Specific erythrocyte binding capacity and biological activity of Plasmodium falciparum erythrocyte binding ligand 1 (EBL-1)-derived peptides

The Cellular and Molecular Interaction Between Erythrocytes and Plasmodium falciparum Merozoites

Plasmodium falciparum is the most lethal human malaria parasite, partly due to its genetic variability and ability to use multiple invasion routes via its binding to host cell surface receptors. The parasite extensively modifies infected red blood cell architecture to promote its survival which leads to increased cell membrane rigidity, adhesiveness and permeability. Merozoites are initially released from infected hepatocytes and efficiently enter red blood cells in a well-orchestrated process that involves specific interactions between parasite ligands and erythrocyte receptors; symptoms of the disease occur during the life-cycle’s blood stage due to capillary blockage and massive erythrocyte lysis. Several studies have focused on elucidating molecular merozoite/erythrocyte interactions and host cell modifications; however, further in-depth analysis is required for understanding the parasite’s biology and thus provide the fundamental tools for developing prophylactic or therapeutic alternatives to mitigate or eliminate Plasmodium falciparum-related malaria. This review focuses on the cellular and molecular events during Plasmodium falciparum merozoite invasion of red blood cells and the alterations that occur in an erythrocyte once it has become infected.

Erythrocyte glycophorins as receptors for Plasmodium merozoites

Glycophorins are heavily glycosylated sialoglycoproteins of human and animal erythrocytes. In humans, there are four glycophorins: A, B, C and D. Glycophorins play an important role in the invasion of red blood cells (RBCs) by malaria parasites, which involves several ligands binding to RBC receptors. Four Plasmodium falciparum merozoite EBL ligands have been identified: erythrocyte-binding antigen-175 (EBA-175), erythrocyte-binding antigen-181 (EBA-181), erythrocyte-binding ligand-1 (EBL-1) and erythrocyte-binding antigen-140 (EBA-140). It is generally accepted that glycophorin A (GPA) is the receptor for P. falciparum EBA-175 ligand. It has been shown that α(2,3) sialic acid residues of GPA O-glycans form conformation-dependent clusters on GPA polypeptide chain which facilitate binding. P. falciparum can also invade erythrocytes using glycophorin B (GPB), which is structurally similar to GPA. It has been shown that P. falciparum EBL-1 ligand binds to GPB. Interestingly, a hybrid GPB-GPA molecule called Dantu is associated with a reduced risk of severe malaria and ameliorates malaria-related morbidity. Glycophorin C (GPC) is a receptor for P. falciparum EBA-140 ligand. Likewise, successful binding of EBA-140 depends on sialic acid residues of N- and O-linked oligosaccharides of GPC, which form a cluster or a conformational structure depending on the presence of peptide fragment encompassing amino acids (aa) 36–63. Evaluation of the homologous P. reichenowi EBA-140 unexpectedly revealed that the chimpanzee homolog of human glycophorin D (GPD) is probably the receptor for this ligand. In this review, we concentrate on the role of glycophorins as erythrocyte receptors for Plasmodium parasites. The presented data support the long-lasting idea of high evolutionary pressure exerted by Plasmodium on the human glycophorins, which emerge as important receptors for these parasites.

Conserved Binding Regions Provide the Clue for Peptide-Based Vaccine Development: A Chemical Perspective

