Targeting the Conserved Fusion Loop of HAP2 Inhibits the Transmission of Plasmodium berghei and falciparum

Abstract

Inhibiting transmission of Plasmodium is a central strategy in malarial eradication, and the biological process of gamete fusion during fertilization is a proven target for this approach. The lack of a structure or known molecular function of current anti-malarial vaccine targets has previously been a hindrance in the development of transmission-blocking vaccines. Structure/function studies have indicated that the conserved gamete membrane fusion protein HAP2 is a class II viral fusion protein. Here, we demonstrate that targeting a function-critical site of the fusion/cd loop with species-specific antibodies reduces Plasmodium berghei transmission in vivo by 58.9% and in vitro fertilization by up to 89.9%. A corresponding reduction in P. falciparum transmission (75.5%/36.4% reductions in intensity/prevalence) is observed in complimentary field studies. These results emphasize conserved mechanisms of fusion in Apicomplexa, while highlighting an approach to design future anti-malarial transmission-blocking vaccines.

Keywords: HAP2; fusion; malaria; transmission; vaccine.

Graphical abstract

Figure 1

Identification of a Putative Plasmodium HAP2 Fusion Loop and Assessment of In Vivo Transmission-Blocking Efficacy by Direct Feeding Assays in Immunized Mice (A) Domain II (DII) schematic (yellow box) above alignment of the cysteine-rich region of HAP2 proteins from P. berghei (Pb), P. falciparum (Pf), Chlamydomonas reinhardtii (Cr), Leishmania major (Lm), Tribolum castaneum (Tc), Tetrahymena thermophile (Tt), Arabidopsis thaliana (At), Toxoplasma gondii (Tg), and Crytosporidium muris (Cm). Conserved and semi-conserved residues are in white font on a red or silver background, respectively. Conserved cysteine residues are numbered sequentially below the alignment, and their disulfide connectivities are drawn in green with disulfide bonds numbered (see Fédry et al., 2017). The conserved salt bridge between R and E (arrowheads above the sequence) is drawn in blue below the alignment. Loops 1 and 2 of the cd loop are highlighted in gray and separated by an alpha helix. A region upstream of the predicted fusion loop to which peptides and antibodies were generated is highlighted in blue and is referred to as Pb upstream. Highlighted in red is the short (18 aa) region within the predicted P. berghei and P. falciparum bipartite fusion loops to which peptides and antibodies were generated and is referred to as the Pb cd loop and Pf cd loop, respectively. (B and C) IFA with serum from mice immunized with either Pf cd loop (aas 178–194), Pb cd loop (aas 174–191), Pb upstream (aas 123–142), or KLH (negative control) recognizes WT P. berghei ANKA male gametocytes/gametes (B). Staining is absent in KLH control serum and in all IFAs performed with P. berghei HAP2-KO gametocytes/gametes (C). Scale bar, 5 μm. (D) Three cohorts of five mice were immunized with KLH, Pf cd loop, Pb cd loop, or Pb upstream (NB 1 mouse from Pf cd loop and Pb upstream were culled on veterinary advice before end of experiment). Each cohort was challenged with WT P. berghei 2.34, and determination of transmission blockade was performed by DFA. Individual data points represent the number of oocysts found in individual mosquitoes 12 days post-feeding. Horizontal bars indicate mean intensity of infection, while error bars indicate SEM within individual samples. Asterisks indicate p value < 0.05 Mann-Whitney U test.

Figure 2

Evaluation of Pb cd Loop and Pb Upstream Antibodies to Inhibit Ookinete Conversion (A and B) IFA of WT P. berghei ANKA and P. berghei HAP2-KO male gametes with (A) anti-Pb cd loop and (B) anti-Pb upstream. (C) Secondary-only control antibodies (green) DAPI (blue). (D) IFA of HAP2-GFP male gametes with anti-GFP shows a similar staining pattern to that obtained using anti-Pb cd loop and anti-upstream antibodies. Scale bars, 5 μm. (E and F) In vitro ookinete development assays with anti-Pb cd loop (E) and anti-upstream (F) antibodies compared to negative control antibody UPC10 at concentrations of 0, 50, 100, 250, and 500 μg/mL. ^∗p value < 0.05 paired t test. Error bars indicated SEM within individual samples.

Figure 3

Transmission-Blocking Efficacy of Pb cd Loop and Pb Upstream Antibodies in SMFA (A–C) Triplicate SMFAs with anti-Pb cd loop and anti-Pb upstream antibodies compared with negative control antibody UPC10 at concentrations of 250 and 500 μg/mL replicate 1 (A), replicate 2 (B), and replicate 3 (C). Individual data points represent the number of oocysts found in individual mosquitoes 12 days post-feeding. Horizontal bars indicate mean intensity of infection, while error bars indicate SEM within individual samples. ^∗p value < 0.05 Mann-Whitney U test. ns, p value not significant.

Figure 4

Assessment of Transmission-Blocking Efficacy of Pf cd Loop Antibodies in Gametocyte-Positive Patient Blood from Burkina Faso Using DMFA (A–C) DMFA at three naturally occurring gametocyte densities; 32 gametocytes/μl (A), 72 gametocytes/μl (B), and 112 gametocytes/μl (C) with anti-Pf cd loop antibody at concentrations of 125 and 250 μg/mL compared with negative control antibody UPC10 at 250 μg/mL. Individual data points represent the number of oocysts found in individual mosquitoes 7 days post-feeding. Horizontal bars indicate mean intensity of infection, while error bars indicate SEM within individual samples. ^∗p value < 0.05 Mann-Whitney U test. ns, not significant.