A Plasmodium falciparum malaria vaccine candidate which contains epitopes from the circumsporozoite protein and a blood stage antigen, 5.1

Abstract

The previously described Plasmodium falciparum blood stage antigen, 5.1 (also referred to as exp-1) was expressed at a high level in Escherichia coli. Saimiri monkeys immunised with purified recombinant antigen 5.1 were partially protected from P. falciparum blood stage parasite challenge. The gene coding for 5.1 was combined with DNA coding for an (Asn-Ala-Asn-Pro)19 sequence (abbreviated (NANP)19 in the one-letter amino acid code). To facilitate purification of the recombinant protein, DNA coding for a hexahistidine (His6) sequence was introduced at the 5' end of the gene (proteins containing His6 have high affinity for Ni(2+)-chelate columns even in the presence of 6 M guanidine HCl). The recombinant protein, His6-5.1-(NANP)19 with an apparent molecular size of 40 kDa could be highly purified by a combination of 4 steps: (1) release and solubilization of the recombinant fusion protein from E. coli in the presence of 6 M guanidine-HCl; (2) precipitation of over 60% of the bacterial proteins by the addition of ammonium sulphate to 50% saturation; (3) affinity chromatography on a Ni(2+)-chelate column in the presence of 6 M guanidine-HCl; (4) adsorption onto a cation exchange resin in the presence of 6 M urea, and elution with an increasing NaCl gradient. Compared with the previously tested tetanus toxoid-(NANP)3 malaria vaccine, this protein elicits an anti-(NANP)n response which more closely resembles that evoked by native sporozoites. The recombinant vaccine also induces the production of antibodies against the blood stages of the malaria parasite.