Sickle cell trait is associated with a delayed onset of malaria: implications for time-to-event analysis in clinical studies of malaria

Naturally acquired antibodies specific for Plasmodium falciparum reticulocyte-binding protein homologue 5 inhibit parasite growth and predict protection from malaria

BACKGROUND

Plasmodium falciparum reticulocyte-binding protein homologue 5 (PfRH5) is a blood-stage parasite protein essential for host erythrocyte invasion. PfRH5-specific antibodies raised in animals inhibit parasite growth in vitro, but the relevance of naturally acquired PfRH5-specific antibodies in humans is unclear.

METHODS

We assessed pre-malaria season PfRH5-specific immunoglobulin G (IgG) levels in 357 Malian children and adults who were uninfected with Plasmodium. Subsequent P. falciparum infections were detected by polymerase chain reaction every 2 weeks and malaria episodes by weekly physical examination and self-referral for 7 months. The primary outcome was time between the first P. falciparum infection and the first febrile malaria episode. PfRH5-specific IgG was assayed for parasite growth-inhibitory activity.

RESULTS

The presence of PfRH5-specific IgG at enrollment was associated with a longer time between the first blood-stage infection and the first malaria episode (PfRH5-seropositive median: 71 days, PfRH5-seronegative median: 18 days; P = .001). This association remained significant after adjustment for age and other factors associated with malaria risk/exposure (hazard ratio, .62; P = .02). Concentrated PfRH5-specific IgG purified from Malians inhibited P. falciparum growth in vitro.

CONCLUSIONS

Naturally acquired PfRH5-specific IgG inhibits parasite growth in vitro and predicts protection from malaria. These findings strongly support efforts to develop PfRH5 as an urgently needed blood-stage malaria vaccine.

CLINICAL TRIALS REGISTRATION

NCT01322581.

Effects of age, hemoglobin type and parasite strain on IgG recognition of Plasmodium falciparum-infected erythrocytes in Malian children

BACKGROUND

Naturally-acquired antibody responses to antigens on the surface of Plasmodium falciparum-infected red blood cells (iRBCs) have been implicated in antimalarial immunity. To profile the development of this immunity, we have been studying a cohort of Malian children living in an area with intense seasonal malaria transmission.

METHODOLOGY/PRINCIPAL FINDINGS

We collected plasma from a sub-cohort of 176 Malian children aged 3-11 years, before (May) and after (December) the 2009 transmission season. To measure the effect of hemoglobin (Hb) type on antibody responses, we enrolled age-matched HbAA, HbAS and HbAC children. To quantify antibody recognition of iRBCs, we designed a high-throughput flow cytometry assay to rapidly test numerous plasma samples against multiple parasite strains. We evaluated antibody reactivity of each plasma sample to 3 laboratory-adapted parasite lines (FCR3, D10, PC26) and 4 short-term-cultured parasite isolates (2 Malian and 2 Cambodian). 97% of children recognized ≥1 parasite strain and the proportion of IgG responders increased significantly during the transmission season for most parasite strains. Both strain-specific and strain-transcending IgG responses were detected, and varied by age, Hb type and parasite strain. In addition, the breadth of IgG responses to parasite strains increased with age in HbAA, but not in HbAS or HbAC, children.

CONCLUSIONS/SIGNIFICANCE

Our assay detects both strain-specific and strain-transcending IgG responses to iRBCs. The magnitude and breadth of these responses varied not only by age, but also by Hb type and parasite strain used. These findings indicate that studies of acquired humoral immunity should account for Hb type and test large numbers of diverse parasite strains.

Biochemical and immunological mechanisms by which sickle cell trait protects against malaria

Sickle cell trait (HbAS) is the best-characterized genetic polymorphism known to protect against falciparum malaria. Although the protective effect of HbAS against malaria is well known, the mechanism(s) of protection remain unclear. A number of biochemical and immune-mediated mechanisms have been proposed, and it is likely that multiple complex mechanisms are responsible for the observed protection. Increased evidence for an immune component of protection as well as novel mechanisms, such as enhanced tolerance to disease mediated by HO-1 and reduced parasitic growth due to translocation of host micro-RNA into the parasite, have recently been described. A better understanding of relevant mechanisms will provide valuable insight into the host-parasite relationship, including the role of the host immune system in protection against malaria.

Relationship between malaria incidence and IgG levels to Plasmodium falciparum merozoite antigens in Malian children: impact of hemoglobins S and C

Heterozygous hemoglobin (Hb) AS (sickle-cell trait) and HbAC are hypothesized to protect against Plasmodium falciparum malaria in part by enhancing naturally-acquired immunity to this disease. To investigate this hypothesis, we compared antibody levels to four merozoite antigens from the P. falciparum 3D7 clone (apical membrane antigen 1, AMA1-3D7; merozoite surface protein 1, MSP1-3D7; 175 kDa erythrocyte-binding antigen, EBA175-3D7; and merozoite surface protein 2, MSP2-3D7) in a cohort of 103 HbAA, 73 HbAS and 30 HbAC children aged 3 to 11 years in a malaria-endemic area of Mali. In the 2009 transmission season we found that HbAS, but not HbAC, significantly reduced the risk of malaria compared to HbAA. IgG levels to MSP1 and MSP2 at the start of this transmission season inversely correlated with malaria incidence after adjusting for age and Hb type. However, HbAS children had significantly lower IgG levels to EBA175 and MSP2 compared to HbAA children. On the other hand, HbAC children had similar IgG levels to all four antigens. The parasite growth-inhibitory activity of purified IgG samples did not differ significantly by Hb type. Changes in antigen-specific IgG levels during the 2009 transmission and 2010 dry seasons also did not differ by Hb type, and none of these IgG levels dropped significantly during the dry season. These data suggest that sickle-cell trait does not reduce the risk of malaria by enhancing the acquisition of IgG responses to merozoite antigens.

Humoral and cellular immunity to Plasmodium falciparum merozoite surface protein 1 and protection from infection with blood-stage parasites

BACKGROUND

 Acquired immunity to malaria develops with increasing age and repeated infections. Understanding immune correlates of protection from malaria would facilitate vaccine development and identification of biomarkers that reflect changes in susceptibility resulting from ongoing malaria control efforts.

