SUMMARY: Naturally acquired immunity to falciparum malaria protects millions of people routinely exposed toPlasmodium falciparuminfection from severe disease and death. There is no clear concept about how this protection works. There is no general agreement about the rate of onset of acquired immunity or what constitutes the key determinants of protection; much less is there a consensus regarding the mechanism(s) of protection. This review summarizes what is understood about naturally acquired and experimentally induced immunity against malaria with the help of evolving insights provided by biotechnology and places these insights in the context of historical, clinical, and epidemiological observations. We advocate that naturally acquired immunity should be appreciated as being virtually 100% effective against severe disease and death among heavily exposed adults. Even the immunity that occurs in exposed infants may exceed 90% effectiveness. The induction of an adult-like immune status among high-risk infants in sub-Saharan Africa would greatly diminish disease and death caused byP. falciparum. The mechanism of naturally acquired immunity that occurs among adults living in areas of hyper- to holoendemicity should be understood with a view toward duplicating such protection in infants and young children in areas of endemicity.

During 1989–1999, 11 volunteers were immunized by the bites of 1001–2927 irradiated mosquitoes harboring infectious sporozoites of Plasmodium falcipa

CD8 ⁺ cytotoxic T lymphocytes (CTLs) are critical for protection against intracellular pathogens but often have been difficult to induce by subunit vaccines in animals. DNA vaccines elicit protective CD8 ⁺ T cell responses. Malaria-naı̈ve volunteers who were vaccinated with plasmid DNA encoding a malaria protein developed antigen-specific, genetically restricted, CD8 ⁺ T cell–dependent CTLs. Responses were directed against all 10 peptides tested and were restricted by six human lymphocyte antigen (HLA) class I alleles. This first demonstration in healthy naı̈ve humans of the induction of CD8 ⁺ CTLs by DNA vaccines, including CTLs that were restricted by multiple HLA alleles in the same individual, provides a foundation for further human testing of this potentially revolutionary vaccine technology.

Abs are central to malaria immunity, which is only acquired after years of exposure to Plasmodium falciparum ( Pf ). Despite the enormous worldwide burden of malaria, the targets of protective Abs and the basis of their inefficient acquisition are unknown. Addressing these knowledge gaps could accelerate malaria vaccine development. To this end, we developed a protein microarray containing ∼23% of the Pf 5,400-protein proteome and used this array to probe plasma from 220 individuals between the ages of 2–10 years and 18–25 years in Mali before and after the 6-month malaria season. Episodes of malaria were detected by passive surveillance over the 8-month study period. Ab reactivity to Pf proteins rose dramatically in children during the malaria season; however, most of this response appeared to be short-lived based on cross-sectional analysis before the malaria season, which revealed only modest incremental increases in Ab reactivity with age. Ab reactivities to 49 Pf proteins measured before the malaria season were significantly higher in 8–10-year-old children who were infected with Pf during the malaria season but did not experience malaria ( n = 12) vs. those who experienced malaria ( n = 29). This analysis also provided insight into patterns of Ab reactivity against Pf proteins based on the life cycle stage at which proteins are expressed, subcellular location, and other proteomic features. This approach, if validated in larger studies and in other epidemiological settings, could prove to be a useful strategy for better understanding fundamental properties of the human immune response to Pf and for identifying previously undescribed vaccine targets.

Sterile protective immunity against challenge with Plasmodium spp. sporozoites can be induced in multiple model systems and humans by immunization with radiation-attenuated Plasmodium spp. sporozoites. The infected hepatocyte has been established as the primary target of this protection, but the underlying mechanisms have not been completely defined. Abs, CD8⁺ T cells, CD4⁺ T cells, cytokines (including IFN-γ and IL-12), and NO have all been implicated as critical effectors. Here, we have investigated the mechanisms of protective immunity induced by immunization with different vaccine delivery systems (irradiated sporozoites, plasmid DNA, synthetic peptide/adjuvant, and multiple Ag peptide) in genetically distinct inbred strains, genetically modified mice, and outbred mice. We establish that there is a marked diversity of T cell-dependent immune responses that mediate sterile protective immunity against liver-stage malaria. Furthermore, we demonstrate that distinct mechanisms of protection are induced in different strains of inbred mice by a single method of immunization, and in the same strain by different methods of immunization. These data underscore the complexity of the murine host response to a parasitic infection and suggest that an outbred human population may behave similarly. Data nevertheless suggest that a pre-erythrocytic-stage vaccine should be designed to induce CD8⁺ T cell- and IFN-γ-mediated immune responses and that IFN-γ responses may represent an in vitro correlate of pre-erythrocytic-stage protective immunity.