Induction of Plasmodium falciparum transmission-blocking antibodies by recombinant vaccinia virus

Malaria transmission-blocking activity in experimental infections of Anopheles gambiae from naturally infected Plasmodium falciparum gametocyte carriers

Experimental infections of anopheline mosquitoes were carried out with Plasmodium falciparum gametocytes from 65 naturally infected patients in Cameroon. A comparison was made between infections with blood containing autologous plasma and blood in which the plasma was replaced with plasma from a donor without previous malaria exposure. A lower infection rate was observed in 50 of 65 autologous plasma samples. Transmission was significantly blocked in 3 infections. This indicates that, in a population living in an area endemic for malaria, blood plasma factor(s) can reduce the transmission capacity of gametocyte carriers to mosquitoes.

Transmission-blocking immunity against malaria and other vector-borne diseases

Antibodies to sexual stage malaria parasites block transmission of Plasmodium by female mosquitoes. With the recent isolation of genes encoding several of the target antigens of transmission-blocking antibodies, the development of a subunit transmission-blocking vaccine against malaria is now a realistic goal.

Structure and expression of a post-transcriptionally regulated malaria gene encoding a surface protein from the sexual stages of Plasmodium berghei

The sexual stage-specific protein Pbs21 of the rodent malaria parasite Plasmodium berghei, expressed on the surface of zygotes and ookinetes, has been shown to induce an effective and long-lasting transmission blocking immunity. The gene encoding Pbs21 was cloned by screening a cDNA library prepared from enriched zygotes and ookinetes using the monoclonal antibody 13.1.15, which is capable of blocking subsequent parasite sexual development in the mosquito vector. The Pbs21 gene encoded a protein of 213 amino acids which contained a putative amino-terminal signal sequence and a putative carboxy-terminal hydrophobic membrane anchor. The amino-acid sequence was characterised by a large number of cysteine residues which were organized into 4 epidermal growth factor-like domains. The spacing of the cysteine residues was highly conserved when compared to the 25-kDa ookinete proteins of Plasmodium falciparum (Pfs25), Plasmodium reichenowi (Prs25) and Plasmodium gallinaceum (Pgs25) which were approximately 45%, 45% and 40% homologous to Pbs21 respectively. The gene is located on chromosome 5 and cross-hybridizes to a similarly defined gene unit in the other rodent malaria species Plasmodium chabaudi, Plasmodium vinckei and Plasmodium yoelii. The gene is internally disposed and not in the subtelomeric region of chromosome 5. The gene is transcribed in a stage-specific manner giving rise to an abundant 1.5-kb transcript. This mRNA is synthesised in the precursor cells to female gametes (gametocytes) however the protein is observed only after activation of the gametes, suggesting that translation of the mRNA is controlled by a post-transcriptional process. The Pbs21 gene and the P. berghei parasite system provide an excellent vehicle for the study of stage-specific transcriptional and post-transcriptional control in malaria.

Pfs2400 can mediate antibody-dependent malaria transmission inhibition and may be the Plasmodium falciparum 11.1 gene product

Monoclonal antibodies (mAb) have been raised against Plasmodium falciparum gametocyte stage protein extracts, in an effort to identify novel parasite antigens that might mediate malaria transmission-blocking immunity. mAb 1A1 identified Pfs2400, a sexual stage-specific antigen of greater than 2 megadaltons, that is associated with the outer leaflet of the parasitophorous vacuole membrane in mature circulating gametocyte-infected red blood cells. Upon induction of gametogenesis, Pfs2400 partitions between the gamete plasmalemma and the degenerating erythrocyte membrane. The antigen is no longer detectable in the fully emerged gamete. mAb 1A1 dramatically reduces the number of oocysts formed in P. falciparum gametocyte-fed mosquitoes. The cognate antigen is probably the product of the Pf11.1 gene (Scherf et al. 1988. EMBO [Eur. Mol. Biol. Organ.]J. 7:1129) on the basis that a peptide composed of two copies of the degenerate nine amino acid repeat sequence in the Pf11.1 protein, can inhibit binding of mAb1A1 to the native antigen. The mechanism of transmission inhibition mediated by the Pfs2400 is discussed.