METHODS

 The relationship between immunoglobulin G (IgG) antibody and both interferon γ (IFN-γ) and interleukin 10 (IL-10) responses to the 42-kD C-terminal fragment of Plasmodium falciparum merozoite surface protein 1 (MSP142) and the risk of (re)infection were examined following drug-mediated clearance of parasitemia in 94 adults and 95 children in an area of holoendemicity of western Kenya.

RESULTS

 Positive IFN-γ enzyme-linked immunosorbent assay (ELISA) and enzyme-linked immunosorbent spot assay (ELISPOT) responses to MSP142 3D7 were associated with delayed time to (re)infection, whereas high-titer IgG antibodies to MSP142 3D7 or FVO alleles were not independently predictive of the risk of (re)infection. When IFN-γ and IL-10 responses were both present, the protective effect of IFN-γ was abrogated. A Cox proportional hazard model including IFN-γ, IL-10, MSP142 3D7 IgG antibody responses, hemoglobin S genotype, age, and infection status at baseline showed that the time to blood-stage infection correlated positively with IFN-γ responses and negatively with IL-10 responses, younger age, and asymptomatic parasitemia.

CONCLUSIONS

 Evaluating combined allele-specific cellular and humoral immunity elicited by malaria provides a more informative measure of protection relative to evaluation of either measure alone.

An intensive longitudinal cohort study of Malian children and adults reveals no evidence of acquired immunity to Plasmodium falciparum infection

BACKGROUND

In experimental models of human and mouse malaria, sterilizing liver stage immunity that blocks progression of Plasmodium infection to the symptomatic blood stage can be readily demonstrated. However, it remains unclear whether individuals in malaria-endemic areas acquire such immunity.

METHODS

In Mali, 251 healthy children and adults aged 4-25 years who were free of blood-stage Plasmodium infection by polymerase chain reaction (PCR) were enrolled in a longitudinal study just prior to an intense 6-month malaria season. Subsequent clinical malaria episodes were detected by weekly active surveillance and self-referral. Asymptomatic P. falciparum infections were detected by blood-smear microscopy and PCR analysis of dried blood spots that had been collected every 2 weeks for 7 months.

RESULTS

As expected, the risk of clinical malaria decreased with increasing age (log-rank test, P = .0038). However, analysis of PCR data showed no age-related differences in P. falciparum infection risk (log-rank test, P = .37).

CONCLUSIONS

Despite years of exposure to intense P. falciparum transmission, there is no evidence of acquired, sterile immunity to P. falciparum infection in this population, even as clinical immunity to blood-stage malaria is clearly acquired. Understanding why repeated P. falciparum infections do not induce sterile protection may lead to insights for developing vaccines that target the liver stage in malaria-endemic populations.

Plasmodium falciparum clearance rates in response to artesunate in Malian children with malaria: effect of acquired immunity

Background. Artemisinin resistance, a long parasite clearance half-life in response to artemisinin, has been described in patients with Plasmodium falciparum malaria in southeast Asia. Few baseline half-lives have been reported from Africa, where artemisinins were recently introduced.

Methods. We treated P. falciparum malaria in 215 Malian children aged 0.5–15 years with artesunate (0, 24, 48 hours) and amodiaquine (72, 96, 120 hours). We estimated half-life by measuring parasite density every 6 hours until undetectable and evaluated the effects of age, sex, ethnicity, and red blood cell (RBC) polymorphisms on half-life. We quantified the proportion of parasitized RBCs recognized by autologous immunoglobulin G (IgG).

Results. The geometric mean half-life was 1.9 hours (95% confidence interval, 1.8–2.0) and did not correlate with parasite ex vivo susceptibility to artemisinins. In a linear model accounting for host factors, half-life decreased by 4.1 minutes for every 1-year increase in age. The proportion of parasitized RBCs recognized by IgG correlated inversely with half-life (r = −0.475; P = .0006).

Conclusions. Parasite clearance in response to artesunate is faster in Mali than in southeast Asia. IgG responses to parasitized RBCs shorten half-life and may influence this parameter in areas where age is not an adequate surrogate of immunity and correlates of parasite-clearing immunity have not been identified.

Clinical Trials Registration. NCT00669084.

Artemisinin-resistant Plasmodium falciparum in Pursat province, western Cambodia: a parasite clearance rate study

BACKGROUND

Artemisinin-resistant Plasmodium falciparum has been reported in Pailin, western Cambodia, detected as a slow parasite clearance rate in vivo. Emergence of this phenotype in western Thailand and possibly elsewhere threatens to compromise the effectiveness of all artemisinin-based combination therapies. Parasite genetics is associated with parasite clearance rate but does not account for all variation. We investigated contributions of both parasite genetics and host factors to the artemisinin-resistance phenotype in Pursat, western Cambodia.

METHODS

Between June 19 and Nov 28, 2009, and June 26 and Dec 6, 2010, we enrolled patients aged 10 years or older with uncomplicated falciparum malaria, a density of asexual parasites of at least 10,000 per μL of whole blood, no symptoms or signs of severe malaria, no other cause of febrile illness, and no chronic illness. We gave participants 4 mg/kg artesunate at 0, 24, and 48 h, 15 mg/kg mefloquine at 72 h, and 10 mg/kg mefloquine at 96 h. We assessed parasite density on thick blood films every 6 h until undetectable. The parasite clearance half-life was calculated from the parasite clearance curve. We genotyped parasites with 18 microsatellite markers and patients for haemoglobin E, α-thalassaemia, and a mutation of G6PD, which encodes glucose-6-phosphate dehydrogenase. To account for the possible effects of acquired immunity on half-life, we used three surrogates for increased likelihood of exposure to P falciparum: age, sex, and place of residence. This study is registered with ClinicalTrials.gov, number NCT00341003.

FINDINGS

We assessed 3504 individuals from all six districts of Pursat province seeking treatment for malaria symptoms. We enrolled 168 patients with falciparum malaria who met inclusion criteria. The geometric mean half-life was 5·85 h (95% CI 5·54-6·18) in Pursat, similar to that reported in Pailin (p=0·109). We identified two genetically different parasite clone groups: parasite group 1 (PG1) and parasite group 2 (PG2). Non-significant increases in parasite clearance half-life were seen in patients with haemoglobin E (0·55 h; p=0·078), those of male sex (0·96 h; p=0·064), and in 2010 (0·68 h; p=0·068); PG1 was associated with a significant increase (0·79 h; p=0·033). The mean parasite heritability of half-life was 0·40 (SD 0·17).

