Protective immunity induced in Aotus monkeys by a recombinant SERA protein of Plasmodium falciparum: further studies using SERA 1 and MF75.2 adjuvant.

We describe the third of three vaccination trials of Panamanian Aotus monkeys with a recombinant blood-stage antigen derived from the malaria parasite Plasmodium falciparum. Immunization was performed with an N-terminal region of the SERA antigen (serine repeat antigen protein), SERA 1, that contains a 262-amino-acid fragment including amino acids 24 to 285 of the 989-amino-acid SERA protein. Vaccinations were carried out with the recombinant protein mixed with either Freund's, MF75.2, or MF59.2 adjuvant. A control group that did not receive SERA 1 but only MF75.2 adjuvant was included. Monkeys vaccinated with the antigen MF59.2 mixture produced low anti-SERA 1 titers and were not protected. Monkeys vaccinated with antigen and Freund's adjuvant had, in general, a higher average anti-SERA 1 titer (107,278) than did monkeys immunized with SERA 1 and MF75.2 (40, 143), yet monkeys in both groups were well protected. Monkeys that received only MF75.2 developed neither detectable anti-SERA 1 nor anti-P. falciparum antibodies prior to or 10 days after parasite challenge, yet were apparently protected against infection. Monkeys vaccinated with either SERA 1 and Freund's, SERA 1 and MF75.2, or MF75.2 alone and that had been challenged but did not develop a countable parasitemia were treated with a curative dose of mefloquine 100 days after parasite challenge and then rechallenged 40 days later. None of the five rechallenged monkeys that had originally received SERA 1 and Freund's developed a countable parasitemia. Only one of five rechallenged monkeys that originally received SERA 1 and MF75.2 developed a high countable parasitemia, while two animals developed a barely countable parasitemia. Four of the rechallenged monkeys that had originally received only MF75.2 developed a moderate to high countable parasitemia. The results indicate that vaccination with SERA 1 and either Freund's or MF75.2 adjuvant provides protection and vaccination with MF75.2 alone can provide a temporary protection unrelated to the induction of anti-SERA 1 or antimalarial antibodies.