Anemia in parasite- and recombinant protein-immunized aotus monkeys infected with Plasmodium falciparum

The human malaria-Aotus monkey model: a historical perspective in antimalarial chemotherapy research at the Gorgas Memorial Laboratory-Panama

The human malaria-Aotus monkey model has served the malaria research community since its inception in 1966 at the Gorgas Memorial Laboratory (GML) in Panama. Spanning over five decades, this model has been instrumental in evaluating the in vivo efficacy and pharmacokinetics of a wide array of candidate antimalarial drugs, whether used singly or in combination. The animal model could be infected with drug-resistant and susceptible Plasmodium falciparum and Plasmodium vivax strains that follow a characteristic and reproducible course of infection, remarkably like human untreated and treated infections. Over the years, the model has enabled the evaluation of several synthetic and semisynthetic endoperoxides, for instance, artelinic acid, artesunate, artemether, arteether, and artemisone. These compounds have been evaluated alone and in combination with long-acting partner drugs, commonly referred to as artemisinin-based combination therapies, which are recommended as first-line treatment against uncomplicated malaria. Further, the model has also supported the evaluation of the primaquine analog tafenoquine against blood stages of P. vivax, contributing to its progression to clinical trials and eventual approval. Besides, the P. falciparum/Aotus model at GML has also played a pivotal role in exploring the biology, immunology, and pathogenesis of malaria and in the characterization of drug-resistant P. falciparum and P. vivax strains. This minireview offers a historical overview of the most significant contributions made by the Panamanian owl monkey (Aotus lemurinus lemurinus) to malaria chemotherapy research.

Sterile protection against P. vivax malaria by repeated blood stage infection in the Aotus monkey model

The malaria parasite Plasmodium vivax remains a major global public health challenge, and no vaccine is approved for use in humans. Here, we assessed whether P. vivax strain-transcendent immunity can be achieved by repeated infection in Aotus monkeys. Sterile immunity was achieved after two homologous infections, whereas subsequent heterologous challenge provided only partial protection. IgG levels based on P. vivax lysate ELISA and protein microarray increased with repeated infections and correlated with the level of homologous protection. Parasite transcriptional profiles provided no evidence of major antigenic switching upon homologous or heterologous challenge. However, we observed significant sequence diversity and transcriptional differences in the P. vivax core gene repertoire between the two strains used in the study, suggesting that partial protection upon heterologous challenge is due to molecular differences between strains rather than immune evasion by antigenic switching. Our study demonstrates that sterile immunity against P. vivax can be achieved by repeated homologous blood stage infection in Aotus monkeys, thus providing a benchmark to test the efficacy of candidate blood stage P. vivax malaria vaccines.

A Plasmodium vivax Plasmid DNA- and Adenovirus-Vectored Malaria Vaccine Encoding Blood-Stage Antigens AMA1 and MSP142 in a Prime/Boost Heterologous Immunization Regimen Partially Protects Aotus Monkeys against Blood-Stage Challenge

Malaria is caused by parasites of the genus Plasmodium, which are transmitted to humans by the bites of Anopheles mosquitoes. After the elimination of Plasmodium falciparum, it is predicted that Plasmodium vivax will remain an important cause of morbidity and mortality outside Africa, stressing the importance of developing a vaccine against P. vivax malaria. In this study, we assessed the immunogenicity and protective efficacy of two P. vivax antigens, apical membrane antigen 1 (AMA1) and the 42-kDa C-terminal fragment of merozoite surface protein 1 (MSP1₄₂) in a plasmid recombinant DNA prime/adenoviral (Ad) vector boost regimen in Aotus monkeys. Groups of 4 to 5 monkeys were immunized with plasmid DNA alone, Ad alone, prime/boost regimens with each antigen, prime/boost regimens with both antigens, and empty vector controls and then subjected to blood-stage challenge. The heterologous immunization regimen with the antigen pair was more protective than either antigen alone or both antigens delivered with a single vaccine platform, on the basis of their ability to induce the longest prepatent period and the longest time to the peak level of parasitemia, the lowest peak and mean levels of parasitemia, the smallest area under the parasitemia curve, and the highest self-cure rate. Overall, prechallenge MSP1₄₂ antibody titers strongly correlated with a decreased parasite burden. Nevertheless, a significant proportion of immunized animals developed anemia. In conclusion, the P. vivax plasmid DNA/Ad serotype 5 vaccine encoding blood-stage parasite antigens AMA1 and MSP1₄₂ in a heterologous prime/boost immunization regimen provided significant protection against blood-stage challenge in Aotus monkeys, indicating the suitability of these antigens and this regimen for further development.

