Failure of recombinant vaccinia viruses expressing Plasmodium falciparum antigens to protect Saimiri monkeys against malaria.

Saimiri sciurus monkeys were immunized at multiple sites with recombinant vaccinia viruses expressing Plasmodium falciparum antigen genes and boosted 4 weeks later. Control monkeys were immunized with a thymidine kinase-negative vaccinia virus mutant. Two weeks later, all of the monkeys were challenged by intravenous inoculation of P. falciparum (Indochina strain) parasites. A group of unimmunized monkeys was challenged in parallel. All of the monkeys that received vaccinia virus recombinants or the control virus produced good anti-vaccinia virus antibody responses. However, those that received a single construct containing ring-infected erythrocyte surface antigen (RESA) given at eight sites did not produce significant antibody to any of the three major RESA repeat epitopes after immunization but were primed for an enhanced antibody response after challenge infection with P. falciparum. Most of the monkeys produced detectable antibodies to the RESA epitopes after challenge infection. One group of monkeys was immunized with four constructs (expressing RESA, two merozoite surface antigens [MSA-1 and MSA-2], and a rhoptry protein [AMA-1]), each given at two sites. While these monkeys failed to produce significant antibody against MSA-2 or AMA-1 after immunization, they produced enhanced responses against these antigens after challenge infection. Immunization involved an allelic form of MSA-2 different from that present in the parasite challenge strain, so that the enhanced responses seen after challenge infection indicated the presence of T-cell epitopes common to both allelic forms. No groups of monkeys showed any evidence of protection against challenge, as determined by examination of the resulting parasitemias.