INTERPRETATION

Heritable artemisinin resistance is established in a second Cambodian province. To accurately identify parasites that are intrinsically susceptible or resistant to artemisinins, future studies should explore the effect of erythrocyte polymorphisms and specific immune responses on half-life variation.

FUNDING

Division of Intramural Research, National Institute of Allergy and Infectious Diseases, National Institutes of Health.

Lack of associations of α(+)-thalassemia with the risk of Plasmodium falciparum and Plasmodium vivax infection and disease in a cohort of children aged 3-21 months from Papua New Guinea

Despite consistent evidence of a protective effect of α(+)-thalassemia against severe Plasmodium falciparum disease, the mechanisms underlying this protection remain unknown. An increase in risk of Plasmodium vivax malaria in early childhood resulting in a cross-species protection against severe P. falciparum malaria has been proposed as a possible mechanism in Melanesian children. The association of α(+)-thalassemia genotypes with a risk of P. falciparum and P. vivax infection and uncomplicated illness was reassessed in a cohort of 1,112 Papua New Guinean children, followed from 3 to 21 months of age. Three hundred and eighty-nine (35.0%) children were homozygous for α(+)-thalassemia (-α/-α), 506 (45.5%) heterozygous (αα/-α) and 217 (19.5%) homozygous for the wild-type allele. No significant differences in the incidence of P. falciparum (Pf) or P. vivax (Pv) malaria were observed between α(+)-thalassemia homozygote (Pf: incidence rate ratio (IRR)=1.13, CI(95) (0.82, 1.56), P=0.45, Pv: IRR=1.15, CI(95) (0.88, 1.50), P=0.31), heterozygote (Pf: IRR=0.98, CI(95) (0.71, 1.34), P=0.93, Pv: IRR=1.14, CI(95) (0.88, 1.48), P=0.33) and wild-type children. The prevalence of infection with either species did not differ between α(+)-thalassemia genotypes, although densities of P. vivax (but not of P. falciparum) infections were significantly higher in α(+)-thalassemia homozygote and heterozygote children. An excessive risk of moderate-to-severe anemia (Hb<8 g/dl) was observed in α(+)-thalassemia homozygote children (IRR=1.54, CI(95) (1.12, 2.11), P=0.008). This study therefore failed to confirm an increased risk of P. vivax or P. falciparum malaria in very young, α(+)-thalassemic children without significant levels of acquired immunity. This confirms the lack of protection by α(+)-thalassemia against uncomplicated P. falciparum and challenges the hypothesis of immunological cross-protection between P. falciparum and P. vivax as a mechanism underlying α(+)-thalassemia protection against severe P. falciparum disease in Melanesian children.

Cytoadherence in paediatric malaria: ABO blood group, CD36, and ICAM1 expression and severe Plasmodium falciparum infection

As a leading cause of childhood mortality worldwide, selection pressure by Plasmodium falciparum continues to shape the human genome. Severe disturbances within the microcirculation result from the adhesion of infected erythrocytes to host receptors on monocytes, platelets, and endothelium. In this prospective study, we compared expression of all major host cytoadhesion receptors among Ugandan children presenting with uncomplicated malaria (n = 1078) versus children with severe malaria (n = 855), including cerebral malaria (n = 174), severe anaemia (n = 522), and lactic acidosis (n = 154). We report a significant survival advantage attributed to blood group O and increased monocyte expression of CD36 and ICAM1 (CD54). The high case fatality rate syndromes of cerebral malaria and lactic acidosis were associated with high platelet CD36 expression and thrombocytopenia, and severe malaria anaemia was characterized by low ICAM1 expression. In a logistic regression model of disease severity, odds ratios for the mitigating effects of blood group O, CD36, and ICAM1 phenotypes were greater than that of sickle haemoglobin. Host genetic adaptations to Plasmodium falciparum suggest new potential malaria treatment strategies.

Systems immunology of human malaria

Plasmodium falciparum malaria remains a global public health threat. Optimism that a highly effective malaria vaccine can be developed stems in part from the observation that humans can acquire immunity to malaria through experimental and natural P. falciparum infection. Recent advances in systems immunology could accelerate efforts to unravel the mechanisms of acquired immunity to malaria. Here, we review the tools of systems immunology, their current limitations in the context of human malaria research, and the human 'models' of malaria immunity to which these tools can be applied.

Abnormal PfEMP1/knob display on Plasmodium falciparum-infected erythrocytes containing hemoglobin variants: fresh insights into malaria pathogenesis and protection

Hemoglobin (Hb) variants are associated with reduced risk of life-threatening Plasmodium falciparum malaria syndromes, including cerebral malaria and severe malarial anemia. Despite decades of research, the mechanisms by which common Hb variants - sickle HbS, HbC, α-thalassemia, fetal HbF - protect African children against severe and fatal malaria have not been fully elucidated. In vitro experimental and epidemiological data have long suggested that Hb variants do not confer malaria protection by restricting the growth of parasites in red blood cells (RBCs). Recently, four Hb variants were found to impair cytoadherence, the binding of P. falciparum-infected RBCs (PfRBCs) to microvascular endothelial cells (MVECs), a centrally important event in both parasite survival and malaria pathogenesis in humans. Impaired cytoadherence is associated with abnormal display of P. falciparum erythrocyte membrane protein 1 (PfEMP1), the parasite's major cytoadherence ligand and virulence factor, on the surface of host RBCs. We propose a model in which Hb variants allow parasites to display relatively low levels of PfEMP1, sufficient for sequestering PfRBCs in microvessels and avoiding their clearance from the bloodstream by the spleen. By preventing the display of high levels of PfEMP1, Hb variants may weaken the binding of PfRBCs to MVECs, compromising their ability to activate endothelium and initiate the downstream microvascular events that drive the pathogenesis of malaria.

α-Thalassemia impairs the cytoadherence of Plasmodium falciparum-infected erythrocytes

BACKGROUND

α-Thalassemia results from decreased production of α-globin chains that make up part of hemoglobin tetramers (Hb; α(2)β(2)) and affects up to 50% of individuals in some regions of sub-Saharan Africa. Heterozygous (-α/αα) and homozygous (-α/-α) genotypes are associated with reduced risk of severe Plasmodium falciparum malaria, but the mechanism of this protection remains obscure. We hypothesized that α-thalassemia impairs the adherence of parasitized red blood cells (RBCs) to microvascular endothelial cells (MVECs) and monocytes--two interactions that are centrally involved in the pathogenesis of severe disease.