Synthetic peptides have become invaluable biomedical research and medicinal chemistry tools for studying functional roles, i.e., binding or proteolytic activity, naturally-occurring regions’ immunogenicity in proteins and developing therapeutic agents and vaccines. Synthetic peptides can mimic protein sites; their structure and function can be easily modulated by specific amino acid replacement. They have major advantages, i.e., they are cheap, easily-produced and chemically stable, lack infectious and secondary adverse reactions and can induce immune responses via T- and B-cell epitopes. Our group has previously shown that using synthetic peptides and adopting a functional approach has led to identifying Plasmodium falciparumconserved regions binding to host cells. Conserved high activity binding peptides’ (cHABPs) physicochemical, structural and immunological characteristics have been taken into account for properly modifying and converting them into highly immunogenic, protection-inducing peptides (mHABPs) in the experimental Aotus monkey model. This article describes stereo–electron and topochemical characteristics regarding major histocompatibility complex (MHC)-mHABP-T-cell receptor (TCR) complex formation. Some mHABPs in this complex inducing long-lasting, protective immunity have been named immune protection-inducing protein structures (IMPIPS), forming the subunit components in chemically synthesized vaccines. This manuscript summarizes this particular field and adds our recent findings concerning intramolecular interactions (H-bonds or π-interactions) enabling proper IMPIPS structure as well as the peripheral flanking residues (PFR) to stabilize the MHCII-IMPIPS-TCR interaction, aimed at inducing long-lasting, protective immunological memory.

Functionally relevant proteins in Plasmodium falciparum host cell invasion

A totally effective, antimalarial vaccine must involve sporozoite and merozoite proteins (or their fragments) to ensure complete parasite blocking during critical invasion stages. This Special Report examines proteins involved in critical biological functions for parasite survival and highlights the conserved amino acid sequences of the most important proteins involved in sporozoite invasion of hepatocytes and merozoite invasion of red blood cells. Conserved high activity binding peptides are located in such proteins’ functionally strategic sites, whose functions are related to receptor binding, nutrient and protein transport, enzyme activity and molecule–molecule interactions. They are thus excellent targets for vaccine development as they block proteins binding function involved in invasion and also their biological function.

Identification of a specific region of Plasmodium falciparum EBL-1 that binds to host receptor glycophorin B and inhibits merozoite invasion in human red blood cells

The malaria parasite Plasmodium falciparum invades human erythrocytes through multiple pathways utilizing several ligand-receptor interactions. These interactions are broadly classified in two groups according to their dependency on sialic acid residues. Here, we focus on the sialic acid-dependent pathway by using purified glycophorins and red blood cells (RBCs) to screen a cDNA phage display library derived from P. falciparum FCR3 strain, a sialic acid-dependent strain. This screen identified several parasite proteins including the erythrocyte-binding ligand-1, EBL-1. The phage cDNA insert encoded the 69-amino acid peptide, termed F2i, which is located within the F2 region of the DBL domain, designated here as D2, of EBL-1. Recombinant D2 and F2i polypeptides bound to purified glycophorins and RBCs, and the F2i peptide was found to interfere with binding of D2 domain to its receptor. Both D2 and F2i polypeptides bound to trypsin-treated but not neuraminidase or chymotrypsin-treated erythrocytes, consistent with known glycophorin B resistance to trypsin, and neither the D2 nor F2i polypeptide bound to glycophorin B-deficient erythrocytes. Importantly, purified D2 and F2i polypeptides partially inhibited merozoite reinvasion in human erythrocytes. Our results show that the host erythrocyte receptor glycophorin B directly interacts with the DBL domain of parasite EBL-1, and the core binding site is contained within the 69 amino acid F2i region (residues 601-669) of the DBL domain. Together, these findings suggest that a recombinant F2i peptide with stabilized structure could provide a protective function at blood stage infection and represents a valuable addition to a multi-subunit vaccine against malaria.

Mycobacterium tuberculosis Rv0679c protein sequences involved in host-cell infection: potential TB vaccine candidate antigen

Background

To date, the function of many hypothetical membrane proteins of Mycobacterium tuberculosis is still unknown and their involvement in pathogen-host interactions has not been yet clearly defined. In this study, the biological activity of peptides derived from the hypothetical membrane protein Rv0679c of M. tuberculosis and their involvement in pathogen-host interactions was assessed. Transcription of the Rv0679c gene was studied in 26 Mycobacterium spp. Strains. Antibodies raised against putative B-cell epitopes of Rv0679c were used in Western blot and immunoelectron microscopy assays. Synthetic peptides spanning the entire length of the protein were tested for their ability to bind to A549 and U937 cells. High-activity binding peptides (HABPs) identified in Rv0679c were tested for their ability to inhibit mycobacterial invasion into cells.