Persistence and immunogenicity of chemically attenuated blood stage Plasmodium falciparum in Aotus monkeys

Malaria is a disease caused by a protozoan of the Plasmodium genus and results in 0.5-0.7million deaths per year. Increasing drug resistance of the parasite and insecticide resistance of mosquitoes necessitate alternative control measures. Numerous vaccine candidates have been identified but none have been able to induce robust, long-lived protection when evaluated in malaria endemic regions. Rodent studies have demonstrated that chemically attenuated blood stage parasites can persist at sub-patent levels and induce homologous and heterologous protection against malaria. Parasite-specific cellular responses were detected, with protection dependent on CD4+ T cells. To investigate this vaccine approach for Plasmodium falciparum, we characterised the persistence and immunogenicity of chemically attenuated P. falciparum FVO strain parasites (CAPs) in non-splenectomised Aotus nancymaae monkeys following administration of a single dose. Control monkeys received either normal red blood cells or wild-type parasites followed by drug treatment. Chemical attenuation was performed using tafuramycin A, which irreversibly binds to DNA. CAPs were detected in the peripheral blood for up to 2days following inoculation as determined by thick blood smears, and for up to 8days as determined by quantitative PCR. Parasite-specific IgG was not detected in monkeys that received CAPs; however, in vitro parasite-specific T cell proliferation was observed. Following challenge, the CAP monkeys developed an infection; however, one CAP monkey and the infection and drug-cure monkeys showed partial or complete resistance. These experiments lay the groundwork for further assessment of CAPs as a potential vaccine against malaria.

Social status, immune response and parasitism in males: a meta-analysis

In male vertebrates, two conflicting paradigms--the energetic costs of high dominance rank and the chronic stress of low rank--have been proposed to explain patterns of immune function and parasitism. To date, neither paradigm has provided a complete explanation for status-related differences in male health. Here, we applied meta-analyses to test for correlations between male social status, immune responses and parasitism. We used an ecoimmunological framework, which proposes that males should re-allocate investment in different immune components depending on the costs of dominance or subordination. Spanning 297 analyses, from 77 studies on several vertebrate taxa, we found that most immune responses were similar between subordinate and dominant males, and neither dominant nor subordinate males consistently invested in predictable immune components. However, subordinate males displayed significantly lower delayed-type hypersensitivity and higher levels of some inflammatory cytokines than dominant males, while dominant males exhibited relatively lower immunoglobulin responses than subordinate males. Despite few differences in immunity, dominant males exhibited consistently higher parasitism than subordinate males, including protozoan blood parasites, ectoparasites and gastrointestinal helminths. We discuss our results in the context of the costs of dominance and subordination and advocate future work that measures both parasitism and immune responses in wild systems.

Malaria induces anemia through CD8+ T cell-dependent parasite clearance and erythrocyte removal in the spleen

Severe malarial anemia (SMA) in semi-immune individuals eliminates both infected and uninfected erythrocytes and is a frequent fatal complication. It is proportional not to circulating parasitemia but total parasite mass (sequestered) in the organs. Thus, immune responses that clear parasites in organs may trigger changes leading to anemia. Here, we use an outbred-rat model where increasing parasite removal in the spleen escalated uninfected-erythrocyte removal. Splenic parasite clearance was associated with activated CD8(+) T cells, immunodepletion of which prevented parasite clearance. CD8(+) T cell repletion and concomitant reduction of the parasite load was associated with exacerbated (40 to 60%) hemoglobin loss and changes in properties of uninfected erythrocytes. Together, these data suggest that CD8(+) T cell-dependent parasite clearance causes erythrocyte removal in the spleen and thus anemia. In children infected with the human malaria parasite Plasmodium falciparum, elevation of parasite biomass (not the number of circulating parasites) increased the odds ratio for SMA by 3.5-fold (95% confidence intervals [CI95%], 1.8- to 7.5-fold). CD8(+) T cell expansion/activation independently increased the odds ratio by 2.4-fold (CI95%, 1.0- to 5.7-fold). Concomitant increases in both conferred a 7-fold (CI95%, 1.9- to 27.4-fold)-greater risk for SMA. Together, these data suggest that CD8(+)-dependent parasite clearance may predispose individuals to uninfected-erythrocyte loss and SMA, thus informing severe disease diagnosis and strategies for vaccine development.

