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

Identification of Babesia microti immunoreactive antigens by phage display cDNA screen

Human babesiosis is a malaria-like illness caused by protozoan parasites of the genus Babesia. Babesia microti is responsible for most cases of human babesiosis in the United States, particularly in the Northeast and the Upper Midwest. Babesia microti is primarily transmitted to humans through the bite of infected deer ticks but also through the transfusion of blood components, particularly red blood cells. There is a high risk of severe and even fatal disease in immunocompromised patients. To date, serology testing relies on an indirect immunofluorescence assay that uses the whole Babesia microti antigen. Here, we report the construction of phage display cDNA libraries from Babesia microti-infected erythrocytes as well as human reticulocytes obtained from donors with hereditary hemochromatosis. Plasma samples were obtained from patients who were or had been infected with Babesia microti. The non-specific antibody reactivity of these plasma samples was minimized by pre-exposure to the human reticulocyte library. Using this novel experimental strategy, immunoreactive segments were identified in three Babesia microti antigens termed BmSA1 (also called BMN1-9; BmGPI12), BMN1-20 (BMN1-17; Bm32), and BM4.12 (N1-15). Moreover, our findings indicate that the major immunoreactive segment of BmSA1 does not overlap with the segment that mediates BmSA1 binding to mature erythrocytes. When used in combination, the three immunoreactive segments form the basis of a sensitive and comprehensive diagnostic immunoassay for human babesiosis, with implications for vaccine development.

Plasmodium falciparum Glutamic Acid-Rich Protein-Independent Polyclonal Antibodies Inhibit Malaria Parasite Growth in Human Erythrocytes

Plasmodium falciparum glutamic acid-rich protein (PfGARP) is a recently characterized cell surface antigen encoded by Plasmodium falciparum, the causative agent of severe human malaria pathophysiology. Previously, we reported that the human erythrocyte band 3 (SLC4A1) serves as a host receptor for PfGARP. Antibodies against PfGARP did not affect parasite invasion and growth. We surmised that PfGARP may play a role in the rosetting and adhesion of malaria. Another study reported that antibodies targeting PfGARP exhibit potent inhibition of parasite growth. This inhibition occurred without the presence of any immune or complement components, suggesting the activation of an inherent density-dependent regulatory system. Here, we used polyclonal antibodies against PfGARP and a monoclonal antibody mAb7899 to demonstrate that anti-PfGARP polyclonal antibodies, but not mAb7899, exerted potent inhibition of parasite growth in infected erythrocytes independent of PfGARP. These findings suggest that an unknown malaria protein(s) is the target of growth arrest by polyclonal antibodies raised against PfGARP.

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.

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.

The Need for Novel Asexual Blood-Stage Malaria Vaccine Candidates for Plasmodium falciparum

Extensive control efforts have significantly reduced malaria cases and deaths over the past two decades, but in recent years, coupled with the COVID-19 pandemic, success has stalled. The WHO has urged the implementation of a number of interventions, including vaccines. The modestly effective RTS,S/AS01 pre-erythrocytic vaccine has been recommended by the WHO for use in sub-Saharan Africa against Plasmodium falciparum in children residing in moderate to high malaria transmission regions. A second pre-erythrocytic vaccine, R21/Matrix-M, was also recommended by the WHO on 3 October 2023. However, the paucity and limitations of pre-erythrocytic vaccines highlight the need for asexual blood-stage malaria vaccines that prevent disease caused by blood-stage parasites. Few asexual blood-stage vaccine candidates have reached phase 2 clinical development, and the challenges in terms of their efficacy include antigen polymorphisms and low immunogenicity in humans. This review summarizes the history and progress of asexual blood-stage malaria vaccine development, highlighting the need for novel candidate vaccine antigens/molecules.

Plasmodium falciparum GCN5 plays a key role in regulating artemisinin resistance-related stress responses