Results

The gene encoding Rv0679c was detected in all strains of the M. tuberculosis complex (MTC), but was only transcribed in M. tuberculosis H37Rv, M. tuberculosis H37Ra and M. africanum. Anti-Rv0679c antibodies specifically recognized the protein in M. tuberculosis H37Rv sonicate and showed its localization on mycobacterial surface. Four HABPs inhibited invasion of M. tuberculosis to target cells by up to 75%.

Conclusions

The results indicate that Rv0679c HABPs and in particular HABP 30979 could be playing an important role during M. tuberculosis invasion of host cells, and therefore could be interesting research targets for studies aimed at developing strategies to control tuberculosis.

Prevalence of 5' insertion mutants and analysis of single nucleotide polymorphism in the erythrocyte binding-like 1 (ebl-1) gene in Kenyan Plasmodium falciparum field isolates

Plasmodium merozoites attach to and invade red blood cells (RBCs) during the erythrocytic cycle. The invasion process requires recognition of RBC surface receptors by proteins of the Plasmodium Duffy binding like erythrocyte binding like (DBL-EBP) family. Clones and isolates of Plasmodium falciparum have varying abilities to utilize different RBC receptors, and multiple distinct pathways so far identified depend on glycophorins A, B, C, and as yet unidentified receptors. At present, five members of the DBL-EBP family have been identified in the P. falciparum genome, based on gene structure and amino acid sequence homology. The cardinal features of this family consist of conserved 5' and 3' cysteine-rich regions (regions II and VI, respectively) whose cysteine residues are highly conserved along with the majority of aromatic amino acids. In contrast to the single DBL-EBP family member in Plasmodium vivax, in P. falciparum all DBL-EBP family members have a duplication of the conserved 5' cysteine-rich region denoted as the F1 and F2 domains. These cysteine-rich regions are considered crucial in recognition of erythrocyte receptors and it has been shown that several bind to glycophorins on the erythrocyte surface. Several studies, on both field isolates and laboratory strains have uncovered a relatively high degree of sequence polymorphism in the DBP-EBL genes. This study is now extended to include field isolates collected from sites within Kenya. DNA isolated from blood samples of infected patients was utilized to amplify the region I sequence of ebl-1 gene in order to investigate polymorphism in the region immediately adjacent to the 5' cysteine-rich domains, and to determine the prevalence of an insertion mutant that effectively knocks out the gene.

Glycophorin B is the erythrocyte receptor of Plasmodium falciparum erythrocyte-binding ligand, EBL-1

In the war against Plasmodium, humans have evolved to eliminate or modify proteins on the erythrocyte surface that serve as receptors for parasite invasion, such as the Duffy blood group, a receptor for Plasmodium vivax, and the Gerbich-negative modification of glycophorin C for Plasmodium falciparum. In turn, the parasite counters with expansion and diversification of ligand families. The high degree of polymorphism in glycophorin B found in malaria-endemic regions suggests that it also may be a receptor for Plasmodium, but, to date, none has been identified. We provide evidence from erythrocyte-binding that glycophorin B is a receptor for the P. falciparum protein EBL-1, a member of the Duffy-binding-like erythrocyte-binding protein (DBL-EBP) receptor family. The erythrocyte-binding domain, region 2 of EBL-1, expressed on CHO-K1 cells, bound glycophorin B(+) but not glycophorin B-null erythrocytes. In addition, glycophorin B(+) but not glycophorin B-null erythrocytes adsorbed native EBL-1 from the P. falciparum culture supernatants. Interestingly, the Efe pygmies of the Ituri forest in the Democratic Republic of the Congo have the highest gene frequency of glycophorin B-null in the world, raising the possibility that the DBL-EBP family may have expanded in response to the high frequency of glycophorin B-null in the population.

