Persistence of Protective Immunity to Malaria Induced by DNA Priming and Poxvirus Boosting: Characterization of Effector and Memory CD8⁺-T-Cell Populations

Abstract

The persistence of immunity to malaria induced in mice by a heterologous DNA priming and poxvirus boosting regimen was characterized. Mice were immunized by priming with DNA vaccine plasmids encoding the Plasmodium yoelii circumsporozoite protein (PyCSP) and murine granulocyte-macrophage colony-stimulating factor and boosting with recombinant vaccinia encoding PyCSP. BALB/c mice immunized with either high-dose (100 g of p PyCSP plus 30 g of pGM-CSF) or low-dose (1 g of p PyCSP plus 1 g of pGM-CSF DNA) priming were protected against challenge with 50 P. yoelii sporozoites. Protection 2 weeks after immunization was 70 to 100%, persisted at this level for at least 20 weeks, and declined to 30 to 40% by 28 weeks. Eight of eight mice protected at 20 weeks were still protected when rechallenged at 40 weeks. The antigen (Ag)-specific effector CD8؉ -T-cell population present 2 weeks after boosting had ex vivo Ag-specific cytolytic activity, expressed both gamma interferon (IFN-␥) and tumor necrosis factor alpha, and constituted 12 to 20% of splenic CD8 ؉ T cells. In contrast, the memory CD8 ؉ -Ag-specific-cell population at 28 weeks lacked cytolytic activity and constituted only 6% of splenic CD8 ؉ T cells, but at the single-cell level it produced significantly higher levels of IFN-␥ than the effectors. High levels of Ag-or parasite-specific antibodies present 2 weeks after boosting had declined three-to sevenfold by 28 weeks. Low-dose priming was similarly immunogenic and as protective as high-dose priming against a 50-, but not a 250-, sporozoite challenge. These results demonstrate that a heterologous priming and boosting vaccination can provide lasting protection against malaria in this model system.

Prime-boost strategies for malaria vaccine development

“…In the DNA-MVA prime-boost regimen in mice, protective efficacy against P. berghei fell from 100% at 14 days post MVA to 60% at day 150 (J. Schneider, personal communication). Durability of protection was characterised in more detail for prime-boost regimens in the P. yoelii model, with a drop in efficacy from 70-100% at 20 weeks to 30-40% at 28 weeks (Sedegah et al, 2002).…”

Successful Induction of CD8 T Cell-Dependent Protection Against Malaria by Sequential Immunization with DNA and Recombinant Poxvirus of Neonatal Mice Born to Immune Mothers

“…We therefore used the Plasmodium yoelii murine malaria model to study the efficacy of immunizing (priming) 7-day-old neonatal mice with a DNA plasmid expressing the P. yoelii circumsporozoite protein (PyCSP) 3 and boosting them at the age of 28 days with a recombinant poxvirus expressing the PyCSP. This specific regimen was chosen for study in neonatal mice, because our extensive previous studies (4,6,14) have demonstrated it to be much more effective than any other regimen we have assessed in eliciting the CD8 T cell responses, we and others consider it to be critical for protective immunity against the liver stages of Plasmodium sp. parasites (15,16).…”

Memory CD8 T cell responses exceeding a large but definable threshold provide long-term immunity to malaria

“…Protective CD8 T cell immunity against liver-stage Plasmodium infection has been demonstrated after vaccination of rodents with irradiated or genetically attenuated parasites and after subunit vaccination against liver-stage antigens (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12). Immunity in rodents can last for 6-12 months (3,4,7,13), but in several studies also seems to wane with time (7,(14)(15)(16).…”