Immunity to malaria after administration of ultra-low doses of red cells infected with Plasmodium falciparum

The pathophysiology of falciparum malaria

Falciparum malaria is a complex disease with no simple explanation, affecting organs where the parasite is rare as well as those organs where it is more common. We continue to argue that it can best be understood in terms of excessive stimulation of normally useful pathways mediated by inflammatory cytokines, the prototype being tumor necrosis factor (TNF). These pathways involve downstream mediators, such as nitric oxide (NO) that the host normally uses to control parasites, but which, when uncontrolled, have bioenergetic failure of patient tissues as their predictable end point. Falciparum malaria is no different from many other infectious diseases that are clinically confused with it. The sequestration of parasitized red blood cells, prominent in some tissues but absent in others with equal functional loss, exacerbates, but does not change, these overriding principles. Recent opportunities to stain a wide range of tissues from African pediatric cases of falciparum malaria and sepsis for the inducible NO synthase (iNOS) and migration inhibitory factor (MIF) have strengthened these arguments considerably. The recent demonstration of bioenergetic failure in tissue removed from sepsis patients being able to predict a fatal outcome fulfils a prediction of these principles, and it is plausible that this will be demonstrable in severe falciparum malaria. Understanding the disease caused by falciparum malaria at a molecular level requires an appreciation of the universality of poly(ADP-ribose) polymerase-1 (PARP-1) and Na(+)/K(+)-ATPase and the protean effects of activation by inflammation of the former that include inactivation of the latter.

Nitric oxide production and mononuclear cell nitric oxide synthase activity in malaria-tolerant Papuan adults

Individuals living in regions of intense malaria transmission exhibit natural immunity that allows them to be without fever and other symptoms for most of the time despite frequent parasitization. Although this tolerance of parasitemia appears to be more effective in children than in adults (as evidenced by lower parasitemia fever thresholds with age), adults do exhibit a degree of tolerance but the mechanism(s) underlying this are unclear. Asymptomatic malaria-exposed children have higher levels of nitric oxide (NO) than children with severe disease, and NO has been proposed as a mediator of malarial tolerance. However, the ability of highly malaria-exposed asymptomatic adults to generate high-level basal NO is unknown, as is the relationship between NO and malaria tolerance in adults. The relationship between NO and malaria parasitemia was therefore determined in asymptomatic adults from Papua, Indonesia. Adults with Plasmodium falciparum parasitemia had markedly increased basal systemic NO production relative to aparasitemic Papuan controls, who in turn produced more NO than healthy controls from a region without malaria. Immunoglobulin E levels were universally elevated in malaria-exposed Papuan subjects, suggesting that the prevalence of intestinal parasitosis may be high and that nonmalarial infection may also contribute to high basal NO production. Basal peripheral blood mononuclear cell (PBMC) NO synthase activity was elevated in Papuans but poorly correlated with systemic NO production, suggesting that NO production in this setting arises not only from PBMCs but also from other tissue and cellular sources. NO production was associated with and may contribute to malaria tolerance in Papuan adults.

The purine salvage enzyme hypoxanthine guanine xanthine phosphoribosyl transferase is a major target antigen for cell-mediated immunity to malaria

Although there is good evidence that immunity to the blood stages of malaria parasites can be mediated by different effector components of the adaptive immune system, target antigens for a principal component, effector CD4(+) T cells, have never been defined. We generated CD4(+) T cell lines to fractions of native antigens from the blood stages of the rodent parasite, Plasmodium yoelii, and identified fraction-specific T cells that had a Th1 phenotype (producing IL-2, IFN-gamma, and tumor necrosis factor-alpha, but not IL-4, after antigenic stimulation). These T cells could inhibit parasite growth in recipient severe combined immunodeficient mice. N-terminal sequencing of the fraction showed identity with hypoxanthine guanine xanthine phosphoribosyl transferase (HGXPRT). Recombinant HGXPRT from the human malaria parasite, Plasmodium falciparum, activated the T cells in vitro, and immunization of normal mice with recombinant HGXPRT reduced parasite growth rates in all mice after challenge.

Malaria vaccines: where are we and where are we going?

Malaria is still killing over one million people each year and its incidence is increasing. The need for an effective vaccine is greater than ever. A major difficulty with vaccine research is that the malaria parasite presents thousands of antigens to the human immune system that vary throughout its life cycle. Identifying those that may prove to be vaccine targets is complicated and time consuming. Most vaccines are targeted at individual stages of the malaria life cycle, although it is likely that only the development of a multistage vaccine will offer complete protection to both visitors to, and residents of, a malaria-endemic area. With the development of a successful vaccine other issues such as cost, distribution, education, and compliance will have to be addressed. This review describes some of the current vaccine candidates for immunising against malaria.

A new NOS2 promoter polymorphism associated with increased nitric oxide production and protection from severe malaria in Tanzanian and Kenyan children

BACKGROUND

Nitric oxide (NO) is a mediator of immunity to malaria, and genetic polymorphisms in the promoter of the inducible NO synthase gene (NOS2) could modulate production of NO. We postulated that NOS2 promoter polymorphisms would affect resistance to severe malaria.

METHODS

We assessed genomic DNA from healthy children and from those diagnosed with malaria from Tanzania (n=47 and n=138, respectively) and Kenya (n=1106) for polymorphisms by single-stranded conformational polymorphism (SSCP) analysis and sequencing. We also measured in-vivo NO production in Tanzanian children.

FINDINGS

We identified a novel single nucleotide polymorphism, -1173 C-->T, in the NOS2 promoter that was significantly associated with protection from symptomatic malaria (odds ratio 0.12, 95% CI 0.03-0.48, p=0.0006) in 179 Tanzanian children, and significantly associated with protection from severe malarial anaemia (adjusted relative risk 0.25, 95% CI 0.09-0.66, p=0.0005) in 1106 Kenyan children studied over 5 years. The risk of parasitaemia was not significantly different in wild-type or -1173 C-->T individuals. -1173 C-->T protection in Tanzanians was independent of the previously recognised NOS2-954 G-->C polymorphism. The (CCTTT)(n) NOS2 polymorphism (Tanzania and Kenya) was not associated with severe malaria outcomes. -1173 C-->T was associated with increased fasting urine and plasma NO metabolite concentrations in Tanzanian children, suggesting that the polymorphism was functional in vivo. Interpretation The NOS2 promoter -1173 C-->T single nucleotide polymorphism is associated with protection against cerebral malaria and severe malarial anaemia. Increased NO production in individuals with the -1173 C-->T polymorphism lends support to a protective role for NO against these syndromes. Targeted interventions to increase NO delivery or production could provide novel preventive and therapeutic strategies against these major causes of mortality in African children.