METHODS AND FINDINGS

We obtained P. falciparum isolates directly from Malian children with malaria and used them to infect αα/αα (normal), -α/αα and -α/-α RBCs. We also used laboratory-adapted P. falciparum clones to infect -/-α RBCs obtained from patients with HbH disease. Following a single cycle of parasite invasion and maturation to the trophozoite stage, we tested the ability of parasitized RBCs to bind MVECs and monocytes. Compared to parasitized αα/αα RBCs, we found that parasitized -α/αα, -α/-α and -/-α RBCs showed, respectively, 22%, 43% and 63% reductions in binding to MVECs and 13%, 33% and 63% reductions in binding to monocytes. α-Thalassemia was associated with abnormal display of P. falciparum erythrocyte membrane protein 1 (PfEMP1), the parasite's main cytoadherence ligand and virulence factor, on the surface of parasitized RBCs.

CONCLUSIONS

Parasitized α-thalassemic RBCs show PfEMP1 display abnormalities that are reminiscent of those on the surface of parasitized sickle HbS and HbC RBCs. Our data suggest a model of malaria protection in which α-thalassemia ameliorates the pro-inflammatory effects of cytoadherence. Our findings also raise the possibility that other unstable hemoglobins such as HbE and unpaired α-globin chains (in the case of β-thalassemia) protect against life-threatening malaria by a similar mechanism.

Haemoglobin variants and Plasmodium falciparum malaria in children under five years of age living in a high and seasonal malaria transmission area of Burkina Faso

Background

Genetic factors play a key role in determining resistance/susceptibility to infectious disease. Susceptibility of the human host to malaria infection has been reported to be influenced by genetic factors, which could be confounders if not taken into account in the assessment of the efficacy of interventions against malaria. This study aimed to assess the relationship between haemoglobin genotypes and malaria in children under five years in a site being characterized for future malaria vaccine trials.

Methods

The study population consisted of 452 children living in four rural villages. Hb genotype was determined at enrolment. Clinical malaria incidence was evaluated over a one-year period using combined active and passive surveillance. Prevalence of infection was evaluated via bi-annual cross-sectional surveys. At each follow-up visit, children received a brief clinical examination and thick and thin blood films were prepared for malaria diagnosis. A clinical malaria was defined as Plasmodium falciparum parasitaemia >2,500 parasites/μl and axillary temperature ≥37.5°C or reported fever over the previous 24 hours.

Results

Frequencies of Hb genotypes were 73.2% AA; 15.0% AC; 8.2% AS; 2.2% CC; 1.1% CS and 0.2% SS. Prevalence of infection at enrolment ranged from 61.9%-54.1% among AA, AC and AS children. After one year follow-up, clinical malaria incidence (95% CI) (episodes per person-year) was 1.9 (1.7-2.0) in AA, 1.6 (1.4-2.1) in AC, and 1.7 (1.4-2.0) in AS children. AC genotype was associated with lower incidence of clinical malaria relative to AA genotype among children aged 1–2 years [rate ratio (95% CI) 0.66 (0.42-1.05)] and 2–3 years [rate ratio (95% CI) 0.37 (0.18-0.75)]; an association of opposite direction was however apparent among children aged 3–4 years. AS genotype was associated with lower incidence of clinical malaria relative to AA genotype among children aged 2–3 years [rate ratio (95% CI) 0.63 (0.40-1.01)].

Conclusions

In this cohort of children, AC or AS genotype was associated with lower risk of clinical malaria relative to AA genotype only among children aged one to three years. It would be advisable for clinical studies of malaria in endemic regions to consider haemoglobin gene differences as a potentially important confounder, particularly among younger children.

Haemoglobinopathies and the clinical epidemiology of malaria: a systematic review and meta-analysis

BACKGROUND

Haemoglobinopathies can reduce the risk of malaria syndromes. We aimed to quantify the relation between different haemoglobin mutations and malaria protection to strengthen the foundation for translational studies of malaria pathogenesis and immunity.

METHODS

We systematically searched the Medline and Embase databases for studies that estimated the risk of malaria in patients with and without haemoglobinopathies up to Sept 9, 2011, and identified additional studies from reference lists. We included studies that enrolled mainly children or pregnant women and had the following outcomes: Plasmodium falciparum severe malaria, uncomplicated malaria, asymptomatic parasitaemia, or pregnancy-associated malaria, and Plasmodium vivax malaria. Two reviewers identified studies independently, assessed quality of the studies, and extracted data. We produced odds ratios (ORs; 95% CIs) for case-control studies and incidence rate ratios (IRRs; 95% CIs) for prospective studies. We did the meta-analysis with a random-effects model when equivalent outcomes were reported in more than one study.

FINDINGS

Of 62 identified studies, 44 reported data for haemoglobin AS, 19 for haemoglobin AC and CC, and 18 for α-thalassaemia. Meta-analysis of case-control studies showed a decreased risk of severe P. falciparum malaria in individuals with haemoglobin AS (OR 0·09, 95% CI 0·06-0·12), haemoglobin CC (0·27, 0·11-0·63), haemoglobin AC (0·83, 0·67-0·96), homozygous α-thalassaemia (0·63, 0·48-0·83), and heterozygous α-thalassaemia (0·83, 0·74-0·92). In meta-analysis of prospective trials only haemoglobin AS was consistently associated with protection from uncomplicated malaria (IRR 0·69, 95% CI 0·61-0·79); no haemoglobinopathies led to consistent protection from asymptomatic parasitaemia. Few clinical studies have investigated β-thalassaemia, haemoglobin E, P. vivax malaria, or pregnancy-associated malaria.

INTERPRETATION

Haemoglobin AS, CC, and AC genotypes and homozygous and heterozygous α-thalassaemia provide significant protection from severe malaria syndromes, but these haemoglobinopathies differ substantially in the degree of protection provided and confer mild or no protection against uncomplicated malaria and asymptomatic parasitaemia. Through attenuation of severity of malaria, haemoglobinopathies could serve as a model for investigation of the mechanisms of malaria pathogenesis and immunity.

FUNDING

US National Institute of Allergy and Infectious Diseases.