Plasmodium falciparum causes the most severe malaria and is exposed to various environmental and physiological stresses in the human host. Given that GCN5 plays a critical role in regulating stress responses in model organisms, we aimed to elucidate PfGCN5's function in stress responses in P. falciparum. The protein level of PfGCN5 was substantially induced under three stress conditions [heat shock, low glucose starvation, and dihydroartemisinin, the active metabolite of artemisinin (ART)]. With a TetR-DOZI conditional knockdown (KD) system, we successfully down-regulated PfGCN5 to ~50% and found that KD parasites became more sensitive to all three stress conditions. Transcriptomic analysis via RNA-seq identified ~1,000 up- and down-regulated genes in the wild-type (WT) and KD parasites under these stress conditions. Importantly, DHA induced transcriptional alteration of many genes involved in many aspects of stress responses, which were heavily shared among the altered genes under heat shock and low glucose conditions, including ART-resistance-related genes such as K13 and coronin. Based on the expression pattern between WT and KD parasites under three stress conditions, ~300-400 genes were identified to be involved in PfGCN5-dependent, general, and stress-condition-specific responses with high levels of overlaps among three stress conditions. Notably, using ring-stage survival assay, we found that KD or inhibition of PfGCN5 could sensitize the ART-resistant parasites to the DHA treatment. All these indicate that PfGCN5 is pivotal in regulating general and ART-resistance-related stress responses in malaria parasites, implicating PfGCN5 as a potential target for malaria intervention.

Plasmodium falciparum GCN5 plays a key role in regulating artemisinin resistance-related stress responses

Plasmodium falciparum causes the most severe malaria and is exposed to various environmental and physiological stresses in the human host. Given that GCN5 plays a critical role in regulating stress responses in model organisms, we aimed to elucidate PfGCN5's function in stress responses in P. falciparum . The protein level of PfGCN5 was substantially induced under three stress conditions (heat shock, low glucose starvation, and dihydroartemisinin, the active metabolite of artemisinin (ART)). With a TetR-DOZI conditional knockdown (KD) system, we successfully down-regulated PfGCN5 to ∼50% and found that KD parasites became more sensitive to all three stress conditions. Transcriptomic analysis via RNA-seq identified ∼1,000 up-and down-regulated genes in the wildtype (WT) and KD parasites under these stress conditions. Importantly, DHA induced transcriptional alteration of many genes involved in many aspects of stress responses, which were heavily shared among the altered genes under heat shock and low glucose conditions, including ART-resistance-related genes such as K13 and coronin . Based on the expression pattern between WT and KD parasites under three stress conditions, ∼300-400 genes were identified to be involved in PfGCN5-dependent, general and stress-condition-specific responses with high levels of overlaps among three stress conditions. Notably, using ring-stage survival assay (RSA), we found that KD or inhibition of PfGCN5 could sensitize the ART-resistant parasites to the DHA treatment. All these indicate that PfGCN5 is pivotal in regulating general and ART-resistance-related stress responses in malaria parasites, implicating PfGCN5 as a potential target for malaria intervention.

IMPORTANCE

Malaria leads to about half a million deaths annually and these casualties were majorly caused by the infection of Plasmodium falciparum . This parasite strives to survive by defending against a variety of stress conditions, such as malaria cyclical fever (heat shock), starvation due to low blood sugar (glucose) levels (hypoglycemia), and drug treatment. Previous studies have revealed that P. falciparum has developed unique stress responses to different stresses including ART treatment, and ART-resistant parasites harbor elevated stress responses. In this study, we provide critical evidence on the role of PfGCN5, a histone modifier, and a chromatin coactivator, in regulating general and stress-specific responses in malaria parasites, indicating that PfGCN5 can be used as a potential target for anti-malaria intervention.

Analysis of sequence diversity in Plasmodium falciparum glutamic acid-rich protein (PfGARP), an asexual blood stage vaccine candidate

Glutamic acid-rich protein of Plasmodium falciparum (PfGARP) binds to erythrocyte band 3 and may enhance cytoadherence of infected erythrocytes. Naturally acquired anti-PfGARP antibodies could confer protection against high parasitemia and severe symptoms. While whole genome sequencing analysis has suggested high conservation in this locus, little is known about repeat polymorphism in this vaccine candidate antigen. Direct sequencing was performed from the PCR-amplified complete PfGARP gene of 80 clinical isolates from four malaria endemic provinces in Thailand and an isolate from a Guinean patient. Publicly available complete coding sequences of this locus were included for comparative analysis. Six complex repeat (RI-RVI) and two homopolymeric glutamic acid repeat (E1 and E2) domains were identified in PfGARP. The erythrocyte band 3-binding ligand in domain RIV and the epitope for mAB7899 antibody eliciting in vitro parasite killing property were perfectly conserved across isolates. Repeat lengths in domains RIII and E1-RVI-E2 seemed to be correlated with parasite density of the patients. Sequence variation in PfGARP exhibited genetic differentiation across most endemic areas of Thailand. Phylogenetic tree inferred from this locus has shown that most Thai isolates formed closely related lineages, suggesting local expansion/contractions of repeat-encoding regions. Positive selection was observed in non-repeat region preceding domain RII which corresponded to a helper T cell epitope predicted to be recognized by a common HLA class II among Thai population. Predicted linear B cell epitopes were identified in both repeat and non-repeat domains. Besides length variation in some repeat domains, sequence conservation in non-repeat regions and almost all predicted immunogenic epitopes have suggested that PfGARP-derived vaccine may largely elicit strain-transcending immunity.

