Malaria Branch, and Office of the Director, Division of Parasitic Diseases, Center for Infectious Diseases, Centers for Disease Control, Chiron Corporation, Department of Immunology, Walter Reed Army Institute of Research, Atlanta, Georgia
The role of circulating peripheral blood momonuclear cells (PBMC) in mediating protective immunity was examined during an immunization trial in Saimiri monkeys. Three engineered constructs representing different but overlapping regions of the circumsporozoite (CS) protein of Plasmodium vivax were used to immunize the Saimiri monkeys. Monkeys were randomly placed into three immunization groups: rPvCS2, rPvCS3, and LCV3 (representing three different but overlapping portions of the P. vivax CS protein) and two control groups: an alum adjuvant control group and an unimmunized control group. Collections of PBMC were made throughout the study at weeks 0, 2, 8, challenge (week 16), and two weeks after challenge. Proliferative responses to all immunogens and pokeweed mitogen were measured in all monkeys. Fourteen of 18 monkeys immunized with either rPvCS2 or rPvCS3 responded on the day of challenge to the appropriate immunogen with a stimulation index > 2. Immunization with LCV3, which represents the repeat region only, elicited a specific response in only one monkey. However, monkeys in both control groups also responded to rPvCS2 and rPvCS3, regardless of immunization, suggesting the presence of epitopes in rPvCS2 and rPvCS3 capable of associating with differing MHC antigens. Furthermore, the frequency of these cells in the periphery was increased by immunization, as demonstrated by a greater number of responding monkeys in the rPvCS2 and rPvCS3 immunized groups.