Evidence for both innate and acquired mechanisms of protection from Plasmodium falciparum in children with sickle cell trait

Sickle cell trait (HbAS) is known to be protective against Plasmodium falciparum malaria, but it is unclear when during the course of infection this protection occurs and whether protection is innate or acquired. To address these questions, a cohort of 601 children 1-10 years of age were enrolled in Kampala, Uganda, and followed for 18 months for symptomatic malaria and asymptomatic parasitemia. Genotyping was used to detect and follow individual parasite clones longitudinally within subjects. Children with HbAS were protected against the establishment of parasitemia, as assessed by the molecular force of infection at older but not younger ages (at 2 years of age: incidence rate ratio [IRR] = 1.16; 95% confidence interval [95% CI], 0.62-2.19; P = .6; at 9 years of age: IRR = 0.50; 95% CI, 0.28-0.87; P = .01), suggesting an acquired mechanism of protection. Once parasitemic, children with HbAS were less likely to progress to symptomatic malaria, with protection again being the most pronounced at older ages (at 2 years of age: relative risk [RR] = 0.92; 95% CI, 0.77-1.10; P = .3; at 9 years of age: RR = 0.68; 95% CI, 0.51-0.91; P = .008). Conversely, the youngest children were best protected against high parasite density (at 2 years of age: relative density = 0.24; 95% CI, 0.10-0.54; P = .001; at 9 years of age: relative density = 0.59; 95% CI, 0.30-1.19; P = .14), suggesting an innate mechanism of protection against this end point.

Sickle cell protection from malaria

A linkage between presence of Sickle Haemoglobin (HbS) and protection from malaria infection and clinical manifestations in certain areas was suspected from early observations and progressively elucidated by more recent studies. Research has confirmed the abovementioned connection, but also clarified how such protection may be abolished by coexistence of sickle cell trait (HbS trait) and alpha thalassemia, which may explain the relatively low incidence of HbS trait in the Mediterranean. The mechanisms of such protective effect are now being investigated: factors of genetic, molecular and immunological nature are prominent. As for genetic factors attention is given to the role of the red blood cell (RBC) membrane complement regulatory proteins as polymorphisms of these components seem to be associated with resistance to severe malaria; genetic ligands like the Duffy group blood antigen, necessary for erythrocytic invasion, and human protein CD36, a major receptor for P. falciparum-infected RBC's, are also under scrutiny: attention is focused also on plasmodium erythrocyte-binding antigens, which bind to RBC surface components. Genome-wide linkage and association studies are now carried out too, in order to identify genes associated with malaria resistance. Only a minor role is attributed to intravascular sickling, phagocytosis and haemolysis, while specific molecular mechanisms are the object of intensive research: among these a decisive role is played by a biochemical sequence, involving activation of haeme oxygenase (HMO-1), whose effect appears mediated by carbon monoxide (CO). A central role in protection from malaria is also played by immunological factors, which may stimulate antibody production to plasmodium antigens in the early years of life; the role of agents like pathogenic CD8 T-cells has been suggested while the effects of molecular actions on the immunity mechanism are presently investigated. It thus appears that protection from malaria can be explained by interaction of different factors: the elucidation of such mechanisms may prove valuable for the prevention and treatment strategy of a disease which still affects large parts of the world.

Hemoglobin S and C heterozygosity enhances neither the magnitude nor breadth of antibody responses to a diverse array of Plasmodium falciparum antigens

BACKGROUND

Heterozygous states of hemoglobin (Hb) A and HbS (HbAS, sickle-cell trait) or HbC (HbAC) protect against Plasmodium falciparum malaria by unclear mechanisms. Several studies suggest that HbAS and HbAC accelerate the acquisition of immunity to malaria, possibly by enhancing P. falciparum-specific antibody responses.

METHODS

We used a protein microarray representing 491 P. falciparum proteins expressed during exoerythrocytic and erythrocytic stages of the life cycle to test the hypothesis that HbAS and HbAC enhance the P. falciparum-specific IgG response compared with normal HbAA. Plasma samples were collected from Malian children aged 2-10 years before and after a 6-month malaria season and were probed against the microarray. Immunoglobulin G (IgG) profiles of children with HbAA (n = 106), HbAS (n = 15), and HbAC (n = 20) were compared.

RESULTS

Although the magnitude and breadth of P. falciparum-specific IgG responses increased with age and from before to after the malaria season in each antigen category, Hb type did not independently predict significant differences in P. falciparum-specific IgG profiles.

CONCLUSIONS

These data do not support the hypothesis that HbAS and HbAC protect against malaria by enhancing P. falciparum-specific antibody responses. It remains possible that HbAS and HbAC protect against malaria by enhancing antibody responses to antigens not studied here or through other immune mechanisms.

CpG DNA as a vaccine adjuvant

Synthetic oligodeoxynucleotides (ODNs) containing unmethylated CpG motifs trigger cells that express Toll-like receptor 9 (including human plasmacytoid dendritic cells and B cells) to mount an innate immune response characterized by the production of Th1 and proinflammatory cytokines. When used as vaccine adjuvants, CpG ODNs improve the function of professional antigen-presenting cells and boost the generation of humoral and cellular vaccine-specific immune responses. These effects are optimized by maintaining ODNs and vaccine in close proximity. The adjuvant properties of CpG ODNs are observed when administered either systemically or mucosally, and persist in immunocompromised hosts. Preclinical studies indicate that CpG ODNs improve the activity of vaccines targeting infectious diseases and cancer. Clinical trials demonstrate that CpG ODNs have a good safety profile and increase the immunogenicity of coadministered vaccines.

Influence of haemoglobins S and C on predominantly asymptomatic Plasmodium infections in northern Ghana

The haemoglobin (Hb) variants HbS and HbC protect against severe malaria. Yet, the influence particularly of HbC on asymptomatic or mild Plasmodium infection is not well established. In a dry season cross-sectional survey among 2108 children aged 0.5-9 years in the Northern Region of Ghana, Plasmodium species and density, as well as Hb, were analysed with respect to Hb genotypes. HbAC occurred in 19.7% and HbAS in 7.4% (HbSC, 0.8%; HbCC, 0.8%; HbSS, 0.3%). Overall, 56% of the children had microscopically visible parasitaemia. By PCR, P. falciparum, P. malariae, and P. ovale were present in 74.5%, 9.7%, and 5.5%, respectively. Febrile parasitaemia was rare (2.8%) but anaemia (Hb<11g/dL) frequent (59.3%). Children with HbAA and HbAC showed virtually identical malariometric parameters. In contrast, children with HbAS had significantly less parasitaemia, lower parasite densities, and a higher proportion of submicroscopic P. falciparum infection. Remarkably, in children with HbCC, P. malariae infection occurred in 37.5% (adjusted odds ratio (aOR), 5.8; 95% CI, 1.8-18.8) and P. ovale in 18.8% (aOR, 3.61; 95% CI, 0.97-13.5). In this population with predominantly asymptomatic Plasmodium infection, HbAC shows no discernible effect on malaria-related parameters. Homozygous HbC, in contrast, confers an increased risk of P. malariae infection which conceivably may modulate falciparum malaria.