IMPORTANCE

Malaria is a major global health problem. Severe malaria anemia (SMA) is a complex disease associated with partial immunity. Rapid hemoglobin reductions of 20 to 50% are commonly observed and must be rescued by transfusion (which can carry a risk of HIV acquisition). The causes and risk factors of SMA remain poorly understood. Recent studies suggest that SMA is linked to parasite biomass sequestered in organs. This led us to investigate whether immune mechanisms that clear parasites in organs trigger anemia. In rats, erythropoiesis is largely restricted to the bone marrow, and critical aspects of the spleen expected to be important in anemia are similar to those in humans. Therefore, using a rat model, we show that severe anemia is caused through CD8(+) T cell-dependent parasite clearance and erythrocyte removal in the spleen. CD8 activation may also be a new risk factor for SMA in African children.

Sequential Plasmodium chabaudi and Plasmodium berghei infections provide a novel model of severe malarial anemia

Lack of an adequate animal model of Plasmodium falciparum severe malarial anemia (SMA) has hampered the understanding of this highly lethal condition. We developed a model of SMA by infecting C57BL/6 mice with P. chabaudi followed after recovery by P. berghei infection. P. chabaudi/P. berghei-infected mice had an initial 9- to 10-day phase of relatively low parasitemia and severe anemia, followed by a second phase of hyperparasitemia, more profound anemia, reticulocytosis, and death 14 to 21 days after infection. P. chabaudi/P. berghei-infected animals had more intense splenic hematopoiesis, higher interleukin-10 (IL-10)/tumor necrosis factor alpha and IL-12/gamma interferon (IFN-γ) ratios, and higher antibody levels against P. berghei and P. chabaudi antigens than P. berghei-infected or P. chabaudi-recovered animals. Early treatment with chloroquine or artesunate did not prevent the anemia, suggesting that the bulk of red cell destruction was not due to the parasite. Red cells from P. chabaudi/P. berghei-infected animals had increased surface IgG and C3 by flow cytometry. However, C3(-/-) mice still developed anemia. Tracking of red cells labeled ex vivo and in vivo and analysis of frozen tissue sections by immunofluorescence microscopy showed that red cells from P. chabaudi/P. berghei-infected animals were removed at an accelerated rate in the liver by erythrophagocytosis. This model is practical and reproducible, and its similarities with P. falciparum SMA in humans makes it an appealing system with which to study the pathogenesis of this condition and explore potential immunomodulatory interventions.

Malaria-related anaemia: a Latin American perspective

Malaria is the most important parasitic disease worldwide, responsible for an estimated 225 million clinical cases each year. It mainly affects children, pregnant women and non-immune adults who frequently die victims of cerebral manifestations and anaemia. Although the contribution of the American continent to the global malaria burden is only around 1.2 million clinical cases annually, there are 170 million inhabitants living at risk of malaria transmission in this region. On the African continent, where Plasmodium falciparum is the most prevalent human malaria parasite, anaemia is responsible for about half of the malaria-related deaths. Conversely, in Latin America (LA), malaria-related anaemia appears to be uncommon, though there is a limited knowledge about its real prevalence. This may be partially explained by several factors, including that the overall malaria burden in LA is significantly lower than that of Africa, that Plasmodium vivax, the predominant Plasmodium species in the region, appears to display a different clinical spectrus and most likely because better health services in LA prevent the development of severe malaria cases. With the aim of contributing to the understanding of the real importance of malaria-related anaemia in LA, we discuss here a revision of the available literature on the subject and the usefulness of experimental animal models, including New World monkeys, particularly for the study of the mechanisms involved in the pathogenesis of 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.

