Continuous cultivation of intraerythrocytic Plasmodium falciparum in a serum-free medium with the use of a growth-promoting factor

Dihydroartemisinin Disrupts Zinc Homeostasis in Plasmodium falciparum To Potentiate Its Antimalarial Action via Pyknosis

Artemisinins have been used as first-line drugs worldwide to treat malaria caused by Plasmodium falciparum; however, its underlying mechanism is still unclear. This study aimed to identify the factors inducing growth inhibition via pyknosis, a state of intraerythrocytic developmental arrest, when exposing the parasite to dihydroartemisinin (DHA). Changes in the expression of genome-wide transcripts were assessed in the parasites treated with antimalarials, revealing the specific downregulation of zinc-associated proteins by DHA. The quantification of zinc levels in DHA-treated parasite indicated abnormal zinc depletion. Notably, the zinc-depleted condition in the parasite produced by a zinc chelator induced the generation of a pyknotic form and the suppression of its proliferation. The evaluation of the antimalarial activity of DHA or a glutathione-synthesis inhibitor in the zinc-depleted state showed that the disruption of zinc and glutathione homeostasis synergistically potentiated the growth inhibition of P. falciparum through pyknosis. These findings could help further understand the antimalarial actions of artemisinins for advancing malaria therapy.

Cultivation of Asexual Intraerythrocytic Stages of Plasmodium falciparum

Successfully developed in 1976, the continuous in vitro culture of Plasmodium falciparum has many applications in the field of malaria research. It has become an important experimental model that directly uses a human pathogen responsible for a high prevalence of morbidity and mortality in many parts of the world and is a major source of biological material for immunological, biochemical, molecular, and pharmacological studies. Until present, the basic techniques described by Trager and Jensen and Haynes et al. remain unchanged in many malaria research laboratories. Nonetheless, different factors, including culture media, buffers, serum substitutes and supplements, sources of erythrocytes, and conditions of incubation (especially oxygen concentration), have been modified by different investigators to adapt the original technique in their laboratories or enhance the in vitro growth of the parasites. The possible effects and benefits of these modifications for the continuous cultivation of asexual intraerythrocytic stages of P. falciparum, as well as future challenges in developing a serum-free cultivation system and axenic cultures, are discussed.

Continuous Cultures of Plasmodium Falciparum Established in Tanzania from Patients with Acute Malaria

Background

Malaria morbidity and mortality, almost entirely from Plasmodium falciparum, are still rampant in Africa: therefore, it is important to study the biology of the parasite and the parasite-host cell interactions. In vitro cultivation of Plasmodium falciparum is most useful for this purpose, as well as for investigating drug resistance and possible new therapies. Here we report that the Trager & Jensen continuous culture of P. falciparum can be established in a laboratory in Tanzania with minimal facilities and with modest expenditure.

Methodology

This was an in-vitro set up of continuous culture of Plasmodium falciparum study, carried out in 2016–2020 at Muhimbili university of health and allied sciences, Dar-es salaam. Parasite samples were obtained from patients with acute malaria, frozen parasites, and live cultures. Data was collected and analyzed using GraphPad Prism version 8.

Results

We have successfully achieved exponential growth of existing strains that are used worldwide, as well as of parasites in clinical samples from patients with acute malaria. In the aim to optimize growth we have compared human serum and bovine serum albumin as components of the culture media. Additionally, culture synchronization has been achieved using sorbitol.

Conclusion

This experimental system is now available to our institution and to researchers aiming at investigating drug sensitivity and mechanisms of protection against Plasmodium falciparum that accrue from various genes expressed in red cells.

From marginal to essential: the golden thread between nutrient sensing, medium composition and Plasmodium vivax maturation in in vitro culture

