Saccharomyces cerevisiae recombinant Pfs25 adsorbed to alum elicits antibodies that block transmission of Plasmodium falciparum

Abstract

Antibodies to Pfs25, a cysteine-rich 25-kDa protein present on the surface ofPlasmodiumfalciparum zygotes, can completely block the transmission of malaria parasites when mixed with infectious blood and fed to mosquitoes through a membrane feeding apparatus. Recently, a polypeptide analog, Pfs25-B, secreted from recombinant Saccharomyces cerevisiae was found to react with conformation-dependent, transmission-blocking monoclonal antibodies and to elicit transmission-blocking antibodies in experimental animals when emulsified in either Freund's or muramyl tripeptide adjuvant. In this study, Pfs25-B adsorbed to alum induced transmission-blocking antibodies in both rodents and primates. Bacterially produced Pfs25, however, did not elicit complete transmission-blocking antibodies in rodents. Furthermore, unlike monoclonal antibodies to Pfs25, which block transmission only after ookinete development, antisera to Pfs25-B adsorbed to alum appeared to block the in vivo development of zygotes to ookinetes as well.

Immunization of Mice with DNA-Based Pfs25 Elicits Potent Malaria Transmission-Blocking Antibodies

“…This suggests that those proteins are synthesized in the appropriate conformations in the DNA vector. Previous studies have shown this to be a critical factor in eliciting TrB antibodies (14).…”

“…This was especially important for the use of Pfs25 as an immunogen since previous studies have clearly established that disulfide-bonded conformational epitopes on Pfs25 are targets of TrB antibodies (3). In fact, E. coli-derived recombinant Pfs25 protein could not elicit complete TrB antibodies in mice since it did not recreate the known conformational epitope(s) that is the target of TrB MAbs (14). The only forms of rPfs25 shown to be effective have been those expressed in vaccinia virus or in yeast (3,9,14).…”

“…Additionally, plasmids used for DNA immunization have been shown to contain immunostimulatory sequences (34), and, thus, such vaccines could be effective without the need for adjuvant formulation. It has previously been shown that the immune response to some of the malarial TrB vaccine candidates was adjuvant dependent (14,25). DNA immunization could thus circumvent the problem of the paucity of adjuvants available for use in humans.…”

“…Some of these, like Pfs230, Pfs48/45, and Pfg27, are synthesized predominantly in the gametocytes (vertebrate host) with some residual expression seen after gametogenesis and fertilization (19,32), while others, like Pfs25 and Pfs28 (9,12,13), are expressed only after initiation of gametogenesis and fertilization in the vector host. Studies on purified Pfs25 recombinant proteins expressed in Escherichia coli or yeast have demonstrated a need for proper conformational folding of target epitopes, administration of adjuvants, and multiple immunizations (3,9,14). In view of the fact that DNA immunization can overcome some of these immunogenicity requirements, we combined the genes coding for two target antigens found on different sexual stages, Pfg27 in gametocytes and Pfs25 in zygotes, and evaluated their potential as experimental DNA-based TrB vaccines as single immunogens, coimmunogens, and a hybrid gene fusion.…”

Structural and Antigenic Properties of Merozoite Surface Protein 4 of Plasmodium falciparum

“…This will be particularly so for antibodies to the EGF-like domain. Whereas the EGFlike domain of MSP1 has been expressed in a conformationally correct form in E. coli (11), it has been necessary to express other proteins containing EGF-like domains in yeast (4,22). It would appear from these studies that MSP4 will require the use of DNA immunization approaches or appropriate eukaryotic host-vector combinations.…”