Pharmacokinetics, safety, and hydrolysis of oral pyrroloquinazolinediamines administered in single and multiple doses in rats

Pyrroloquinazolinediamine (PQD) derivatives such as tetra-acetamide PQD (PQD-A4) and bis-ethylcarbamyl PQD (PQD-BE) were much safer (with therapeutic indices of 80 and 32, respectively) than their parent compound, PQD (therapeutic index, 10). Further evaluation of PQD-A4 and PQD-BE in single and multiple pharmacokinetic (PK) studies as well as corresponding toxicity studies was conducted with rats. PQD-A4 could be converted to two intermediate metabolites (monoacetamide PQD and bisacetamide PQD) first and then to the final metabolite, PQD, while PQD-BE was directly hydrolyzed to PQD without precursor and intermediate metabolites. Maximum tolerant doses showed that PQD-A4 and PQD-BE have only 1/12 and 1/6, respectively, of the toxicity of PQD after a single oral dose. Compared to the area under the concentration-time curve for PQD alone (2,965 ng.h/ml), values measured in animals treated with PQD-A4 and PQD-BE were one-third (1,047 ng.h/ml) and one-half (1,381 ng.h/ml) as high, respectively, after an equimolar dosage, suggesting that PQD was the only agent to induce the toxicity. Similar results were also shown in multiple treatments; PQD-A4 and PQD-BE generated two-fifths and three-fifths, respectively, of PQD concentrations, with 8.8-fold and 3.8-fold safety margins, respectively, over the parent drug. PK data indicated that the bioavailability of oral PQD-A4 was greatly limited at high dose levels, that PQD-A4 was slowly converted to PQD via a sequential three-step process of conversion, and that PQD-A4 was significantly less toxic than the one-step hydrolysis drug, PQD-BE. It was concluded that the slow and smaller release of PQD was the main reason for the reduction in toxicity and that the active intermediate metabolites can still maintain antimalarial potency. Therefore, the candidate with multiple-step hydrolysis of PQD could be developed as a safer potential agent for malaria treatment.

Malarial anemia: of mice and men

Severe malaria is manifest by a variety of clinical syndromes dependent on properties of both the host and the parasite. In young infants, severe malarial anemia (SMA) is the most common syndrome of severe disease and contributes substantially to the considerable mortality and morbidity from malaria. There is now growing evidence, from both human and mouse studies of malaria, to show that anemia is due not only to increased hemolysis of infected and clearance of uninfected red blood cells (RBCs) but also to an inability of the infected host to produce an adequate erythroid response. In this review, we will summarize the recent clinical and experimental studies of malaria to highlight similarities and differences in human and mouse pathology that result in anemia and so inform the use of mouse models in the study of severe malarial anemia in humans.

Intravascular infiltrates and organ-specific inflammation in malaria pathogenesis

Malaria infects 5-10% of humanity and causes around two million deaths annually, mostly in children. The disease is of significant interest to immunologists, as acquired host immunity can limit the clinical impact of infection and partially reduces parasite replication; however, immunological reactions also contribute significantly to pathogenesis and fatalities. This review addresses the view that immunopathology in severe malaria arises predominantly from intravascular lesions resulting from a pathogen-initiated cascade of activated immune effector and regulatory cells infiltrating the vascular beds of diverse target organs, including bone marrow, spleen, brain, placenta and lungs. The main feature distinguishing these processes from classical cellular inflammation is the absence of extravasation, resulting from the intravascular location of the pathogen. Clinical and epidemiological observations combined with experimental infections in animal models suggest that parasite 'molecular patterns' or toxins cause cytokine and chemokine enhancement of infiltrates, composed of macrophages, neutrophils, natural killer (NK) cells, invariant natural killer T (iNKT) cells, gamma/delta T cells and both CD4(+) and CD8(+) effector T cells, leading to local vascular and organ derangement. Diverse pattern recognition and NK receptors crucially regulate these responding cell populations. Thus, innate immune mechanisms lie at the heart of this massive global public health problem.