PdpC, a secreted effector protein of the type six secretion system, is required for erythrocyte invasion by Francisella tularensis LVS

Francisella tularensis is a gram negative, intracellular pathogen that is the causative agent of the potentially fatal disease, tularemia. During infection, F. tularensis is engulfed by and replicates within host macrophages. Additionally, this bacterium has also been shown to invade human erythrocytes and, in both cases, the Type Six Secretion System (T6SS) is required for these host-pathogen interaction. One T6SS effector protein, PdpC, is important for macrophage infection, playing a role in phagolysosomal escape and intracellular replication. To determine if PdpC also plays a role in erythrocyte invasion, we constructed a pdpC-null mutant in the live vaccine strain, F. tularensis LVS. We show that PdpC is required for invasion of human and sheep erythrocytes during in vitro assays and that reintroduction of a copy of pdpC, in trans, rescues this phenotype. The interaction with human erythrocytes was further characterized using double-immunofluorescence microscopy to show that PdpC is required for attachment of F. tularensis LVS to erythrocytes as well as invasion. To learn more about the role of PdpC in erythrocyte invasion we generated a strain of F. tularensis LVS expressing pdpC-emgfp. PdpC-EmGFP localizes as discrete foci in a subset of F. tularensis LVS cells grown in broth culture and accumulates in erythrocytes during invasion assays. Our results are the first example of a secreted effector protein of the T6SS shown to be involved in erythrocyte invasion and indicate that PdpC is secreted into erythrocytes during invasion.

Evolving perspectives on rosetting in malaria

The ability of the intraerythrocytic Plasmodium spp. to form spontaneous rosettes with uninfected red blood cells (URBCs) has been observed in the medically important malaria parasites. Since the discovery of rosettes in the late 1980s, different formation mechanisms and pathobiological roles have been postulated for rosetting; most of which have focused on Plasmodium falciparum. Recent breakthroughs, including new data from Plasmodium vivax, have highlighted the multifaceted roles of rosetting in the immunopathobiology and the development of drug resistance in human malaria. Here, we provide new perspectives on the formation and the role of rosetting in malaria rheopathobiology.

A multi-epitope vaccine designed against blood-stage of malaria: an immunoinformatic and structural approach

Malaria is a complex disease caused by parasites of the genus Plasmodium and is the leading cause of morbidity and mortality worldwide. The most severe form of malaria disease is caused by Plasmodium falciparum. Thus, a combination of different approaches is needed to control malaria. Resistance to first-line drugs and insecticides, on the other hand, makes the need for an effective vaccination more urgent than ever. Because erythrocyte parasites cause the most clinical symptoms, developing a vaccination for this stage of infection might be highly beneficial. In this research, we employed various bioinformatics methods to create an efficient multi-epitope vaccine that induces antibodies against the blood stage of malaria infection. For this purpose, we selected the malaria PfGARP protein as the target here. The B, HTL epitopes, and epitope conservation were predicted. The predicted epitopes (including 5 B and 5 HTL epitopes) were connected using suitable linkers, and the flagellin molecule was used as an adjuvant to improve its immunogenicity. The final construct vaccine with 414 amino acids long was designed. The vaccine's allergenicity, antigenicity, solubility, physicochemical characteristics, 2D and 3D structure modeling, molecular docking, molecular dynamics simulation, in silico cloning, and immunological simulation were tested. In silico immune simulation results showed significantly elevated IgG1 and IgM and T helper cells, INF γ, IL 2, and B-cell populations after the injection of the designed vaccine. These significant computational analyses indicated that our proposed vaccine candidate might activate suitable immune responses against malaria. However, in vitro and in vivo studies are essential for further validation.