Anaemia in a phase 2 study of a blood stage falciparum malaria vaccine

Background

A Phase 1-2b study of the blood stage malaria vaccine AMA1-C1/Alhydrogel was conducted in 336 children in Donéguébougou and Bancoumana, Mali. In the Phase 2 portion of the study (n = 300), no impact on parasite density or clinical malaria was seen; however, children who received the study vaccine had a higher frequency of anaemia (defined as haemoglobin < 8.5 g/dL) compared to those who received the comparator vaccine (Hiberix). This effect was one of many tested and was not significant after adjusting for multiple comparisons.

Methods

To further investigate the possible impact of vaccination on anaemia, additional analyses were conducted including patients from the Phase 1 portion of the study and controlling for baseline haemoglobin, haemoglobin types S or C, alpha-thalassaemia, G6PD deficiency, and age. A multiplicative intensity model was used, which generalizes Cox regression to allow for multiple events. Frailty effects for each subject were used to account for correlation of multiple anaemia events within the same subject. Intensity rates were calculated with reference to calendar time instead of time after randomization in order to account for staggered enrollment and seasonal effects of malaria incidence. Associations of anaemia with anti-AMA1 antibody were further explored using a similar analysis.

Results

A strong effect of vaccine on the incidence of anaemia (risk ratio [AMA1-C1 to comparator (Hiberix)]= 2.01, 95% confidence interval [1.26,3.20]) was demonstrated even after adjusting for baseline haemoglobin, haemoglobinopathies, and age, and using more sophisticated statistical models. Anti-AMA1 antibody levels were not associated with this effect.

Conclusions

While these additional analyses show a robust effect of vaccination on anaemia, this is an intensive exploration of secondary results and should, therefore, be interpreted with caution. Possible mechanisms of the apparent adverse effect on haemoglobin of vaccination with AMA1-C1/Alhydrogel and implications for blood stage vaccine development are discussed. The potential impact on malaria-associated anaemia should be closely evaluated in clinical trials of AMA1 and other blood stage vaccines in malaria-exposed populations.

A positive correlation between atypical memory B cells and Plasmodium falciparum transmission intensity in cross-sectional studies in Peru and Mali

BACKGROUND

Antibodies that protect against Plasmodium falciparum (Pf) malaria are only acquired after years of repeated infections. The B cell biology that underlies this observation is poorly understood. We previously reported that "atypical" memory B cells are increased in children and adults exposed to intense Pf transmission in Mali, similar to what has been observed in individuals infected with HIV. In this study we examined B cell subsets of Pf -infected adults in Peru and Mali to determine if Pf transmission intensity correlates with atypical memory B cell expansion.

METHODOLOGY/PRINCIPAL FINDINGS

In this cross-sectional study venous blood was collected from adults in areas of zero (U.S., n = 10), low (Peru, n = 18) and high (Mali, n = 12) Pf transmission. Adults in Peru and Mali were infected with Pf at the time of blood collection. Thawed lymphocytes were analyzed by flow cytometry to quantify B cell subsets, including atypical memory B cells, defined by the cell surface markers CD19(+) CD20(+) CD21(-) CD27(-) CD10(-). In Peru, the mean level of atypical memory B cells, as a percent of total B cells, was higher than U.S. adults (Peru mean: 5.4% [95% CI: 3.61-7.28]; U.S. mean: 1.4% [95% CI: 0.92-1.81]; p<0.0001) but lower than Malian adults (Mali mean 13.1% [95% CI: 10.68-15.57]; p = 0.0001). In Peru, individuals self-reporting ≥1 prior malaria episodes had a higher percentage of atypical memory B cells compared to those reporting no prior episodes (≥1 prior episodes mean: 6.6% [95% CI: 4.09-9.11]; no prior episodes mean: 3.1% [95% CI: 1.52-4.73]; p = 0.028).

CONCLUSIONS/SIGNIFICANCE

Compared to Pf-naive controls, atypical memory B cells were increased in Peruvian adults exposed to low Pf transmission, and further increased in Malian adults exposed to intense Pf transmission. Understanding the origin, function and antigen specificity of atypical memory B cells in the context of Pf infection could contribute to our understanding of naturally-acquired malaria immunity.

High-throughput prediction of protein antigenicity using protein microarray data

MOTIVATION

Discovery of novel protective antigens is fundamental to the development of vaccines for existing and emerging pathogens. Most computational methods for predicting protein antigenicity rely directly on homology with previously characterized protective antigens; however, homology-based methods will fail to discover truly novel protective antigens. Thus, there is a significant need for homology-free methods capable of screening entire proteomes for the antigens most likely to generate a protective humoral immune response.

RESULTS

Here we begin by curating two types of positive data: (i) antigens that elicit a strong antibody response in protected individuals but not in unprotected individuals, using human immunoglobulin reactivity data obtained from protein microarray analyses; and (ii) known protective antigens from the literature. The resulting datasets are used to train a sequence-based prediction model, ANTIGENpro, to predict the likelihood that a protein is a protective antigen. ANTIGENpro correctly classifies 82% of the known protective antigens when trained using only the protein microarray datasets. The accuracy on the combined dataset is estimated at 76% by cross-validation experiments. Finally, ANTIGENpro performs well when evaluated on an external pathogen proteome for which protein microarray data were obtained after the initial development of ANTIGENpro.

AVAILABILITY

ANTIGENpro is integrated in the SCRATCH suite of predictors available at http://scratch.proteomics.ics.uci.edu.