Phase 1 randomized double-blind safety and immunogenicity trial of Plasmodium falciparum malaria merozoite surface protein FMP1 vaccine, adjuvanted with AS02A, in adults in western Kenya

We report the first trial of candidate malaria vaccine antigen FMP1, a 42kDa fragment from the C-terminus of merozoite surface protein-1 (MSP-1) from the 3D7 strain of Plasmodium falciparum, in an endemic area. Forty adult male and female residents of western Kenya were enrolled to receive 3 doses of either FMP1/AS02A or Imovax rabies vaccine by intra-deltoid injection on a 0, 1, 2 month schedule. Thirty-seven volunteers received all three immunizations and 38 completed the 12-month evaluation period. Slightly more recipients of the FMP1/AS02A vaccine experienced any instance of pain at 24 h post-immunization than in the Imovax group (95% versus 65%), but otherwise the two vaccines were equally safe and well-tolerated. Baseline antibody levels were high in both groups and were boosted in the FMP1/AS02A group. Longitudinal models revealed a highly significant difference between groups for both the average post-baseline antibody responses to MSP-1(42) (F1,335=13.16; P<0.001) and the Day 90 responses to MSP-1(42) (F1,335=16.69; P<0.001). The FMP1/AS02A vaccine is safe and immunogenic in adults and should progress to safety testing in children at greatest risk of malaria.

Safety and reactogenicity of an MSP-1 malaria vaccine candidate: a randomized phase Ib dose-escalation trial in Kenyan children

OBJECTIVE

Our aim was to evaluate the safety, reactogenicity, and immunogenicity of an investigational malaria vaccine.

DESIGN

This was an age-stratified phase Ib, double-blind, randomized, controlled, dose-escalation trial. Children were recruited into one of three cohorts (dosage groups) and randomized in 2:1 fashion to receive either the test product or a comparator.

SETTING

The study was conducted in a rural population in Kombewa Division, western Kenya.

PARTICIPANTS

Subjects were 135 children, aged 12-47 mo.

INTERVENTIONS

Subjects received 10, 25, or 50 microg of falciparum malaria protein 1 (FMP1) formulated in 100, 250, and 500 microL, respectively, of AS02A, or they received a comparator (Imovax (rabies vaccine).

OUTCOME MEASURES

We performed safety and reactogenicity parameters and assessment of adverse events during solicited (7 d) and unsolicited (30 d) periods after each vaccination. Serious adverse events were monitored for 6 mo after the last vaccination.

RESULTS

Both vaccines were safe and well tolerated. FMP1/AS02A recipients experienced significantly more pain and injection-site swelling with a dose-effect relationship. Systemic reactogenicity was low at all dose levels. Hemoglobin levels remained stable and similar across arms. Baseline geometric mean titers were comparable in all groups. Anti-FMP1 antibody titers increased in a dose-dependent manner in subjects receiving FMP1/AS02A; no increase in anti-FMP1 titers occurred in subjects who received the comparator. By study end, subjects who received either 25 or 50 microg of FMP1 had similar antibody levels, which remained significantly higher than that of those who received the comparator or 10 microg of FMP1. A longitudinal mixed effects model showed a statistically significant effect of dosage level on immune response (F(3,1047) = 10.78, or F(3, 995) = 11.22, p < 0.001); however, the comparison of 25 microg and 50 microg recipients indicated no significant difference (F(1,1047) = 0.05; p = 0.82).

CONCLUSIONS

The FMP1/AS02A vaccine was safe and immunogenic in malaria-exposed 12- to 47-mo-old children and the magnitude of immune response of the 25 and 50 microg doses was superior to that of the 10 microg dose.

New potential antimalarial agents: therapeutic-index evaluation of pyrroloquinazolinediamine and its prodrugs in a rat model of severe malaria

