding activity. Synthetic peptides derived from the erythrocyte-binding repeats of PfGARP induced erythrocyte aggregation reminiscent of the rosetting phenomenon. Using peptides derived from the immunogenic repeats, a quantitative immunoassay was developed to detect a selective immune response against PfGARP in human plasma samples obtained from patients in rural Mali, suggesting the feasibility of PfGARP as a potential biomarker of disease progression. Collectively, our results suggest that PfGARP may play a functional role in enhancing the adhesive properties of human erythrocytes by engaging band 3 as a host receptor. We propose that immunological and pharmacological inhibition of PfGARP may unveil new therapeutic options for mitigating lesions in cerebral and pregnancy-associated malaria.

Human Cyclophilin B forms part of a multi-protein complex during erythrocyte invasion by Plasmodium falciparum

Invasion of human erythrocytes by Plasmodium falciparum merozoites involves multiple interactions between host receptors and their merozoite ligands. Here we report human Cyclophilin B as a receptor for PfRhopH3 during merozoite invasion. Localization and binding studies show that Cyclophilin B is present on the erythrocytes and binds strongly to merozoites. We demonstrate that PfRhopH3 binds to the RBCs and their treatment with Cyclosporin A prevents merozoite invasion. We also show a multi-protein complex involving Cyclophilin B and Basigin, as well as PfRhopH3 and PfRh5 that aids the invasion. Furthermore, we report identification of a de novo peptide CDP3 that binds Cyclophilin B and blocks invasion by up to 80%. Collectively, our data provide evidence of compounded interactions between host receptors and merozoite surface proteins and paves the way for developing peptide and small-molecules that inhibit the protein−protein interactions, individually or in toto, leading to abrogation of the invasion process.

Invasion of red blood cells by Plasmodium falciparum is a complex process and relies on several receptor-ligand interactions. Here, the authors show that human cyclophilin B binds Plasmodium surface protein PfRhopH3 and that interruption of this interaction reduces invasion by 80%.

Multiple Plasmodium vivax proteins of Pv-fam-a family interact with human erythrocyte receptor Band 3 and have a role in red cell invasion

Elucidation of molecular mechanisms of receptor-ligand biology during host-parasite interaction helps in developing therapeutic targets. Several Pv-fam-a family proteins of Plasmodium vivax bind to host erythrocytes but their erythrocyte receptors remains to be explored. Here, we show that three merozoite proteins (PvTRAg36, PvATRAg74, and PvTRAg38) of this family interact with Band 3 on human erythrocytes through its three exofacial loops (loop 1, loop 3, and loop 6). These parasite proteins also interfered with the parasite growth in in-vitro, and the inhibition rate seems to be associated with their binding affinity to Band 3. This redundancy in receptor-ligand interaction could be one of the probable mechanism parasite utilizes to invade the host erythrocyte more efficiently.

Broadly neutralizing epitopes in the Plasmodium vivax vaccine candidate Duffy Binding Protein

Plasmodium vivax Duffy Binding Protein (PvDBP) is the most promising vaccine candidate for P. vivax malaria. The polymorphic nature of PvDBP induces strain-specific immune responses, however, and the epitopes of broadly neutralizing antibodies are unknown. These features hamper the rational design of potent DBP-based vaccines and necessitate the identification of globally conserved epitopes. Using X-ray crystallography, small-angle X-ray scattering, hydrogen-deuterium exchange mass spectrometry, and mutational mapping, we have defined epitopes for three inhibitory mAbs (mAbs 2D10, 2H2, and 2C6) and one noninhibitory mAb (3D10) that engage DBP. These studies expand the currently known inhibitory epitope repertoire by establishing protective motifs in subdomain three outside the receptor-binding and dimerization residues of DBP, and introduce globally conserved protective targets. All of the epitopes are highly conserved among DBP alleles. The identification of broadly conserved epitopes of inhibitory antibodies provides critical motifs that should be retained in the next generation of potent vaccines for P. vivax malaria.