No abstract

The development of an effective preerythrocytic vaccine against Plasmodium falciparum malaria is likely to require inclusion of components from several preerythrocytic antigens. The association of HLA-B53 with resistance to severe malaria in West Africa provided evidence that HLA class I-restricted CD8 ؉ T-cell responses play a role in protective immunity in African children, supporting data from rodent models of malaria. Previously, a single epitope from liver-stage-specific antigen 1 (LSA-1) has been shown to be recognized by HLA-B53-specific cytotoxic T lymphocytes (CTL), but HLA-B53 epitopes were not found in four other antigens. In this study we measured CTL responses to peptides from the recently sequenced antigen liver-stage antigen 3 (LSA-3) and identified in it a new epitope restricted by HLA-B53. Several CTL epitopes restricted by other class I types were also identified within LSA-3 in studies in The Gambia and Tanzania. CTL were also identified to an additional P. falciparum antigen, exported protein 1 (Exp-1), the homologue of which is a protective antigen in a rodent model of malaria. These findings emphasize the diversity of P. falciparum antigens recognized by CD8 ؉ T cells in humans and support the inclusion of components from several antigens in new CTL-inducing vaccines against malaria.Preerythrocytic immunity to Plasmodium falciparum infection is mediated in part by T lymphocytes acting against the liver-stage parasite. These T cells must recognize parasitederived peptides on infected host cells in the context of major histocompatibility complex antigens. T-cell-mediated immunity appears to target several parasite antigens expressed during the sporozoite and liver stages of the infection (13). Complete protection against sporozoite challenge observed in irradiated Plasmodium berghei sporozoite-immunized mice and P. falciparum sporozoite-immunized humans results from immune responses to antigens expressed by the parasite at the preerythrocytic stages of its life cycle (20). Although antibody and CD4 ϩ and CD8 ϩ T cells all have been implicated in preerythrocytic immunity, protection mainly or entirely dependent on CD8 ϩ T cells (25, 27) is found in several rodent host-parasite combinations. Recently, it has been found that it is possible to immunize perforin-and Fas-deficient mice with irradiated Plasmodium berghei sporozoites, indicating that CD8 ϩ T-cell-mediated protection against this parasite is probably not related to the lytic functions of these cells (23). However, in humans, lysis assays correlate well with other measures of CD8ϩ T-cell function, such as gamma interferon (IFN-␥) secretion (15).We (1, 12, 17, 21) and others (4, 6, 7, 26) have previously identified in malaria-endemic areas several epitopes of cytotoxic T-lymphocytes (CTL), restricted by a variety of HLA class I molecules, in each of five preerythrocytic P. falciparum antigens: circumsporozoite protein, thrombosponding related adhesion protein, liver-stage antigen 1 (LSA-1), Pfs16, and sporozoite threonine and asparagine...

Field studies of malaria in endemic areas frequently. use the presence or levels of parasitaemia, together with the measurement of fever, as the primary criteria Rlth which to identify cases. However, since malaria cases do not always present with measurable fever, and since asymptomatic parasitaemia occurs, additional episode markers might be useful epidemiological tools. We have measured the C-reactive protein and haptoglobin levels in paediatric patients presenting to a village health post in the Kilombero District in Tanzania and in convalescent sera from the same patients, in order to evaluate these acute-phase reactants as alternative markers of Plasmodium falciparum episodes. Among afebrile patients, C-reactive protein levels were highly correlated with parasite density. High C-reactive protein levels are therefore probably indicative of recent clinical malaria cpisodcs in currently afebrile individuals with high parasite densities. An appropriate case definition for malaria in epidemiological studies in endemic areas might therefore be hyperparasitaemia accompanied by either, or both, measurable fever and raised C-reactive protein levels. This would give less biased estimates of the overall burden of malaria morbidity than does a definition which requires measurable fever. Levels of haptoglobin were highly negatively correlated with parasitaemia, but did not appear to be useful episode markers because this correlation was probably not related to acute morbidity. However, haproglobin can be useful to assess at community level The impact of interventions on parasitaemia.

Studies in The Gambia have provided indirect evidence that cytotoxic T lymphocytes (CTL) play a protective role against malaria in humans and recently, using allele-specific HLA class I peptide motifs, several peptide epitopes for CTL in four pre-erythrocytic Plasmodium falciparum antigens have been identified in naturally exposed Gambians. However, CTL levels were low, suggesting that boosting these low levels by immunization might provide substantial protection. In the Kilombero valley of Tanzania, malaria transmission is holoendemic and 300 times more intense than in The Gambia. We report here that several of the epitopes identified in The Gambia are also recognized in naturally exposed, partially immune Tanzanian adults and that levels of CTL are similar to or slightly higher than in Gambian subjects, despite the much higher inoculation rate. We report a new HLA-A2.1-restricted epitope from the thrombospondin-related anonymous protein (TRAP) and we demonstrate that peptide epitopes in TRAP are naturally processed for recognition by CTL from naturally exposed humans. The common allele of a variable HLA-B7-restricted epitope in the circumsporozoite protein behaved as an altered peptide ligand (APL) with respect to CTL cognate for a rarer allelic variant of this epitope, suggesting that APL antagonism may occur in natural CTL responses to P. falciparum. The moderate levels of CTL observed, even in this area of intense malaria transmission, points to the need to assess candidate vaccines aimed at increasing CTL levels.

The SPf66 synthetic vaccine is safe and partly efficacious against Plasmodium falciparum malaria among children 1-5 years old. The estimated vaccine efficacy [VE] for all clinical episodes over a period of 18 months after the third dose is 25% (95% confidence interval [CI], 1%-44%; P = .044). The observed temporal variations in efficacy could have been due to chance (likelihood ratio chi 2 = 13.8, 8 df; P = .086). Efficacy against clinical malaria did not vary significantly with age (chi 2 = 1.07, 4 df; P = .90). Overall parasite density was 21% lower in vaccine recipients than in the placebo group (95% CI, 0%-38%; P = .044). Further development of SPf66 may require trials to evaluate safety, immunogenicity, and efficacy when administered in the first year of life, together with other vaccines contained in the Expanded Programme of Immunization schedule.