Historically neglected, due to its biological peculiarities, the absence of a continuous long-term in vitro blood stage culture system and a propensity towards high morbidity rather than mortality, Plasmodium vivax was put back on the agenda during the last decade by the paradigm shift in the fight against malaria from malaria control to malaria eradication. While the incidence of the deadliest form of malaria, Plasmodium falciparum malaria, has declined since this paradigm shift took hold, the prospects of eradication are now threatened by the increase in the incidence of other human malaria parasite species. Plasmodium vivax is geographically the most widely distributed human malaria parasite, characterized by millions of clinical cases every year and responsible for a massive economic burden. The urgent need to tackle the unique biological challenges posed by this parasite led to renewed efforts aimed at establishing a continuous, long-term in vitro P. vivax blood stage culture. Based on recent discoveries on the role of nutrient sensing in Plasmodium’s pathophysiology, this review article critically assesses the extensive body of literature concerning Plasmodium culture conditions with a specific focus on culture media used in attempts to culture different Plasmodium spp. Hereby, the effect of specific media components on the parasite’s in vitro fitness and the maturation of the parasite’s host cell, the reticulocyte, is analysed. Challenging the wide-held belief that it is sufficient to find the right parasite isolate and give it the right type of cells to invade for P. vivax to grow in vitro, this review contends that a healthy side-by-side maturation of both the parasite and its host cell, the reticulocyte, is necessary in the adaptation of P. vivax to in vitro growth and argues that culture conditions and the media in particular play an essential role in this maturation process.

Profiling molecular factors associated with pyknosis and developmental arrest induced by an opioid receptor antagonist and dihydroartemisinin in Plasmodium falciparum

Malaria continues to be a devastating disease, largely caused by Plasmodium falciparum infection. We investigated the effects of opioid and cannabinoid receptor antagonists on the growth of intraerythrocytic P. falciparum. The delta opioid receptor antagonist 7-benzylidenenaltrexone (BNTX) and the cannabinoid receptor antagonists rimonaband and SR144528 caused growth arrest of the parasite. Notably BNTX and the established antimalarial drug dihydroartemisinin induced prominent pyknosis in parasite cells after a short period of incubation. We compared genome-wide transcriptome profiles in P. falciparum with different degrees of pyknosis in response to drug treatment, and identified 11 transcripts potentially associated with the evoking of pyknosis, of which three, including glutathione reductase (PfGR), triose phosphate transporter (PfoTPT), and a conserved Plasmodium membrane protein, showed markedly different gene expression levels in accordance with the degree of pyknosis. Furthermore, the use of specific inhibitors confirmed PfGR but not PfoTPT as a possible factor contributing to the development of pyknosis. A reduction in total glutathione levels was also detected in association with increased pyknosis. These results further our understanding of the mechanisms responsible for P. falciparum development and the antimalarial activity of dihydroartemisinin, and provide useful information for the development of novel antimalarial agents.

Copper Homeostasis for the Developmental Progression of Intraerythrocytic Malarial Parasite

Malaria is one of the world’s most devastating diseases, particularly in the tropics. In humans, Plasmodium falciparum lives mainly within red blood cells, and malaria pathogenesis depends on the red blood cells being infected with the parasite. Non-esterified fatty acids (NEFAs), including cis-9-octadecenoic acid, and phospholipids have been critical for complete parasite growth in serum-free culture, although the efficacy of NEFAs in sustaining the growth of P. falciparum has varied markedly. Hexadecanoic acid and trans-9-octadecenoic acid have arrested development of the parasite, in association with down-regulation of genes encoding copper-binding proteins. Selective removal of Cu⁺ ions has blockaded completely the ring–trophozoite–schizont progression of the parasite. The importance of copper homeostasis for the developmental progression of P. falciparum has been confirmed by inhibition of copper-binding proteins that regulate copper physiology and function by associating with copper ions. These data have provided strong evidence for a link between healthy copper homeostasis and successive developmental progression of P. falciparum. Perturbation of copper homeostasis may be, thus, instrumental in drug and vaccine development for the malaria medication. We review the importance of copper homeostasis in the asexual growth of P. falciparum in relation to NEFAs, copper-binding proteins, apoptosis, mitochondria, and gene expression.

In vitro adaptability of Plasmodium falciparum to different fresh serum alternatives

To reduce the dependency on fresh AB⁺ serum in continuous culture of Plasmodium falciparum, a comparative study was undertaken to assess the in vitro adaptability of P. falciparum to media supplemented with fresh AB⁺ serum from whole blood, AB⁺ plasma, serum derived from AB⁺ plasma, AB⁺ human serum from Sigma, Albumax II, fetal bovine serum and new born calf serum, independently and in different combinations. Combinations were used to analyze whether two different substitutes demonstrate any synergistic effect on the growth of the parasites. Our findings exhibited that the combination of fresh human serum and Albumax II showed good growth pattern in comparison to that of fresh serum and can thereby be instrumental in reducing the role of fresh human serum in continuous parasite maintenance. Culture maintained with Albumax II with or without hypoxanthine showed average growth.

