Structural vaccinology of malaria transmission-blocking vaccines

Abstract

Introduction: The development of effective vaccines remains a major health priority to combat the global burden of malaria, a life-threatening disease caused by Plasmodium parasites. Transmission-blocking vaccines (TBVs) elicit antibodies that neutralize the sexual stages of the parasite in blood meals ingested by the Anopheles mosquito, interrupting parasite development in the vector host and preventing disease spread to other individuals.Areas covered: The P. falciparum gametocyte surface antigens Pfs230, Pfs48/45, and Pfs47, the parasite ookinete surface protein Pfs25, and the male gametocyte specific protein PfHAP2 are leading TBV candidates, some of which are in clinical development. The recent expansion of methodology to study monoclonal antibodies isolated directly from humans and animal models, coupled with effective measures for parasite neutralization, has provided unprecedented insight into TBV efficacy and development.Expert opinion: Available structural and functional data on antigen-monoclonal antibody (Ag-mAb) complexes, as well as epitope classification studies, have identified neutralizing epitopes that may aid vaccine development and improve protection. Here, we review the clinical prospects of TBV candidates, progress in the development of novel vaccine strategies for TBVs, and the impact of structural vaccinology in TBV design.

Keywords: Malaria; fertilization; gamete; gametocyte; monoclonal antibodies; ookinete; pfhap2; pfs230; pfs25; pfs47; pfs48/45; plasmodium falciparum; standard membrane feeding assay; structural vaccinology; transmission blocking vaccine; vaccine design; zygote.

Figure 1.

(a) Different life cycle stages of the P. falciparum parasite. It involves two hosts. In the exo-erythrocytic cycle, sporozoites infect hepatocytes and mature into schizonts inside them which rupture and release merozoites. These released merozoites infect RBCs and develop into trophozoites and schizonts in the erythrocytic cycle. Some parasites differentiate into gametocytes that develop inside erythrocytes and are ingested by female Anopheles mosquito during a blood meal. Parasite development in the mosquito is known as the sporogonic cycle (mosquito/transmission stages). The ingested gametocytes develop into zygotes and ookinetes which invade the midgut wall of the mosquito where they develop into oocysts. The oocysts rupture and release sporozoites that make their way to the salivary glands. Inoculation of the sporozoites into a new human host continues the malaria life cycle. TBVs elicit antibody responses against the target antigen of mosquito/transmission stages of the parasite, the gametes and ookinetes, to prevent infection in mosquito and further transmission of sporozoites into a new human host. This figure is created with BioRender.com. Overall view of the structure of (b) Schematic domain organization of all proteins described in this article. TBV candidates are colored in blue and the rest are in gray.

Figure 2.

Structural basis for transmission-reducing activity of mAb 4F12 against Pfs230D1M. (a) Domain organization of full-length Pfs230 and its truncations. SP signal peptide (b) Overview of the Pfs230D1M structure (PDB: 6OHG). Overlay of Pfs230 domain I with the Pfs48/45–6 C (PDB: 6E62), both domains of Pf12 (PDB: 2YMO) and Pf41 (PDB: 4YS4). Root mean square deviations (rmsd) between Pfs230 and the structures are indicated in brackets. Disulfide bridges are shown as sticks. (c) Binding of Fab fragment of 4F12 on the Pfs230D1M. The Pfs230D1M is represented as a gray surface and epitope is colored in blue. mAb 4F12 recognizes conformational epitope on Pfs230 DI by LCDR1–3, HCDR1 and HCDR3. (d) Polymorphic residues mapped onto the Pfs230 surface. The 4F12 epitope is colored in blue. Amino acid substitutions are shown in yellow and the observed substitutions are also indicated. This figure is modified and adapted from [10].

Figure 3.

Structural basis for transmission-reducing activity of mAb 85RF45.1 against Pfs48/45. (a) Domain organization of Pfs48/45 immunogen. SP signal peptide, GPI glycosylphosphatidylinositol anchor. (b) Overview of the Pfs48/45–6C structure (PDB: 6E62). Overlay of Pfs48/45–6C with the domain II of Pf12 (PDB: 2YMO) and Pf41 (PDB: 4YS4). Root mean square deviations (rmsd) between Pfs48/45–6C and the structures are indicated in brackets. Disulfide bridges are shown as sticks. (c) Binding of Fab fragment of 85RF45.1 on the Pfs48/45–6C domain. The Pfs48/45–6C domain is represented as gray surface and epitope is colored in blue. mAb 85RF45.1 recognizes conformational epitope on Pfs48/45–6C by all CDRs (LCDR1–3 and HCDR1–3). (d) Polymorphism mapped onto the Pfs48/45 surface. The 85RF45.1 epitope is colored in blue. Amino acid substitutions are shown in yellow and the observed substitutions are also indicated. This figure is modified and adapted from [5].

Figure 4.

Structural basis for transmission-reducing activity of mAbs 1269, 1245, and 2544 against Pfs25. (a) Domain organization of Pfs25 immunogen. SP signal peptide, GPI glycosylphosphatidylinositol anchor. (b) An overview of the Pfs25 structure (PDB: 6PHC) and its EGF-like domains 1 to 4 are colored in blue, green, orange, and firebrick, respectively. Disulfide bridges are shown as sticks. (c) Surface representation of Pfs25 displaying conformational epitopes of 1269 (site I) (PDB: 6B0A), 1245 (site II) (PDB: 6B0G), 2544 (near to site I) (PDB: 6PHC), 2530 (site III) (PDB: 6PHB) and 2586 (between sites I and II) (PDB: 6PHD). (d) Binding of Fab fragments of 1269, 1245, 2544, 2530 and 2586 on the Pfs25 protein. Pfs25 is represented as a gray surface and epitopes for 1269, 1245, 2544, 2530 and 2586 are colored in red, blue, green, pink and yellow orange, respectively. For each mAb, K_D (nM) and IC₈₀ (µg/ml) values are also indicated. mAbs 1269, 1245, 2544, 2530 and 2586 recognize different conformational epitopes on Pfs25. This figure is modified and adapted from [6,9].

Figure 5.

Sequence similarity, structure homology and molecular details of Pvs25–2A8 Fab structure. (a) Sequence alignment of Pfs25 with Pvs25. (b) An overview of the Pvs25 structure (PDB: 1Z3G) and its EGF-like domains 1–4 are colored in blue, green, orange, and firebrick, respectively. (c) Overlay of Pfs25 (PDB: 6PHC) with the Pvs25 (PDB: 1Z3G) structure. Inset shows conserved residues between Pfs25 and Pvs25 making H-bond contact that promote the triangular arrangement. Root mean square deviations (rmsd) between Pfs25 and Pvs25 is indicated in brackets. (d) Binding of Fab fragment of 2A8 on the Pvs25 protein. Pvs25 is represented as a gray surface and epitope is colored in blue. mAb 2A8 recognize conformational epitope on B loop and central β-strands of the EGF-like domain II by CDRs H1-H3 and hypervariable forth loop. Disulfide bridges are shown as sticks. This figure is modified and adapted from [9,32].