Tetra-acetamide pyrroloquinazolinediamine (PQD-A4) and bis-ethylcarbamyl pyrroloquinazolinediamine (PQD-BE) are new derivatives of pyrroloquinazolinediamine (PQD) and are being investigated as potential chemotherapeutic agents for the treatment of malaria. Comparative studies to assess the therapeutic indices of PQD-A4, PQD-BE, and PQD were conducted in Plasmodium berghei-infected rats following daily intragastric dosing for three consecutive days. Artesunate (AS), a standard drug for treatment of severe malaria, was used as a comparator. The minimum doses required to clear malaria parasitemia were 156 micromol/kg of body weight for AS and 2.4 micromol/kg for PQD, PQD-4A, and PQD-BE. The maximum tolerated dose (MTD) of AS was 625 micromol/kg, and its therapeutic index was calculated to be 4. The MTDs of PQD-A4, PQD-BE, and PQD were found to be 190, 77, and 24 micromol/kg, respectively, yielding therapeutic indices of 80, 32, and 10, respectively. Although PQD-A4 and PQD-BE are only half as potent as PQD based on their curative effects, the two new derivatives, PQD-4A and PQD-BE, are 8.0-fold and 3.2-fold safer, respectively, than their parent compound when they are dosed for three consecutive days. Oral PQD-A4 and PQD-BE are 44 to 70 times more potent on an mg basis than intravenous AS. As assessed from the therapeutic index over 3 days, PQD-A4, PQD-BE, and PQD administered orally are 20.0, 8.0, and 2.5 times safer than AS given intravenously. The results indicate that PQD-4A is a promising candidate for antimalarial treatment.

Surgical bone marrow aspiration in Aotus lemurinus griseimembra

Aotus lemurinus griseimembra are highly susceptible to infection by human malaria parasites and reproduce some of its clinical manifestations, including anemia. We developed a new surgical technique to obtain bone marrow samples from Aotus by surgical aspiration of the femur. First, we determined that the femur offered advantages over other bones, primarily due to lower fracture vulnerability. We tested a surgical technique using 20 G IV catheters in formaldehyde-preserved animals, then conducted the procedure on 27 live animals. This technique provided easy, quick surgical access to adequate volumes of bone marrow and was safe for almost all animals: only one died; another developed nervous impairment of the lower limb. Adequate cell samples were obtained in all animals and allowed cytological studies. This procedure offers a useful tool for bone marrow research in Aotus and helps overcome current limitations of such research in human where these studies are limited by ethical and technical issues.

A critical role for the host mediator macrophage migration inhibitory factor in the pathogenesis of malarial anemia

The pathogenesis of malarial anemia is multifactorial, and the mechanisms responsible for its high mortality are poorly understood. Studies indicate that host mediators produced during malaria infection may suppress erythroid progenitor development (Miller, K.L., J.C. Schooley, K.L. Smith, B. Kullgren, L.J. Mahlmann, and P.H. Silverman. 1989. Exp. Hematol. 17:379–385; Yap, G.S., and M.M. Stevenson. 1991. Ann. NY Acad. Sci. 628:279–281). We describe an intrinsic role for macrophage migration inhibitory factor (MIF) in the development of the anemic complications and bone marrow suppression that are associated with malaria infection. At concentrations found in the circulation of malaria-infected patients, MIF suppressed erythropoietin-dependent erythroid colony formation. MIF synergized with tumor necrosis factor and γ interferon, which are known antagonists of hematopoiesis, even when these cytokines were present in subinhibitory concentrations. MIF inhibited erythroid differentiation and hemoglobin production, and it antagonized the pattern of mitogen-activated protein kinase phosphorylation that normally occurs during erythroid progenitor differentiation. Infection of MIF knockout mice with Plasmodium chabaudi resulted in less severe anemia, improved erythroid progenitor development, and increased survival compared with wild-type controls. We also found that human mononuclear cells carrying highly expressed MIF alleles produced more MIF when stimulated with the malarial product hemozoin compared with cells carrying low expression MIF alleles. These data suggest that polymorphisms at the MIF locus may influence the levels of MIF produced in the innate response to malaria infection and the likelihood of anemic complications.

Severe malarial anemia of low parasite burden in rodent models results from accelerated clearance of uninfected erythrocytes

Severe malarial anemia (SMA) is the most frequent life-threatening complication of malaria and may contribute to the majority of malarial deaths worldwide. To explore the mechanisms of pathogenesis, we developed a novel murine model of SMA in which parasitemias peaked around 1.0% of circulating red blood cells (RBCs) and yet hemoglobin levels fell to 47% to 56% of baseline. The severity of anemia was independent of the level of peak or cumulative parasitemia, but was linked kinetically to the duration of patent infection. In vivo biotinylation analysis of the circulating blood compartment revealed that anemia arose from accelerated RBC turnover. Labeled RBCs were reduced to 1% of circulating cells by 8 days after labeling, indicating that the entire blood compartment had been turned over in approximately one week. The survival rate of freshly transfused RBCs was also markedly reduced in SMA animals, but was not altered when RBCs from SMA donors were transferred into naive recipients, suggesting few functional modifications to target RBCs. Anemia was significantly alleviated by depletion of either phagocytic cells or CD4+ T lymphocytes. This study demonstrates that immunologic mechanisms may contribute to SMA by promoting the accelerated turnover of uninfected RBCs.