Strategies for developing multi-epitope, subunit-based, chemically synthesized anti-malarial vaccines

An anti-malarial vaccine against the extremely lethal Plasmodium falciparum is desperately needed. Peptides from this parasite's proteins involved in invasion and having high red blood cell-binding ability were identified; these conserved peptides were not immun genic or protection-inducing when used for immunizing Aotus monkeys. Modifying some critical binding residues in these high-activi binding peptides' (HABPs') attachment to red blood cells (RBC) allowed them to induce immunogenicity and protection against expermental challenge and acquire the ability to bind to specific HLA-DRp1* alleles. These modified HABPs adopted certain characterist structural configurations as determined by circular dichroism (CD) and ¹H nuclear magnetic resonance (NMR) associated with certain HLA-DRβ1* haplotype binding activities and characteristics, such as a 2-Å-distance difference between amino acids fitting into HLA-DRp1 Pockets 1 to 9, residues participating in binding to HLA-DR pockets and residues making contact with the TCR, suggesting haplotyp and allele-conscious TCR. This has been demonstrated in HLA-DR-like genotyped monkeys and provides the basis for designing high effective, subunit-based, multi-antigen, multi-stage, synthetic vaccines, for immediate human use, malaria being one of them.

Functional, structural, and immunological compartmentalisation of malaria invasive proteins

Conserved Plasmodium falciparum merozoite high activity binding peptides (HABPs) involved in red blood cell (RBC) invasion which are present in merozoite surface proteins (MSPs) involved in attachment, rolling over RBC, those derived from soluble proteins loosely bound to the membrane, and those present in microneme and rhoptry organelles have an alpha-helical structure and bind with high affinity to HLA-DR52 molecules. On the contrary, conserved HABPs belonging to molecules anchored to the membrane by a GPI tail, or a transmembranal region, or those molecules presenting PEXEL motifs have a strand, turn or unordered configuration and bind with high affinity to HLA-DR53 molecules. Such functional, cellular, structural, and immunological compartmentalisation has tremendous implications in subunit-based, multi-epitope, synthetic, anti-malarial vaccine development.

Identifying putative Mycobacterium tuberculosis Rv2004c protein sequences that bind specifically to U937 macrophages and A549 epithelial cells

Virulence and immunity are still poorly understood in Mycobacterium tuberculosis. The H37Rv M. tuberculosis laboratory strain genome has been completely sequenced, and this along with proteomic technology represent powerful tools contributing toward studying the biology of target cell interaction with a facultative bacillus and designing new strategies for controlling tuberculosis. Rv2004c is a putative M. tuberculosis protein that could have specific mycobacterial functions. This study has revealed that the encoding gene is present in all mycobacterium species belonging to the M. tuberculosis complex. Rv2004c gene transcription was observed in all of this complex's strains except Mycobacterium bovis and Mycobacterium microti. Rv2004c protein expression was confirmed by using antibodies able to recognize a 54-kDa molecule by immunoblotting, and its location was detected on the M. tuberculosis surface by transmission electron microscopy, suggesting that it is a mycobacterial surface protein. Binding assays led to recognizing high activity binding peptides (HABP); five HABPs specifically bound to U937 cells, and six specifically bound to A549 cells. HABP circular dichroism suggested that they had an alpha-helical structure. HABP-target cell interaction was determined to be specific and saturable; some of them also displayed greater affinity for A549 cells than U937 cells. The critical amino acids directly involved in their interaction with U937 cells were also determined. Two probable receptor molecules were found on U937 cells and five on A549 for the two HABPs analyzed. These observations have important biological significance for studying bacillus-target cell interactions and implications for developing strategies for controlling this disease.