Vaccination of experimental monkeys against Plasmodium falciparum: a possible safe adjuvant.

Owl monkeys (Aotus trivirgatus griseimembra) were effectively immunized against a human malaria parasite, Plasmodium falciparum. Two injections of antigen, primarily mature segmenters with fully developed merozoites, mixed with adjuvant (6-O-stearoyl-N-acetylmuramyl-L-alanyl-D-isoglutamine and liposomes) were administered intramuscularly at a 4-week interval. Approximately 2 weeks after the second vaccination, the monkeys were challenged with the homologous strain of P. falciparum. All immunized monkeys survived the challenge. The substitution of Freund's complete adjuvant is an encouraging step toward the development of an effective and safe vaccine for human malaria.

Effects of red cell extract on in vitro growth and multiplication of malarial parasites.

An extract of red blood cells was prepared from monkey blood. Red cell extract (RCE) of Aotus monkeys was beneficial for the in-vitro cultivation of 2 strains of P. falciparum. The active material(s) in Aotus RCE was not sedimented by centrifugation at 34,800 g for 1 hr. Rhesus monkey RCE improved the growth of P. knowlesi for in-vitro cultivation through 4 schizogonous cycles (88 hr).

In vitro production and partial purification of Plasmodium falciparum antigen.

A simple technique for achieving high yields of Plasmodium falciparum parasites on a continuous basis is described. The technique is applicable in any laboratory. The culture apparatus is also simple and inexpensive and allows multiple cultures to be run simultaneously. A total of approximately 1-2 x 10(9) parasites can be harvested per culture flask per week requiring the use of only 40.0 ml of culture medium (RPMI 1640), 5.0 ml of human sera, and 2.0 ml of outdated human whole blood. P. falciparum parasites (segmenters containing individual merozoites) are cultured in vitro and concentrated 10-15 fold through the use of discontinuous bovine serum albumin gradient centrifugation.Commercial saponin is purified on a Sephadex G-25 column. The haemolytic effect of purified saponin related to human red blood cell concentration is studied. Preliminary observations on the action of some synthetic detergents and enzymes on human erythrocytes are also reported. Purified saponin is used to lyse red blood cells infected with in vitro cultured P. falciparum for the preparation of merozoite antigen. Further purification of parasite material is carried out by sucrose density gradient centrifugation.

Immunization of experimental monkeys against Plasmodium falciparum: use of synthetic adjuvants.

The replacement of Freund's adjuvant by a possible safe adjuvant for effective immunization of owl monkeys (Aotus trivirgatus griseimembra) against a human malaria parasite, Plasmodium falciparum, has been investigated. Experiments involved the use of two synthetic adjuvants: MDP (N-acetylmuramyl-L-alanyl-D-isoglutamine) and stearoyl-MDP (6-O-stearoyl-N-acetylmuramyl-L-alanyl-D-isoglutamine). In both cases, P. falciparum merozoites obtained through short-term in vitro cultivation were used as antigen. MPD was used as adjuvant in 5 owl monkeys; 2 control monkeys died and of the 3 experimental monkeys only 1 survived. In contrast, in another experiment where stearoyl-MDP was used as adjuvant, there was 100% protection of 4 immunized monkeys against a challenge with the homologous strain of P. falciparum. The results of the second experiment are encouraging for the development of an effective and safe vaccine for human malaria.