Perturbation of copper homeostasis is instrumental in early developmental arrest of intraerythrocytic Plasmodium falciparum

Background

Malaria continues to be a devastating disease. The elucidation of factors inducing asexual growth versus arrest of Plasmodium falciparum can provide information about the development of the parasite, and may help in the search for novel malaria medication. Based on information from genome-wide transcriptome profiling of different developmental stages of P. falciparum, we investigated the critical importance of copper homeostasis in the developmental succession of P. falciparum with regard to three aspects of copper function. These were:1) inhibition of copper-binding proteins, 2) copper-ion chelation, and 3) down-regulated expression of genes encoding copper-binding proteins associated with a specific growth-promoting factor.

Results

Inhibition of copper-binding proteins with tetrathiomolybdate (TTM) caused cessation of growth of the parasite. TTM arrested the parasite irreversibly during the trophozoite to schizont stage progression. Target molecules for TTM may be present in P. falciparum. The involvement of copper ions in developmental arrest was also investigated by copper-ion chelating methods, which indicated a critical function of reduced copper ions (Cu¹⁺) in the parasite during the early developmental stage. Copper ions, not only in the parasite but also in host cells, were targets of the chelators. Chelation of Cu¹⁺caused blockage of trophozoite progression from the ring stage. Profound growth arrest was detected in parasites cultured in a chemically defined medium containing hexadecanoic acid alone as a growth-promoting factor. This developmental arrest was associated with down-regulated expression of genes encoding copper-binding proteins. Cis-9-octadecenoic acid completely prevented the down-regulation of gene expression and developmental arrest that were observed with the use of hexadecanoic acid.

Conclusions

The critical importance of copper homeostasis in early developmental stages of P. falciparum was confirmed. Perturbation of copper homeostasis induced profound and early developmental arrest of P. falciparum. These findings should help to elucidate the mechanisms behind the development of P. falciparum, and may be applied in the development of effective antimalarial strategies.

Novel inhibitor cystine knot peptides from Momordica charantia

Two new peptides, MCh-1 and MCh-2, along with three known trypsin inhibitors (MCTI-I, MCTI-II and MCTI-III), were isolated from the seeds of the tropical vine Momordica charantia. The sequences of the peptides were determined using mass spectrometry and NMR spectroscopy. Using a strategy involving partial reduction and stepwise alkylation of the peptides, followed by enzymatic digestion and tandem mass spectrometry sequencing, the disulfide connectivity of MCh-1 was elucidated to be CysI-CysIV, CysII-CysV and CysIII-CysVI. The three-dimensional structures of MCh-1 and MCh-2 were determined using NMR spectroscopy and found to contain the inhibitor cystine knot (ICK) motif. The sequences of the novel peptides differ significantly from peptides previously isolated from this plant. Therefore, this study expands the known peptide diversity in M. charantia and the range of sequences that can be accommodated by the ICK motif. Furthermore, we show that a stable two-disulfide intermediate is involved in the oxidative folding of MCh-1. This disulfide intermediate is structurally homologous to the proposed ancestral fold of ICK peptides, and provides a possible pathway for the evolution of this structural motif, which is highly prevalent in nature.

Molecular factors that are associated with early developmental arrest of intraerythrocytic Plasmodium falciparum

Malaria continues to be a devastating disease. We investigated the factors that control intraerythrocytic development of the parasite Plasmodium falciparum by using a chemically defined medium (CDM) containing non-esterified fatty acid(s) (NEFA) and phospholipids with specific fatty acid moieties, to identify substances crucial for parasite development. Different NEFAs in the CDM played distinct roles by altering the development of the parasite at various stages, with effects ranging from complete growth to growth arrest at the ring stage. We used genome-wide transcriptome profiling to identify genes that were differentially expressed among the different developmental stages of the parasite, cultured in the presence of various NEFAs. We predicted 26 transcripts that were associated with the suppression of schizogony, of which 5 transcripts, including merozoite surface protein 2, a putative DEAD/DEAH box RNA helicase, serine repeat antigen 3, a putative copper channel, and palmitoyl acyltransferase, were particularly associated with blockage of trophozoite progression from the ring stage. Furthermore, the involvement of copper ions in developmental arrest was detected by copper-ion-chelating methods, implying a critical function of copper homeostasis in the early growth stage of the parasite. These results should help to elucidate the mechanisms behind the development of P. falciparum.

