Protective immunization with a novel membrane protein of Plasmodium yoelii-infected erythrocytes

Immunization with a particulate fraction of blood-stage antigens was shown previously to protect mice against Plasmodium yoelii malaria, To identify a ntigens inducing the protective response, sera from immunized mice were use d to screen a P. yoelii cDNA expression library. Sequence analysis of one 2 ,6-kb cDNA clone indicated that the identified gene, pypag-1, encoded a nov el plasmodial antigen. Two nonoverlapping regions of pypag-1 were expressed in Escherichia coli. The first recombinant antigen, pAg-1N, contained the N-terminal 337 residues, which included a putative transmembrane domain and a region relatively rich in tryptophan residues. The second recombinant an tigen, pAg-1C, contained the remaining C-terminal 211 residues, which inclu ded 31 copies of a 5-amino-acid degenerative repeat. Immunoblot studies usi ng rabbit antiserum raised against recombinant pAg-1N showed that the nativ e pypAg-1 protein migrated at approximately 98 kDa, considerably slower tha n its predicted molecular mass of 66 kDa, Immunofluorescence studies locali zed the expression of the native pypAg-1 protein both to the cytoplasm and at the surface of P, yoelii-infected erythrocytes, Immunization with either pAg-1N or pAg-1C induced a four- to sevenfold reduction in P. yoelii blood -stage parasitemia, As such, pypAg-1 appears to contain at least two distin ct protective epitopes, To our knowledge, this is the first characterizatio n of a protective antigen of P. yoelii that is associated with the erythroc yte membrane.