CONTACT

pfbaldi@ics.uci.edu

Plasmodium falciparum clearance with artemisinin-based combination therapy (ACT) in patients with glucose-6-phosphate dehydrogenase deficiency in Mali

Background

Artemisinin-based combination therapy (ACT) is currently the most effective medicine for the treatment of uncomplicated malaria. Artemisinin has previously been shown to increase the clearance of Plasmodium falciparum in malaria patients with haemoglobin E trait, but it did not increase parasite inhibition in an in vitro study using haemoglobin AS erythrocytes. The current study describes the efficacy of artemisinin derivatives on P. falciparum clearance in patients with glucose-6-phosphate dehydrogenase deficiency (G6PD), a haemoglobin enzyme deficiency, not yet studied in the same context, but nonetheless is a common in malaria endemic areas, associated with host protection against uncomplicated and severe malaria. The impact of G6PD deficiency on parasite clearance with ACT treatment was compared between G6PD-deficient patients and G6PD-normal group.

Methods

Blood samples from children and adults participants (1 to 70 years old) with uncomplicated P. falciparum malaria residing in Kambila, Mali were analysed. Study participants were randomly assigned to receive either artemether-lumefantrine (Coartem^®) or artesunate plus mefloquine (Artequin™). A restriction-fragment length polymorphism analysis of PCR-amplified DNA samples was used to identify the (A-) allele of the gene mutation responsible for G6PD deficiency (G6PD*A-). 470 blood samples were thus analysed and of these, DNA was extracted from 315 samples using the QIAamp kit for PCR to identify the G6PD*A- gene.

Results

The DNA amplified from 315 samples using PCR showed that G6PD*A- deficiency was present in 56 participants (17.8%). The distribution of the specific deficiency was 1%, 7% and, 9.8% respectively for homozygous, hemizygous, and heterozygous genotypes. Before treatment, the median parasitaemia and other baseline characteristics (mean haemoglobin, sex and age groups) between G6PD deficiency (hemizygous, heterozygous, and homozygous) and G6PD-normal participants were comparable (p > 0.05). After treatment, parasite clearance did not change significantly whether the participants were G6PD deficient or G6PD normal on day 1 (OR = 1.3; CI = 0.70-2.47; p > 0.05) and on day 2 (OR = 0.859; CI = 0.097-7.61; p > 0.05).

Conclusions

The presence of G6PD deficiency does not appear to significantly influence the clearance of P. falciparum in the treatment of uncomplicated malaria using ACT.

The Plasmodium falciparum-specific human memory B cell compartment expands gradually with repeated malaria infections

Immunity to Plasmodium falciparum (Pf) malaria is only acquired after years of repeated infections and wanes rapidly without ongoing parasite exposure. Antibodies are central to malaria immunity, yet little is known about the B-cell biology that underlies the inefficient acquisition of Pf-specific humoral immunity. This year-long prospective study in Mali of 185 individuals aged 2 to 25 years shows that Pf-specific memory B-cells and antibodies are acquired gradually in a stepwise fashion over years of repeated Pf exposure. Both Pf-specific memory B cells and antibody titers increased after acute malaria and then, after six months of decreased Pf exposure, contracted to a point slightly higher than pre-infection levels. This inefficient, stepwise expansion of both the Pf-specific memory B-cell and long-lived antibody compartments depends on Pf exposure rather than age, based on the comparator response to tetanus vaccination that was efficient and stable. These observations lend new insights into the cellular basis of the delayed acquisition of malaria immunity.

Plasmodium falciparum (Pf) is a mosquito-borne parasite that causes over 500 million cases of malaria annually, one million of which result in death, primarily among African children. The development of an effective malaria vaccine would be a critical step toward the control and eventual elimination of this disease. To date, most licensed vaccines are for pathogens that induce long-lived protective antibodies after a single infection. In contrast, immunity to malaria is only acquired after repeated infections. Antibodies play a key role in protection from malaria, yet several studies indicate that antibodies against some Pf proteins are generated inefficiently and lost rapidly. The cells that are responsible for the maintenance of antibodies over the human lifespan are memory B-cells and long-lived plasma cells. To determine how these cells are generated and maintained in response to Pf infection, we conducted a year-long study in an area of Mali that experiences a six-month malaria season. We found memory B-cells and long-lived antibodies specific for the parasite were generated in a gradual, step-wise fashion over years despite intense Pf exposure. This contrasts sharply with the efficient response to tetanus vaccination in the same population. This study lends new insights into the delayed acquisition of malaria immunity. Future studies of the cellular and molecular basis of these observations could open the door to strategies for the development of a highly effective malaria vaccine.

A prospective analysis of the Ab response to Plasmodium falciparum before and after a malaria season by protein microarray

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 approximately 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.

Differing effects of HbS and HbC traits on uncomplicated falciparum malaria, anemia, and child growth

The high prevalence of hemoglobin S (HbS) in Africa and hemoglobin C (HbC) in parts of West Africa is caused by the strong protection against severe falciparum malaria during childhood. Much less is known about the effect of HbS and especially HbC on Plasmodium falciparum infection, uncomplicated malaria, and anemia. A total of 1070 children from the Ashanti Region, Ghana, were enrolled at the age of 3 months and visited monthly until 2 years of age. The effects of the beta-globin genotype on the age-dependent incidence of malaria, levels of parasitemia, and hemoglobin as well as physical development were analyzed by population-averaged models. Infants with HbAS were protected from uncomplicated malaria (P < .005) and anemia (P < .001), had lower age-adjusted parasite densities (P < .001), and higher age-adjusted hemoglobin levels compared with children with the HbAA genotype (P = .004). In contrast, HbAC carriers had lower hemoglobin levels (P < .033) and were not protected against malaria or anemia. Notably, infants with HbAS were also significantly protected against stunting compared with carriers of HbAA or HbAC. This indicates differing mechanisms of protection against malaria of HbAS and HbAC and might help to understand why HbC is restricted to distinct areas of West Africa.