Plasmodium falciparum: differing effects of non-esterified fatty acids and phospholipids on intraerythrocytic growth in serum-free medium

Different combinations of non-esterified fatty acids (NEFA) had variable effects on intraerythrocytic growth of Plasmodium falciparum. All stages of the parasite cultured in medium supplemented with cis-9-octadecenoic acid (C18:1-cis-9), hexadecanoic acid (C16:0), phospholipids (Pld) and bovine albumin free of NEFA were similar to those grown in complete growth medium. Three typical growth patterns indicating suppressed schizogony (SS), suppressed formation of merozoites (SMF), and inhibited invasion of merozoites (IMI) resulted from culture in other combinations of lipids. Unsaturated or saturated NEFA with longer or shorter carbon chains than C18:1-cis-9 or C16:0, higher degree of unsaturation, and trans-forms mainly resulted in SS and SMF effects. However, IMI or partial IMI was observed with tetradecanoic acid or octadecanoic acid enriched with C18:1-cis-9, and cis-9-hexadecenoic acid plus C16:0. Isoforms of C18:1-cis-9 also mainly resulted in partial IMI. SMF also occurred with C18:1-cis-9 plus C16:0 in the absence of Pld. Thus different NEFA exerted distinct roles in erythrocytic growth of the parasite by sustaining development at different stages.

Plasmodium falciparum: Chemically defined medium for continuous intraerythrocytic growth using lipids and recombinant albumin

Dioleoylphosphatidylcholine and other phosphatidylcholines containing different fatty acid moieties were found to increase the ability of nonesterified fatty acids (NEFA) to sustain continuous intraerythrocytic growth of Plasmodium falciparum in the presence of specific proteins. Other phospholipids, including phosphatidylethanolamine, phosphatidylserine, and phosphatidic acid, were beneficial to parasite growth. Different combinations and concentrations of NEFA tested in the presence of phospholipids and bovine albumin had variable effects on parasite growth. The most effective combination for promoting parasite growth consisted of 30 microg/ml cis-9-octadecenoic acid (oleic acid) plus 15 microg/ml hexadecanoic acid (palmitic acid). Recombinant human albumin could replace bovine or human albumin in culture media enriched with structurally defined lipids. This study therefore established a chemically defined culture medium suitable for sustaining the growth of P. falciparum.

The prevalence and degree of resistance of Plasmodium falciparum to first-line antimalarial drugs: an in vitro study from a malaria endemic region in Yemen

BACKGROUND

Unpublished studies on antimalarial drug efficacy have found low levels of chloroquine resistance in Yemen. This study was carried out to determine the current prevalence of drug resistance in Plasmodium falciparum in Yemen to the main anti-malarial drugs and to determine the effective concentration(EC) values.

METHODS

The WHO standard protocol was used for the selection of subjects, collection of blood samples, culture techniques, examination of post-culture blood slides and interpretation of results. The in vitro micro-test Mark III was used for assessing susceptibility of P. falciparum isolates.

RESULTS

The criteria for blood parasite density was met by 219 P. falciparum malaria patients. Chloroquine resistance was found in 47% of isolated P. falciparum schizonts. Mefloquine resistanfce was found in 5.2%. In addition, the EC50 and EC95 values in blood that inhibited schizont maturation in resistant isolates were higher than the normal therapeutic level of mefloquine. No resistance occurred against quinine or artemisinin, with no growth at hte cut-off level of quinine and inhibition of low concentrations of artemisinin.

CONCLUSION

Our study confirmed the occurrence of chloroquine-resistant P. falciparum and a slow increase in the rate of this resistance; it is likely that resistance will increase further and spread over all the foci of malaria in Yemen. The low rate of mefloquine-resistant P. falciparum, was lower than that reported in Africa or Southeast Asia, but it is the first report of mefloquine resistance in Yemen. Finally, the isolates were sensitive to low-concentrations of quinine and artemisinin.

In vitro cultivation of Plasmodium falciparum: studies with modified medium supplemented with ALBUMAX II and various animal sera

RPNI, a combination of three commercially available growth media (RPMI-1640, NCTC-135 and IMDM) has been found to support long term continuous cultivation of 3D7 strain of Plasmodium falciparum in the presence of 10% bovine calf serum. During the present study, the suitability of this medium was evaluated for the development of P. falciparum in the presence of horse, goat and rabbit sera as well as various concentrations of ALBUMAX II. RPNI medium supplemented with 10% bovine calf serum (RPNI-BCS) was used as control. The cultures were maintained in candle jars protocol and parasitaemia was monitored daily up to day 7. Horse, goat and rabbit sera all supported the development of P. falciparum. Horse serum gave best results in RPNI medium and supported continuous culture up to day 100. The parasitaemia in the presence of ALBUMAX was significantly higher in RPNI than in RPMI-1640. Addition of hypoxanthine in RPMI-1640 caused an increase in parasitaemia whereas no obvious advantage could be observed in RPNI. The findings exhibited that medium RPNI has an edge over conventional RPMI-1640 medium for in vitro cultivation of P. falciparum.

