Immunization of Saimiri sciureus monkeys with a recombinant hybrid protein derived from the Plasmodium falciparum antigen glutamate-rich protein and merozoite surface protein 3 can induce partial protection with Freund and Montanide ISA720 adjuvants

Abstract

The immunogenicity and efficacy of a hybrid recombinant protein derived from the N-terminal end of the glutamate-rich protein (GLURP) and the C-terminal portion of the merozoite surface protein 3 (MSP3) of Plasmodium falciparum was evaluated in Saimiri sciureus monkeys. The GLURP/MSP3 hybrid protein, expressed in Lactococcus lactis, was administered in association with alum, Montanide ISA720, or complete or incomplete Freund adjuvant (CFA/IFA) in groups of five animals each. The three formulations were shown to be immunogenic, but the one with alum was shown to be weak compared to the other two, particularly CFA/IFA, which provided very high antibody titers (enzyme-linked immunosorbent assay titers of >3,000,000 and immunofluorescence antibody test titers of 6,400). After a challenge infection with P. falciparum FUP strain, all five monkeys from the GLURP/MSP3-alum group showed a rapid increase in parasitemia, reaching 10% and were treated early. The two monkeys with the highest antibody titers in group GLURP/MSP3-Montanide ISA720 had a delay in the course of parasitemia and were treated late due to a low hematocrit. In the GLURP/MSP3-CFA/IFA group, parasitemia remained below this threshold in four of the five animals and, after it reached a peak, parasitemia started to decrease and monkeys were treated late. When all animals were grouped according to the outcome, a statistically significant association between high antibody titers and partial protection was observed. The challenge infection boosted the antibody titers, and the importance of this event for vaccine efficacy in areas where this parasite is endemic is discussed. In conclusion, these data suggest that GLURP and MSP3 can induce protection against malaria infection if antibodies are induced at properly high titers.

FIG. 1.

Coomassie blue-stained 12% polyacrylamide gel of purified GLURP/MSP3 hybrid protein (lane 1) and of L. lactis supernatant proteins (lane 2) used in the immunization experiments.

FIG. 2.

Follow-up of antibody titers (ELISA) against the hybrid GLURP/MSP3 protein during immunization of S. sciureus monkeys with different antigen-adjuvant formulations. Black arrows indicate immunization injection points; gray arrows indicate the challenge infection point. The code names and associate symbols for each monkey are shown.

FIG. 3.

Titers of antibody (as determined by ELISA) to GLURP_27-500 (GLURP-R0) and MSP3_212-380 (MSP3-C-terminal) proteins after three injections of the hybrid GLURP/MSP3-derived or control formulations in S. sciureus monkeys.

FIG. 4.

Course of parasitemia of S. sciureus monkeys immunized with different formulations containing the different GLURP/MSP3 or control formulations, inoculated with 50,000 P. falciparum-parasitized red blood cells (FUP strain). The code names of each monkey and associated symbol are shown.

FIG. 5.

Follow-up of titers of antibody (as determined by ELISA) to the hybrid GLURP/MSP3 protein during challenge infection of S. sciureus monkeys with P. falciparum FUP strain.