In vitro growth-inhibitory activity and malaria risk in a cohort study in mali

Immunity to the asexual blood stage of Plasmodium falciparum is complex and likely involves several effector mechanisms. Antibodies are thought to play a critical role in malaria immunity, and a corresponding in vitro correlate of antibody-mediated immunity has long been sought to facilitate malaria vaccine development. The growth inhibition assay (GIA) measures the capacity of antibodies to limit red blood cell (RBC) invasion and/or growth of P. falciparum in vitro. In humans, naturally acquired and vaccine-induced P. falciparum-specific antibodies have growth-inhibitory activity, but it is unclear if growth-inhibitory activity correlates with protection from clinical disease. In a longitudinal study in Mali, purified IgGs, obtained from plasmas collected before the malaria season from 220 individuals aged 2 to 10 and 18 to 25 years, were assayed for growth-inhibitory activity. Malaria episodes were recorded by passive surveillance over the subsequent 6-month malaria season. Logistic regression showed that greater age (odds ratio [OR], 0.78; 95% confidence interval [95% CI], 0.63 to 0.95; P = 0.02) and growth-inhibitory activity (OR, 0.50; 95% CI, 0.30 to 0.85; P = 0.01) were significantly associated with decreased malaria risk in children. A growth-inhibitory activity level of 40% was determined to be the optimal cutoff for discriminating malaria-immune and susceptible individuals in this cohort, with a sensitivity of 97.0%, but a low specificity of 24.3%, which limited the assay's ability to accurately predict protective immunity and to serve as an in vitro correlate of antibody-mediated immunity. These data suggest that antibodies which block merozoite invasion of RBC and/or inhibit the intra-RBC growth of the parasite contribute to but are not sufficient for the acquisition of malaria immunity.

Retention of erythrocytes in the spleen: a double-edged process in human malaria

PURPOSE OF REVIEW

Splenomegaly is frequent in acute or chronic forms of Plasmodium falciparum malaria, and splenectomy is associated with more frequent fever and parasitaemia. A novel role for the spleen in malaria is indicated by recent epidemiological and experimental data, bringing about a novel paradigm on severe malaria pathogenesis.

RECENT FINDINGS

In Sudanese children, severe malarial anaemia was associated with larger spleen, longer fever duration, and lower parasitaemia than cerebral malaria. These findings are consistent with evolution toward severe malarial anaemia being linked to the presence of a spleen-dependent mechanism that is absent or inefficient in cerebral malaria. An isolated-perfused human spleen model revealed unexpected retention of numerous erythrocytes harbouring young parasite stages (rings), probably through an innate mechanical process.

SUMMARY

A new paradigm is discussed, whereby the extent of erythrocyte retention in the spleen conditions not only haemoglobin concentration and spleen size but also the rate of parasite load increase. The prediction is that, in nonimmune children, stringent splenic retention of rings and uninfected erythrocytes reduces the risk of cerebral malaria (a complication associated with high parasite loads) but increases the risk of severe malarial anaemia. This hypothesis casts new light on epidemiological, genetic, and experimental studies in malaria pathogenesis.

Atypical memory B cells are greatly expanded in individuals living in a malaria-endemic area

Epidemiological observations in malaria endemic areas have long suggested a deficiency in the generation and maintenance of B cell memory to Plasmodium falciparum (Pf) in individuals chronically reinfected with the parasite. Recently, a functionally and phenotypically distinct population of FCRL4(+) hyporesponsive memory B cells (MBCs) was reported to be expanded in HIV-infected individuals with high viral loads. In this study, we provide evidence that a phenotypically similar atypical MBC population is significantly expanded in Pf-exposed Malian adults and children as young as 2 years of age as compared with healthy U.S. adult controls. The number of these atypical MBCs was higher in children with chronic asymptomatic Pf infections compared with uninfected children, suggesting that the chronic presence of the parasite may drive expansion of these distinct MBCs. This is the first description of an atypical MBC phenotype associated with malaria. Understanding the origin and function of these MBCs could be important in informing the design of malaria vaccines.

World Malaria Day 2009: what malaria knows about the immune system that immunologists still do not

Malaria kills >1 million children each year, and there is little doubt that an effective vaccine would play a central role in preventing these deaths. However, the strategies that proved so successful in developing the vaccines we have today may simply not be adequate to confront complex, persistent infectious diseases, including malaria, AIDS, and tuberculosis. We believe that the development of a highly effective vaccine will require a better understanding of several features of the immune response to malaria. At the top of the list is the complex and ancient relationship between the parasite that causes malaria and the immune system that enables the parasite to persist in an otherwise functional immune system. A close second is the antigenic targets in malaria and how to overcome the enormous polymorphism of these targets. Meeting these challenges represents a call to arms of basic immunologists to advance our knowledge of malaria immunity.

The TLR9 ligand CpG promotes the acquisition of Plasmodium falciparum-specific memory B cells in malaria-naive individuals

Despite the central role of memory B cells (MBC) in protective immune responses, little is understood about how they are acquired in naive individuals in response to Ag exposure, and how this process is influenced by concurrent activation of the innate immune system's TLR. In this longitudinal study of malaria-naive individuals, we examined the MBC response to two candidate malaria vaccines administered with or without CpG, a TLR9 ligand. We show that the acquisition of MBC is a dynamic process in which the vaccine-specific MBC pool rapidly expands and then contracts, and that CpG enhances the kinetics, magnitude, and longevity of this response. We observed that the percentage of vaccine-specific MBC present at the time of reimmunization predicts vaccine-specific Ab levels 14 days later; and that at steady-state, there is a positive correlation between vaccine-specific MBC and Ab levels. An examination of the total circulating MBC and plasma cell pools also suggests that MBC differentiate into plasma cells through polyclonal activation, independent of Ag specificity. These results provide important insights into the human MBC response, which can inform the development of vaccines against malaria and other pathogens that disrupt immunological memory.

Plasmodium falciparum signal peptide peptidase is a promising drug target against blood stage malaria

The resistance of malaria parasites to current anti-malarial drugs is an issue of major concern globally. Recently we identified a Plasmodium falciparum cell membrane aspartyl protease, which binds to erythrocyte band 3, and is involved in merozoite invasion. Here we report the complete primary structure of P. falciparum signal peptide peptidase (PfSPP), and demonstrate that it is essential for parasite invasion and growth in human erythrocytes. Gene silencing suggests that PfSPP may be essential for parasite survival in human erythrocytes. Remarkably, mammalian signal peptide peptidase inhibitors (Z-LL)(2)-ketone and L-685,458 effectively inhibited malaria parasite invasion as well as growth in human erythrocytes. In contrast, DAPT, an inhibitor of a related gamma-secretase/presenilin-1, was ineffective. Thus, SPP inhibitors specific for PfSPP may function as potent anti-malarial drugs against the blood stage malaria.