Immunological processes in malaria pathogenesis

Malaria is possibly the most serious infectious disease of humans, infecting 5-10% of the world's population, with 300-600 million clinical cases and more than 2 million deaths annually. Adaptive immune responses in the host limit the clinical impact of infection and provide partial, but incomplete, protection against pathogen replication; however, these complex immunological reactions can contribute to disease and fatalities. So, appropriate regulation of immune responses to malaria lies at the heart of the host-parasite balance and has consequences for global public health. This Review article addresses the innate and adaptive immune mechanisms elicited during malaria that either cause or prevent disease and fatalities, and it considers the implications for vaccine design.

Gamma interferon responses to Plasmodium falciparum liver-stage antigen 1 and thrombospondin-related adhesive protein and their relationship to age, transmission intensity, and protection against malaria

Gamma interferon (IFN-gamma) responses to the Plasmodium falciparum antigens liver-stage antigen 1 (LSA-1) and thrombospondin-related adhesive protein (TRAP) are thought to be important in protection against malaria. Optimal methods of testing and the effects of age and transmission intensity on these responses are unknown. IFN-gamma responses to LSA-1 and TRAP peptides were assessed by the enzyme-linked immunospot assay (ELISPOT) and enzyme-linked immunosorbent assay (ELISA) in children and adults from areas of stable and unstable malaria transmission in Kenya. Adults in the areas of stable and unstable transmission had similar frequencies and levels of IFN-gamma responses to LSA-1 and TRAP as determined by ELISPOT and ELISA. In contrast, IFN-gamma responses to the LSA-1 T3 peptide (assessed by ELISPOT) and to any LSA-1 peptide (assessed by ELISA) were less frequent in children in the area of unstable transmission than in children in the area of stable transmission. IFN-gamma responses to LSA-1 were more frequently detected by ELISA than by ELISPOT in the stable-transmission area. IFN-gamma responses detected by ELISA and ELISPOT did not correlate with each other. In children in the stable-transmission area, IFN-gamma responses to LSA-1 peptides assessed by ELISA, but not by ELISPOT, were associated with protection against clinical malaria and anemia. IFN-gamma responses to LSA-1 appear to require repeated P. falciparum exposure and/or increased age and, as measured by ELISA, are associated with protection against clinical malaria and anemia.

The etiology of severe anemia in a village and a periurban area in Mali

Severe anemia is one of the major complications of malaria in Africa. We studied 2 populations, one in a village and the second in a periurban area in Mali, to understand the preventable factors in the disease. The 2 correlates of disease were parasitemia above 100 000 parasitized red blood cells per microliter (0.1 x 10(12)/L) and a low baseline hemoglobin level. All cases of moderate to severe anemia occurred in children under 3.2 years of age. Raising the baseline hemoglobin level and lowering peak parasitemia in infants and young children may reduce the incidence of severe anemia resulting from malarial infection.

Severe anemia affects both splenectomized and non-splenectomized Plasmodium falciparum-infected Aotus infulatus monkeys

Severe anemia is the earliest and a frequently fatal complication of Plasmodium falciparum infection. Here we describe Aotus infulatus as a primate model suitable to study this malaria complication. Both non-splenectomized and splenectomized monkeys receiving different inocula of P. falciparum FVO strain presented large (> 50%) decreases in hematocrit values during infection. Non-splenectomized animals were able to control parasite growth (parasitemia did not exceed 4%), but they had to be treated because of severe anemia. Three of 4 splenectomized monkeys did not control parasitemia and were treated, but developed severe anemia after treatment when presenting a negative blood film. Destruction of parasitized red blood cells alone cannot account for the degree of anemia. Non-splenectomized monkeys repeatedly infected with homologous parasites became rapidly and progressively resistant to reinfection and to the development of severe anemia. The data presented here point to A. infulatus as a suitable model for studying the pathogenesis of severe malarial infection.