PROTECTION OF AOTUS MONKEYS AFTER IMMUNIZATION WITH RECOMBINANT ANTIGENS OF PLASMODIUM-FALCIPARUM

The genus Aotus spp. (owl monkey) is one of the WHO recommended experi mental models for Plasmodium falciparum blood stage infection, especia lly relevant for vaccination studies with asexual blood stage antigens of this parasite. For several immunization trials with purified recom binant merozoite/schizont antigens, the susceptible Aotus karyotypes I I, III, IV and VI were immunized with Escherichia coli derived fusion proteins containing partial sequences of the proteins MSAI (merozoite surface antigen 1), SERP (serine-stretch protein) and HRPII (histidine alanine rich protein II) as well as with a group of recombinant antig ens obtained by an antiserum raised against a protective 41 kD protein band. The subcutaneous application (3x) of the antigen preparations w as carried out in intact animals followed by splenectomy prior to chal lenge, in order to increase the susceptibility of the experimental hos ts to the parasite. A partial sequence of HPPII, the combination of th ree different fusion proteins of the 41 kD group and a mixture of two sequences of SERP in the presence of a modified Al(OH)3 adjuvant confe rred significant protection against a challenge infection with P. falc iparum blood stages (2-5 x 10(6)) i. RBC). Monkeys immunized with the MS2-fusion protein carrying the N-terminal part of the 195 kD precurso r of the major merozoite surface antigens induced only marginal protec tion showing some correlation between antibody titer and degree of par asitaemia. Based on the protective capacity of these recombinant antig ens we have expressed two hybrid proteins (MS2/SERP/HRPII and SERP/MSA I/HRPII) in E. coli containing selected partial sequences of SERP, HRP II and MSAI. Antibodies raised against both hybrid proteins in rabbits and Aotus monkeys recognize the corresponding schizont polypeptides. In two independent immunization trials using 13 animals (age 7 months to 3 years) we could show that immunization of Aotus monkeys with eith er of the two hybrid proteins administered in an oil-based well tolera ted formulation protected the animals from a severe experimental P. fa lciparum (strain Palo Alto) infection.