Pgs28 belongs to a family of epidermal growth factor-like antigens that are targets of malaria transmission-blocking antibodies

Although Pgs28, a 28-kD surface protein of Plasmodium gallinaceum oökinetes, was previously thought not to be a target of transmission-blocking antibodies, we found that polyclonal antisera to Pgs28 completely blocked parasite infectivity to Aedes aegypti mosquitoes. Antisera raised against reduced Pgs28 were less effective in blocking transmission than were antisera to nonreduced Pgs28; thus, the target epitope(s) of transmission-blocking antibodies appears to be conformation dependent. In stage-specific assays, polyclonal antisera impaired the in vitro transformation of zygotes to mature oökinetes, as well as the in vivo development of mature oökinetes to oöcysts. Using microsequence of immunoaffinity-purified Pgs28, we cloned the 666-bp open reading frame of the Pgs28 gene. The deduced amino acid sequence of Pgs28 is strikingly similar to that of a P. gallinaceum zygote surface protein, Pgs25, and its P. falciparum analogue, Pfs25. Pgs28, like Pgs25 and Pfs25, has a presumptive secretory signal sequence, followed by four epidermal growth factor-like domains, and a terminal hydrophobic region.

Malaria transmission-blocking vaccines

Antibodies to surface proteins of the sexual stages of Plasmodium falciparum block completely the transmission of these malaria parasites. Transmission-blocking vaccines therefore represent a powerful and novel approach to controlling the spread of this lethal disease.

Transmission blocking antibody of the Plasmodium falciparum zygote/ookinete surface protein Pfs25 also influences sporozoite development

The Plasmodium falciparum zygote/ookinete surface protein, Pfs25, persists in the oocyst wall throughout its development. Anti-25 kD transmission blocking antibody, given to infected Anopheles stephensi or A. gambiae mosquitoes in an additional bloodmeal, 3-6 days after being fed gametocyte infected blood, penetrated the oocyst and reacted with the 25 kD protein within it. This reaction caused a significant reduction in the number of developing sporozoites. Mouse serum containing antibodies raised by immunization with a recombinant 25 kD yeast product showed a similar effect.

A family of erythrocyte binding proteins of malaria parasites

Malaria erythrocyte binding proteins use the Duffy blood group antigen (Plasmodium vivax and Plasmodium knowlesi) and sialic acid (Plasmodium falciparum) on the erythrocyte surface as receptors. We had previously cloned the one P. vivax gene, the one P. falciparum gene, and part of one of the three P. knowlesi genes encoding these erythrocyte binding proteins and described the homology between the P. knowlesi and P. vivax genes. We have completed the cloning and sequencing of the three P. knowlesi genes and identified introns in the P. vivax and P. falciparum genes that correct the previously published deduced amino acid sequences. All have similar structures, with one or two exons encoding the signal sequence and the erythrocyte binding domain, an exon encoding the transmembrane domain, and two exons encoding the cytoplasmic domain with the exception of the P. knowlesi beta gene. The regions of amino acid sequence homology among all the genes are the 5' and 3' cysteine-rich regions of the erythrocyte binding domain. On the basis of gene structure and amino acid homology, we propose that the Duffy binding proteins and the sialic acid binding protein are members of a gene family. The level of conservation (approximately 70%) of the deduced amino acid sequences in the 5' cysteine-rich region between the P. vivax protein and the three P. knowlesi proteins is as great as between the three P. knowlesi proteins themselves; the P. knowlesi beta protein just 3' to this cysteine-rich region is homologous to the P. vivax protein but not to the other P. knowlesi proteins. Conservation of amino acid sequences among these organisms, separated in evolution, may indicate the regions where the adhesin function resides.

Recombinant Pfs25 protein of Plasmodium falciparum elicits malaria transmission-blocking immunity in experimental animals

Pfs25 is a sexual stage antigen of Plasmodium falciparum that is expressed on the surface of zygote and ookinete forms of the parasite. Monoclonal antibodies directed against native Pfs25 can block completely the development of P. falciparum oocysts in the midgut of the mosquito vector. Thus, this 25-kD protein is a potential vaccine candidate for eliciting transmission-blocking immunity in inhabitants of malaria endemic regions. We have synthesized, by secretion from yeast, a polypeptide analogue of Pfs25 that reacts with conformation-dependent monoclonal antibodies, and elicits transmission-blocking antibodies when used to immunize mice and monkeys in conjunction with a muramyl tripeptide adjuvant. Our results suggest the further evaluation of recombinant DNA-derived Pfs25 in transmission-blocking vaccination studies in humans.