Investigating serum factors promoting erythrocytic growth of Plasmodium falciparum

The elucidation of factors inducing the growth of Plasmodium falciparum can provide critical information about the developmental mechanisms of this parasite and open the way to search for novel targets for malaria chemotherapy. The ability of components of a growth-promoting factor derived from bovine serum and various related substances to sustain growth of P. falciparum was characterized. A simple total lipid fraction (GFS-C) containing non-esterified fatty acids (NEFAs) as essential factors was noted to promote the parasite's growth. Various proteins from a variety of animals were tested, indicating the importance not only of GFS-C, but also of specific proteins, such as bovine and human albumin, in the parasite growth. Several combinations of the NEFAs tested sustained low parasite growth. Among various phospholipids and lysophospholipids tested, lysophosphatidylcholine containing C-18 unsaturated fatty acids was found to sustain the complete development of the parasite in the presence of bovine albumin. Several other lysophospholipids can partially support growth of P. falciparum.

Cultivation of plasmodium spp

Cultivation of both human and non-human species of Plasmodium spp., the causal agent of malaria, has been a major research success, leading to a greater understanding of the parasite. Efforts at cultivating the organisms in vitro are complicated by the parasites' alternating between a human host and an arthropod vector, each having its own set of physiological, metabolic, and nutritional parameters. Life cycle stages of the four species that infect humans have been established in vitro. Of these four, P. falciparum remains the only species for which all stages have been cultured in vitro; different degrees of success have been achieved with the other human Plasmodium spp. The life cycle includes the exoerythrocytic stage (within liver cells), the erythrocytic stage (within erythrocytes or precursor reticulocytes), and the sporogonic stage (within the vector). Culture media generally consist of a basic tissue culture medium (e.g., minimal essential medium or RPMI 1640) to which serum and erythrocytes are added. Most of the efforts have been directed toward the stage found in the erythrocyte. This stage has been cultivated in petri plates or other growth vessels in a candle jar to generate elevated CO(2) levels or in a more controlled CO(2) atmosphere. Later developments have employed continuous-flow systems to reduce the labor-intensive nature of medium changing. The exoerythrocytic and sporogonic life cycle stages have also been cultivated in vitro. A number of avian, rodent, and simian malarial parasites have also been established in vitro. Although cultivation is of great help in understanding the biology of Plasmodium, it does not lend itself to use for diagnostic purposes.

Simple isoquinoline and benzylisoquinoline alkaloids as potential antimicrobial, antimalarial, cytotoxic, and anti-HIV agents

Twenty-six simple isoquinolines and 21 benzylisoquinolines were tested for antimicrobial, antimalarial, cytotoxic, and anti-HIV activities. Some simple isoquinoline alkaloids were significantly active in each assay, and may be useful as lead compounds for developing potential chemotherapeutic agents. These compounds include 13 (antimicrobial), 25, 26, and 42 (antimalarial), 13 and 25 (cytotoxic), and 28 and 29 (anti-HIV). A quaternary nitrogen atom of isoquinolium or dihydroisoquinolinium type may contribute to enhanced potency in the first three types of activities. In contrast, anti-HIV activity was found with tetrahydroisoquinoline and 6,7-dihydroxyisoquinolium salts.

Serum factors governing intraerythrocytic development and cell cycle progression of Plasmodium falciparum

Malaria is clinically manifested only when the human malaria parasites in the genus Plasmodium enter the obligatory intraerythrocytic life cycle. Elucidation for the roles of the serum, the key nutrient, and its components is then deemed essential for thorough understanding of the proliferation of Plasmodium cells at the erythrocytic stage. Fractionation and analysis of serum and its components was performed by chromatography, solvent extraction, and subsequent reconstitution experiments. Only fractions containing serum albumin (SA) from the serum and purified intact bovine serum albumin (BSA) showed comparable growth promoting activity with human serum (HS). Delipidated BSA can only effect parasite growth after reconstitution with lipids extracted from intact BSA. Fatty acid (FA) species in the neutral lipid fraction from intact BSA proved likewise when reconstituted with delipidated BSA. Furthermore, the involved FA species have to come in a pair of one saturated and one unsaturated, with palmitic and oleic acids as the best combination. The results were further substantiated by morphological analysis as well as biochemical analysis of the DNA synthesis during the intraerythrocytic development. This study can be a basis to explore the molecular mechanism of lipid traffic within the parasitized red blood cell (RBC), which can be an important adjunct to the development of drugs for malaria therapy.