Glycosylphosphatidylinositol-anchored micronemal antigen (GAMA) interacts with the band 3 receptor to promote erythrocyte invasion by malaria parasites

Glycosylphosphatidylinositol-anchored micronemal antigen (GAMA) is an erythrocyte binding protein known to be involved in malarial parasite invasion. Although anti-GAMA antibodies have been shown to block GAMA attachment to the erythrocyte surface and subsequently inhibit parasite invasion, little is known about the molecular mechanisms by which GAMA promotes the invasion process. In this study, LC-MS analysis was performed on the erythrocyte membrane to identify the specific receptor that interacts with GAMA. We found that ankyrin 1 and the band 3 membrane protein showed affinity for GAMA, and characterization of their binding specificity indicated that both Plasmodium falciparum and Plasmodium vivax GAMA bound to the same extracellular loop of band 3 (loop 5). In addition, we show the interaction between GAMA and band 3 was sensitive to chymotrypsin. Furthermore, antibodies against band 3 loop 5 were able to reduce the binding activity of GAMA to erythrocytes and inhibit the invasion of P. falciparum merozoites into human erythrocytes, whereas antibodies against P. falciparum GAMA (PfGAMA)-Tr3 only slightly reduced P. falciparum invasion. The identification and characterization of the erythrocyte GAMA receptor is a novel finding that identifies an essential mechanism of parasite invasion of host erythrocytes.

Cell Physiology and Molecular Mechanism of Anion Transport by Erythrocyte Band 3/AE1

The major transmembrane protein of the red blood cell, known as band 3, AE1, and SLC4A1, has two main functions: 1) catalysis of Cl^-/HCO₃^- exchange, one of the steps in CO₂ excretion; 2) anchoring the membrane skeleton. This review summarizes the 150 year history of research on red cell anion transport and band 3 as an experimental system for studying membrane protein structure and ion transport mechanisms. Important early findings were that red cell Cl^- transport is a tightly coupled 1:1 exchange and band 3 is labeled by stilbenesulfonate derivatives that inhibit anion transport. Biochemical studies showed that the protein is dimeric or tetrameric (paired dimers) and that there is one stilbenedisulfonate binding site per subunit of the dimer. Transport kinetics and inhibitor characteristics supported the idea that the transporter acts by an alternating access mechanism with intrinsic asymmetry. The sequence of band 3 cDNA provided a framework for detailed study of protein topology and amino acid residues important for transport. The identification of genetic variants produced insights into the roles of band 3 in red cell abnormalities and distal renal tubular acidosis. The publication of the membrane domain crystal structure made it possible to propose concrete molecular models of transport. Future research directions include improving our understanding of the transport mechanism at the molecular level and of the integrative relationships among band 3, hemoglobin, carbonic anhydrase, and gradients (both transmembrane and subcellular) of HCO₃^-, Cl^-, O₂, CO₂, pH, and NO metabolites during pulmonary and systemic capillary gas exchange.

A newly characterized malaria antigen on erythrocyte and merozoite surfaces induces parasite inhibitory antibodies

We previously identified a Plasmodium falciparum (Pf) protein of unknown function encoded by a single-copy gene, PF3D7_1134300, as a target of antibodies in plasma of Tanzanian children in a whole-proteome differential screen. Here we characterize this protein as a blood-stage antigen that localizes to the surface membranes of both parasitized erythrocytes and merozoites, hence its designation as Pf erythrocyte membrane and merozoite antigen 1 (PfEMMA1). Mouse anti-PfEMMA1 antisera and affinity-purified human anti-PfEMMA1 antibodies inhibited growth of P. falciparum strains by up to 68% in growth inhibition assays. Following challenge with uniformly fatal Plasmodium berghei (Pb) ANKA, up to 40% of mice immunized with recombinant PbEMMA1 self-cured, and median survival of lethally infected mice was up to 2.6-fold longer than controls (21 vs. 8 d, P = 0.005). Furthermore, high levels of naturally acquired human anti-PfEMMA1 antibodies were associated with a 46% decrease in parasitemia over 2.5 yr of follow-up of Tanzanian children. Together, these findings suggest that antibodies to PfEMMA1 mediate protection against malaria.

A Thermal Exhaust Port on the Death Star of Plasmodium falciparum-Infected Erythrocytes

Eliciting reliable and effective immunity against Plasmodium falciparum parasites remains an elusive goal in malaria control. Raj and colleagues recently described a naturally occurring human antibody response to a parasite antigen that initiates apoptosis-like cell death of parasites, adding fascinating insight into host-pathogen dialog that may furnish actionable targets for antiparasite therapies or vaccines.

Malaria According to GARP: A New Trail towards Anti-disease Vaccination

Naturally acquired anti-Plasmodium falciparum immunity protects first and foremost against severe disease. Raj et al. have established a tantalizing path towards an anti-disease vaccine by identifying glutamic acid-rich protein (GARP) antibodies as signatures of protection against severe malaria in Tanzanian children and demonstrating efficacy in blood cultures and monkey trials.