In vitro culture and drug sensitivity assay of Plasmodium falciparum with nonserum substitute and acute-phase sera

The short-term in vitro growth of Plasmodium falciparum parasites in the asexual erythrocytic stage and the in vitro activities of eight standard antimalarial drugs were assessed and compared by using RPMI 1640 medium supplemented with 10% nonimmune human serum, 10% autologous or homologous acute-phase serum, or 0.5% Albumax I (lipid-enriched bovine serum albumin). In general, parasite growth was maximal with autologous (or homologous) serum, followed by Albumax I and nonimmune serum. The 50% inhibitory concentrations (IC50s) varied widely, depending on the serum or serum substitute. The comparison of IC50s between assays with autologous and nonimmune sera showed that monodesethylamodiaquine, halofantrine, pyrimethamine, and cycloguanil had similar IC50s. Although the IC50s of chloroquine, monodesethylamodiaquine, and dihydroartemisinin were similar with Albumax I and autologous sera, the IC50s of all test compounds obtained with Albumax I differed considerably from the corresponding values obtained with nonimmune serum. Our results suggest that Albumax I and autologous and homologous sera from symptomatic, malaria-infected patients may be useful alternative sources of serum for in vitro culture of P. falciparum isolates in the field. However, autologous sera and Albumax I do not seem to be suitable for the standardization of isotopic in vitro assays for all antimalarial drugs.

Hypoxanthine: a low molecular weight factor essential for growth of erythrocytic Plasmodium falciparum in a serum-free medium

A low molecular weight factor in a basal medium essential for erythrocytic Plasmodium falciparum development in a serum-free medium using a cell growth-promoting factor derived from adult bovine serum was detected. The factor was hypoxanthine. The optimal hypoxanthine concentration for parasite growth was between 15 and 120 microM. The contribution of hypoxanthine to increased parasite growth was clearly evident in cultures on day 4. Among various low molecular weight supplements tested, adenine, adenosine, AMP, ATP, cyclic AMP, guanine, guanosine, inosine, inosine monophosphate, xanthine, NAD, NADH, NADP, NADPH and deoxyguanosine triphosphate showed a similar effect to that of hypoxanthine in the serum-free culture system. On the other hand, the addition of uric acid, FAD, thymidine, uridine, orotic acid, deoxythymidine triphosphate, deoxycytidine triphosphate, deoxyadenosine triphosphate, ribose-1-phosphate, or ethanolamine was not beneficial to the parasite growth. The results presented here will not only be of practical value, but will provide important information about the developmental requirements of the parasite.

Relationship between maternally derived anti-Plasmodium falciparum antibodies and risk of infection and disease in infants living in an area of Liberia, west Africa, in which malaria is highly endemic

In areas where Plasmodium falciparum is endemic, immunoglobulin G is acquired by the fetus in utero, mainly during the third trimester of pregnancy. The potential protective effect of transferred anti-P. falciparum maternal antibodies was examined in a longitudinal study of 100 infants from birth to 1 year of age. The probability of acquiring a P. falciparum infection and developing an episode of clinical malaria was determined in relation to the P. falciparum-specific antibody level of the infant at birth against P. falciparum schizont antigen or recombinant merozoite surface protein MSP1(19) antigen. The risk of acquiring an episode of clinical malaria increased from birth to 6 months of age, after which it decreased. The overall prevalence of P. falciparum parasitemia was highest (48.9%) in the 6-month-old infants. The age-specific hematocrit value showed the lowest mean value (30.2) from 6 to 9 months, and the spleen rate was the highest (69.8%) at the same age. There was a lower risk of developing an episode of clinical malaria during the first year of life in the infants with high levels of anti-MSP1(19) antibodies at birth. The level of maternally derived overall anti-schizont antigen antibodies did not seem to play a role in the relative risk of developing malaria infection or disease during the first year of life, though the level of specific anti-MSP1(19) antibodies may be associated with protection.